Children look up at the night sky, at the twinkling stars and occasional colorful bursts of planets, and wonder where they come from, what they were made of, and how far away they really are.
Astronomers do too.
Every day scientists across the globe stare endlessly at the printouts from huge telescopes or from satellites deep in the uncharted regions of the galaxy, searching for the slightest clue. Among them is Lee Carkner, a doctoral candidate in astronomy and astrophysics at Penn State, who, along with his adviser Eric Feigelson, is studying stars that closely resemble our own sun.
Stars are spawned by large clouds of gas and dust which eventually collapse, entering into the primary stage of stardom, the protostar phase. Protostars differ from stars like the sun in that they do not burn hydrogen by nuclear fusion, but rather garner their energy from gravitational collapse. Some of the material does not fall directly into the protostar, but orbits it, creating a disk: The star now enters the T Tauri (pronounced TEE-tor-ee) phase. T Tauri stars are strong x-ray emitters, although most of their surface is only a few thousand degrees warm. (X-rays are normally associated with temperatures of several million degrees.)
"What we've figured out," Carkner says, "is that these x-rays are being emitted by a process very similar to that in the sun." Floating above the yellow surface of the sun are large coronal regions, thousands of times hotter than the surface itself. "These regions are heated," Carkner explains, "by the crackling of large magnetic loops rooted in the surface, heating the plasma so that it produces x-rays."
Because x-rays are blocked by the Earth's atmosphere, a ground-based telescope does not suffice to study T Tauri stars. Therefore, Carkner enlisted the help of the orbital satellite ROSAT, or Roentgensatellit (named after Wilhelm Conrad Roentgen, the man who discovered x-rays). ROSAT has long cylindrical mirrors wrapped in gold foil. X-rays enter these cylinders, are reflected along the mirrors, and interact with a box of gas (called a proportional counter), which detects the x-rays. Through a tedious process that involves taking a snapshot of a large region of the sky and searching for an x-ray emitter, Carkner is hoping to discover T Tauri stars that might have gone undetected by other methods. "You find a good x-ray emitter," Carkner says, "and that's a good candidate for a T Tauri star."
There are two types of T Tauri stars, classical and weak, he explains. The classical T Tauri is characterized by a large disk of dust and gas orbiting the young star. "Eventually," Carkner notes, "we think these disks will conglomerate into planets and planetary systems." Weak T Tauri stars lack any visible disk, though Carkner believes there was once one around every star. "These disks are ubiquitous," Carkner adds. "T Tauri stars have them, protostars have them. Something's got to happen to them."
All stars pass through the protostar and T Tauri stages, as our sun once did. "There are no Peter Pan T Tauri stars," Carkner jokes. "They always grow up." But the sun is midway through its 10 billion year life; the stars Carkner is looking at are a spry 10 million years young. Studying them, he believes, will lead to a better understanding of the time frame involved in the dissipation of the disk and the formation of planetary systems like our own.
Since any planetary system needs a sun (a former T Tauri) in order to support life, Carkner's study will also narrow down the possibilities of where other life could exist. Although it may not lead to the discovery of habitable solar systems and planets, Carkner says, "I think we'll be able to say with a lot more confidence that X number of stars have planetary systems. We'll have a more secure base to speculate how many civilizations might be out there."
Lee Carkner is a doctoral student in astronomy and astrophysics. His adviser is Eric Feigelson, Ph.D., professor of astronomy and astrophysics in the Eberly College of Science, 518 Davey Lab, University Park, PA 16802; 814-865-0162. This project is funded by NASA.