Over a century ago, what is probably the best-known chart in the whole of astronomy was produced so that we could understand and study the stars better. This was the Hertzsprung-Russell diagram (HRD), named after the two astronomers Eijnar Hertzsprung and Henry Norris Russel, which orders stars by their brightness and color. Stars are hot balls of gas that exhibit different properties. The HRD makes these differences visible by displaying the stars in different colors and sizes, which indicate their luminosity, temperature and distance away from us. Many of the things that we know about stars and how they change over time come from studies into how and why they are arranged in groups in this diagram. For instance, our own Sun is in the “main sequence” together with most of the other stars.
For a long time, it was hard to make a clear distinction between the various groups of stars because conventional telescopes were not accurate enough. However, recent observations through the Hubble Space Telescope reveal some interesting insights: “We think that all the stars in the star cluster were created at the same time out of the same cloud of gas,” explains Chen Wang, the principal author of the study published in Nature Astronomy, who completed her doctorate at the University of Bonn and only recently started as a researcher at the Max Planck Institute for Astrophysics (MPA). “They should all be the same age and have the same chemical composition. But if that’s the case, then how can there be a second sequence of stars that are bluer?”
This strange phenomenon left astronomers scratching their heads, with many even choosing to ignore it as an explanation seemed hard if not impossible to come by. Drawing on her experience as a theoretical astrophysicist, however, Chen combined two clues to put forward one possible origin of these blue main-sequence stars. First of all, she used computer simulations to determine that the blue stars could be explained if they were rotating more slowly than others in the cluster. Second, recent models of two stars merging have shown that the stars formed in the process become strongly magnetic and rotate very slowly. Chen put these two ideas together and suggested that the blue stars are actually ones rotating very slowly that emerged from the merger of other stars.
When a binary star system (two individual stars that are gravitationally bound to each other) fuses together, it produces a star that is more massive than both of its predecessors. It also contains more hydrogen in its core than a single star of the same age and mass. In other words, although the merged stars may be just as old as all the other ones in the cluster, they appear younger on the color/brightness chart because of their blueness.
“The merger hypothesis that Chen is proposing is a very compelling explanation for the blue main-sequence stars, because it combines various puzzling findings in a logical way,” says Selma de Mink, Director and Head of the Stellar Astronomy Department at the MPA. This would mean that a significant proportion of the stars merge with a companion before their life has even really begun. If Chen is right—which she could well be—this could shine a new light on many questions about how stars are born, why some rotate fast and others slowly, and why some have magnetic fields and others do not.
“Data from star clusters suggests that stars can be formed in two ways, either by attracting more material to them through gravity—as has long been assumed is the case. Or, following the new theory, by the merger of two existing stars, which is the case for around 30 percent of them,” says Professor Norbert Langer from the Argelander Institute for Astronomy at the University of Bonn and the Max Planck Institute for Radio Astronomy. “This is casting star birth in a whole new light.”
Although more tests are naturally still required, as with any hypothesis, we may finally have an explanation for the mystifying riddle of the blue main sequence more than a hundred years after Hertzsprung and Russell created their famous diagram.
