A star suddenly darkened in the sky. What do scientists say about this?

What star are we talking about?

We are talking about Betelgeuse. This is a bright star in the constellation Orion. red supergiant,

intensively losing gas from the atmosphere.

The star is one of the largest and brighteststars in the sky. It is located in the constellation Orion. Its size and mass are 15-25 times that of the sun. Because of this, as well as the short distance to Earth, Betelgeuse can be easily seen with the naked eye.

Constellation orion

Betelgeuse is about 1,000 times our sizeThe sun. If instead of the Sun there was Betelgeuse, then it would, on average, extend approximately to the asteroid belt and periodically reach the orbit of Jupiter. It is so large that we can even make a kind of "maps" of its surface using telescopic observations.

Also, a star can pulsate: it changes its size, sometimes increasing, then decreasing in diameter. The diameter of Betelgeuse during pulsations varies from 500 to 800 times the diameter of our Sun. In its smallest phase, the star, if it were in our system, would extend to the orbit of Mars, and at its maximum, to the orbit of Jupiter.

Due to the pulsations, Betelgeuse is alsovariable star - its brightness also changes. During its maximum brightness, it is the eighth brightest star in the night sky and the brightest star in the constellation Orion. At the minimum, it becomes the 20th brightest (right after Deneb).

Betelgeuse is about 640 light years away. Technically, it could be that it has already exploded, but the light from its explosion has not yet reached us.

What happened?

In October 2019, the star began to fade, and byFebruary 2020, its brightness decreased unprecedentedly. This was unexpected even for a red supergiant (this class of luminaries is known for its inconsistent brilliance). By April 2020, Betelgeuse's brightness had recovered to normal levels.

The specialists had two versions of what happened. First, the star could have ejected a cloud of dust that temporarily eclipsed it. This often happens with red supergiants, although usually such "eclipses" are not so large. Secondly, the surface of a celestial body could be covered with dark spots.

A group of astronomers led by Andrea Dupree from the Harvard-Smithsonian Center for Astrophysics published the results of an analysis of observational data on Betelgeuse in 2019-2020.

The data was collected with the help of Hubble, which monitoredstar in the ultraviolet range, the ground-based STELLA observatory, which received information about the movement of the outer layers of the star, the STEREO space observatory, and ground observers and observatories (for example, TrES), which tracked changes in the brightness of Betelgeuse.

The first version was soon confirmed: dust clouds were discovered above the convective cells of the star. 

What does it mean?

Scientists concluded that last year fromA large convective cell on the visible surface of the giant ejected plasma, which was accelerated by the expanding layers of the star during the next cycle of long-term pulsations. The expanding plasma bubble passed through the stellar hot atmosphere into cooler outer regions where the plasma cooled, resulting in the formation of dust particles that created the dust cloud observed in the southern hemisphere of Betelgeuse.

From September to November 2019, just beforelarge-scale drop in brightness, Hubble recorded a flow of matter that was rushing outward from the star’s atmosphere at a speed of more than 300 thousand km/h. According to the calculations of the authors of the article, Betelgeuse lost twice as much mass every day as it usually flows out of it in the form of stellar wind.

Photo: NASA

Ultraviolet data showed thatthe outer layers of the star's atmosphere returned to their previous state by February 2020, despite the fact that the star's brightness in the visible range had not yet returned to its previous level.

At a distance of millions of kilometers from the surface of the star, this material has cooled enough to form dust from it, eclipsing Betelgeuse.

True, the question remains why this cloud was not detected during observations at submillimeter waves. Researchers have yet to answer it.

The next minimum brightness is expected to be a starwill be held in April 2021, observations will be carried out using space observatories. The closeness of the star makes it possible, in the course of long-term observations, to study in detail the processes of mass loss by the supergiant and its circumstellar medium.

What's going on now?

Since the end of April, observations of Betelgeusestopped. The sun in its annual movement across the celestial sphere came too close to this star, and now it is not in the night, but in the daytime sky. It is clear that ground-based telescopes cannot study it at this time. However, astronomers have found a way to observe the star and find out that it has begun to fade again.

Scientists used the STEREO-A satellite,designed for solar wind research. In the summer of 2020, its position in orbit made it possible to observe Betelgeuse (although to do this, the telescope had to be rotated 180 degrees). A brief report on these observations was published in Astronomer's Telegram.

In June and July, STEREO-A performed a total of five brightness measurements on the star. They showed that the brightness of the luminary began to decline again.

The new fall in the brilliance of Betelgeuse came as a surprise. Usually, the brightness of this star changes in a 420-day cycle, and in August-September 2020, it should have reached another maximum.

Currently, scientists are closely monitoringdevelopment of the situation. Betelgeuse is the closest star to Earth, living its last millennia (and maybe even years) before a supernova explosion. By studying it, humanity can learn a lot about the life and death of the luminaries that disperse the darkness of the Universe.

Note that Betelgeuse is not the onlyof semi-regular variable stars that exhibit inhomogeneous variations in brightness. Last year, similar behavior was detected in the star V Canes Venatici, which was explained by an asymmetric dust envelope.

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