Monday, June 08, 2009

Building Planets

Artist's conception of star formation around EX Lupi
Artist’s conception: EX Lupi builds its planetary system out of its cirumstellar disk.
Image Credit: NASA/JPL-Caltech/T. Pyle (SSC)
Note: I'm away on vacation right now. This blog post was authored by my friend and colleague Joel Green, a postdoc at the University of Texas at Austin.

I’m in planet construction.

When I’m out at a bar, I see that people in the real world have business cards. I’m now designing a business card with a hardhat on it. This might prove very handy should I ever run into Slartibartfast or anyone else from Magrathea.

But in practical terms – remember, this is practical for an astronomer -- what I study is the assembly of solar systems, born out of gas and dust inside of a giant stellar nursery, alongside a new generation.

Stars form in clusters; the light from star formation in other galaxies can be picked up en masse by our telescopes. Except for some new studies of the very nearest of our satellite galaxies (in particular the Large and Small Magellanic Clouds), we can infer very broad properties from these observations. And these large-scale questions are very interesting: what is the general rate of star formation? How is it distributed around a galaxy? Does star formation occur spontaneously (via some sort of small-scale turbulence) where gas clusters, or does it require a triggering event like a giant supernova or a local shockwave?

Spitzer image of star formation in the constellation Serpens
Spitzer-IRAC image of a cluster of stars forming in Serpens.
Image credit:
NASA/JPL-Caltech/L. Allen (Harvard-Smithsonian CfA) & Gould's Belt Legacy Team

But I don’t pay attention to other galaxies; I like to see the individual stars, and watch them develop. Of course I won’t live long enough to see an individual star go through its life cycle over millions or billions of years (in fact, some stars are so small that they can survive to several times the current age of the Universe). So rather than watching a single star, I collect data on thousands of stars that are in different stages of their life cycle, focusing principally on stars that are in their infancy -- less than about 25 million years old, all the way down to stars that are in their first 10,000 years. Some of these are hardly even definable as stars, rather as slowly compressing agglomerations of gas and dust surrounded by slowly spiraling and infalling material.

I study star formation that occurs in the local region, within a mere kiloparsec of our Sun. A kiloparsec is 3260 light years, or 19,000 trillion miles. I realize this sounds like a long way, but consider that our galaxy is perhaps 25,000 light years across. Our local group of galaxies is perhaps 2 million light years in extent. The observable Universe is 14 billion light years across. So really we are talking about a stone’s throw away; a kiloparsec hardly even gets us out of our local spiral arm, the Orion Arm, and its nearest neighbors.

So really this is just our backyard. Yet within this region we see stars of all ages and masses, of differing compositions and in a wide variety of local environments. All of these factors contribute to the development of a solar system; we survey these systems to disentangle these factors, much as doctors do in pharmaceutical studies.

The reason why we look at nearby stars, rather than surveying the galaxy, is simple. We want to be able to see them! Telescope time is precious, and we want to have as large of a sample size as possible. The instrument that we used to obtain this data, the Spitzer Space Telescope, just completed its 5 and a half year mission on May 15th, 2009. There is no instrument currently planned that will be able to examine the inner portions of solar systems to the extent that Spitzer could, and now we are left with a grand archive on which to conduct our studies.

Spitzer being assembled in 2003
Spitzer being assembled before launch in August 2003.
Image Credit:
Russ Underwood, Lockheed Martin Space Systems

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