Our Sun is often described as an “average yellow dwarf star,” but there is enormous variety among stars. Their masses, temperatures, luminosities, sizes and ages can differ by extraordinary amounts, producing objects that push the known limits of stellar physics.
Identifying record holders is not always straightforward. A star’s calculated size or luminosity can change when astronomers obtain a better distance measurement, while massive stars can lose substantial amounts of material during their lifetimes. Some of the objects below are therefore better regarded as leading contenders rather than permanent record holders.
Largest Star Contender – Stephenson 2-18
Stephenson 2-18 is an enormous red supergiant in the direction of the constellation Scutum and is frequently cited as one of the largest stars known. Estimates of its radius place it among a small group of red supergiants and hypergiants whose outer layers would extend far beyond Earth’s orbit if they replaced the Sun.
There is an important catch, however. Determining the radii of very distant red supergiants is difficult because their distances, temperatures and surrounding material introduce substantial uncertainty. Stephenson 2-18 should therefore be treated as a leading contender for the largest known star, rather than an undisputed record holder.
Most Massive Star – R136a1
R136a1 lies in the extraordinarily dense R136 star cluster within the Tarantula Nebula in the Large Magellanic Cloud, about 160,000 light-years from Earth.
The star is one of the most massive ever measured. Estimates have placed its current mass at well over 200 times that of the Sun, while models indicate that it may have formed with more than 300 solar masses. R136a1 is losing material rapidly through an immensely powerful stellar wind, so its present mass is already substantially lower than its initial mass.
Stars this massive are extremely rare and short-lived. R136a1 is only around a million years old, yet it has already lost tens of solar masses of material.
Most Luminous Star – R136a1
R136a1 is remarkable for another reason: it is also among the most luminous stars known. Estimates place its energy output at several million times that of the Sun, with some calculations approaching ten million solar luminosities.
That tremendous output comes from the star’s exceptional mass and extremely high temperature. Massive stars consume their nuclear fuel at a furious rate, which is why a star such as R136a1 can shine millions of times more brightly than the Sun while surviving for only a tiny fraction of the Sun’s lifetime.
Smallest True Star – EBLM J0555-57Ab
At the opposite end of the stellar scale is EBLM J0555-57Ab, located roughly 600 light-years away. It is one of the smallest hydrogen-burning stars ever measured and has a radius comparable to that of Saturn.
Its mass is only about 8 percent that of the Sun, placing it very close to the lower limit at which an object can maintain hydrogen fusion in its core. Had it formed with slightly less mass, it would have become a brown dwarf rather than a true star.
The discovery illustrates an intriguing feature of stellar physics: a genuine star can actually be smaller in radius than some giant planets while containing many times more mass.
One of the Hottest Known Stars – WR 102
WR 102 in Sagittarius is an oxygen-sequence Wolf-Rayet star and one of the hottest stars known. Its temperature is estimated at around 200,000 K, compared with about 5,800 K at the Sun’s visible surface.
Despite that extraordinary temperature, WR 102 is physically quite compact. It is an evolved massive star that has already shed its outer hydrogen-rich layers, exposing intensely hot material that was once deep inside the star.
Wolf-Rayet stars of this type are rare and represent a brief late stage in the lives of massive stars. WR 102 is expected eventually to end its life in a core-collapse supernova, although predicting exactly when that will happen is not possible.
Hypervelocity Star – HE 0437-5439
HE 0437-5439 is a massive hypervelocity star associated with the region around the Large Magellanic Cloud. Measurements show it traveling at hundreds of kilometers per second, making it a striking example of a star moving fast enough to challenge the gravitational hold of a galaxy.
It was once widely described as the fastest known star, but that title is no longer useful as an absolute record. Astronomers have since discovered other extreme high-velocity stars, while stars orbiting close to the supermassive black hole at the center of the Milky Way can attain even greater speeds during parts of their orbits.
Hypervelocity stars are scientifically valuable because their trajectories can reveal how violent gravitational encounters, binary systems and massive black holes can accelerate stars to extraordinary velocities.
Fastest-Rotating Massive Star – VFTS 102
VFTS 102 lies in the Large Magellanic Cloud near the Tarantula Nebula and is one of the most rapidly rotating massive stars ever discovered.
Its equatorial rotation speed is roughly 600 km/s, or more than 2 million km/h. That is close to the point at which centrifugal effects become so strong that a massive star risks losing material from its equator.
VFTS 102 is especially interesting because such extreme rotation may have resulted from interaction with a former binary companion. Astronomers have investigated whether the star was spun up by receiving material from a companion before that companion exploded as a supernova.
Among the Reddest Naked-Eye Stars – Mu Cephei and CE Tauri
Color is another way in which stars can reach extremes. Two particularly striking naked-eye examples are Mu Cephei in Cepheus and CE Tauri in Taurus.
Mu Cephei is traditionally known as Herschel’s Garnet Star because of its deep orange-red appearance, while CE Tauri is sometimes called the Ruby Star. Both are cool, evolved red supergiants whose relatively low surface temperatures shift much of their visible output toward the red end of the spectrum.
A star’s color can be quantified using its color index, although variability, interstellar dust and the method of measurement complicate attempts to declare a permanent “reddest star.” These two nevertheless rank among the most intensely colored stars accessible to naked-eye observers.
One of the Oldest Known Milky Way Stars – HE 1523-0901
HE 1523-0901 is an extremely metal-poor star in the Milky Way’s halo and one of the oldest stars for which astronomers have obtained a detailed age estimate.
Using ESO’s Very Large Telescope, astronomers measured radioactive uranium and thorium in the star and compared their abundances with other elements. This form of stellar radioactive dating produced an estimated age of about 13.2 billion years.
That means HE 1523-0901 formed relatively soon after the Big Bang, making it a surviving fossil from the early history of the Milky Way. Its extremely low abundance of heavy elements also reflects an era when relatively few previous generations of stars had enriched the galaxy with heavier material.
Most Distant Star – Earendel
The most distant individual star yet detected is Earendel, formally designated WHL0137 LS. Its light has taken about 12.9 billion years to reach Earth, allowing astronomers to see the star as it existed when the universe was only a small fraction of its present age.
Even an exceptionally luminous star would normally be impossible to detect individually at such a distance. Earendel became visible because a massive galaxy cluster between Earth and the star bends and magnifies its light through gravitational lensing.
The Hubble Space Telescope first identified Earendel, and subsequent observations with the James Webb Space Telescope provided astronomers with additional information about the star and its surroundings. Its discovery pushed observations of individual stars far deeper into cosmic history than had previously been possible.