GALAXIES

There are Three main classes (as classified by Hubble). A. Spiral B. Elliptical C. Irregular 1. Spiral galaxies. Disk + central bulge. M51 M51 Whirlpool Galaxy [type Sc]. M31 M31 Andromeda Galaxy [type Sb]. M77 M77 in Cetus [type SBp]. M104 M104 Sombrero Galaxy [type Sa]. M85 M85 a ``lenticular'' galaxy (on left) -- mostly bulge, a little disk [type S0]. NGC5866 NGC5866 a lenticular galaxy, mostly bulge [type S0].

Spiral Galaxies characterized by a flat thin rotating disk gas and dust evident -- implies ongoing star formation spiral arms classified as Sa, Sb, Sc, SBa, SBb, SBc star formation outlines spiral arms billion stars in a typical spiral galaxy some spiral galaxies (SB's) have ``bars'' bar-shaped features crossing the center of the galaxy. spiral arms begin at the ends of the bars.

Barred spiral galaxies. Disk + central bulge with bar. M83 M83 in Hydra [type SBc if you think it has enough of a bar]. M91 M91 in Virgo Cluster [type SBb]. M95 M95 [type SBb]. M83 M91

2. Elliptical galaxies. All bulge, elliptical shape, no disk; stars but no gas. M32 M32 dwarf elliptical galaxy, satellite of M31. M60 M60 giant elliptical galaxy in Virgo Cluster (on right, with NGC4647). M87 M87 giant elliptical galaxy, the dominant galaxy in Virgo Cluster. M32 M87

Elliptical Galaxies similar in form to globular clusters (i.e. nearly spherical) little gas and dust -- no disk -- little or no star formation little or no rotation normal ellipticals can contain from 1 to 100 billion stars. giant elliptical galaxies are found at the center of dense clusters of galaxies. a giant elliptical galaxy may have 100 times as many stars as the biggest normal galaxies Giant elliptical galaxies have grown and continue to grow by gravitationally merging with smaller galaxies that come too close This process is called galactic canabalismgalactic canabalism

3. Irregular Galaxies small amorphous galaxies normal galaxies disrupted by collisions The fundamental difference between Spirals and Ellipticals is rotation. Determines whether a disk forms or not. If a disk is present, the disk can collect gas and dust and star formation can be an ongoing process throughout the galaxy. Irregular galaxies. Irregular shape. M82

M-82 Galaxy M-82 is also known as the "Cigar Galaxy". It is not exactly edge-on, but it is very close. The galaxy is rich in star birth activities and shows a nice dark dusk region. The galaxy is located near the Big dipper and can only be seen from the northern hemisphere. Constellation: Ursa Major Size: 11.3 x 4.2 arcmin Magnitude: 9.3 RA: 09h 55m 54.0s Dec: +69d 40m 57s

Classification Galaxies come in a range of different shapes and sizes. There is a system by which we can describe what a galaxy looks like. It's called the Hubble "tuning fork" diagram and it classifies galaxies into characteristic shapes On the left of the diagram are the Elliptical galaxies, named by E0 to E7. The E stands for elliptical, and the number indicates how egg- shaped the ellipse is - 0 means a ball shape, and 7 looks rather like a thowing discus.

An E0 galaxy An E7 galaxy

After these simple types of galaxies the diagram splits into two. On the upper branch are the S0 galaxies first - called lenticular galaxies because they are shaped like a lens in a magnfiying glass. The description is made up of the "S", meaning lenticular, the "0", meaning no arms, and the subscript number indicates how heavily a stripe is absorbed out of the image of the galaxy by dust in the galactic disc. A lenticular galaxy with no absorbing dust - an S01 A lenticular galaxy with a lot of absorbing dust - an S03

Continuing along that branch the next 3 types are all have spiral arms, and they are grouped by how tightly those arms are wound and how large the central bulge is - the two happen to be closely related. The name is defined by the "S" and the lower case letter after which indicates how wound up the arms are: from "a" to "d".

An Sa galaxy - note the tightly- wound, smooth arms, and the central bright disc An Sb galaxy - better defined spiral arms An Sc galaxy - much more loosely wound spiral arms, both of which are clearly defined An Sd galaxy - very loose arms, with much of the luminosity in the arms, not the disc

The lower branch of the tuning fork diagram is largely a copy of the upper branch, but its occupants all have a line of stars through the centre - a bar. The first two pictures shown on the diagram are the SB0 galaxies. The B stands for barred, and a subscript number indicates how heavily defined the bar is: to the right is an SB02 galaxy.tuning fork diagram These pictures are of Barred Spirals - the centers of all spiral galaxies are not necessarily round, in some cases they can have an elongated "bar-like" structure to them (such as this galaxy - in fact our own Milky Way is a barred spiral galaxy)

An SBa galaxy - NGC Note the bright centre and tight spirals An SBb galaxy - NGC Better defined arms which are more loosely wound An SBc galaxy - NGC Even looser arms, and a much dimmer central portion of the galaxy