Universe Today

Image credit: Harvard CfA

New calculations by a pair of Harvard astronomers predict that the first “Sun-like” stars in the Universe were alone; devoid of planets or life. The very first generation of stars was hot and massive; they lived hard and died young. After they exploded as supernovae and seeded the Universe with heavier materials, other stars formed in stellar nurseries. The next generation of stars was probably similar in mass and composition to our own Sun, but there weren’t enough minerals to create rocky planets like the Earth. It took a succession of supernovae before there was enough heavy material that planets could form – probably 500 million to 2 billion years after the Big Bang.

To most people, the phrase “Sun-like star” calls to mind images of a friendly, warm yellow star accompanied by a retinue of planets possibly capable of nurturing life. But new calculations by Harvard astronomers Volker Bromm and Abraham Loeb (Harvard-Smithsonian Center for Astrophysics), which were announced today at the 203rd meeting of the American Astronomical Society in Atlanta, show that the first Sun-like stars were lonely orbs moving through a universe devoid of planets or life.

“The window for life opened sometime between 500 million and 2 billion years after the Big Bang” says Loeb. “Billions of years ago, the first low-mass stars were lonely places. The reason for that youthful solitude is embedded in the history of our universe.”

In The Beginning
The very first generation of stars were not at all like our Sun. They were white-hot, massive stars that were very short-lived. Burning for only a few million years, they collapsed and exploded as brilliant supernovae. Those very first stars began the seeding process in the universe, spreading vital elements like carbon and oxygen, which served as planetary building blocks.

“Previously, with Lars Hernquist and Naoki Yoshida (also at the CfA), I have simulated those first supernova explosions to calculate their evolution and how much heavy elements (elements heavier than hydrogen or helium) they produced,” says Bromm. “Now, in this work, Avi Loeb and I have determined that a single first-generation supernova could produce enough heavy elements to enable the first Sun-like stars to form.”

Bromm and Loeb showed that many second-generation stars had sizes, masses, and hence temperatures similar to our Sun. Those properties resulted from the cooling influence of carbon and oxygen when the stars formed. Even elemental abundances as low as one-ten thousandth those found in the Sun proved sufficient to allow smaller, low-mass stars like our Sun to be born.

Yet those same low abundances prohibited rocky planets from forming around those first Sun-like stars due to a lack of raw materials. Only as further generations of stars lived, died, and enriched the interstellar medium with heavy elements did the birth of planets, and life itself, become possible.

“Life is a recent phenomenon,” Loeb states unequivocally. “We know that it took many supernova explosions to make all the heavy elements we find here on Earth and in our Sun and our bodies.”

Recent observational evidence corroborates their finding. Studies of known extrasolar planets have found a strong correlation between the presence of planets and the abundance of heavy elements (“metals”) in their stars. That is, a star with higher metallicity and more heavy elements is more likely to possess planets. Conversely, the lower a star’s metallicity, the less likely it is to have planets.

“We’re now just beginning to investigate the metallicity threshold for planet formation, so it’s hard to say when exactly the window for life opened. But clearly, we’re fortunate that the metallicity of the matter that birthed our solar system was high enough for the Earth to form,” says Bromm. “We owe our existence in a very direct way to all the stars whose life and death preceded the formation of our Sun. And this process began right after the Big Bang with the very first stars. As the universe evolved, it progressively seeded itself with all the heavy elements necessary for planets and life to form. Thus, the evolution of the universe was a step-by-step process that resulted in a stable G-2 star capable of sustaining life. A star we call the Sun.”

Original Source: Harvard CfA News Release

Image credit: Hubble

New research from the Hubble Space Telescope indicates that the majority of large dying Wolf-Rayat stars have a smaller companion star orbiting nearby. This discovery will help astronomers understand how these unique stars evolve in the Universe, and could provide new a new method to estimate their size. Wolf-Rayat stars start out at least 20 times the mass of the Sun, last only a few million years, and then explode as supernovae. It’s now believed that these stars and their companions transfer mass as they orbit one another.

The majority of massive and brilliant but dying “Wolf-Rayet” stars have company – a smaller companion star orbiting nearby, according to new observations using the Hubble Space Telescope. The result will help astronomers understand how the biggest stars in the Universe evolve. It may also resolve the mystery of impossibly massive stars, and calls into question a certain kind of distance estimate that uses the apparent brightness of starlight.

Wolf-Rayet (WR) stars begin life as cosmic titans, with at least 20 times the mass of the Sun. They live fast and die hard, exploding as supernova and blasting vast amounts of heavy elements into space for use in later generations of stars and planets. “I tell people I study the stars that made a lot of the carbon in their bodies and the gold in their jewelry,” says Dr. Debra Wallace of NASA’s Goddard Space Flight Center, Greenbelt, Md. “Understanding how Wolf-Rayet stars evolve is a critical link in the chain of events that ultimately led to life.” Wallace is lead author of papers on this research to be published in the Astronomical Journal and the Astrophysical Journal.

By the time these stars are near the end of their brief lifetimes, during the “Wolf-Rayet” phase, they are fusing heavy elements in their cores in a frantic bid to prevent collapsing under their own immense mass. This generates intense heat and radiation that drives fierce, 2.2 million to 5.4 million mile-per-hour (3.6 million to 9 million km/hr) stellar winds characteristic of WR stars (Image 1). These winds blow off the outer layers of WR stars, greatly reducing their mass and compressing nearby interstellar clouds, triggering their gravitational collapse and igniting a new generation of stars.

Because cosmic distances are so great, what appears as a single star even when viewed through large telescopes (Image 2) may in fact be two or more stars orbiting each other (Images 3 and 4). In the new research, Wallace and her team used the superior resolving power of the Planetary Camera in the Wide-Field Planetary Camera 2 instrument on board Hubble to identify new potential companion stars for 23 of 61 WR stars in our galaxy. Although the apparent companion stars need to be confirmed with a light-analysis technique called spectroscopy, the team was conservative in declaring nearby stars companions.

“The portion of Wolf-Rayet stars having visually identified companion stars zoomed from 15 percent before Hubble to 59 percent with our observations, which included a quarter of the known WR stars in our galaxy,” said Wallace. “I wouldn’t be surprised if future observations reveal companions around an even greater percentage of them.”

The presence of a companion star should significantly influence how these stars evolve, according to the team. One of many possible influences is mass transfer. If the stars come close together at some point in their orbits, their gravitational interaction could cause one to transfer gas to the other, significantly altering their masses over time. Since more massive stars use up their fuel much faster than less massive stars, such a mass transfer could significantly change their lifetimes. Other influences include altering orbits, rotation rates, or mass-loss rates through the pull of their gravity, and the impact of stellar winds. “Astronomers assumed Wolf-Rayet stars were single when trying to calculate how they evolve, but we are finding most have company,” said Wallace. “It’s like thinking married life will be the same as life as a bachelor. A companion star has got to change the life of these stars somehow.”

Since what is seen as one star may in fact be two or even more, stupendous mass estimates of more than a hundred times that of the Sun for certain stars may have to be revised downward. “This actually helps clear up an apparent mystery, because astronomers believe there is a limit to how big a star can be,” said Wallace. “The more massive a star, the faster it consumes its fuel and the brighter it shines. Above about 100 solar masses, a star should essentially blow itself apart through its intense radiation.”

The result also makes a common technique for estimating distances to these stars more uncertain. To get a distance estimate to a star, one gets the spectral type of the star, an analysis of the star’s light that reveals its unique characteristics, like a fingerprint. For a given spectral type, one knows the star’s average absolute luminosity (how bright it would be if it were a certain distance – 32.6 light-years – away). By measuring its apparent luminosity (how bright it appears to be at its actual, but unknown, distance), one can then use the relationship between its apparent and absolute luminosity to determine the actual distance. If there are really two (or more) stars there that you don’t see, the WR star will appear to be brighter than it should for its spectral type and real distance, causing the distance to be misestimated.

The team includes Wallace; Dr. Douglas R. Gies of the Department of Physics and Astronomy, Georgia State University, Atlanta, Ga.; Anthony F. J. Moffat, D?partement de Physique, Universit? de Montr?al, Quebec, Canada; and Michael M. Shara, Department of Astrophysics, American Museum of Natural History, New York, N.Y. The research was funded by NASA.

Original Source: NASA News Release

Image credit: University of Florida

A team of astronomers from the University of Florida have found what could be the brightest star ever seen in the Universe. Located 45,000 light years away across our galaxy, LBV 1806-20 could be 40 million times brighter and 150 times larger than our own Sun. This gigantic and bright star isn’t long for the Universe; however, it’s only a couple of million years old, and will blow up as a supernova in a few million more. This star defies current theories about how large stars should be able to get.

A University of Florida-led team of astronomers may have discovered the brightest star yet observed in the universe, a fiery behemoth that could be as much as much as seven times brighter than the current record holder.

But don?t expect to find the star — which is at least 5 million times brighter than the sun — in the night sky. Dust particles between Earth and the star block out all of its visible light. Whereas the sun is located only 8.3 light minutes from Earth, the bright star is 45,000 light years away, on the other side of the galaxy. It is detectable only with instruments that measure infrared light, which has longer wavelengths that can better penetrate the dust.

In a National Science Foundation-funded study scheduled to be presented today at the American Astronomical Society national conference in Atlanta, the team says the star is at least as bright as the Pistol Star, the current record holder, so named for the pistol-shaped nebula surrounding it. Whereas the Pistol Star is between 5 million and 6 million times as bright as the sun, however, the new contender, LBV 1806-20, could be as much as 40 million times the sun?s brightness.

?We think we?ve found what may be the most massive and most luminous star ever discovered,? said Steve Eikenberry, a UF professor of astronomy and the lead author of a paper on the discovery that was recently submitted to the Astrophysical Journal.

Eikenberry will discuss his findings in a news conference to be held by the society at 12:30 p.m. today at the Courtland Room in the Hyatt Regency Atlanta, where the conference is being held.

One longstanding problem with gauging the brightness of stars at great distances is that what seems at first to be one amazingly bright star turns out on closer examination to be a cluster of nearby stars. Don Figer, an astronomer at the Baltimore-based Space Telescope Science Institute who led the team that discovered the Pistol Star in 1997, said the high-quality data collected by the UF-led team reduced but did not eliminate this possibility.

?The high-resolution data prove that the object is not simply a cluster of lower mass stars, although it is possible that it is a collection of a few stars in a tight orbit around each other,? Figer said. ?More study will be needed to determine the distance and singularity of the object in order to establish whether the object is truly the most massive star known.?

Astronomers have known about LBV 1806-20 since the 1990s. At that time, it was identified as a ?luminous blue variable star? – a relatively rare, massive and short-lived star. Such stars get their names from their propensity to display light and color variability in the infrared spectrum.

Luminous blue variable stars are extremely large, with LBV 1806-20 probably at least 150 times larger than the sun, Eikenberry said. The stars are also extremely young by stellar time. LBV 1806-20 is estimated at less than 2 million years old. The sun in our solar system, by contrast, is 5 billion years old. Typical stars, such as the sun, live 10 billion years.

LBVs have ?short and troubled lives,? as Eikenberry put it, because ?the more mass you have, the more nuclear fuel you have, the faster you burn it up. They start blowing themselves to bits.?

Eikenberry?s team made several key advances that led to the estimate of the star?s oversized mass and brightness, he said.

One, they sharpened infrared images obtained from the Palomar 200-inch telescope at the California Institute of Technology?s Palomar Observatory using a camera equipped with ?speckle imaging,? a relatively new technology for improving resolution of objects at great distances.

?The shimmering that you see coming off a hot blacktop road in the summer – the upper atmosphere kind of does that with star light,? Eikenberry said. ?Speckle imaging kind of freezes that motion out, and you get much better images.?

Composed of 17 astronomers and graduate students, the team also came up with an accurate estimate for the distance from the Earth to the bright star. Team members further determined its temperature and how much of the star?s infrared light gets absorbed by dust particles as the light makes its way toward Earth. The scientists relied on data collected by the Blanco 4-meter telescope at the National Optical Astronomy Observatory?s Cerro Tololo Inter-American Observatory in Chile.

Each of these variables contributed to the estimate of the star?s remarkable candlepower. ?You correct for dust absorption, then you correct for temperature of the star, you correct for distance of the star – all of those things feed into luminosity,? Eikenberry said.

One of the mysteries about LBV 1806-20 is how it got so big. Current theories of star formation suggest they should be limited to about 120 solar masses, or 120 times as large as the sun, because the heat and pressure from such big stars? cores force matter away from their surfaces. Eikenberry said one possibility is that the big star was formed in a process called shock-induced star formation, which occurs when a supernova blows up and slams the gaseous material in a molecular cloud together into a massive star.

The star?s size is not its only distinguishing characteristic. It is located in a small cluster of highly unusual or extremely rare stars, including a so-called ?soft gamma ray repeater,? a freakishly magnetic neutron star that is one of only four identified in the entire galaxy of 100 billion stars. With a magnetic field hundreds of trillions of times more powerful than Earth?s magnetic field, this type of star gets its name from its periodic bursts of gamma rays. The cluster also apparently includes an infant or newly formed star.

?We?ve got this zoo of freak stars, all crammed together, really nearby, and they?re all part of the same cluster of stars,? Eikenberry said. ?It?s really kind of weird.?

Also buried within the cluster is an extremely young infant star, Eikenberry said. The presence of the infant star, the luminous blue variable and the soft gamma ray repeater are vivid examples of an important emerging fact about stellar evolution: All stars in a single cluster don?t form at the same time, he said. ?We?re seeing what I think is going to become a textbook example of the fact that stars aren?t all born in an instant, even in a small cluster,? he said.

Figer, the Pistol Star discoverer, said the research makes an important contribution to astronomers? understanding of the star formation process.

?The findings are significant because such massive stars are very rare and define the upper limits of the star formation process,? he said. ?The team has made a remarkable contribution to our understanding of the most extreme stars.?

The team carrying out this work also included UF?s Jessica LaVine; Keith Matthews, with the California Institute of Technology; Stephane Corbel, with the Universite de Paris; John-David Smith, with the University of Arizona; John Wilson, with the University of Virginia; Donald Barry, Michael Colonno and James Houck, all with Cornell University; and undergraduate research students Shannon Patel, Malia Jackson, and Dounan Hu of Cornell University; and Megan Garske of Northwestern Nazarene University.

Original Source: University of Florida News Release