Star formation efficiency and the initial mass function

4 problems

• Do massive elliptical galaxies contain excess low-mass stars?

  Contested

  Spectra and motions of stars in the centers of the biggest galaxies suggest they hold more small, dim stars than the Milky Way does. Some gravitational lensing measurements disagree.

• What sets the typical stellar mass near 0.2 solar masses?

  Open

  The most common birth mass of stars is about a fifth of the Sun's mass, and this peak is found almost everywhere we look. Why nature prefers this mass, and whether it changes in very different environments, is not explained.

• Why does gas form stars at only one percent per free-fall time?

  Open

  If gas clouds simply collapsed under their own weight, galaxies would form stars about a hundred times faster than they do. Turbulence, magnetic fields and the push of newborn stars all slow it down, but which one sets the value is unknown.

• Was the IMF top-heavy in the first galaxies?

  Contested

  Metal-poor gas in the early universe cools less well, which might have favored more massive stars. This would change how bright and how heavy early galaxies appear.