This star exploded half a century ago.  Since then, it has continued to surprise scientists

HM Sge is a two-star system called the symbiotic system. This means that we are dealing here with a white dwarf, which is the remains of a sun-like star, which during its evolution had already passed through the red giant stage and then shed its outer layers, leaving only a dense, hot core and the red giant. In this case, the white giant gradually strips its great companion of its outer layers. The fall of the gas stream from the red giant into the white dwarf creates an accretion disk around it. The increasing mass of the white dwarf and the disk surrounding it causes an increase in pressure and temperature there, reaching over time the point where a thermonuclear explosion occurs on the surface of the white dwarf, i.e. a nova explosion.

Such an explosion occurred in this system in 1975. As a result, the system increased its brightness by about 250 times. In this particular case, it was only after the explosion that things started to get interesting. Normally, after a nova explosion, the brightness of the system begins to decrease after only a few days. HM Sge maintained its increasing brightness for about a decade. Before the explosion, its brightness was +17 degrees, and after the explosion it was +10.5 degrees. Yes, it’s less bright today than it was at maximum, but it’s still +12, so it’s still brighter than it was before the explosion.

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In 2021 and 2022, HM Sge’s symbiotic system will end It was observed by a team of scientists using the Hubble Space Telescope and an infrared telescope mounted aboard a modified Boeing 747, the SOFIA Observatory.

The HM Sge design began to dim in 1985. Scientists suspect that the reason for this behavior could be the red giant losing mass due to pulsation (this red giant is a variable star) and ejecting a large amount of dust from it, which then partially obscured the system, Or the separation of two components of the system moving around the center of mass of the system after an orbit of 90 years. This separation would have reduced the amount of mass transferred from the red giant to the white dwarf.

Observations with Hubble in 2021 determined that this system emits a strong emission line of ionized magnesium. This is an extraordinary discovery, because in 1990, fifteen years after a new line exploded, there was no such line in this system. At that time, the temperature of the white dwarf was estimated at 200,000 degrees Celsius. However, now that the line has appeared, it means that the temperature of the white dwarf has risen to 250,000 degrees Celsius. She is currently one of the most famous stars of this genre. Moreover, we do not yet know what causes such an increase in temperature.

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Scientists eventually concluded that the entire system stabilized at the new level relatively quickly after the 1975 explosion. However, over the years, the system slowly dimmed, partly because its components were moving away from each other, and partly because of the red giant’s pulsating behavior. This situation may continue for many years, until the two components approach each other again, causing an increase in the rate of matter transfer between the two components, and a new one explodes.

Over time, the red giant here will also shed its outer layers and become a white dwarf. The shedding of the fragile shell will likely bring the two white dwarfs closer together. If they collide over time, a supernova Ia explosion will occur, but such an event may not occur for hundreds of millions or even billions of years. So we may not see it anymore.

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