The Solar System

BAR

a

b Outline of an

E6 galaxy

10(a – b)————

a

Sa

Sb

Sc

Visual NGC 3623

Visual-wavelength image

Visual

NGC 3627 Visual

NGC 1365 Visual NGC 2997 Visual

Anglo-Australian Telescope Board

Anglo-Australian Telescope Board

Anglo-Australian Telescope Board

1 Elliptical galaxies are round or elliptical, contain no visible AURA/NOAO/NSF

gas and dust, and have few or no bright stars. They are

classified with a numerical index ranging from 1 to 7; E0s are round,

and E7s are highly elliptical. The index is calculated from the largest

and smallest diameter of the galaxy used in the following formula

and rounded to the nearest integer.

2 Spiral galaxies contain a disk and spiral arms. Their halo stars

are not visible, but presumably all spiral galaxies have halos. Spirals

contain gas and dust and hot, bright O and B stars, as shown at right and

below. The presence of short-lived O and B stars alerts us that star

formation is occurring in these galaxies. Sa galaxies have larger nuclei,

less gas and dust, and fewer hot, bright stars. Sc galaxies have small

nuclei, lots of gas and dust, and many hot, bright stars. Sb galaxies are

intermediate.

Roughly 2/3 of all spiral galaxies

arebarred spiral galaxies

classified SBa, SBb, and SBc. They

have an elongated nucleus with spiral

arms springing from the ends of the

bar, as shown at left. Our own galaxy

is a barred spiral.

2a

M87 is a giant elliptical galaxy

classified E1. It is several times

larger in diameter than our

own galaxy and is surrounded

by a swarm of over 500 globular

clusters.

The Leo 1 dwarf elliptical

galaxy is not many times

bigger than a globular

cluster.