Chasing the Universe's First Generation of Stars

Home»April»Chasing the Universe's First Generation of StarsA few years ago, Avi Loeb spent some time with his family near Cradle Mountain in the highlands of central Tasmania, a rugged island 150 miles south of Australia. Their cabin had no Internet connection, affording Loeb some free time after dinner to step outside and look up at the clear night sky, untainted by any trace of urban glow. He was bowled over by a dazzling spectacle: the countless stars of our galaxy, the Milky Way, airily stretched across the heavens. Off to the side, he could see our... Subscribe and get 10 issues packed with:The latest news, theories and developments in the world of scienceCompelling stories and breakthroughs in health, medicine and the mindEnvironmental issues and their relevance to daily lifeCutting-edge technology and its impact on our futureRegistration is FREE and takes only a few seconds to complete. If you are already registered on DiscoverMagazine.com, please log in.

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Why Do Stars Twinkle?

Stars appear to twinkle, planets don’t. Here's why.

Did you know you can distinguish between stars and planets in the sky?

Stars twinkle, planets don’t.

Okay, that’s not actually correct. The stars, planets, even the sun and moon twinkle, all in varying amounts. Anything outside the atmosphere is going to twinkle.

If you’re feeling a little silly using the word twinkle over and over again, we can also use the scientific term: astronomical scintillation.

You can’t feel it, but you’re carrying the entire weight of the atmosphere on your shoulders. Every single square inch of your skin is getting pushed by 15 pounds of pressure. And even though astronomers need our atmosphere to survive, it still drives them crazy. As it makes objects in space so much harder to see.

Stars twinkle, I mean scintillate, because as light passes down through a volume of air, turbulence in the Earth’s atmosphere refracts light differently from moment to moment. From our perspective, the light from a star will appear in one location, then milliseconds later, it’ll be distorted to a different spot.

We see this as twinkling.

So why do stars appear to twinkle, while planets don’t?

Stars appear as a single point in the sky, because of the great distance between us and them. This single point can be highly affected by atmospheric turbulence. Planets, being much closer, appear as disks.

We can’t resolve them as disks with our eyes, but it still averages out as a more stable light in the sky.

Astronomers battle atmospheric turbulence in two ways:

First, they try to get above it. The Hubble Space Telescope is powerful because it’s outside the atmosphere. The mirror is actually a quarter the size of a large ground-based observatory, but without atmospheric distortion, Hubble can resolve galaxies billions of light-years away. The longer it looks, the more light it gathers.

Second, they try to compensate for it.

Some of the most sophisticated telescopes on Earth use adaptive optics, which distorts the mirror of the telescope many times a second to compensate for the turbulence in the atmosphere.

Astronomers project a powerful laser into the sky, creating an artificial star within their viewing area. Since they know what the artificial star should look like, they distort the telescope’s mirror with pistons canceling out the atmospheric distortion. While it’s not as good as actually launching a telescope into space, it’s much, much cheaper.

Now you know why stars twinkle, why planets don’t seem to twinkle as much, and how you can make all of them stop.

Universe Today has written many articles about stars. Here’s an article that talks about a technique astronomers use to minimize the twinkle of the Earth’s atmosphere.

If you’d like more information on stars, check out Hubblesite’s news releases about stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

This article was republished with permission from Universe Today.

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Violent birth of neutron stars: Computer simulations confirm sloshing and spiral motions as stellar matter falls inward

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New Study Shows Very First Stars Not Monstrous

Scientists are simulating how the very first stars in our universe were born. This diagram shows a still from one such simulation. The cube on the right is a blown up region at the center of the box on the left. Image credit: NASA/JPL-Caltech/Kyoto Univ.
› Full image and caption November 10, 2011

PASADENA, Calif. -- The very first stars in our universe were not the behemoths scientists had once thought, according to new simulations performed at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Astronomers "grew" stars in their computers, mimicking the conditions of our primordial universe. The simulations took weeks. When the scientists' concoctions were finally done, they were shocked by the results -- the full-grown stars were much smaller than expected.

Until now, it was widely believed that the first stars were the biggest of all, with masses hundreds of times that of our sun. The new research shows they are only tens of times the mass of sun; for example, the simulations produced one star that was as little as 43 solar masses.

"The first stars were definitely massive, but not to the extreme we thought before," said Takashi Hosokawa, an astronomer at JPL and lead author of the new study, appearing online Friday, Nov. 11 in the journal Science. "Our simulations reveal that the growth of these stars is stunted earlier than expected, resulting in smaller final sizes."

The early universe consisted of nothing more than thin clouds of hydrogen and helium atoms. A few hundred million years after its birth, the first stars began to ignite. How these first stars formed is still a mystery.

Astronomers know that all stars form out of collapsing clouds of gas. Gravity from a growing "seed" at the center of the cloud attracts more and more matter. For so-called normal stars like our sun, this process is aided by heavier elements such as carbon, which help to keep the gas falling onto the budding star cool enough to collapse. If the cloud gets too hot, the gas expands and escapes.

But, in the early universe, stars hadn't yet produced heavy elements. The very first stars had to form out of nothing but hydrogen and helium. Scientists had theorized that such stars would require even more mass to form, to compensate for the lack of heavy elements and their cooling power. At first, it was thought the stars might be as big as one thousand times the mass of our sun. Later, the models were refined and the first stars were estimated to be hundreds of solar masses.

"These stars keep getting smaller and smaller over time," said Takashi. "Now we think they are even less massive, only tens of solar masses."

The team's simulations reveal that matter in the vicinity of the forming stars heats up to higher temperatures than previously believed, as high as 50,000 Kelvin (90,000 degrees Fahrenheit), or 8.5 times the surface temperature of the sun. Gas this hot expands and escapes the gravity of the developing star, instead of falling back down onto it. This means the stars stop growing earlier than predicted, reaching smaller final sizes.

"This is definitely going to surprise some folks," said Harold Yorke, an astronomer at JPL and co-author of the study. "It was standard knowledge until now that the first stars had to be extremely massive."

The results also answer an enigma regarding the first stellar explosions, called supernovae. When massive stars blow up at the end of their lives, they spew ashes made of heavier elements into space. If the very first stars were the monsters once thought, they should have left a specific pattern of these elements imprinted on the material of the following generation of stars. But, as much as astronomers searched the oldest stars for this signature, they couldn't find it. The answer, it seems, is that it simply is not there. Because the first stars weren't as massive as previously thought, they would have blown up in a manner akin to the types of stellar explosions that we see today.

"I am sure there are more surprises in store for us regarding this exciting period of the universe," said Yorke. "NASA's upcoming James Webb Space Telescope will be a valuable tool to observe this epoch of early star and galaxy formation."

For technical details and videos visit http://www-tap.scphys.kyoto-u.ac.jp/~hosokawa/firststarstop_e.html .

The California Institute of Technology manages JPL for NASA.  More information about JPL is online at www.jpl.nasa.gov .

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

2011-348

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Water Hit With Young Star's Best Shot

A jet of gas firing out of a very young star can be seen ramming into a wall of material in this infrared Spitzer image. Image credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA
› Full image and caption September 18, 2008

Water is being blasted to pieces by a young star's laser-like jets, according to new observations from NASA's Spitzer Space Telescope.

The discovery provides a better understanding of how water -- an essential ingredient for life as we know it -- is processed in emerging solar systems.

"This is a truly unique observation that will provide important information about the chemistry occurring in planet-forming regions, and may give us insights into the chemical reactions that made water and even life possible in our own solar system," said Achim Tappe, of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.

A young star forms out of a thick, rotating cloud of gas and dust. Like the two ends of a spinning top, powerful jets of gas emerge from the top and bottom of the dusty cloud. As the cloud shrinks more and more under its own gravity, its star eventually ignites and the remaining dust and gas flatten into a pancake-like disk, from which planets will later form. By the time the star ignites and stops accumulating material from its cloud, the jets will have died out.

Tappe and his colleagues used Spitzer's infrared eyes to cut through the dust surrounding a nascent star, called HH 211-mm, and get a better look at its jets. These particular jets are exceptionally young at 1,000 years old, and they are some of the most collimated, or focused, known. An instrument on Spitzer called a spectrometer analyzed light from one of the jets, revealing information about its molecules.

To the astronomers' surprise, Spitzer picked up the signature of rapidly spinning fragments of water molecules, called hydroxyl, or OH. In fact, the hydroxyl molecules have absorbed so much energy (through a process called excitation) that they are rotating around with energies equivalent to 28,000 Kelvin (27,700 degrees Celsius). This far exceeds normal expectations for gas streaming out of a stellar jet. Water, which is abbreviated H2O, is made up of one oxygen atom and two hydrogens; hydroxyl, or OH, contains one oxygen and one hydrogen atom.

The results reveal that the jet is ramming its head into a wall of material, vaporizing ice right off the dust grains it normally coats. The jet is hitting the material so fast and hard that a shock wave is also being produced.

"The shock from colliding atoms and molecules generates ultraviolet radiation, which will break up water molecules, leaving extremely hot hydroxyl molecules," said Tappe.

Tappe said this same process of ice being vaporized off dust occurs in our own solar system, when the sun vaporizes ice in approaching comets. In addition, the water that now coats our world is thought to have come from icy comets that vaporized as they rained down on a young Earth.

Tappe is the lead author of a paper on this topic, which was published in a recent issue of the Astrophysical Journal. Co-authors on the paper include Charlie Lada, and August Muench, also of the Harvard-Smithsonian Center for Astrophysics; and J. H. Black, of the Chalmers University of Technology, in Onsala, Sweden.

Media contact: Whitney Clavin/JPL
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0850

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Dying supergiant stars implicated in hours-long gamma-ray bursts

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