Monday, June 15, 2009

Building Planets, Part 2

I am returning tomorrow from vacation. In the meantime, here's a continuation of guest author Joel Green's discussion on his work in star and planet formation. Joel will be writing more in the future as his time allows.

Anyone familiar with the beautiful artist conceptions of the Milky Way might picture our galaxy as a spinning ceiling fan of stars, with the outer edges of the arms trailing off behind. This picture seems a little strange upon consideration: why are there no stars in between the arms?

Artists Conception of the Milky Way seen from above

Image Credit: NASA/JPL-Caltech/R. Hurt

The answer is that there are stars there. Spiral arms represent compression waves – shocks of great magnitude orbiting the bulge of our galaxy. The arms are illuminated by star formation as the great windmilling shock brushes the gas and causes compression and expansion, stirring up the material. The Galactic Ceiling Fan turns once 500 million years or so; the Milky Way is about 20 orbits old. In Galactic years, we are in our infancy!

This compression cascades down from the largest size scales through clusters and associations down to individual star-forming regions known as molecular clouds – so named for their abundance of molecular hydrogen (H2) gas. There are other less abundant elements in these regions as well, cast out by dying stars of earlier generations, and interstellar dust in a pristine state. These clouds are the sites of star birth in groups of a few to a few thousand at a time.

The Orion Nebula as seen by the Hubble Space Telescope

The Orion Nebula as observed by the Hubble Space Telescope.
Image Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

When instabilities, shocks, or turbulence stir up a quiescent cloud, some small parcel of the gas will be forced closer together. Because gravity is always an attractive force and gets stronger the closer things come together, an initial clump of gas and dust will continue to compress and draw in all the nearby material as it spirals into a more solid core that will eventually yield a star and, in all likelihood, a planetary system.

As the material spirals inward, it spins up faster. This is the same reason that dancers can suddenly spin faster if they pull their extended arms back to their body – the same amount of angular momentum transported to a tighter ring around the center leads to a faster orbit. So any particle with even the tiniest initial angular at large distances from the core will orbit very quickly when it is close to the star, and if there is a slightly preferred direction, then the core will begin to spin under all the impacts of these particles raining down on it. If the core were to absorb all of this energy without any release, it would spin so quickly and violently that it would blow itself apart, and star formation would be impossible. Instead, much of the momentum is carried off in the form of laser-like columns of jets shot out from the north and south pole of the spinning core, blasting into the environment.

Hubble Space Telescope image of Herbig-Haro 47

A Hubble Space Telescope image of HH 47. The source protostar is veiled in the center of the two symmetric flows.
Image Credit: STSci / NASA

Let’s pause for a moment in our slow zoom to the protostar, and back up to that initial clump. How did it get compressed? We have already seen that large-scale movement on the galactic scale can cause galaxy-wide compression, but how does this occur in an individual cloud? Judging from recent studies, it would seem that the causes of these clumps are actually TOO numerous. It is suspected that just by existing, molecular clouds have enough inherent turbulence to trigger star formation (usually referred to as “spontaneous” star formation). But there are smaller scale triggers, and we can actually observe these in action. For my thesis, I used the Spitzer Space Telescope to study a particularly hot and violent jet launched by a protostar, and traced its path through the Cepheus A molecular cloud, a mere 2000 light years from our doorstep.

To be continued...

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