Greg Chaffins Part of the NerdBeach Series “How Space Works“
Galaxies are one of the universe’s greatest examples of scale being deeply unfair. You look up at the night sky and see a few scattered stars, maybe a planet if you’re lucky, and think, “Aw, space seems manageable.” But then science arrives and says:
“Actually, every star you can see is part of a galaxy containing hundreds of billions more stars, and there are likely trillions of galaxies in the universe.”
And suddenly your entire sense of importance takes a mild emotional hit.
Galaxies are enormous collections of stars, gas, dust, planets, black holes, and cosmic nonsense all held together by gravity. They’re the giant building blocks of the universe, the neighborhoods where stars live, and the reason the cosmos looks like an overachieving glitter project when photographed.
So how do galaxies work, and why did the universe decide to organize stars into these absurdly massive spinning structures?
Let’s zoom way, way out.
A Galaxy Is Basically a Giant Star City
- Galaxy Definition: A huge collection of stars, gas, dust, and dark matter bound together by gravity.
- Galaxy Size: Ranges from a few million stars to hundreds of billions of stars.
- Milky Way Galaxy: Contains an estimated 100–400 billion stars.
At its simplest, a galaxy is a huge collection of stars, gas, dust, and dark matter all bound together by gravity. Think of it like a city. Except instead of:
- houses, you have stars
- roads, you have glowing gas clouds
- traffic, you have stars orbiting at absurd speeds
- annoying neighbors, you have black holes
Galaxies range wildly in size. Some smaller galaxies have only a few million stars. Large galaxies can contain:
- hundreds of billions of stars.
Our galaxy, the Milky Way, contains an estimated 100–400 billion stars. And yes, our Sun is just one random middle-management star among all that.
Humbling.
Gravity Holds the Whole Thing Together
- Reason for Galaxy Stability: Gravity from stars, clouds, and matter within a galaxy keeps it bound together.
- Star Distribution in Galaxies: Stars within galaxies are very far apart, often separated by trillions of miles.
- Analogy for Galaxies: Galaxies are like a group of glowing objects spread far apart but still gravitationally connected.
The reason galaxies don’t simply fly apart is gravity. Every star, cloud, and chunk of matter inside a galaxy pulls on everything else. Together, all that gravity keeps the galaxy bound into one giant structure.
But galaxies are so huge that their stars are not packed tightly together like many people imagine. In reality, stars inside galaxies are:
- very far apart
- often separated by trillions of miles
Which means galaxies are less like crowded cities and more like: “A bunch of glowing objects spread absurdly far apart but still technically hanging out together.”
Space loves unnecessary distance.
Galaxies Spin Constantly
- Galactic Rotation: Most galaxies rotate, with stars orbiting their centers.
- Sun’s Orbital Speed and Time: The Sun orbits the Milky Way at 514,000 mph, taking 230 million years per orbit.
- Galactic Movement Perspective: Slow by human standards but incredibly fast in a cosmic context.
Galaxies don’t just sit still looking pretty. Most of them rotate. Stars orbit around the galactic center much like planets orbit stars. For example:
- Our Sun orbits the center of the Milky Way at roughly 514,000 mph.
That sounds fast until you realize it still takes:
- about 230 million years
to complete one orbit.
Meaning the last time our Sun was in this exact spot around the galaxy: Dinosaurs hadn’t even shown up yet. Galactic movement is extremely slow when measured by human standards and absurdly fast when measured literally any other way.
Most Galaxies Have a Giant Black Hole in the Middle
- Supermassive Black Hole Location: Most galaxies have a supermassive black hole at their center.
- Supermassive Black Hole Mass: These black holes can contain millions or even billions of times the Sun’s mass.
- Supermassive Black Hole Influence: The black hole likely helps shape the galaxy’s structure and influences nearby stars.
Because apparently galaxies weren’t dramatic enough already. Scientists believe most galaxies have a supermassive black hole at their center. These black holes can contain:
- millions
- or even billions of times the Sun’s mass.
Our own galaxy has one called:
Sagittarius A*
Which is just sitting in the center of the Milky Way being horrifying. The black hole doesn’t “hold the galaxy together” by itself, but it likely helps shape the galaxy’s structure and influences nearby stars. So yes:
At the center of our galactic home lies a giant invisible gravity monster.
Sleep well.
Galaxies Come in Different Shapes
- Galaxy Types: There are three main types: spiral, elliptical, and irregular.
- Spiral Galaxies: Flat spinning disks with swirling arms.
- Spiral Galaxy Examples: Milky Way and Andromeda.
Not all galaxies look alike. There are three main types:
Spiral Galaxies
Flat spinning disks with swirling arms.
Examples:
- Milky Way
- Andromeda
These are the pretty “classic galaxy” shapes you usually see in pictures.
Elliptical Galaxies
Big rounded blobs of stars.
Less flashy, more “cosmic potato.”
Irregular Galaxies
Galaxies with no clear shape.
Basically: “Gravity tried its best.”
The universe loves variety.
Galaxies Formed From Early Matter Clumping Together
- Early Universe Composition: Hot gas, energy, and chaotic physics.
- Galaxy Formation Process: Gravity pulled matter together, forming stars, star clusters, and eventually galaxies.
- Reason for Galaxy Formation: Gravity’s attraction to loose matter.
After the Big Bang, the universe was mostly:
- hot gas
- energy
- chaos
- and physics doing weird startup things
Over time, gravity pulled matter together into denser regions. Those dense areas became:
- stars
- star clusters
- and eventually entire galaxies.
Basically, galaxies formed because: Gravity kept seeing loose matter and saying “mine.”
Over billions of years, those clumps grew larger and larger until full galaxies emerged.
Galaxies Can Collide
- Galactic Collision Outcome: Galaxies passing through each other, gravitational distortion, and eventual merging.
- Star Collision Rarity: Direct star collisions are uncommon due to vast distances between stars.
- Milky Way and Andromeda Collision: The Milky Way is expected to collide with the Andromeda Galaxy in approximately 4.5 billion years.
Despite how massive space is, galaxies sometimes crash into each other. And yes, that sounds terrifying. But because stars are so spread out, direct star-on-star collisions are actually rare. Instead, galaxies mostly:
- pass through each other
- distort each other with gravity
- merge into larger galaxies over time
The Milky Way is actually on course to collide with the Andromeda Galaxy in about:
4.5 billion years
So future Earth (if it still exists) may eventually have a very different night sky. Again: Future humanity problem.
Dark Matter Makes the Math Weird
- Dark Matter Definition: Unseen matter that adds extra gravity to galaxies.
- Dark Matter Characteristics: Makes up most of the universe’s matter and its exact nature is unknown.
- Dark Matter Role: Keeps galaxies together despite insufficient visible matter.
Here’s where galaxies get extra strange. When scientists calculate how galaxies rotate, the math doesn’t add up. There’s not enough visible matter to explain why galaxies stay together the way they do. Meaning something unseen is adding extra gravity.
Scientists call this mystery:
We don’t know exactly what it is. We only know:
- it exists somehow
- it makes up most of the universe’s matter
- and it sounds like a villain from a superhero movie
So galaxies may actually be floating inside giant halos of invisible mystery matter. Space remains aggressively weird.
We Live in Just One Tiny Corner
- Humanity’s Place in the Universe: Humanity occupies a microscopic dot in one average neighborhood of one galaxy among countless others.
- The Scale of the Universe: The Milky Way is just one galaxy among likely trillions.
The craziest part? The Milky Way is just one galaxy among likely trillions. And inside our galaxy:
- we’re in one spiral arm
- orbiting one average star
- on one medium-sized planet
Basically: Humanity occupies one microscopic dot in one average neighborhood of one galaxy among countless others.
Existential, but fun.
The View From One Tiny Speck
Galaxies work because gravity pulls together enormous collections of stars, gas, dust, and dark matter into giant rotating structures that span unimaginable distances. They spin, evolve, collide, form stars, house black holes, and help shape the large-scale structure of the universe.
So the next time you look up at the night sky, remember: You’re staring out from inside one tiny part of an enormous spinning galaxy filled with hundreds of billions of stars… and that galaxy itself is just one among trillions.
Nature remains unbelievably cool and deeply rude to our sense of scale.
This article is part of the NerdBeach series: How Space Works
