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What Are Galaxies? (Jargon Free!)

What Are Galaxies

With the successful launch and operation of the James Webb Space Telescope, you may have seen some of the amazing images it has captured so far. One of James Webb’s first images can be seen below. 

galaxy

Image Credit: NASA/ESA/STSci

This image is of Stephans’ Quintet, a visual grouping of five galaxies. While the resolution of these five galaxies is amazing, James Webb is so powerful we can see thousands of galaxies in the background of this image as well.  What are galaxies? How many are there? How are they made? 

What Are Galaxies?

Galaxies are objects made up of billions of stars, their solar systems, gas, dust, and dark matter, all held together by gravity. Galaxies are the building blocks of the Universe. They are billions of years old and have been shaped over their lifetimes by interactions within the galaxy and with other galaxies. Though it is impossible to count every galaxy and get an exact number, astronomers estimate that there are between 100 billion and 200 billion galaxies in our Universe. 

What Types of Galaxies Are There?

There are three classifications for galaxies: spiral, elliptical, and irregular. Spiral galaxies have curved arms. Elliptical galaxies are smooth and oval shaped. Irregular galaxies are galaxies with no definite shape. Galaxies in any category span a wide range of sizes. Dwarf galaxies only contain as few as 100 million stars whereas the largest galaxies contain trillions of stars. 

What Types of Galaxies Are There

Image Credit: A. Feild (STScI)

Elliptical Galaxies

Approximately one third of galaxies are elliptical galaxies. They can vary in shape from almost circular to elongated like a cigar. In comparison to other types of galaxies elliptical galaxies contain little gas and dust and are no longer actively forming stars. The stars in elliptical galaxies are older and often give them a yellow orange color. The smallest elliptical galaxies are only a few thousand light years across. Giant elliptical galaxies are rare and are approximately 300,000 light years across. 

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Spiral Galaxies

Spiral galaxies are divided into two groups: barred spirals and normal spirals. Barred spirals stand out from normal spirals because their arms do not reach all the way into the center of the galaxy. The arms are connected to the two ends of a straight bar through the center of the galaxy. 

Left: The Great Barred Sprial Galaxy (Credit: Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA) Right: Spiral galaxy (Credit: ESA/Hubble & NASA, ESO, J. Lee and the PHANGS-HST Team)

Spiral galaxies are still actively forming stars, this gives them a blue-white appearance. A large fraction of the local Universe is spiral galaxies. Our galaxy, the Milky Way is a spiral galaxy. Our solar system is on the outskirts of one of its spiral arms.

Spiral Galaxies

Image Credit: NASA

Irregular Galaxies

Irregular galaxies have very little dust and are very abundant in the early Universe. As we look at the early Universe we are looking back in time. It is suspected that there are so many irregular galaxies in the early Universe because they have not formed into spiral or elliptical galaxies. 

In addition to these three categories, astronomers often see many unusually shaped galaxies. These galaxies spear to be in a transition phase of their evolution. This could be from interacting or colliding with another galaxy, or the galaxies central active galactic nuclei ejecting material from the galaxy. 

How Do Galaxies Form?

Scientists do not know for sure how galaxies formed and evolved into the shapes we see today, but they have theories about their origins and evolutions. With the help of supercomputers scientists can create and run simulations to look back in time to the early Universe and simulate how a galaxy may have formed and evolved over its lifetime. 

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Scientists have concluded that the oldest galaxies formed fairly soon after the Big Bang. Most galaxies formed during the early stages of the Universe, though some galaxies have formed in the past few billion years. Which is relatively recent on the scale of a 13.8-billion-year-old Universe.

Scientists predict that the early Universe was mainly filled with hydrogen and helium, though not uniformly dispersed. Some areas were denser than others. These dense regions allowed for hydrogen and helium to accumulate into small clouds that eventually collapse under their gravity. This formed the first generation of stars.

These clouds continued to collapse and form stars as well as be gravitationally attracted to other clouds. As these clouds started to combine with each other they began spinning and further collapsing creating stars along the way. This gas, dust, stars, and dark matter formed into the galaxies we see today. 

Galaxy Evolution

Understanding how galaxies evolve is a major field of study. There are three key ways galaxies evolve. 

Passive Evolution – This occurs when a galaxy remains undisturbed by interactions or mergers with other galaxies. Star formation eventually stops, and these galaxies slowly become fainter and redder as the stars age and eventually die. 

Interactions and Mergers – These processes may or may not spark the formation of new stars. If these interactions do not bring new stars, then only the shape of the galaxies evolve into something new. If however, these interactions spark the birth of new stars, the luminosity and color of the galaxies will change as well as the shape. 

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Secular Evolution – This evolution is driven by internal factors in the galaxy such as star formation or active galactic nuclei activity. 

Galaxy Collisions

While the space between galaxies seems enormous, it is all relative. Galaxies are extremely large, so compared to stars, galaxies are relatively close to each other. This causes galaxies to interact and collide with one another. As galaxies collide, they pass through one another, their stars do not collide into each other.

Because stars are so small compared to galaxies, they have relatively large distances between them, making collision during a merger extremely rare. The interaction of the galaxies gravitational forces can sometimes cause new star formation, supernovas, or even form black holes

Galaxy Collisions

Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration NEWS RELEASE: 2006-46 >

Will the Milkey Way Collide with Another Galaxy?

Yes! Our Milky Way galaxy is on track to collide with our neighbor the Andromeda galaxy in four billion years. While the initial collision with Andromeda will take place in four billion years, computer simulations predict that it will take another two billion years for the galaxies to completely merge into an elliptical galaxy. Below is an artist’s depiction of what our sky will look like in 3.75 billion years, just before our Milkey Way collides with Andromeda.

Will the Milkey Way Collide with Another Galaxy

Illustraion Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), T. Hallas, and A. Mellinger NEWS RELEASE: 2012-20 >

References

https://www.space.com/25303-how-many-galaxies-are-in-the-universe.html
https://astronomy.swin.edu.au/cosmos/E/evolution+of+galaxies
https://noirlab.edu/public/images/iotw2127a/
https://esahubble.org/images/potw2114a/
https://esahubble.org/wordbank/spiral-galaxy/#:~:text=Barred%20spirals%20differ%20from%20normal,to%20be%20barred%20spiral%20galaxies.
https://www.nasa.gov/webbfirstimages
https://hubblesite.org/science/galaxies
https://svs.gsfc.nasa.gov/11534#:~:text=Galaxies%20are%20thought%20to%20begin,them%20into%20larger%20spinning%20packs.
https://spaceplace.nasa.gov/galaxy/en/
https://spaceplace.nasa.gov/galactic-explorer/en/

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Cassie Hatcher

Cassie Hatcher

Cassie is a lifelong learner with a passion for communicating high level science in a conversational matter. She holds a B.S. and M.S. in physics and has written two astronomy theses, one of which is published. She earned an internship at NASA’s Goddard Space Flight Center in 2016 and got the chance to see the James Webb Space Telescope while it was being built.

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