What is a galaxy, anyway?

What is a galaxy, and what are they made of?

Looking up into the dark of the night sky, you will see stars scattered in every direction (provided there isn’t too much light pollution where you happen to be looking up from). Generally speaking, they are fairly evenly spread out across our shared sky.

If you find yourself in a really dark place, with very little or no light pollution at all, then what you see will look very different. What you will be able to see is a faint band of light, stretching across the night sky. Now, if you were to look at this band with the help of a telescope, you would see many many… Many more stars.

Italian astronomer and physicist Galileo Galilei was the first person on record to do just that. When he did, he confirmed a belief many had held for over 2,000 years.

This belief, which had held for over two millennia, was that this band was comprised of a huge collection of distant stars. Throughout this time it was called the ‘Milky Way’, which was taken from the Greek “galaxías kýklos”. And that is, essentially, the answer to ‘What is a galaxy?’; it is a vast collection of stars.

What is a galaxy to a solar system?

The idea that the Solar System was in a flattish clump of stars started to form. This happened because when we look along the ‘disk’, we see lots and lots of stars, but fewer when we look away from it.

Also, as well as the stars, there are several ‘fuzzy blobs’ that were observed that at first were thought to be in the Milky Way. In the beginning, people just thought that the Milky Way was everything there was, that it was the whole of the Universe.

A German philosopher, Immanuel Kant, floated the idea that while the Milky Way was indeed our Universe, the fuzzy blobs (which turned out to be nebulae) were other universes that were far away from the one we live in.

These thoughts were merely speculation at the time, but with the development of the camera and telescopes, astronomers started to realise things. Just 100 years ago they began to understand that our galaxy is just one among, what seemed like, an infinite number of others.

These ‘other universes’, they learned, were galaxies just like ours. It is safe to presume that, just like ours, they guessed these other galaxies of stars that were held together by gravity, also contained solar systems. Just like ours.

Does size matter? Distances in space are huge

The distance between stars within a galaxy is usually a few lightyears, but the lightyears between galaxies are measured in the millions. In between these galaxies, so far as we know, is basically nothing. It is estimated that there is around 1 atom per cubic meter of space.

Even the smallest of galaxies are home to a few million stars, while the largest contains trillions of these twinkling beauties.

Our galaxy, which is considered ‘typical’, plays host to a few hundred billion stars.

Answering the question, what is a galaxy?

Let’s take a look at what galaxies are made of. There are a few things that can be isolated as being the building blocks of galaxies, and these are…

• Stars
• Gas
• Dust
• Supermassive black holes
• Dark matter

Planets may be considered a building block of galaxies, but that would largely depend on who you ask.

Stars, as previously mentioned, are abundant celestial objects. They endure for extended periods, yet their existence is not eternal. As they deplete their energy and reach the end of their life cycles, they give rise to white dwarfs, neutron stars, or black holes.

However, despite being the primary source of illumination for galaxies, stars constitute only a minor portion of the galaxy’s overall mass. While stars consist of gas, significant quantities of gas also exist between these stars in what we refer to as the interstellar medium.

In small galaxies, there can be just as much interstellar gas as there is star gas. This difference between the two decreases with the size of the galaxy.

The interstellar medium (matter and radiation that exist in the space between the star systems in a galaxy) is divided into different phases. Some regions are diffuse and extremely hot, with temperatures in the millions of degrees.

In these areas, the gas is ionized which means that most atoms have had one or more electrons stripped away due to the high temperature.

Other regions are denser and cooler; around 10,000 K.

When gas is heated, its pressure increases, causing it to expand. This process is analogous to how a hot air balloon ascends.

Conversely, when gas significantly cools (reaching temperatures around 100 degrees Kelvin or –170 °C), it condenses into dense, cold molecular clouds. These regions serve as the birthplaces of new stars.

During the time of the Big Bang, the universe mostly birthed hydrogen and helium atoms, and ‘What is a galaxy’ was on the way to being answered.

However, as stars reach the end of their life cycles and fade away, they release a portion of their gas back into the interstellar medium. This gas, now enriched with heavier elements, is affectionately referred to as “metals” by astronomers (although chemists may disagree with this term).

Today, approximately two percent of the total gas mass has transformed into these metals, and roughly one-third of them have coalesced into dust particles.

Planets — a small fraction of what makes a galaxy

When a star is born, it gives rise to a disk composed of gas and dust. Within this disk, dust particles can aggregate, forming rocks. Over time, these rocks grow larger, eventually evolving into planets.

Although planets constitute only a small fraction of the total mass in the cosmic budget, their significance cannot be overstated. They likely play a crucial role in supporting life, making them exceptionally intriguing.

In the past 25 years, our understanding has expanded significantly. We’ve discovered over 4,000 exoplanets—planets orbiting stars beyond our own Solar System—revealing that most stars are accompanied by planetary companions.

Supermassive black holes and dark matter (great name for a band, write that down)

When we think of the answer to “what is a galaxy”, we also have to take black holes into account, since most galaxies have one.

Within the heart of most galaxies lies a colossal entity known as a ‘supermassive black hole’. These cosmic behemoths tip the scales at millions or even billions of times the mass of our Sun. In the vast realm of astronomy, where everything dwarfs earthly measurements, we employ a unit called ‘Solar masses’—equivalent to approximately two billion billion trillion kilograms—to quantify their immense weight.

Despite their staggering mass, the gravitational influence of these black holes remains relatively minor compared to the overall galaxy. However, during certain periods, these enigmatic entities can ignite an ‘active galactic nucleus’ or transform into a quasar. In this fiery state, they unleash powerful forces that expel significant portions of the galaxy’s matter into the cosmic void.

In some instances, these quasars can strip the galaxy of its gas to such an extent that it stifles the birth of new stars. The interplay between these cosmic giants and their galactic surroundings continues to captivate astronomers and deepen our understanding of the universe.

However, what we have observed so far constitutes only about one-sixth of a galaxy’s total mass. The predominant portion of a galaxy’s mass is composed of something entirely distinct: dark matter.

Dark matter fundamentally differs from “normal” matter in that it exclusively interacts through gravity. Unlike ordinary matter, it remains impervious to electromagnetic or nuclear forces. Consequently, dark matter neither emits light nor collides with other particles.

Where Dark Matter sits relative to a galaxy

Its very name—dark matter—stems from its elusive nature. We cannot directly perceive it; instead, we discern its effects on the visible matter around it. Gravitational interactions with luminous matter reveal its presence.

While normal matter can cool and coalesce into dense galaxies, dark matter faces greater challenges in clumping together. As a result, it forms an extensive “halo” enveloping the visible portion of the galaxy.

What we perceive as a galaxy represents merely a fraction of its true composition. The accompanying figure illustrates the approximate dimensions of the Milky Way’s components.

The Milky Way, when viewed edge-on, reveals its intricate components. What is a galaxy? Let’s explore the cosmic tapestry:

• Thin Disk (Dark Blue): Within this flattened region, most of the Milky Way’s stars and gas clouds reside. It’s akin to the galactic dance floor, where celestial bodies twirl in harmony.

• Thick Disk (Cyan): Like a celestial sandwich, the Milky Way boasts a thick disk containing a fraction of its stars. These stars follow a different groove, adding depth to the galactic choreography.

• Bulge (Orange): At the heart of our galaxy lies the bulge—a dense, spherical congregation of stars. It’s the cosmic nucleus, where stellar crowds gather for a grand celestial soirée.

• Halo (Red and Yellow): Encircling the disk, a halo of hot gas (red) mingles with ancient stars and approximately 150 ‘globular clusters’ (yellow). These clusters are like cosmic pearls, each containing thousands of tightly huddled stars.

• Dark Matter Halo (Grey): Beyond all visible components, an enigmatic shroud envelops the Milky Way—the dark matter halo. Its gravitational influence extends far and wide, shaping the galactic destiny.

Remember, this model only approximates reality, but in truth, the boundaries between these components blur as we venture farther from the galactic center. The Milky Way’s cosmic ballet continues, weaving a captivating tale across the vast expanse of space.

That pretty much sums up what a galaxy is, and next time we will be looking at the different types of galaxies!