The question of the brightest galaxy in the universe is complex because the term “brightest” has two distinct meanings in astronomy. Astronomers seeking the true record holder look for the object that generates the most energy, known as intrinsic luminosity, regardless of its distance. This separates familiar, nearby galaxies from the distant, energetic phenomena that represent the cosmos’s most powerful light sources.
Defining Galactic Brightness
Astronomers use two primary metrics to quantify how bright a celestial object appears or truly is. Apparent magnitude measures the brightness of an object as seen from Earth, which is heavily influenced by the object’s distance from us. A relatively faint, nearby object can easily have a lower (brighter) apparent magnitude than a distant, highly energetic one.
The more meaningful measure is intrinsic luminosity, which defines the total amount of energy an object emits per unit of time. This physical property is independent of the observer’s location. To make this measurement comprehensive, astronomers use bolometric luminosity, which accounts for energy emitted across the entire electromagnetic spectrum, not just visible light.
Intrinsic luminosity is often expressed in units of solar luminosities (\(L_\odot\)), representing how many times brighter an object is than our own Sun. The brightest galaxies far outshine the light produced by their billions of stars combined. This is why the search for the brightest galaxy focuses on objects with extraordinary power-generating mechanisms at their core.
Identifying the Most Luminous Galaxy
The current record holder for the most luminous object in the known universe is a quasar, a type of Active Galactic Nucleus (AGN), named J0529-4351. Its light has traveled over 12 billion years to reach us, meaning we observe it as it existed when the universe was less than a billion years old. Its energy output is estimated to be over 500 trillion times that of the Sun.
Quasars represent the extremely bright core of a distant galaxy. J0529-4351 was initially miscategorized as a foreground star due to its extreme apparent brightness in sky surveys, illustrating its powerful intrinsic luminosity despite its enormous distance. Identified in early 2024, this quasar now holds the title of the fastest-growing black hole and the most luminous object observed to date.
Another notable class of contenders are the Extremely Luminous Infrared Galaxies (ELIRGs), such as WISE J224607.57-052635.0 (W2246-0526). This galaxy, previously the record holder, shines with the energy of approximately 300 trillion Suns, primarily in the infrared spectrum. While slightly less luminous than J0529-4351, ELIRGs are important because their energy is largely obscured by dense dust clouds, which re-radiate the immense power as infrared light.
The Engines of Extreme Brightness
The light from these record-breaking objects does not come from the combined glow of their stars. Instead, the overwhelming luminosity is generated by an Active Galactic Nucleus (AGN), powered by a central supermassive black hole (SMBH). The quasar J0529-4351, for instance, hosts a black hole estimated to be 17 billion times the mass of the Sun.
This black hole actively consumes surrounding matter, including gas and dust, at an extraordinary rate—over one solar mass every day. As this material spirals inward, it forms a massive structure called an accretion disk. Intense gravitational and frictional forces heat the matter to millions of degrees, causing it to emit vast amounts of radiation across the entire electromagnetic spectrum, from radio waves to X-rays.
The accretion disk in J0529-4351 is thought to be the largest known, spanning seven light-years across. This mechanism is far more efficient at producing light than nuclear fusion in stars, allowing the galactic core to completely outshine its stellar population. For some ELIRGs, intense bursts of star formation, often triggered by galactic mergers, also contribute to the infrared output by heating surrounding dust.
Contextualizing Other Bright Galaxies
While quasars hold the record for intrinsic luminosity, other galaxies are considered “bright” under different, less technical criteria. The most familiar example is the Andromeda Galaxy (M31), which is the closest large spiral galaxy to the Milky Way. With an apparent magnitude of 3.4, Andromeda is the brightest external galaxy visible to the unaided eye in the night sky.
Within our Local Group of galaxies, which includes the Milky Way and Andromeda, Andromeda is also the largest member. The Magellanic Clouds, though smaller, are also prominent and are sometimes the brightest apparent galaxies when viewed from the Southern Hemisphere. These galaxies are considered bright because of their proximity, not because of extreme energy generation.
Another phenomenon that complicates brightness measurement is gravitational lensing. This occurs when the gravity of a massive object, such as a foreground galaxy, bends and magnifies the light from a distant object. Lensing can temporarily inflate a quasar’s apparent brightness, creating a misleadingly high magnitude reading. Astronomers must account for this effect to calculate the true intrinsic luminosity of objects like J043947.08+163415.7, whose actual energy output is significantly less than its magnified appearance.