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Space Facts

Star Life Cycle

Star Life Cycle: Eagle Nebula, birthplace of stars.
A portion of the Eagle Nebula, birthplace of new stars and the beginning of the star life cycle. From Hubble telescope.

Stars are the powerhouses of the universe. These mighty balls of gas give us light. The larger ones give us the heavier elements when, at the end of their lives, they explode as supernovae. All of this power comes from the "burning" of lighter elements to form heavier ones. This "burning," however, comes not from chemical combination, as when wood burns in oxygen; it comes from the fusing of lighter elements, like hydrogen, into heavier elements, like helium, carbon, oxygen and nitrogen.

The star life cycle begins with a cloud of gas and sometimes dust containing heavier elements. But hydrogen—the lightest element—makes up the bulk of each new star. Shock waves in the clouds from collisions with other clouds, or disturbance from passing stars, compress the gas enough to allow gravity to take over. Knots of higher density compress under mutual attraction to form spheres. As the gas compresses further, temperatures go up. If the mass of the new sphere is great enough, the temperature at the core reaches the critical point of nuclear ignition. The newborn star has started to burn hydrogen.

The Mass of a Star

The future fate of a star is determined largely by the mass it accumulates by its birth. A heavier star will burn its fuel far faster, and will run out of that fuel much sooner. The chemical make-up of a star also controls stellar evolution, but to a lesser degree.

Stars range in mass from about 4% the mass of Sol (our sun) to about 100 times its mass. The most light-weight stars burn with a deep reddish-orange color. The more massive a star, the hotter its core and thus its outer layers or "surface." A star does not have a solid surface; this is merely the visual surface that we see. The colors range through bright orange, yellow-orange, bright yellow, pale green, vibrant blue, and scintillating violent for the most massive stars.

The Main Sequence

Once the newborn star has settled down and reached a point of equilibrium, where it is no longer collapsing, it is said to have entered the "main sequence"—the bulk of its life as a shining star. During its stay in the main sequence, the light it emits remains fairly stable, though it increases slightly over time. In the next billion years, our own sun will have increased in brightness about ten percent, all but destroying life on Earth.

Chart on Main Sequence Ages

The chart, below, shows the estimated life-spans of stars based on their initial mass (click on the image for a larger, more detailed view of Main Sequence Ages). This mass determines how hotly the star burns which determines its surface color. The range of colors has been divided into spectral classes from the most massive to the least, by the letters O, B, A, F, G, K and M. This system of classification is a holdover from previously erroneous ideas of stellar evolution. On this scale of letters, a finer scale of numbers has been added so that stars slightly dimmer and cooler than our own sun—which is a G2 star—are labeled "G3," "G4" and "G5."

Star Life Cycle: Stellar Life Spans - star evolution based on mass

The letter and number combination give us the color or temperature of the star. We add to this a Roman numeral indicating the star's luminosity class. For main sequence stars, also called "dwarfs," we use "V." For under-luminous sub-dwarfs, we use "VI" and degenerate white dwarfs, we occasionally use "VII." For sub-giants, at the beginning of old age, we use "IV," and for giants, "III." For exceptionally bright giants, we give "II," while for super-giants we give "I." For an exceedingly rare breed of hyper-giant, we give the luminosity class "0."

Only in recent years have scientists been able to determine the age of a star that was not a part of a star cluster. We have long known the life span of a star based on its spectral class which is determined by its mass. This only gives us a maximum age for each star in the main sequence and says nothing about stars which have entered old age as a sub-giant, red giant or beyond. Any G0V star, for instance, has a maximum age of about ten billion (1010) years. That star, however, could be as little as a few hundred thousand years old or as much as its maximum main sequence age or anything in between.

Many techniques have been used to estimate an individual star's age, and the techniques are becoming gradually more refined. With more accurate distance measurement, the brightness of a main sequence star gives us one clue. The chemistry of a star, including lithium content, as well as chromosphere turbulence also tell us something of the star's evolution. A star's path through the galaxy has also been used to estimate ages, because stars of similar age travel in clusters, groups and streams.

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


Click on this chart to see a larger, more detailed view of Main Sequence Ages (999 x 606).