It is almost a year since the T Coronae Borealis star has been in the science news with enviable regularity. It is not visible to the naked eye presently — but it is expected to produce a flare in the near future, after which it will become visible in the sky for some time.
In this case, we’re talking about a new flare. Such events, although relatively rare, are not unique. In the twenty-first century, more than a dozen new stars flared in the Milky Way, which, under suitable observing conditions, could be seen in the sky without optical instruments. However, there is another, much more rare and spectacular phenomenon. We’re talking about supernovae. Some come from the collapse of giant stars. Others arise from the thermonuclear explosion of a white dwarf. The result is the same: the star, with few exceptions, completely collapses, accompanied by a grandiose firework show.
The supernova explosion releases such a gigantic amount of energy that astronomers regularly find them in other galaxies. Their observations are of great value because they allow us to refine intergalactic distances and test theoretical models. But those are supernovae in other galaxies. What about the Milky Way? When was the last time it had a supernova?
Supernovae that humans have seen
Over the last thousand years, five supernovae have flared up in the Milky Way, which could be seen in the Earth’s sky. The first and brightest of them appeared in May 1006. It was several times brighter than Venus and, according to some sources, it could be observed even during the day. The supernova is believed to have been formed by the merger of two white dwarfs that occurred at a distance of about 7,000 light-years from Earth.
The next supernova in the Milky Way flared up in the summer of 1054 in the constellation Taurus. For a while, it could also be seen in the sky during the day. The supernova produced the famous Crab Nebula, which has a pulsar at its center.
In 1181, Chinese and Japanese observers noticed the appearance of a supernova in the constellation Cassiopeia. According to the records, it had a brilliance comparable to that of Saturn. It was observed in the sky for six months, after which it disappeared. Already in modern times astronomers have managed to solve its mystery. It turned out that the supernova of 1181 belonged to a very rare type Iax. It was formed by the merger of two white dwarfs. Usually, in such events, they completely explode with no remnants. In this case, however, the thermonuclear reaction affected only a fraction of the matter, resulting in a fainter-than-usual flare and the birth of a so-called zombie star.
The next known supernova in the Milky Way flared up in 1572 in the constellation Cassiopeia. This supernova is now commonly referred to as the supernova of Tycho Brahe, who contributed greatly to its study. By attempting to calculate the parallax of the “new star”, the scientist determined that it was much farther away than the Moon. As for brilliance, it was comparable to that of Venus. Currently, we know that the source of the supernova was the explosion of a white dwarf that occurred at a distance of 8,000 to 10,000 light-years from Earth.
In the autumn of 1604, the last Milky Way supernova, so far visible to the naked eye, flared up in the constellation Ophiuchus. It was also observed by Johannes Kepler and Galileo Galilei. The source of the supernova was the explosion of a white dwarf whose mass exceeded the Chandrasekar limit due to the absorption of the companion star’s matter.
Supernovae that remain invisible
Theoretical models suggest that a galaxy like our Milky Way should see one or two supernovae flare per century. In reality, however, no person on Earth has ever seen a supernova appear in our galaxy after 1604. The reason is most likely due to dust clouds blocking the visible light they emitted.
And there is no doubt that supernovae continued to flare up in the Milky Way after 1604. Astronomers are reliably aware of at least two such events. The first supernova flared up in the constellation Cassiopeia. Its light was supposed to reach Earth in 1680. However, there is no reliable historical record of a new star ever being observed in the sky at that time.
The next supernova flared up in the late 19th century at the center of the Milky Way. Once again, its light was unable to reach Earth because of dust clouds. The existence of this supernova was not confirmed until 2008.
However, due to the fact that supernovae explosions release huge amounts of energy and become gigantic in brightness, sometimes even supernovae in galaxies closest to us can be seen in the sky. For example, the supernova that flared up in 1885 in the Andromeda Galaxy was on the verge of being visible to the naked eye.
But a far more famous example is supernova SN 1987A, which flared up in 1987 at a distance of 160,000 light-years from Earth in the Large Magellanic Cloud. It became the first supernova since 1604 to be seen in the sky without the aid of a telescope. Its source was the gravitational collapse of a blue supergiant. The observations of SN 1987A provided, and still provide, unique data on the nature of such events and how exactly supernova remnants evolve.
Next supernova in the Milky Way
When will the next supernova flare in the Milky Way? Unfortunately, there is no concrete answer to this answer. It’s quite common to hear about Betelgeuse. But, although this star will indeed explode in the future, most astronomers believe it will happen not in our lifetime, but many thousands of years from now.
It is much more likely that the next supernova in the Milky Way will be spawned by a white dwarf in a close binary system that absorbs too much matter and passes the Chandrasekhar limit. But where and when it will happen is impossible to predict at this point. As well as whether the flare will be visible in Earth’s sky, or whether it will be interfered with by interstellar dust.
So, based on available statistics, the probability of seeing a supernova in the sky in our lifetime is comparable to winning a big lottery. All the more interesting is the expected T Coronae Borealis flare. Although it will not become a supernova, but just the appearance of a new star in the sky, it is also quite a non-trivial event.
