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.
It is widely believed that the first stars were formed when the age of the universe was a few hundred million years old. At its birth, a first star is just a tiny embryo - a protostar - with a mass of about one percent of the sun (see the figure below). The protostar is then expected to grow by accumulating the surrounding hot gas, but how much gas it can acquire has been largely unknown.
Credit: Takashi Hosokawa (1,2), Kazuyuki Omukai (2), Naoki Yoshida (3), Harold W. Yorke (1)
Movies: (from a different angle) 3 (from a different angle) 4 (2-dimensional slice) 5 (central region of 3000 AU)
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.
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.
The stars we see today formed out of collapsing clouds of gas and dust. In the very early universe, however, the stars had fewer ingredients available. There wasn't any dust yet, or heavy elements, both of which help cool the gas in a cloud so that it can collapse. Instead, the very first stars formed from nothing but hydrogen and helium gas. Astronomers theorize that, in order to overcome their lack of cooling ingredients, these stars would have needed more mass to form. The first stars were thought to have been more massive than even the most massive stars observed today.
At the very center of each box shown here is the forming star – the star itself is too small to see at this scale. The red areas show hot gas with temperatures as high as 50,000 Kelvin (90,000 degrees Fahrenheit), heated by, and surrounding, the forming star. Blue shows much cooler gas, with the darkest blue showing the densest portions of cool gas, shaped like a disk surrounding the seedling star.
Astrophysicists thought so far that the first stars could grow huge, as much as a few hundred times the sun in mass.
Credit: Takashi Hosokawa (1,2), Kazuyuki Omukai (2), Naoki Yoshida (3), Harold W. Yorke (1)
As the star pulls matter from the disk onto it, it grows more massive. Meanwhile, some gas -- shown in the red areas -- is so hot that it expands and escapes. Eventually, large amounts of the surrounding gas become too hot and escape. At this point, the star stops growing -- it has finished "baking."
Scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif., have used these simulations to show that the first stars, during their formation, had a greater impact on surrounding gas than previously thought. The higher gas temperatures would have caused the stars to stop growing sooner. As a result, the first stars were not likely hundreds of times the mass of sun, but only tens of times its mass.
None of these first stars still exist today. After a few million years they exploded in fiery supernovae, spewing heavier elements cooked in their interiors out into the surrounding gas.
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.
Numerical simulations showed us the very exciting evolution of the first stars in the deep universe beyond our observations. However, observers have found signatures of the first stars in the elemental abundances of very old stars in our Galaxy (see the figure below). They searched for the signatures of the "monstrous" stars of hundreds of solar-masses, but could not find them. Instead, recent observations all support that the first stars should have been the massive stars of several tens of solar-masses. Such ordinary massive stars, in the sense there are indeed such stars with similar masses in the present-day Universe, are really resulting from our simulations.
Credit: Takashi Hosokawa (1,2), Kazuyuki Omukai (2), Naoki Yoshida (3), Harold W. Yorke (1)
"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."
The California Institute of Technology manages JPL for NASA. More information about JPL is online athttp://www.jpl.nasa.gov .
Contacts and sources:
Whitney Clavin/Alan Buis
Jet Propulsion Laboratory, Pasadena, Calif.
1: Jet Propulsion Laboratory, California Institute of Technology,
2: Department of Physics, Kyoto University,
3: Institute for the Physics and Mathematics of the Universe, TODIAS, The University of Tokyo
Read more at Nano Patents and Innovations
It is widely believed that the first stars were formed when the age of the universe was a few hundred million years old. At its birth, a first star is just a tiny embryo - a protostar - with a mass of about one percent of the sun (see the figure below). The protostar is then expected to grow by accumulating the surrounding hot gas, but how much gas it can acquire has been largely unknown.
Credit: Takashi Hosokawa (1,2), Kazuyuki Omukai (2), Naoki Yoshida (3), Harold W. Yorke (1)
Movies: (from a different angle) 3 (from a different angle) 4 (2-dimensional slice) 5 (central region of 3000 AU)
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.
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.
The stars we see today formed out of collapsing clouds of gas and dust. In the very early universe, however, the stars had fewer ingredients available. There wasn't any dust yet, or heavy elements, both of which help cool the gas in a cloud so that it can collapse. Instead, the very first stars formed from nothing but hydrogen and helium gas. Astronomers theorize that, in order to overcome their lack of cooling ingredients, these stars would have needed more mass to form. The first stars were thought to have been more massive than even the most massive stars observed today.
At the very center of each box shown here is the forming star – the star itself is too small to see at this scale. The red areas show hot gas with temperatures as high as 50,000 Kelvin (90,000 degrees Fahrenheit), heated by, and surrounding, the forming star. Blue shows much cooler gas, with the darkest blue showing the densest portions of cool gas, shaped like a disk surrounding the seedling star.
Astrophysicists thought so far that the first stars could grow huge, as much as a few hundred times the sun in mass.
Credit: Takashi Hosokawa (1,2), Kazuyuki Omukai (2), Naoki Yoshida (3), Harold W. Yorke (1)
As the star pulls matter from the disk onto it, it grows more massive. Meanwhile, some gas -- shown in the red areas -- is so hot that it expands and escapes. Eventually, large amounts of the surrounding gas become too hot and escape. At this point, the star stops growing -- it has finished "baking."
Scientists at NASA's Jet Propulsion Laboratory, Pasadena, Calif., have used these simulations to show that the first stars, during their formation, had a greater impact on surrounding gas than previously thought. The higher gas temperatures would have caused the stars to stop growing sooner. As a result, the first stars were not likely hundreds of times the mass of sun, but only tens of times its mass.
None of these first stars still exist today. After a few million years they exploded in fiery supernovae, spewing heavier elements cooked in their interiors out into the surrounding gas.
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.
Numerical simulations showed us the very exciting evolution of the first stars in the deep universe beyond our observations. However, observers have found signatures of the first stars in the elemental abundances of very old stars in our Galaxy (see the figure below). They searched for the signatures of the "monstrous" stars of hundreds of solar-masses, but could not find them. Instead, recent observations all support that the first stars should have been the massive stars of several tens of solar-masses. Such ordinary massive stars, in the sense there are indeed such stars with similar masses in the present-day Universe, are really resulting from our simulations.
Credit: Takashi Hosokawa (1,2), Kazuyuki Omukai (2), Naoki Yoshida (3), Harold W. Yorke (1)
"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."
The California Institute of Technology manages JPL for NASA. More information about JPL is online athttp://www.jpl.nasa.gov .
Contacts and sources:
Whitney Clavin/Alan Buis
Jet Propulsion Laboratory, Pasadena, Calif.
3: Institute for the Physics and Mathematics of the Universe, TODIAS, The University of Tokyo
Read more at Nano Patents and Innovations
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