Galaxy - Spiral Galaxies
Spiral galaxies have a disk shape with a bulging central nucleus, so that they look like an astronaut's pancake floating in midair with a fried egg in the center on both sides. Surrounding the disk is a spherical halo consisting of globular clusters—spherical clusters of roughly 100,000 stars each. The astronaut's breakfast has drops of syrup floating in a spherical distribution around the pancake.
The disk of a spiral galaxy contains the spiral arms that give class of galaxy its name. There are usually two spiral arms that wind around each other several times in a whorl from the nucleus to the edge of the disk. A few spiral galaxies have more than two spiral arms.
There are two types of spiral galaxies, normal spirals and barred spirals. In the normal spiral galaxies, the spiral arms wind outward from the nucleus. In barred spirals, there is a central bar structure extending out on either side of the nucleus. The spiral arms wind outward from the edge of this bar structure.
In his classification scheme, Hubble denoted normal spiral galaxies by S and barred spiral galaxies by SB. He then subclassified spirals according to how tightly the spiral arms wind around the nucleus, using a, b, and c. Galaxies denoted Sa are the most tightly wound and therefore have a relatively small disk compared to the spiral arms. Sc galaxies are the most loosely wound. They therefore extend well beyond the nucleus and have a relatively larger disk compared to the nucleus. Sb galaxies are intermediate between the Sa and Sc galaxies. Hubble used a similar scheme for barred spirals, producing the classifications SBa, SBb, and SBc.
Some galaxies have a disk surrounding a nucleus, but do not have spiral arms in the disk. Hubble classified these galaxies as SO. They are now also called lenticular galaxies. As for elliptical galaxies, modern astronomers also add luminosity classes (I, II, III, IV, V) to Hubble's classification scheme. Luminosity class I galaxies are the most luminous and are referred to as supergiant spirals. Luminosity class V galaxies are the least luminous.
The luminosity classes of spiral galaxies do not have as wide a range as elliptical galaxies, so there are no dwarf spiral galaxies. Spiral galaxies are typically about the size of the Milky Way, roughly 100,000 light years in diameter. They will typically have a mass of about 100 billion times the mass of the sum, so will contain roughly 100 billion stars. The largest supergiant spirals can have as much as several trillion times the mass of the sun.
Spiral galaxies contain fairly young stars in their disks and spiral arms and older stars in their nuclei and halos. The disks and spiral arms also contain interstellar gas and dust, which are the raw materials for forming new stars. The halos like elliptical galaxies contain very little gas and dust. This difference in the distribution of the contents of spiral galaxies tells us that they were originally spherical in shape. The rotation of these galaxies caused them to flatten out and form their disks.