Suppose you live in a very big house, with lots of families, each one with many children. Every month or so, a new baby is born, so you know all about it. You wonder about childbirth in other houses. But the nearest big house is many miles away, and the houses in your neighborhood are all small, with just one small family – nothing too much is happening there.
That’s more or less the situation with astronomers trying to study star formation in other galaxies. We know about the birth of stars in our own Milky Way, but what about other galaxies? Hard to say: the large ones are very far away, and the nearby ones are tiny, with not much star formation going on, so it’s hard to see anything happening there.
Thanks to the sharp vision of ALMA, astronomers have now been able to study small regions of star formation in a nearby dwarf galaxy in great detail. The galaxy, known as NGC 6822, is much smaller and less massive than our home galaxy. If you’d compare the Milky Way to a large adult person weighing 100 kilograms, NGC 6822 would be like a new-born kitten, weighing just 200 grams or so. Surprisingly, star formation in the small galaxy is not very different.
Of course, the number of new stars that are born in the dwarf galaxy is much lower. Also, the ‘stellar nurseries’ – the clouds of gas and dust from which new stars are born – are smaller. But in the coldest and densest parts of these clouds, molecules are formed, just as in our Milky Way galaxy. The millimeter and submillimeter radiation emitted by these molecules can be observed by ALMA.
The dense cores of cold, molecular gas in NGC 6822 turn out to be much smaller than similar cores in the Milky Way. Still, the molecules move around in a very similar way and with very similar velocities. This suggest that the formation of new stars in a dwarf galaxy happens in the same way and takes about the same amount of time as in the Milky Way.
Large galaxies, like the Milky Way and the Andromeda galaxy, grew from mergers of small building blocks like NGC 6822. Since astronomers now know how star formation works in a small dwarf galaxy, they may also better understand the process in larger galaxies, even if those can’t be studied in so much detail.
What? NGC 6822 is a small, irregularly-shaped galaxy in the Local Group – the group of a few dozen galaxies to which our own Milky Way and the Andromeda galaxy belong. The dwarf galaxy was discovered in 1884 by American astronomer Edward Barnard – it’s sometimes called the Barnard galaxy. It is located in the constellation Sagittarius (the Archer), at a distance of approximately 1.5 million light-years.
The ALMA observations of NGC 6822 were carried out by a team of astronomers led by Andreas Schruba. Andreas is an astronomer at the Max Planck Institute for Extraterrestrial Physics in Germany. He worked together with colleagues from Germany, the United States, the Netherlands and South Africa. The team published the new results in The Astrophysical Journal – a professional astronomy magazine.