What are three distinguishing characteristics that can differ among spiral galaxies?
the arms? i think is one.
i would guess that its disklike structure, with the buldge at the center, and arms .
The best know classification scheme is the Hubble system. It establishes two basic types - regular and barred - and three subclasses of each type based on how tightly the arms are wrapped. The de Vaucouleurs classification system is a more detailed version of the Hubble scheme which profiles spirals 3 dimensionally. The DDO, or van den Bergh scheme factors luminosity into the mix and the Yerkes, or Morgan system factors in the integrated spectrum. For an overview see:
Of course there are other systems at well, but these are the most widely used.
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If you mean things that distinguish them from ellipticals, then there are a lot:
1. They have disks with organized rotation.
2. They have a lot of gas and dust.
3. They have active star formation.
4. They're bluer (consequence of #3).
5. They can have spiral arms (density waves).
6. They're more common outside of clusters.
7. They sometimes have bulges.
8. They sometimes have bars.
From one spiral to another, the following things are known to vary and can be used to classify them:
1. The size of the bulge.
2. Whether or not there's a bar.
3. The number of spiral arms.
4. The continuity of the spiral arms.
5. The angle about which the arms wrap.
6. How blue they are (rough measure of amount of star formation)
7. Amount of gas and dust.
There are other things, but those can all be visually apparent. You can also measure rotation curves, ellipticities, etc.
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