Recipe for Stars

(Section Not Complete)

The adjacent image shows one of the most famous images in astronomy, the dark nebula called the Horsehead Nebula. It is about 1500 light years away in the constellation Orion. It is part of a larger molecular cloud called Barnard 33. The darkness of the nebula is primarily because of dust in it that obscures the light from behind it. The red glow is caused by hydrogen in the region behind the Horsehead which has been ionized by the nearby hot star Sigma Orionis. [Source: N.A.Sharp/AURA/NOAO/NSF]

Map of neutral hydrogen in galaxy using 21 cm line

The adjacent European Southern Observatory image shows a molecular cloud in the constellation Ophiuchus (the Serpent Holder). It is about 500 light years away and about a half light year in diameter. A high concentration of molecular hydrogen and dust causes the molecular cloud to absorb all light from the stars behind it. The centers of such molecular clouds are probably among the coldest places in the Universe. The formation of many stars begins by condensation of regions of such molecular clouds (Ref).

The following image shows the molecular cloud Barnard 68 at six different wavelengths (Ref). The wavelength 0.44 micrometers corresponds to blue light, 0.55 micrometers to yellow-green, and the longer wavelengths to the near IR portion of the spectrum. Clearly the longer wavelength IR light penetrates the cloud much better than visible light. This illustrates an important property of interstellar light extinction by dust: the amount of extinction is strongly wavelength dependent. In particular, we see that IR observations are must less prone to distortion by absorption in intervening dust. Here is a streaming video movie that morphs the images at these different wavelengths to illustrate the change in absorption with wavelength.

Molecular clouds, which are the origination of many stars, are very difficult to study because they are primarily composed of molecular hydrogen at temperatures only about 10 degrees above absolute zero. Since hydrogen is virtually invisible under those conditions with normal detection methods, 99% or more of a molecular cloud cannot be seen with standard methods. Prior studies have had to rely on using radio telescopes to detect trace molecules such as carbon monoxide or ammonia in these clouds and then using this information to determine indirectly the distribution of molecular hydrogen. Advances in IR detection illustrated by the image give now for the first time a way to peer directly into the center of such stellar birthplaces.

Life and Death of Stars

Java Applet: Ideal Gas Law

Java Applet: Maxwell Velocity Distribution

Formation of Stars

Images of different kinds of nebulae

The following image shows a region of starbirth in the Large Magellanic Cloud about 170,000 light years distant and 150 light years across called N 119 and a blowup of its central region, which is a nebula called the Papillon Nebula (because of its shape: papillon is the French word for "butterfly"). Stellar winds from hot newborn massive stars within the nebula are responsible for the ridges, arcs, and filaments. One possible explanation for the unusual shape of the Papillon Nebula is that there are massive stars (10 solar masses or more) forming in the nebula and the radiation pressure from these stars is halting the infall of gas onto these stars and directing it away in a bipolar outflow (Ref).