Proxima Centauri

Proxima Centauri is the closest known star to Earth after the Sun, lying approximately 4.25 light-years away in the constellation Centaurus. Identified in 1915 by Robert T. A. Innes, it forms part of the Alpha Centauri triple star system, where it is designated Alpha Centauri C. Although physically associated with the system, it lies about 13,000 astronomical units from the Alpha Centauri A and B pair, orbiting them roughly once every 550,000 years. Because of its extremely faint apparent magnitude of 11.13, Proxima Centauri is not visible to the naked eye.

Physical Characteristics

Proxima Centauri is a red dwarf of stellar classification M5.5, placing it at the lower-mass end of main-sequence stars. It has a mass of around 12.5 per cent that of the Sun and an average density approximately 33 times higher. Its diameter is about one-seventh that of the Sun, a figure determined from direct interferometric measurements of its angular size.
Despite its very low luminosity—only around 0.16 per cent of the Sun’s total output—Proxima Centauri exhibits strong magnetic activity. It is a flare star, occasionally producing dramatic bursts of energy that significantly increase its brightness for short periods. These flares arise from the star’s fully convective interior, which continuously circulates plasma from core to surface. Because helium produced by hydrogen fusion does not accumulate at the core, the star will remain on the main sequence for about four trillion years, far longer than the Sun.
Its surface gravity is considerably stronger than that of Earth, and its effective temperature gives it a red-yellow hue. The star’s rotational period has been measured at roughly 83 days, with some variation in estimates. Observations indicate a complex magnetic activity cycle that may span around seven years.

Stellar Structure and Fusion

As a fully convective star, Proxima Centauri transfers energy through the movement of plasma, unlike the Sun, which has a radiative interior. This convection supports the generation of an extensive magnetic field. Frequent flares can reach temperatures exceeding 27 million Kelvin and produce significant X-ray emission. Even during quiescent phases, the star’s chromosphere and corona remain active, yielding X-ray output comparable to that of the Sun despite Proxima’s much smaller size.
The star’s stellar wind appears relatively weak, no more than a fifth of the solar mass-loss rate, though because of its small radius, its mass-loss rate per unit area may be higher than the Sun’s.

Life Cycle

Proxima Centauri will continue hydrogen fusion for trillions of years. As helium accumulates throughout the star, it will gradually contract and heat, ultimately transitioning into a more luminous phase sometimes described as a blue dwarf stage. When it finally exhausts its hydrogen supply, it will bypass the red-giant phase, cooling directly into a helium white dwarf.

Planetary System

Proxima Centauri hosts one confirmed and two candidate exoplanets:

• Proxima Centauri b is a terrestrial-mass planet orbiting within the star’s habitable zone at a distance corresponding to an 11.2-day orbital period. Its minimum mass is estimated at slightly above one Earth mass. Although its orbit falls within the theoretical region where liquid water could exist, the planet’s habitability is uncertain due to the star’s intense flare activity and high-energy radiation.
• Proxima Centauri c is a candidate super-Earth orbiting much farther from the star with an orbital period of several years. Its existence remains under debate.
• Proxima Centauri d is a candidate sub-Earth with a very short orbital period of 5.1 days, inferred through precise radial-velocity measurements.

These detections make Proxima Centauri a key object of interest in the study of potentially habitable planets around low-mass stars.

Variability and Activity Cycles

Proxima Centauri’s variability is well documented. Light-curve observations show:

• brief, intense superflares,
• brightness changes over its rotational period,
• longer-term variations linked to a magnetic cycle of approximately seven years.

A major flare recorded on 6 May 2019 produced unprecedented ultraviolet emission, underscoring the star’s potential effect on surrounding planetary environments.

Significance in Stellar Studies

Proxima Centauri serves as an important laboratory for understanding red dwarf physics, stellar magnetism, and the characteristics of low-mass stellar evolution. Its proximity enables direct measurements—such as angular diameter and detailed spectroscopic analysis—that are challenging for more distant red dwarfs. Research into its planetary system contributes to broader questions concerning the habitability of planets orbiting small, active stars.
Proxima Centauri also offers insights into the dynamics of the Alpha Centauri system. One hypothesis suggests that Proxima may have been gravitationally captured by the more massive A–B pair during the early dispersal of their birth cluster, though more precise radial-velocity data are needed to confirm this.