The Differences Between Galaxies, Solar Systems, and the Universe

The vastness of space is huge and practically unimaginable to the human brain. Even though galaxies are mostly empty space, they can still contain over 100 billion stars.

To bring this vast area into some kind of order we can comprehend, we use terms to describe the various components of space, such as a galaxy, universe, and solar system.

These are each well defined but to the layperson can feel interchangeable in everyday conversation. Without experience in the field, it is hard to know which is bigger: the Universe, a galaxy, or a solar system.

Happily, size is the easiest way to differentiate between these three things.

In this article, we’ll describe galaxies, solar systems, and the Universe, and we’ll set out which of them is the largest and the size order they run in. We’ll also take a quick look at constellations and where they fit in too.

Distinguishing Features of Universes, Galaxies, and Solar Systems

In this first section, we’re looking at what each object is and how it’s defined. In the next section, we’ll compare the three main ones in a table to make the differences between galaxies and the Universe clear.

The Universe

This is ‘The Big One’!

Containing everything we know of, the Universe is the whole cosmos.

It formed with the Big Bang 13.8 billion years ago.

There is no doubt that it is huge. The visible universe extends out for 46 billion light-years, but we are pretty sure that there are parts of the Universe beyond this boundary that we can’t see because the light from them hasn’t had enough time to reach us.

We know that the Universe is also expanding. Other than the gravitationally bound galaxies of our Local Group, all other galaxies are receding from us due to the creation of new space between us and them. This is causing the Universe to expand in all directions.

What we don’t know is whether or not the Universe is infinite in size or if it will ever stop expanding.

All evidence available today tells us there is only one Universe, but there is a theory that there might be many more of them which are invisible to us. This would make us part of a multiverse.

Galaxies

A galaxy is a gravitationally bound collection of stars, gas, and dark matter. They can be staggeringly huge, like the largest known galaxy Alcyoneus, or they can be relatively small, with just a few thousand stars.

When galaxies were originally discovered they were known as ‘island universes’ because the word universe was originally used to mean our galaxy. We now use those words for very different meanings.

Galaxies come in three different forms: elliptical, spiral, and irregular. The ones you’re used to seeing in astronomy magazines are the more picturesque spiral variety, such as the NGC 772, shown below.

Example of a spiral galaxy, NGC 772
Spiral galaxy NGC 772, imaged by the Hubble Space Telescope (source)

Our home galaxy is called the Milky Way. We can see it at night as the lighter band of stars stretching overhead. When we see this band, we are actually looking out over the plain of our galaxy.

Galaxies began forming shortly after the universe, so the oldest of them is only a few hundred million years younger than it. However, there is evidence that galaxy formation was still happening less than a billion years ago.

When a galaxy is first formed, it is relatively small, not much different from a large star cluster. Over time, these small galaxies collide and merge to become much larger. Eventually, 4.5 billion years from now, our galaxy will merge with the Andromeda Galaxy to create a real giant with 1.5 trillion stars in it.

Whereas there is only one universe, there are an estimated 2 trillion galaxies within it. That’s 2,000,000,000,000! So, despite their terrific scale, they are tiny when compared to the Universe as a whole.

Solar Systems

Our sun has been known by its Latin name, Sol, for millennia. This has given rise to the term ‘solar’, which we use to describe things relating to it, such as solar panels, solar eclipse (an eclipse of the sun), and the solar system.

The solar system describes the system of planets rotating around the sun. And since there are many suns (stars) in a galaxy, there are also many solar systems.

We have discovered planets orbiting more than 3,200 different stars, so far. These exoplanets (planets outside of our own solar system) are signs that other solar systems are very common. The number of exoplanets discovered doubles every couple of years.

Given there are trillions of galaxies within the universe, there is an almost countless number of solar systems within it too.

Constellations

Finally, for this section, let’s look at the difference between a constellation and a galaxy.

We just discovered that a galaxy is a collection of stars and other matter that is gravitationally bound together.

Constellations are patterns of stars that are visible in the night sky. The stars within them are rarely gravitationally linked. They are usually many hundreds, or even thousands, of light-years apart. It is only our line of sight that makes them look close together.

Galaxies are not part of constellations because a constellation is defined by its pattern of stars. However, we define a galaxy’s location by which constellation it is in, giving an area of the sky to look for it in.

Universe vs. Galaxy vs. Solar System

Scale is the main difference between solar systems, galaxies, and the Universe. Solar systems are based around a single star. Galaxies are made of millions-trillions of stars, including those with planets going around them. The Universe contains all two trillion galaxies and their countless solar systems.

The table below characterizes the other key differences.

Table of Differences Between Galaxies, Solar Systems, and the Universe

UniverseGalaxySolar System
ScaleContains everything – all galaxies, stars, planets, and space.A few thousand to trillions of starsOne star with planets orbiting
QuantityOneTwo trillionCountless
ExpansionConstantly expandingNot expandingNot expanding
Age13.8 billion years500 million to 13.4 billion years4.6 billion years (our solar system)
FormationBig BangClouds of dust & gas + collisions and mergersSmaller pockets of gas within galaxies

A Word on Galaxy Clusters

Before wrapping this article up, it’s worth spending a minute looking at galaxy clusters. On the scale chart, these sit between galaxies and the Universe.

Galaxy clusters are, as the name suggests, groups of two or more galaxies that are gravitationally bound together. Superclusters are clusters of clusters. Superclusters don’t share a common center of gravity like a cluster.

The Milky Way galaxy, home to our solar system, is part of the Local Group of galaxies. This cluster has more than 50 galaxies in it but most are dwarf galaxies. The largest are Andromeda Galaxy, the Milky Way, and the Large and Small Magellanic Clouds.

Our Local Group is part of the Virgo Supercluster, which itself is thought to be part of an even larger cluster known as the Laniakea Supercluster. You can learn more about Laniakea and The Great Attractor here.

These massive groups of galaxies form a network that, on the largest scale, looks like a galactic sponge. All of this, stretching across many billions of light-years in all directions, is still within the single Universe.

The galactic sponge
Galactic sponge at a massively macro-scale (source)

Summary

Scale is the biggest difference between a solar system, galaxy, and the Universe.

The solar system, involving a single star, is the smallest of these three. Galaxies, such as our Milky Way, contain billions of stars, many with their own solar systems

The Universe is so large that we don’t even know whether it is finite. We do know that it is big enough to contain every one of the two trillion galaxies we believe to be in it.


Written by Adam Kirk

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