Astronomers have discovered the Milky Way galaxy astonishingly - our galactic home cannot hold onto all its stars. Some of its stellar members get ejected into intergalactic space and spend their lives on an uncertain journey.
Velocity Distribution of Star Population
In observing the field of stars in the Milky Way, one of the things astronomers measure is the velocity distribution. The overall velocity distribution of the stellar population provides insight into the galaxy's rotation.
When a star is not harmonized with the galaxy's rotation, it catches experts' attention.
Some stars move with a significantly peculiar velocity concerning their environment. This characteristic is shared by a relative fraction of massive stars in the Milky Way galaxy.
Solving the Mystery of Runaway Stars
A group of researchers discovered dozens of stars that move differently than the Milky Way. They are considered runaway stars found to be on their way out of the galaxy.
Mar Carretero Castrillo leads the team from the Department of Quantum Physics and Astrophysics, Institute of Cosmos Sciences, University of Barcelona. They based their work on two stellar catalogs, the Galactic O-Star Catalog (GOSC) and the Be Star Spectra (BeSS). These two are catalogs of various types of massive stars, O-type and Be-type stars, and their subtypes.
The astronomers also used data from Gaia, the powerful star-measuring spacecraft from ESA. This space observatory employs astrometry in measuring the positions, distances, and motions of one billion stars.
The number of runaway stars getting out of their way of the Milky Way is still unknown, but experts keep finding more of them. Some estimates suggest that 100 million rogue stars escape our host galaxy, but it can depend on the mechanism that drives them away.
Castrillo and his team cross-referenced Gaia data with the GOSC and BeSS catalogs and came up with 417 O-type stars and 1335 Be-type stars. Of this population, 106 are type O runaway stars, comprising 25.4% of the stars in the GOSC catalog.
Scientists are baffled as to why massive stars make up such a high proportion of runaway stars. Two competing theories, the dynamical ejection scenario (DES) and binary supernova scenario (BSS) are used to explain this phenomenon.
BSS suggests that one star explodes as a supernova, and the explosion pushes the other star. The surviving star gets enough energy in the right direction to escape from its bond with its partner, which has become a neutron star or a black hole. It can also resist the gravitational pull of the Milky Way and goes on a long journey into galactic space.
DES, on the other hand, does not involve any dramatic supernova explosion. This theory suggests that a star in a compact, densely packed region experiences gravitational interactions with other celebrities. As the binary stars encounter single stars, runaways are produced.
For several decades, scientists have been debating about the two theories. Both scenarios can create stars with a velocity that is enough to leave the galaxy, although it is suggested that DES is more likely than the BSS.
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