Courtesy Richard Mushotzky and S. Snowden, Laboratory for High Energy Astrophysics, Goddard Space Flight Center.

Through a glass, darkly
Here's an unsettling fact about the universe: After thousands of years of looking, we still can't see most of it. Sure, we see a few comets and asteroids, lots of stars, gobs of gas, even the odd extrasolar planet or two. Still, most of the universe remains absolutely invisible. It's called "dark matter," and it doesn't emit, reflect or absorb light. In fact, dark matter does not seem to be composed of the familiar electrons, neutrons and protons.

As to what we can say about what dark matter actually is, the answer is -- some is probably composed of black holes, but beyond that, not much is known.

So it doesn't take an astrophysicist to realize that if we want to understand the universe, we've got to transcend today's bumper-sticker level of knowledge about dark matter.

Cluster truster
Astrophysicist Richard Mushotzky searches for dark matter not in galaxies but in clusters of galaxies. A cluster can contain as many as 100,000 galaxies, and since one galaxy can contain a billion stars, simple math says that one cluster has more stars than Los Angeles does cars. "These are the largest things that know about each other, which are physically connected to each other," as Mushotzky explains.

If something is true of such a vast array of matter, it's a significant truth about the universe.

So far, most of what we know about clusters comes from studying visible light from stars and galaxies. But they may not be the best source of data because they are only a small portion -- maybe 3 percent -- of the mass in a cluster. Tracking dark matter with visible galaxies, Mushotzky says, "Does not work on a large scale... It's like trying to trace the distribution of matter in Colorado by looking only at the mountain peaks -- you would get the wrong image of the land."

Preparatory blather aside, what's Chandra's role?
You knew X-rays would enter this picture, and here we go. Much more of the matter in a cluster -- say 17 percent - is gas, not stars. This gas -- mainly hydrogen and helium -- is composed of the familiar protons, electrons and neutrons. Much of it is hotter than this summer's heat wave -- say one million degrees Celsius. And that's hot enough to glow in the X-ray part of the spectrum, which makes it fair game for a telescope with Chandra's resolution.

Chandra has a second advantage: It will not rely on the traditional "how fast is that yo-yo spinning" approach to dark matter. Instead of measuring galactic rotation, the new X-ray telescope will look at gas pressure.

As with the rotation studies, the reasoning comes straight from physics 101. Chandra can identify the temperature and identity of the hot gas. The various lines on a spectrum present tell us what elements are emitting radiation, University of Wisconsin X-ray expert Wilt Sanders points out, the brightness ratio of two lines of the same element can tell us the temperature, and the absolute brightness tells us how many emitting atoms (actually ions) are along the line of sight.

From that we can determine the density and pressure if we know the size of the cluster and assume that it is roughly spherical. Even though the cluster gas is perhaps one-hundredth as dense as the gas in a galaxy (which measures at roughly one atom per cubic centimeter) random interactions between the gas atoms will cause it to "try " to spread further apart. From the pressure, it's fairly easy to measure the gravitational field required to counterbalance that pressure, and then calculate the mass and location of the dark matter itself.

In a way, Chandra will improve on a TV set. The ol' boob tube allows us to see something (radio-frequency electromagnetic radiation) that's invisible to the naked eye. X-rays, on the other hand, should allow us to see something that's invisible in all wavelengths.

To Mushotzky, who's been involved with planning the X-ray telescope from the beginning, Chandra is part of a "revolution" that will produce better images of dark matter. "The numbers are more robust," he says, and better numbers should lead to a better picture of one of the most profound mysteries of the cosmos --- what it actually is.

Stars 'R us
Knowing what makes up the universe should help us understand how it became what it is today. According to present estimates, the universe is about 12 to 14 billion years old. Our galaxy, on the other hand, is just 10 billion years young. What preceded our galaxy?

Nobody knows. But to approach the issue, Chandra will track the remains of supernovas to supply another slant to the enduring question of the universe's evolution. Supernovas, immense explosions that occur when some stars run out of fuel, produce giant nebulas, clouds of glowing dust that remain visible for thousands of years afterwards.

Past nebulas contained something quite important -- the atoms in our bodies. Heavy elements, from carbon, the life-giving element six on the periodic table through uranium, element 92, are made only in the furious burst of fusion that precedes a supernova.

Says astrophysicist Wilt Sanders, "Everything heavier than lithium is made in stars." Only after the explosion of a supernova can these atoms "get into space and get swept up in the next generation of stars and planets." Most atoms in Earth and other planets are actually "recycled" material made in supernovas. We -- and old Apple IIs and baritone saxophones, too -- are made of material from the stellar junkyard. The entire solar system, in fact, is part of a generation of objects assembled from dust and gas left by a previous generation of exploding stars.

Sanders, who plans to use Chandra to study supernova remnants, says the superheated gas and dust could offer data on the past, and clues to the fate of the current generation of stars and planets.

One thing's for sure. Neutron stars are almost as strange as dark matter.

©1999, University of Wisconsin, Board of Regents.