Black Holes: Big Ones, Small Ones, (and now Middle-Sized Ones)

by Nicholas Mee on August 4, 2012

Small, Intermediate or Large?

As I’m sure you know, a black hole is a region of space containing matter that has been squeezed to an incredible density. The gravitational pull of a black hole is so strong that nothing can escape (not even light), and any object coming within a certain distance of the black hole (the “event horizon”) will be sucked irresistibly into it.

Until recently scientists knew of two sizes of black holes – “small” or “stellar mass” black holes and “large” or “supermassive” black holes. The first of these types form from collapsing stars and are typically between 10 and 30 times the mass of the sun (so they’re not really that “small”, although if the mass of the sun were a black hole it would be under 3 kilometres across). Large ones can be up to several billion times the mass of the sun, and are found at the centres of galaxies.

Anyway, scientists recently announced that they have discovered a third size of black hole (which they have termed “intermediate mass”), and this has some exciting implications for our understanding of how the universe was formed.

Intermediate Mass Black Holes

What is an intermediate mass black hole, you might ask? It is a class of objects intermediate in mass between the stellar mass black holes whose mass is typically around ten times the mass of the sun and supermassive black holes whose masses run in the range of millions to billions of solar masses.

The life cycle of stars has been well understood for several decades. It is thought that stars with a mass of around ten or more times the mass of the sun will end their lives in a cataclysmic collapse that results in the generation of a black hole. Most of the mass of the star is crushed out of existence in the death throes of the star and all that is left is a warped region of spacetime whose gravitational attraction is so strong that even light cannot escape.


In 1970 NASA placed its first X-ray telescope in Earth orbit. The satellite was launched from Kenya on 12th December, the seventh anniversary of Kenyan independence from Britain and in honour of their hosts, NASA named the satellite Uhuru, Swahili for freedom. One of its first discoveries was a powerful X-ray source in the constellation of Cygnus the Swan, now known as Cygnus X-1. It proved to be a very interesting celestial object – the first stellar mass black hole ever to be identified.

Astronomers have been studying Cygnus X-1 for almost forty years with a variety of observatories detecting light at a range of wavelengths – radio, optical and X-ray. Using the US National Radio Observatory’s Very Long Baseline Array, the distance to Cygnus X-1 has recently been determined as 6,070 light years. This accurate figure has helped to pin down the properties of the black hole. We now know that the black hole was born just six million years ago. The mass of the black hole is a whopping 14.8 times the mass of the sun and it is spinning round at an incredible 800 times per second.

Astronomers believe that the Cygnus X-1 system began as a binary star system consisting of two very massive stars bound together by their mutual gravitational attraction. The heavier of the two would have burnt its nuclear fuel at a faster rate until it was exhausted and no further energy could be released by nuclear reactions. The star would then have undergone a terminal collapse in which the material forming the star was transformed into a black hole.

Although the original star would have been millions of kilometres in diameter, the black hole is tiny, with a diameter of a few kilometres (less than a millionth of its original diameter), but still retaining a mass almost 15 times the mass of the sun. The black hole and its companion star remain gravitationally bound together and continue to perform their orbital dance. Surrounding the black hole is an “accretion disc” of material that is gradually being drawn from its neighbour. As its swirls in towards the black hole this material is heated to extremely high temperatures, which causes it to emit the intense X-rays that were first detected by Uhuru.

The following artist’s impression gives us a good idea of what the Cygnus X-1 system looks like:

Cygnus-X1 (courtesy of Harvard University)

As impressive as Cygnus X-1 is, it is a mere baby compared to the behemoths that appear to inhabit the central region of most galaxies. The 1960s saw the discovery of a new class of astronomical objects that appeared to be star-like, but from the red-shifts of their spectra it was clear that they were located at immense distances. In order to be visible to us, their energy output had to be gargantuan.


These objects were named quasars (quasi-stellar objects). We now know that quasars are the sites of extremely violent activity at the centre of distant galaxies. Quasars fluctuate in brightness over very short periods of times, as short as a few hours, which means that their active regions must be very small indeed – just a few light hours across. It is believed that the enormous energy output of a quasar is the result of material falling into a supermassive black hole situated at the heart of a distant galaxy. There is no other known object that could conceivably produce such a vast output of energy from such a small region of space. A quasar might have a trillion times the energy output of the sun, which implies that the supermassive black hole at its core is consuming several stars worth of material every year.

Quasars were much more common in the early universe. It seems likely that all galaxies go through a quasar phase in their early history, including our own Milky Way galaxy, before settling into a quieter existence, when their central black hole has consumed all the material in its neighbourhood. Our own galaxy contains a relatively quiet supermassive black hole at its centre, 30,000 light years distant, with a mass of around 3.7 million solar masses. Astronomers are now tracking a collection of stars right at the centre of the galaxy that are in orbit around this supermassive black hole:

The next goal of scientists is to produce an image of the black hole. This is the ambitious challenge of the Event Horizon Telescope:

They Don’t Come Much Bigger Than This…

There is a much larger supermassive black hole in our galactic neighbourhood. The giant elliptical galaxy M87 in the Virgo cluster of galaxies, which is 53.5 million light years away, has a distinctive jet of material spewing from its core. In visible light, this jet extends around five thousand light years into intergalactic space. But radio telescopes can detect lobes of material produced by the jet that extend as far as 250,000 light years from the galaxy.

The M87 jet is thought to be produced by a supermassive black hole located at the heart of the galaxy with a mass of six and a half billion times the mass of the sun. The size of this black hole would be about the size of the orbit of Neptune around the sun. Astrophysicists believe that as the accretion disc swirls around and finally falls into the black hole some of its material is shot out from the poles of the black hole to form a pair of jets. (There is almost certainly an oppositely directed jet that is not visible from Earth.)

Explaining the origin of the huge black holes at the centre of galaxies has proved to be a problem for astronomers. There doesn’t seem to be enough time for millions of stellar mass black holes to coalesce into a supermassive behemoth in the 13.7 billion years since the origin of the universe.

The latest discovery of intermediate mass black holes offers to shed light on the mysterious origin of their enormous relatives.

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