Proxima Centauri has a radius roughly 14% that of the Sun, making it only slightly larger than Jupiter in physical size. Its mass is about 12% of the Sun’s. These numbers come from measurements tied to the star’s orbit within the Alpha Centauri triple-star system and refined by parallax data, and they place Proxima firmly in the category of red dwarfs, the smallest hydrogen-fusing stars in the galaxy.1The Astrophysical Journal. Three-dimensional Climate Simulations for the Detectability of Proxima Centauri b Despite being our nearest stellar neighbor at just 4.24 light-years away, Proxima is so faint that it was not discovered until 1915, and you still cannot see it without a telescope.
Putting the Size Difference in Perspective
A radius of 0.141 times the Sun’s means Proxima Centauri’s diameter is about 194,000 kilometers. The Sun’s diameter is roughly 1.39 million kilometers. If you shrank the Sun to the size of a front door, Proxima would be about the size of a softball. Jupiter, for comparison, has a radius of about 0.10 solar radii, so Proxima is only modestly larger than the biggest planet in our own solar system. The difference is that Proxima sustains hydrogen fusion in its core and Jupiter does not, which is why one is a star and the other is a planet.
That size gap has real consequences for nearly everything about how the star behaves. A star’s luminosity, surface temperature, internal structure, lifespan, and the kind of environment it creates for any orbiting planets all scale with its mass and radius. Proxima Centauri sits near the bottom of the stellar main sequence, which means it operates under physical rules that differ from the Sun in ways that go well beyond just being smaller.
Mass and What It Means for How the Star Works
Proxima Centauri’s mass is estimated at about 0.120 solar masses, give or take roughly 0.015 solar masses.1The Astrophysical Journal. Three-dimensional Climate Simulations for the Detectability of Proxima Centauri b That makes it roughly one-eighth as massive as the Sun. For a star, mass is the single most important number. It determines how hot and dense the core becomes, which in turn sets the rate of nuclear fusion and therefore the star’s brightness and color.
With so little mass pressing inward, Proxima’s core is much cooler and less pressurized than the Sun’s. The result is a surface temperature of only about 3,000 Kelvin, compared to the Sun’s roughly 5,800 Kelvin. That lower temperature shifts the star’s light output heavily toward the red and infrared end of the spectrum. To human eyes, Proxima would look dim and deeply red, almost orange-red, rather than the yellowish-white we see when we look at the Sun.
Astronomers have also measured Proxima Centauri’s mass through gravitational microlensing, a technique where the star’s gravity bends the light of a more distant background star as Proxima passes in front of it. The amount of bending reveals the foreground star’s mass independently of orbital calculations.2Monthly Notices of the Royal Astronomical Society. The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event These independent measurements are valuable because they provide a check on the estimates derived from the Alpha Centauri system’s orbital dynamics.
A Completely Different Interior
The Sun has a layered internal structure: a radiative core where fusion happens, surrounded by a radiative zone where energy moves outward as photons, and then a convective zone in the outer third where hot plasma physically churns upward and cooler plasma sinks back down. This two-zone arrangement is typical for stars above roughly a third of the Sun’s mass.
Proxima Centauri, at 12% of the Sun’s mass, falls well below that threshold. Stars this small are believed to be fully convective, meaning the churning, boiling motion extends from the surface all the way down to the core. There is no radiative zone and no tachocline, the boundary layer between the radiative and convective zones that plays a key role in generating the Sun’s magnetic field.3The Astrophysical Journal Letters. MAGNETIC CYCLES IN A DYNAMO SIMULATION OF FULLY CONVECTIVE M-STAR PROXIMA CENTAURI
This matters because it changes how efficiently the star uses its fuel. In the Sun, only the hydrogen in the core is hot enough to fuse. The rest of the Sun’s hydrogen just sits in the outer layers and never participates. In a fully convective star like Proxima, material from the outer layers gradually circulates down to the core, so the star can eventually burn through a much larger fraction of its total hydrogen supply. That is one reason why small red dwarfs live so extraordinarily long.
A Lifespan Dwarfing the Sun’s
The Sun is roughly halfway through its estimated 10-billion-year main-sequence lifetime. Proxima Centauri, by contrast, will keep fusing hydrogen for trillions of years. Stellar lifetime is inversely correlated with mass: the less massive the star, the more slowly it burns through its fuel, and the longer it lasts.4The Astrophysical Journal Letters. Reduced Diversity of Life around Proxima Centauri and TRAPPIST-1 Combined with the full-body convection that gives Proxima access to nearly all its hydrogen, the result is a star that could theoretically shine for four trillion years or more. The universe itself is only about 13.8 billion years old, so no red dwarf that has ever formed has yet come close to dying of old age.
This extreme longevity is one reason astrobiologists are interested in planets around red dwarfs. A planet that remains in a habitable zone for trillions of years has, at least in principle, far more time for life to develop than a planet orbiting a Sun-like star. Whether the other conditions around a red dwarf are actually hospitable is a separate and more complicated question.
A Surprisingly Active Surface
Given how small and cool Proxima Centauri is, you might expect it to be a quiet, placid star. It is not. Proxima is known for powerful stellar flares that can temporarily increase its brightness by large factors, particularly in ultraviolet and X-ray wavelengths. This flaring activity is linked to the star’s strong magnetic field, which is thought to be generated by its fully convective interior through a dynamo mechanism that works differently from the Sun’s.
One marker of this magnetic activity is the coverage of starspots, regions on the surface where strong magnetic fields suppress convection and create cooler, darker patches, analogous to sunspots. Estimates of Proxima’s starspot coverage vary depending on the method used. Analysis of TESS light curves suggests a spot area of roughly 4% of the visible stellar surface when estimated from brightness variations.5The Astrophysical Journal. Flaring Activity of Proxima Centauri from TESS Observations: Quasiperiodic Oscillations during Flare Decay and Inferences on the Habitability of Proxima b However, when researchers calculate spot area based on the energy released in the star’s largest flares, the implied coverage can be as high as 15 to 30% of the surface, and some earlier analyses have cited around 20%.6The Astrophysical Journal. Impact of Stellar Superflares on Planetary Habitability
The Sun, by comparison, rarely has spot coverage above 0.5% of its visible disk, even during solar maximum. The fact that a star with only 14% of the Sun’s radius can have proportionally far more of its surface magnetically disrupted is one of the more striking contrasts between the two stars. Proxima packs a disproportionate amount of magnetic punch into a very small body.
Light Output and How It Reaches Proxima b
Proxima Centauri’s total luminosity is only about 0.17% of the Sun’s. To put that differently, you would need roughly 600 Proximas to match the Sun’s energy output. This extreme faintness is the reason it is invisible to the naked eye despite being the closest star.
The planet Proxima Centauri b orbits much closer to its star than Earth orbits the Sun, at about 0.049 astronomical units (roughly 7.5 million kilometers, compared to Earth’s 150 million kilometers from the Sun). Even at that close distance, the total irradiance reaching the top of Proxima b’s atmosphere is only about 877 watts per square meter, or roughly 64% of what Earth receives from the Sun.7Astronomy & Astrophysics. The full spectral radiative properties of Proxima Centauri That places the planet in a range where liquid water on the surface is theoretically possible, depending on atmospheric conditions.
The quality of that light, though, is very different. Because Proxima is so cool, most of its energy comes out as infrared radiation rather than visible light. A hypothetical observer on Proxima b would see a dim, reddish sun that appears about three times larger in the sky than our Sun does from Earth (because the planet is so much closer), but the landscape would be bathed in a ruddy, low-contrast light quite unlike what we experience.
The High-Energy Catch
While Proxima b receives less total energy than Earth, the story flips dramatically at the high-energy end of the spectrum. The average X-ray and extreme ultraviolet irradiance hitting the top of Proxima b’s atmosphere is nearly 60 times higher than what Earth receives from the Sun.7Astronomy & Astrophysics. The full spectral radiative properties of Proxima Centauri During major flares, this bombardment spikes even higher.
This imbalance is a direct consequence of Proxima’s strong magnetic activity. Flares and the star’s magnetically heated upper atmosphere pump out far more high-energy radiation, relative to the star’s total output, than the Sun does. For any planet in the habitable zone of a star this active, the question is whether an atmosphere could survive the constant stripping effect of that radiation, or whether it would be eroded away over time. Many models suggest that a planet without a strong magnetic field of its own would struggle to retain a thick atmosphere under these conditions, which complicates the habitability picture considerably.
A Slow Start for Habitability
Red dwarfs create another problem for habitability that stems directly from their small size. When a star like Proxima Centauri first forms, it is brighter and hotter than it will be for most of its life, because it is still contracting under gravity. The contraction phase for low-mass stars lasts much longer than for Sun-like stars. Simulations suggest that Proxima Centauri took roughly 169 million years after formation before it dimmed enough for a planet at Proxima b’s current orbital distance to enter the habitable zone for the first time.8PubMed Central. The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants
During that long pre-habitable phase, the planet would have been too hot. If it had surface water, that water could have been lost to a runaway greenhouse effect, with steam in the upper atmosphere being broken apart by ultraviolet light and the hydrogen escaping to space. Whether Proxima b retained enough water to be habitable after this extended baking period is one of the open questions in the field. The Sun, being more massive, contracted to its main-sequence brightness much faster, giving Earth a comparatively gentle start.
Why Red Dwarfs Like Proxima Dominate the Galaxy
Proxima Centauri is not an oddity. Red dwarfs are by far the most common type of star in the Milky Way, making up roughly 70 to 75% of all stars. Their small size and low luminosity just make them hard to see. Before modern surveys that could detect very faint stars, the galaxy appeared to be dominated by brighter Sun-like stars and giants, but that was a selection bias. In reality, the typical star in the universe looks much more like Proxima Centauri than like the Sun.
This has real implications for the search for life beyond Earth. If habitable planets exist around red dwarfs, the sheer number of these stars means there could be far more potentially habitable worlds than if life were limited to planets orbiting Sun-like stars. On the other hand, if the intense flaring activity, atmospheric stripping, and prolonged pre-habitable periods typical of red dwarfs make their planets generally uninhabitable, then the vast majority of stars in the galaxy are essentially sterile hosts regardless of how long they live.
The debate is far from settled. Proxima Centauri, as the closest example, has become a key test case. Every new observation of its flaring behavior, magnetic field strength, or the properties of its planets feeds into models that extrapolate to billions of similar stars across the galaxy. The fact that such an unassuming star, one you could barely distinguish from Jupiter in physical size, sits at the center of some of the biggest questions in astrobiology is a reminder that size and importance do not always track together in astronomy.
How Astronomers Keep Refining the Measurements
Measuring the radius and mass of a star like Proxima Centauri is harder than it might seem. For nearby bright stars, interferometry can directly resolve the disk of the star and yield a geometric size. For a star as faint as Proxima, direct measurement is more challenging, and astronomers have historically relied on combining luminosity measurements with models of stellar structure. The radius of 0.141 solar radii cited in most recent literature comes from this kind of modeling, constrained by the star’s measured brightness and distance.1The Astrophysical Journal. Three-dimensional Climate Simulations for the Detectability of Proxima Centauri b
Mass measurements have an additional tool available for Proxima: its membership in the Alpha Centauri system. The orbital dynamics of the triple-star system provide a gravitational mass estimate. Independently, the microlensing event observed with the SPHERE instrument on the Very Large Telescope offered a direct gravitational measurement that does not depend on stellar models at all.2Monthly Notices of the Royal Astronomical Society. The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event The agreement between these independent methods gives confidence that the 12% solar mass figure is solid.
The uncertainty bars on the radius, about ±0.021 solar radii, are worth noting because they reflect a genuine challenge with low-mass stars. The relationship between mass, radius, and luminosity for red dwarfs is less cleanly calibrated than for Sun-like stars, partly because so few red dwarfs are in eclipsing binary systems where both mass and radius can be measured directly. As more red dwarfs get precise measurements from missions like Gaia and from long-baseline interferometry, these uncertainties should shrink, and any surprises in the mass-radius relationship for the smallest stars will become clearer.