Types of Galaxies: The Brilliant Collections of Stars

There are four primary types of Galaxies in our Universe; Spiral, Elliptical, Peculiar, and Irregular. Scientists estimate that there are approximately two trillion Galaxies in our universe.  

In 1926 Edwin Hubble developed a process to classify the shape of galaxies into four primary categories. With advances in telescopes and the invention of computers, we’re better able to discern the differences between galaxies’ visual appearances.

What’s a Galaxy? 

A Galaxy is a collection of stars, dark matter, dust, and gasses held together by gravity. A single galaxy may have billions of stars in it. 

A galaxy’s behavior is how its planets, stars, dust, and gasses interact with the gravity of other galaxies.

 Example: Milky Way

The Milky Way is one galaxy in the universe and is our home galaxy!

• Scientists have discovered, defined as definitive proof, approximately three thousand two hundred stars in the Milky Way.
• Scientists estimate that there are over two billion stars in the Milky Way.
• Each star may have planets revolving around it (called exoplanets)
• Scientists estimate that there are between one hundred and two hundred billion planets in the Milky Way.
  • Food for thought: Scientists also estimate that ⅕ of the exoplanets in the Milky Way can sustain life, similar to Earth. 
• The Milky Way is a spiral galaxy.

Galaxy Classification 

Hubble created an optical morphology process to classify Galaxies into distinct categories based on their appearance. Hubble placed each galaxy into a class in his “tuning fork.”  

Hubble was incorrect in interpreting the results of his Tuning Fork in relationship to a galaxies age. The basic methodology pioneered by Hubble for Galaxy identification is used today.

The diagram has two different categories. 

1. Elliptical galaxies. Identified with a value of zero to seven. The higher the number, the greater the elliptic value. (E7 has a very elliptical shape and E0 has a round shape.)  
2. Spiral galaxies. Identified with the letter “S” followed by a letter between “a” and “c.” The “a, b, c” classification identifies how tightly the spirals, or arms, are wound around the galaxy.
   1. Normal Spirals-Pinwheel or whirlpool shaped. Sa, Sb, Sc. 
   2. Barred Spirals have the letter “B” in their name/classification, such as SBa. A bar of stars runs through the center bulge of the galaxy. 
   3. S0 identifies a spiral-shaped galaxy with no visible arms that pinwheel out from the bulge at the galaxy’s center. 

Three Types of Galaxies (or Perhaps Four)

Spiral

Seventy-seven percent of all the known galaxies in the universe are spiral galaxies. Spiral Galaxies have three features:a

1. Disk: Stars, dust, and gas that move in a circular orbit around the center of the galaxy
2. Buldge: Located in the center of the galaxy. The bulge is spherical shaped and is home to only old stars that are dimmer than their neighbors. 
3. Halo: Located at the outer edge of the galaxy, the halo is home to the galaxy’s older stars. The older stars are globular clusters. 

The image above of M51, also known as the Whirlpool Galaxy, is an example of a Spiral Galaxy. The Milky Way is also a Spiral Galaxy.

Elliptical

Elliptical galaxies typically contain older stars that are dimmer and have a redder color than other stars. Elliptical galaxies contain the oldest stars in a galaxy. The star’s orbits in their galaxies are elongated and somewhat random. 

Generally located near galaxy clusters, Elliptical Galaxies are separated into three categories. 

1. Ordinary: Shaped anywhere from a perfect oval shape to a highly elliptical sphere.
2. Giant: May stretch thousands of light years across.
3. Dwarf: May “only” stretch a few hundred light miles from side to side.

Many characterizations help to identify Elliptical galaxies.

1. Classified based on their elliptical share (E0 to E7)
2. Lack a bulge in the center of the galaxy
3. The center of the galaxy is (typically) the brightest
4. Older stars with lower mass and fewer new stars than other galaxy types
5. A Black hole is located in the center of the galaxy. The black hole may prevent the formation of new stars by consuming the ingredients required for star creation: Dust and gasses. 

The Hubble Space Telescope captured the image above of the elliptical galaxy NGC-1316. Scientists believe two spiral galaxies combined approximately two billion years ago and formed one elliptical galaxy. 

Peculiar

A Peculiar galaxy is in the active process of colliding with another galaxy. It may be an Elliptical or Spiral galaxy. The shape of the galaxies distorts during the collision. The collision causes their shapes to warp, and they look “peculiar.”  

The “collision” will take billions of years to occur. There may be actual collisions when the collision occurs, but most likely not too many.  

• Think in terms of a freeway with 1000 lanes of traffic. One planet in the approaching galaxy may be traveling in lane one, and another planet from the resident galaxy may be traveling in lane eight hundred. The odds of the two planets colliding are very low. Outer space is relatively empty!
• Gas clouds and dust are on the same freeway. They’re traveling in lanes one through five hundred on the approaching galaxy and lanes three hundred through eight hundred on the resident galaxy. Gas clouds and dust collide. New stars and planets will be formed.
• Think positive! The Milky Way and the Andromeda galaxies are on a collision course. We’ll have a front-row seat in a few billion years. 

NGC 7469 is a barred spiral galaxy that’s in the process of colliding with IC 5283. The merging galaxies are named ARP 298. 

Irregular

Need a bucket to deposit all the galaxies that don’t easily fit into the Spiral, Elliptical or Peculiar categories? Enter the Irregular Galaxy category.  

Irregular galaxies are small dwarf galaxies that lack a discernable shape. There are no spiraling arms, a bulge in the middle of the galaxy, or a disk to help classify the galaxy.

Scientists have created three categories for irregular galaxies.

1. Irr I-There’s something there, but it doesn’t fall into Hubble’s Classification chart.
2. Irr II-No discernable structures
3. DIrr III-A dwarf irregular galaxy

Irregular Galaxies are challenging to locate! IC 10 is currently in the star formation process in the image above. 

Wrap up!

There are differing scientific opinions about what types of galaxies exist. There’s an agreement concerning the kinds of galaxies noted on Hubble’s flowchart, but more understanding is needed regarding irregular galaxies. Some scientists believe there are three types of galaxies, while others think there are four types of galaxies.  

What isn’t up for debate is that there are over two trillion galaxies in our universe. With enough data collection and discussion, we believe that we’ll select the ideal classification system.