Stars vary greatly in size, from colossal giants to tiny celestial bodies. This article explores the characteristics of the smallest known stars and the methods used to discover these dim, distant objects.
The Smallest Star Identified
The current record holder for the smallest known star is EBLM J0555-57Ab, discovered in 2017 by a team from the University of Cambridge. This star resides approximately 670 light-years from Earth in the constellation Pictor. It is part of a triple star system, orbiting a much larger primary star, EBLM J0555-57Aa, every 7.8 days.
EBLM J0555-57Ab is slightly smaller than Saturn, with a radius of about 59,000 kilometers. It holds approximately 88 times the mass of Jupiter, or about 0.084 times the mass of our Sun. Its gravity is about 300 times stronger than Earth’s, and it shines 2,000 to 3,000 times dimmer than our Sun.
What Determines a Star’s Size?
A star is defined by its ability to sustain nuclear fusion of hydrogen in its core, a process that releases immense energy. This process requires a delicate balance between the inward pull of gravity, which tries to compress the star, and the outward pressure generated by the fusion reactions. For hydrogen fusion to ignite and be sustained, a celestial body must have sufficient mass to generate the necessary temperature and pressure at its core.
A lower mass limit for a true star is around 0.07 to 0.08 times the mass of our Sun, equivalent to approximately 75 to 80 times the mass of Jupiter. Objects below this limit do not achieve the core conditions for sustained hydrogen fusion and are classified as brown dwarfs.
How Astronomers Pinpoint Tiny Stars
Detecting very small and dim stars presents significant observational challenges for astronomers. Their faintness and the overwhelming glare from brighter nearby stars make direct imaging extremely difficult. Consequently, astronomers primarily rely on indirect methods to identify and characterize these elusive objects.
The transit method involves observing periodic dips in a star’s brightness. These dips occur when a celestial body, such as a planet or a smaller star, passes directly in front of its host star from our perspective, temporarily blocking some of its light. The duration and depth of these brightness changes provide information about the transiting object’s size and orbital period. EBLM J0555-57Ab, for instance, was discovered using this method, as it transited its larger companion star.
Radial velocity, also known as Doppler spectroscopy, is another method. This technique measures slight “wobbles” in a star’s movement caused by the gravitational tug of an orbiting body. By analyzing shifts in the star’s light spectrum due to the Doppler effect, astronomers can deduce the presence and mass of the orbiting object. This method is often used to confirm discoveries made by the transit method and to determine the mass of the orbiting body.
Placing Small Stars in Context
The smallest true stars, like EBLM J0555-57Ab, are classified as red dwarfs. These stars are the most common type in the Milky Way galaxy, making up between 70% and 85% of all stars. Individual red dwarfs are not visible to the naked eye due to their low luminosity and cool surface temperatures, typically ranging from 2,000 to 3,900 Kelvin.
Red dwarfs have exceptionally long lifespans, often trillions of years, far exceeding the Sun’s estimated 10-billion-year lifespan. This longevity is a result of their low mass and slow rate of hydrogen fusion, as well as their fully convective interiors, which allow them to efficiently burn a larger proportion of their hydrogen fuel.
Their extended existence makes them targets in the search for exoplanets, as orbiting worlds would have ample time for life to develop. Many red dwarfs have been found to host planets, some of which reside in the star’s habitable zone. While the habitable zone around a red dwarf is much closer due to its lower luminosity, their number and stability make them important for understanding stellar populations and the potential for life.