Perhaps no other cosmic phenomenon is as fascinating as black holes, places where the force of gravity is so strong that it twists the nature of space and time around them – so strong that not even light can escape its gravitational pull. Black holes are ubiquitous throughout the universe, with more than 100 million believed to be in our Milky Way alone. They have been the subject of many sci-fi thrillers, including the 2014 film Interstellar, and they are often portrayed as dangerous monsters that consume everything around them. Fortunately, black holes are thought to be “out there” and far away.
But a black hole recently discovered by astronomers is the closest ever found, effectively an astronomical neighbor. This black hole is called “Gaia BH1,” and its discovery offers astronomers a nearby laboratory that can help them better understand how black holes form.
Gaia BH1: our astronomical neighbor
At a distance of 1,600 light-years, the black hole poses no threat to humanity; So you can still sleep peacefully at night. However, the story of how the black hole was discovered is interesting.
Black holes are the remnants of long dead stars. Their name comes from the fact that not only do they not emit light, but they also absorb any light that falls on them. Because of this, they are invisible. “Black holes” is a literal name for these cosmic ninjas that are undetectable in the way we see other stars.
But that only applies to isolated black holes. When black holes are surrounded by gas or are close to a nearby star, the hole’s gravity pulls the gas in. When it falls into the hole, it heats up to incredible temperatures. This blisteringly hot gas emits light that can be detected, an interstellar flare easily visible with the right scientific instruments. Perhaps ironically, “feeding” black holes, as they are called, are among the brightest things in the cosmos. This is how most known black holes have been observed, although this is not the case for Gaia BH1.
Gaia BH1 has not been observed directly, nor is it a feeding black hole. Instead, it was discovered by observing its effect on a second star in its planetary system. Perhaps half of all planetary systems containing a Sun-like star contain two stars orbiting each other. Gaia BH1 is an example of such a system, with the twist that one of the two stars is a black hole.
In December 2013, a Soyuz ST-B carrying the Gaia telescope was launched from a launch site in French Guiana. Gaia’s mission is to map the position and motion of a billion nearby stars in the galaxy. It’s a breathtakingly ambitious mission that has been amazingly successful and made many advances in astronomical knowledge.
Relevant to Gaia BH1, however, was the observation of a particular star that “wobbled” in place as if orbiting another star — one that scientists could not see. This data piqued the curiosity of a group of astronomers who rotated the Gemini North telescope in Hawaii, one of the International Gemini Observatory’s twin telescopes, to study the mysterious behavior. The group released a paper that not only confirmed the Gaia observation, but also showed that what they saw was a sun-like star (class G) with a mass 93% the mass of the Sun orbiting a black hole a mass of 9.62 ± 0.18 solar masses. The two stars orbit a place between them about twice a year.
That’s how it works
Suppose you have a basketball-sized ball, and suppose that this ball weighs about the weight of a small child. The ball glows red, but quite dimly, so it doesn’t glow much around it, although it’s easy to see in a dimly lit location. Now connect this ball with a strong string. Finally, you slip into an all-black burglar costume and take everything to a dark room and turn off the lights.
Now comes the fun part. They grab the string and start swinging it around like a hammer throw at the Olympics. What does an outsider see? You see the ball circling something invisible (you). And from the movement of the ball they can conclude that you are there.
And that is the basic idea of how this measurement was achieved. By looking at a star’s behavior near the black hole, astronomers were able to determine that the black hole was there even though they had never seen it directly.
So what did this new observation teach us? Well, several things. While the black hole has about 10 times the mass of the Sun, its mass as a star was closer to 20 times that of the Sun. This means that the progenitor star of the black hole only lived a few million years. (By contrast, our own much smaller star has already lived for 4.5 billion years and has only reached about half its lifespan.)
Before becoming a black hole, the progenitor star first grew to a supergiant stage, in which a star puffs up to large sizes. It’s as if our star had grown as big as Earth’s orbit.
The progenitor star was so large that it completely swallowed the existing star. Then, after a brief stint as a supergiant, the progenitor star ran out of fuel and exploded in a supernova, leaving behind the black hole that astronomers discovered.
That’s odd, because astronomers would have expected that the friction experienced by the currently visible star while embedded in the supergiant progenitor would have slowed the stars’ orbits and brought them closer together than they see today. This will require astronomers to reconsider their theories about the growth and evolution of binary star systems like that with Gaia BH1.
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There’s so much we don’t yet know about how star systems evolve over time, but discoveries like Gaia BH1 are bringing us ever closer to understanding the universe around us.
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