Why Black Holes Eject Stars

Dec 9, 2019 By Deepa Gopal
Deepa Gopal's picture

Image of two stars in a binary-star systemWhen our ancestors were starting to learn how to walk upright, a star was ejected from the black hole at the center of the Milky Way Galaxy.

This star, traveling at 3.7 million miles per hour, was captured by researchers as it traveled close to Earth -- five million years later!

A research team, led by Sergey Koposov of Carnegie Mellon University’s McWilliams Center for Cosmology, found the star (known as S5-HVS1) in the constellation Grus.

The star, ten times more luminous than our sun and about twice as enormous, was traveling over 10 times faster than most other stars in the galaxy. It was spotted just 29,000 light-years away from Earth!

A Star Ejected: How Do We Know?

Astronomer Jack Hills first proposed that black holes can eject superfast stars at high velocities. However, S5-HVS1 is the first time scientists have observed it happen. 

Picture of black holeA black hole is an extremely dense region in space where matter is packed tightly within a very small area. This dense matter exerts a very strong gravitational pull - so strong that nothing can escape, not even light! Scientists have found proof that every large galaxy contains a supermassive black hole at the center. The supermassive black hole at the center of the Milky Way, known as Sagittarius A*, has a mass equal to around 4 million suns.

What caused our black hole to eject a star? When scientists launch a spacecraft to explore the outer reaches of our solar system, they use a principle called gravity assist. The spacecraft is steered close to a planet, and the gravitational pull of the planet would give the spacecraft extra speed to propel it forward. 

If a star happens to move close to a black hole, it could get flung out of the galaxy, similar to what scientists observed with S5-HVS1. The Hills Mechanism (named after Jack Hills) works on binary or two-star systems where the black hole would capture the closer of the two stars and eject the other partner.

What Exactly Are Binary Stars?

Star flung out of black holeMost twinkling stars that you see in the night sky are in fact a part of two or more clusters.

A cluster of two stars orbiting around a central mass is known as a binary star system and is the most common. How binary stars form is still a mystery. It is possible that a passing star is captured by a stationary star, making it into a two-star system. However, it is more likely that during the formation of the solar system, two separate stars evolved next to each other.

There are two kinds of binary stars -- wide binaries where the two stars are quite far apart and don't interact with each other, except for the gravitational pull between them, that keeps them in orbit. Or there can be narrow binaries, where the stars are very close to each other and exchange mass. Scientists can learn a lot from binary star systems such as the mass, size, and temperature of each of the planetary bodies. This information is useful in estimating the characteristics of single stars as well.

The fast-moving S5-HVS1 is well on its way out of our galaxy. However, its discovery has captured the attention of researchers and stargazers around the world!

Sources: LiveScience, Space