A star 47 times the mass of the Sun will soon become a hypergiant

The supergiant, spinning wildly, may have already eaten its neighbor.
Grant Currin
Cygnus OB2 (left), Gran Telescopio Canarias (right)NASA/CXC/SAO/J.Drake et al / Wikipedia/Pachango

It is not an inconspicuous star.

It's nearly a million times brighter than the Sun and dozens of times more massive, with an internal temperature of roughly 43 thousand degrees Fahrenheit.

It is J20395358+4222505, and recently it came into focus for a team of curious astronomers.

A team of researchers from Europe and Latin America got a look at the gigantic star using a spectrograph in the Canary Islands and they report their findings in a paper published last month in Monthly Notices of the Royal Astronomical Society.

They found that it’s burning hot, spinning fast, and poised to grow even larger.

“[W]e speculate that the star will join the group of B hypergiants in the near future (astronomically speaking),” they write in the paper.

This blue giant is hiding in the Cygnus star nursery

Despite being such an incredibly bright object, J20395358+4222505 was barely known to science until 2018, when many of the same researchers trained their telescopes to an intriguing patch of sky. They were looking deep into the constellation Cygnus, at an immense star cluster called Cygnus OB2. That star cluster — 5,000 lightyears from Earth — is easily the largest in our part of the Milky Way galaxy.

Cygnus OB2 isn’t just close, it’s also exciting. The researchers describe the stellar nursery as ​​”one of the most powerful star-forming regions” near the Sun. It’s packed with young, very big stars that burn extremely hot.

When the researchers took a close look five years ago, they found 42 massive stars that had been hidden behind huge clouds of gas and dust. The bright star would be easily visible from Earth, but the gas reduces the visible light by a factor of roughly 10,000, according to the researchers. 

Of the 42 gigantic stars they found in the cluster, 20395358+4222505 turned out to be the largest.

Astronomers uncovered plenty of surprises

The researchers were taken by surprise in three big ways. First was the sheer size of 20395358+4222505. It's not just big as stars in our astronomical backyard go — it's one of the largest and brightest stars in the Milky Way!

The second surprise is that the star is in a life stage that astronomers rarely see. It's just finishing up the stable part of its life. With roughly 90 percent of its days behind it, the blue supergiant is in for a thrilling last chapter. Up to this point, the star has been burning lighter elements, like hydrogen and helium. As it gets older and denser, the star will star burning heavier elements, turning them into still heavier elements.

Eventually, the star will transform into a blue hypergiant. Researchers don't know much about them as they're so rare.

Finally, the researchers couldn't believe that 20395358+4222505 changed speeds dramatically over the handful of days they observed it. It was traveling roughly 130,000 miles per hour faster on the first day of observation than the second. There are two potential explanations for the incredible discrepancy, the researchers say. Either it's in a binary system with another star, or with a very dense object, like a neutron star or a black hole.

It seems unlikely that it's a binary star system because the researchers didn't see anything. If it's a smaller object, that neutron star or black hole must've started off as one of the biggest in the Milky Way.

Taken together, these findings show that 20395358+4222505 is "one of the most peculiar objects in the Galaxy," according to co-author Sara Rodríguez Berlanas. 

Study abstract

2MASS J20395358+4222505  is an obscured early B supergiant near the massive OB star association Cygnus OB2. Despite its bright infrared magnitude (Ks = 5.82) it has remained largely ignored because of its dim optical magnitude (B = 16.63, V = 13.68). In a previous paper, we classified it as a highly reddened, potentially extremely luminous, early B-type supergiant. We obtained its spectrum in the UB and R spectral bands during commissioning observations with the instrument MEGARA at the Gran Telescopio CANARIAS. It displays a particularly strong Hα emission for its spectral type, B1 Ia. The star seems to be in an intermediate phase between supergiant and hypergiant, a group that it will probably join in the near (astronomical) future. We observe a radial velocity difference between individual observations and determine the stellar parameters, obtaining Teff = 24 000 K and log gc = 2.88 ± 0.15. The rotational velocity found is large for a B supergiant, vv sin i = 110 ± 25 kms−1kms−1⁠. The abundance pattern is consistent with solar, with a mild C  underabundance (based on a single line). Assuming that J20395358+4222505  is at the distance of Cyg OB2, we derive the radius from infrared photometry, finding R = 41.2 ± 4.0 R, log(L/L) = 5.71 ± 0.04 and a spectroscopic mass of 46.5 ± 15.0 M. The clumped mass-loss rate (clumping factor 10) is very high for the spectral type, M˙M˙ = 2.4 × 10−6 M a−1. The high rotational velocity and mass-loss rate place the star at the hot side of the bi-stability jump. Together with the nearly solar CNO abundance pattern, they may also point to evolution in a binary system, J20395358+4222505  being the initial secondary.

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