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The cosmos is filled with stars, each one distinct. The variety is truly mind-blowing. Here’s a look at some of the most extreme stars in the observable universe.
10. The Stars That Live the Longest
How long does a star survive? For this, we define a star's lifespan by its duration of nuclear fusion, as the remnants of a star can persist long after its nuclear reactions cease.
Stars with smaller masses tend to have longer lifespans. The least massive stars are the red dwarfs, with masses ranging from 7.5 to 50 percent of the Sun’s. Anything lighter wouldn't be able to sustain nuclear fusion, which is what makes a star. According to current models, the tiniest red dwarfs could undergo fusion for as long as 10 trillion years, far surpassing the Sun's fusion lifespan of around 10 billion years—1,000 times shorter. After most of their hydrogen is fused, these red dwarfs are predicted to become blue dwarfs. Once they run out of hydrogen, their core fusion ceases, transforming them into white dwarfs.
9. The Most Ancient Stars
The oldest stars are those formed just after the Big Bang, roughly 13.8 billion years ago. Astronomers can estimate a star's age by analyzing its light, which reveals the proportions of elements like hydrogen, helium, and lithium it contains. Older stars typically have high amounts of hydrogen and helium, with much less of the heavier elements.
The oldest known visible star is SMSS J031300.36-670839.3, discovered and published in February 2014. Estimated to be 13.6 billion years old, it’s not one of the first stars from the beginning of the universe. None of those original stars have been found yet, but some may still exist. Red dwarfs, after all, can live for trillions of years. If any of those ancient stars are out there, they would be extremely rare, so searching for them would be a monumental challenge.
8. The Least Bright Stars
What defines the dimmest stars? To answer this, we need to clarify what we mean by 'dimmest.' The further away a star is, the less bright it appears. To eliminate this effect, we focus on measuring luminosity—the total energy emitted by the star in the form of photons (light particles).
When considering only stars still undergoing fusion, red dwarfs have the lowest luminosity. The faintest star with the lowest known luminosity is the red dwarf 2MASS J0523-1403. Any star with even less brightness would fall into the category of brown dwarfs, which aren't technically stars at all.
Then there are stellar remnants: white dwarfs, neutron stars, and black holes. Just how dim can they be?
White dwarf stars emit some light, but they gradually cool as time passes. After an extended period, they transform into cold, carbon-rich remnants emitting almost no light at all—becoming 'black dwarfs.' Since it takes an incredibly long time for white dwarfs to cool down, there are no black dwarfs yet.
Astrophysicists are unsure about the fate of matter in cooling neutron stars. By studying supernovae in distant galaxies, they estimate that several hundred million neutron stars have formed in our galaxy. However, they can only detect a small fraction of them. The rest must have cooled to the point of being nearly invisible.
What about black holes floating in the vastness of intergalactic space, with no nearby objects orbiting them? These black holes would still emit a minimal amount of radiation, called Hawking radiation, but it would be almost undetectable. Such isolated black holes would be the faintest of stellar remnants. Do they exist? Maybe.
7. The Brightest Stars
The brightest stars are typically the most massive. They are often Wolf-Rayet stars, known for their high temperatures and powerful stellar winds that eject significant amounts of mass. These stars don't last long—they burn bright and burn fast.
The star currently recognized as the most luminous (and most massive) is R136a1. Announced in 2010, this Wolf-Rayet star boasts a luminosity about 8.7 million times that of the Sun and a mass roughly 265 times that of our Sun. Due to its mass loss, it once had a mass as high as 320 Suns.
R136a1 is part of a dense stellar cluster known as R136. As Paul Crowther, one of its discoverers, stated, 'Planets take longer to form than these stars take to live and die. Even if there were planets, there would be no astronomers on them because the night sky would be almost as bright as the day in these clusters.' This is a scenario that Isaac Asimov envisioned in his 1941 short story 'Nightfall.'
6. The Biggest Stars
Although R136a1 has an enormous mass, it isn't the largest star in terms of size. Many stars are far larger, and they are all red supergiants. These stars spent most of their lives as smaller versions of themselves, only expanding when they ran out of hydrogen to fuse, began fusing helium, became hotter, and grew larger. Our Sun will eventually run out of hydrogen and swell up, but it will become a red giant, not a red supergiant. To reach the red supergiant phase, a star must be at least ten times the mass of the Sun. This phase is short-lived, lasting anywhere from a few thousand to a billion years—brief by stellar standards, though not by human ones.
The most famous red supergiants are Antares A and Betelgeuse, but they’re relatively small compared to the largest red supergiants. Determining the exact size of the largest red supergiants is tricky because their sizes are hard to measure accurately. The largest can be up to 1,500 times the diameter of the Sun, or possibly even larger.
5. The Stars with the Most Explosive Luminosity
The most energetic photons are gamma rays, which are produced in nuclear explosions. To detect these, the United States launched the Vela satellites to track gamma rays from Soviet nuclear tests. In July 1967, the satellites picked up a gamma ray burst (GRB) that didn’t resemble the kind produced by a nuclear explosion. Many more GRBs were detected after that, and they were found to be incredibly brief, lasting from mere milliseconds to several minutes. Despite their short duration, they were far brighter than the most luminous stars, though their source wasn’t on Earth.
What causes gamma ray bursts (GRBs)? Many theories exist, but most today are believed to come from the explosive deaths of massive stars (supernovae or hypernovae) on their way to becoming neutron stars or black holes. Some GRBs are thought to be produced by magnetars, a type of neutron star with an exceptionally powerful magnetic field. Others might occur when two neutron stars merge, or when a neutron star falls into a black hole.
4. The Most Mind-Blowing Former Stars
Black holes are not technically stars—they’re the remnants of stars—but comparing them to stars emphasizes just how extraordinary they are.
A black hole forms when a star’s gravity becomes so intense that it overpowers all other forces, causing the star to collapse to a point mass. With mass but no volume, it theoretically possesses infinite density, although that's just because we don't yet have a complete theory for what happens inside a black hole.
Black holes can be incredibly massive. Some of the black holes found at the centers of galaxies weigh tens of billions of times the mass of the Sun. Additionally, the matter swirling around supermassive black holes can shine with a brilliance that outshines all the stars in the galaxy combined. In some cases, there are powerful jets of matter blasting from the region near a black hole, moving at nearly the speed of light.
3. The Most Uncommon Stars
Some types of stars are truly peculiar. They may not hold extreme values for traits like luminosity or mass, but they are simply odd in other ways.
Thorne-Zytkow objects are a bit out of the ordinary. Named after physicists Kip Thorne and Anna Zytkow, who first proposed their existence, these objects form when a neutron star falls into the core of a red giant or supergiant. Sounds far-fetched, doesn’t it? Yet, one has recently been discovered.
Sometimes, two massive yellow stars can orbit so closely that matter flows between them, creating a structure resembling a giant cosmic peanut. Currently, only two such systems are known to exist.
Przybylski’s Star is often cited as an example of an oddball star due to its unique starlight. Astronomers usually analyze the intensity of different wavelengths to determine a star’s composition, a task that’s typically straightforward. However, scientists are still puzzled by the spectrum of Przybylski’s Star.
2. The Most Unpredictable Stars
A large number of stars experience significant fluctuations in their brightness as observed from Earth. These stars are classified as variable stars. There are thousands of them, with the General Catalogue of Variable Stars listing more than 45,000 within our Milky Way galaxy alone.
According to astrophysicist Coel Hellier, the most extreme fluctuations in brightness occur in cataclysmic variable (CV) stars. These stars can brighten by up to 100 times in less than a day, only to fade, then brighten again, in a repeating cycle. This constant change in luminosity makes CVs a favorite among amateur astronomers.
We now have a clearer understanding of CVs: they are binary stars, with one being a regular star and the other a white dwarf. The regular star sheds material onto a disk orbiting the white dwarf. Once the disk accumulates enough mass, fusion begins, causing the brightness to spike. This surge doesn’t last, however, as the fusion fades and the cycle starts anew. Occasionally, the white dwarf may even be destroyed in the process.
1. The Fastest-Moving Stars
In 2005, Warren Brown and his colleagues at the Harvard-Smithsonian Center for Astrophysics revealed the discovery of a star traveling at such a high velocity that it will escape the Milky Way galaxy forever. Officially named SDSS J090745.0+024507, Brown refers to it as 'the outcast star.'
Since then, other fast-moving stars have been identified and are known as hypervelocity stars. By May 2014, 20 such stars had been discovered, with many of them appearing to originate from the galaxy's center. One theory suggests that a binary star system ventured too close to the black hole at the galaxy's core, resulting in one star being captured while the other was ejected at an incredible speed.
Some stars seem to be moving even faster. Generally, the farther a star is from our galaxy, the faster it appears to be moving away from us. This apparent speed is due to the expansion of the universe rather than the star's actual motion through space.
