The Galaxy Evolution Explorer: Strength in numbers

When astronomers try to piece together the life cycle of stars, or even entire galaxies, they run up against a substantial problem: In the course of a single human life, nothing much changes. It's no wonder ancient people saw the cosmos as eternal and immutable; the night sky has looked pretty much the same ever since we began recording it.

Of course there are some exceptions, and historically, they have been greeted with widespread panic. Comets are thought of as harbingers of doom, or once in a while a star grows inexplicably brighter and gets written down into the chronicles. This brings up an essential question in astronomy: How can we understand how the stars are born, live, and die, when no single person (or single species, for that matter), has ever existed long enough to observe the whole process?

The answer lies in observing as many stars and galaxies as possible at the same time. A new NASA mission is setting out to do just this.

Consider this analogy. If you wanted to study the life cycles of human beings, but you could only do so for one day, then you would want to observe, say, a million people, selected at random. You might notice that some humans are noticeably smaller and less developed. In that population, you'd find babies. Maybe, if you were lucky, you'd even observe someone being born. Then you might notice toddlers, school-age children, and a few teenagers.

Is there a sequence suggested between these groups? Perhaps so. And even though there is a large variation in size and shape in mature adults, you could probably identify these humans as being full grown by other biological and social traits. Some of them might even be pregnant, which would help explain those babies. And finally, you'd notice that some of the humans didn't seem to work so well anymore: Aging has set in. You'd notice that old people are more likely to die, but young people sometimes die, too. Could you recognize the pattern?

Galaxies, which are interacting collections of hundreds of billions of stars, must have a life cycle too. The search is now on to find the youngest galaxies, with the most powerful telescopes in the world competing to find the tiny, faintest images of galaxies billions of years ago. (Remember, it takes light time to travel through space. A galaxy we observe at a distance 13 billion light years away, looks to us as it appeared 13 billion years ago.)

On April 28, NASA's Galaxy Evolution Explorer (Galex, for short) shot into orbit to join the hunt. Galex's goal is to study how galaxies have changed, specifically how star formation rates have changed, in the last 10 billion years. It will look at galaxies near and far (older and younger populations) and ask pretty much the same questions we asked about humans. How do younger galaxies compare to older ones? How are they similar? Different? At what point in a galaxy's lifetime does star formation take place? Is it an even-keeled process, or do stars form in fits and spurts? How does star formation relate to a galaxy's size, shape, and chemical composition?

Galex's technique to explore star birth rates is quite ingenious. It will look at galaxies with ultraviolet light. The reason UV light is a good probe of star formation is that hotter stars live shorter lives. Stars with significantly higher surface temperatures than the Sun give off most of their radiation in UV. They also lead significantly shorter lives, as they have to burn through their nuclear fuel faster to maintain their high temperatures. The hottest and most UV-bright stars live amazingly short lives of only a few million years. It's reasonable, then, to assume that a galaxy with an overabundance of UV light is going through a burst of star formation.

Once Galex establishes the galaxies with a lot of UV light, the pattern-hunting begins. Since Galex will be able to observe distant galaxies, astronomers will be looking to pinpoint the exact phase in the life of a galaxy when the UV turns on and star formation kicks into high gear. The high-mass, high-temperature stars Galex is sensitive to are also the sorts of stars that go supernova when they die, creating all the heavier elements of the periodic table. And those happen to be the elements that make up planets, people, and all the building-blocks of life. So, in a way, Galex is probing not only the presence of massive stars, but the point in a galaxy's evolution when life becomes possible.

One of the key aspects of Galex's mission is to correlate its findings with other observatories. Galex will also do detailed studies of nearby galaxies, in order to understand how a fully-grown, mature galaxy looks like. We can then compare that to what an extremely young galaxy looks like in the UV, but to do that, we'll use visible light observatories like the Hubble Space Telescope.

So, at the end of Galex's 28-month mission, we should have a much better idea what the life cycle of a galaxy is really like. No one could ever watch an entire galaxy change and evolve, but when you've got a few million to play with, you can piece together a story. In the end, it's a powerful way for tiny, limited things like ourselves to reach into the larger universe.

Michelle Thaller is an astronomer at the California Institute of Technology. A massive-star specialist by trade, she dedicates most of her time to education and public outreach for the Space Infrared Telescope Facility.

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