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.