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Advanced equipment aboard Hubble reveals galaxies' most elusive secrets

New, high-precision equipment orbiting Earth aboard the Hubble Space Telescope is now sending such rich data back to astronomers, some feel they are crossing the final frontier toward understanding galaxy evolution, says Todd Tripp, leader of the team at the University of Massachusetts Amherst.

The illustration shows how quasistellar object (QSO) absorption lines are used to study the vast (and effectively invisible) gaseous halos of galaxies. When the light from a distant QSO passes through the gas surrounding a foreground galaxy (schematically indicated with a red dashed circle), some of the colors of the QSO light are absorbed by the foreground material. Consequently, the Hubble Space Telescope observes that some of the colors are “missing.” By studying the absorbed colors, astronomers can determine many things about the gaseous halo, such as composition, temperature, density and mass. This technique has revealed that the gaseous halos of galaxies as much larger and more massive than the distribution of stars within the galaxy. These large halos are produced by “winds” of matter rapidly moving away from the galaxies [Credit: Courtesy of Todd M. Tripp, UMass Amherst and NASA Galaxy Evolution Explorer]

Galaxies are the birthplaces of stars, each with a dense, visible central core and a huge envelope, or halo, around it containing extremely low-density gases. Until now, most of the mass in the envelope, as much as 90 percent of all mass in a galaxy, was undetectable by any instrument on Earth.

But Hubble's sensitive new Cosmic Origins Spectrograph (COS), the only one of its kind, has dramatically improved the quality of information regarding the gaseous envelope of galaxies, Tripp says. This huge gain in precision is one of the enormous accomplishments of the COS mission. "Even 10 years ago, most of the mass of a galaxy was invisible to us and such detailed investigations were impossible." the UMass Amherst astronomer points out. "With COS, in a sense we now have the ability to see the rest of the iceberg, not just the tip. This is a very exciting time to be an astronomer."

Tripp, postdoctoral researcher Joe Meiring and theoretical astronomer Neil Katz are co-authors of several companion articles reporting advances in understanding galaxy evolution based on the new COS data in the Nov. 18 issue of Science. Other lead investigators are Nicolas Lehner of the University of Notre Dame and Jason Tumlinson of the Space Telescope Science Institute, Baltimore.

"With the new spectrograph we can see galaxy halos out to at least 150,000 parsecs," says Tripp. One kiloparsec is about 19 trillion miles. "Where once we saw only the framework we are now getting a more complete picture, including the composition and movement of gases in the envelope, varying temperatures in different locations and the chemical structure, all in incredible detail," Tripp adds.

In particular, data on the chemical composition and temperature in the gas clouds allow the astronomers to calculate a galaxy's halo mass and how the gaseous envelope regulates the galaxy's evolution.

Another overall mission focus is to explore how galaxies gather mass for making stars. The astronomers have found that heavy elements in the envelopes surrounding the most vigorous star-forming galaxies continuously recycle material, as supernovae explode and shoot hot gas for trillions of miles. Faster-moving material escapes the envelope, but slower- moving particles collapse back into the center and restart the cycle.

Tripp and his UMass Amherst team specialize in studying how the fast-moving gases and matter from exploding supernovae circulate in galaxies. It was a surprise to discover how much mass extends far outside each galaxy, he says. "Not only have we found that star-forming galaxies are pervasively surrounded by large halos of hot gas," says Tripp, "we have also observed that hot gas in transit. We have caught the stuff in the process of moving out of a galaxy and into intergalactic space."

Further, the speed at which gases are moving in different parts of a galaxy is critical. Slower speeds may mean cooling gases, ready to collapse back into the core. Hotter gases are likely expanding and might escape the envelope.

Because the light emitted by this hot plasma is so faint that it is effectively invisible, astronomers use a trick to illuminate it from behind, like studying a misty fog bank by looking through lighthouse beams. In this case the lighthouse is usually a quasar, a super bright object behind the galaxy of interest. Gathering several sightings through the fog, scientists can piece together a map of the gaseous envelope.

Certain wavelengths of light emitted by the quasar are absorbed by the ions in a galaxy's envelope. With COS, a whole new area of the electromagnetic spectrum has become visible. To learn more, Tripp and colleagues also calculate concentrations of the many elements such as hydrogen, oxygen, sulfur, carbon and neon in the envelope, plus up to five ions of each. One of the neon ions has turned out to be particularly important.

"In detecting the neon ions we find that there's a lot of gas at several hundred thousand degrees Kelvin, which we've never been able to see unambiguously before," says Tripp. "It means we can characterize the total mass distribution in the envelope, setting more precise constraints on the temperatures overall. We can now access more diverse ions, and we have new leverage on determining whether stuff is heating up or cooling off. We're gaining new insights."

The neon ion will also play a role in testing theoretical models of galaxy evolution. Theorists including Katz at UMass Amherst construct model galaxies on a computer, simulating its make-up and how it evolves over time. Tripp says, "Now we have hard data to plug into the model and test their ideas. They've got a lot of detailed predictions we can now compare to the real universe. It's a new day for all of us."

Source: University of Massachusetts at Amherst [November 17, 2011]

Chandra adds to black hole birth announcement

New details about the birth of a famous black hole that took place millions of years ago have been uncovered, thanks to a team of scientists who used data from NASA's Chandra X-ray Observatory as well as from radio, optical and other X-ray telescopes.

Cygnus X-1 is located near large active regions of star formation in the Milky Way. An artist's illustration depicts what astronomers think is happening within the Cygnus X-1 system. Cygnus X-1 is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star. The black hole pulls material from a massive, blue companion star toward it. This material forms a disk (shown in red and orange) that rotates around the black hole before falling into it or being redirected away from the black hole in the form of powerful jets [Credit: NASA/CXC/M. Weiss]

Over three decades ago, Stephen Hawking placed -- and eventually lost – a bet against the existence of a black hole in Cygnus X-1. Today, astronomers are confident the Cygnus X-1 system contains a black hole, and with these latest studies they have remarkably precise values of its mass, spin, and distance from Earth. With these key pieces of information, the history of the black hole has been reconstructed.

"This new information gives us strong clues about how the black hole was born, what it weighed and how fast it was spinning," said author Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass. "This is exciting because not much is known about the birth of black holes."

Reid led one of three papers -- all appearing in the November 10th issue of The Astrophysical Journal -- describing these new results on Cygnus X-1. The other papers were led by Jerome Orosz from San Diego State University and Lijun Gou, also from CfA.

Cygnus X-1 is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star. The black hole is in close orbit with a massive, blue companion star.

Using X-ray data from Chandra, the Rossi X-ray Timing Explorer, and the Advanced Satellite for Cosmology and Astrophysics, a team of scientists was able to determine the spin of Cygnus X-1 with unprecedented accuracy, showing that the black hole is spinning at very close to its maximum rate. Its event horizon -- the point of no return for material falling towards a black hole -- is spinning around more than 800 times a second.

An independent study that compared the evolutionary history of the companion star with theoretical models indicates that the black hole was born some 6 million years ago. In this relatively short time (in astronomical terms), the black hole could not have pulled in enough gas to ramp up its spin very much. The implication is that Cygnus X-1 was likely born spinning very quickly.

Using optical observations of the companion star and its motion around its unseen companion, the team made the most precise determination ever for the mass of Cygnus X-1, of 14.8 times the mass of the Sun. It was likely to have been almost this massive at birth, because of lack of time for it to grow appreciably.

"We now know that Cygnus X-1 is one of the most massive stellar black holes in the Galaxy," said Orosz. "And, it's spinning as fast as any black hole we've ever seen."

Over three decades ago, Stephen Hawking placed -- and eventually lost - a bet against the existence of a black hole in Cygnus X-1. Today, astronomers are confident the Cygnus X-1 system contains a black hole. In fact, a team of scientists has combined data from radio, optical, and X-ray telescopes including Chandra to determine the black hole's spin, mass, and distance more precisely than ever before. With these key pieces of information, the history of the black hole has been reconstructed. This is an X-ray image of Cygnus X-1 from the Chandra X-ray Observatory [Credit: NASA/CXC]

Knowledge of the mass, spin and charge gives a complete description of a black hole, according to the so-called "No Hair" theorem. This theory postulates that all other information aside from these parameters is lost for eternity behind the event horizon. The charge for an astronomical black hole is expected to be almost zero, so only the mass and spin are needed.

"It is amazing to me that we have a complete description of this asteroid-sized object that is thousands of light years away," said Gou. "This means astronomers have a more complete understanding of this black hole than any other in our Galaxy."

The team also announced that they have made the most accurate distance estimate yet of Cygnus X-1 using the National Radio Observatory's Very Long Baseline Array (VLBA). The new distance is about 6,070 light years from Earth. This accurate distance was a crucial ingredient for making the precise mass and spin determinations.

The radio observations also measured the motion of Cygnus X-1 through space, and this was combined with its measured velocity to give the three-dimensional velocity and position of the black hole.

This work showed that Cygnus X-1 is moving very slowly with respect to the Milky Way, implying it did not receive a large "kick" at birth. This supports an earlier conjecture that Cygnus X-1 was not born in a supernova, but instead may have resulted from the dark collapse of a progenitor star without an explosion. The progenitor of Cygnus X-1 was likely an extremely massive star, which initially had a mass greater than about 100 times the sun before losing it in a vigorous stellar wind.

In 1974, soon after Cygnus X-1 became a good candidate for a black hole, Stephen Hawking placed a bet with fellow astrophysicist Kip Thorne, a professor of theoretical physics at the California Institute of Technology, that Cygnus X-1 did not contain a black hole. This was treated as an insurance policy by Hawking, who had done a lot of work on black holes and general relativity.

By 1990, however, much more work on Cygnus X-1 had strengthened the evidence for it being a black hole. With the help of family, nurses, and friends, Hawking broke into Thorne's office, found the framed bet, and conceded.

"For forty years, Cygnus X-1 has been the iconic example of a black hole. However, despite Hawking's concession, I have never been completely convinced that it really does contain a black hole -- until now," said Thorne. "The data and modeling described in these three papers at last provide a completely definitive description of this binary system."

Source: Chandra X-ray Center [November 17, 2011]

New FASTSAT discoveries paint detailed view of region near Earth

Space around Earth is anything but a barren vacuum. The area seethes with electric and magnetic fields that change constantly. Charged particles flow through, moving energy around, creating electric currents, and producing the aurora. Many of these particles stream in from the solar wind, starting out 93 million miles away on the surface of the sun. But some areas are dominated by particles of a more local source: Earth's atmosphere.

This artist's concept drawing shows the Fast, Affordable, Science and Technology SATellite (FASTSAT) -- NASA's first microsatellite, which launched on Nov. 19, 2010 and has been collecting data on the dynamic atmosphere surrounding Earth [Credit: NASA]

These are the particles being watched by FASTSAT's Miniature Imager for Neutral Ionospheric Atoms and Magnetospheric Electrons (MINI-ME) instrument. For one well-defined event, scientists have compared MINI-ME's observations to those from two other instruments. The event shows a detailed picture of this dynamic region, with a host of interrelated phenomena -- such as electric current and outflowing particles – occurring together.

"We're seeing structures that are fairly consistent throughout a handful of instruments," says Michael Collier at NASA's Goddard Space Flight Center in Greenbelt, Md., who is the principal investigator for MINI-ME. "We put all of these observations together and it tells a story greater than the sum of its parts."

Unlike the hotter hydrogen coming from the sun, Earth's upper atmosphere generally supplies cooler oxygen ions that course outward along Earth's magnetic field lines. This "ion outflow" occurs continuously, but is especially strong during periods when there is more solar activity such as solar flares and coronal mass ejections that burst off the sun and move toward Earth. Such activity drives oxygen ions out of our planet's upper atmosphere, particularly in regions where aurora displays are strong.

"These ion outflow events are important because they help us understand the space weather environment around Earth," says Goddard's Doug Rowland who is the principal investigator for FASTSAT's Plasma Impedance Spectrum Analyzer, or PISA instrument. "The heavy ions flowing away from Earth can act as a brake, or damper, on incoming energy from the solar wind. The flow also indicates ways in which planets can lose their atmospheres – something that happens slowly on Earth, but more quickly on smaller planets with weaker magnetic fields, like Mars."

MINI-ME has been successfully spotting such outflows since the instrument first began to collect data in the winter of 2010. The instrument counts ions as it moves through a part of Earth's atmosphere called the ionosphere. This is the region where the particles gain enough speed and energy to overcome Earth's gravity, so it's an ideal place to study the first step in the outflow process.

Late on March 31, 2011, the FASTSAT spacecraft flew through an ion outflow with well-defined areas of increased fast moving, or "energetic," particles.

Simultaneous observations from PISA, which measures the density of material in the atmosphere, also showed that this was a highly structured auroral zone. In addition, the scientists turned to the National Science Foundation's Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), a mission managed by the Johns Hopkins Applied Physics Laboratory, which measures current flow and magnetic features through a network of instruments placed on commercial satellites owned by Iridium Communications. AMPERE data showed current structures that were also consistent with what is expected for an auroral zone.

"This is just one event," says Collier. "But it helps confirm the idea that the current and ion-outflows are all connected. As we continue to go through the data, there will be many more events to follow. We'd like to be able to pin down the origin of all these mechanisms in the ionosphere."

Over time, data like this will allow scientists to determine where these ions come from, what drives them, and how their intensity varies with incoming solar activity.

Source: NASA/Goddard Space Flight Center [November 14, 2011]

The methane habitable zone

In the search for life elsewhere, many studies focus on finding liquid water. But what if life could exist with some other solvent? Saturn’s smoggy moon Titan makes scientists question the possibilities for methane-based life in the galaxy.

Our planet is at just the right distance from the Sun for liquid water to exist on its surface and play a role in life [Credit: NASA]

The search for life is largely limited to the search for water – we look for exoplanets at the correct distance from their stars for liquid water to splash and flow freely on their surfaces, we ‘follow the water’ on the red planet Mars, and SETI scans radio frequencies in the ‘water hole’ between the 1,420MHz emission line of neutral hydrogen and the 1,666MHz hydroxyl line.

There are two very good reasons why our attention is so strongly focused on water. First, it’s an efficient solvent for biological chemistry, allowing molecules to move around in cells, and has properties that are friendly to life – a high heat capacity, the ability to remain in liquid form across a wide temperature range, and a molecular density that forces molecules to organize themselves, rather than the water organizing around the molecules. Secondly, the biosignatures of a water-based chemistry are a lot easier for us to identify remotely.

Moreover, the most important fact about water’s relationship with life is that it is here on Earth. “Some argue that is the only important thing about water,” says Chris McKay of NASA’s Ames Research Center. McKay is an astrobiologist and planetary scientist who specializes in hunting down alien environments and then asking the question, ‘could something live here?’, rather than deciding what is and isn’t habitable beforehand.

“We live on a planet where water is a liquid and we have adapted and evolved to work with that liquid,” he says. “Life has very cleverly used the properties of water to do things not just in terms of solution, but in using the strong polarity of that solution to its advantage in terms of hydrophobic and hydrophilic bonds, and using the very structure of water to help align molecules.”

Suppose, though, that life needn’t be constrained to a water-based chemistry; would we be able to recognize the signatures of such life and the habitats in which it lives? From one perspective, water seems such a good match for life because it may be the only match – no other liquid has the properties or abundance that water has. On the other hand there is another point of view that says there is more to the story, and that life simply works with whatever materials it has at hand. On Earth, that material is water, but on other planets it may be something else. Already we have discovered another world, in our Solar System, where rivers and lakes are made with a quite different liquid.

Titan’s Liquid for Life?

One point four billion kilometers from the Sun orbits Saturn, the majestic ringed planet. Saturn is a gaseous world with an atmosphere of hydrogen and helium, and no discernible rocky surface below. However, among its retinue of icy moons is Titan, bigger than the planet Mercury and swathed in a dense cloak of hydrocarbon smog suspended in its nitrogen-rich atmosphere. It’s the only moon in the Solar System to have an atmosphere, and it has intrigued astronomers ever since Gerard Kuiper detected methane there in 1944.

Sunlight glints off a methane lake at near Titan’s north pole in this five-micron infrared image taken by the Cassini spacecraft [Credit: NASA/JPL/University of Arizona/DLR]

When the joint NASA–ESA Cassini–Huygens mission arrived in the Saturnian system in 2004, the truth about Titan was revealed. Infrared cameras and radar on Cassini showed a world riven by black, oily rivers and lakes, whilst the Huygens probe plunged through the opaque atmosphere to land on a soggy floodplain, but not one damp with water. On Titan, where the temperature is just 94 degrees above absolute zero (–179 degrees Celsius) water is as solid as rock and liquid methane runs through the river valleys and into the high latitude polar lakes. Instead of a water cycle, Titan has a methane cycle, and a complex molecular soup formed from reactions in the upper atmosphere between ultraviolet radiation from the Sun and methane.

Suppose life could exist in an environment like this; it would be a whole new category of habitable planet, one where liquid methane replaces liquid water, consequently leading to an entirely different habitable zone, one that is farther out from a star than the liquid water zone.

McKay is already ahead of the game. Along with Ashley Gilliam of NASA Ames and the University of California, Santa Cruz, he published a paper in the journal Planetary and Space Science in April that describes where a world with temperatures suitable for liquid methane could be found around a red dwarf star.

Methane Worlds

Red dwarfs – also called M-dwarfs after of their classification on the Hertzsprung–Russell diagram – are stars that are smaller and cooler than our Sun, and therefore the planetary systems around them are scaled down accordingly. McKay and Gilliam calculate that a planet would have a surface temperature of –179 degrees Celsius in a zone between 0.63 and 1.66 astronomical units (99 million and 248 million kilometers around the star Gliese 581, an M3-type red dwarf located 20.5 light years away. Four planets have already been confirmed orbiting Gliese 581, but none within the ‘liquid methane habitable zone’. Two more planets have been claimed to exist in the system, and one of these would fall within the zone, at 0.76 astronomical units, but evidence for the existence of this world has proven highly controversial. Alternatively, the liquid methane habitable zone around a cooler M4 type red dwarf would be even closer in, between 0.084 astronomical units and 0.23 astronomical units (12.6 million kilometers to 34.4 million kilometers.

Cold, methane-dominated worlds could easily exist around Sun-like stars, and Titan is proof of that. But there are advantages to finding these worlds around red dwarfs instead. First, their small orbital radii make them easier to detect, whether by transits or via radial velocity Doppler shifts. Second, Titan’s atmosphere is opaque to blue and ultraviolet light, but transparent to red and infrared light, and red dwarfs produce more of the latter than the former. If Titan orbited a red dwarf, more red light would seep through to its surface, warming the planet and extending the range of the liquid methane habitable zone. (Interestingly, a red giant, which is close to the endpoint in the life cycle of a Sun-like star, produces light of similar red wavelengths. When our Sun expands into a bloated red giant in about five billion years, engulfing all the planets up to Earth and possibly Mars, Titan may well reap the benefits – for a short while at least before the red giant puffs away to leave behind a white dwarf star.)

Red dwarfs are also often highly magnetically active, and experience large stellar flares that emit powerful bursts of ultraviolet radiation. While these flares should not irreversibly damage exoplanet atmospheres, according to research led by Antigona Segura of Universidad Nacional Autonoma de Mexico, they could have a different kind of impact on Titan-esque atmospheres, disassociating molecules to create a haze like we see shrouding Saturn’s largest moon. The more active the red dwarf, the more hazy the atmosphere of a Titan-like planet becomes, and the thicker the haze, the colder the surface and the closer to the star the liquid methane habitable zone has to be.

Greater or lesser haze also would alter the outward appearance of such a world, and if we are to one day hunt for habitable planets in the liquid methane habitable zone, we’ll need to know what they will look like, as well as what biosignatures to search for. It is here that the main stumbling block lies.

“We just don’t know what the tell-tale signs for life would be in such an atmosphere because it is so vastly different from ours,” says Lisa Kaltenegger, who conducts research into habitable exoplanets at the Max Planck Institute in Germany and at the Harvard–Smithsonian Center for Astrophysics. “That said, it will change in a flash if Chris [McKay] finds life on Titan and can tell us what it produces and what we could look for remotely with a telescope.”

Proof of Life

However, McKay already has a few suggestions and, tantalizingly, there may even be supporting evidence from Titan. In 2005 he published a paper with Heather Smith of the International Space University in Strasbourg, building on work by Steven Benner of the University of Florida, describing how methane-based life forms on Titan (‘methanogens’) could consume hydrogen, acetylene and ethane, and exhale methane instead of carbon-dioxide. If such a life form existed, it could reveal itself through a depletion of hydrogen, acetylene and ethane at the surface.

Comparison of the habitable zone in the Solar System and the habitable zone around Gliese 581. Because Gliese 581 is a cooler red dwarf, its liquid water habitable zone is closer in, while its liquid methane habitable zone would be between 0.63 and 1.66 astronomical units [Credit: ESO]

Amazingly, this is what studies of this moon have actually shown, but McKay himself is dubious that these measurements necessarily mean there is life on Titan. Rather, he points to other, more likely explanations including mistakes in the modeling of Titan’s atmosphere upon which these measurements are partially derived, to unknown physical processes occurring on Titan that are not related to life. In regards to using this formula to search for life elsewhere, the depletion of hydrogen, acetylene and ethane on the surface of a hazy planet are not conducive to remote spectroscopic imaging from many light years away.

“It’s not clear that we would be able to see the depletion of hydrogen over interstellar distances,” says McKay. “On Earth, of course, the big biosignature that is visible over interstellar distances is the build-up of oxygen, but even that is not an unambiguous indicator for life, because for most of Earth’s history there has been life but no build-up of oxygen.”

Nevertheless, Jonathan Lunine of the University of Arizona has speculated that there are many more exo-Titans than exo-Earths out there. If Chris McKay is right about methane-based life, such habitats could vastly outnumber planets with water-based life. The trouble is, the cold temperatures in which liquid methane exists would result in life forms with very slow metabolisms. Life would be sluggish. Is there any way it could be warmed up?

“You would have to invoke a lot of pressure to keep methane liquid at warmer temperatures,” says Kaltenegger. “And if you think about it, water and carbon are extremely abundant. So if you make the planet warmer, you are much more likely to get carbon dioxide than methane, and methane would go out of its liquid phase so you wouldn’t have the triple point [where for a given temperature and pressure a material can exist as a liquid, solid or gas] like the triple point of water on Earth.”

Other Solutions

If warm liquid methane is out of the question, what other potential substitutes for water are there? Hydrogen fluoride comes close to the properties of liquid water, but fluoride is relatively rare in the Universe and so is unlikely to play a major factor in life. More common is salt: if temperatures are hot enough, salts will become liquid.

“I can imagine a world where there is sodium chloride liquid and salt is somehow the basis for life,” says McKay, but he admits it is speculative. Rather than spending too much time on outlandish ideas, he believes it’s better to search for the environments first, and then ask, can anything live there?

Lisa Kaltenegger shares the same philosophy. “First we have to figure out using models what it takes for a planet to be habitable, and then we look at the data that comes in about such worlds and see how far we can stretch the habitable definition.”

Yet of the nearly 700 confirmed exoplanets so far (not counting all the unconfirmed candidate worlds detected by NASA’s Kepler mission) only two are potentially suitable for liquid water, and that’s only on the condition that circumstances on the planets themselves are perfect. As for liquid methane, without some hard data on the properties of methane-based life, astronomers are going to err on the side of caution. Even McKay still favors searching for water-based life and habitats.

“We know water works, it’s something that we understand,” he says. “Although I argue that we should consider liquid methane, we don’t know that it works. In that sense we’re still guessing.”

We may be kept guessing until the next mission to Titan, which may not happen until the 2030s. Just as Earth is the template for water-based habitats, Titan is the template for methane-based habitats. However, unless life can be shown to exist there, liquid methane habitable zones will always be passed over in favor of watery zones. Unfortunately, we may be missing a huge chunk of life in the process.

Author: Keith Cooper | Source: Astrobio.net [November 11, 2011]

The Tarantula glows with X-rays and infrared light

This spiderweb-like tangle of gas and dust is a star-forming region called 30 Doradus. It is one of the largest such regions located close to the Milky Way galaxy, and is found in the neighboring galaxy Large Magellanic Cloud.

The star-forming region, 30 Doradus, is one of the largest located close to the Milky Way and is found in the neighboring galaxy Large Magellanic Cloud. About 2,400 massive stars in the center of 30 Doradus, also known as the Tarantula Nebula, are producing intense radiation and powerful winds as they blow off material [Credit: X-ray: NASA/CXC/PSU/L. Townsley et al.; Infrared: NASA/JPL/PSU/L. Townsley et al]

About 2,400 massive stars in the center of 30 Doradus, also known as the Tarantula nebula, are producing intense radiation and powerful winds as they blow off material.

Multimillion-degree gas detected in X-rays (blue) by NASA's Chandra X-ray Observatory comes from shock fronts -- similar to sonic booms -- formed by these stellar winds and by supernova explosions. This hot gas carves out gigantic bubbles in the surrounding cooler gas and dust shown here in infrared light from NASA's Spitzer Space Telescope (orange).

Source: JPL/NASA [November 10, 2011]

Close encounter of the rocky kind

Discovered by a UA astronomer six years ago, a city-block sized space rock will race past the Earth closer than the moon in what will be the closest encounter of an object of this size in more than 60 years.

A radar image of asteroid 2005 YU55 taken by the Arecibo radio telescope in Puerto Rico [Credit: NASA/Cornell/Arecibo]

When an asteroid the size of a city block zips past the Earth about 29,000 miles per hour on Nov. 8, it will seem like an encounter with an old acquaintance to Univeristy of Arizona astronomer Robert McMillan.

Six years ago, McMillan was taking images of the night sky with an 83-year-old telescope on Kitt Peak searching for asteroids, chunks of rock that weren't swept up into one of the nascent planets during the formation of our solar system and have traveled around the sun ever since. That is how he discovered 2005 YU55.

"2005 YU55 is one of the potentially hazardous asteroids that make close approaches from time to time because their orbits either approach or intersect the orbit of the Earth," said McMillan, who is an associate research scientist with the UA's Lunar and Planetary Laboratory with a joint appointment in the UA's Steward Observatory.

The asteroid – or more correctly, minor planet – will approach the Earth within about 202,000 miles, closer than the distance to the moon. No object of comparable size has come this close since 1976, and none is going to until 2028, when another asteroid dubbed 2001 WN5 will pass about halfway between the moon and the Earth.

Although 2005 YU55's orbit takes it into Earth's neighborhood every once in a while, there is no chance of it hitting our planet for at least another 100 years. However, because asteroids' trajectories change over time, there is a slight chance it may do so at some point in the future.

Enter SPACEWATCH and the Catalina Sky Survey, two research programs at the UA's Lunar and Planetary Laboratory dedicated to the study of small objects like asteroids and comets. Founded in 1980 by the late Tom Gehrels and McMillan, who is now its principal investigator, SPACEWATCH aims to find objects that might pose a hazard to Earth and gain a better understanding of how the solar system came to be.

Begun by LPL senior staff scientist Steve Larson and now led by senior staff scientist Ed Beshore, the Catalina Sky Survey is a NASA-supported project to discover and catalog Earth-approaching and potentially hazardous asteroids.

"We complement each other in what we focus on," McMillan said. "With SPACEWATCH, we spend a lot of our time doing follow-ups on objects that have already been discovered through other programs. Because we can go after fainter and dimmer objects, we can chase them longer after their time of discovery as they are hurtling out deeper into space."

SPACEWATCH was the first program dedicated to the discovery and tracking of asteroids that made use of charge-coupled devices, or CCDs, which are now found in many digital cameras, instead of photographic plates to scan the skies. The 0.9-meter telescope, built in 1921, was the first telescope the UA's Steward Observatory ordered. Originally housed on campus, it was moved to Kitt Peak in 1962, where it has been in operation ever since.

The search for asteroids is challenging and tedious.

"When you look through a telescope, asteroids don't look any different from stars," McMillan explained. "The only difference is that they're moving, and to detect that motion we have to take a series of images. Usually we take three images spaced 20 or 30 minutes apart."

Next, the observers run specialized software to examine those images for any star-like images that are moving from one image to the next. The software compiles them into a list of candidates that is presented to the observer for their approval.

"We have to double-check the images, because the software is not perfect and sometimes picks up things that aren't really asteroids," McMillan said.

Objects that pass the review are then forwarded to the Minor Planet Center, or MPC, at the Smithsonian Astrophysical Observatory in Cambridge, Mass.

In determining which ones could potentially slam into Earth, asteroid hunters look for those that are traveling faster than the more common Main Belt Asteroids, which orbit the sun between the orbits of Mars and Jupiter and do not pose a threat. Objects meeting those criteria are sent to the MPC separately with special designations as candidates for Near Earth Objects, or NEOs. The MPC reviews them and decides which ones should be designated as NEOs.

That is how 2005 YU55 went from being a faint speck of light to a Potentially Hazardous Asteroid.

"The MPC posted it on their confirmation page, which is monitored by everybody who follows up newly discovered Near Earth Objects," McMillan said. "So we followed it up on subsequent nights and over the following month. Over time, we refined its orbit to the point that NASA's Jet Propulsion Laboratory listed a large number of potential close encounters with the Earth."

"Now, after 767 observations by ground-based observers, we have the orbit of that asteroid really nailed down, so we know it's not going to hit the Earth on Nov. 8."

In addition to observation with optical telescopes, radar measurements revealed 2005 YU55's distance, velocity and size. The coal-black asteroid, which is almost spherical in shape and measures about 1,300 feet in diameter, slowly spins as it travels through space, completing a rotation every 18 hours.

According to McMillan, the gravity of other planets, as well as the pressure of sunlight affect the paths of asteroids. In addition, a phenomenon known as the Yarkovsky Effect – slight asymmetries in heat distribution that arise as the object soaks up sunlight and radiates it back into space as it turns – plays a role. Those asymmetries exert forces that add up over time to cause the asteroid to veer off its orbit ever so slightly.

"We now know that 2005 YU55 is a carbonaceous chondrite asteroid that is relatively dark and contains carbon," McMillan said, "But long-range trajectories are very difficult to predict when you don't know the exact physical properties of the asteroid."

Author: Daniel Stolte | Source: University of Arizona [November 07, 2011]

Hubble directly observes the disc around a black hole

An international team of astronomers has used a new technique to study the bright disc of matter surrounding a faraway black hole. Using the NASA/ESA Hubble Space Telescope, combined with the gravitational lensing effect of stars in a distant galaxy, the team measured the disc's size and studied the colours (and hence the temperatures) of different parts of the disc. These observations show a level of precision equivalent to spotting individual grains of sand on the surface of the Moon.

This picture shows a quasar that has been gravitationally lensed by a galaxy in the foreground, which can be seen as a faint shape around the two bright images of the quasar. Observations of one of the images show variations in color over time. This is caused by stars within the lens galaxy passing through the path of the light from the quasar, magnifying the light from different parts of the quasar's accretion disc as they move. This has allowed a team of scientists to reconstruct the color and temperature profile of the accretion disc with unprecedented precision. The level of detail involved is equivalent to being able to study individual grains of sand on the surface of the Moon while standing on Earth [Credit: NASA, ESA and J.A. Munoz (University of Valencia)]

While black holes themselves are invisible, the forces they unleash cause some of the brightest phenomena in the Universe. Quasars — short for quasi-stellar objects — are glowing discs of matter that orbit supermassive black holes, heating up and emitting extremely bright radiation as they do so.

"A quasar accretion disc has a typical size of a few light-days, or around 100 billion kilometres across, but they lie billions of light-years away. This means their apparent size when viewed from Earth is so small that we will probably never have a telescope powerful enough to see their structure directly," explains Jose Munoz, the lead scientist in this study.

Until now, the minute apparent size of quasars has meant that most of our knowledge of their inner structure has been based on theoretical extrapolations, rather than direct observations.

The team therefore used an innovative method to study the quasar: using the stars in an intervening galaxy as a scanning microscope to probe features in the quasar's disc that would otherwise be far too small to see. As these stars move across the light from the quasar, gravitational effects amplify the light from different parts of the quasar, giving detailed colour information for a line that crosses through the accretion disc.

The team observed a group of distant quasars that are gravitationally lensed by the chance alignment of other galaxies in the foreground, producing several images of the quasar.

They spotted subtle differences in colour between the images, and changes in colour over the time the observations were carried out. Part of these colour differences are caused by the properties of dust in the intervening galaxies: the light coming from each one of the lensed images has followed a different path through the galaxy, so that the various colours encapsulate information about the material within the galaxy. Measuring the way and extent to which the dust within the galaxies blocks light (known to astronomers as the extinction law) at such distances is itself an important result in the study.

For one of the quasars they studied, though, there were clear signs that stars in the intervening galaxy were passing through the path of the light from the quasar. Just as the gravitational effect due to the whole intervening galaxy can bend and amplify the quasar's light, so can that of the stars within the intervening galaxy subtly bend and amplify the light from different parts of the accretion disc as they pass through the path of the quasar's light.

By recording the variation in colour, the team were able to reconstruct the colour profile across the accretion disc. This is important because the temperature of an accretion disc increases the closer it is to the black hole, and the colours emitted by the hot matter get bluer the hotter they are. This allowed the team to measure the diameter of the disc of hot matter, and plot how hot it is at different distances from the centre.

They found that the disc is between four and eleven light-days across (approximately 100 to 300 billion kilometres). While this measurement shows large uncertainties, it is still a remarkably accurate measurement for a small object at such a great distance, and the method holds great potential for increased accuracy in the future.

"This result is very relevant because it implies we are now able to obtain observational data on the structure of these systems, rather than relying on theory alone," says Munoz. "Quasars' physical properties are not yet well understood. This new ability to obtain observational measurements is therefore opening a new window to help understand the nature of these objects."

Source: ESA/Hubble Information Centre [November 04, 2011]

Nonterrestrial artifacts hard to pin down

Two Pioneer probes left our solar system carrying plaques about humankind, and two Voyager probes will soon join them to gather information about places far out in our galaxy. We can and will send more autonomous probes into outer space, but why have we never found evidence of other civilizations doing the same? A pair of postdoctoral researchers at Penn State, approaching the problem mathematically, shows that we have not looked in enough places to ensure that no extraterrestrial artifacts exist in our solar system.

The famed Pioneer plaque [Credit: NASA]

"The vastness of space, combined with our limited searches to date, implies that any remote unpiloted exploratory probes of extraterrestrial origin would likely remain unnoticed," report Jacob Haqq-Misra, Rock Ethics Institute, and Ravi Kumar Kopparapu, Earth and Environmental Systems Institute, in a paper accepted by Acta Astronautica and posted online on ArXiv.

So far, we have not found any nonterrestrial artifacts in our solar system. The Fermi paradox, originally formulated by Enrico Fermi, asks, if intelligent life is common, why have no technological civilizations been observed. Answers to this question could include life is rare, intelligent cultures inevitably destroy themselves, intelligent beings have not gotten here yet or they are here but not revealing themselves. Even without actual contact, like us, other civilizations could be sending unpiloted probes to quietly peek at our civilization.

These probes, like ours, would be small and might be hidden in a variety of places. In the asteroid belt they would probably go unnoticed, especially if these nonterrestrial objects are only 3 to 33 feet in size, weighing little more than a ton.

"Extraterrestrial artifacts may exist in the solar system without our knowledge simply because we have not yet searched sufficiently," said Haqq-Misra and Kopparapu. "Few if any of the attempts would be capable of detecting a 1 to 10 meter (3 to 33 foot) probe."

Haqq-Misra and Kopparapu use a probabilistic method to determine if we have looked closely enough anywhere in the solar system to definitively say there are no nonterrestrial objects here. The analysis is based on answering the question, how sure can we be that we should have already found any nonterrestrial objects lurking in the solar system.

They view the solar system as a fixed volume and figure out the percentages of that volume that would need to be thoroughly searched using a discovery capability small enough to detect these probes, assuming that the probes are not consciously camouflaged. The researchers note that most searches to date have not been fine enough to locate such small probes or to totally rule out anywhere.

After taking into account a variety of potential biases, such as "the universe is teeming with life" or "life is rare," the team developed an equation that can be applied to a portion of the volume of the solar system and determine whether sufficient searching has been done to ensure that we can say there are no nonterrestrial objects within that volume.

The researchers found that it is, at this point, difficult to say that there are not nonterrestrial objects in our solar system.

"The surface of the Earth is one of the few places in the solar system that has been almost completely examined at a spatial resolution of less than 3 feet," said Haqq-Misra and Kopparapu.

But even as humans have spread across the solid surfaces of the Earth, there are still caves, jungles and deserts as well as the ocean floor and subsurface areas that have not been explored. Even with this, the Earth does have a high confidence that no nonterrestrial artifacts exist.

The moon and Mars have been searched to a small extent. An ongoing mapping project, the Lunar Reconnaissance Orbiter, is looking at the moon at a resolution of about 20 inches, so we may eventually be able to determine if there are no nonterrestrial objects on the moon. The researchers caution that surface maps may not be sufficient to distinguish between a space probe and a rock.

The surface of Mars is still mostly unsurveyed and the researchers' confidence in the probability of no nonterrestrial artifacts is low. Similarly, locations like the Earth-moon Lagrange points, the asteroid belt and the Kuiper belt might also shelter extra solar system probes, but the vast majority of the solar system's volume is uninvestigated.

"Searches to date of the solar system are sufficiently incomplete that we cannot rule out the possibility that nonterrestrial artifacts are present and may even be observing us," said Haqq-Misra and Kopparapu. They add that "the completeness of our search for nonterrestrial objects will inevitably increase as we continue to explore the moon, Mars and other nearby regions of space."

Source: Pennsylvania State University [November 07, 2011]

Solving Einstein’s theory

A team of University researchers will get their hands on some of Europe’s fastest supercomputers in a bid to crack Einstein’s theory of relativity and help describe what happens when two black holes collide.

Experts in gravitational waves from the School of Physics and Astronomy have secured almost 16.7 million hours worth of supercomputer time to simulate and map the most violent events in the universe since the big bang – namely, collisions of black holes.

The team will use more than 1,900 computer processors over the next year to try and solve the equations of Einstein’s general theory of relativity.

The ultimate goal of the simulations is the direct observation of black-hole collisions through the gravitational waves they emit.

"Gravitational waves are ripples in space and time – predicted by Einstein almost 100 years ago," according to Mark Hannam, School of Physics and Astronomy, who will lead the Cardiff research team.

"However, despite Einstein’s predictions – they have not yet been directly detected. Gravitational waves are generated by accelerating masses, such as orbiting black holes, similar to the way accelerating electrical charges emit electromagnetic waves, like light, infra-red and radio waves - with the important difference that gravitational waves are far weaker.

"For this reason it is electromagnetic waves that have told us everything we have learnt about the cosmos since ancient times. If we could also detect gravitational waves, that would push open a new window on the universe, and tell us about its `dark side'," he added.

Over the past decade a network of gravitational wave detectors has been built, including the US Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European GEO600 and Virgo detectors, with the ambitious goal of not only making the first direct detection of the gravitational waves, but also to observe the entire Universe through gravitational radiation.

Cardiff's researchers work on theoretical modelling of black-hole-binary collisions using state-of-the-art numerical techniques and high performance computer clusters, strong field tests of gravity with gravitational-wave observations and the development of algorithms and software to search for gravitational waves.

Researchers at Cardiff play leading roles within the LIGO Scientific Collaboration, in particular in gravitational-wave searches for compact binary coalescences, supernovae, gamma-ray bursts, and other transient sources.

Coalescing black holes are prime candidates for the first observations. The results of this project will help to identify the sources of these signals, and contribute to answering important open questions in astrophysics and fundamental physics, such as whether the objects created in these cosmic collisions are really black holes, or even more exotic objects like naked singularities.

In the process the team hope to be able to test if Einstein's theory of gravity is correct, or whether, just as Newton's gravity gave way to Einstein's, perhaps Einstein's relativity gives way to even deeper insights into the nature of space and time.

The research team comprises more than 20 physicists working at Cardiff, the Universities of Jena, Vienna, and the Balearic Islands, the Albert Einstein Institute in Potsdam, and the California Institute of Technology. Solving Einstein's equations on supercomputers to accurately describe black holes became possible only after a series of breakthroughs in 2005, and the mostly young researchers are excited to be part of a scientific revolution.

"The detectors are pushing against the limits of current technology, and now we will help them with simulations that are at the cutting edge of computing power. Access to such vast computing resources is a fantastic boost for scientific research in Wales," Dr. Hannam added.

While supercomputing resources in Europe used to be relatively scarce, the PRACE Research Infrastructure now provides access to world-class supercomputers for European research projects, which undergo a competitive peer review process.

The PRACE infrastructure currently consists of three world-class supercomputers, which can each perform about 1 Petaflop which is a thousand billion arithmetic operations per second. The first machine in the network, the German Jugene, started operation in 2010, and it was joined in early 2011 by the French machine Curie, and the German system Hermit is about to officially start operation on November 1.

Future computers in the PRACE network are planned in Germany, Italy, and Spain.

Source: Cardiff University [November 03, 2011]

Biography of a star

Nuclear fusion is a virtually inexhaustible source of energy, and for decades now scientists have been working on exploiting it. A process that continues to present difficulties in laboratories on Earth has been running smoothly in stars like our own Sun for billions of years. But how do the stars work? How are they born? How do they die? Achim Weiss at the Max Planck Institute for Astrophysics in Garching tracks the life cycle of the cosmic plasma spheres - not with a telescope, but by using computer model calculations.

Plasma laboratory in the firmament: Dozens of young stars shine in the NGC 3603 nebula. Today, astrophysicists replay the births and biographies of stars on computers [Credit: NASA, ESA and the Hubble Heritdge (STSci/AURA) - ESA/Hubble Collaboration]

For a glimpse of the world’s largest laboratory, you need only look into the clear night sky far from the bright city lights. And if, at the same time, you take a deep breath of fresh country air, you will be supplying your body with the very substances that are produced in this laboratory. Elements such as nitrogen, oxygen and carbon originate in nurseries that have sparkled on the terrestrial firmament since time immemorial: sometimes brighter, sometimes less bright; sometimes white, sometimes in shades of yellow, blue or red.

The stars have always fascinated man. As recently as the 1850s, however, researchers were still speculating over the nature of these flickering lights. “We do not know what the stars are, and never will,” one professor is reported to have answered when asked by a young physics student whether there might not perhaps be some way of learning more about the universe than merely the position, distance and brightness of the Sun, moon and stars. The student’s name was Karl Friedrich Zöllner, and he was by no means satisfied with his professor’s answer. Undeterred, he continued his studies and became one of the first astrophysicists– a profession that he played a part in shaping.

Achim Weiss shares the same profession, and works, appropriately, at the Max Planck Institute for Astrophysics. He has a surprisingly uncomplicated answer to Zöllner’s question: “Stars are simple plasma spheres that are subject to their own gravity.” A plasma is a gas consisting of ions, electrons and neutral particles; over 99 percent of the visible matter in the universe is in this state. For its part, gravitation is the dominating force in space, acting upon all objects that are substantially larger than molecules. Little else is needed in the way of parts to build a star. Ingredients such as magnetic fields, vibration or electrical phenomena are rarely significant – either in nature or in the computer in Garching on which Weiss models stars.

In space, the birth of a star begins with a giant gas cloud. The mass of this cloud must be so great that gravity prevails against the internal pressure and the turbulence that would drive the filigree structure apart. For its birth to proceed, the star presumably needs a little gentle help from outside, such as the pressure wave of a nearby supernova, that is, an exploded sun.

At some point, the cloud breaks up into smaller lumps, each of which collapses. Shackled by gravitation, the particles within such a fragment bunch up. “If this were to continue indefinitely, the star’s birth would end in a black hole,” says Achim Weiss. How does the inside of the emerging gas sphere withstand the growing gravitational pressure? What stops the stellar embryo from breaking up?

The compressive work of gravity generates heat and pressure. The heat causes the electrons to separate from the cores of their atoms – a plasma is produced. And the pressure enables the gas to build up a “counter-force” against the gravitation: at any given distance from the sphere’s center, the pressure is exactly equal to the weight of the gas masses lying above it. The star has become a stable structure. Or as an astrophysicist would put it: it is in a state of hydrostatic equilibrium.

Such a state can be reproduced by a simple experiment: carefully press in a bicycle pump, then block off the outlet with your finger. Since air in the pump is no longer able to flow out, pressure builds up in the tube and prevents the piston from moving. If the right amount of pressure is applied to the piston, it remains stationary in the tube of the pump and a form of equilibrium is produced.

“What happens next in the star’s life depends entirely on its mass,” says Achim Weiss. The mass is therefore the decisive parameter in the model calculations. In a perfectly normal, average star like our own Sun (mass: 1.989 x 1030 kg), an event with far-reaching consequences occurs after its birth, which lasts a few hundred thousand years. In the center, the gas – primarily hydrogen – heats up to a temperature of over ten million degrees Celsius. At this astronomically high temperature, a fusion reactor ignites, and nucleosynthesis begins: four hydrogen nuclei (protons) combine to form a nucleus of helium-4.

Only now has the cosmic gas sphere become a full member of the star family. The reason is that stars have another property that differentiates them crucially from planets: they shine, because they derive energy from nucleosynthesis. The fusion reactor also ensures that the gas remains hot and delivers sufficient pressure to maintain the hydrostatic equilibrium.

Some stars, however, do not possess sufficient substance at birth. If their mass is less than 75 times that of the planet Jupiter, or in other words less than 8 percent of the mass of the Sun, fusion reactions may still occur on a limited scale within them; a proton, for example, may fuse with deuterium nucleus, consisting of one proton and one neutron, to form a helium-3 nucleus. However, lightweights such as these among the stars never reach the stage of steady hydrogen burning. (The term “burning” is used for historical reasons and is usual in astrophysics; it actually refers to “fusion” and is unrelated to chemical combustion.)

These “black sheep” of the star family are called brown dwarfs. Their lives are fairly unspectacular: owing to their low core temperature, the gas pressure is not sufficient to keep the gas spheres in equilibrium in the long term. Ultimately, gravity gains the upper hand. The brown dwarfs shrink and convert their gravitational energy into heat. Incidentally, this process, known as the Kelvin-Helmholtz contraction, was discussed by astronomers as one of the possible sources of stars’ energy, before they solved the riddle in the 20th century with the aid of nuclear fusion.

As the brown dwarfs shrink and cool down, however, the properties of the gases composed of free electrons change: they degenerate, as physicists say. This state has a peculiar feature: the temperature becomes decoupled from the pressure and density, and the star is able to cool down without the pressure dropping. The star remains stabilized, and therefore does not vanish as a small black hole; instead, it becomes progressively colder and darker.

But back to stars of normal weight. A few million years after birth, the young star checks the deluge of matter from its parent cloud by means of increasingly intense radiation and a rising wind of charged particles that it spits off its surface into space. With these mechanisms, the star avoids a further increase in mass and reaches the nuclear fusion phase. At this point, it enters the main sequence in the Hertzsprung-Russell diagram.

A star might be expected to respect its place in this society forever, according to its initial mass. But this is by no means the case. The population density in the Hertzsprung-Russell diagram (HRD) reflects the relative frequency with which individual star types occur at a particular point in time. If, however, the data from the same stars were to be entered in an HRD every couple hundred thousand years, and the measurements repeated over a period of several billion years, we would notice movement: in the resulting time-lapse movie, some stars would enter the main sequence and remain in it for a long time, only to leave it very quickly toward the giant sequence, finally “crashing” into the dwarfs. In other words, stars are by no means static plasma spheres – they develop. “I am interested in these differences in stars’ biographies for my calculations,” says Max Planck researcher Weiss.

Let us consider a star of the same type as our Sun. Nuclear fusion functions smoothly only when the external conditions such as pressure, density and temperature are right, and sufficient fuel is also available. At this point, the Sun has consumed about half of the hydrogen at its core by nuclear fusion; around 70 percent of its mass lies within half the solar radius of 350,000 kilometers. Over time, the hydrogen reserves are completely exhausted, and increasing quantities of helium collect at the heart of the Sun until it consists entirely of helium, something that will happen in around six billion years’ time. Since the Sun is already four and a half billion years old, it will have had a fairly stable life of ten billion years by that point.

When hydrogen burning at the Sun’s center ceases, the star has a problem. It loses energy, but tries to maintain the hydrostatic equilibrium. Fusion in the interior no longer delivers energy. The Sun uses a trick to compensate for this deficit: the core begins to contract, and converts gravitational energy into heat. In the process, it heats up, becoming so hot that the layers outside the burnt-out core reach a sufficiently high temperature to maintain the hydrogen fusion. Calculations show that this burning of the shell eats its way progressively outward over time. And something is also happening on the inside: the core contracts further still and heats up so much that, ultimately, the helium ignites.

Nuclear fusion takes a detour

At this point, the Sun draws its energy from two sources. Whereas in the shell, the hydrogen is fusing to form helium, the triple-alpha process is taking place in the core: a carbon nucleus is created from each set of three helium nuclei (alpha particles). This takes place in a roundabout way, however. The fusion of two helium nuclei first produces an unstable beryllium nucleus with a half- life of only 10-16 seconds.

Stellar element cuisine: From simple hydrogen fusion (1) in the core of a star, the process passes through the various stages of shell burning (2, 3), ending in the creation of heavy elements up to and including iron (4) [Credit: S&T: Casey Reed / Source: J. Hester & others]

Only when, during its extremely brief existence, this helium nucleus collides with another helium nucleus is stable carbon produced. The capture of further helium nuclei may also cause oxygen and neon nuclei to form. In order to ignite the helium, the core contracts, as already mentioned, becoming hotter in the process. At the same time, however, the outer shell greatly expands, causing the surface temperature to fall from values of some 6,000 degrees to around 3,000 degrees Celsius. The Sun has increased its radius a hundredfold, and shines with a reddish light up to 5,000 times as brightly as it does at present: it has become a red giant. Accordingly, it migrates in the Hertzsprung-Russell diagram to the giant sequence.

“Recording such a biography requires numerical programs that describe the star as an ideal gas sphere,” says Achim Weiss. In principle, the task is to divide the star mathematically into “onion skins,” and to determine the chemical composition, physical structure (mass, temperature, density, energy flow) and type of nuclear reaction for each of them. In order for a star to be analyzed for a particular point in time, Weiss and his colleagues typically require a thousand layers. The result is a snapshot of the stellar glass sphere: a model of a star.

In the second step, Weiss then calculates the changes that take place in this model, for example as a result of the nuclear fusion processes, over a given time. He then generates the next, slightly older model. In this way, the researcher tracks the development of a star in the computer. In order to test the calculations in practice, some kind of initial model is first required. For this purpose, Weiss uses the measurable state parameters of an actual, undeveloped star as approximate values – so its mass, luminosity and radius. He then sets these state parameters to zero for the center and begins to calculate in stages from the inside out. “Only once we have found a solution in this way for the initial model do we begin the actual calculations,” says the astrophysicist.

What is the subsequent fate of a star with the mass of our Sun? Achim Weiss solves this time problem by calculating a further model for a point later on in the Sun’s life, for example a million years from now. “Approximately 10,000 individual models are needed in order to describe the entire life of a star,” says Weiss. The time interval between these models must not be too great, however, particularly at an advanced stage in the star’s life: at the giant stage, events follow in quick succession – once the helium in the core has transformed completely into carbon and oxygen. The core is then surrounded by two shells: in the inner shell, helium burns to form carbon; in the outer shell, hydrogen burns to form helium.

In the space of a few tens of thousands of years, a star goes through a wild phase. First, the carbon/oxygen core contracts, while at the same time, the envelope expands. This process does not take place evenly, however, but rather in bursts of greater or lesser regularity during which the star inflates, once again increasing strongly in size and luminosity. During this process, the two outer shells do not burn simultaneously, but alternately.

And an astonishing process takes place within the star: “The complicated interplay of forces creates the conditions for the nucleosynthesis of heavy elements,” explains Achim Weiss, “and violent convection flows are generated within the star.” These flows use particles to transport energy, and thoroughly mix the gas. The heat given off by a radiator is transported in the same way: hot air rises, while cool air falls. You need only hold your hand above a hot radiator to experience this phenomenon for yourself.

The resulting “eddies” in the star cause a certain amount of hydrogen from the outer layer to reach the helium that is burning in the shell beneath it. There, the protons are able to react with the carbon, resulting in neutrons being released. The neutrons are captured by the iron particles that were present in the star in small quantities from the beginning, resulting in the formation of neutron- rich iron isotopes.

If too many neutrons accumulate, radioactive beta decay occurs, which in turn creates stable cobalt nuclei. The neutrons are thus captured progressively by the atomic nuclei, which then become progressively heavier. This “s-process” (s for slow) produces all elements up to and including lead. According to Achim Weiss, “one day, the Sun will produce barium and other rare earths such as lanthanum.”

At any rate, the star’s death is now imminent. In the final phase, it loses several tenths of its mass within the space of a few tens of thousands of years, at the end of which 99 percent of its mass is accounted for by its carbon/oxygen core and only half a percent each by the thin hydrogen envelope and the helium shell. The carbon/oxygen core is effectively blasted clear in much the same way that the desert wind blasts a stone free of sand. The material that is carried off forms an expanding envelope surrounding the star; it is lit by the star, and it assumes the most diverse shapes, such as rings, spheres or asymmetrical structures. In the “hard core,” the fusion processes ultimately grind to a complete halt.

The star’s meager remains have a temperature of a few tens of thousands of degrees, and are now only as large as the Earth. The star now appears in the Hertzsprung-Russell diagram as a white dwarf: at first still hot and bright, but in the absence of nuclear fusion, cooling down and becoming dark, first quickly, then more and more slowly – just like the brown dwarfs. When the computer has churned out the state parameters for such a white dwarf – endless columns of figures for values such as the density, radius, mass and temperature – Weiss’ work is normally over; a white dwarf is the final stage of a star of low or medium mass.

Life for the heavyweights is faster and more dramatic: while a star such as the Sun remains on the main sequence of the Hertzsprung-Russell diagram for 10 billion years, a star with ten times its mass stays there for only 20 million years. It is much more wasteful of its fuel reserves, and ultimately fuses elements in its core up to and including iron. Should it experience an energy crisis, it bursts. At the Max Planck Institute for Astrophysics, a dedicated research group is studying the simulation of supernovae of this kind.

What connection exists between a white dwarf and the star from which it developed? This is one of the problems that Achim Weiss is studying with the aid of his models. For this purpose, the researcher obtains from catalogs the data of suns belonging to a cluster. Clusters are collections of several hundreds or thousands of suns that were born almost simultaneously many millions of years ago. Since they were not all endowed with the same mass at birth, their lives have taken different paths. Their ages can be determined from the “population density” at various points in the Hertzsprung- Russell diagram.

Let us assume that a cluster is 500 million years old, and that Weiss finds within it a white dwarf with a cooling age of 100 million years. The cooling age is the time that has elapsed since the star developed into a white dwarf. In this example, this means that the star had previously lived normally for 400 million years. “The problem to be solved now, says Weiss, is: What star takes 400 million years to develop into a white dwarf?” In this case, it could be a star with approximately three solar masses. Using his models, the researcher examines this “initial final mass relationship” and obtains results that are sometimes confusing.

All stars with the same initial mass would normally be assumed to have the same final mass as well. As an example, however, the final masses of the white dwarfs in the Beehive Cluster differ by a factor of two. “I have no idea why this is the case,” says Achim Weiss. The form of energy transport within the gas spheres and the mass loss from the surfaces are evidently decisive factors: “Models with greater mass, which have large convective cores, deliver clearer results.” Achim Weiss intends to continue the search for an answer to this question. By no means do we already know everything in astrophysics – even if we do now have a pretty good idea of “what the stars are.”

Author: Helmut Hornung | Source: Max-Planck-Gesellschaft [November 02, 2011]