Massive stars, which have 10-150 times the mass of the Sun, are relatively rare. Although there are not many of these stars, they produce the bulk of the heavy elements in a galaxy when they explode in supernovas. Their extreme brightness makes them signposts of star formation in distant galaxies. In this latest study, researchers modeled the formation of these stars. Recently, they looked at the conditions inside cold clouds of molecular hydrogen; apparently, these conditions favor the formation of massive stars over low-mass stars like the Sun. The researchers concluded that early formation of a few low-mass stars in a cloud paves the way for later formation of a stellar “big brother”.
Astrophysicist Christopher F. McKee, Professor of physics and astronomy at UC Berkeley, explains: “It’s only the formation of these low-mass stars that heats up the cloud enough to cut off the fragmentation. It is as if the cold molecular cloud starts on the process of making low-mass stars but then, because of heating, that fragmentation is stopped and the rest of the gas goes into one large star.” Mark R. Krumholz, a Hubble postdoctoral fellow in the Department of Astrophysical Sciences at Princeton, further explains that if a cloud is cold, it tends to break up into many small pieces, which later become low-mass stars; on the other hand, as the cloud gets warmer, it can produce bigger objects.
Although the cloud gradually becomes warmer, its temperatures are still cold. The estimated temperature within a typical interstellar hydrogen cloud is between 10-20 degrees Celsius above absolute zero (about -430 degrees Fahrenheit). Stopping the entire cloud from collapsing requires an increase of many hundreds of degrees, according to McKee. Each small star within a hydrogen cloud has a zone of influence where the gas is warmed, preventing the cloud from collapsing into small fragments. In low density clouds, each zone of influence is small and contains very little mass, so this effect is insignificant. As the cloud’s density increases, however, the gas and small stars get packed closer and closer together. Eventually, the zones of influence of the few low-mass stars encompass the entire cloud, and therefore, cloud fragmentation is prevented. The cloud is forced to collapse, resulting in the production of a massive star.
This collapse occurs within an even larger interstellar cloud, which may have a mass that is more than a million times the mass of the Sun. Therefore, as in our galaxy’s Orion Nebula, many massive stars may be forming simultaneously inside a giant molecular cloud. The density above which massive stars can form is about a million hydrogen molecules per cubic centimeter. On Earth, this would be considered a very extreme vacuum. Nevertheless, over a long enough period of time, these density rates are sufficient to cause the cloud to collapse into a massive star. By comparison, the particle density in Earth’s atmosphere is 10 trillion times greater.
According to the study, the density limitation can lead us to the conclusion that in the outer reaches of galaxies, where the density may not reach this threshold, low-mass stars may be forming in the absence of any massive stars. Because we can see only the big, bright stars from Earth, astronomers may be underestimating the amount of star formation taking –place in distant galaxies. “In fact, there may be many stars forming in the outer reaches of distant galaxies, just not the bright ones we can see,” McKee said. “Star formation could be occurring that is essentially invisible.”
More information on the Princeton/Berkeley research can be found here.
Image: Star formation simulation (Credit: Mark Krumholz, Princeton)