A starburst galaxy is a galaxy producing new stars at an extraordinarily high rate, often tens to hundreds of times faster than a typical galaxy like the Milky Way. The Milky Way forms roughly 2 new stars per year (measured in solar masses). Starburst galaxies can exceed 20 solar masses per year, with the most extreme examples reaching into the hundreds. These episodes are temporary, lasting millions to perhaps a billion years before the galaxy exhausts its fuel or blows it away.
How Star Formation Rates Compare
The Milky Way converts about 1.9 solar masses of gas into stars each year. That’s a fairly typical pace for a large spiral galaxy. Normal star-forming galaxies generally stay below about 20 solar masses per year. Starburst galaxies occupy the upper end of the scale, spanning the top two to three orders of magnitude in star formation activity. Some reach rates of 100 or even 1,000 solar masses per year.
There’s no single hard cutoff that separates a “starburst” from a regular galaxy. Astronomers generally use the term when a galaxy is forming stars so quickly that it would burn through its entire gas supply in a fraction of the galaxy’s lifetime. That unsustainable pace is really the defining feature: a starburst is a temporary frenzy, not a steady hum.
What Triggers a Starburst
The most common trigger is a collision or close encounter between two galaxies. Nearly all starbursting galaxies in the nearby universe are mergers or interacting systems. When two gas-rich galaxies pass close to each other or collide outright, gravitational forces compress and churn the gas in both galaxies, creating the dense conditions needed for rapid star formation. On average, a merger increases a galaxy’s specific star formation rate by a factor of three to four.
The starburst doesn’t always happen only at the galaxy’s center. Many interacting systems show star formation spread across tidal tails, bridges of gas between the two galaxies, and other extended structures far from the nucleus. The Antennae Galaxies, a famous merging pair, produce stars across their overlapping disks and long tidal streamers. Another interacting pair, NGC 2207 and IC 2163, has a star formation rate around 15 solar masses per year, almost entirely in extended regions outside the galactic cores.
The key ingredient is gas. “Dry” mergers between galaxies that have already used up their gas don’t produce starbursts. At least one of the colliding galaxies needs a substantial reservoir of cold gas to fuel the process.
What Happens Inside the Gas Clouds
The interstellar medium inside a starburst region looks very different from what exists in the Milky Way’s neighborhood. In the starburst cores of galaxies like NGC 253 (the Sculptor Galaxy), molecular gas is packed into thousands to hundreds of thousands of small, dense clouds. These clouds are typically only about 0.5 to 2 light-years across, with gas densities around 10,000 particles per cubic centimeter. For comparison, the average density of gas in the Milky Way’s disk is closer to 1 particle per cubic centimeter.
Turbulence plays a major role. Galaxy interactions drive supersonic turbulence through the gas, increasing it by a factor of several compared to a quiet spiral galaxy. This turbulence creates pockets of compressed gas that collapse under their own gravity and form stars. So the violence of the collision doesn’t just scatter gas around; it actively squeezes some of it into the dense knots where stars ignite.
Supernovae and Galactic Winds
All those new stars have consequences. Massive stars burn through their fuel quickly and explode as supernovae. In the starburst galaxy M82, the supernova rate is estimated at roughly one explosion every 10 to 15 years. The Milky Way, by contrast, averages about one or two per century. That’s an order of magnitude difference, and M82 is a relatively modest starburst.
When enough supernovae go off in a concentrated region, their combined energy drives a “superwind,” a massive outflow of gas, dust, and energetic particles streaming out of the galaxy. These winds are complex mixtures of cool, warm, and hot gas. The entrained material moves at 100 to 1,000 kilometers per second, while the hottest wind fluid can reach speeds up to 3,000 kilometers per second. That’s roughly 1% the speed of light.
These superwinds do two important things. First, they enrich the space between galaxies with heavy elements forged in stellar interiors, seeding intergalactic gas with metals to roughly 10 to 30 percent of what’s found in our Sun. Second, they gradually rob the galaxy of the raw material it needs to keep forming stars. This is one of the main ways a starburst eventually shuts itself off.
How Long a Starburst Lasts
Starburst phases involve timescales spanning three orders of magnitude. Individual star-forming regions within the burst are active for about 1 to 10 million years. The broader starburst episode, including the wind-driven clearing of gas, plays out over roughly 100 million to 1 billion years. At typical starburst rates, a galaxy can convert 10 billion solar masses of molecular gas into stars in about 100 million years. That’s comparable to the entire gas supply of even the most gas-rich galaxies, which is why the starburst can’t last.
Gas depletion sets a hard ceiling. Once the available gas is consumed by star formation or expelled by superwinds, the starburst fades. The galaxy transitions into a quieter phase, sometimes becoming a “red and dead” elliptical galaxy with very little ongoing star formation.
Why They Glow in Infrared
Starburst galaxies are among the brightest objects in the infrared sky, and the reason comes down to dust. Young, massive stars produce enormous amounts of ultraviolet light, but the dense clouds of dust surrounding them absorb much of that radiation before it can escape. The dust heats up and re-emits the energy as infrared light, with a peak around 50 micrometers wavelength. The result is that a starburst galaxy’s energy output is dominated by this thermal glow from warm dust rather than by visible starlight.
Astronomers classify the most infrared-bright galaxies by their total infrared luminosity. Luminous Infrared Galaxies (LIRGs) exceed 100 billion times the Sun’s luminosity in infrared. Ultra-Luminous Infrared Galaxies (ULIRGs) surpass 1 trillion solar luminosities. Almost all ULIRGs are starburst galaxies powered by major mergers.
M82: The Textbook Example
Messier 82, the Cigar Galaxy, is the closest and best-studied starburst galaxy, sitting about 12 million light-years away in the constellation Ursa Major. Its central region is forming stars roughly 10 times faster than the entire Milky Way. The gravitational interaction with its larger neighbor, M81, triggered the burst by funneling gas toward M82’s core.
The galactic wind streaming from M82 is visible in dramatic Hubble images as plumes of glowing hydrogen gas extending thousands of light-years above and below the galaxy’s disk. Radiation and particles from newborn stars carve into the surrounding gas, and the resulting wind compresses enough material to trigger yet more star formation in a feedback loop. At magnitude 8.4, M82 is bright enough to spot with a small telescope and is best viewed in April.
Starbursts in the Early Universe
The James Webb Space Telescope has been pushing observations of starburst-like activity back to when the universe was only a few hundred million years old. A recent analysis of over 2,400 galaxy candidates at redshifts of 7 to 14 (corresponding to roughly 600 to 300 million years after the Big Bang) found something unexpected: the rate of star formation relative to existing stellar mass stays roughly constant across that entire epoch, rather than spiking dramatically at the earliest times.
This suggests that any dust-free starburst phase in the very early universe was short-lived. Some of the galaxies observed at these extreme distances already show signs of being relatively mature, with high mass-to-light ratios indicating they formed the bulk of their stars at least 500 million years before they were observed. Others appear to have grown through intermittent bursts of star formation, remaining dim for most of their early history before undergoing a rapid growth spurt. The variety of conditions found at these early epochs is one of the key puzzles Webb is helping to unravel.