Many stars are single, just like our own Sun. But there are also many twins and even triples in the Universe – two or three stars that were born together and keep each other company. Using ALMA, astronomers have now witnessed the birth of a triple star system. The new observations confirm the theory that there are two ways to form multiple stars.

Stars are born from large clouds of gas and dust. Gravity pulls the particles in a cloud together. As a result, the cloud gets smaller and denser. Eventually, it becomes hot enough to produce heat and light – it has turned into a star.

Large clouds fragment into smaller pieces during their contraction. Each fragment can turn into a star. That’s how star clusters are born – large groups of many dozens of individual stars.

At a later stage, a cloud fragment can break down further into two or three pieces. The resulting stars are born very close to each other. Their mutual gravity keeps them together. That’s how binary or multiple stars are born. The individual stars belong to each other, but they’re usually well over a hundred billion kilometers apart.

But the new ALMA observations reveal that there’s another way to form stellar twins or triples, at much closer separations. Newborn stars are usually surrounded by flat, rotating disks of gas and dust. Eventually, this material can clump together into planets. But in the case of a baby star known as L1449 IRS3B, something else is going on.

The ALMA images show that the disk has a spiral structure, indicating that it is unstable. Indeed, the disk is fragmenting into three separate blobs. Each blob will probably turn into a star on its own. The result: a triple star system with separations of just a few tens of billion kilometers.

Astronomers already knew that some multiple star systems are rather ‘wide’, while others are pretty ‘close’. They had speculated that this could be the result of two different ways to produce them. ALMA has now confirmed this idea.



L1448 IRS3B is a young ‘proto-star’ in the northern constellation Perseus, at a distance of 750 light-years. It’s probably less than 150,000 years old – very young for a star. Like most other proto-stars, it is surrounded by a flat disk of gas and dust. But in the case of L1448 IRS3B, the disk is unstable and fragmenting into three separate blobs. In the future, the system will probably evolve into a triple star system.


The ALMA observations of L1448 IRS3B were carried out by a team of astronomers led by John Tobin of the University of Oklahoma in Norman and the University of Leiden in the Netherlands. John worked together with Kaitlin Kratter from the University of Arizona in Tucson and with other colleagues from the United States, Germany and the Netherlands. Their results have been published in the 27 October 2016 issue of the scientific magazine Nature.