A dead galaxy is one that has stopped forming new stars. While it still contains billions of existing stars, no new ones are being born, and the galaxy is slowly fading as its oldest stars burn out over billions of years. Astronomers often call these galaxies “red and dead” because their light shifts toward red wavelengths as their hot, blue, short-lived stars die off and only cooler, longer-lived red stars remain.
Why Galaxies Stop Making Stars
Stars form when clouds of cold gas collapse under their own gravity. A galaxy “dies” when it runs out of this cold gas, either by using it all up, having it stripped away, or losing the ability to pull in fresh supplies. Astronomers call this process quenching, and it can happen through several different routes depending on a galaxy’s size, location, and history.
Galaxies in the local universe fall broadly into two camps: blue, star-forming spiral galaxies typically found in less crowded regions of space, and red, dead elliptical galaxies concentrated in dense galaxy clusters. The red ones evolved from blue ones. Every galaxy dominated by old stars today must have built up its stellar mass through active star formation at some earlier point, then had that formation shut down.
How Black Holes Kill Massive Galaxies
In the largest galaxies, the supermassive black hole at the center plays a decisive role. As gas spirals into the black hole, it releases enormous amounts of energy in two distinct ways. In younger, more active galaxies, the black hole’s intense radiation pushes gas outward in powerful winds, physically ejecting the raw material needed for star formation. In older, quieter galaxies, the black hole launches jets of material at nearly the speed of light, heating the surrounding gas so it can never cool down enough to collapse into new stars.
Without this black hole feedback, computer simulations massively overproduce giant galaxies. The energy output from these central black holes is essentially the thermostat that keeps the universe’s largest galaxies from growing without limit. In smaller galaxies, exploding stars can do a similar job, blowing gas out of the galaxy before it has a chance to form the next generation of stars. But for the most massive galaxies, only a supermassive black hole packs enough punch.
How Neighbors Can Kill a Galaxy
A galaxy doesn’t have to destroy itself from within. The environment matters enormously. Galaxy clusters, the densest neighborhoods in the universe, are filled with extremely hot gas between the galaxies. When a galaxy moves through this hot medium at high speed, the pressure acts like a cosmic headwind, stripping cold gas right out of the galaxy’s outer edges. This process, called ram-pressure stripping, peels away the low-density gas from the outskirts of a galaxy’s disk, leaving behind a shrunken gas reserve that quickly gets used up.
Gravitational interactions add to the damage. Close encounters with neighboring galaxies can pull gas into long tidal tails, stretching it thin enough that even weak ram pressure can sweep it away. Studies of galaxy groups have shown that a combination of tidal forces and ram pressure is often needed to fully explain how much gas a galaxy has lost. Other processes contribute too: galaxy harassment (repeated high-speed flybys), major mergers that violently rearrange a galaxy’s structure, and strangulation, where a galaxy’s surrounding gas reservoir is cut off so no fresh fuel can fall inward. Strangulation is slower and more subtle than stripping. The galaxy continues forming stars for a while using whatever gas it already has, then gradually winds down as that supply is exhausted.
This is why dense galaxy clusters have far more elliptical, non-star-forming galaxies than the emptier regions of space. The cluster environment is hostile to star formation in multiple overlapping ways.
What a Dead Galaxy Looks Like
Living galaxies glow blue because massive, young stars burn extremely hot and bright. These stars live fast and die young, sometimes in just a few million years, so their presence signals ongoing star formation. When star formation stops, these blue stars are the first to vanish. What remains are smaller, cooler, redder stars that can burn for tens of billions of years. Over time, the galaxy’s overall color shifts from blue to red.
Astronomers classify galaxies as quiescent (the formal term for “dead”) using color measurements. A common method plots galaxies on a diagram based on their ultraviolet and infrared light. Star-forming galaxies cluster in one region, dead galaxies in another. About 20% of galaxies that appear quiescent through simple color cuts turn out to be dusty star-forming galaxies in disguise, their blue light hidden by dust rather than absent. More sophisticated techniques using multiple color filters can weed out these impostors.
Dead galaxies also tend to have a distinctive shape. Most are elliptical or spheroidal, lacking the defined spiral arms that characterize active galaxies. Major mergers between galaxies can scramble orderly disk structures into these rounder shapes while simultaneously triggering a final burst of star formation that burns through remaining gas reserves.
Dead Galaxies in the Early Universe
One of the more surprising discoveries in recent years is that some galaxies died remarkably early in cosmic history. The James Webb Space Telescope has observed ZF-UDS-7329, an extremely massive dead galaxy seen as it existed when the universe was only about 3.5 billion years old (at a redshift of roughly 3.2). This galaxy had already assembled a huge population of stars and then stopped forming new ones, all within a relatively short window of cosmic time.
Finding massive dead galaxies this early challenges models of galaxy evolution, which generally predict that it takes longer for galaxies to accumulate so much mass and then shut down. Studying these early relics at high resolution, something only now possible with Webb’s instruments, gives astronomers a sharper picture of how quickly quenching can occur and what physical conditions drive it.
The Milky Way’s Eventual Death
Our own galaxy is still very much alive, forming roughly one to two new stars per year. But it has an expiration date. The Milky Way and the Andromeda galaxy are being pulled together by their mutual gravity and will collide head-on in about 4 billion years. Over the following 2 billion years, the two spirals will merge into a single large elliptical galaxy.
NASA visualizations depict this transition using the same color language astronomers apply universally: blue for the active spiral galaxies before the collision, orange-yellow for the elliptical remnant afterward. The merger will likely trigger an initial burst of intense star formation as gas clouds smash together, followed by a rapid exhaustion of remaining fuel. The end result will be a massive, round, red galaxy with no spiral structure and no significant star formation. In other words, a dead galaxy.