High above the spiral Milky Way, astronomers have spotted two clusters of new stars growing at the fringes of our galaxy. The discovery, published in the journal Monthly Notices of the Royal Astronomical Society, appears to be the first such stellar cradles found outside the galactic disk.
Living so far off the disk has its perks: If aliens lived on planets around these young stars, they'd get a spectacular bird's-eye view of the Milky Way, pinwheel arms and all. (By comparison, Earthlings with a clear view see the Milky Way as a band across the night sky – a beautiful but less-shapely sight.)
The clusters, called Camargo 438 and Camargo 439, were found inside a high-latitude molecular cloud made mostly of hydrogen and named HRK 81.4-77.8. The giant cloud is thought to be about 2 million years old and sits about 16,600 light-years away, high above the galactic disk. Out in the interstellar boonies, this cloud should be boring – but instead, it's buzzing with young stars.
"The molecular cloud HRK 81.4-77.8 is currently forming stars, apparently an unprecedented event detected so far among high latitude clouds," study leader and astronomer Denilso Camargo of the Federal University of Rio Grande do Sul in Brazil said in an email.
Though giant molecular clouds can occasionally be found high above the galactic disk, none of those few distant clouds appeared to hold stellar cradles. Such star-birthing giant molecular clouds are typically found in the thick of the inner galactic disk, where there's a decent density of star stuff that gravity can start pulling together. Finding them high above the hubbub, in the sparsely populated halo around the galaxy, isn't just rare – it hasn't been seen before, Camargo said.
Camargo and colleagues had been looking for new star clusters using data from NASA's Wide-field Infrared Survey Explorer spacecraft, or WISE for short, when they came across the strange objects.
"I usually spend the nights at home looking for new clusters in the WISE's infrared sky maps," Camargo said. On a night like any other, he spotted two very faint objects, and emailed their description to his colleagues. In the morning, he said, his colleague Eduardo Bica told him that he had "discovered something very important."
Using the star clusters, the researchers were able to pin down the distance to these high-latitude clouds, which Camargo said was an unprecedented feat. The study shows that star formation can happen in very unexpected places in the galaxy.
Why do these star clusters exist? There are two possibilities, Camargo said. First off, it's possible that stars exploding in powerful supernovae shoot gas and dust far out from the galactic disk, and as that material starts falling back, it comes together to form these giant molecular clouds. The second explanation is a little more complex: A supernova explosion could produce what Camargo called a "continuous superwind" that blows gas out from the disk in an "expanding super-bubble" that could actually lead to not just one, but multiple episodes of star formation.
But the scientists also need to continue to examine the clusters to make sure they're of galactic origin, Camargo added.
"Are Camargo 438 and Camargo 439 alien star clusters?" he said. Only future study to determine their origin will tell.