Fat Stars Grow Just Like Skinny Stars

How much did you weigh when you were born? Most newborns are between 3 and 4 kilograms at birth. But not all. Some babies weigh much more or much less. It all depends on how fast they grew.

With baby stars, it’s the same. Most newborn stars can have different masses, and most weigh less than the Sun. But some are much heavier. So did they grow in the same way?

To answer that question, astronomers have used ALMA to study a very massive baby star. The star is known as G353. It already weighs in at ten times the mass of our own Sun. And it’s still growing, so in the end, it will be even more massive.

Astronomers already knew how lightweight baby stars are growing. When they are very young, they are surrounded by a large disk of gas. The gas falls onto the star, thanks to the star’s gravity. As a result, the star becomes ever more massive. The growth of the star only stops when there is no more gas left in the disk, or when the star’s radiation blows the remaining gas into space.

But no one knew how the heftiest baby stars grow. These massive protostars, as they are also called, are quite rare. Even the nearest ones are many hundreds of light-years away – too far for detailed study.

However, thanks to ALMA’s sharp vision and high sensitivity, astronomers have now studied the growth of G353. And it turns out to grow in just the same way as smaller baby stars. The only difference is that the surrounding disk contains more gas.

The disk of G353 is a little bit brighter on one side than on the other side. Also, it appears to be unstable. In the future, it will probably fall apart in many smaller blobs. Similar things have been observed in the disks of less massive stars. That’s another indication that heavy stars and lightweight stars grow in the same manner.

You might think that this is an obvious result – something you would expect. But remember that no one knew whether or not there would be a difference. That’s also important in science: even if you believe something to be true, you first have to prove it!

What?

G353 is actually called G353.273+0.641. The numbers refer to the star’s position in the sky, in the constellation Scorpius. The protostar is 5,500 light-years away and just some 3,000 years old. Luckily, we see the star’s disk almost face-on. As a result, ALMA had a clear view of what goes on in the disk.

Who?

The ALMA observations of the massive protostar G353 were carried out by a team of Japanese astronomers, led by Kazuhito Motogi of Yamaguchi University. The new results have been published in the Astrophysical Journal Letters.