Let’s discuss what is happening in the star forming regions by looking at a nearby place where stars are currently forming. One of the best-studied stellar nurseries is located in the Orion constellation, The Hunter, about 1,500 light years away. The hunter’s pattern can be easily recognized by the eye-catching “belt” of three stars that mark his waist. The Orion Molecular Cloud is much larger than the star pattern and is really an impressive structure. In its long dimension, it spans a distance of about 100 light years. The total amount of molecular gas is about 200,000 times the mass of the sun. Most of the cloud does not glow in visible light, But it exposes its existence by emitting dusty gas in infrared & radio wavelengths.
Stars in Orion’s belt are typically around 5 million years old, while stars near the center of the “sword” hanging on Orion’s belt are only between 300,000 and 1 million years old. The region roughly in the middle of the sword where a star is still forming is called the Orion Nebula. About 2,200 young stars can be found in this region, which is only slightly larger than a dozen light years in diameter. The Orion Nebula also contains tight star cluster called the Trapezoid. The brightest stars in the trapezoid can easily be seen with a small telescope.
Compare this to our own solar neighborhood, where the typical distance between stars is around 3 light years. Few stars can be seen in the Orion Cluster in visible light, but infrared images that penetrate dust better recognize more than 2,000 stars that are part of the group.
Studies at Orion and other star formation regions show that star formation is not a very efficient process. In the Orion Nebula region, about 1% of the material in the cloud was converted to stars. Because of this, we still see a significant amount of gas and dust near the trapezoidal stars. The leftover material eventually heats up, either from radiation and winds from hot stars forming, or from explosions from most massive stars.
Whether gently or explosively, the material within the neighborhood of the new stars is blown away into celestial body space. The oldest groups of stars or star clusters can now be observed well in visible light because they are no longer enveloped by dust and gas.
While we do not know what originally caused star formation in Orion, there is good evidence that the first generation of stars triggered the formation of additional stars, which in turn led to the formation of more stars.
The basic idea of triggered star formation is this: When a massive star forms, it emits a large amount of ultraviolet radiation and expels gas in the form of a stellar wind at high speed. This injection of energy heats the gas around the stars & causes it to expand. When massive stars run out of fuel, they explode, and the energy of the explosion also heats the gas. The hot gases accumulate in the surrounding cold molecular cloud, compress the material and increase its density. If this increase in density is large enough, gravity will overcome the pressure and stars will form in the compressed gas. Such a chain reaction, in which the brightest and hottest stars in 1 area become the cause of star formation “next door”, seems to have occurred not only in Orion, but also in many other molecular clouds.
There are many molecular clouds that only (or mainly) form low-mass stars. Since low-mass stars. Since low-mass stars do not have strong winds and do not die when they explode, triggered star formation cannot take place in these clouds. There are also stars that form relatively isolated in small-cores. Therefore, not all star formation is originally triggered by the death of massive stars. However, there are likely other possible triggers, such as spiral density waves and other processes, that we do not understand yet.