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What is a Galaxy?

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What is a Galaxy?

“A long time ago in a galaxy far, far away….”

So began one of the most celebrated movie epics of the 20th Century. Perhaps because of its prevalence in popular science-fiction, the word “galaxy” summons up images of spaceships and alien worlds filled with strange wonders beyond our wildest imaginings. The reality, though, is even more astonishing than our collective fictions let on.

A galaxy, by definition, is “a large system of stars held together by mutual gravitation and isolated from similar systems by vast regions of space.”

The word comes from the Greek “galaxi­as,” meaning “milky.” The first known galaxy was our own, the Milky Way. The ancient Greeks believed that the bright patch in the night sky, which is actually the center of our galaxy, was milk spilled by the goddess Hera when she was nursing the hero Hercules, so they named it “kyklos galaktikos,” or “milky circle.”

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Over time, as astronomers developed more powerful tools for observing the night sky, they began to realize that ours was not the only such milky circle in the Universe.

The earliest recorded observation of another galaxy was made by Persian astronomer Al-Sufi. He discovered the Andromeda Galaxy in the 10th century, describing it only as a “small cloud,” and, later, another galaxy now known as the Large Magellanic Cloud. Al-Sufi observed both with the naked eye centuries before the invention of the telescope during the 17th Century. Not coincidentally, these are the only two galaxies outside the Milky Way that are easily visible to the unaided eye.

Al-Sufi’s discovery faded to obscurity for more than 600 years later, until German astronomer Simon Marius re-discovered the Andromeda Galaxy in 1612, also without a telescope. Because of their cloudy, distant appearance, astronomers began to refer to these deep sky objects as nebulae, from the Latin word for “mist.”

Even after the invention of the telescope made it easier to view these murky figures, no one was really sure what they were. It wasn’t until 1750 that English astronomer Thomas Wright theorized that the Milky Way was actually flattened disk of stars. He believed these other nebulae could be the same thing – other Milky Ways out in distant space.

Seizing on this idea, German philosopher Immanuel Kant began referring to these distant nebulae as “Island Universes,” separated from one another by vast oceans of empty space.

Around this same time, astronomers began discovering more of these “islands” at a rapid pace. French astronomer Charles Messier found so many that he began compiling a catalog of them. He discovered 109 nebulae, only to be eclipsed mere decades after his death by German astronomer William Herschel’s catalog of more than 2,400 nebulae.

It wasn’t until the early 20th Century, though, and after centuries of intense debate, that the matter of what these distant objects were was was conclusively settled. In 1922, the Estonian astronomer Ernst Öpik was able to determine the distance of the so-called Andromeda Nebula, proving that it existed outside of our own galaxy.

Not long after, American astronomer Edwin Hubble invented a telescope that was advanced enough to bring other so-called “spiral nebulae” into sharp relief, making it possible, for the first time, to make out individual star formations within these distant shapes. It was Hubble who eventually created the classification system for galaxies, the Hubble sequence, that remains in use to this day.

Here’s some interesting trivia about galaxies:
– Astronomers estimate that there are at least 100 billion galaxies in the Universe.

– The largest known galaxy in the Universe, IC 1101, is about 5.5 million light years in diameter and is located one billion light years away from Earth. In contrast, our own Milky Way galaxy is about 100,000 light years in diameter. (One light year is about 5.9 trillion miles).

– Though dwarfed by IC 1101, the Milky Way is actually among the larger galaxies in the Universe. Most galaxies range from about 3,260 light years to 326,000 light years across.

– There are three main types of galaxies, named for their shape: spiral, elliptical, and irregular. Elliptical and irregular galaxies can be either average sized or dwarf galaxies. The most abundant type of galaxy is dwarf ellipticals

– The nearest galaxies to our own are the Magellanic Clouds, two dwarf irregulars that are thought to be orbiting the Milky Way at a distance of about 200,000 light years. The next closest is the Andromeda Galaxy, a spiral galaxy 2.6 million light years away.

– The farthest visible galaxy from us is approximately 14 billion light years away.

– An average galaxy contains about 40 billion stars, while a large galaxy, such as our own, contains 200 to 400 billion stars.

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1 frank chalykoff { 04.03.14 at 10:15 am }

Does each galaxy have a black hole at its center?

2 Evelyn Padin { 03.27.14 at 7:50 am }

Reading this, after having read my usual Biblical/Spiritual readings this morning, makes me believe that we are truly eternal. I had been wondering what to do as a hobby, because I love “doing” and I had no idea what would fulfill my curiosity. I am going to get a telescope. Now that we are going into the warmer weather, yes, a telescope may give me the feeling of wonder and surprise. Carl Sagan said “if you want to see what’s going on in the sky, all you have to do is look up.” Thank you for this lovely inspiring article.

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