Astronomers have found a particularly sneaky 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 actual images of the black hole at the center of the Milky Way. In a recent paper, a team of astronomers describes another exciting new discovery: the first “dormant” black hole observed outside the galaxy.

I am an astrophysicist who has studied black holes – the densest objects in the universe – for almost two decades. Dormant black holes are black holes that emit no detectable light. Thus, they are notoriously hard to find. This new discovery is exciting because it provides insight into the formation and evolution of black holes. This information is essential for understanding gravitational waves as well as other astronomical events.

What exactly is VFTS 243?

VFTS 243 is a binary system, meaning it is made up of two objects that orbit 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 in the Large Magellanic Cloud, a Milky Way satellite galaxy located approximately 163,000 light-years from Earth.

This video begins with a view of the Milky Way and zooms in to VFTS 243, which is located in the Large Magellanic Cloud.

The black hole in VFTS 243 is considered dormant because it emits no detectable radiation. This is in stark contrast to other binary systems in which strong X-rays are detected from the black hole.

The black hole is about 33 miles (54 kilometers) in diameter and is dwarfed by the energetic star, which is about 200,000 times larger. Both rapidly revolve 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 binary systems with black holes that don’t emit X-rays hidden in the Milky Way and the Large Magellanic Cloud. Black holes are most easily seen when they remove matter from a companion star, a process known as “feeding”.

The power supply produces a disk of gas and dust that surrounds the black hole. As material from the disk falls toward 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.

On the left is an optical image showing Cygnus X-1 surrounded by a red frame. On the right is an artist’s rendition showing the outer layers of the black hole siphoning material from the companion star and forming an accretion disk.
X-rays: NASA/CXC; Optics: digitized sky survey, CC BY-NC

Astronomers have known for years that VFTS 243 is a binary system, but whether the system is a pair of stars or a dance between a single star and a black hole was unclear. To determine which 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 two separate stars. With no detectable radiation emanating from the star’s companion, the only possible conclusion was that the second body in the binary is a black hole and thus the first dormant black hole found outside the Milky Way galaxy.

A black dot and a big blue star rotate around each other.
In the VFTS 243 system, the stellar companion and the black hole (which are not shown to scale) orbit each other. Note that there is no accretion disk present.
ESO/L. Calcada, CC BY

Why is VFTS 243 important?

Most black holes with a mass of less than 100 Suns are formed from the collapse of a massive star. When this happens there is often a huge 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 otherwise could have kicked the black hole out of the system – or at the very least disrupt the orbit. Instead, it appears the progenitor star collapsed directly to form the black hole without an explosion.

The massive star in the VFTS 243 system will only live another 5 million years – a nod to astronomical timescales. The death of the star should lead to the formation of another black hole, turning the VFTS 243 system into a black hole binary.

To date, astronomers have detected nearly 100 events where binary black holes merge and produce ripples in spacetime. But how these binary black hole systems form is still unknown, which is why VFTS 243 and similar systems yet to be discovered are so vital for future research. Maybe nature has a sense of humor – because black holes are the darkest objects in existence and emit no light, but they illuminate our basic understanding of the universe.

About Armand Downs

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