Supermassive Black Hole Ejects Star From Milky Way Galaxy


About five million years ago, when our earliest Australopithecine ancestors had just branched off from the ancestors of modern chimpanzees, a cosmic catastrophe occurred. A star from the Milky Way galaxy experienced an encounter that would doom it to extragalactic oblivion.

A consortium of astronomers called the S5 collaboration (Southern Stellar Stream Spectroscopic Survey) have announced the serendipitous discovery of what is called a hypervelocity star. A hypervelocity star is one that is moving much, much, faster than ordinary ones. Such stars are moving at such a high velocity that they are no longer captured by the gravitational force of the galaxy in which they inhabit. Basically, it’s very similar to what happens when you throw a ball upward. For most velocities, the ball will fall back to Earth; but, for a fast-enough throw, the ball will escape the Earth and fly off into space.

The hyper-velocity star, called S5-HVS1, was measured to be moving with a speed of 1755 ± 50 kilometers per second (about 3.9 million miles per hour) compared to the Milky Way galaxy as a whole. That is to be contrasted with the speed of our own Sun, which is about 225 kilometers per second. Thus S5-HVS1 is moving with a speed of about 7.8 times faster than the Sun.

S5-HVS1 is an A-type star, which means that it has a mass of about 1.4 – 2.5 times that of the Sun. This particular star is on the larger side – about 2.4 solar masses. It is also hotter and therefore is a bluer star, which emits more ultraviolet. Familiar A-type stars are Altair, Sirius A, and Vega.

Because S5-HVS1 passed within 29,000 light years of the Earth (which is quite near from a galactic point of view), astronomers were able to determine the star’s trajectory and found that the star unambiguously originated from the center of the Milky Way galaxy, in very close proximity to the supermassive black hole called Sgr A*. Sgr A* has a mass of about four million times that of our Sun.

A star with such a high velocity is exceedingly rare and can be created only through the interaction of a binary star with a supermassive black hole. About 30 years ago, an astronomer by the name of Jack Hill proposed that if a binary star system passed near a supermassive black hole like Sgr A*, that the two stars would be split apart, with one of them slowed a great deal and the other given enormous velocity. This is now called the Hill mechanism.

Astronomers believe that S5-HVS1 was created in this way. About 4.8 million years ago, a pair of stars passed by the center of our galaxy (the data suggests within about 500 light years, probably much less). The two stars seemed to be following a path similar to others currently orbiting in a disk around Sgr A*. It may be that this binary pair was once part of the same disk, but they seemed to have come too close to the black hole behemoth.

And the rest, as they say, is history. The star was ejected with an initial velocity of about 1800 kilometers per second and it has slowed somewhat over the eons due to the gravitational tug of the Milky Way. In about 100 million years, it will leave the Milky Way for good, a lonely star, doomed to live out the rest of its life in the cold darkness of extragalactic space. An exciting life will lead to a lonely death.

Astronomers will continue to measure the trajectory of S5-HVS1 and search for similar stars. Additional observations will tell researchers about the population of stars near the center of the Milky Way.

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