Evidence found in 1924 meant that the Milky Way galaxy was not the only galaxy. With the discovery of other galaxies, Hubble began to observe the properties of these galaxies, noting the similarities and the differences. The main types are spiral, elliptical, and irregular galaxies. Examples of spiral galaxies are Milky Way, Andromeda, Pinwheel, Sunflower, and Triangulum Galaxies. Elliptical Galaxies examples include Leo I Dwarf Spheroid and Galaxy M60. The two best-known irregular galaxies are the Large Magellanic Cloud and the Small Magellanic Cloud. (Fraknoi, Morrison, &Wolff, 2018, p.943) Below is a comparison and contrast of the spiral and the elliptical galaxies.
Similarities
The spiral and elliptical galaxies are astronomical systems containing stars, dust, gas, and dark matter. The components of both the spiral galaxies and the elliptical galaxies are held together by the force of gravity. Stars orbit a galactic center in both spiral and elliptical galaxies. Spiral galaxies and elliptical galaxies are believed to have massive black holes at their centers. Population II stars are found in spiral galaxies as well as in elliptical galaxies.
Differences
Spiral galaxies are shaped like a flat disk and have a bulging center with spiral arms, while elliptical galaxies are shaped like spheres or ellipsoids, and their internal structure is not visible. The diameter of the luminous parts of the spiral galaxies is estimated to range from 20,000 to more than 100,000 light-years light-years. On the other hand, the diameters of large elliptical galaxies are more than several hundred thousand light-years and are larger than the largest spirals. Moreover, Spiral galaxies contain dust and emission nebulae, and hence new star formation is still occurring, however, the elliptical galaxies have very little or no dust and emission nebulae; hence, there is no new star formation. Spiral galaxies consist of the population I star with a bluish light and population II stars, while elliptical galaxies only consist of population II stars dominated by a reddish light.
The Hubble Sequence
The Hubble sequence is a morphological classification scheme invented by Edwin Hubble in 1925, where astronomers and researchers use visual appearance to classify galaxies. Hubble observed the features of galaxies and looked for order among their characteristics. The correlations found in the characteristics of galaxies by Hubble formed a basis for his classification scheme. The scheme is often referred to as the Hubble tuning fork diagram.
In modern astronomy, the Hubble Sequence is still used. The physical properties of galaxies such as color and luminosities are still useful in the field of extragalactic astronomy. Modern improvements to the sequence have been made, and hence it remains relevant. However, the criteria for assigning classes to galaxies in the Hubble sequence is subjective and leads to different observers assigning galaxies to different classes. For faint or distant galaxies, visual classification becomes unreliable. The wavelength in which galaxies are observed can change the appearance of galaxies.
Galactic evolution began shortly after the Big Bang, and it is a process in which galaxies form and evolve steadily over time. In about 4 billion years, the Milky Way Galaxy is predicted to merge with the Andromeda Galaxy. This observation is proof of galactic evolution. However, in recent observations, there have been discoveries of very big structures of galaxies, which are too large to have been formed in the time since the Big Bang.
Conclusion
Today, we have learned that the Milky Way is a barred spiral galaxy. In addition, the Wolfe Disk galaxy, which is more than 12 billion years old, was discovered to be the oldest galaxy in the Spiral Galaxies. Moreover, we now know that elliptical galaxies are formed from the collisions and merges of spiral galaxies. With the universe ever-expanding universe and more merges occurring, we hope to learn whether it will lead to the end of the universe or it will be an eternal universe.
Reference
Fraknoi, A., Morrison, D., &Wolff, S.C. (2018). Astronomy.