Irregular, Elliptical, & Normal & Barred Spiral Galaxies
Edwin Hubble devised the scheme that astronomers use to classify galaxies outside the Milky Way galaxy.
The Milky Way is only one of billions of galaxies in the observable universe. Based on their shape, Edwin Hubble classified the types of galaxies we observe into normal spiral, barred spiral, elliptical, and irregular galaxies.
Spiral Galaxies typically contain a few hundred billion stars distributed in a disk a few thousand light years thick and over 100,000 light years in diameter. Their disks contain spiral arms that wind around each other several times. The stars, nebulae, and galactic (or open) clusters in the disks tend to be younger and contain about 2% of elements heavier than hydrogen and helium in their composition. These stars are Population I stars. A spherical halo surrounds the disk. The halo contains globular clusters of old, Population II, stars with few elements heavier than hydrogen and helium.
Both normal and barred spiral are similar in size, number of stars, and structure to the Milky Way galaxy. The difference is a central bar structure. In normal spiral galaxies the spiral arms originate in the center. In barred spiral galaxies, the spiral arms originate at the end of a central bar like structure that extends outward from either side of the nucleus.
Traditionally astronomers considered the Milky Way to be a normal spiral galaxy, but there is now evidence, not proof, that it is actually a barred spiral galaxy.
In his classification scheme, Hubble denoted normal spiral galaxies with the capital letter S and barred spiral galaxies with the capital letters SB. He then added a small a, b, or c based on the extent of the spiral arms. The letter a indicated that the spiral arms were not extensive, tightly wound, and poorly defined. At the other extreme the letter c indicated extensive, loosely wound, and well defined spiral arms. the classifications for spiral galaxies would then look like Sc, SBb, or something similar.
Elliptical galaxies have either a spherical or an elliptical (American football like) shape.
Giant elliptical galaxies are larger and more massive than the Milky Way. They can contain about a trillion stars and be about 150,000 light years in diameter. Dwarf elliptical galaxies only contain in the neighborhood of a few million stars. Like the stars in the halos of spiral galaxies, the stars in elliptical galaxies are generally very old Population II stars. Elliptical galaxies contain very little interstellar matter and very few nebulae, so new stars are no longer being formed.
Dwarf elliptical galaxies are hard to find because they are so faint. They are however very common. Our Local Group of galaxies contains many more dwarf elliptical galaxies than spiral galaxies, and they are often companion galaxies to spiral galaxies.
In his classification scheme, Hubble denoted elliptical galaxies with a capital E. He also added a number from 0 to 7. A 0 indicates the galaxy is spherical and a 7 indicates the most elongated elliptical galaxies. Hubble’s initial scheme did not distinguish between giant and dwarf elliptical galaxies.
Hubble classified galaxies that have no regular shape and do not fit into the scheme of spiral or elliptical galaxies as irregular galaxies. Irregular galaxies contain mostly fairly young Population I stars. They also contain a fairly large percentage of nebulae and other forms of interstellar matter, which is the raw material for making new stars.
In the 1920s Hubble proved that galaxies were indeed outside the Milky Way galaxy and invented the classification scheme that astronomers still use today.