Devouring stars: The science of supermassive black holes

Artist's impression of a flare emanating from a supermassive black hole.

Image credit: NASA/JPL-Caltech/Public domain.

First published on 27th December 2015. Last updated on 5th June 2017 by Dr Helen Klus

In November 2015, a team of scientists led by Sjoert van Velzen of Johns Hopkins University and Gemma Anderson of ICRAR (the International Centre for Radio Astronomy Research) in Perth discovered what happens when a supermassive black hole devours a star[1a]. Their paper is published in Science, and can be read for free here.

1. Black holes

Black holes form when massive stars stop nuclear fusion and collapse in on themselves.

Stars are able to exist because of a balance between two forces. They are fuelled by nuclear fusion, but are not torn apart by nuclear explosions because they are contained by their own gravitational field.

When all potential matter has been fused, and there's no longer a force to balance gravity, they expel most of their mass and collapse inwards, becoming extremely dense. If what remains is over about 1.4 times the mass of the Sun, then it will become a black hole.

A black hole is defined as an object with a radius from which light cannot escape. This radius is known as the Schwarzschild radius, or event horizon.

The more mass that falls into a black hole, the more massive it becomes. If its mass doubles, its Schwarzschild radius doubles, and its volume increases by a factor of eight. This means that the more matter that falls into the black hole, the less dense it becomes on average.

Classical theories predict that all of the mass in a black hole is contained in the centre, in a space with no volume, and an infinite density. This is unlikely to be correct, however, as a theory of quantum gravity is needed to describe something so massive and so small, and these are still being developed.

Ordinary black holes have masses that are not much higher than the mass of the Sun, but the most massive black holes are millions to billions of times as massive. These are known as supermassive black holes[2].

1.1 Escape velocity

To escape from a gravitational field, you need to achieve a kinetic energy equal to the gravitational potential energy:

mV2 =

Here m is your mass, V is your velocity, G is the gravitational constant (G = 6.67408 × 10-11 Nm2kg-2), M is the mass of the object you are trying to escape from, and r is the distance between you and the centre of mass of the object you are trying to escape from.

Ignoring air resistance, the velocity you need to escape is therefore:

Vesc =
The escape velocity of the Sun is:
Vesc =
2G × (1.99 × 1030)/696,000,000
= 617,623 m/s.
The Average density =
The Volume of a sphere =
πr3, and so the average density of the Sun is:
1.99 × 1030/4⁄3π × 696,000,0003
= 1,408 kg/m3.

1.2 The Schwarzschild radius

The speed of light, c, is 299,792,458 m/s. In order for the gravitational field to be so strong that not even light can escape, Vesc must be higher than this:

Vesc =
, and so if Vescc,

This is known as the Schwarzschild radius, RSch, or event horizon:

≥ RSch.

A black hole twice as massive as the Sun will have a Schwarzschild radius of:

2G × (2×1.99×1030)/c2
5,908 m.
The average density is:
2 × 1.99×1030/4⁄3π × 5,9083
= 4.6 billion billion kg/m3.

Sagittarius A* has a Schwarzschild radius of:

2G × (4×106 × 1.99×1030)/c2
11.8 billion m.
The average density of Sagittarius A* is:
4×106 × 1.99×1030/4⁄3π × (1.2×1010)3
= 1,151,000 kg/m3.

1.3 Density

Black holes become less dense as they increase in mass. This is because if the mass doubles, the Schwarzschild radius also doubles, which means the volume increases by a factor of 23 = 8.

Average density =
, and so if the mass doubles:
Average density =
2 × Mass/8 × Volume
1 × Mass/4 × Volume

This means that when the mass doubles, the average density decreases by a factor of 4.

Supermassive black hole H1821+643, has a Schwarzschild radius of:

2G × (3×1010×1.99×1030)/c2
88,620 billion m.
The average density of H1821+643 is:
3×1010× 1.99×1030/4⁄3π × (8.9×1013)3
= 0.02 kg/m3,

which is less dense than hydrogen gas.

2. Supermassive black holes

Supermassive black holes are thought to exist at the centre of almost all massive galaxies. Our own galaxy the Milky Way contains a supermassive black hole known as Sagittarius A*, which is thought to be about 4 million times as massive as the Sun[3].

While all the stars in the Galaxy orbit around Sagittarius A*, it's also directly orbited by a number of stars from well beyond the Schwarzschild radius[4]. It's likely that most other supermassive black holes are also directly orbited by stars.

Supermassive black holes may also be orbited by dust and gas, in what is known as an 'accretion disc'. This can become heated by friction, producing X-rays.

At the rotation axis of the black hole, matter from the accretion disc can be pushed away at the speed of light, in jets that can extend for thousands of light-years. As these jets run out of energy, they flare out, creating radio lobes. Supermassive black holes like this are known as active galactic nuclei[5].

Jets can also be produced from dormant black holes if stars get too close, and are pulled within the Schwarzschild radius. Evidence that stars can fall into supermassive black holes came in 2005, when hyper-velocity stars were discovered[6]. These are thought to have been part of a binary star system that broke apart as it approached a supermassive black hole. As one star was captured, the other was pushed away at a velocity exceeding the escape velocity of the Galaxy.

In 2011, a jet was produced from an otherwise dormant supermassive black hole in a galaxy 3.9 billion light-years away[7][8]. This was attributed to it capturing a star, in an event known as Swift J1644+57[9]. This supermassive black hole is thought to be about 8 million times the mass of the Sun, and twice the mass of Sagittarius A*.

Animation showing a star falling towards a supermassive black hole, as it approaches, it is destroyed, creating an accretion disc and a jet.

Animation showing supermassive black hole Swift J1644+57 producing a jet after destroying a star. Image credit: NASA/Goddard Space Flight Center/CI Lab/Public domain.

Annotated stills from the animation shown above. The first panel states: ‘A Sun-like star on an eccentric orbit plunges toward the supermassive black hole in the heart of a distant galaxy.' The second panel states: ‘Strong tidal forces near the black hole increasingly distort the star. If the star passes too close, it is ripped apart.' The third panel states: ‘The part of the star facing the black hole steams toward it and forms an accretion disc. The remainder of the star just expands into space.' The forth panel states: ‘Near the black hole, magnetic fields power a narrow jet of particles moving near the speed of light. Viewed head-on, the jet is a brilliant X-ray and radio source.'

Annotated stills from the animation showing how supermassive black hole Swift J1644+57 produced a jet after destroying a star (click to enlarge). Image credit: NASA/Goddard Space Flight Center/Swift/Public domain.

3. ASASSN-14li

Last month, Dr Sjoert van Velzen's team showed that supermassive black hole ASASSN-14li produced a jet within the first few weeks of consuming a star[1b]. ASASSN-14li is about 3 million times the mass of the Sun, about 3/4 of the mass of Sagittarius A*. It is situated in the galaxy PGC 043234, which is about 300 million light-years from Earth.

ASASSN-14li was first discovered in December 2014 using ASAS-SN (the All-Sky Automated Survey for Supernovae)[10]. It was first observed using AMI-LA (the Arcminute Microkelvin Imager Large Array) that same month, and has been monitored for about four hours a week since then.

ASASSN-14li was found to be in the process of devouring a star earlier this year[11], and Dr van Velzen's team began an even more detailed monitoring campaign within 22 days using AMI (the Arcminute Microkelvin Imager), and WSRT (the Westerbork Synthesis Radio Telescope).

They found that the star formed an accretion disc around the supermassive black hole before being devoured, and produced a jet that was far less powerful than the jet produced by Swift J1644+57.

It's commonly suggested that supermassive black holes produce more powerful jets the faster the black hole spins[1c].

This does not appear to be the case for ASASSN-14li, however, and may instead be related to the magnetic field strength near the black hole's event horizon[1d].

Scientists hope to gain a better understanding of ASASSN-14li, and other supermassive black holes, from future observations using more precise telescopes like the Square Kilometre Array (SKA). SKA is a radio telescope project that is due to be built in Australia and South Africa between 2018 and 2030. It will be 50 times more sensitive than any other radio instrument.

SKA will also be used to map a billion galaxies and measure dark energy, to test Einstein's theory of general relativity, to map magnetic fields across the universe, and to search for evidence of life on potentially habitable exoplanets.

4. References

  1. (a, b, c, d) van Velzen, S., et al, 2016, 'A radio jet from the optical and x-ray bright stellar tidal disruption flare ASASSN-14li', Science, 351, pp.62-65.

  2. Luminet, J. P., 1992, 'Black Holes', Cambridge University Press.

  3. Schödel, R., et al, 2002, 'A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way', Nature, 419, pp.694-696.

  4. University of Chicago, 'Stars orbiting the Supermassive Black Hole at our Galactic Center', last accessed 01-06-17.

  5. Blandford, R. D., Netzer, H., Woltjer, L., Courvoisier, T. J. L., and Mayor, M., 2013, 'Active Galactic Nuclei', Springer.

  6. Brown, W. R., Geller, M. J., Kenyon, S. J., and Kurtz, M. J., 2005, 'Discovery of an unbound hypervelocity star in the Milky Way halo', The Astrophysical Journal Letters, 622, pp.33-36.

  7. Burrows, D. N., et al, 2011, 'Relativistic jet activity from the tidal disruption of a star by a massive black hole', Nature, 476, pp.421-424.

  8. Zauderer, B. A., et al, 2011, 'Birth of a relativistic outflow in the unusual gamma-ray transient Swift J164449.3+573451', Nature, 476, pp.425-428.

  9. NASA, 'Researchers Detail How a Distant Black Hole Devoured a Star', last accessed 01-06-17.

  10. Holoien, T. S., et al, 2016, 'Six months of multiwavelength follow-up of the tidal disruption candidate ASASSN-14li and implied TDE rates from ASAS-SN', Monthly Notices of the Royal Astronomical Society, 455, pp.2918-2935.

  11. Miller, J. M., et al, 2015, 'Flows of X-ray gas reveal the disruption of a star by a massive black hole', Nature, 526, pp.542-545.

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