Hypervelocity Stars in Milky Way Galaxy: Renegades and Orphans

Hypervelocity stars are a small number of stars in the Milky Way galaxy moving with high velocity, greater than 200 kilometers per second. Normal stars do not exceed this speed; and by latest estimate there are about 400 billion stars in our galaxy.  While all of them are revolving around the galactic center, the hypervelocity stars are running away from the galaxy. The first hypervelocity star was discovered by Warren Brown of the Harvard-Smithsonian Institute known as SDSS J090745.0+24507 is moving at about 1.5 million miles per hour (670 kilometers per second), which is twice the galactic escape velocity. Another star called HE 0437-5439 originated from the neighboring dwarf galaxy, Large Magellanic Cloud (LMC), is receding from Milky Way galactic center at a velocity of 723 km/s. It is known to have originated as one of a pair of stars; when the pair was too close to the massive black hole in the LMC galactic center, it was ejected at the expense of its twin, and entered the Milky Way, and now well on it’s out of our galaxy.

Hypervelocity stars may also be born after an interaction between a single star and a binary black hole (BBH). Sagittarius A* is associated with the super massive black hole at Milky Way galactic center is proposed to be such a BBH. Neutron stars born after the collapse of a massive star could be pushed to similar speeds by the supernova explosions, but hypervelocity stars are not old. The existence of hypervelocity stars was first proposed by Jack Hills, an astronomer who considered hypervelocity ejections as a natural consequence of galaxies hosting super massive black holes (SMBH). The interaction between a SMBH and a stellar binary can result in the dynamical capture of one of the binary components at the expense of the high velocity ejection of its companion star. It is believed that there are about 1000 hypervelocity stars exist in our Galaxy, but only nine have been known. Each of the hypervelocity stars seems to have been ejected at a different time, ranging from 30 million to 160 million years ago. Since these stars defy the norms of galactic stars and are on their way to escape into the intergalactic medium, they have to pass the galactic halo that consists of dark matter. The furthest hypervelocity star lies 360,000 light years away in the outer reaches of the halo where dark matter outweighs normal matter by a factor of six. The Astronomers are hopeful that studying these unique stars could shed some light into the nature of black holes and components of the galactic halo.