Star formation history, however, does not always take place in a single burst, as occurs in spiral and irregular galaxies where star formation is continuous or in successive bursts.
It is clear that spiral and irregular galaxies are systems more complex than those represented by SSP's, and that, in particular, their chemical evolution must be taken into account for a precise interpretation of the spectrophotometric data.
Bekki, "Stripping of nitrogen-rich AGB ejecta from interacting dwarf
irregular galaxies," Monthly Notices of the Royal Astronomical Society, vol.
Garnett, "Nitrogen in
irregular galaxies," The Astrophysical Journal, vol.363, pp.142-153, 1990.
The small
irregular galaxies were born about 100 million years before reionization began and had just started to churn out stars.
In addition, recent work by Erik Tollerud (University of California, Irvine) and his colleagues show that the Clouds are much bluer and younger than comparably sized
irregular galaxies, and they form stars at a much higher rate.
Most are spiral and
irregular galaxies of normal mass, and more than 30 percent are colliding.
But they also arise in dwarf and
irregular galaxies, and they surge forth in bursts of activity when galaxies collide.
Ellipticals include both the biggest and smallest galaxies in the universe, and they differ from spirals and
irregular galaxies because almost all their stars are very old.
Nothing in my notes, however, suggests that one of these
irregular galaxies is more difficult to observe than the other, so the selection must have been arbitrary.
The Magellanic-type
irregular galaxies are defined as having no nucleus at all: whereas ellipticals are "all bulge," Magellanic irregulars are "all disk."
It is nearly invisible in the elliptical galaxies that have only old stars but practically all you see in
irregular galaxies like the Magellanic Clouds.
