Finding Answers to Big Questions

Dr. Michael Muno is an astrophysicist who uses Chandra, among other telescopes, to study some of the most exotic objects in the Universe: white dwarfs, neutron stars, and black holes. He's currently a post-doctoral fellow at the Space Radiation Laboratory at the California Institute of Technology. He has spent time at both UCLA and MIT after receiving his Ph.D. from MIT in 1997.
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In my work, I use Chandra to study the black holes and neutron stars that are left behind when the largest stars exhaust their fuel and collapse. They fascinate me because they are the densest objects in the known universe. As such, their formation and the way that they interact with their surroundings involve physical processes that have few good analogies on Earth. They are the best laboratories to study Einstein's theory of gravity (general relativity) and the structure of matter at extremely high densities.

These objects can also provide answers to some big questions in astronomy. For instance, astronomers aren't really sure which stars form neutron stars and black holes, so I am searching for ways to determine what were their long-dead progenitors. I am also studying the process by which gravitational bodies can collect matter from their surroundings, which astronomers refer to as "accretion." Accretion is the key process in a number of phenomena, including how stars form out of collapsing clouds of interstellar gas. It's also how the supermassive black holes at the centers of nearly every galaxy get to be so large. Finally, the explosions that accompany the formation of black holes and neutron stars affect the surrounding gas in interstellar space. In some cases, the impact of the explosion compresses the gas and triggers the formation of more stars. When the explosions are too large, they may end up ejecting gas from the Galaxy, halting further star formation.

Currently, I am working with Chandra observations of the inner thousand light years of the Galaxy. I am interested in this region because I can find there just about every important process that occurs in our Galaxy. Massive clouds of gas are funneled into the Galactic center through their interaction with stars and gas. Over hundreds of millions of years, these clouds will collapse and form stars. The most massive stars exhaust their fuel quickly, shedding matter as a fast wind of plasma as they become hotter, and then they eventually explode. The energy from the winds and subsequent supernova explosions causes new stars to form in some places. This energy also expels the gas from the Galactic center in others. Eventually, what remains are the cinders of the dead stars: hundreds of neutron stars and black holes (and hundreds of thousands of white dwarfs left by less massive stars).

The Chandra X-ray Observatory is ideal for studying these processes. The fast winds of the largest young stars produce X-rays when they collide with surrounding gas, heating plasma to tens of millions of degrees. After the death of a star, the white dwarfs, neutron stars, and black holes can be seen in X-rays when they are tearing matter from binary companions. As the matter falls onto the "compact object", it is heated to tens of millions of degrees, making these "X-ray binaries" the brightest objects in the sky as seen in X-rays.

I think that the most compelling of the goals of this project involve black holes and neutron stars. There is a million-solar-mass black hole at the center of our survey of the inner Galaxy called Sagittarius A*. Already, my colleagues have identified small outbursts that occur as matter falls into that black hole . I recently have found evidence for HREF="http://chandra.harvard.edu/press/07_releases/press_011007.html">larger bursts of X-rays in the past that have been reflected towards us from nearby clouds of gas. I have also found evidence that the supermassive black hole is surrounded by a HREF="http://chandra.harvard.edu/press/05_releases/press_011005.html">swarm of smaller, stellar-mass black holes. My current plan is to identify a large sample of black holes and neutron stars that are in orbit with massive, young stars, so that I can measure their masses, using observations with ground-based infrared telescopes.

-Mike Muno
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Mike will share his thoughts on where he wants to go from here in the next post.