How does the composition between irregular, spiral, and elliptical galaxies compare?

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They vary in a number of things, types of stars they contain, the amounts of metal, dust, and gas they contain, and the ages of the stars within them.


Typically, elliptical galaxies tend to be composed of older, 'lighter' stars, and are thought to be old galaxies, so there is little inter-stellar matter, and hence little star-formation taking place. They also tend to have a number of globular clusters around them.

Spirals, as you might guess, have a spiral structure (about 60% of them seem to be barred spirals, which means they have a bar across them). The stars in the arms especially tend to lie more or less in a rough plane, and orbit the galactic bulge (which is in the centre).

A spiral's arms tend to contain a number of hotter, younger, blue stars, whereas the galactic bulge (the bit in the middle) may be more like an elliptical galaxy, and may contain more older, metal-poor, redder stars, this seems to vary though, depending on how tightly wrapped the arms are. The more tightly wrapped ones (Sa) contain older stars, and the more open ones (Sc) contain younger, bluer, more metal-rich stars.

The bulge may well also contain a very massive black hole, though this is still not fully confirmed. It has also been speculated that Spirals may contain a dark matter halo of some sort.

As for irregular galaxies, some may once have had a structure, which got deformed by some external gravitational force. A lot of them contain great amounts of dust and gas (unless they are dwarf irregular galaxies).

The irregular galaxies fall into a 3 basic classes: Irr-I, Irr-II, and dlrrs. Irr-I has some structure, but not enough to be classified as a spiral, barred-spiral, or elliptical; the Irr-II has no structure at all; and the dlrrs are dwarf galaxies with low levels of metals and high levels of gas. They may be very similar to the first galaxies to form in the Universe.

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