Since the early 2000s, extensive astrometric observations of the sky began, which gave an accurate idea of the speed and direction of movement of stars. We began to see the Universe around us in dynamics. About 20 years ago, the first star leaving our galaxy was discovered. It turned out that there are quite a lot of runaway stars and most of them are heavy, the study showed.
An example of a rogue star creating a shock wave as it moves through interstellar gas. Image source: NASA/JPL-Caltech
There are two main hypotheses about how runaway stars appear, the speeds and directions of movement of which do not coincide with the carousel circling of the rest of the matter in the galaxy. One theory suggests that a star is given momentum as a result of a supernova explosion in a binary system, after which the star, freed from the shackles of its partner’s gravity, flies into the distance. The second theory speaks of a dynamic ejection, when a pair of stars in a close binary system flies past a third massive object, for example, a black hole.. The hole tears off one of the stars, which gives a second impulse of movement.
Scientists can only argue which scenario dominates. However, a group of astronomers decided to find out the most likely of them, since the same astrometric European telescope “Gaia” collected data on millions of stars in our galaxy.
The researchers used two catalogs of O- and B-type stars and data from Gaia.. Stars of these types, including the Be subtype, are massive, young and hot, which is why they occur frequently, in groups and usually in the form of binary systems. Finally, among the discovered runaway stars, massive stars are the most common.
Comparison of Gaia data and the GOSC and BeSS catalogs revealed 417 O-type stars and 1335 Be-type stars that were present in all sources. This made it possible to calculate 106 O-type runaway stars and 69 similar Be-type stars. The percentage of runaway O-type stars turned out to be much higher (25.4%) than B and Be type stars (5.2%). In other words, more massive O-type stars escape more often and generally move faster than B-type stars.. Thanks to the Gaia data, by the way, previously unknown runaway stars were discovered during the study: 42 among O stars and 47 among B and Be stars. Most of them will remain in our galaxy, but about a dozen have become so fast that they will eventually leave it.
Based on the data obtained, scientists concluded that the dynamic ejection scenario appears much more often and is more widespread in the Universe than the appearance of runaway stars in a binary system with the formation of a supernova. In order for the most massive stars to begin to fly in an arbitrary direction at supergalactic speeds, and not move in a circle around the disk of the galaxy, it is necessary to give them as much energy as cannot be obtained from the destruction of a binary system by a supernova explosion. And this happens much more often than previously thought. According to the most conservative estimates, there are about 10 million rogue stars wandering around our galaxy alone.