$$\require{cancel}$$

# 13.10 Young Star Clusters

Star formation takes place in collapsing molecular clouds of gas and dust. As this material fragments into new star systems, thousands of stars can end up forming in a very tightly packed region. While star formation is still going on, these regions are called star forming regions but as the stars finished consuming the gas and begun to glow brightly as a tightly packed ball of main sequence stars, they take on the name open cluster. Over time the differential rotation of the galaxy (which will carry stars closer to the Galactic center faster along their orbits and will carry stars further from the center of the galaxy slower longer orbits) will cause the open clusters to fragment apart into streams of stars. Eventually it won't even be possible to tell which stars originated side-by-side in the same molecular cloud.

Star forming region NGC 3603 showing young stars surrounded by gas and dust. Click here for original source URL.

To see ongoing star formation we only need turn to the constellation Orion. The Orion star-forming region spreads across this large region of stars. This region, which is about 400 to 700 parsecs away, is a dense, cloudy hotbed of star-forming activity. When we look in this direction on a starry night, we can see the Orion Nebula as part of Orion's sword — this is just one rich knot of star formation in a much more expansive region. It contains the results of recent and ongoing star formation. Many newly forming objects lie hidden from our eyes inside dust clouds in this region of interstellar space. Using detectors sensitive to the infrared light that passes through the clouds, we are able to know what sorts of star formation is going on within the clouds, even though the clouds block visible light.

Star forming regions allow us to study stars that haven't yet settled into smooth hydrogen burning (fusion) along the main sequence. From slight over densities in the gas that thermally glow in millimeter wavelengths as they gravitationally contract, to well-defined stellar proplyds that resemble cocoons among the stars, to violent pre-main sequence stars, star-forming regions provide us a host of objects to examine, each representing a different stage in star formation.

We can see stars at different stages of star formation because different mass stars form at different rates. Mass drives stellar evolution and massive stars form first and evolve fastest. In systems like the Orion nebula, the most massive stars — stars that will someday die in a supernova explosion — have yet to die and they daily demonstrate the youth of these systems. These brilliant, massive, bluish, O-type giants and super giants last only a few million years before running out of fuel. For the time being, these giants dominate binocular and small telescope views of the Orion Nebulae. This is only temporary, however, and the future of the Orion Nebulae can be seen in the nearby  Pleiades open cluster. This roughly 150 million year old system has already lost its O type stars to the fates of stellar evolution, and what dust it still has can't be easily seen by the human eye.

While the O-type stars may dominate in light output in star forming regions, they are scarce in number, and these systems are also filled with a multitude of more Sun-like stars that are still in the process of forming. Some of the best examples of more typical young stars are the T Tauri stars (named after their prototype in the constellation Taurus.) These not-yet-stable stars vary in brightness, often possess jets, and are still accreting material from their stellar nebulae.

The central part of the Orion Nebula. Click here for original source URL.

The Orion nebula and other star-forming regions like it will not last forever. The tight groups of stars will disperse. The brightest stars will exhaust their nuclear fuel and die. The surrounding gas will no longer be energized by young stars — it will cool and dim. Millions of years from now, the curtain will fall on this spectacular light show