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Astronomers discover a star so massive that its forming a tiny star around it rather than planets

The universe is full of surprises, and a colossal young star has been hiding a stellar one.

While observing infant star MM 1a, astronomers found that its massive disk was actually forming another star instead of planets. The much smaller companion, dubbed MM 1b, was detected just outside the behemoth star’s dusty disk, and could actually house a planet-forming disk of its own. The discovery of the new star, published on December 14 in the Astrophysical Journal Letters, marks one of the first times astronomers saw a star forming in the fragmented disk of another.

Stellar Siblings

Binary stars are pretty common in the universe, and it’s thought that they form the same way single stars do: from a massive cloud of dust and gas that collapses under its own gravity. If the molecular cloud is large enough, it can birth two similar-sized stars instead of one.

And since binary pairs are easily detectable, astronomers from the University of Leeds were rather surprised when they observed MM 1a. Homing in on the seemingly single star, they found an unexpected, much smaller companion star lurking in the outskirts of its dense disk — the region of dust and gas where planets typically form.

“In this case, the star and disk we have observed is so massive that, rather than witnessing a planet forming in the disk, we are seeing another star being born,” said John Ilee, a researcher at the University of Leeds and head of the study, in a news release.


These images show the dust and gas emissions surround the strange pair. Dust emissions are shown in green, while gas is shown in red and blue (red is moving away from the star; blue is approaching). The smaller star, MM 1b, is visible in the lower left.
ALMA (ESO/NAOJ/NRAO)/JD Ilee (University of Leeds)
To shed light on the peculiar pair, the researchers used the Atacama Large Millimetre/submillimetre Array (ALMA) to probe the star system — observing the light frequencies that emit from the disk’s gas and measuring the radiation that emits from the disk’s dust. They used this information to calculate the mass of the stars, and found that MM 1a is about 40 times the mass of our Sun, and that MM 1b weighs just half our Sun’s mass.