‘The early chemical soup of the galaxy'

IU's inaugural Kirkwood Chair, AAS president-elect studies data from when the first stars were forming in the Milky Way–some 14 billion years ago

By Susan Williams

Photo by Paul Martens
Bill Kopp cleans the Kirkwood Observatory's telescope pier while the actual telescope is out being cleaned and repaired.

'Still and always (I am awed), especially at the sight of the Milky Way from a clear, dark site on a moonless night. That is one of the best parts of being an astronomer!'

--C. Pilachowski

Such enthusiasm for the night skies is no surprise, even to those of us who don't function well until noon. Human beings always have been fascinated by the heavens—the mystery of who else might be out there, the wonder at how the planets came to be, the powers exerted by the moon upon the Earth, the information held for us within the stars.

Catherine"Caty" Pilachowski arrived in Bloomington this past August as professor of astronomy and the holder of the inaugural Kirkwood Chair in astronomy at Indiana University. Her interest in astronomy goes back to childhood, and she will become president of the American Astronomical Society next June, the highest such office in U.S. astronomy.

"I read the books of George Gamow and Fred Hoyle, famous scientists, and found the story of stellar evolution to be the most fascinating. Of course, that is the field I work in now," said Pilachowski, who researches the origin of chemical elements, a specialty that involves both stellar evolution and nuclear chemistry.

"I study the earliest phases of galactic chemical enrichment, back when the first stars were forming in the Milky Way some 14 billion years ago," she said."Most elements are produced in stars, and different kinds of stars make different elements. Over its long history, the composition of our galaxy—the Milky Way—has gradually been enriched as elements have been produced in stars and then mixed back in."

According to Pilachowski and other scientists, only hydrogen and helium, and maybe a trace of lithium, existed in the galaxy's beginning, so the first stars were very low in"metals," less than one ten-thousandth of the heavy elements that our sun has today.

"Since more than 99 percent of the atoms in the sun are hydrogen and helium, this means that more than 99.999 percent of these first stars were hydrogen and helium," Pilachowski said."By studying the compositions of stars formed at this early time in the galaxy—and some of these early stars are still around—we can learn about the very early history of the Milky Way."

Pilachowski uses a spectrograph, a camera attached to a telescope, to produce and record a band of colors in order to study the compositions of stars. In a process she likens to"making rainbows of starlight," she studies the amount of light at each color to determine of what the star is made.

"We classify stars by their luminosity and temperature," said Pilachowski."Massive stars are hot and bright. Stars with little mass are cool and dim. Most stars change color during their evolution. Color is just a manifestation of temperature. Hot stars are blue, mid-temperature stars like the sun are white or yellow, and cool stars are orange or red.

"We can detect very small traces of exotic elements like europium and barium and lanthanum using spectroscopy, and these elements tell us about the types of stars that formed and contributed to the early chemical soup of the galaxy."

Obviously, star formation is a complex process. While scientists don't completely understand, the star-making process is thought to begin with clouds of gas and dust called nebulae. Through contraction and collapse under the force of gravity, the cloud of gas becomes a protostar. A protostar, which in turns evolves into a main sequence star, like the sun.

Once a cloud of gas starts to contract, said Pilachowski, it will continue to do so as long as there is a way for the gas to stay cool.

"For clouds of little or no metal elements, we think hydrogen molecules radiate the heat away to keep the cloud cool as it contracts," said Pilachowski."Elements are made by nuclear reactions in the cores of stars. The sun is converting hydrogen into helium, for example, and when it runs out of hydrogen, it will start converting helium into carbon and oxygen. Some elements are made only in supernovae."

Supernovae, high-mass stars, occur when massive stars run out of nuclear fuel in their cores. At least eight or 10 times the mass of the sun and capable of generating as much energy in a few weeks as the sun puts out in its entire 10-billion year lifetime, a supernova suddenly increases in brightness due an explosion. The explosion may leave behind a tiny neutron star, or in the case of a sufficiently huge explosion, a black hole. Or maybe, nothing at all.

Red giants, low mass stars on which Pilachowski does most of her work, evolve more peacefully, morphing into a planetary nebula. Then they become white dwarfs.

"As stars evolve, they cool somewhat to become red giants, and then get hot again as the outer layers are stripped off and they become white dwarfs," said Pilachowski."Then the white dwarfs cool and fade away as the stars finally die. They die when they've exhausted their nuclear fuel, and there is nothing left to burn to keep them shining."

Does knowing the stars intimately detract from their magic? No way. In fact, being familiar with the sheer magnificence of a star's life cycle may even enhance its charisma.

"Still and always (I am awed), especially at the sight of the Milky Way from a clear, dark site on a moonless night," said Pilachowski."That is one of the best parts of being an astronomer!"

Another good part is sharing the beauty.

"The first thing I'd show a beginning astronomer would be the telescope," she said,"because telescopes are awesome! The second thing would be the instrumentation, because our modern instruments (such as the spectrograph) to detect and measure the very faint light of the stars and galaxies are incredibly powerful.

"And then I'd take a peek at a globular cluster. These are old clusters of stars that are really beautiful to see. They contain as many as 1 million stars, and with a large telescope, you can actually see the colors of the stars. You need to see a picture of a globular cluster!"

Pilachowski suggests we try the following site:

http://hubble.stsci.edu/news .and. views/pr.cgi.1999+26f