Runaway stars may have surrounded the galaxy with a "fire curtain."

As cosmologists have established, all matter that is understood by science makes up only 4% of the Universe. Interestingly, the countless galaxies within it constitute a mere tenth of this tiny fraction. The remaining nine-tenths consist of the "raw material" from which galaxies are "constructed": intergalactic gas.

This is primarily hydrogen, the first substance that emerged after the Big Bang. The rest of the periodic table is a result of thermonuclear fusion in stars, supernova explosions, and various other processes in space. These are secondary products.

Consequently, galaxies exist within enormous "cocoon" structures of their primordial hydrogen, and in size, these gas spheres far exceed the dimensions of the galaxies themselves. For instance, the disk of the Milky Way stretches across about a hundred thousand light-years, but the surrounding gas sphere is seven times larger.

Astronomers can determine the nature of particles and their states based on the radiation emitted by particles in space. According to their observations, the matter surrounding the Milky Way is significantly heated: often to tens of thousands or even up to a million degrees. It is noteworthy that this is much higher than the temperature of the solar surface (five and a half thousand degrees).

In principle, scientists can explain this through the gravitational influence of the galaxy: for example, it sometimes collides with another galaxy, "devouring" its unfortunate victim, resulting in a shockwave and, consequently, heating. However, it has recently been discovered that there is also plasma in the intergalactic medium with temperatures on the order of ten million degrees, which is not entirely understood. Scientists are trying to figure out what could heat the surrounding matter of the galaxy to such an extent.

Astrophysicists from India proposed an intriguing scenario. They noted that among these overheated particles, elements like sulfur, magnesium, and neon can be traced, all of which are products of supernova explosions. Therefore, scientists suspect that the additional heating of the intergalactic plasma may be caused by stars that have moved beyond the Milky Way—so-called "runaway stars." They have accelerated to such speeds that they have escaped the galaxy's grasp and are fleeing away.

The current investigation focuses on how they achieve this. It is suggested that these stars could be located near black holes and gained gravitational acceleration from them. Another scenario being considered is that they were once part of binary systems, where a companion star exploded as a supernova, and the force of that explosion "pushed" the second star away, giving it "super speed."

As scientists write in The Astrophysical Journal, there are many hot, massive stars among the "runaways." Such stars will not escape for long. They have a short lifespan: within ten to twenty million years (very quickly on cosmic scales), they completely "burn out," exhaust all their thermonuclear fuel, and explode as supernovae. It is believed that this extragalactic "firework" surrounds the Milky Way with scorching plasma.