Proof That Stars Form When Clouds Collide

The birth of stars is one of wonders of the cosmos but it is also a puzzle.

Astronomers have a rough idea of how it happens. Stars clearly form inside huge clouds of gas and dust. The thinking is that ordinary turbulence causes some parts of the cloud to become more dense than others.

When this happens, gravity takes over, drawing in more mass and creating a dense knot of gas and dust. This begins to heat up until the pressures and temperatures at the centre are so great that atoms begin to fuse. At this point, a new star switches on.

But this model raises a number of questions, in particular, why stars of a certain type tend to form in clusters within a cloud rather then evenly throughout it.

Clearly the process is more complicated and astronomers think they know why. The best theories assume that these clouds, driven by supernovas or other processes, often smash into each other as they expand.

At the interface where this collision takes place, there is a sudden rapid heating of gas which triggers star formation here. Again that seems perfectly sensible but the experimental evidence to back up this idea is sparse–it’s just very hard to tease apart the motions of gas clouds after they’ve collided.

Today. Kazufumi Torii at Nagoya University and a few pals say they’ve found two clouds that appear to be colliding in the Trifid nebula, M20. What’s more, they say that young stars appear to be forming at the interface. “We argue that the formation of the first generation stars…was triggered by the collision between the two clouds,” they say.

These guys used the NANTEN2 4-metre telescope in Chile to measure how the light emitted by carbon monoxide in these clouds is redshifted. That showed a number of different clouds in the Trifid Nebula, but two of them with different velocities seem to be superimposed.

The observations also show that the temperature of other clouds in the region is about 10K. But the two clouds of interest are much warmer at about 50K. Clearly these clouds must have collided, heating each other up in the process.

Crucially, the mass of each cloud is about 1000 solar masses but this is spread over a vast region of space some 2 parsecs across. “The total stellar and molecular mass is too small by an order of magnitude to gravitationally bind the system,” say Torri and buddies.

And yet, stars are forming in this region. This star formation must have been triggered by the collision, which took place about a million years ago (not long by astrophysical standards).

That’s quite a coup. Identifying clouds that have collided is obviously tricky. Back in 2009, astronomers (including some of these guys) spotted a similar cloud collision that also seems to be triggering cloud formation in a star cluster called Westerlund 2.

So this is just the second time anybody has seen this kind of starbirth, which must otherwise be common throughout the Universe.

Expect to see more examples from now on. Astronomers suspect that the chemical and physical characteristics of these clouds are an important factor in determining the size and type of stars that form.

Clearly, cloud collisions change the chemistry and physics of this process. The big question now is how.

Ref: arxiv.org/abs/1106.3603: Molecular Clouds in The Trifid Nebula M20; Possible Evidence For A Cloud-Cloud Collision In Triggering The Formation Of The First Generation Stars