1. Galaxy Types and Classification

Standard picture: Hubble Sequence

ellipticals

spirals

barred
un-barred

irregulars

An evolutionary sequence?

Any relation between types?

Continuity of properties?

Ellipticals

``all bulge''

smooth distribution of stars

spherical to elliptical shape

red color

stars have random motions

Spirals

bulge plus disk, in varying proportions

bulge as above

disk bumpy with dust, gas, and young stars and star clusters

bluer color

stars have ordered motions: rotation

barred vs. unbarred: so what?

Irregulars

``train wreck''

``all disk''

but a very messy, lumpy disk

lots of dust, gas, and young stars

very blue color

stars have less ordered motion

2. How are Galaxies Classified?

Hubble's scheme (as applied today):

(i) relative amount of bulge and disk

(ii) openness of spiral arms

(iii) presence of lack of bar (so what?)

But this has been done by eye -- literally

by just a handful of people.

- totally subjective

- private ``club'' of Classifiers

we can do better!

Should be redone (and is) quantitatively with computer

pattern-recognition algorithms,

or by some other objective criterion.

For example:

Classify by colors , or spectra , i.e. by the types of stars found in the galaxies: Spectral Classification.

W. W. Morgan, 1958

Spectral classification is now coming back into vogue.

Other important quantities: (for classification)

mass

luminosity

size

These give us the scale of a galaxy:

3. Even Larger Scales!

Are galaxies the largest structures of matter in the Universe?

No

Galaxy groups and clusters:

The Local group

You are sitting in it

1 Mpc in size

20 galaxies, 3 of which are substantial:

(M31 = Andromeda, M33, and the Milky Way)

Virgo cluster: nearest large galaxy cluster

20 Mpc away 1 Mpc in size

2500 galaxies!

... and that's not all:

clusters of cluster: Super-clusters

20 Mpc in size and bigger

Does the large scale structure continue ?

And if so, to what scales?

How do we measure this structure?

In other words, how do we measure distances?

(a) A scheme which classifies galaxies by some apparent features, but not necessarily in a physical order.

(b) An evolutionary sequence of galaxies.

(c) An objective classification scheme which can be interpreted as a physically related sequence of objects.

(d) A relation between galaxy type and galaxy evolution.

(e) Another name for the HR diagram.

(a) Ellipticals, spirals, and irregulars form a color sequence from red to blue.

(b) Ellipticals, spirals, and irregulars form a sequence from pure bulge to pure disk.

(c) Spirals have the most oredered motion in rotation because of prominent, well-ordered disks.

(d) Hot young stars exist only in ellipticals.

(e) Spirals and irregulars both have dust and hot, young stars.