Black Holes and Stars


An artist’s representation of this star system.

Astronomers have just found a star that’s orbiting a black hole once every 28 minutes. The binary star system X9 is now suspected to consist of a white dwarf star orbiting a massive black hole. While this isn’t particularly unusual, what is notable is exactly how close the two objects are to each other — separated by only two and a half times the distance between the Earth and its moon. This radius of orbit combined with the period of rotation is incredibly fast, as it requires the star to be whipping around the black hole at 8 million miles per hour, or about 1/100th of the speed of light. Of course, this doesn’t bode well for the star: the black hole is constantly ripping matter away from the white dwarf, which means there’s a good chance it’ll end up losing so much mass it completely evaporates.

Black holes, one of the more interesting objects in the universe, are born when stars collapse into themselves, having so much mass that the internal gravity pulls the matter into a single point, creating a singularity that can pull light into it, permanently. This usually happens while a star is dying. Many larger stars have enough mass that, if it were condensed into a point, a black hole could be created, but the forces caused by the fusion at the center of a star counteract the pull of internal gravity. When a star reaches the end of its life however, this fusion ends, and there’s no longer anything pushing the star outwards. If the star is smaller, or up to around 3 times the mass of our sun, it becomes a neutron star, but if it has any more mass than that, a black hole is created.

A black hole passing in front of a galaxy.

Once a star turns into a black hole, not much changes in the surrounding area. While black holes do exert a tremendous gravitational force at the singularity, once you get far enough away, they exert the exact same gravitational force as the star they were born from, since after all, it’s the exact same amount of mass, in roughly the same location as the star. For example, if our sun were to be replaced with a black hole of the same mass, the planets would continue to orbit in exactly the same pattern as before. Of course, the lack of light and heat would change the planets indelibly, but their paths would stay the same.

Sagittarius A*, with the black hole and reflected light from it.

One of the interesting effects of the relative benignity of black holes, is that scientists currently believe that all galaxies have a supermassive black hole at their center, which the star systems all orbit around. Again, this in no way means that the systems are going to end up being sucked into the black hole, the physics is the same as if there were a colossal star, or even just a really big planet at the center. The one at the center of the Milky Way is known as Sagittarius A*, pronounced “Sagittarius A-star.”

Find out more about this black hole!