New discoveries Massive binary stars are born as real twins Read time: 3 minutes All people look different from each other. Except identical twins. They have the same eyes, the same nose, the same mouth and the same hair. Very often, they even wear the same clothes. When two people are so much alike, you can be sure they were born as twins. But what if all people would look like each other? In that case, it would be hard to recognize real twins. Two children of the same age that are always playing together might have been born as twins. But they could also have been born separately, and just became friends at a very young age. It would be hard to tell. That’s the situation with stars. Stars can look very much alike. So if you see two stars orbiting each other – a so-called binary star – it’s hard to know for sure if they were born together like stellar twins. Maybe they were born separately, and only teamed up at a later stage. In particular, astronomers don’t know the birth history of very massive binary stars. There’s one way to find out, of course. If you could witness the birth of stars, you might be able to see if stars are born single or in pairs. However, that’s difficult. Stars are born in large clouds of gas and dust, and it’s hard to see what happens inside. ALMA comes to the rescue. Observing at millimeter wavelengths, ALMA can peer through the obscuring dust clouds and see what goes on inside. It’s a bit like a doctor using ultrasound to ‘look’ inside the belly of a pregnant woman. The ALMA astronomers studied a dusty star-forming region in our Milky Way galaxy. They discovered that the pregnant dust cloud is already carrying twin ‘baby stars’ orbiting each other once every 600 years. Together, the two stars weigh in at 80 times the mass of our own sun. In the future, when the surrounding dust is blown away, the two stars will shine as a luminous, massive binary. And thanks to the ALMA observations, we now know that they were really born as twins. The same is probably true for most other massive binary stars in the Milky Way. What? The star-forming region studied by ALMA is known as IRAS 07299-1651. It is some 5,500 light-years away on the border of the constellations Canis Major (the Greater Dog) and Puppis (the Stern). The cloud of gas and dust was first discovered by the Infra-Red Astronomical Satellite (IRAS) in 1983. ALMA was able to peer through the dust and detect an orbiting pair of baby stars – as if ALMA could see twin babies growing in the belly of a pregnant woman. The two stars are some 12 billion kilometers apart and orbit each other once every 600 years or so. Together, they are about 80 times more massive than our own sun. Who? The ALMA observations of IRAS 07299-1651 were carried out by an international team of astronomers, led by Yichen Zhang of the RIKEN Cluster for Pioneering Research in Japan. Yichen worked together with colleagues from Japan, Sweden, the United States, Italy, and Chile. The team published their results in Nature Astronomy on March 18, 2019. ALMA URL