Stars form as gravitational forces coalesce the gas and dust in interstellar clouds until the material forms clumps dense enough to become stars. But how this happens, and whether or not the processes are the same for all stars remains very uncertain. Astronomers have been studying those clumps, the stellar wombs called "pre-stellar cores," in an attempt to sort out these questions. But precisely because the cores have no stars in them yet, or at best only very young stars, they are faint and difficult to study.
SAO astronomers Erik Rosolowsky and Phil Myers, together with four of their colleagues, have completed the first unbiased census of 200 cores in three relatively nearby clouds of gas and dust. They combined observations from a millimeter wavelength study with their infrared images from the Spitzer Space telescope. The former observations are able to identify the dense material (dense in this case means about 20,000 molecules per cubic centimeter), while the latter can probe inside the clumps for any evidence of warming, thereby signifying the presence of a young star.
Their findings are striking. First, the cores without embedded stars are not all the same, but come in a least two types. In one cloud they are larger in size but with the same mass as the cores that do have stars, while in the other case they are smaller and have less mass than cores with stars. This presumably means that cores in the second category will one day end up making smaller stars. Even more notable, the scientists report that when considered all together, the starless cores have a distribution of masses that is remarkably similar to the distribution of the masses of stars themselves. This finding strongly suggests that the masses of stars are determined by the masses of the cores from which they form and not, for example, by subsequent processes like random fragmentation that might take place after the cores develop.
The astronomers reach once more significant conclusion. Because the sample is about equal in the numbers of cores that have or do not have a embedded star, the team's analysis concludes that the lifetimes of the two cases should be comparable. Once a star forms in a core, it blows away the placental material, and emerges in a few hundred thousand years, and so the pre-stellar cores must also be only a few hundred thousand or so years old. And that means that late stages of the actual birth process is not slow and gradual ("quasi-static" is the technical term), but instead moves in a dynamic way towards the birth of a new star.