New insight in birth of stellar twins Read time: 3 minutes Many stars in the Milky Way galaxy are actually twins – two stars that were born at the same time. Sometimes, such stellar twins form a ‘close binary’: they orbit each other at a small mutual distance. But astronomers have also discovered many ‘wide binaries’, where the twin stars are much further apart. New ALMA observations have now revealed how these wide binaries are born. Stars are born from clouds of gas and dust. Thanks to its own gravity, the cloud contracts. It becomes flattened and starts to spin more rapidly. Eventually, the star that forms in the center of the cloud is surrounded by a flat, rotating disk. (The dust in this disk may clump together into planets). In some cases, the flattened cloud breaks up in two separate parts. The result: two disks, each with its own star at the center. The two disks lie in the same plane, and the spin axes of the two stars are aligned. That’s what is observed in close binary stars. Some astronomers thought that a close binary might evolve into a wide binary: for some reason, the stars could possibly drift apart. But if that is the case, their spin axes would remain aligned. Instead, ALMA observations of a newborn wide binary system have revealed that the two stars have very different orientations. That suggests that they each formed from their own small gas cloud. Apparently, the original cloud of gas and dust was very turbulent. As a result, it broke apart in various fragments, each with their own orientation. Astronomers want to understand the birth of twin stars, because the majority of all stars in the universe are part of a binary. The new ALMA results shed light on the very first phase of this birth process. What? The binary star that was studied by ALMA is known as IRAS 04191+1523. It was first discovered by the American-Dutch Infra-Red Astronomical Satellite (IRAS) in the early 1980s. Both stars are very light-weight: at most 10 percent of the mass of the Sun. They’re also very young: just a few hundred million years old. They orbit each other at a distance of some 135 billion kilometers – 30 times the distance between the Sun and the planet Neptune. Who? The ALMA observations of IRAS 04191+1523 were carried out by a team of astronomers led by Jeong-Eun Lee of the Kyung Hee University in Korea. Jeong-Eun worked together with colleagues from Korea, Japan, and the United States. Their results were published in Nature Astronomy on June 30, 2017. ALMA URL