Any star roughly eight or more times the mass of our Sun has enough gravitational pressure to eventually collapse and explode as a supernova. That covers a small percentage of all stars, but several well-known ones in our night sky fit the bill. The two most famous candidates are Betelgeuse in Orion and Eta Carinae in the southern sky, though a completely different type of supernova can also come from much smaller stars under the right conditions.
What Makes a Star Supernova Material
Stars spend most of their lives fusing hydrogen into helium. Massive stars, those above about eight solar masses, keep going after the hydrogen runs out. They fuse helium into carbon, carbon into neon, and so on, building heavier and heavier elements in layers like an onion. This process ends at iron. Iron fusion doesn’t release energy; it absorbs it. Once a massive star builds an iron core, it loses the outward pressure that held it up against gravity, and the core collapses in a fraction of a second. The outer layers slam inward, bounce off the ultra-dense core, and blast outward in a core-collapse supernova (often called a Type II).
Stars below about eight solar masses, including our Sun, don’t generate enough pressure to reach that iron dead end. They shed their outer layers gently and fade into white dwarfs. But white dwarfs can still explode under special circumstances, which is the basis for a second type of supernova covered below.
Betelgeuse: The Most Watched Star in the Sky
Betelgeuse is the bright reddish star marking Orion’s shoulder, sitting about 500 light-years from Earth. It weighs between 15 and 20 times the mass of the Sun and has swollen so dramatically that if you placed it where our Sun is, its surface would stretch out to roughly the orbit of Mars. It’s a red supergiant, which means it has already burned through its core hydrogen and is working through heavier fuels.
Estimates based on its mass, rotation rate, chemical composition, and the age of the star cluster it was born with suggest Betelgeuse will go supernova sometime in the next few hundred thousand years. That’s a blink in astronomical terms, though “a few hundred thousand years” is obviously not imminent on a human scale. The honest caveat is that it could also happen tomorrow. Astronomers can’t pin down the exact moment because they can’t directly observe what’s happening in the core.
When Betelgeuse does explode, it will briefly rival the full Moon in brightness and be visible during the day. At 500 light-years, though, it poses no danger to Earth (more on safe distances below). It is considered the closest potential core-collapse supernova candidate.
Eta Carinae: A Far More Violent Prospect
Eta Carinae is a massive, unstable star system in the southern constellation Carina, roughly 7,500 light-years away. Its primary star tips the scales at over 100 solar masses, making it one of the most massive stars known in our galaxy. Stars this heavy live fast and die young, burning through their fuel in just a few million years.
What makes Eta Carinae especially interesting is its instability. In the 1840s it erupted in a giant outburst, briefly becoming the second-brightest star in the sky and ejecting huge lobes of gas that are still expanding today. It has continued to flare unpredictably since. Once a star this massive begins fusing carbon in its core, the intense gamma rays produced can create pairs of matter and antimatter particles, robbing the core of the pressure it needs to stay stable. This process can trigger a catastrophic collapse that would produce not just a supernova but potentially a hypernova, an explosion many times more energetic than a standard core-collapse event.
At 7,500 light-years, Eta Carinae is also far enough away to be safe for Earth, though it would put on an extraordinary light show.
Type Ia Supernovae: When Small Stars Explode
Not every supernova comes from a giant star. A white dwarf, the dense remnant of a smaller star, can also explode if it has a companion star orbiting close enough. In these binary systems, the white dwarf’s gravity slowly pulls material off its companion. As mass piles on, the white dwarf approaches a critical threshold of about 1.4 solar masses, known as the Chandrasekhar limit. At that point, the compressed carbon and oxygen in the white dwarf ignite in a runaway thermonuclear explosion that completely destroys the star.
These Type Ia supernovae release enormous energy, about 2 × 10⁵¹ ergs, converting nearly the entire mass of the white dwarf into heavier elements in an instant. They’re actually brighter than most core-collapse supernovae and are so consistent in their peak brightness that astronomers use them as “standard candles” to measure cosmic distances.
The closest known Type Ia candidate is IK Pegasi, a binary system about 150 light-years from Earth. Its white dwarf companion is gradually gaining mass, but it’s still well below the explosion threshold. It won’t pose any threat for millions of years, and by that time the system will have drifted much farther from us.
How Close Is Too Close
A supernova would need to be within about 30 to 50 light-years of Earth to trigger a mass extinction through cosmic rays, which can penetrate the atmosphere and bombard the surface with dangerous radiation for thousands of years. Closer still, within 15 to 20 light-years, a supernova’s initial blast could strip away 30 to 50 percent of the ozone layer, letting our own Sun’s ultraviolet radiation do the real damage to surface life during the long recovery period.
In special cases where the exploding star sits inside a thick region of surrounding gas, the aftermath can radiate X-rays for hundreds to thousands of years. That scenario extends the danger zone out to roughly 150 light-years. Fortunately, no known supernova candidates are anywhere near these thresholds. Betelgeuse at 500 light-years and Eta Carinae at 7,500 light-years are both well outside the kill zone. A supernova close enough to directly threaten Earth is estimated to occur on the order of once per billion years.
How We’d Know It’s Coming
The final stage of a massive star’s life, from the onset of silicon burning in the core to total collapse, takes only about a day. But we wouldn’t need to watch the star itself. When a star’s core collapses, the first signal to escape is a burst of neutrinos, ghostly particles that pass through the outer layers of the star almost instantly. These neutrinos would reach Earth hours before any visible light from the explosion. Detectors like IceCube at the South Pole are set up to catch exactly this kind of burst and send an automatic alert to astronomers worldwide, giving them time to point every available telescope at the right patch of sky.
For the stars we can observe directly, there are also surface-level clues. Betelgeuse’s dramatic dimming in late 2019 and early 2020 sparked widespread speculation, though it turned out to be caused by a cloud of dust expelled from the star’s surface rather than any sign of imminent collapse. Still, changes in a star’s brightness, pulsation patterns, or chemical signatures can all hint at what’s happening deeper inside, even if they can’t tell us the exact timeline.