Spiral vs Elliptical Galaxies
Galaxies are massive collections of stars. They also contain large interstellar gas clouds known as nebulae. These large superstructures of stars were not identified and studied properly until the late 18th and 19th centuries. Even then these were considered as nebulae. These collections of stars lie beyond the vicinity of Milky Way, which is our collection of stars. Majority of the objects in the night sky belong to this galaxy but, if you observe closely, you can identify the twin galaxy of the Milky Way; The Andromeda Galaxy. However, the limited strength of the telescopes allowed only little penetration into the deeper skies; therefore, understanding of these distant astronomical objects was vague. Real explanation into the structure of these magnificent astronomical bodies came much later.
In the early 20th century, Edwin Hubble made an extensive study of galaxies and classified those based on their shape and structure. The two main categories of the galaxies were spiral and the elliptical galaxies. Based on the shape of the spiral arms, spiral galaxies were further classified into two sub categories as Spiral Galaxies (S) and Barred Spiral Galaxies (SB). (Refer to following illustration)
Spiral galaxies are named in such a way because of the winding spiral arms clearly visible in this type of galaxies. These galaxies are flat disk shaped with roughly circular perimeter and bulging center. The spiral galaxies are the most common type of galaxies observable in the universe (around 75%), and our own galaxy, the Milky Way, is also a spiral galaxy. The spiral galaxies were the first type of galaxies to be observed by human, and that was our neighboring galaxy, the Andromeda.
In general, spiral galaxies contain roughly 109 to 1011 solar masses and have luminosity between 108 and 2×1010 solar luminosity. The diameter of the spiral galaxies can vary from 5 kilo parsecs to 250 kilo parsecs. The disk of the spiral galaxies contain younger, Population I stars, whereas the central bulge and the halo contain both Population I and Population II stars.
Theoretically, spiral arms are created by density waves sweeping through the galaxy disk. These density waves create areas of stellar formation and the brighter younger stars in high density within these areas result in a higher luminosity from the area.
The two sub categories of spiral galaxies, Spiral galaxies and Barred Spiral Galaxies are further divided into three subclasses each, based on the shape and the structure of the spiral arms. Sa, Sb and Sc are Spiral galaxies subclasses, while SBa, SBb and SBc are barred spiral subclasses.
Elliptical galaxies have the characteristic oval shape in their outer perimeter and any formation such as spiral arms are not visible. Even though elliptical galaxies display no internal structure, they also have a denser nucleus. Roughly 20% of the galaxies in the universe are elliptical galaxies.
An elliptical galaxy may contain 105 to 1013 solar masses and may create luminosity between 3×105 to 1011 solar luminosities. The diameter may range from 1 kilo parsec to 200 kilo parsecs. An elliptical galaxy contains a mixture of Population I and Population II stars within the body.
Elliptical galaxies have eight subclasses E0-E7, where eccentricity increase in the direction of E0 to E7, and E0 is roughly spherical in shape.
What is the difference between Spiral and Elliptical Galaxies?
• Spiral galaxies have a flat disk like shape and a bulging center with spiral arms consisting the disk. Elliptical galaxies are ellipsoids with no clearly visible internal structure.
• Spiral galaxies have a very dense nucleus and a region of stars bulging outwards from the disks and, therefore, called the central bulge. Elliptical galaxies also have dense centers, but they do not protrude from the body of the galaxy.
• Spiral galaxies are the most common type of galaxies and contain three quarters of all the galaxy population. Elliptical galaxies are relatively rare and contains only one fifth of the galaxy population.
• Spiral galaxies have star forming regions in spiral arms; therefore have majority Population I stars. There are both Population I and II stars in the halo and the central bulge. Elliptical galaxies, having no structure have a mixture of Population I and II stars.