Star Clusters Unexpectedly Spawning New Stars

Robert Gendler and Stephane Guisard. If you lived on a planet in Camargo 438 or 439, most of the sky would look dark. However, the Milky Way would be magnificent in one direction, looking much like this photo of M83.

While the discovery of two star clusters far above the Milky Way was a surprise, the finding that new stars are forming there right now was even more unexpected.

Young galaxies are full of stray gas in the process of being converted to stars, some of which are huge and blue, others small and red. Big stars live fast and die young (and leave very beautiful corpses indeed), so after a while they're all gone and all that is left is smaller, more sedate celestial bodies. The rare exceptions occur when new sources of gas keep the production line of bright stars rolling.

One place we don't get new stars is among the globular clusters dotted around a galaxy. These are always filled with old stars of small to medium size, and are stable places without the excitement that comes with new star formation.

Or at least that is what we thought until Camargo 438 and 439 were discovered. The pair of clusters lie within the gas cloud HRK 81.4-77.8, which is located 16,000 light-years from the disk of the Milky Way. The clusters of stars turned up on a survey of gas clouds conducted by a team at the Federal University of Rio Grande do Sul, Brazil.

Finding a molecular cloud such as HRK 81.4-77.8 so far from the galactic plane is a rare event. In a paper to be published in the Monthly Notices of the Royal Astronomical Society and currently available at ArXiv.org, team leader Dr. Denilso Camargo describes the pair as “the most distant known embedded clusters from the [Galactic] plane.” This is not a case of incremental record breaking; most extra-galactic clusters lie less than a twentieth of this distance from the plane of the galaxy.

While clouds such as HRK 81.4-77.8 are made up of gasses that could become stars, their density is very low, which has been thought to be the reason we don't see star formation occurring inside them. Researchers still don't know where these clouds come from, although Camargo and his co-authors favor the “Galactic fountain” model where supernovae within the galactic disk throw off material that eventually becomes clouds like these.

An alternative proposal is that the clouds come from outside the galaxy and are slowly being captured by the Milky Way, possibly as a result of interaction with the Magellenic clouds.

Either way, the process appears to be very new by astronomical standards. The largest stars in the two clusters have yet to reach the main sequence, indicating we are witnessing the very beginnings of star formation in the region with ages estimated at 2 million years. Although the two clusters each have around 40 stars, C439 is thought to contain 260 solar masses, whereas C438 has barely a fifth of that mass.

H/T: LA Times

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