Stars Form Much More Readily Than Astronomers Thought

By Bill Andrews | April 10, 2014 1:06 pm
Figure 1: Two of the molecular clouds studied by Kainulainen and colleagues: The Pipe Nebula (left) and the Rho Ophiuchi cloud (right) in the Milky Way. In the background, an ordinary image of the Milky Way; each inset map shows to what extent the light of background stars is dimmed as it passes through the cloud in question. These maps form the basis of the three-dimensional reconstruction of cloud structure from which the astronomers derived their "recipe for star formation". Credit: Background: ESO/S. Guisard

The Pipe Nebula (left) and the Rho Ophiuchi cloud (right) in the Milky Way. Each inset map shows how much the light of background stars is dimmed as it passes through the cloud in question. Credit: Background: ESO/S. Guisard // Column-density maps: J. Kainulainen, MPIA

Understanding stars is fundamental to the science of astronomy: the “astro” in “astronomy” means star, after all. And courtesy of a new study researchers have a better understanding of how these things form — providing insights not just into the stars themselves, but also into galactic and planetary evolution.

A Star Is Born

Astronomers weren’t totally in the dark about star formation. They knew that stars form within giant clouds of molecular gas (mostly hydrogen) and dust. When an area within the cloud becomes too full of molecules, it undergoes gravitational collapse — the area attracts more stuff, which makes it denser and more massive, which attracts yet more stuff — until enough stuff is there to ignite nuclear fusion, the process that fuels a star.

The problem was figuring out how to know when an area is “too full” of molecules. What’s the “critical density” at which gas clouds turn into stars?

Astronomers had long been trying to figure this out, and have devised many models of star formation. The only problem was, it’s pretty hard to measure how dense a gas cloud is to begin with.

Star Search

Now astronomers have shown that they can determine a gas cloud’s density by analyzing how it distorts the light from distant stars “behind” it. The more a star dims, the more gas it’s traveling through, and therefore the denser the cloud is at that point.

Armed with a way of calculating a cloud’s density, the team carried out direct observations of 16 nearby star-forming gas clouds (within about 850 light-years of Earth) to determine a value for the critical density necessary to set off a new star: around 5,000 hydrogen molecules per cubic centimeter. That’s a lot lower than the theories had predicted (a whole order of magnitude lower, in some cases), which is surprising. It’s unclear yet why the guesses were so far off. The findings appeared in this week’s Science.

Now that astronomers have this technique at their disposal, they’ll be better able to test and refine their theories on star formation. That could mean a solution to one of the greatest challenges of astrophysics: being able to look at a gas cloud and predict how many stars will come out of it, and what kind of stars they’ll be.

This gives researchers great predictive power in figuring out how galaxies (including our own) will behave in the future. It’s also a boon to researchers who study planet formation, since star birth is the first step in the process of creating a solar system.

CATEGORIZED UNDER: Space & Physics, top posts
MORE ABOUT: stars
  • Michael Orfield

    5,000 seems small to me. Even in a cubic centimeter. Does this figure compare at all with the densities needed for nuclear fusion in a hydrogen bomb? Or am I talking apples and oranges?

    • http://blogs.discovermagazine.com Jonathan Tracey

      It’s important to realize that this also requires a lot of cubic centimeters to occur.

      • Michael Orfield

        Good point.

    • http://www.hackcraft.net/ Jon Hanna

      Yes, apples and oranges. It’s not the density necessary for fusion. Indeed, if you ever made hydrogen as a schoolchild you’d have had much denser hydrogen in that test-tube. Rather, it’s the density necessary to start attracting more hydrogen in, and from that starting point to begin to become massively denser again.

      • Michael Orfield

        Thank you

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