An Unusual Black Hole Could Change What We Know about Astrophysics

James Maynard
Nov 29, 2019 · 5 min read

Astronomers in China have recently found an unexpected black hole, much larger than any thought to exist in our galaxy — what’s its story, and what could it mean for the future of astronomy?

The Milky Way galaxy is thought to be home to around 100 million stellar-mass black holes, formed during the deaths of massive stars. Astronomers and astrophysicists concluded years ago that such black holes should have masses no greater than around 20 times that of the Sun. That idea has now been shattered by the discovery of a black hole with three-and-a-half times that projected upper mass.

Sitting 15,000 light years from Earth sits a black hole with 70 times the mass of the Sun. Discovered by researchers at the National Astronomical Observatory of China (NAOC) (managed by the Chinese Academy of Sciences), this object, dubbed LB-1, shatters ideas about large, stellar-mass black holes.

Stellar-mass black holes come in sizes much larger than previously believed, a new discovery reveals. Image credit: Geralt/Pixabay

“We thought that very massive stars with the chemical composition typical of our Galaxy must shed most of their gas in powerful stellar winds, as they approach the end of their life. Therefore, they should not leave behind such a massive remnant... Now theorists will have to take up the challenge of explaining its formation,” explained LIU Jifeng of the National Astronomical Observatory of China.

Making the Invisible Visible

Until recently, astronomers were only able to detect stellar-mass black holes by spotting powerful emissions of X-ray light produced as matter spiraled around the enigmatic object. However, most black holes formed in the deaths of stars do not produce powerful X-ray signals, and only a couple dozen stellar-mass black holes have been found in the Milky Way.

Using the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), researchers examined stars orbiting with an invisible companion, in a quest to find additional black holes which formed from massive dead stars.

The idea of finding black holes using this method is not new — the concept was first proposed by English scientist John Michell in 1783. However, the technology needed to perform these searches has only been available to astronomers in the last few years.

A black hole surrounded by gas.
A black hole surrounded by gas.
Gas spirals into the region of a black hole, heating the gas and dust in an accretion disk. Image credit: YU Jingchuan, Beijing Planetarium, 2019

Just one in a thousand stars are thought to orbit in a binary system with black holes, making the search challenging for astronomers. However, such a system was detected, and the world’s largest telescopes — the Gran Telescopio Canarias in the Canary Islands and the 10-meter Keck I telescope in Hawaii — aimed their sights toward the unlikely pair.

Soon, the system was resolved as a star eight times more massive than our Sun orbiting a black hole of 70 solar masses. The pair orbit around their common center of mass once every 79 days.

“The long orbital period of 78.9 days shows that this is a wide binary system. Gravitational-wave experiments have detected black holes of similar mass, but the formation of such massive ones in a high-metallicity environment would be extremely challenging within current stellar evolution theories,” researchers reported in an article published in the journal Nature, describing their findings.

Where Have I Seen this Before?

The LAMOST observatory, used in the initial discovery of LB-1. Image credit: LAMOST, National Astronomical Observatories, China

Black holes come in four classes — primordial, supermassive, intermediate, and stellar. Primordial black holes were formed during the early era of matter in the Universe, while supermassive black holes (which exist near the centers of nearly all galaxies) are the most massive. Intermediate class objects are the least-understood of all of these objects, and stellar-mass black holes, formed during the deaths of the largest stars, are the most common.

“Why are black holes so different from all other objects in the macroscopic Universe? Why are they, and they alone, so elegantly simple? If I knew the answer, it would probably tell me something very deep about the nature of physical laws. But I don’t know.”
― Kip S. Thorne, Black Holes & Time Warps: Einstein’s Outrageous Legacy

The first notions of black holes came in the late 18th Century, but our modern ideas of these regions began with German astronomer Karl Schwarzschild who developed his notions from the work of Albert Einstein. It was Schwarzschild who first developed the concept of an event horizon — the border around a black hole beyond which nothing, not even light — could escape.

“For a non-rotating black hole, the radius of the event horizon is known as the Schwarzschild radius, and marks the point at which the escape velocity from the black hole equals the speed of light. In theory, any mass can be compressed sufficiently to form a black hole. The only requirement is that its physical size is less than the Schwarzschild radius. For example, our Sun would become a black hole if its mass was contained within a sphere about 2.5 km [1.5 miles] across,” the Swinburne Centre for Astrophysics and Supercomputing describes.

A computer simulation showing what it would be like to approach a stellar-mass black hole. Video credit: NASA Goddard.

In recent years, astronomers have detected ripples in spacetime resulting from collisions of massive black holes in distant galaxies. Calculations of these events show that the black holes involved in these collisions are much more massive than astronomers had predicted could exist in our own galaxy.

Measurements of LB-1 show that similar massive black holes also exist in our own galaxy, causing astrophysicists to rethink how such stellar-mass black holes form. This finding could result in significant changes to the way we think of black holes, potentially forever altering the way we think about the densest objects in the Universe.

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James Maynard

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