ALMA witnesses birth of stellar baby twins

Our sun is a single star. It lives out its life all by itself. But the majority of stars in the universe are twins: two stars that were born more or less at the same time. For most of their lives, stellar twins stay together. They orbit each other in an ever-lasting cosmic waltz. Astronomers call them binary stars.

Surprisingly, not much is known in detail about the birth of binary stars. All stars are born in cosmic clouds of gas and dust. The clouds collapse under their own gravity. In the case of a single star, the material ends up in a flat, rotating disk. The gas in the disk flows inward. At the centre of the disk, it collects into a huge ball. Eventually, the ball of gas becomes hot enough to start shining. A star is born.


But in the case of a binary star, no one knew how the gas from the disk falls toward the centre. Taiwanese and Japanese astronomers have now solved this puzzle. Using ALMA, they made detailed observations of a pair of baby twin stars that are forming right now. ALMA revealed that the surrounding disk is not smooth. Instead, it has a spiral structure.

The astronomers also measured the rotation velocity in the spiral disk. They found that the gas between the spiral arms rotates slower. The result is that it falls towards the centre, where it piles up on the two baby stars.

So it looks like the newborn twins shake up the material in the disk, because of their orbital motion. Thanks to this shake-up, gas from the disk can flow inward, where it ‘feeds’ the growing baby stars.

To understand the observed spiral structure of the disk, the astronomers simulated the birth of the baby twin stars, using a huge supercomputer. The outcome of the supercomputer simulations accurately matches the ALMA observations.

So thanks to ALMA, astronomers now have a better understanding of the birth of stellar baby twins. That means of the birth of most of the stars in the universe.

ALMA observed a pair of baby twin stars known as L1551 NE. The pair lies at a distance of 460 light-years, in the constellation Taurus the Bull. The two stars are smaller and less massive than the sun. They orbit each other at a distance of some 22 billion kilometers – 145 times the distance between the sun and the Earth.

The new ALMA observations were carried out by an international team of astronomers led by Shigehisa Takakuwa of the Institute of Astronomy and Astrophysics at the Academia Sinica in Taiwan. The results have been published in The Astrophysical Journal on November 20, 2014.