A quasar is the blazing core of a distant galaxy, powered by a supermassive black hole actively pulling in surrounding gas and dust. As that material spirals inward, it heats up and radiates enormous amounts of energy, often outshining every star in its host galaxy combined.
The name is short for quasi stellar radio source, coined because early quasars were first picked up as compact radio emitting points that looked deceptively like ordinary stars through a telescope, only far more powerful and far more distant.
Because quasars are so bright, they can be seen across billions of light years, giving astronomers a rare window into what galaxies looked like when the universe was young. More detail is available from NASA’s Imagine the Universe.
Because a quasar’s brightness can flicker noticeably over just days or weeks, astronomers realized early on that its energy source had to be extremely compact, no larger than our solar system, since nothing could coordinate a brightness change across a larger region faster than light itself could cross it. That simple timing argument was one of the first strong hints that something as small as a black hole could plausibly generate as much power as an entire galaxy. Quasars were also far more common in the young universe than they are today, most of the ones we observe are billions of light-years away, giving astronomers a direct window into an era when galaxies were still actively feeding their central black holes at a much higher rate.
Quasars are thought to play an active role in shaping their host galaxies rather than just sitting passively at the center. The intense radiation and outflowing winds a quasar generates can blow surrounding gas out of a galaxy entirely, a process called feedback, which can shut down future star formation and helps explain why the most massive galaxies in the universe tend to have relatively few young stars left.