MadSci Network: Astronomy
Query:

Re: How old is the Orion Nebula?

Date: Fri Jun 23 18:44:24 2000
Posted By: John Dreher, Staff Astronomer, SETI Institute
Area of science: Astronomy
ID: 961431769.As
Message:

The Orion Nebula is between 300,000 and 1,000,000 years old. Unlike the Crab Nebula, which is all that remains of a massive star that blew up recently (by cosmic standards), the Orion Nebula is a vast cloud of gas and dust that has formed hundreds of stars already and continues to do so. Gas and dust are everywhere between the stars of our Milky Way, but in some regions the gas and dust are much colder and denser than in other regions; we call these relatively dense areas Giant Molecular Clouds.

Gravity tends to cause these clouds to collapse, but pressure from the internal heat and from magnetic fields pushes back creating a balance. Sometimes, for reasons that are still not completely understood, gravity gets the upper hand and a molecular cloud, or at least the densest parts of the cloud, start to collapse to even greater density. This in turn triggers stars to condense out of the cold gas. Once these stars have reached a sufficient size, they can ignite internal fusion burning, and they start to shine. Some of the new stars will be massive and very bright; the light from these bright stars burns away the dust and ionizes the gas creating a region of hot, high pressure gas that then blows out a hole in the molecular cloud. What you see in Orion is a huge cavity in a cold, dark cloud that has been blown out by several new, massive stars. Less conspicuous, but much more plentiful, are many new stars that have masses more like that of our own sun. Using infrared and radio telescopes that can see through dust, astronomers can look inside the remaining cold dark cloud and see yet more stars that are still in the process of collapsing out of the gas. Eventually the energy from all the new stars, including perhaps some supernovas when the most massive of the new stars blow up in a million years or so, will destroy the original cloud entirely and disperse all the remaining gas and dust into the wider interstellar medium.


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