A blue supergiant star is moving at enormous speed in the Milky Way

Modern astronomical instruments continuously scan the sky , obtaining images and data about the stars.


We have discovered a blue colossus that orbits at enormous speed hidden behind a wall of interstellar gas and dust. Astronomically, in the garden of our house, the Milky Way.

Modern astronomical instruments continuously scan the sky, obtaining images and data about the stars, trying to reach ever greater distances and more distant moments in time. It could be thought that nothing close to us escapes this surveillance, and that surprises always come from remote regions of space.

And yet it is not. Even in our vicinity there are scientific gems to discover. Among them J20395358+40222505, a blue supergiant star, with a surface temperature of about 24,000 K, which moves at an enormous speed in the Milky Way.

A star of 46 solar masses

The amount of energy a star emits per unit area depends on its temperature, and J20395358+40222505 emits about 300 times more energy per unit area than the Sun.

With a radius 42 times larger, it emits 600,000 times more energy than our star. To reach that luminosity, it must have an enormous mass that allows it to reach sufficient temperatures in its center for nuclear reactions to generate a lot of energy. So much energy that when the radiation manages to escape, the temperature is still tens of thousands of degrees, which gives its surface the blue color that distinguishes this type of star .

J20395358+40222505 is a massive star, very massive, it has 46 solar masses. The fate of massive stars is to die as supernovae, when the nuclear reactions at their core, consuming the nuclear fuel represented by elements from hydrogen to iron, run out, and cannot provide the energy needed to sustain the gravitational pull of stars. the upper layers.

The usual fate (although not the only possible one) is to form a neutron star or a black hole, with masses growing with the mass of the parent star (although not always; the formation of stellar black holes is a complex and fascinating field) .

At a distance of about 5,700 light-years from Earth, J20395358+40222505 is in our galactic-scale neighborhood. We might expect such a luminous star, close to us, to be a relatively bright object in the sky.

Due to its characteristics, J20395358+40222505 should be a magnitude 4 star, visible to the naked eye (the human eye is capable of reaching, in good conditions, up to magnitude 6, more than six times fainter). However, it barely reaches magnitude 14. Why don’t we see it?

Hidden in a cloud of gas and dust

The reason is that J20395358+40222505 is located in Cygnus-X , the closest region of intense massive star formation to the Milky Way.

These regions are filled with clouds of gas and dust from which new stars form, but which also hide the stars behind them. Dust clouds between J20395358+40222505 and us reduce its light by about 10,000 times, making it an irrelevant dot in telescope images.

During tests of the MEGARA multi-object spectrograph at the Gran Telescopio Canarias, we decided to obtain a spectrum of J20395358+40222505: “Hey, I have an interesting object here that could be used for a test. Let’s take a look at it,” I suggested.

The observation showed a star with a peculiar spectrum, whose analysis allowed us to determine the physical parameters of J20395358+40222505 indicated above: its mass, radius, temperature and luminosity.

J20395358+40222505 was thus revealed to be a massive star with a large radius, a supergiant, in a brief intermediate phase of its evolution, close to changing to the hypergiant state, where its radius will grow rapidly and its atmosphere will lose mass at an even higher rate. .

An acceleration of 60 km/s. What pushes her?

During the tests of an instrument the first objective is to check its capabilities. So at that time, obtaining the spectrum of J20395358+40222505 was not a priority, so it was taken in chunks: two chunks were taken on August 29 and a third the next day.

The surprise jumped during the analysis when verifying that the spectrum of August 30 showed a radial velocity clearly different from that of the previous two, with a difference of about 60 km/s (or 260,000 km/h).

Getting a star of about 50 solar masses to move at that speed is not easy. The usual explanation for these speed variations is the presence of a companion, whose gravitational influence rotates the observed star around the common center of mass. To spin a star with the mass of J20395358+40222505 at that speed, you need a companion of relatively high mass. However, neither the images nor the spectra reveal the presence of a companion.

Massive stars , because of their high mass, have strong gravitational fields, and tend to form binary or even multiple star systems when they are born. But if we don’t see the companion it means that it can’t be very bright.

The options are not many. It is most likely the remnant of a companion even more massive than J20395358+40222505. Not much chance for a residue like that. Although there are other possible scenarios, with the mass needed to force J20395358+40222505 to spin at that speed, it is most likely a black hole. The parent star must have been even more massive than J20395358+40222505 and both must have formed a supermassive binary system.

After an unexpected discovery, with an exciting probable interpretation, but others still possible, our team is in that much-needed phase of Science of testing hypotheses with new data that allows us to distinguish which of them is correct. The first data collected are promising. We will closely follow the speedy journey of J20395358+40222505.The Conversation