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Globular Cluster in Canes Venatici
Globular Cluster M3. Some 500,000 stars are crammed into an region about 100 light years across in this globular cluster.


On a clear dark night, a few thousand stars are visible to the naked eye. With binoculars and powerful telescopes, we can see so many stars that we could never hope to count them. Even though each individual star is unique, all stars share much in common. The Sun, which is the source of virtually all light, heat and energy reaching the Earth, is the nearest star. Today, we know that stars are born from interstellar gas clouds, shine by nuclear fusion and then die, sometimes in dramatic ways.

Birth of Stars

Stars are born in cold interstellar clouds like the Orion Nebula and the Eagle Nebula. In these stellar nurseries, dense regions undergo gravitational collapse to form a rotating gas globule. As the globule collapses, the temperature and pressure increase and it spins faster. This causes the globule to have a central core and a surrounding flattened disk of dust. The central core becomes a star, while the disk may coalesce into planets and asteroids. The process of collapse takes between 10,000 and 1,000,000 years.

Lives in the Balance

A star's life is an extended battle between two opposing forces: gravity and pressure. A star can maintain its internal pressure only if it continually generates energy to replace the energy that it radiates into space. This energy comes primarily from nuclear fusion of light elements into heavier elements, through which a star shines for millions or billions of years.

All stars spend a significant amount of their lifetime fusing hydrogen to helium. This phase of the star's life is called the main sequence. Examples of main sequence stars are the Sun, Vega, Sirius and Spica. When the hydrogen in the core of the star is depleted, the envelope of the star expands tremendously and the star becomes a red giant. Examples of red giants are Betelguese, Arcturus, Aldebaran and Antares.

As a star has a limited amount of material in its core, it cannot rely on thermal energy to resist gravity forever and its ultimate fate depends on whether something other than thremal pressure manages to halt the unceasing crush of gravity.

Death of Stars

The outcome of a star's struggle between gravity and pressure depends entirely on its birth mass. Stars with masses below about 5 solar masses swell into red giants near the ends of their lives, after which the envelope is ejected as a planetary nebula, while the core becomes a white dwarf. Examples of planetary nebulae are the Ring Nebula, Eskimo Nebula, Helix Nebula and the Cat's Eye Nebula.

High mass stars with masses above 5 solar masses end their lives much more violently. The stronger gravity of high mass stars compress their cores to higher temperatures, and consequently, these stars are much brighter than low mass stars. In the final stages of their lives, they proceed to fuse increasingly heavier elements until they have exhausted all possible fusion sources. When fusion ceases, gravity drives the core to implode, resulting in a titanic supernova explosion, leaving behind a neutron star or a black hole. Examples of supernova remnants are Vela, Crab Nebula, Veil Nebula and Supernova 1987A.

Star Clusters

Most stars are believed to have their origin in clusters. There are two kinds of star clusters: open clusters and globular clusters.

Open Clusters

Open clusters are physically related groups of stars held together by mutual gravitational attraction. Open clusters populate about the same regions of the Milky Way and other galaxies as diffuse nebulae, and are found along the band of the Milky Way in the sky. Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster due to velocity changes from tidal interactions with other objects. Examples of open clusters are M37 and M52.

Globular Clusters

Globular clusters are gravitational bound concentrations of approximately ten thousand to one million stars, spread over a volume of several tens to 200 light years in diameter. They are believed to be very old, estimates of their ages being 12 to 20 billion years. Examples of globular clusters are M13 and M28.

The Ask an Astronomer team's favorite links about Stars:

  • Stars: This website gives you a lot of information about individual stars. Look here if you want details (like age, color, distance etc.) of prominent stars.
  • Constellations and their Stars: This site tells you about constellations and and gives information about the stars associated with them. It also has interactive sky charts and information about Messier objects.
  • The Hipparcos Astrometry Mission: Hipparcos is a satellite which measured the distances to thousands of stars through stellar parallax. This website has links to the Hipparcos catalog and other links related to Hipparcos.
  • The HR diagram: This site gives a schematic HR diagram showing evolutionary tracks of stars of different masses.
  • 3-D Star Chart Viewer: This website has links to a downloadable 3-D Star Chart Viewer for Linux.

Previously asked questions about Stars:

General questions:


Stellar remnants:


"Measuring" the stars:

Star formation:

Nuclear burning:

How to ask a question:

If you have a question about Stars which isn't answered above, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.

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