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.
In the Milky Way, most stars move in roughly circular orbits inside a disk with a radius of 60 000 light-years. The galaxy also has a central elliptical “bulge,” about 6000 light-years in radius and more tightly packed with stars, and a sparse outer region, called the halo, that extends to a radius of 800 000 light-years. The Milky Way’s mass is thus spread over a large volume. Figure 1a quantifies the distribution in terms of the circular orbital velocity of stars, decomposed into the contributions from a supermassive black hole at the center of the Milky Way and from the bulge, disk, and halo. That model is based on measurements made at different distances from the galactic center.2 That the stars’ circular orbital velocities do not slow as a function of radius R" role="presentation" style="box-sizing: border-box; display: inline; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">RR means that the galaxy is dominated by visible matter inside the Sun’s orbit.
Hypervelocity stars in the 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.
In the Milky Way, most stars move in roughly circular orbits inside a disk with a radius of 60 000 light-years. The galaxy also has a central elliptical “bulge,” about 6000 light-years in radius and more tightly packed with stars, and a sparse outer region, called the halo, that extends to a radius of 800 000 light-years. The Milky Way’s mass is thus spread over a large volume. Figure 1a quantifies the distribution in terms of the circular orbital velocity of stars, decomposed into the contributions from a supermassive black hole at the center of the Milky Way and from the bulge, disk, and halo. That model is based on measurements made at different distances from the galactic center.2 That the stars’ circular orbital velocities do not slow as a function of radius R" role="presentation" style="box-sizing: border-box; display: inline; line-height: normal; word-spacing: normal; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative;">RR means that the galaxy is dominated by visible matter inside the Sun’s orbit.
Hypervelocity stars in the Milky Way
