Astronomers have found an "impossible" pair of vampire stars exhibiting some truly unusual behavior.
Astronomers using the Einstein Probe space telescope have detected a burst of X-ray light from a nova explosion on a white dwarf. It turns out that this white dwarf is part of a very unusual binary star system located in a satellite galaxy of the Milky Way known as the Large Magellanic Cloud. The study has been published in The Astrophysical Journal Letters, reports Space.
A Very Strange Binary Star System
Analysis of the X-ray light reveals the existence of a highly unusual binary star system in the neighboring galaxy. One star belongs to the class of Be stars, which are very hot and have high brightness. This star has a mass approximately 12 times greater than that of the Sun. The second component of the system is a white dwarf, which has a mass about 1.2 times that of the Sun but is much smaller in size. White dwarfs form after Sun-like stars shed their outer layers, leaving behind cores that are compressed under the force of gravity.
However, there is a problem. Sun-like stars can exist for up to 10 billion years, but a star with a mass 12 times greater than that of the Sun should have lived no more than 20 million years before exploding as a supernova. Given the vast difference in lifespan, how can this Be star coexist in the same orbit with its companion, the white dwarf? Scientists believe the answer lies in the fact that the Be star and the white dwarf are sharing material, taking turns feeding off each other like vampires.
Stars That "Eat" Each Other
The authors of the study suggest that the binary system originally consisted of two stars with masses 6 and 8 times greater than that of the Sun. The more massive a star is, the faster it exhausts its fuel for nuclear fusion in its core, and the shorter its lifespan.
Therefore, the more massive star in this pair reached the end of its life first. As a result, it expanded and became a giant star. At this moment, the outer layers of the giant star began to be attracted to the less massive star due to their close proximity. Eventually, the star with a mass 6 times greater than that of the Sun grew to a mass 12 times greater than that of the Sun.
At the same time, all that remains of the star with a mass 8 times greater than that of the Sun is its core, which has collapsed into a white dwarf.
After this, the white dwarf began to reclaim the material it had lost to the giant star through its intense gravity. As more new material accumulates on the white dwarf, a thermonuclear explosion occurs. This is known as a nova explosion. The explosion results in a very bright flash, including X-ray light, which is what the Einstein Probe detected.
Vampire Stars Changed Their Fate
According to the scientists, the exchange of material has altered the fate of the stars in this binary system. Typically, a star with a mass 6 times greater than that of the Sun reaches the end of its life by expanding into a red giant before shedding its outer layers, leaving behind a white dwarf. However, by accumulating so much mass from its companion, it is doomed to explode as a supernova.
The star with a mass 8 times greater than that of the Sun is situated right on the boundary between stars that evolve into red giants and those that become supernovae. But instead, it transformed into a white dwarf, which is more typical for less massive stars.
This does not mean that it will not eventually explode as a supernova. Type Ia supernova explosions destroy white dwarfs that have accumulated too much mass; specifically, such a star must have a mass 1.44 times that of the Sun. Considering that it currently has a mass 1.2 times greater than that of the Sun, it won't take long for the star to accumulate the necessary material for an explosion. However, the white dwarf could survive if the neighboring giant star explodes as a supernova first.