Galaxies

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Podcast Transcript

For most of human existence, we looked up at the night sky and thought that was all there was to the universe.

However, in the 20th century, as telescopes improved, we made a shocking discovery. The universe was much, much larger than we supposed, and some of those points of light in the sky were, in fact, collections of stars themselves. 

Learn more about galaxies, what they are made of, and all about our own, on this episode of Everything Everywhere Daily.


If we want to provide a very rough taxnomy of the universe, we can start with things like planets, planetoids, asteroids, meteors, and other such bodies. These are usually rocky objects or can sometimes be amalgamations of gas, like Jupiter. 

All of these objects are objects whose matter is gravitationally, or in the case of very small objects, chemically or electrostatically, bound to itself. 

Then you have stars, which I’ve talked about in a previous episode. Most planets and planet-like objects that we know of are under the gravitational influence of a star. They orbit around it and are part of a star’s solar system. 

A star, of course, is simply an amalgamation of gasses, usually hydrogen, and helium, that is massive enough for fusion to occur. 

For the longest time, this is what we assumed the universe consisted of. We had objects in our solar system of which we were aware, and then there were other stars in the sky that we could see. 


However, there was something else in the sky that wasn’t points of light. There was a band of faint light that could be seen in the sky. It didn’t extend over the entire sky, it was as if it was a faint smear of light which extended across it. 


We know this band of faint light as the Milky Way. 

Over the centuries, there were theories as to what this smear of light was. The Greek philosopher Democritus, as early as the 5th century BC, thought that it was a collection of very distant stars. 

The Arab astronomer Alhazen in the 10th century, tried to measure the parallax of the Milky Way and realized that there was no parallax. Parallax is the change in the apparent position of an object relative to more distant objects caused by a change in the observer’s line of sight toward the object.

So, a star will be in a slightly different position when the Earth is on one side of the sun than it will be when the Earth is on the other side, six months later. 

You can observe parallax for very close objects by looking at nearby objects with one eye closed and then the other. 

Because the Milky Way had no measurable parallax, it meant that it was very, very far away. 

In the early 17th century, Galileo looked at the Milky Way through a telescope and found that it did, in fact, consist of a great many very faint stars. 

One of the biggest insights as to what the Milky Way was, was made by the English astronomer Thomas Wright in 1750.  In his book, “An Original Theory or New Hypothesis of the Universe,” he theorized that the Milky Was a collection of stars that was gravitationally held together. 

There were other questions that arose as telescopes became better. Not all of the lights in the sky were stars. Some of them were known as nebulae. 

There were different types of nebula. Some appeared to be clouds, and others appeared to be spirals. 

These spiral nebulae were very perplexing. The aforementioned Thomas Wright proposed that some of these nebula were, in fact, separate Milky Ways.

What exactly these spiral nebulae were, became one of the most hotly contested controversies in astronomy in the early 20th century. 

It came to a head on April 26, 1920, when the two proponents for each side of the debate presented their cases at the Smithsonian Museum of Natural History. This became known as the Great Debate. At stake was nothing less than the size of the universe.

On one side was Harlow Shapley, head of the Harvard Observatory. He contended that spiral nebulae, such as the Andromeda Nebula, were a part of the same system as all the other stars we could see. 

On the other side was Herber Curtis, the director of the Allegheny Observatory. He contended that spiral nebulae were, in fact, completely separate systems of stars located outside of our own and vastly farther away. 

Curtis came to this conclusion after observing 10 different novas take place within the Andromeda nebula, all of which were an order of magnitude fainter than those otherwise observed in the sky.

No one walked away a winner from the debate that evening, but over time the evidence accumulated, and Curtis was right. Andromeda and other spiral nebulae were, in fact, separate collections of stars, vastly further away from the individual stars we can see in the sky. 

At first, they were called island universes, but that was sort of awkward, so they were eventually just called galaxies. 

Thomas Wright’s theories from way back in the 18th century were also generally proven correct. The Milky Way that we see in the sky is our galaxy, and it is a flat disk of stars that we can see from the edge. 

As astronomical observations improved, more and more galaxies were discovered. There wasn’t just one type of galaxy, there were, in fact, a variety of galactic types. 

Edwin Hubble, the astronomer who ended the Great Debate by measuring the distance to several spiral nebulae, categorized the various types of galaxies. There are spiral galaxies with different shapes and types of arms, there are elliptical galaxies, which are mostly smooth without arms, and there are irregular galaxies without form. 

After a century of disocvering the existence of galaxies, what have astronomers learned about them?

The size of a galaxy can vary dramatically. A dwarf galaxy can consist of as little as a few thousand stars up to a billion stars. Most galaxies, such as our own, consist of 100 to 400 billion stars.

The largest galaxies, known as supergiant ellipticals, may consist of trillions of stars, and were likely created by swallowing up other galaxies. 

The closest galaxy to the Milky Way might be the Canis Major Dwarf Galaxy. It is approximately 25,000 light-years away, and I say maybe because some speculate that it isn’t a separate galaxy at all but actually part of the Milky Way. 

The Large and Small Magellanic Clouds are located about 160,000 and 200,000 light-years away.  As with the Canis Major Dwarf Galaxy, both of the  Magellanic Clouds are gravitationally bound to the Milky Way, so they are sort of like satellite galaxies. 

The closest galaxy which is not gravitationally bound to the Milky Way is the Andromeda galaxy. It is located about 2.5 million light-years away from us in the constellation Andromeda.

In fact, Andromeda is on course to collide with the Milky Way in about 4.5 billion years. 

Galactic collisions are not the violent events that they might sound like. It is entirely possible for galaxies to collide but for no stars to actually touch. It is more a matter of gravitational influence, not physical collision. 

The Milky Way has a diameter of about 100,000 lightyears, and it is a spiral galaxy. Our solar system is located about midway on one of the spiral arms, about 25,000 lightyears from the galactic core. It takes the sun about 250 million years to complete an orbit around the center of the galaxy.

This period of time is known as a “galactic year” or “cosmic year.”

Our galaxy, like most galaxies, has a supermassive black hole at its center, something which I covered in the episode on black holes. These are the gravitational sources that gravitationally bind objects in the galaxy together.

The number of galaxies in the universe is an estimate which keeps getting updated as more and better observations are made. Telescopes like the James Webb are now able to peer about as far back as possible. 

One of the observations which demonstrated the mind-boggling number of galaxies in the universe was the Hubble Deep Field Image. 

10% of Hubble Space Telescope’s observation time is devoted to what is known as director’s discretion. In 1995, the then-director of the Space Telescope Science Institute, Robert Williams, decided to point the Hubble toward a relatively empty patch of sky that was 1/24,000,000 the area of the total sky.

Over a period of 10 days and hundreds of exposures, the result was an image that change astronomy. This tiny empty patch of sky was filled with galaxies. 

Subsequent deep-field images have shown the same thing no matter where they point the camera. 

So, while we know there are a lot of galaxies, the total number is a wild estimate, because we can’t see most of the universe. The current best guess is that there are between 100 billion to 1 trillion galaxies in the universe, but don’t be surprised if that estimate is updated in the future. 

Galaxies aren’t the end when it comes to structures in the universe. Galaxies can themselves be bound together to form galactic clusters. 

There may be hundreds to thousands of galaxies in a galactic cluster. Even large clusters, known as superclusters, may have tens of thousands of galaxies in them. 

These clusters are shaped into sheets and filaments, with enormous voids with no galaxies between them. 

The Hercules–Corona Borealis Great Wall is the largest structure in the universe. It is 10 billion light-years long and 7 billion light-years wide. To put that into perspective, the observable universe is only 93 billion lightyears in diameter.

It consists of tens of thousands of galaxies, each of which may contain hundreds of billions of stars. 

Unlike the Great Wall in China, this great wall can be seen from space. 

In some ways, galaxies, not stars, can be thought of as the fundamental units of the universe if you take a big enough perspective.

Not only are their sizes awe-inspiring and mind-bending, but just one century ago, no one was even sure if they existed. Today we know that our galaxy isn’t alone, but there may be hundreds of billions to trillions of others out there similar to ours. 


The Executive Producer of Everything Everywhere Daily is Charles Daniel.

The associate producers are Thor Thomsen and Peter Bennett.

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