Imagine being placed inside a completely dark and large room. There are people inside the room and they are holding faint penlights. Some of them may be holding their lights above their heads while others may be letting their lights swing with their hands. The people are moving. How then can a new person in the room make sense of his surroundings? This new person is facing a task that is similar to that of scientists – to make sense of an immense universe populated by stars and galaxies. But, unlike one lone person who feels no need to report what he sees, the scientists are pressured to come up with theories about how the universe came about and how the galaxies were formed. And so far, here are the most widely accepted ideas about galactic formation and evolution.
A galaxy is composed of numerous stars. Through a series of complex geometric and mathematical processes, astronomers estimate that there are about 100 billion stars that make up the Milky Way galaxy (Frommert & Kronberg). This number may be within an error margin of a million or so stars. But for most people on earth, 100 billion or 200 billion stars in the Milky Way does not make any significant difference. These stars are too far and other galaxies do not have a significant impact on the people’s daily lives. Yet, for astronomers who wanted to make sense of the universe, the number of stars in a galaxy may provide clues to its formation and evolution. The number of stars or stellar mass is a distinct characteristic among the types of galaxies.
According to shape, there are three major types of galaxies. These are the spiral, the elliptical, and the irregular. Edwin Powell Hubble established this classification during the early part of the twentieth century and it is still being followed today. This is because as time went by, other characteristics of the galaxies supported this classification according to shape. These other characteristics include the stellar and gas content, the internal motions, the history of formation, and possibly, the most likely path of evolution.
Spiral Galaxy. The estimated mass of spiral galaxies is 109 to 1012 times of the mass of the sun. The spiral galaxy shows two known components, which are called Population I and Population II. The Population I is composed of a large amount of interstellar matter and of open star clusters. The Population II, on the other hand, has very little interstellar matter and globular clusters. The interstellar matter, which is seen as nebulae or dust clouds, is considered quite important in the formation of stars. In fact, the open star clusters that accompany interstellar matter in Population I are young stars. These young stars are arranged either into spiral patterns or bar-like structures. Scientists estimate that 75% of all galaxies in the universe belong to the spiral type (Pogge). Aside from the Milky Way, two other known galaxies were found to be of the spiral type: the Whirlpool galaxy and the Andromeda galaxy.
Within the classification of the spiral galaxy, there are sub-types, the ordinary spiral galaxies (Type S) and the barred spiral galaxies (Type SB). And within the Type S classification, the ordinary spiral galaxies are further grouped into Type Sa, Type Sb, and Type Sc. The Type Sa galaxies show a distinct and large bulge at the center but the spiral arms are not distinct. The Type Sc, on the other hand, has a small bulge and the spiral arms can be clearly seen. And finally, the Type Sb is the intermediate between the Type Sa and the Type Sc. A similar sub-classification is found among the barred-spiral galaxies. There are Type Sba, Type SBb and Type SBc. But, to differentiate the ordinary spiral galaxy from the barred-spiral galaxy, the latter has a central bar-like structure and from the two ends of this structure, the spiral arms are found. The ordinary spiral galaxy has the spiral arms emerging from all directions of the spherical disc-like center. In an intensive astronomical study of the Milky Way galaxy, scientists found that it is classified as a Type SBb.
A special type of spiral galaxy is known as the lenticular galaxy, which is assigned with the name Type S0. It appears to be spiral, but it has no spiral arms. Scientists assumed that the absence of the spiral arms indicates that no new stars are being formed. The most probable reason why no new stars are formed is because the galaxy ran out of interstellar matter. Thus, a lenticular galaxy is an old galaxy and mostly made up of the Population II component.
Elliptical Galaxy. The estimated mass of an elliptical galaxy is significantly lesser than that of the spiral galaxy. It is about 105 to 1013 times of the mass of the sun. But, the elliptical galaxy can be four times wider in diameter than the spiral galaxy. This means that the elliptical galaxy may appear bigger but it has lesser interstellar mass. The size of the elliptical galaxy might be the reason why, despite the lesser interstellar mass, it is almost as bright as the spiral galaxy. Scientists estimate that about 20% of the galaxies in the universe are of the elliptical type. Some examples of elliptical galaxies are M87 and M32. The naming of galaxies using letters and numbers has simplified the process of mapping the universe since there will be less time taken up by debates about what to name a discovered celestial body.
A typical elliptical galaxy is composed of very old stars and the instruments utilized by astronomers can detect very little gas or dust. This means that the majority of the stars in elliptical galaxies are red in color. Compared to those found in spiral galaxies, the motion of the stars in elliptical galaxies are slower. Some scientists also propose that there seems to be no existing unifying pattern or overwhelming force that govern or control the motion of the stars, except that they go around the central region. However, there is still a sub-classification of elliptical galaxies. These are the Type E0 and the Type E7. The former indicates that the elliptical galaxy is more spherical whereas the latter indicates that the galaxy is flat. From the perspective of evolution, some scientists theorize that the elliptical galaxies are the earliest galaxies that were formed and their present characteristics predict how the spiral galaxies will appear, billions of years from the now.
Irregular Galaxy. The estimated interstellar mass of irregular galaxies is roughly similar to that of the spiral galaxies. However, unlike the spiral galaxies, the irregular galaxies are smaller. This is probably because there is more gas than stars in irregular galaxies. The majority of the stars in irregular galaxies have been found to be blue in color. This implies that the irregular galaxies are composed mostly of young stars. But similar to elliptical galaxies, the stars in irregular galaxies have chaotic motions, following no particular pattern. Scientists estimate that about 5% of all galaxies in the universe are irregular galaxies. Some examples of irregular galaxies are the large and small Magellanic Clouds and the NGC1427A. If the elliptical galaxies are largely composed of Population II stars, the irregular galaxies are largely composed of Population I stars. From the perspective of galactic evolution, it can be surmised that the irregular galaxies exhibit the appearance of young galaxies that started appearing right after the Big Bang (Lochner). However, the irregular type of galaxies cannot be considered as the youngest galaxies because there is another newly discovered type of galaxy called the dwarf galaxies.
Too few information has been obtained about dwarf galaxies but scientists have found that the dwarf galaxies are also irregular in shape, with very little brightness or luminosity, and, most important of all, at the beginning of forming new stars. Thus, if the types of galaxies are arranged from the youngest to the oldest, the sequence would be this: first, dwarf galaxies; second, irregular galaxies; third, spiral galaxies; and last, elliptical galaxies.
There is no agreement or consensus among scientists about how galaxies are formed (Dey). Some proposed that clumps of mass, after the Big Bang, were attracted to each other and eventually formed galaxies. But, perhaps, the puzzle to the formation of galaxies may be solved because of the discovery of supermassive black holes. Black holes are supposed to be the result of a large star that imploded. It is considered a black hole because no light can escape from it. But scientists have found that all galaxies have supermassive black holes at the center. This implies that these black holes may have played an important role in the formation of galaxies. These supermassive black holes may be considered the nuclear engine for the creation of galaxies (White, et al).
These existing ideas and theories may be well accepted today, but as technology advances and more sensitive and sophisticated instruments are developed, there would be additional discoveries that would contradict or topple down the said existing theories. This means that the ideas about galaxies that will be presented in this paper may no longer be considered relevant five years from now. Nevertheless, the quest for knowledge is a continuous journey and only the courageous can finally hold the truth. Why is courage needed? This is because the journey would be besieged by uncertainty, false logic, and wrong assumptions. However, it is through these mistakes that scientists learn and find the truth about the universe. Intellect also undergoes an evolution.
Dey, Arjun. Formation and Evolution of Galaxies. 29 May 1999. 28 July 2007. <http://www.noao.edu/swift/proposal/node6.html>.
Frommert, Hartmut & Kronberg, Christine. Galaxies, The Messier Catalog. 12 July 2007. Students for the Exploration and Development of Space. 28 July 2007. <http://www.seds.org/messier/galaxy.html>.
Lochner, Jim. The Hidden Lives of Galaxies – Formation of Galaxies. 19 November 2004. NASA Goddard Space Flight Center. 28 July 2007. <http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/imagine/page22.html>.
Pogge, Richard. Astronomy 162: Introduction to Stars, Galaxies, & the Universe, 12 February 2006. Ohio State University. 28 July 2007. <http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Unit4/types.html>.
White, Andrew, Scott, Katherine, Hurr, Susan, & Hurley, Jon. Theories Surrounding the Supermassive Black Holes. 2007. 28 July 2007. <http://www.users.muohio.edu/whiteak1/index.html>.