Experts on the births and deaths of stars have an intriguing new puzzle piece to fit into their picture of how some stars die. It involves the lucky number seven and the fact that black holes appear to come in cookie-cutter sizes.
Yale astronomer Charles Bailyn and his colleagues have discovered a remarkable uniformity in the masses of black holes formed when dying stars collapse and explode. Instead of having a wide range of masses, like the stars that spawned them, six of the seven black holes in his study were very close to the same size -- seven times the mass of our own sun. Professor Bailyn announced his discovery on June 10 at the annual meeting of the American Astronomical Society in Winston-Salem, North Carolina,
The observation could help scientists better understand the complex reactions in these bright explosions, which are called supernovas. It may also aid their understanding of how all black holes are formed, including gigantic quasars that are believed to reside at the core of many galaxies.
"Something about the collapse of a star in a supernova explosion seems to favor retaining about seven solar masses of matter in the black hole, and blowing the rest of the star back out into space," says Professor Bailyn. He adds that the seventh black hole he analyzed had a mass between 10 and 14 times that of the sun, which suggests the possibility of a second subgroup. "This new finding will send astrophysicists back to their supernova computer models to try to figure out why."
The relatively small black holes he has been studying are believed to be scattered by the thousands throughout our galaxy, although only seven have been identified in the last decade. "According to EinsteinUs general theory of relativity, any collapsed star with a mass at least three times that of our sun must create a black hole," says Professor Bailyn, who has been observing the activity surrounding a stellar-sized black hole in the constellation Scorpius about 10,000 light years from earth. His research was featured in April on the PBS documentary series "Mysteries of Deep Space."
Astronomers have labeled the black hole in the Scorpius constellation a "microquasar" because, like quasars found at the core of galaxies, it is emitting mysterious jets of matter at close to the speed of light. The jets may be created when matter falls onto a black hole too quickly to be sucked in all at once and is regurgitated in "a kind of cosmic burp,S says the Yale astronomer.
Professor Bailyn has observed several of these stellar-sized black holes paired with a companion star. The paired objects are locked in a tight orbit around each other while the denser black hole slowly sucks streams of matter from the companion star into its invisible depths. By calculating the speed of the companion star's orbit, he and former Yale graduate student Jerome Orosz, now at Pennsylvania State University, determined the minimum mass of the black hole in the Scorpius constellation.
Their observations, published 18 months ago in the journal Nature, were made with YaleUs one-meter telescope and the National Observatory's four-meter telescope at the Cerro Tololo Interamerican Observatory in Chile.
Based on more recent observations of the companion star's brightness at various points in its orbit, the two researchers were able to pinpoint the black hole's mass at seven solar masses, plus or minus one-fourth of a solar mass, according to an article they published in April's issue of the Astrophysical Journal. With the help of Yale astrophysicist Paulo Coppi and his graduate student Raj K. Jain, Professor Bailyn also analyzed the six other known stellar-sized black holes using Bayesian statistics. Their research was funded by the National Science Foundation.
Steps in a supernova explosion
All the elements crucial to life as we know it, including oxygen and carbon, are forged in the internal furnace of dying stars, which then explode into bright supernovas, spewing the elements into space. Astrophysicists believe our own solar system condensed about 5 billion years ago from the cosmic dust of just such a supernova explosion.
Scientists believe that only hydrogen, helium and traces of lithium were spawned by the "big bang" explosion that created the universe. From these elements, stars and galaxies began to condense after about a billion years of cooling. All other elements were created inside massive stars -- those at least eight times the mass of our sun -- after the stars' outer hydrogen and helium layers began to burn off.
For example, scientists calculate that a star 25 times the mass of the sun has a life span of about 7.5 million years and that it burns only hydrogen for the first 7 million years or so. Helium burning occurs in the last 500,000 years, according to theories. Elements lighter than iron are formed either from the burning of carbon in the last 600 years, neon in the last year, oxygen in the last 6 months or silicon during the last day of the star's life. All the other elements are created at the moment the star collapses and explodes into a supernova.
Following a supernova explosion, the remnants of the dying star collapse into either a dense neutron star or an even denser black hole only a few kilometers in diameter. The black hole is so dense not even light can escape its huge gravitational pull, thereby making it invisible.
"In this new era of black hole studies, we are now able to observe phenomena which, until now, have just been the subject of theoretical speculation," Professor Bailyn says. "We have gone beyond the stage of simply finding examples of black holes to prove they exist, and now we are studying their detailed characteristics, both individually and as a group of objects. In this study, we found that they separate into two subgroups, one of which seems to be associated with a very specific mass."