Red dwarfs are the smallest and most common stars in the Milky Way, yet not a single one is bright enough to be seen from Earth with the naked eye. These cool, faint stars burn their nuclear fuel so slowly that the least massive could survive for trillions of years — far longer than the universe has existed so far. They are also important targets in the search for planets beyond our Solar System, making these seemingly modest stars some of the most interesting objects in modern astronomy.
Red Dwarfs Are the Smallest Stars
Red dwarfs are small, relatively cool main-sequence stars that fuse hydrogen into helium just as the Sun does, but with far less mass and energy output. Most belong to the M spectral class, the coolest of the main stellar classes, and have masses ranging from roughly 0.08 to 0.6 times that of the Sun. Their surface temperatures are generally below about 3,900 K (3,600°C / 6,500°F) — still extraordinarily hot by everyday standards, but considerably cooler than the Sun’s roughly 5,800 K (5,500°C / 9,900°F). This lower temperature helps give red dwarfs their reddish appearance. Below about 0.08 solar masses, an object cannot sustain normal hydrogen fusion and is instead classified as a brown dwarf.
Most Stars in the Milky Way Are Red Dwarfs
Despite being too faint to see individually with the naked eye, red dwarfs are by far the most common type of star in the Milky Way. Astronomers estimate that they make up roughly three-quarters of the galaxy’s stars. Their apparent absence from the night sky is simply a consequence of their low luminosity: even nearby red dwarfs are easily outshone by much more distant, brighter stars. In fact, Proxima Centauri, the closest star to the Sun, is a red dwarf but is around 100 times too faint to be seen without a telescope.
A Red Dwarf Can Shine for Trillions of Years
Red dwarfs are extraordinarily long-lived because they consume their nuclear fuel much more slowly than larger stars. The smallest red dwarfs are also fully convective, continually circulating material between their cores and outer layers and allowing them to use a much greater proportion of their hydrogen. While the Sun is expected to spend about 10 billion years on the main sequence, the least massive red dwarfs could continue shining for trillions of years. In fact, the universe is only about 13.8 billion years old, so no red dwarf has yet had enough time to reach the natural end of its life.
The Smallest May Eventually Turn Into Blue Dwarfs
The future of the lowest-mass red dwarfs is especially unusual. Because they use their hydrogen so slowly and efficiently, they are not expected to swell into red giants like the Sun eventually will. Instead, models predict that after trillions of years they will gradually become hotter and more luminous, passing through a theoretical blue dwarf stage before ultimately ending their lives as white dwarfs. No blue dwarfs exist today because the universe is nowhere near old enough for even the earliest red dwarfs to have reached this stage.
Red Dwarfs Can Be Surprisingly Violent
Their small size and low luminosity can make red dwarfs sound like quiet stars, but many are magnetically very active. They can produce powerful stellar flares that suddenly release large amounts of energy, including ultraviolet and X-ray radiation. This is especially common among younger red dwarfs, although some remain active for billions of years. These outbursts can be far more intense relative to a star’s normal brightness than those produced by the Sun, making stellar activity one of the defining characteristics of many red dwarfs.
Their Planets Are Easier to Find
Red dwarfs have become important targets in the search for planets beyond our Solar System. Because these stars are relatively small and low in mass, an orbiting planet has a more noticeable effect on them than it would on a larger star, making planets easier to detect using methods such as transits and measurements of a star’s motion. Their lower energy output also means that their habitable zones are much closer to the star than the Sun’s. A planet around a red dwarf therefore needs to orbit relatively close to receive enough stellar energy for liquid water to potentially exist on its surface. Astronomers have discovered many planets around red dwarfs, including numerous rocky, roughly Earth-sized worlds.
Living Around a Red Dwarf Could Be Difficult
The close-in orbits that place planets within a red dwarf’s habitable zone can also create problems for life. Many such planets may become tidally locked, with the same side permanently facing their star, although an atmosphere or ocean could potentially redistribute heat between the day and night sides. More seriously, powerful stellar flares and high-energy radiation can threaten planetary atmospheres, particularly around young and highly active red dwarfs. This does not mean that red-dwarf planets cannot support life, but whether they can maintain habitable conditions remains an important open question in astronomy.
The TRAPPIST-1 System Has Seven Earth-Sized Planets
One of the most remarkable red dwarf systems known is TRAPPIST-1, located about 40 light-years away in the constellation Aquarius. This small, cool red dwarf is orbited by seven roughly Earth-sized rocky planets, several of which lie at distances where temperatures could potentially allow liquid water under the right atmospheric conditions. The planets orbit extremely close to their star — all seven would fit comfortably inside Mercury’s orbit around the Sun. TRAPPIST-1 has therefore become an important natural laboratory for studying what planets around red dwarfs are really like and whether any could maintain habitable environments.
The Closest Star to the Sun Is a Red Dwarf
Proxima Centauri is especially valuable to astronomers because it is both our nearest stellar neighbor and home to its own planetary system. With only about 12% of the Sun’s mass, it is the smallest member of the wider Alpha Centauri system and is known to host at least two confirmed planets. These include the roughly Earth-mass Proxima Centauri b, which lies within the star’s habitable zone and completes an orbit in just 11.2 days. Its combination of proximity, planets and powerful stellar flares gives astronomers a unique opportunity to study red dwarfs, their planetary systems and the challenges planets may face around these long-lived but active stars.
Small Stars With an Enormous Future
Red dwarfs may be faint and easy to overlook, but their extraordinary abundance and lifespans make them anything but insignificant. Long after stars like the Sun have exhausted their fuel, countless red dwarfs should still be shining across the cosmos. And with so many of them surrounded by planets, these small stars may remain important places to search for other worlds for an almost unimaginable length of time.