A small star can outlive the Sun by hundreds of times
Models give Proxima Centauri a remaining main-sequence lifetime near four trillion years because low-mass red dwarfs consume hydrogen slowly.
Scale Guide
Stars to scale
Explore the scaleStellar diameters span red dwarfs, main-sequence stars, expanded giants and unstable supergiants whose outer boundaries can be difficult to define.
Shared scale
From red dwarfs to red supergiants
Shared physical scale
1.44billion km
A star’s diameter depends on mass, evolutionary state, wavelength and the definition of its photosphere. Low-mass dwarfs consume fuel slowly; evolved stars can expand far beyond their main-sequence size; the most extended stars lose material into winds and dusty surroundings that blur any single outer edge.
Proxima Centauri is about a seventh of the Sun’s diameter; the largest estimates here exceed a thousand times it. Stars that big have swollen late in life: even the most massive main-sequence stars are at most a few dozen times wider than the Sun.
Small stars
The long-lived majority sits below the Sun
Proxima Centauri, Lalande 21185 and 61 Cygni A are cooler and smaller than the Sun. Luhman 16 B is a brown dwarf below the hydrogen-fusion threshold. Red dwarfs like Proxima are the most common stars in the Milky Way, roughly three of every four.
Shared physical scale
1.14million km
Middle scale
Stars near the Sun’s size can be at different life stages
The Sun and Sirius A are main-sequence stars with different masses and temperatures; Procyon A is a subgiant beginning to expand after core hydrogen depletion. Sirius A’s brightness in our sky reflects both its luminosity and its distance of only 8.6 light-years.
Giants
Stars of modest mass can swell into giants
Pollux, Arcturus and Aldebaran expanded after exhausting hydrogen in their cores. Their cooler outer layers cover much more area than the Sun’s, although these stars began with only about one to two solar masses.
Massive stars
Hot giants and supergiants
Spica A is a hot blue giant of about eleven solar masses, yet only about seven times the Sun’s diameter. Polaris Aa is a yellow supergiant about 46 times the Sun’s width; Rigel and Deneb are blue-white supergiants about 74 and 200 times wider.
Shared physical scale
181million km
Extreme stars
At the upper end, a star's edge is hard to define
Eta Carinae A is embedded in a dense wind; Betelgeuse pulsates and loses material; VY Canis Majoris is surrounded by asymmetric ejecta; the sizes of Stephenson 2 DFK 49 and Stephenson 2-18 depend strongly on distance and atmospheric models. Their quoted diameters are useful estimates, not a secure ranking of hard-edged spheres.
Shared physical scale
2.37billion km
Scale anchors
Worth remembering
Betelgeuse would swallow Mars
Placed where the Sun is, Betelgeuse would extend well past the orbit of Mars, out to about the far edge of the asteroid belt.
Giants are usually ordinary stars grown old
Pollux is about 9 times the Sun’s diameter, Arcturus about 25 and Aldebaran about 45, yet none began with much more than two solar masses.
The North Star helps measure the universe
Polaris Aa is the nearest classical Cepheid, a pulsating class used to calibrate astronomical distances.
The largest estimate reaches Saturn
A star of Stephenson 2-18’s estimated size, placed at the Sun’s position, would reach about as far as Saturn’s orbit.
Profiles
Stars across the full diameter range
Quoted diameters, stellar classes and measurement caveats from brown dwarfs to mass-losing supergiants.
Luhman 16 B is the cooler T-type member of the nearest known brown-dwarf binary system. It is massive enough to glow from its own heat but too light to sustain ordinary hydrogen fusion, with patchy clouds that make its brightness change as it rotates.
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Proxima Centauri is the nearest known star to the Sun, just over four light-years away. ESA describes it as a flare star prone to dramatic brightening, yet because it burns fuel so slowly it may remain on the main sequence for another four trillion years.
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Lalande 21185 is a nearby red dwarf in Ursa Major, also cataloged as Gliese 411 and HD 95735. It is the brightest red dwarf in the northern celestial hemisphere, yet it is still too dim for unaided eyes and is better known through telescopes, surveys, and its nearby planetary system.
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61 Cygni A is a nearby orange dwarf star in one of the sky’s most famous binary systems. The 61 Cygni pair became the first stellar system with a measured parallax, and 61 Cygni A is also a standard reference star for the K5 V spectral class.
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The Sun is the star at the center of the Solar System and the source of almost all the energy reaching its planets. It fuses about 600 million metric tons of hydrogen each second, and energy released in the core takes on the order of 100,000 years to work its way out to the visible surface.
Visual creditSolar Dynamics Observatory 2024 / NASA / SDO / HMI science team / Public domainSource: Wikimedia Commons
Sirius A is the brightest star in Earth’s night sky and the luminous primary of the Sirius binary. The same system also contains Sirius B, a white dwarf.
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Procyon A is the bright F-type primary of the Procyon binary and has begun to swell beyond an ordinary main-sequence star. Its faint companion, Procyon B, is already a white dwarf.
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Pollux is the brightest star in Gemini and the nearest giant star to the Sun. It has already left the main sequence and swelled into an orange giant, making it a useful nearby preview of what a Sun-like star looks like after exhausting core hydrogen.
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Arcturus is a bright orange giant and the brightest star in the northern celestial hemisphere. It also races unusually fast across the sky for such a bright star, shifting by about two arcseconds per year because of its high motion relative to the Sun.
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Aldebaran is a red giant branch star in Taurus with a cool orange surface and a diameter about 45 times the Sun’s. It lies in front of the Hyades cluster by chance, so it appears to sit among the bull’s-head stars without actually belonging to that group.
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Spica A is the hot blue primary in the Spica binary and a Beta Cephei variable near the end of its main-sequence life. Its rapid rotation and close companion distort the star into an ellipsoidal shape, so the unresolved system brightens and dims slightly as the stars orbit.
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Polaris Aa is the yellow supergiant at the heart of the North Star system and the nearest classical Cepheid to Earth. Cepheids are standard candles for measuring cosmic distances, which makes this nearby example an important calibrator.
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Rigel is the blue-white supergiant that marks Orion’s foot. Although it looks like a single star to the eye, it is the luminous primary of a multiple-star system.
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Deneb is the blue-white supergiant at one corner of the Summer Triangle and among the most luminous stars visible to the naked eye. It is also the prototype of the Alpha Cygni variables, whose surfaces pulse in multiple overlapping rhythms.
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Eta Carinae A is the luminous blue variable primary in the Eta Carinae system, buried inside the Carina Nebula. Its dense wind makes the star’s surface hard to define, so this record uses an approximate visible stellar disk rather than a clean hydrostatic radius.
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Betelgeuse is a red supergiant nearing the end of its stellar life. Its dramatic dimming in 2019 and 2020 turned out to be caused by a dusty veil ejected from the star, giving astronomers a rare direct look at how giant stars shed material into space.
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Stephenson 2 DFK 49 is an extreme red supergiant whose quoted size remains highly uncertain because its outer layers are diffuse and difficult to define. Even so, plausible estimates make it so enormous that its photosphere could approach or even engulf Jupiter’s orbit if placed at the center of the Solar System.
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VY Canis Majoris is an extreme red hypergiant wrapped in a dense, asymmetric cloud of its own ejecta. Much of its light reaches us in the infrared after being absorbed and re-emitted by that dust.
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Stephenson 2-18 is a very cool, luminous red supergiant candidate in the direction of the Stephenson 2 stellar association. It has been given one of the largest published stellar size estimates, but its exact diameter remains uncertain because the star is distant, dust-affected, and difficult to separate cleanly from its surrounding region.
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If the Sun were a bead one centimeter wide, Proxima Centauri would be a grain about 1.5 millimeters across, Aldebaran a ball about 45 centimeters wide and Stephenson 2-18 a sphere about 21 meters across.
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About
Scale of Space is a scroll-based journey through the universe, placing objects on a single logarithmic scale so you can compare size across an unbroken range.
Guides explain selected parts of that range, while focused views filter the objects by subject.