Classification of celestial bodies began with Charles Messier in the 18th century. He began by cataloging celestial bodies and calling each of the unknown formations “nebulas” until it was deciphered as otherwise. Many nebulas today have named that begin with a capital M followed by a number. This is from Messier’s nebula classification system.
Today, it is easier to distinguish between nebulas, galaxies, and other celestial bodies. There are three different categories in which astronomers classify galaxies: spiral, elliptical, and irregular.
Spiral galaxies are composed of three main components: a central bulge called the nucleus, arms twisting outwards from the nucleus, and a halo around the galaxy where globular clusters are located. Research estimates that between 75-85% of galaxies are spiral, including our own galaxy, the Milky Way.
The nucleus is made up of dust, gas, star clusters, nebulas, and older stars. The nucleus is the hottest and brightest part of the galaxy because this is where the highest concentration of energy is located. The nucleus contains no new stars and no active star formation takes place here. The nucleus of our galaxy is located near the Sagittarius constellation.
The arms spiraling outward are where active star formation takes place. The arms are made up of stars, planets, and other celestial bodies. More young than old stars are found here. The arms spiral out from the nucleus in such a way that the galaxy is shaped like a frisbee or a flat disk. The diameter of the “face of the disk,” if you will, is much larger than the width of the disk. Our sun is located about 26,000 light years away from the center of the galaxy (about halfway out) on the arm named Orion. The diameter of the Milky Way is about 150,000 light years across, while the width ranges from about 400-2000 light years.
view from above a spiral galaxy
view from the side of a spiral galaxy
Surrounding the disk is a halo of gases where globular clusters near the galaxy reside. Globular clusters are masses with high concentrations of old stars. Astronomers measure the distance between us and the globular clusters in order to gain understanding about the size and shape of our galaxy.
A variation of the spiral galaxy with an elongated central bulge is known as a “barred spiral galaxy.”
Barred spiral galaxies often may only have two arms, one protruding from each side of the nucleus.
Elliptical galaxies can be oval in shape or spherical. Elliptical galaxies may or may not be in the shape of a flat disk. They tend to be smaller and made up of mostly old stars. There are usually no regions of concentrated active star formation in these galaxies.
a photograph of the elliptical galaxy M87
Elliptical galaxies are usually found in clusters of other elliptical galaxies. However, dwarf elliptical galaxies, which you can imagine are smaller, can usually be found near large spiral galaxies. Dwarf elliptical galaxies are more numerous than regular sized ones.
Any galaxies that cannot be described as spiral or elliptical are lumped under this category. Irregular galaxies are usually blob-shaped and appear to be in disarray, and theoretically could take on almost any shape.
two examples of irregular galaxies
You can usually tell what types of stars are in a galaxy by the galaxy’s color. Irregular galaxies, generally speaking, tend to be white and dusty (similar to the photo above on the left.) This indicates that they contain lots of young, massive stars.
As far as the local scene goes, irregular galaxies aren’t very common in our neck of the woods. Research finds that irregular galaxies are more common farther away from our own. Since looking out into space is looking into the past, this indicates that irregular galaxies were more common when the universe was younger.
OUR GALAXY VS. THE OTHERS
Commonality: Spiral galaxies are by far the most common type, with an estimated 75-85% of galaxies falling under this category. Elliptical galaxies are pretty common on our neck of the woods, but outside of that, only make up about 15% of galaxies. Irregular galaxies are less common near ours but more common farther out in space, suggesting that they occurred more regularly in the past than they do now.
Shape: While elliptical galaxies tend to be round and aren’t necessarily flat, spiral galaxies are shaped like a flat disk with a central bulge from which twisting arms protrude. Irregular galaxies can be any shape that isn’t spiral or elliptical and often appear as random gas in disarray.
Size: Elliptical galaxies are brighter and smaller while spirals are larger and dimmer. Among local galaxies, only a small percentage of those as large as the Milky Way take irregular forms, so these are also usually smaller. The Milky Way is a relatively large galaxy, stretching 150,000 light years across its diameter.
Star Formation: Elliptical galaxies tend to appear red, orange, or yellow in color since they are made up of older stars. The nucleus of spiral galaxies often appears white or yellow since the older stars are located there, but the arms are usually blue since active star formation takes place there. Elliptical galaxies don’t have very many young stars and spiral galaxies don’t have very many old ones. Irregular galaxies often appear white in color and in general are made up of many hot, young stars.
Presence of Dust: Elliptical galaxies do not have much dust since there is no active star formation. Since spiral galaxies undergo lots of star formation, these galaxies often have lots of dust. The dust in the Milky Way is so abundant that obstructs our view of the rest of the galaxy (although you can see a band of the galaxy from Earth on a clear night.) The dust in spiral galaxies appears as a darkened spot in images/telescopes and can usually be spotted around the edge of the disk when looking at the width of the galaxy. Irregular galaxies often appear as mostly dust because of the high rate of active star formation.
Star Types: M-spectral-type stars are the coolest and oldest on the spectral type scale, while O-spectral-type stars are the youngest and the hottest. Elliptical galaxies contain more M-spectral type stars than O-spectral types because there is no active star formation and they are composed of mostly older stars. Spiral galaxies more often than not contain both M-types and O-types because they have old stars located in the nucleus but also make new stars in the arms. Irregular galaxies, in general, are more likely to have mostly M-type stars because they do not usually have very many old stars and they undergo lots of active star formation.
Hubble’s “tuning fork” classification system for stars, used by astronomers around the world today.
Although it’s commonly believed that the Andromeda galaxy is our closest neighbor, this article I found on universetoday.com titled “What is the Closest Galaxy to the Milky Way?” begs to differ. Apparently, Andromeda is our closest spiral galaxy, but the closest galaxy in general is the Canis Major Dwarf galaxy, located about 25,000 light years away from the Milky Way (about the same distance between our sun and the nuclear bulge of our galaxy.)
The galaxy was discovered in 2003 when a massive experiment that surveyed about 70% of our sky lead researchers to find a concentration of M-type stars in the same part of the sky where the Canis Major constellation resides.
The Canis Major Dwarf galaxy is an elliptical galaxy believed to contain one billion stars, many of which are red giants. The galaxy has a complex structure of trailing filaments known as the Monoceros Ring, a unique feature which wraps around Canis Major Dwarf three times.
Scientists speculate that this galaxy is currently being ripped apart by gravitational forces much larger than that of the Milky Way, and that one day the remaining shreds of the galaxy will merge with our own.
This article relates to the conceptual objective because it discusses the discovery of and information about an elliptical galaxy. Not only does this particular galaxy have a unique ring wrapped around it three times–but it’s also the closest known galaxy to our own, even closer to our own! This is exciting because there’s an entire galaxy of old, cool, M-type stars very close to us, and we’ve only discovered its existence and proximity quite recently. Studying this galaxy can give us insight on other similar elliptical galaxies.
REFLECTION: I thought this was a really interesting article because we’ve recently discovered an entire galaxy that is much closer to us than Andromeda is. I also thought that this galaxy in particular was interesting because of the ring of star dust and space debris that wraps around it three times. I wish that the article had included a picture, but they only showed an illustration of its location relative to the Milky Way.
While searching for an article to include in this conceptual objective, I learned of a concept foreign to me known as “galactic cannibalism.” Galactic cannibalism can be defined as “the collision of two galaxies and the subsequent absorption of parts of one into the other.” (Source: x) Galactic cannibalism is not the same as galactic collision. In galactic collision, the two galaxies merge at low-impact speeds and then stop moving once they run out of momentum, often retaining much of their original shape. In galactic cannibalism, a larger galaxy with a more powerful gravitational pull rips apart and consumes a smaller galaxy, resulting in growth and an irregular shape for the larger galaxy.
Galactic cannibalism is a slow process that takes thousands of years, so scientists are unable to study a galaxy being swallowed whole from start to finish. Instead, they look at different galaxies currently in the process of being cannibalized and piece together the “snap shots” from various galaxies consuming other galaxies.
I found a very interesting (article) on the official Hubble Space Telescope website which features an image of a galaxy called NGC 4388, which is one of over 1300 galaxies in the Virgo cluster.
the image of NGC 4388 included in the Hubble Space Telescope article
This galaxy is particularly fascinating for scientists because it exhibits features of both an elliptical galaxy and a spiral galaxy. While the outer edges of the galaxy appear smooth, featureless, and orange in color, (characteristic of elliptical galaxies,) it also possesses a center bulge attached to two blue spiraling arms (which we know are undergoing active star formation due to their color.)
Having features of both an elliptical and spiral galaxy is thought to be caused by various gravitational interactions with the rest of the Virgo star cluster. Gravitational interactions include small blows, head-on collisions, tidal influencing*, galactic merging, and galactic cannibalism.
This article about NGC 4388 connects with the conceptual objective because the galaxy exemplifies features of both elliptical and spiral galaxies. (Dare I call this strange phenomenon…irregular?) It calls attention to the fact that there are only two real categories of galaxies plus a third lazy man’s category where the rest of the galaxies are lumped. The article raises questions about the way we think about galaxies–clearly, categorizing galaxies is not always so black and white.
REFLECTION: I really liked the article because it just goes to show you that there are exceptions to any rule. When it comes to most things in life, when you try to fit things into neat boxes and come up with rules and regulations for how things work, it usually doesn’t work. Although the galaxy is officially classified as a spiral, it really looks to me like it’s about 50/50. I also thought that the article was cool because of how much gravitational stress/damage the galaxy had to endure in order to transform into this awesome hyrbid galaxy. What doesn’t kill you makes you stronger, right?
*speaking of tidal influencing, I found this image (here) on the Hubble Space Telescope website shows galaxies whose shapes were distorted due to tidal interactions. I couldn’t resists including the photo in this post because it looks like a rose and it’s absolutely beautiful! This pair of galaxies is called Arp 273.