What Are Quasars and How Do They Shape Galaxies?

Cosmic Beacons Powered by Black Holes

Somewhere in the distant universe, objects no larger than our solar system outshine entire galaxies containing hundreds of billions of stars. These are quasars—quasi-stellar radio sources—and they rank among the most extreme phenomena astronomers have ever observed. The brightest known quasar, J0529-4351, blazes with a luminosity more than 500 trillion times that of the Sun.

Despite their extraordinary power, quasars remained a mystery for decades after their discovery. Understanding how they work reveals fundamental truths about galaxies, black holes, and the evolution of the cosmos itself.

How Quasars Work

A quasar is the luminous core of a galaxy where a supermassive black hole—one with a mass ranging from millions to billions of Suns—is actively feeding on surrounding gas and dust. As material spirals inward, it forms a rapidly rotating accretion disk. Gravitational and frictional forces heat this disk to millions of degrees, causing it to radiate enormous amounts of energy across the electromagnetic spectrum, from radio waves to X-rays.

Some quasars also launch relativistic jets—narrow beams of ionized matter accelerated to nearly the speed of light—that can extend hundreds of thousands of light-years into intergalactic space. According to NASA, the most powerful quasars emit up to a thousand times the total energy output of the Milky Way.

Not every galaxy hosts an active quasar. The phenomenon requires a generous supply of gas funneled toward the central black hole, which is why quasars were far more common in the early universe, when galaxies were gas-rich and collisions were frequent.

A Discovery That Changed Astronomy

Quasars were first detected as puzzling radio sources in the late 1950s. The breakthrough came on February 5, 1963, when Caltech astronomer Maarten Schmidt examined the optical spectrum of a radio source called 3C 273. He recognized the familiar hydrogen emission lines—but shifted dramatically toward the red end of the spectrum.

That enormous redshift meant 3C 273 lay roughly three billion light-years away, far beyond the Milky Way. For an object at such a distance to appear so bright, its intrinsic luminosity had to be staggering. Schmidt's discovery, published in Nature in March 1963, opened an entirely new window on the distant universe and earned quasars their nickname: beacons of the deep cosmos.

How Quasars Shape Their Surroundings

Quasars do not merely shine—they actively sculpt the galaxies around them through a process called AGN feedback. Radiation, winds, and jets from an active quasar can heat and expel the cold gas that galaxies need to form new stars, effectively throttling stellar birth in the host galaxy.

Recent research using the James Webb Space Telescope has pushed this picture further. A team led by Yongda Zhu at the University of Arizona found that the quasar J0100+2802 suppresses star formation not just in its own galaxy but in neighbouring galaxies within a radius of at least one million light-years. The intense radiation splits molecular hydrogen in surrounding gas clouds, stripping away the raw material needed to build new stars.

This discovery suggests that galaxies do not evolve in isolation. A single powerful quasar can influence the development of an entire cosmic neighbourhood.

Why Quasars Still Matter

Because quasars are so luminous, they serve as cosmic backlights. Their light passes through intervening gas clouds on its way to Earth, and by analysing the absorption lines imprinted on that light, astronomers can map the composition and distribution of matter across billions of light-years—a technique critical to understanding the large-scale structure of the universe.

JWST continues to reveal surprises: dust-shrouded quasars hidden in the early universe suggest bright quasars were at least twice as common in the first billion years after the Big Bang as previously thought. Each new finding refines our models of how supermassive black holes and their host galaxies co-evolved from the cosmic dawn to the present day.

From mysterious radio blips in the 1960s to JWST's infrared gaze into the earliest epochs, quasars remain one of astronomy's most powerful tools—and most spectacular phenomena.