Supermassive Black Holes

A supermassive black hole is a black hole with the mass on the order of hundreds of thousands to billions of solar masses. Most galaxies are believed to contain supermassive black holes at their centers.


This artist's concept depicts a supermassive black hole and its
accretion disk at the center of a galaxy (Credit: NASA)

Supermassive black holes have properties which distinguish them from lower-mass classifications:

• The average density of a supermassive black hole can be very low, and may actually be lower than the density of air. This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object is directly proportional to the cube of the radius, and mass merely increases linearly, the volume increases at a greater rate than mass. Thus, average density decreases for increasingly larger radii of black holes.
• The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut travelling towards the black hole center would not experience significant tidal force until very deep into the black hole.

There are several models for the formation of black holes of this size. The most obvious is by slow accretion of matter starting from a black hole of stellar size. Another model of supermassive black hole formation involves a large gas cloud collapsing into a relativistic star of perhaps a hundred thousand solar masses or larger. The star would then become unstable to radial perturbations due to electron-positron pair production in its core, and may collapse directly into a black hole without a supernova explosion, which would eject most of its mass and prevent it from leaving a supermassive black hole as a remnant. Yet another model involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, primordial black holes may have been produced directly from external pressure in the first instants after the Big Bang.

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a region called Sagittarius A* because:

• The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter of 17 light hours from the central object.
• Early estimates indicated that the central object contains 2.6 million solar masses and has a radius of less than 17 light hours. Only a black hole can contain such a vast mass in such a small volume.
• Further observations strengthened the case for a black hole, by showing that the central object's mass is about 3.7 million solar masses and its radius no more than 6.25 light-hours.

The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group have provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole, based on data from the ESO and the Keck telescope. Our galactic central black hole is calculated to have a mass of approximately 4.1 million solar masses.


Sagittarius A* (centre) and two light echoes
from a recent explosion (Credit: NASA)

It is now widely accepted that the center of nearly every galaxy contains a supermassive black hole. The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M-sigma relation, strongly suggests a connection between the formation of the black hole and the galaxy itself.

The explanation for this correlation remains an unsolved problem in astrophysics. It is believed that black holes and their host galaxies coevolved between 300-800 million years after the Big Bang, passing through a quasar phase and developing correlated characteristics, but models differ on the causality of whether black holes triggered galaxy formation or vice versa, and sequential formation cannot be excluded. The unknown nature of dark matter is a crucial variable in these models.

At least one galaxy, Galaxy 0402+379, appears to have two supermassive black holes at its center, forming a binary system. Should these collide, the event would create strong gravitational waves. Binary supermassive black holes are believed to be a common consequence of galaxy mergers. As of November 2008, another binary pair, in OJ 287, contains the most massive black hole known, with a mass estimated at 18 billion solar masses.

IK Pegasi B: The Nearest Supernova Candidate

IK Pegasi is a binary star system in the constellation Pegasus. White dwarf IK Pegasi B, a massive star that is no longer generating energy through nuclear fusion, is the nearest known supernova candidate. When the primary evolves into a red giant, it will grow to a radius where the white dwarf can attract more matter from the expanded envelope. When the white dwarf approaches the limit of 1.44 solar masses, it is going to explode as a Type Ia supernova.


In IK Pegasi binary system, gas is being stripped away
from a giant star to form an accretion disc around
a compact companion (NASA image).

The primary is a main sequence star that displays minor pulsations in luminosity. It is categorized as a Delta Scuti variable star with a period of about an hour. Its companion is a massive white dwarf — a star that has evolved past the main sequence. They orbit each other every 21.7 days with a separation of about astronomical units. This is smaller than the orbit of Mercury around the Sun.

The distance to the IK Pegasi system can be measured directly by observing its parallax shifts against the distant stellar background as the Earth orbits around the Sun. This shift was measured to high precision by the Hipparcos spacecraft, and the distance was estimated as 150 light years. Hipparcos also measured the proper motion — the small angular motion of IK Pegasi across the sky because of its motion through space. The combination of the distance and proper motion of this system was used to compute the transverse velocity of IK Pegasi as 16.9 km/s.

The interior of IK Pegasi B may be composed wholly of carbon and oxygen, or alternatively, it may have a core of oxygen and neon, surrounded by a mantle enriched with carbon and oxygen. The exterior is covered by an atmosphere of almost pure hydrogen. Any helium in the envelope will have sunk beneath the hydrogen layer. The entire mass of the star is supported by electron degeneracy pressure — a quantum mechanical effect that limits the amount of matter that can be squeezed into a given volume.


A comparison between the IK Pegasi B (center), its companion
IK Pegasi A (left) and the Sun (right). (Credit: RJHall)

IK Pegasi B is considered to be a high-mass white dwarf, at an estimated 1.15 solar masses. Its radius can be estimated from known theoretical relationships between the mass and radius of white dwarfs, giving a value of about 0.60% of the Sun's radius. Thus this star packs a mass greater than the Sun into a volume roughly the size of the Earth. The massive, compact nature of a white dwarf produces a strong surface gravity — over 900,000 times the gravitational force on the Earth. The surface temperature is about 35,500K, making it a strong source of ultraviolet radiation. Under normal conditions this white dwarf would continue to cool for more than a billion years, while its radius would remain unchanged.

At some point in the future, IK Pegasi A will consume the hydrogen fuel at its core and form a red giant. The envelope of a red giant can extend up to a hundred times its previous radius. Once IK Pegasi A expands to the point where its outer envelope overflows the Roche lobe of its companion, a gaseous accretion disk will form around the white dwarf. This mass transfer between the stars will also cause their mutual orbit to shrink. Should the white dwarf's mass approach the Chandrasekhar limit of 1.44 solar masses it will no longer be supported by electron degeneracy pressure and it will undergo a collapse. If the core is made of carbon-oxygen, increasing pressure and temperature will initiate carbon fusion in the center prior to attainment of the Chandrasekhar limit. The dramatic result is a runaway nuclear fusion reaction that consumes a substantial fraction of the star within a short time. This will be sufficient to unbind the star in a cataclysmic, Type Ia supernova explosion.

A supernova would need to be within about 26 light years of the Earth to effectively destroy the Earth's ozone layer, which would severely impact the planet's biosphere. IK Pegasi system is not likely to pose a threat to life on the Earth, however. It is thought that the primary star is unlikely to evolve into a red giant in the immediate future. As shown previously, the space velocity of this star relative to the Sun is 20.4 km/s. This is equivalent to moving a distance of one light year every 14,700 years. After 5 million years, this star will be separated from the Sun by more than 500 light years. This is outside the radius where a Type Ia supernova is thought to be hazardous.


This video shows a thermonuclear flame burning its way through a white dwarf star. The flame produces hot ash, which buoyantly rises as the flame burns. The ash breaks out of but remains gravitationally bound to the surface of the star and collides at a point on the opposite side of the star from the breakout location. The blue shows the approximate surface of the star and the orange shows the interface between the star and the hot ash produced by the flame. (Credit: DOE NNSA ASC/Alliance Flash Center at the University of Chicago)

The Story of Gamma-ray Bursts

Gamma-ray bursts are flashes of gamma rays connected with extremely energetic explosions in distant galaxies. They are the most luminous electromagnetic events occurring in the universe.

This illustration shows the life of a massive star as nuclear fusion converts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses into a black hole. Energy may be released during the collapse along the axis of rotation to form a gamma-ray burst. (Credit: Nicolle Rager Fuller/NSF)

"The death star": BBC documentary on the first stars in the universe and gamma-ray bursts.

The Key to the Cosmos

Prof. Jim Al-Khalili investigates the atom by looking at radioactivity, the Atom Bomb and the Big Bang, and even why we are here and how we were made. He shows that, in the quest to understand the atom, the mystery of how the entire universe was created became revealed.

The Hawking Paradox: Video

The strange and exciting mix of general relativity and quantum mechanics results in the so called black hole information paradox. The Discovery Channel show "The Hawking Paradox” explains that information lost in black holes could be saved in parallel universes where no black holes exist. See the complete 49 min video here: