A star’s “heaviness” refers to its mass, a fundamental property representing the amount of matter it contains. Unlike weight, which depends on the gravitational pull of another object, mass is an intrinsic characteristic of a star itself. Understanding a star’s mass is paramount because it dictates nearly every aspect of its existence, from its luminosity and lifespan to its eventual demise. It determines a star’s structure, evolution, and ultimate fate.
Measuring Stellar Mass
Scientists primarily determine a star’s mass by observing binary star systems, which consist of two stars orbiting a common center of mass. Around half of all stars exist in such pairs, providing a direct means for mass calculation. By applying Newton’s reformulation of Kepler’s Third Law of planetary motion, astronomers can calculate the combined mass of the two stars.
Kepler’s Third Law relates the orbital period of the stars to the average distance between them. Measuring the individual orbital speeds of the stars through the Doppler effect, which causes shifts in their light, allows astronomers to determine how much of the total mass each star possesses.
For single stars, determining mass is more indirect and relies on stellar models and the mass-luminosity relationship. More massive stars typically burn hotter and are significantly more luminous. By analyzing a single star’s observed luminosity and temperature, scientists can infer its mass, though these estimates are less direct than those derived from binary systems.
The Range of Star Masses
The masses of stars vary across an immense spectrum, typically measured in “solar masses” (M☉), with our Sun serving as the standard unit (1 M☉). At the lower end, objects with insufficient mass to sustain stable hydrogen fusion are classified as brown dwarfs. These “failed stars” have masses generally less than 0.075 M☉, or approximately 75 times the mass of Jupiter.
The minimum mass for a star to initiate and sustain hydrogen fusion in its core is approximately 0.08 M☉. Stars just above this limit, like red dwarfs, are the smallest and coolest true stars. They fuse hydrogen very slowly, granting them extraordinarily long lifespans, potentially trillions of years.
At the upper extreme, the most massive stars can reach hundreds of solar masses. While theoretical models once suggested an upper limit of around 150 M☉ due to intense stellar winds that would blow away excess material, observations have challenged this. Stars like R136a1 have been measured with masses between 170 and 230 M☉, pushing the boundaries of what was thought possible.
Mass and a Star’s Life
More massive stars require higher core temperatures and pressures to counteract their stronger gravitational forces, leading to faster nuclear fusion rates. This rapid energy production makes massive stars significantly more luminous than their lower-mass counterparts.
The accelerated rate of fusion in massive stars also means they consume their hydrogen fuel much more quickly, resulting in considerably shorter lifespans. For instance, a star with 40 solar masses might only live for a few million years, whereas our Sun, at 1 M☉, has a main-sequence lifespan of about 10 billion years. The least massive stars, red dwarfs, can burn for trillions of years.
Low-mass stars, up to about 8 M☉, will eventually evolve into red giants, shed their outer layers to form planetary nebulae, and leave behind a dense, cooling white dwarf. If a white dwarf’s mass exceeds approximately 1.4 solar masses, known as the Chandrasekhar limit, it can become unstable.
Conversely, high-mass stars, typically those above 8 M☉, undergo a more violent end. After exhausting their nuclear fuel, their cores collapse catastrophically, triggering a supernova explosion. This spectacular event can leave behind either an incredibly dense neutron star or, for the most massive stars (those originally around 30 M☉ or more), a black hole.