Star forming regions
Where do stars form? When do they form?The Orion Nebula is one of the most spectacular star forming regions that can be easily observed, since it is at about 1,400 light-years from us and spans more than 10 light-years across. It consists of stars, dust and clouds of gas. There are 4 very massive and bright stars at the center, the Trapezium stars. These stars emit copious amount of ultraviolet radiation (l = wavelength < 4,000 Angstrom) that ionizes hydrogen and produces the glow that can be seen with the naked eye. Smaller stars are embedded in the cloud or behind it, and are barely visible, although they set the cloud aglow giving its wispy appearance.
Orion Nebulae. Credits: C. R. O'Dell (Rice University) and NASA
Our understanding of how protostars (already formed stars, but before thermonuclear ignition) come to be from a cloud of gas and dust is still sketchy. Thus, a lot of effort is dedicated to the study of this Nebula and its associated molecular clouds, see below. Imaging the Nebula at different wavelengths helps to understand some of the processes occurring in the cloud.
The Nebula consists of gas (mostly hydrogen) and dust. Dust is made of small (0.1 micron; 1 micron = 10-6 meters) size grains. Electromagnetic radiation of comparable wavelength, such as ultraviolet and visible light (4,000-7,000 Angstrom which is equivalent to 0.4 to 0.7 micron) gets effectively scattered and absorbed by dust, thus obscuring what is in the center of the cloud. Actually, part of the electromagnetic radiation is absorbed by dust; dust heats up and re-radiates in the infrared, as a black-body of temperature between 30-100 K would. Radiation with much larger wavelength, such as infrared radiation of several to tens of microns, doesn't "see" the grains (because they are smaller than its wavelength). Thus, the cloud is transparent to infrared radiation.
Ultraviolet radiation (Violet light in the picture) coming from the left gets absorbed and scattered by dust giving out infrared radiation (Red light)
There are two important regions, called Orion Molecular Cloud 1 and Orion Molecular Cloud 2 which are behind the Orion Nebula proper, and contain star forming regions. They are powerful emitters of infrared radiation. These clouds are much denser (106 atoms/cm3) than the Nebula (600 atoms/cm3). Note that our atmosphere has 1019 atoms/cm3. As mentioned previously, the most abundant element is hydrogen.
The two pictures are taken in visible and infrared light.
In diffuse clouds, such as in most of Orion, most of the hydrogen is in atomic form, while in dense molecular clouds most of it is in molecular form (two hydrogen atoms bound together, or H2). There are also more complex molecules as well, including CO (carbon monoxide). See also the Section about the interstellar medium below.
Another spectacular stellar nursery is the Eagle Nebula, 7,000 light-years away. Massive young stars illuminate and heat the nebula.
Eagle nebula.
Credits: AURA/NOAO/NSF
Copyright WIYN Consortium, Inc., all rights reserved.
Lagoon nebula.
Credits: AURA/NOAO/NSF
Copyright WIYN Consortium, Inc., all rights reserved.
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