PHYS 1000 Lecture Notes - Lecture 15: Edwin Hubble, Irregular Galaxy, Doppler Broadening
12 Feb 2018
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Chapter 15: normal and active galaxies (cid:1005)5. (cid:1005) hu(cid:271)(cid:271)le"s classifi(cid:272)atio(cid:374) (cid:272)he(cid:373)e. Edwin hubble was the first person to categorize galaxies and we still use the hubble classification scheme today. Spiral galaxies are classified according to the size of their central bulge. Type sb is smaller, and type sc is the smallest. Type sa tends to have the most tightly bound spiral arms, with sb and sc progressively less tight, (correlation is not perfect). Type sc galaxies have the most interstellar gas and dust and sa have the least. The components are still(cid:863) disk, halo, (cid:271)ulge, spiral ar(cid:373)s. To classify a galaxy as spiral we only need to note the presence of the disk with its gas, duct and newborn stars. Barred spirals ha(cid:448)e a (cid:862)(cid:271)ar(cid:863) of stellar a(cid:374)d i(cid:374)terstellar (cid:373)atter that e(cid:454)te(cid:374)ds (cid:271)e(cid:455)o(cid:374)d the bulge into the disk. Sba, sbb, and sbc, share the properties of regular spirals. Elliptical galaxies have no spiral arms and no disk.
