Astronomy 1021 Chapter Notes - Chapter 16: Irregular Galaxy, Magellanic Clouds
29 May 2018
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Astronomy 1021
Chapter 16: A Universe of Galaxies
Galaxy Types- Spirals, Lenticulars, Ellipticals, Irregulars
•Spiral Galaxies- ex. Milky Way, Andromeda, M100; flat white disks with yellowish bulges at the
centre, disks are filled with cool gas and dust mostly in their spiral arms (where new stars are
formed)
-Barred spiral galaxies- appear to have a bar of stars through the center with spiral arms curling away
from ends of the bar (Milky Way)
-Edge on Spirals, can’t see the spiral arms, see a band of dust stretching across the sky
-Spiral galaxy components- disk where you have spiral arms, bulge, halo, ‘spheroidal’ component
(bulge + halo)
-Spheroidal vs Disk- hard to see where the bulge ends and halo begins; ex. sombrero galaxy, old
stars, few gas/dust clouds; disk component- stars of all ages, many gas/dust clouds; red-yellow
colour indicates older stars, spiral arm stars are blue b/c still forming
•Ellipticals- only have a halo and no disk component, yellow-red colour, spheroidal component;
often longer in one direction; many globular clusters
-Older star population b/c no gas and dust so no star formation possible, random motion throughout
just like the motion of halo stars
•Lenticular Galaxies- intermediate galaxies; shape resembles elliptical galaxies, disk but no spiral
arms; more gas/dust than ellipticals but less than spirals, star formation happens throughout
•Irregular Galaxies: don’t have a shape, anything that doesn’t fit into the other groups, catch-all
group- makes the most stars, then spiral, then lenticular, then elliptical which doesn’t form any
stars, ex. magellanic clouds
•Elliptical galaxies have a huge range of values for mass, diameter, and luminosity compared to
the others, and irregular galaxies are much smaller, fainter, and less massive
•Hubble’s Tuning Fork Diagram is not a physically meaningful classification; sorted galaxies
according to their shape, on the left are elliptical galaxies, right is spherical, in the middle is
lenticular, no irregular galaxies
-Elliptical galaxies- E + number, the larger the number, the more elongated the galaxy; so E0 is
round, E5 is more elongated
-Lenticular galaxies- S0 or SB0 (b/c they are halfway) have disks like spirals but elongated like
elliptical
-Spiral galaxies- S or SB (when there is a bar), followed by a/b/c; classified based on size of bulge, as
you from a-c the bulge becomes smaller, and how tightly wrapped the stars are around the bulge; ex.
SBa is very tightly, in c the stars are looser
•Classifying Galaxies vs. Stars
-Stellar Classification- led to insights about the nature of stars; temp, mass, and sequence
-Galaxy classification- tells us what galaxies look like, done solely on the basis of their shape; not a
sequence in properties (age, mass); more difficult to interpret
