Sneaky black hole discovery explains star death, black hole formation, and gravitational waves
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
There is always something new and exciting going on in the field of black hole research.
Albert Einstein first published his book explaining the general theory of relativity—which postulates black holes—in 1922. One hundred years later, astronomers captured the true image 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 a galaxy.
I am an astrophysicist who has studied black holes—the densest objects in the universe—for nearly two decades. An inactive black hole is a black hole that does not emit detectable light. Because of this, 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 very important for understanding gravitational waves and other astronomical events.
What exactly is VFTS 243?
VFTS 243 is a binary system, meaning that it consists of two objects orbiting the same center of mass. The first object is a very hot blue star with 25 times the mass of the Sun, and the second is a black hole nine times as massive as 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 emits no detectable radiation. This is in stark contrast to other binary systems where strong X-rays are detected from black holes.
The black hole is about 33 miles (54 kilometers) in diameter and is dwarfed by an energetic star, which is about 200,000 times as massive. Both rapidly revolve around the same center of mass. Even with the most powerful telescopes, visually the system looks like a single blue dot.
Finding dormant black holes
Astronomers suspect there are hundreds of such binary systems with black holes that do not emit X-rays lurking in the Milky Way and the Large Magellanic Cloud. Black holes are most easily seen when they release matter from a companion star, a process known as “eating”.
Eating produces a disk of gas and dust that surrounds the black hole. When material in the disk falls inward toward the black hole, friction heats the accretion disk to millions of degrees. These discs of hot matter emit large amounts of X-rays. The first black hole detected this way was 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 whether the system is a pair of stars or a dance between a single star and a black hole is unclear. To determine which is correct, the team studying binary used a technique called spectral decoding. This technique separates light from VFTS 243 into its constituent wavelengths, which is similar to what happens when white light enters a prism and produces a different color.
This analysis revealed that the light from VFTS 243 came from a single source, not two separate stars. In the absence of detectable radiation emanating from the companion star, the only possible conclusion is that the second object in the binary is a black hole and thus the first inactive black hole discovered outside the Milky Way galaxy.
Why is VFTS 243 important?
Most black holes with masses less than 100 Suns form from the collapse of a massive star. When this happens, there are often violent explosions known as supernovae.
The fact that the black hole in the VFTS 243 system is in a circular orbit with the star is solid evidence that there was no supernova explosion, which otherwise might have kicked the black hole out of the system—or at least disrupted the orbit. Instead, it seems that the ancestral star instantly collapsed to form a black hole without an explosion.
The massive star in the VFTS 243 system will only live for another 5 million years—the blink of an eye in astronomical timescales. The death of the star will result in the formation of another black hole, turning the VFTS 243 system into a binary black hole.
To date, astronomers have detected nearly 100 events in which binary black holes merge and produce ripples in space-time. But how this binary black hole system formed is still unknown, which is why VFTS 243 and similar undiscovered systems are so important for future research. Maybe nature has a sense of humor—because black holes are the darkest objects ever and emit no light, but black holes illuminate our basic understanding of the universe.
‘Black hole cop’ finds dormant black hole outside the Milky Way galaxy
Tomer Shenar et al, X-ray silent black hole born with negligible kick in massive binary within the Large Magellanic Cloud, Natural Astronomy (2022). DOI: 10.1038/s41550-022-01730-y
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