Fast-growing cities wrestle with the problem of finding the optimum density to encourage life. Cities of stars have an analogous challenge, and a study of the magnificent Tarantula Nebula in the Large Magellanic Cloud has allowed astronomers to explore one of the factors keeping them from huddling so tightly together that life becomes impossible.
The Tarantula Nebula is the most active star formation region in the local group of galaxies. Located in one of the nearest galaxies to our own, the stars it is forming are both numerous and often very large. In the heart of the Tarantula, there is R136 – a dazzling cluster of hot young stars that offer us insight into the extremes of star formation.
Impressive as R136 is, it would be a hostile place for life to exist. The stars are so close that supernova explosions would sterilize planets around neighbors, if gravitational interactions hadn't disrupted their orbits first. Simple models suggest this super high-density star formation should be common, but we know star formation is usually more sedate, indicating some form of self-regulation.
Astronomers have proposed a number of factors that probably contribute to this, including what is known as “indirect radiation” pressure. The Australian National University's Dr Roland Crocker explained to IFLScience that most of hot stars' energy is optical or ultraviolet radiation. “When this scatters off dust, it is downshifted to the infrared, and that then bounces around inside molecular clouds and provides indirect radiation pressure,” he said.
Using R136 as a test case, Crocker shows in the Monthly Notices of the Royal Astronomical Society that indirect radiation pressure puts a ceiling on how densely stars can be packed, one consistent with the most tightly bunched clusters we see.
However, while this was probably important in the early universe and in gas- and dust-rich ultraluminous infrared galaxies, it probably has little effect in more sedate circumstances. The Milky Way, for example, forms just two stars a year, and indirect radiation is probably not a major influence in keeping this low.
With indirect pressure found to be marginal outside extreme cases, the search for the important self-regulating processes goes on. Direct radiation pressure of the UV and optical light is certainly part of it, as are stellar winds. Crocker told IFLScience that cosmic rays and heat from supernova explosions are currently being studied, along with turbulence produced by stars' movement.
“Close to the galactic center, where the Milky Way's prime star-forming regions are, tidal forces pull molecular clouds apart, so if stars don't form there quickly, the opportunity is lost,” Crocker said. Elsewhere, limiting effects don't change the number of stars that form, but spread them over a greater area and range of ages.