|Image Source: Physics Today|
Shooting out of the galaxy at speeds greater than the escape velocity, hypervelocity stars provide a window on black holes and the distribution of dark matter surrounding the glowing Milky Way.
Because gravity keeps stars on their orbits, astronomers can use the motions of stars to infer the mass distribution of the visible and invisible constituents of the Milky Way. The Milky Way is the only galaxy whose visible mass distribution we can see in three dimensions and in which we can accurately measure the velocities of millions of individual stars. Gravitational accelerations in the galaxy are usually small, however. Our sun, for instance, experiences a gravitational acceleration of just 2 Å/s2 as it orbits the Milky Way. That’s 10−11 of what we experience on Earth’s surface. It’s also the gravitational- acceleration regime of dark matter—the unseen material inferred to exist in and around galaxies.
Some of the initial evidence for dark matter came in 1932 after Dutch astronomer Jan Oort developed the first modern theory of stellar motions.1 Oort compared the velocity dispersion of stars near the Sun with their number density and inferred the existence of more mass than could be accounted for by the visible stars. In more recent times, radio astronomers have measured the rotation speeds of gas—specifically neutral hydrogen—in the outer parts of the Milky Way and other disk galaxies with much higher accuracy than could be done in Oort’s era. Intriguingly, they found that rotation speeds do not decline with increasing distance outward but stay constant. To keep galaxies like the Milky Way bound together requires the gravitational pull of dark matter, if not a modified theory of gravity.
The focus of this article is a new class of astronomical objects, known as hypervelocity stars, that uniquely connect the center of the galaxy to its outer halo. A decade ago I and my colleagues Margaret Geller, Scott Kenyon, and Michael Kurtz at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, unexpectedly discovered a star moving away from Earth at 850 km/s, roughly 2 million miles per hour.3 The speed is astonishing: The star is racing outward with at least twice the galactic escape velocity at its distance of 300 000 light-years from the galactic center.
Physics Today: Hypervelocity stars in the Milky Way, Warren R Brown