An Introduction To Galaxies And Their Interactions

Rishabh Nakra

Admin and Founder of ‘The Secrets Of The Universe’ and former intern at Indian Institute of Astrophysics, Bangalore, I am a science student pursuing a Master’s in Physics from India. I love to study and write about Stellar Astrophysics, Relativity & Quantum Mechanics.

Having done a couple of research projects on galaxies, I was most excited about this article and here it is! From our school days itself, we know that we live on Earth, which lies in our solar system, which is further a small part of a larger family called The Milky Way Galaxy. Studies show that there are about 2 trillion galaxies in the universe. Milky Way is just one of them. But this was not known 100 years ago. It was believed that the Milky Way is the only galaxy in the universe. Of course, we had images of Andromeda galaxy, the LMC and SMC, they were thought to be a part of the Milky Way.

So, in the 21st article of the series, Basics of Astrophysics, we aim to study the meaning of the word “Galaxy” and the basic classifications of the different galaxies found to date in our Universe.

Read all the articles of the Basics of Astrophysics series here

What Is A Galaxy?

The word galaxy basically refers to a huge collection of gas, dust, billions of stars and their solar systems, all held together by gravitational forces. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy’s center of mass. The number of stars in some galaxies is more than the total number of grains of sand on planet Earth!

Classification Of Galaxies

As more and more galaxies were discovered, there was a need to systematically classify them. The first serious effort to classify the galaxies came from Edwin Hubble. The Hubble’s tuning fork classification is still used today. In this diagram, Hubble classified the galaxies on the basis of their visual appearance into spirals, ellipticals, lenticulars, and irregulars, as shown in the image below.

Although this scheme is now considered somewhat too simple, the basic ideas still hold. The diagram is roughly divided into two parts: elliptical galaxies (ellipticals) and spiral galaxies (spirals). Hubble gave the ellipticals numbers from zero to seven, which characterize the ellipticity of the galaxy – “E0” is almost round, “E7” is very elliptical. The spirals were assigned letters from “a” to “c,” which characterize the compactness of their spiral arms. “Sa” spirals, for example, are tightly wound whereas “Sc” spirals are more loosely wound. Barred spirals have a “B” in their classification. An “SBc” is thus a loosely wound barred spiral galaxy. “S0,” or lenticular galaxies, are in the transition zones between ellipticals and spirals and bridge these two types.

Spiral Galaxies

Spiral galaxies make up roughly 72 percent of the galaxies that scientists have observed. Spirals are large rotating disks of stars and nebulae, surrounded by a shell of dark matter. An important feature of spiral galaxies is that they have a high star-formation (SFR) rate as compared to elliptical galaxies. The SFR is an important parameter in the research of galaxies. Spiral galaxies have three main components: a bulge, disk, and halo.

Bulge: Bulge refers to a spherical structure that lies in the center of the galaxy. This feature mostly contains older stars.
Disk: The disk mostly consists of dust, gas, and younger stars. The disk forms arm structures. Our Sun is located in an arm of our galaxy, the Milky Way.
Halo: Halo of a galaxy is a loose, spherical structure located around the bulge and some of the disk. The halo contains old clusters of stars, known as globular clusters.
A galaxy with poorly defined arms is known as a flocculent spiral galaxy, in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.

Barred spiral galaxies share the same features and functions as regular spiral galaxies, but they also have a bar of bright stars that lies along the center of the bulge and extends into the disk. The bright bulge has very little activity and contains mostly old, red stars. The bar and arms have lots of activity including star formation. Astronomers believe that the Milky Way is a barred spiral galaxy.

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Hubble denoted the normal spirals by S and the barred spirals by SB. The subclasses belonging to each are represented by lower case letters, s, b, and c. Here, Sa and SBa have a large luminous nucleus and tightly coiled arms, whereas Sc and SBc posses small nuclei and loose extended arms.

Elliptical Galaxies

Elliptical galaxies appear like ellipses, ie. stretched circles. Hubble classified them further according to their degree of flattening, known as ellipticity. There are eight types ranging from E0-E7. E0 ellipticals are nearly circular, while E7s are very stretched out. Elliptical galaxies mostly contain very old stars and do not have much gas and dust. Consequently, very little new star formation takes place in these galaxies. Thus they have a low SFR.

The largest galaxies in our Universe are giant ellipticals. Many elliptical galaxies are believed to have been formed due to the interaction of galaxies, resulting in a collision and merger of spiral galaxies. There are two observations to back this hypothesis. Firstly, stars in the elliptical galaxies have a high velocity dispersion. Velocity dispersion, in astrophysics, is a parameter that describes the statistical dispersion of velocities about the mean velocity for a group of astronomical objects. So the stars are moving in a “disordered” fashion (chaotic motion) as compared to spiral galaxies that have low velocity dispersion.

This can very likely be a result of the collision of two spiral galaxies. When the stars in one spiral galaxy having a particular angular momentum will collide with another spiral having different angular momentum, the result will be a larger galaxy with chaotic motion of stars.

The second is, of course, the size and mass of the ellipticals. They have a high stellar mass as compared to the spirals that show they might be formed as a result of a merger.

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Giant elliptical galaxies often exist near the core of large galaxy clusters. Elliptical galaxies have a subclass named Shell galaxies. A Shell galaxy is basically an elliptical galaxy, surrounded by faint arcs or shells of stars, often more blue than the galaxy as a whole.

Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies. Elliptical galaxies make up approximately 10%–15% of galaxies in the Virgo Supercluster, and they are not the dominant type of galaxy in the universe overall. 

Lenticular Galaxies

Apart from the above classification of spiral and elliptical galaxies, there exist some disk-shaped galaxies with no trace of elliptical arms. Hubble assumed these galaxies to be intermediate between spirals and elliptical and designated then as S0 or lenticular galaxies.

Irregular Galaxies

Unlike elliptical and spiral galaxies, irregular galaxies have no regular or symmetrical structure. We can further divide these into two groups, Irr I and Irr II. Irr I type galaxies have H II regions, which are regions of elemental hydrogen gas, and are home to Population I stars, which are young hot stars. Whereas, Irr II galaxies contain large amounts of dust that block most of the light from the stars. All this dust makes is almost impossible to see distinct stars in the galaxy.

Unlike elliptical and spiral galaxies, irregular galaxies have no regular or symmetrical structure. We can further divide these into two groups, Irr I and Irr II. Irr I type galaxies have H II regions, which are regions of elemental hydrogen gas, and are home to Population I stars, which are young hot stars. Whereas, Irr II galaxies contain large amounts of dust that block most of the light from the stars. All this dust makes is almost impossible to see distinct stars in the galaxy.

Interaction of Galaxies

The couple of projects I did in my master’s degree mainly focused on galaxy interactions. So I thought of writing a paragraph on it.

When Hubble started classifying the galaxies, he divided them into 4 groups as we discussed above. But later, as more observations were made with better telescopes, many ‘peculiar galaxies’ were also discovered. Most of them did not fit well into Hubble’s classification scheme. A particular catalog of such peculiar galaxies was the Arp catalog by Prof. Halton Arp. (You can read about my project on the Arp catalog here). A major portion of these galaxies was taken up interacting galaxies. In the last 50 years, a lot of research has been done to know what happens when two galaxies interact or merge with each other.

If a galaxy comes in the vicinity of another galaxy, it starts ‘feeling’ the gravitational effects. The tidal forces that act on these galaxies change their shape. In most cases, tidal tails start forming, as shown in the case of mice galaxies above. A major impact of galaxy interactions is on the star formation rate. It has been shown that galaxy interactions lead to an enhanced star formation rate. This enhancement is maximum in the case of merging galaxies. A classic example of mergers is NGC 4038 (the antennae galaxies) shown below.

Clouds of gas are seen in bright pink and red, surrounding the bright flashes of blue star-forming regions — some of which are partially obscured by dark patches of dust. The rate of star formation is so high that the Antennae Galaxies are said to be in a state of starburst, a period in which all of the gas within the galaxies is being used to form stars. This cannot last forever and neither can the separate galaxies; eventually the nuclei will coalesce, and the galaxies will begin their retirement together as one large elliptical galaxy.

The NED

Before ending this article, I want to share with you a very important website: NASA/IPAC Extragalactic Database or the NED. This is a database of objects outside the Milky Way galaxy and is used by many professional astronomers in their research. You just have to enter the name of the galaxy and every detail of it will be there on your screen. So suppose, you want to know about the Andromeda galaxy. You just have to type ‘Andromeda galaxy’ or M31 or any other name that is designated to this galaxy and there will be plenty of information waiting for you.

Some of that includes the redshift of the galaxy, the spectrum of the galaxy, its distance from us, its luminosity in different bands, magnitude, the FITS image, detailed morphology, position, etc. So if you are interested in research, you might wanna check it out. It will help you someday.

Author’s Message

Since the beginning, I was always fascinated by stellar astrophysics. I had no interest in galaxies. But thanks to the Indian Institute of Astrophysics, I am now doing research on galaxies. This is where internships help you. If you want to become a scientist, make sure you keep yourself open to learning new things every day. Research in astronomy not only requires a good hold on physics and astrophysics but also a knowledge of technical skills such as computer programming. The most widely used language is Python. We will more talk about this at the end of the series. I have written an article that answers the question, ‘how to become an astrophysicist?’. Stay tuned 🙂