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A self-luminous celestial body consisting of a mass of gas held together by its own gravity in which the energy generated by nuclear reactions in the interior is balanced by the outflow of energy to the surface, and the inward-directed gravitational forces are balanced by the outward-directed gas and radiation pressures.

Stellar Evolution

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years for the most massive, to trillions of years for the least massive.

Stellar evolution is not studied by observing the life of a single star, since most stellar changes occur too slowly to be detected, even over many centuries. Instead astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating their structure using computer models.


Protostar: the preliminary stage in the evolution of a star, coming after the beginning of the collapse of the molecular cloud from which it is formed, but before sufficient gravtitational contraction has occurred to allow the ignition of nuclear reactions in its core.

All stars are born from collapsing clouds of gas and dust, often called nebulae or giant molecular clouds. Typical molecular clouds are roughly 100 lightyears across, and contain up to 6,000,000 solar masses. As a giant molecular cloud collapses, it breaks into smaller and smaller clumps. In each of these fragments, the collapsing gas and dust release gravitational potential energy as heat. As its temperature and outwardly expanding pressure rise, a fragment condenses into a rotating sphere of super-hot gas called a protostar.

Carina Nebula - NGC 3372

NGC 3372 - Click for larger view

Stellar Nursery

"A stellar nursery known as Lambda Centauri shines brightly from a distance of about 6,500 light-years in this image from the European Southern Observatory. The bright blue stars in the image, which are quite young, are much hotter than the Sun, so they pump out enormous amounts of ultraviolet energy. That energy causes the surrounding clouds of hydrogen gas to glow bright red. The clouds are permeated by dark lanes and blobs of dust, where new stars are being born. The energy from the young stars is eroding both the dust and gas clouds, though, which eventually will halt the process of starbirth. Lambda Centauri, also known as IC 2944, is in the constellation Centaurus."

Text and photo by: European Southern Observatory

Lambda Centauri

IC 2944 - Click for larger view


A relatively brief period of accelerated star formation which some galaxies undergo. Starbursts occur usually in the central regions of galaxies which have a rich supply of interstellar gas. Areas experiencing starburst produce stars at a rate between ten to a hundred times faster than typical stellar nurseries. The stars formed in these hydrogen-rich pockets are massive and very luminous, but shortlived. An example of a galaxy undergoing starburst is NGC 3034.

Star Cluster

R136 is a massive star cluster in the 30 Doradus Nebula located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way.

It is a young star cluster, estimated to be 1 to 2 million years old, and contains giant and super-giant stars. At the cluster’s core are twelve very massive and luminous stars, initially calculated as ranging in size from 37 to 76 solar masses.

Three very bright stars, R136a1, R136a2 and R136a3, dominate. R136a1 is the most massive star found so far, 265 times our Sun's mass, and 8.7 million times more luminous.

The cluster is approximately 35 light years in diameter. This infrared image of it was taken by the Hubble Space Telescope in October 2009.


R136 - Click for larger view

Globular Cluster

A large compact, spherical star cluster, typically of old stars found in the outer region of a galaxy known as the galactic halo. Catalogued object NGC 6752 in the constellation Pavo is a typical example of a globular cluster.

Open Cluster

Within a galaxy, there are areas where stars are concentrated in groups, held together by mutual gravitational attraction. An open cluster is the term used for a loose, irregular grouping of stars originating from a single nebula, also called a galactic cluster. Examples are NGC 411, NGC 265, and NGC 6819.


NGC 265 - Click for larger view