3.9.3.7
Quasars are supermassive black holes with accretion discs at the centres of extremely distant galaxies
Astrophysics | AQA A-Level Physics
Key Definition
Quasar: An extremely luminous, star-like source of radiation with a very high redshift, now understood to be a supermassive black hole surrounded by an accretion disc at the centre of a distant galaxy.
Discovery and properties
- Quasars (quasi-stellar objects) were first discovered in the 1960s due to their strong radio emissions.
- They were also notable for their:
- Extremely high luminosities (they can outshine entire galaxies)
- Very large redshifts (indicating enormous distances)
- Surprisingly small size (they appear point-like, similar to stars)
- It is now known that quasars are strong emitters of all wavelengths of electromagnetic radiation, not just radio waves.
Formation and structure
- Quasars are a type of active galactic nucleusThe compact, extremely luminous centre of a galaxy, powered by a supermassive black hole that is actively accreting matter. (AGN). Fundamentally, they consist of:
- A supermassive black hole at the centre of a galaxy
- An accretion disc of matter spiralling into the black hole
- Powerful jets of radiation emitted from the poles
- As matter falls into the black hole, its gravitational potential energy is transferred to electromagnetic radiation. The equivalent of 100 solar masses of matter can fall into a quasar each year, releasing enormous amounts of energy.
Redshift and distance
- Quasars are thought to be some of the most distant measurable objects in the known Universe.
- This is evidenced by the extremely large redshifts they show.
- Crucially, because they are so far away, the light we observe from quasars was emitted when the Universe was very young. This allows astronomers to look back to the early Universe, not long after the Big Bang.
Common Mistake
Students sometimes describe quasars as "bright stars." They are not stars at all. Quasars are supermassive black holes that are actively consuming matter. The enormous luminosity comes from the gravitational potential energy of infalling material being converted to radiation in the accretion disc, not from nuclear fusion as in a star.