Quasar

A quasar—a contraction of quasi-stellar object—is the active nucleus of a distant galaxy with extraordinary luminosity. Powered by a supermassive black hole that devoursenormous amounts of matter, a quasar can shine hundreds of times brighter than its entire host galaxy, making it visible from billions of light-years away. They are the brightest persistent objects in the observable universe.

A cosmic beacon

A quasar’s brightness comes from the accretion disk surrounding its central black hole. The spiraling matter releases colossal gravitational energy— up to 40% of its mass converted into energy, far exceeding that of nuclear fusion. For the brightest quasars, this power exceeds that of a thousand galaxies combined, yet it originates from a region not much larger than our solar system.

Witnesses to the Past

Quasars are primarily observed at great distances, which means we see them as they were in the distant past. Most known quasars existed 10 to 13 billion years ago— a time when the universe was still young and supermassive black holes were rapidly growing. Today, these same galaxies have much calmer nuclei, as their black holes have exhausted their reserves of matter.

The connection to galaxies

It is now believed that nearly all large galaxies went through a quasar phase at some point in their history. The Milky Way itself may have been a quasar billions of years ago. Sagittarius A*, its central black hole, is now nearly dormant—but could be reawakened if a large amount of gas were to fall toward it.

Did you know?

The brightest quasar in the sky, 3C 273, is visible through an amateur telescope despite its distance of 2.4 billion light-years. If it were placed at the distance of Proxima Centauri (4.2 light-years), it would shine in the sky as brightly as the Sun.

Frequently Asked Questions

What is the difference between a quasar and a black hole?

A black hole is the compact object itself. A quasar refers to the entire system—a supermassive black hole plus an active accretion disk—that produces extreme brightness. A black hole with no matter to accrete does not form a quasar.

Are there any quasars near us?

Active quasars are rare in the local universe. The closest one is Markarian 231, about 600 million light-years away. The rarity of nearby quasars confirms that they were much more common in the early universe.

Can a quasar affect its host galaxy?

Yes, significantly. The jets and winds produced by a quasar can expel gas from the entire galaxy, halting star formation. This feedback mechanism is considered crucial to the evolution of massive galaxies.