The search for the “darkest star” in the universe is complicated because the word “star” can refer to several different celestial objects, including true stars, failed stars, and the dead remnants of former stars. The concept of “darkness” in space is also relative, as every object with a temperature above absolute zero emits some form of light, typically radiation in the infrared spectrum. Astronomers therefore interpret this question as a search for the objects that produce the least amount of intrinsic light or heat. The answer depends on whether the object is currently powering itself through sustained nuclear fusion, or if it is a cold, cooling object that has long since ceased stellar status.
Defining Stellar Darkness
Astronomers quantify a celestial object’s true brightness using a measurement called luminosity, which is the total amount of energy, or light, a star emits from its surface every second. Luminosity is an intrinsic property of the star itself and does not depend on an observer’s location in space. This is different from apparent magnitude, which is how bright a star appears from Earth, a measure affected by both its luminosity and its great distance from us.
A star’s luminosity is fundamentally determined by its mass, which dictates the temperature and pressure at its core. More massive stars have a stronger gravitational pull, which compresses their cores, leading to significantly higher temperatures and faster rates of nuclear fusion. This accelerated fusion process generates enormous amounts of energy, making massive stars much hotter and more luminous than lower-mass stars. The relationship is so dramatic that a slight decrease in mass results in a huge drop in light output.
Low-Mass Red Dwarfs
When searching for the dimmest objects that still qualify as true stars, the focus falls on low-mass M-dwarfs, commonly called red dwarfs. These stars are defined by their ability to sustain the fusion of hydrogen into helium in their core over billions of years. Red dwarfs are the most numerous type of star in the galaxy, making up about three-quarters of the stellar population.
These stars have masses ranging from approximately 0.075 to 0.6 times the mass of the Sun, which is just enough to trigger and maintain core fusion. The low mass results in relatively weak gravitational pressure and correspondingly low core temperatures, meaning their fusion rate is incredibly slow. Their surface temperatures typically range from about 2,100 to 3,900 Kelvin, which is roughly half that of our Sun.
Because their fusion is so slow, red dwarfs are only about one ten-thousandth to six percent as luminous as the Sun. The minimal energy output of red dwarfs makes their lifespans extraordinarily long, far exceeding the current age of the universe. They emit most of their light in the red and infrared parts of the spectrum, which is why they are difficult to observe with optical telescopes. This category represents the absolute lower limit for a stable, light-producing celestial body that meets the scientific definition of a star.
Brown Dwarfs The Failed Stars
The objects that most closely match the intent of a “darkest star” are brown dwarfs, often described as “failed stars” because they lack the necessary mass to ignite sustained hydrogen fusion. These substellar objects occupy the mass range between the largest gas giant planets and the smallest true stars, typically between 13 and 80 times the mass of Jupiter. While they are massive enough to briefly fuse deuterium, a heavy isotope of hydrogen, this process is not self-sustaining and quickly fades, causing the object to cool and dim over time.
Brown dwarfs are classified into spectral types, with the coolest and dimmest being the T-dwarfs and Y-dwarfs. T-dwarfs, sometimes called “methane dwarfs,” have surface temperatures between 800 and 1,300 Kelvin. Their spectra are dominated by the absorption bands of methane and water vapor, and these objects emit almost all of their energy in the infrared region.
The Y-dwarfs represent the coldest class of brown dwarfs, with temperatures falling below 500 Kelvin, often cooler than the boiling point of water. Some Y-dwarfs have estimated atmospheric temperatures as low as 250 to 400 Kelvin, comparable to the temperature of Earth’s atmosphere. Their spectra feature strong absorption lines from water, methane, and ammonia, closely resembling the atmospheric composition of giant planets like Jupiter. These frigid, dark objects are the dimmest self-luminous bodies that form like stars.
Truly Invisible Stellar Remnants
While red dwarfs are the dimmest true stars and Y-dwarfs are the coldest self-luminous failed stars, the most truly “dark” objects are the stellar remnants that no longer produce any light of their own. These objects do not fit the definition of a star because they have ceased all forms of energy generation. Neutron stars and black holes are the ultra-dense end-products of the catastrophic collapse of massive stars.
A neutron star is the collapsed core left after a supernova. While newly formed ones can be very hot, they cool rapidly over time. These objects are extremely difficult to detect unless they are rapidly spinning as a pulsar or are actively pulling in matter from a companion star, which causes them to emit intense X-rays. Without external energy sources, a solitary, old neutron star will radiate very little electromagnetic energy, only thermal radiation from its cooling surface.
Black holes are the ultimate dark objects, representing a region of spacetime where gravity is so intense that nothing, not even light, can escape from beyond its event horizon. A black hole emits no light of its own, though it can be detected indirectly by the high-energy radiation from a surrounding accretion disk of infalling matter. A black hole that is completely isolated in space is essentially invisible, detectable only by its gravitational influence on nearby objects.