Will Betelgeuse's supernova affect Earth?

No. At several hundred light-years it is far too distant to harm life. The explosion would be a spectacular sight, briefly rivalling the half Moon in brightness.

Why did Betelgeuse dim in 2019 and 2020?

The star ejected a large bubble of gas, which cooled and formed a cloud of dust that partly blocked its light. A temporarily cooler region on its surface helped the dust to condense.

How big is Betelgeuse compared with the Sun?

Its radius is roughly 700 to 1,000 times the Sun's. If it sat at the centre of the Solar System, its surface would reach beyond the orbit of Mars.

Does Betelgeuse have a companion star?

Probably. A faint companion nicknamed Siwarha was tentatively imaged in 2025, and in early 2026 astronomers reported evidence of its wake in Betelgeuse's atmosphere. Confirmation is expected when it is easiest to see again in late 2027.

Orion's red shoulder, a giant nearing its end

Betelgeuse

Red supergiant · ≈ 550–700 ly

Betelgeuse is the bright orange-red star marking the shoulder of Orion, the hunter. It is a red supergiant, one of the largest stars visible to the naked eye. Placed where the Sun is, its swollen outer layers would reach beyond the orbit of Mars and perhaps as far as Jupiter. Its distance is hard to pin down, but most estimates fall between about 550 and 700 light-years.

Betelgeuse is a massive star in the last stages of its life. Within roughly the next 100,000 years its core is expected to collapse, triggering a supernova bright enough to be seen in daylight from Earth. Between late 2019 and early 2020 it dimmed dramatically, a puzzle that turned out to be caused by a cloud of dust.

Key facts

Distance: ≈ 550–700 light-yearsUncertain; a 2020 study favoured about 550
Radius: ≈ 700–1,000 × the SunEstimates vary with method
Mass: ≈ 16–19 solar masses
Surface temperature: ≈ 3,500 K≈ 3,200°C
Luminosity: ≈ 100,000 × the SunRange of estimates roughly 90,000–150,000
Spectral type: M1–M2 Ia–abRed supergiant
Age: ≈ 8–10 million years
Apparent magnitude: ≈ 0.0 to 1.6Variable; faintest during the Great Dimming
Constellation: Orionα Orionis, the hunter's shoulder

A star of enormous size

Betelgeuse is so large and close that it was one of the first stars other than the Sun whose disc could be measured directly. Its radius is thought to be several hundred to about a thousand times the Sun's, with many studies settling near 750. Yet the star is surprisingly tenuous: its outer layers are thinner than the air we breathe, and its edge is not sharp but fades into an extended atmosphere of gas and dust.

The surface of Betelgeuse is dominated by a few gigantic convection cells, each comparable in size to a large fraction of the star, that churn hot gas up from the interior. These cells, together with pulsations lasting around 400 days and a longer cycle of roughly 2,100 days, make the star's brightness vary. Betelgeuse is also losing mass in a steady wind, enriching its surroundings with material that will help form future stars and planets.

The red giant and supergiant stage

Every star spends most of its life fusing hydrogen into helium in its core. When that core hydrogen runs out, the core contracts and heats up while hydrogen keeps burning in a shell around it. The extra energy makes the outer layers swell enormously and cool, so the star grows larger and redder. Stars like the Sun become red giants this way, while stars of more than about eight solar masses become red supergiants such as Betelgeuse.

In a massive star the contracting core grows hot enough to fuse helium into carbon and oxygen, then heavier elements in turn: neon, oxygen, silicon and finally iron. Each stage is faster than the last. Fusing iron releases no energy, so once an iron core builds up, nothing can hold it against gravity. It collapses in less than a second, and the rebound blows the star apart as a core-collapse supernova.

Where Betelgeuse is now

Models suggest Betelgeuse is currently fusing helium in its core, a phase that lasts a few hundred thousand years for a star of its mass. A 2020 study estimated that it has roughly 100,000 years left before its core collapses. It will probably leave behind a neutron star rather than a black hole.

The future supernova

When Betelgeuse explodes, it will shine for weeks about as bright as a half Moon, visible in daylight, before fading over months. At its distance it is far too remote to harm life on Earth. Afterwards Orion will lose its shoulder, and a glowing supernova remnant will slowly spread across the area.

The Great Dimming of 2019–2020

In late 2019 Betelgeuse began to fade far more than usual. By February 2020 it had lost well over half of its normal visible brightness, the deepest drop in more than a century of careful records. The change was obvious even to the naked eye, and it prompted speculation that the star was about to explode.

Observations with the Hubble Space Telescope and ESO's Very Large Telescope solved the mystery. Hubble saw a large outflow of hot, dense gas leaving the star's southern hemisphere in late 2019. As this material moved outwards and cooled, it condensed into a cloud of dust that blocked part of the star's light. A cool patch on the surface, caused by the star's convection, helped the dust to form. Betelgeuse recovered its usual brightness within months.

A hidden companion: Siwarha

For decades astronomers suspected that Betelgeuse's roughly 2,100-day brightness cycle was caused by a small companion star orbiting just outside its outer layers. In 2025 a team led by Steve Howell of NASA's Ames Research Center reported a probable direct detection using speckle imaging on the Gemini North telescope. The faint source appeared close to the predicted position, about six magnitudes fainter than Betelgeuse, with an estimated mass of roughly one and a half Suns. The team proposed the name Siwarha, meaning 'her bracelet'.

Because the detection was at the limit of what the instrument could measure, it was treated as tentative. Supporting evidence arrived in early 2026, when a team led by Andrea Dupree used years of Hubble and ground-based spectra to detect a trail of disturbed gas, a wake, swept through Betelgeuse's atmosphere by the orbiting companion. The name remains unofficial until the International Astronomical Union approves it. The companion should next be at its widest separation from Betelgeuse in late 2027.

Name and observation history

The name Betelgeuse comes from medieval Latin renderings of an Arabic phrase usually interpreted as 'the hand of al-Jawzā', a figure associated with Orion. Its variability was noted by John Herschel in the 1830s. In 1920 Albert Michelson and Francis Pease used an interferometer on the Mount Wilson telescope to measure its angular size, the first such measurement of any star beyond the Sun.

In 1996 the Hubble Space Telescope produced the first direct image of the surface of a star other than the Sun, showing Betelgeuse as a resolved disc. Since then radio telescopes such as ALMA and infrared interferometers have mapped its atmosphere and its slow rotation. The star is also moving rapidly through space, ploughing a bow shock into the surrounding gas.

Surprising facts about Betelgeuse

If Betelgeuse replaced the Sun, Earth would orbit deep inside the star.
Its outer layers are so thin that they are less dense than Earth's atmosphere.
Betelgeuse was the first star other than the Sun to have its angular size measured, in 1920.
When it explodes as a supernova, it should be visible in broad daylight for weeks.
Its suspected companion appears to orbit within a few times Betelgeuse's own radius, inside its extended atmosphere.

Frequently asked questions

When will Betelgeuse explode?: Nobody can give a date. Current models suggest Betelgeuse has roughly 100,000 years left before its core collapses, so an explosion in our lifetime is possible but very unlikely.
Will Betelgeuse's supernova affect Earth?: No. At several hundred light-years it is far too distant to harm life. The explosion would be a spectacular sight, briefly rivalling the half Moon in brightness.
Why did Betelgeuse dim in 2019 and 2020?: The star ejected a large bubble of gas, which cooled and formed a cloud of dust that partly blocked its light. A temporarily cooler region on its surface helped the dust to condense.
How big is Betelgeuse compared with the Sun?: Its radius is roughly 700 to 1,000 times the Sun's. If it sat at the centre of the Solar System, its surface would reach beyond the orbit of Mars.
Does Betelgeuse have a companion star?: Probably. A faint companion nicknamed Siwarha was tentatively imaged in 2025, and in early 2026 astronomers reported evidence of its wake in Betelgeuse's atmosphere. Confirmation is expected when it is easiest to see again in late 2027.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources