The first-ever sighting of an ‘invisible’ black hole could teach us how stars form
The project took years of watching for light to bend in space
New, dark horizons
To discover such an invisible black hole, the team of scientists had to combine two different types of observations over several years. This impressive achievement promises a new way of finding the previously elusive class of isolated black holes.
Einstein’sGeneral Theory of Relativitypredicted that massive objects will bend light as it travels past them. That means that any light passing very close to an invisible black hole – but not close enough to end up inside it – will be bent in a similar way to light passing through a lens. This is calledgravitational lensing, and can be spotted when a foreground object aligns with a background object, bending its light. The method has already been used to study everything from clusters of galaxies to planets around other stars.
The authors of this new research combined two types of gravitational lensing observations in their search for black holes. It started with them spotting light from a distant star suddenly magnify, briefly making it appear brighter before going back to normal. They could not see any foreground object that was causing the magnification via the process of gravitational lensing, though. That suggested the object might be a lone black hole, something which had never been seen before. The problem was that it could also just have been a faint star.
Figuring out if it was a black hole or a faint star required a lot of work, and that’s where the second type of gravitational lensing observations came in. The authors repeatedly took images with Hubble for six years, measuring how far the star appeared to move as its light was deflected.
Eventually this let them calculate the mass and distance of the object which caused the lensing effect. They found it was about seven times the mass of our Sun, located about 5,000 light years away, which sounds far away but is actually relatively close. A star that size and that close should be visible to us. Since we can’t see it, they concluded it must be an isolated black hole.
Taking that many observations with an observatory like Hubble isn’t easy. The telescope is very popular and there is a lot of competition for its time. And given the difficulty of confirming an object like this, you might think the prospects for finding more of them aren’t great. Luckily, we’re at the beginning of a revolution in astronomy. This is thanks to a new generation of facilities, including the ongoingGaia survey, and upcomingVera Rubin ObservatoryandNancy Grace Roman Space Telescope, all of which will take repeated measurements of large parts of the sky in unprecedented detail.
That’s going to be huge for all areas of astronomy. Having regular, high-precision measurements of so much of the sky will let us investigate en masse things which change on very short timescales. We’ll study things as varied as asteroids, exploding stars known as supernovas, and planets around other stars in new ways.
When it comes to the search for invisible black holes, that means rather than celebrating finding just one, we could soon be finding so many that it becomes routine. That will let us fill in the gaps in our understanding of the deaths of stars and the creation of black holes.
Ultimately, the galaxy’s invisible black holes are about to find it much harder to hide.
This article byAdam McMaster, Postgraduate Research Student (PhD) in Astronomy,The Open UniversityandAndrew Norton, Professor of Astrophysics Education,The Open University, is republished fromThe Conversationunder a Creative Commons license. Read theoriginal article.
Story byThe Conversation
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