Solar mass
The solar mass is the unit of reference used in astronomy to express the mass of stars, black holes, galaxies, and other celestial objects. It is equal to the mass of the Sun: approximately 1.989 × 10³⁰ kilograms, or about 333,000 times the mass of Earth. Using this unit allows astronomical masses to be expressed with manageable numbers.
Why this unit?
The Sun’s mass is a natural reference point for stellar astronomy. Rather than writing astronomical numbers in kilograms, astronomers simply express masses as multiples of the solar mass. Saying that a star weighs 20 solar masses is much easier to read than writing 3.978 × 10³¹ kg. The solar mass is denoted by M with a solar symbol as a subscript.
Mass Limits of Stars
The solar mass is used to define important thresholds in the life of stars. A star must have at least 0.08 solar masses to initiate hydrogen fusion—anything less is a brown dwarf. Stars with masses less than 8 solar masses end up as white dwarfs. Those with masses between 8 and about 20 solar masses leave behind a neutron star. The most massive stars, exceeding 20 solar masses, can form a black hole.
Mass extremes
The solar mass provides a sense of scale. The smallest confirmed star, EBLM J0555-57Ab, weighs about 0.085 solar masses. The most massive known stellar black hole, Cygnus X-1, weighs about 21 solar masses. Sagittarius A*, the supermassive black hole at the center of the Milky Way, weighs 4 million solar masses. TON 618, one of the most massive known quasars, exceeds 66 billion solar masses.
Did you know?
The Sun is constantly losing mass: every second, it converts 4 million metric tons of mass into energy through nuclear fusion and ejects several billion kilograms of matter in the form of the solar wind. Over its entire lifetime, it will lose about 0.1% of its total mass.
Frequently Asked Questions
How is a star’s mass measured?
Mainly through Kepler’s laws as applied to binary systems. By observing how two stars orbit each other, we can calculate their respective masses. For isolated stars, we use empirical relationships between luminosity and mass.
Does the Sun’s mass change over time?
Yes, slightly. The Sun loses mass through nuclear fusion and the solar wind. But over the course of its entire lifetime, this loss represents less than 0.1% of its initial mass— a negligible change for standard astronomical calculations.
Can a galaxy’s mass be expressed in solar masses?
Yes. The total mass of the Milky Way is estimated to be about 1,000 billion solar masses, including dark matter. The mass of stars alone is about 60 billion solar masses. These figures give an idea of the scale of galactic structures.