Clusters, Galaxies and Clusters of Galaxies

First off:

Globular clusters.

These aren't galaxies
A lot of the Messier objects are globular clusters of stars: relatively bright and close, mostly old stars e.g. M2 in Aquarius: about 100000 stars Globular Cluster M2

Credit & Copyright: D. Williams, N. A. Sharp, AURA, NOAO, NSF
e.g. M3 (note lots of red giants)

Credit & Copyright: S. Kafka & K. Honeycutt (Indiana University), WIYN, NOAO, NSF

There are about 200 round our galaxy: all galaxies seem to have them. THis is M87 (more about it later). It has about 1000 globulars.

Credit: Anglo-Australian Telescope photograph by David Malin Copyright: Anglo-Australian Telescope Board

THese are quite easy to understand: stars swing in and out of centre.

Galaxies

Approx. 109 (one billion) observed (!!!).
Approx. 105 well studied

Messier list contains about 100 galaxies.
New Galactic Catalog ∼ 10,000

Different shapes

Different Sizes 105 -> 1012 M₀

Elliptical

Huge range of sizes.
M87 is one of the largest: Note the spiral behind. (Stars have points on them) .
Looks simple but...short exposure shows jet + very small core
And the jet looks more complicated the smaller the scale
and it seems to be related to radio "lobes"
Best interpretation is that a very large black hole at the centre of the galaxy is producing a jet which pulsates and interacts with surrounding gas.

Spirals

The classic is M31 (Andromeda): tightly wound spiral with 2 satellites, a bit bigger than the Milky Way
Core maybe has two black holes and billions of stars: superimposed are knots of dust

Credit & Copyright: Robert Gendler
Very bright in X-ryas, with an intense course at the centre
This shows M51 along with Hurricane Julia:
NGC 1512 is an unusual spiral: tightly wound at the centre, with "star-burst" proceeding in a narrow ring
NGC 6946: The Fireworks Galaxy. SHows the spiral structure for what it is: in this case, lots of new stars, (blue) and huge hydrogen clouds (red), and very bright small nucleus

Credit & Copyright: T. Rector (U. Alaska Anchorage), Gemini Obs., AURA

M81 and M82 in Ursa Major make an intriguing pair: M81 seems to be destroying M82 via collisions

GALEX Full Field Credit: GALEX Team, Caltech, NASA
Closeup of centre of M82 shows massive wind blowing out of centre of galaxy

Credit: M. Westmoquette (UCL), J. Gallagher (U. Wisconsin-Madison), L. Smith (UCL), WIYN/NSF, HST, NASA/ESA

Irregulars

come in many varieties.
Some (Irr. I) seem to be similar to spirals, e.g. Magellanic clouds.
i.e. show pop I stars, dust, H gas...

This is SMC ∼ 106 stars
Since galaxies are quite large and move fast, ,it is not surprising they interact and collide: e.g.
These are 3 galaxies (in a group called Stefan's quintet) which are colliding
"The Cartwheel Galaxy". Ring round (100000 lightyears across) consists of new stars produced in the collision with a smaller galaxy, which has now vanished.

Credit: Kirk Borne (STScI), NASA

THe antennae galaxies are two very large galaxies in a violent collision: lats of stars being formed B. Whitmore (STScI), F. Schweizer (DTM), NASA
Not surprisingly the very hot gas produces lots of X-rays: this is the internal part of the antenna galaxies

Credit: G. Fabbiano (CfA) et al., CXC, SAO, NASA

Centaurus A may be the result of the collision of two galaxies as well: lots of dust

Credit: Marina Rejkuba (ESO-Garching) et al., ISAAC, VLT ANTU telescope, ESO Paranal Obs.
Closeup shows the star formation round the dust The Center of Centaurus A

Credit: E.J. Schreier (STScI) et al., HST, NASA

But there are some much weirder things out there...