Big bruiser stars form like their wimpy little siblings

Contributed by
Jul 2, 2010

Astronomers using the UK's Infrared Telescope (UKIRT) have made an important discovery: massive stars form much like lower mass stars do.

That doesn't seem surprising, does it? Ah, but it is. It turns out that massive stars are different than the hoi polloi like the Sun, and it's a bit puzzling that they can form at all!

As I've written many times before, stars form from clouds of gas and dust. A cloud can be huge, light years across, its massive gravity balanced by internal heat. But collisions between clouds or nearby exploding stars can disturb that equilibrium, compressing the cloud. Random eddies and whorls in the cloud can be accelerated, amplified by the collapse, giving the entire complex an overall rotation. Random collisions between pockets of matter inside the cloud slowly rob them of energy, causing them to move inward, falling toward the cloud's equator. What was once a giant amorphous mass is now a relatively rapidly spinning disk of material. The disk gets denser toward the center. As the material compresses there, it heats up. Eventually, if enough matter piles on, the temperature and pressure at the very center can be enough to fuse hydrogen into helium, and a star is born.

But there's more to this story. Massive stars are far hotter than lower mass stars. As the material from the disk accretes onto the protostar, the protostar gets hotter. If it gets too hot, it'll blow away the disk, preventing the star from gaining any more mass. Yet we see more massive stars. How do they form then? Is it the merging of two lower mass stars, or some other process? Any scenario like that would mean that massive stars form differently than lower mass stars, and we should be able to see that. Enter the UKIRT observations. It's really hard to see what's going on deep in the heart of a star-forming cloud, because they're opaque to optical light. Infrared light, however, gets out. Astronomers used UKIRT to look at 50 high-mass young stellar objects, as they're called, to see if they reveal themselves to be different than lower mass stars.

The result they found surprised them: these hefty stars form pretty much like lighter-weight stars.

The key to this was the presence of jets, beams of matter and energy blasting out from the poles of the stars. These jets are focused by a lot of forces, but critical to them is the presence of the disk of material from which the star forms. They saw these jets coming from massive young stars (like in the picture above; the red glow is from material in the jets) and they look pretty much like they do in their low mass star counterparts. Not only that, the jets seen in massive stars are well-defined and don't appear to depend in any way on the amount of energy the star is producing; fiercely hot and bright stars make jets as easily and as nicely as relatively calmer stars.

A study last year indicated that instabilities in the cloud as it collapses help the massive stars form, but that was done in a computer by modeling the physics. These new observations show that this process may yet be correct, since there's no need to make a massive star by colliding smaller ones, for example. It's not proof, but it doesn't contradict the earlier test either.

Still, what's clear is that whatever process is going on in the heart of a star-forming cloud appears to work the same way for stars up to a mass of at least 30 times the Sun's mass (the astronomers didn't observe any stars more massive than that). This is good news! The puzzle of the formation of massive stars just had another puzzle piece fall into place.

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