A quasar is the extremely bright core of a distant galaxy, powered by a supermassive black hole swallowing gas. A single quasar can outshine all the stars of its host galaxy combined, which makes quasars visible across billions of light-years.
How quasars were found
In the 1950s, radio astronomers found sources that looked like stars in optical telescopes, hence the name “quasi-stellar radio source”. In 1963, Maarten Schmidt identified the spectrum of 3C 273 and realized its lines were strongly redshifted: it was about 2 billion light-years away, and therefore enormously luminous. See how spectroscopy reveals distances and composition.
What powers a quasar?
Gas falling toward a black hole of millions to billions of solar masses forms a spinning accretion disk. Friction heats the disk to very high temperatures, so it shines brightly from X-rays to infrared. Some quasars also launch jets of particles at nearly the speed of light. See how black holes form.
Quasars, blazars and other active nuclei
Quasars belong to the broader family of active galactic nuclei (AGN). Many differences come from how we see them:
- Radio-loud quasars: strong radio jets. Only about 10 percent of quasars.
- Radio-quiet quasars: the majority, bright in optical and X-ray light but weak in radio.
- Blazars: quasars with a jet pointed almost straight at Earth, highly variable in brightness. BL Lac objects are a subtype with weak spectral lines.
- Broad absorption line (BAL) quasars: their spectra show strong outflowing winds.
- Red or obscured quasars: hidden behind dust, often found in infrared surveys.
Quasars and galaxy evolution
Quasars were most common around 10 billion years ago, when galaxies had plenty of gas. Energy from quasars can heat or expel gas from their host galaxies, which may help explain why some galaxies stopped forming stars, see galaxy formation and evolution.
Useful tools for astronomers
- The most distant known quasars existed less than a billion years after the Big Bang, challenging ideas on how black holes grew so fast.
- Their light passes through intergalactic gas, revealing it through absorption lines.
- Because they are so far away, quasars serve as fixed reference points for precise sky positions.
- Some are lensed into several images by foreground galaxies, see lensing by galaxies and clusters.
