Based on its amount of hydrogen-alpha emission, a proxy for the rate of star formation, 30 Doradus ranks in the top 1% of 21,000 H II regions in 57 spiral galaxies, and it tops the 614 H II regions found in the study's 13 irregular galaxies
by a factor of 10
And if we leave aside the so-called irregular galaxies
, there are the lenticular galaxies, among which are the very unusual but not-so-rare spindle galaxies.
Further out are a number of similarly sized and slightly misshaped dwarf irregular galaxies
, which are not gravitationally bound to the Milky Way and may be relative newcomers to our galactic neighborhood.
Not only are these conditions typical for dwarf irregular galaxies
like the SMC, they also mimic ones that would have existed in the early universe.
have no particular shape: the Small Magellanic Cloud is an example.
Modern-day galaxies come in three types: spirals with swirling, starlit arms; football-shape ellipticals; and irregular galaxies
, which have no organized structure.
Unlike symmetrical spiral and elliptical galaxies, or known irregular galaxies
such as the Magellanic Clouds, many of these merging pairs display strange structures such as loops, narrow streamers, and rings.
As NASA points out, spiral, elliptical and irregular galaxies
can all be seen in the photo of NGC 4866.
The mix of galaxy structures in the early universe is quite different, with a much greater diversity and larger fraction of irregular galaxies
, Shapley said.
NGC 2403 has two unusual neighbors--the Magellanic-type dwarf irregular galaxies
Holmberg II (6.