HiSET Science: Characteristics of the Milky Way and Other Galaxies

Milky Way
The Milky Way, which houses the Earth's solar system, is a spiral galaxy. It consists of a central bulging disk, the center of which is referred to as a nucleus. Most of a galaxy's visible light comes from stars in this region. The disk is surrounded by a halo of stars and star clusters that spread above, next to, and beneath the nucleus. Globular clusters (dense, spherical clusters of ancient stars) are often found in the halo. Spiral arms of high-luminosity stars (from which this type of galaxy gets its name) fan out from the nucleus as well, with stars that are less bright in between. Interstellar dust populates the entire galaxy between celestial bodies. The entire galaxy rotates about the center.
While Earth, the Sun, and its solar system are located on the disk, we are far from the center of the Milky Way. The galaxy's mass, determined through the application of Kepler's third law to the Sun's orbit, is about 1,011 solar masses.

Structures of Galaxies
Elliptical galaxies are roughly spherical. Within this category, subgroups based on the degree of flattening exhibited in the galaxy's shape range from E0 (spherical) to E7 (flat). A dwarf elliptical galaxy has a spheroidal shape, with low mass and low luminosity. An S0 galaxy is similar in shape to a spiral galaxy, but lacks spiral arms. Spiral galaxies such as the Milky Way are characterized by disk-like nuclei with spiral arms. Subtypes of this category are determined by the tightness of the spiral arms and the size of the nucleus; a spiral galaxy of Sa type has a large nucleus and tightly wound arms, and an Sc-type galaxy consists of a small nucleus with open spiral arms. A barred spiral galaxy exhibits an elongated nucleus. The subtypes of barred spiral galaxies are determined like those of spiral galaxies. Some irregular galaxies (type I) display a loose spiral structure with high levels of disorganization. Other irregular galaxies (type II) can be of any shape.