The Milky Way: Our Galactic Home

The Milky Way. Most people have heard of it. It’s that string of stars in the night sky. Some people also know that the Milky Way is the name for our galaxy, the spiral disk of stars that our sun sits within. However, the Milky Way is a lot more than that. It has a complex structure, with a central dense region and a series of interlocking arms that branch out across thousands of light-years. And it’s full of wonderfully weird stuff from breathtaking nebulas to enormous black holes gobbling up everything in their sight. Let’s dive in, to the Milky Way.

The Milky Way: An Introduction

This is what first comes to mind when most people say the ‘Milky Way’ – a splash of light in the night sky. However, this is only one small part of one arm of the Milky Way galaxy which we sit within (Image of La Silla Observatory by the European Space Agency and S Brunier).

We live on the planet Earth. Earth orbits around our sun, the closest star. Our Sun is a star, at the center of our solar system. Our solar system sits within our galaxy, which is called the ‘Milky Way’, which is made up of many other stars, solar systems and clouds of gas.

The Milky Way is a spiral disk galaxy. That means it’s a flat disk with long arms wrapped around a central point, in the shape of a spiral, disturbingly like a fidget spinner. There are a lot of galaxies shaped like that in our Universe, it tends to be the shape that is formed from spinning a loosely connected clump of matter around (think pizza dough being spun by a chef). Andromeda, the nearest galaxy to us is also a spiral disk galaxy.

To picture our galaxy, it is good to consider some numbers. Most distances in space are measured in what are called ‘light years’ – the distance that light travels over a year. For instance, it takes light eight minutes for light to travel from the Sun to the Earth – so the distance is 8 light minutes. It takes light about 1.5 years for light from our sun to exit the solar system, beyond all the planets and asteroids that float around our sun. The nearest star to us, Proxima Centuari, is about 4 light-years away.

The Milky Way is about 100,000 light-years across. That means it’s about 100,000 times larger than our solar system. It contains roughly 100 billion stars, of all sorts of different types and ages. Our solar system is a tiny spec of dust, to the Olympic swimming pool of the Milky Way. For more on the structure of our Universe more generally, see our post here.

Structure of the Milky Way

This is the Milky Way in its entirety. You can see the central region, referred to as the ‘galactic bar’. Then each of the arms of the galaxy branching out. The arms aren’t quite as neat and distinct as you might like – you can see that they kind of fade in and out of existence and even into each other. The outer arm doesn’t even start in the center of the galaxy. The shadow is the parts of our galaxy we can’t see very well, due to the denser core of the galaxy getting in the way (Image: Artist’s conception by NASA/JPL-Caltech/R).

Like most galaxies, the Milky Way is most dense at the center. That means there are lots of stars and clouds of gas there, including probably a supermassive black hole right in the middle. Then as you get further away the Milky way gets less dense, until you get to the outer rim, where there are almost no stars at all.

There are two major spiral arms of the Milky Way – the Perseus arm and the Scutum-Centaurus Arm. There are also smaller less distinct arms that branch off from the main arms (Norma and Sagittarius). The arms are named after the famous constellations that are in the same directions as the arm in the nights-sky (i.e. the constellations of Centaurus and Sagittarius). Our solar system is in the Sagittarius arm.

Almost all of the famous stars and nebula, come from within the Milky Way, and most of them within the Sagittarius arm. For example, the Eagle nebula, the Crab nebula, Proxima Centauri, Betelgeuse and Sirius are all in our galaxy and the Sagittarius arm.

This is the Milky Way from the perspective of Earth. The closest stars are shown largest with the rest of the Sagittarius arm of our galaxy fading off into the distance, with the galactic center at their end. It doesn’t actually look like this in the night sky because the further away stars get to dim, we only see the closest stars. But this is what the structure of the Milky Way would actually look like from Earth (Image: Wikimedia, P C Budassi).

The Milky Way is also moving – as are almost all galaxies. Just as the Earth moves around the sun, all the stars in the galaxy orbit around the galactic center. In fact, the orbiting of stars around in a galaxy is a lot faster and more significant than the movement of stars and planets – imagine a spinning top on a table and then spinning the table – it’s the table that makes things move really fast and has more energy. It even gets more complex, galaxies can also orbit each other, or spin around in clumps, the same way a star system can have a few stars orbiting each other. The Universe is more like an ocean of currents with floating stars, all spinning around each other, where nothing is really still, rather than a pool table with clear structured motion.

A map of the Milky Way, and the surrounding galaxies. It’s really hard to do maps about galaxies because there is just so much space between galaxies. They essentially end up just being dots with lables. The other problem is that there is no real structure to how galaxies are arranged in the Universe, they very uniformly scattered throughout, as if the Universe was a thin galaxy soup. (see our post on the structure of the Universe generally here). But this map at least gives you the sense that the Milky Way is surrounded by a bunch of other little dwarf galaxies, but the nearest proper galaxy is Andromeda (Image: A Z Colmon, Wikimedia).

The Milky Way: Some final thoughts

This is the best image we have of the very center of our galaxy, the Milky Way. It’s called ‘Sagittarius A’. It is the source of a very bright and strong radio signal, probably meaning it is a supermassive black hole. However, all around this black hole is a dense cloud of gas of hydrogen and helium (Image: *I Zhuravleva* *NASA and Standford)*.

It’s really strange that more people don’t understand the shape of our Universe. At primary school, we learn about the Sun, the Earth and the planets. Maybe you’ll cover comets and asteroids if you’re lucky. But not a lot of kids get all the way to the Milky Way or other galaxies, let alone galactic filaments or the real large-scale structure of our Universe (see our post here).

We, here at The Universal Story, reckon we should all learn more about galaxies. The structure of the Universe is so weird and wonderful. If you were a person, thousands of years ago, looking up at the sky and someone told you what was out there, you would not have believed them. Understanding the stars – giant burning nuclear reactors spread out across billions of miles is weird enough. Realizing there is a whole other level of structure – massive chasms of mostly empty space full of enormous spiral fidget spinners of stars that take billions of years for even light to get to, is just mind-blowing. We should appreciate the Milky Way, our galactic home, so much more.

*A final, very detailed image of the Milky Way and some of the interesting objects it contains* *(Image: Wikimedia, P C Budassi*).

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