Stars are born inside freezing clouds that can outweigh the Sun millions of times
The birthplaces of stars sit at about ten degrees above absolute zero. Giant clouds of molecular hydrogen, some a hundred light-years across and holding up to six million solar masses, slowly break into dense clumps that collapse under their own gravity. Most stars emerge in crowds rather than alone.
The space between stars in a spiral galaxy like the Milky Way is not empty. It holds a thin mix of gas and dust, between ten thousand and a million particles per cubic centimetre, made up by mass of roughly 70 percent hydrogen, 28 percent helium and 1.5 percent heavier elements. Those heavier elements were forged inside earlier stars and scattered when they died. Where this medium thickens into clouds, new stars can form. Elliptical galaxies, by contrast, lose their cold gas within about a billion years and struggle to make new stars unless they merge with another galaxy.
In the densest clouds most hydrogen exists as molecules, hence the name molecular clouds. The Herschel Space Observatory showed that these clouds are laced with long filaments of dense gas, which fragment into gravitationally bound cores; most of those cores become stars. In some filaments the cores line up in near-regular chains, spaced about as far apart as the filament is wide, with young protostars already blowing out jets of material.
Temperature matters. The coldest clouds tend to make low-mass stars, first seen by their infrared glow and then in visible light once the surrounding cloud clears. Warmer giant clouds produce stars of every size. Roughly half the Milky Way's interstellar gas lies in molecular clouds, and the galaxy holds around 6,000 of them heavier than 100,000 Suns. The nearest place forming massive stars is the Orion Nebula, 1,300 light-years away, while smaller stars are forming just 400 to 450 light-years off in the Rho Ophiuchi complex.
Smaller nurseries exist too. Bok globules, named for the astronomer Bart Bok, are dark knots of gas and dust up to a light-year wide containing a few solar masses. They show up as silhouettes against bright nebulae, and more than half of those known are forming stars.
Source: Star formation
