Star formation and distribution in spiral galaxies

[hellfire]

In the nucleus of a spiral galaxy the most stars are old population II stars, and there is no star formation (which occurs in the spiral arms). Why?

Thanks.

 

[meteor]

With respect to the no star formation, I've found this explanation

"The strong tidal forces created by the supermassive black hole require material to have a larger than normal density in order for it to collapse from its own gravity. The rate of the cloud collapse is further restricted by the larger than normal magnetic fields which permeate throughout the galactic center. Finally, the material at the galactic center contains a larger amount of metals (all atoms besides hydrogen or helium) than the rest of the galactic disk. This increase in metallicity impedes the collapse of the star since the material has an large opacity which prevents energy from escaping the collapsing cloud."
http://www.astro.ucla.edu/~tanner/gcsf.html#SF
Hope it helps

 

[sol2]

Vey interesting http://www.astro.ucla.edu/~tanner/pictures/gc_1meter_big.gif Meteor

 

[Nereid]

In the nucleus of a spiral galaxy the most stars are old population II stars, and there is no star formation (which occurs in the spiral arms). Why?

Thanks.

Some clarifications ... the 'nucleus' of a galaxy - spiral or otherwise - isn't a particularly well defined region. Historically, it meant something like 'the bright point we see on images, at the apparent centre of the galaxy; it's too small to resolve, so we don't know what's really there' Some astronomers made (then) very tricky observations of the radial brightness and colour profiles of the central few arcsec of nearby galaxies; these days the VLT, Hubble, etc make nice images of what older astronomers could 'see' only as a fuzzball. The central part of a spiral is the 'bulge', which varies in size from tiny (in Sc spirals) to huge (in Sa and lenticulars); it is the bulge which is comprised of Population II stars.

As the link which meteor provided makes very clear, lots of star formation is occurring in the central few hundred pc of the Milky Way; an interesting finding is that new clusters (Arches, Quintuplet) seem to contain large numbers of quite massive stars (e.g. the Pistol Star). Indeed, the central parts of many spirals clearly contain lots of gas, from which new stars may form!