Stealth black hole discovery sheds light on star death, black hole formation, and gravitational waves

There is always something new and exciting happening in the field of black hole research.

Albert Einstein first published his book explaining the theory of general relativity, which postulated black holes, in 1922. One hundred years later, astronomers captured real images of the black hole at the center of the Milky Way. In a recent article, a team of astronomers describes another exciting new discovery: the first “dormant” black hole observed outside the galaxy.

Dormant black holes are black holes that do not emit any detectable light. Therefore, they are notoriously difficult to find. This new discovery is exciting because it provides insight into the formation and evolution of black holes. This information is vital to understanding gravitational waves, as well as other astronomical events.

What exactly is VFTS 243?

VFTS 243 is a binary system, which means that it is made up of two objects orbiting around a common center of mass. The first object is a very hot blue star with 25 times the mass of the Sun, and the second a black hole nine times more massive than the Sun. VFTS 243 is located in the Tarantula Nebula within the Large Magellanic Cloud, a satellite galaxy of the Milky Way located about 163,000 light years from Earth.

The black hole in VFTS 243 is considered inactive because it does not emit any detectable radiation. This is in stark contrast to other binary systems where strong X-rays are detected from the black hole.

The black hole has a diameter of about 33 miles (54 kilometers) and is dwarfed by the energetic star, which is about 200,000 times larger. Both rotate rapidly around a common center of mass. Even with the most powerful telescopes, visually the system appears to be a single blue dot.

Find dormant black holes

Astronomers suspect that there are hundreds of these non-X-ray-emitting binary black hole systems hidden in the Milky Way and the Large Magellanic Cloud. Black holes are most easily seen when they are pulling matter from a companion star, a process known as “feeding.”

The feed produces a disk of gas and dust that surrounds the black hole. As material from the disk falls into the black hole, friction heats the accretion disk to millions of degrees. These hot disks of matter emit an enormous amount of X-rays. The first black hole to be detected in this way is the famous Cygnus X-1 system.

VFTS 243 is a binary system of a large hot blue star and a black hole orbiting each other, as seen in this animation. Credit: ESO/L. Calçada, CC BY

Astronomers have known for years that VFTS 243 is a binary system, but it was unclear whether the system is a pair of stars or a dance between a single star and a black hole. To determine which one was true, the team studying the binary used a technique called spectral disentanglement. This technique separates the light from VFTS 243 into its constituent wavelengths, which is similar to what happens when white light enters a prism and the different colors are produced.

This analysis revealed that the light from VFTS 243 came from a single source, not from two separate stars. With no detectable radiation emanating from the star’s companion, the only possible conclusion was that the second body within the binary is a black hole, and thus the first dormant black hole found outside the Milky Way galaxy.

Why is VFTS 243 important?

Most black holes with a mass of less than 100 Suns form from the collapse of a massive star. When this happens there is often a tremendous explosion known as a supernova.

The fact that the black hole in the VFTS 243 system is in a circular orbit with the star is strong evidence that there was no supernova explosion, which might otherwise have ejected the black hole from the system or, at the very least, interrupted orbit. Instead, it appears that the parent star collapsed directly to form the black hole without an explosion.

The massive star in the VFTS 243 system will live for only another 5 million years, the blink of an eye on astronomical timescales. The star’s death should result in the formation of another black hole, transforming the VFTS 243 system into a black hole binary.

To date, astronomers have detected almost 100 events in which binary black holes merge and produce ripples in space-time. But how these binary black hole systems form is still unknown, which is why VFTS 243 and similar yet-to-be-discovered systems are so vital for future research. Perhaps nature has a sense of humor, since black holes are the darkest objects in existence and emit no light, but illuminate our fundamental understanding of the universe.