Ancient Suns logo, alien star and two planets

Space Facts

Star Life Cycle—Detail

The chart, below, uses a logarithmic scale to present the ages of stars by spectral class. The globe of Earth represents the age of our Solar system at about 4.5 billion years. The vertical red line farther to the right represents the current age of the universe at about 13.7 billion years.

For each lettered range of spectral types, the approximate percentage of all stars is given. As you can see, the M-types, or "red dwarfs" are the most common type of star.

The first million years of a star's life is off the chart to the left. The most massive, O-type star, only lasts about a million years in main sequence—the period of greatest stability during the life of a star, and the largest portion of its life. For the brightest (hottest) star in each lettered class, the chart gives the typical mass relative to the mass of Sol (our sun). The chart also gives the temperature in degrees Kelvin (°C = °K - 273.15), as well as the typical radius relative to the radius of Sol.

Infancy to Maturity

The chart is divided into "infancy" and "maturity." In the 1960's, a research study by the Rand Corporation gave three billion years as the time it would take to form a planet habitable to humans. A good portion of the time prior to this, a potentially Earth-like planet would be bombarded by meteors, asteroids and comets—say for the first two billion years. After that, it would take a billion years or so for life there to produce sufficient oxygen to support human life. As you can see from the chart, the more massive stars do not stay in the main sequence long enough to reach maturity.


M-type stars have a less certain future, though they will likely remain in the main sequence for hundreds of billions, if not trillions of years. Even the first "M" dwarfs will not have left the main sequence, because the universe is not yet old enough. The smaller the mass of a star, the more other effects play a large role in determining the life of a star—the internal structure and the chemistry.

After the brighter stars have left the main sequence, they become sub-giants and then red giants. Stars up to about 1.23 times the mass of our sun can die quietly as a white dwarf which may take many billions or even trillions of years to dwindle down to a black, degenerate husk. Above this mass, a star may end its life with a bang—a supernova with a bright expanding cloud, highly enriched with heavier elements, and a neutron star or black hole at the center.

Planets for Humanity

If a star is too young, then the system and its planets are too young to support human life. For the first one to two billion years, each planet in a system will suffer repeated bombardment—the kind which killed the dinosaurs, only much more frequently. The less massive stars, from about F2V down to the M-dwarfs, will have a chance of producing a habitable planet. But this accounts for roughly 90% of all stars in the disk of the galaxy. And with the recent estimate of nearly a trillion stars in the Milky Way, this would mean hundreds of millions of opportunities for worlds friendly to humans.

Astronomy Data Book, by J.H. Robinson & J. Muirden — 1979, John Wiley & Sons, New York
A Field Guide to the Stars and Planets, by D.H. Menzel — 1964, Houghton Mifflin Company, Boston
The Collapsing Universe, Isaac Asimov — 1977, Walker and Company, New York
Planets for Man, Stephen H. Dole and Isaac Asimov — 1964, Random House, New York
"2 - Stellar Masses and Radii," retrieved 2011:0330
Stars, Their Birth, Life, and Death, Iosif S. Shklovskii — 1978, W. H. Freeman and Company, San Francisco