Welcome to the Neighbourhood
The universe is vast.
Are you inside a building? Walk outside for a moment. Look up at the sky. Smell the fresh air. Jump into that air. Don’t come down yet. Keep travelling up until you are about 600 kilometres above the ground. Stop and take a look around.
In front of you is a wall of oceans, land and clouds. Stretch your view up further and you can make out where the clouds and oceans give way to the blackness of space. There’s a gravitas to the Earth when you’re this close to it. It fills your vision.
Now, take a few steps backward. Do it slowly; the emptiness can be overwhelming. When you’re used to living with your feet on the ground it is disorienting not to feel connected to any kind of fixed point.
Keep on stepping backwards for about 350000 kilometres and you’ll find yourself standing on the moon. Hanging low over the lunar horizon is a full Earth, perfectly round. A few degrees to the right Jupiter and Saturn shine brightly.
Take a few more steps back. Once you’ve travelled 38 million kilometres or so you will be passing Mars to the right, about 150 million kilometres away. A little further and you will pass Venus—much closer this time, only about 30 million kilometres away.
An Astronomical Unit, or AU, is a unit of measure which represents the approximate distance between the Earth and Sun. It’s roughly 150 million kilometres. If you keep backing up until you’re one AU from Earth you’ll see the Sun to your right and Comet Encke over your left shoulder.
Scoot back further still and you’ll find yourself just outside the orbit of Neptune, the last of Sol’s planets. Here, after travelling 20 AU from Earth, you enter the Kuiper belt. The Kuiper belt is a ring of floating bits and pieces left over from the formation of the Solar System. Pluto lives here.
Keep travelling for another 100 AU or so and you’ll eventually reach the heliopause, the point where the Sun’s solar wind can no longer be felt. You have officially left the Solar System. Say hello to Voyager 1 on your way out.
The Interstellar Medium
The closest star system to our Sun is Alpha Centauri. It is comprised of three stars: Rigil Kentaurus and Toliman, together forming the binary star Alpha Centauri AB, and the much smaller Proxima Centauri. Although they’re part of the same system, Proxima Centauri is about 13,000 AU away from Alpha Centauri AB.
Alpha Centauri isn’t particularly large as far as stars go, Rigil Kentaurus and Toliman each being about the same mass as our own Sun, but because it’s a mere 4.4 light-years from earth it’s one of the brightest stars in our night sky. For something a little more dramatic we should visit the excellently-named Betelgeuse.
Betelgeuse is 430 light-years from Earth, and is classified as a red supergiant. Red supergiants are among the largest stars which we know about. For a sense of scale, the diameter of Betelgeuse is 887 times that of our sun. The name Betelgeuse is from the Arabic for “armpit of Orion”.
Betelgeuse is a runaway star, which means it is bucking the gravitational trend and hurtling through the galaxy on its own terms. As it moves it creates a bow shock four light-years wide in the interstellar medium.
Betelgeuse is expected to explode in a supernova in the next 100,000 years, so watch this space.
Foamy Voids
The 430 light-years to Betelgeuse is a long way to travel. Of course, in the grand scheme of things you haven’t travelled very far at all. If you zip along for another 7500 light-years you’ll reach the edge of the Milky Way galaxy. Such a shortcut is possible because we live right on the edge of the galaxy—the whole Milky Way is about 200,000 light-years in diameter.
The nearest galaxy to the Milky Way is Andromeda, 2.5 million light-years distant. The Milky Way and Andromeda are both part of the 36-member Local Group. The Local Group is in turn part of the Virgo Supercluster, which spans 110 million light-years and contains more than 100 clusters like our Local Group.
The Virgo Supercluster is one of four members of a group of superclusters called Laniakea. Laniakea is part of the Pisces-Cetus Supercluster Complex, which is one of the largest structures in the known universe. It contains around 60 superclusters like Laniakea and stretches for an estimated one billion light-years. The scale of these structures is such that it pushes the limits of our comprehension, so take a deep breath; the Pisces-Cetus Supercluster Complex is a galactic filament.
The structure of the universe appears to be like that of foam. That is, the arrangements of galaxies, clusters and clusters of clusters are actually sheets, walls and filaments in a gigantic mass of bubbles. The bubbles in between these supporting structures mainly consist of… nothing. Big bubbles of nothing1 ranging in size from tens to hundreds of millions of light-years in diameter. This foamy formation is known as the cosmic web.
Of course, this is only the observable universe. Some parts are invisible to us because they’re so far away that there hasn’t been enough time since the Big Bang for the light to reach us. Some of those will become visible over time as the light travels to us, but because of the accelerated expansion of the universe some of that light will simply never reach us.2
The observable universe today is a sphere with a diameter of about 93 billion light-years, with us at the centre.3 There are at least 2 trillion galaxies in that sphere. But really, the scale is just beyond comprehension. To give a sense of this, if our sun was a ping pong ball then Proxima Centauri, our closest star neighbour at 4.2 light-years distance, would be a pea 1,100 kilometres away.
What is the Cosmos?
Western views of the nature of the cosmos have developed over time, but we still have remnants of early world views in our intuition. Plato had his celestial spheres, which evolved in early European minds into a higher world which was fixed in place after creation and made of an invisible substance known as quintessence.
The Chinese approach was closer to our modern understanding. They did not hold to an idea of a fixed higher world but instead believed that everything was made up of qi. This included not only physical objects like planets and trees, but also intangible concepts such as thoughts and ideas.
Critically, eastern views saw the universe as infinite whereas western views saw it as finite.
- What is a bubble, really? Is it a volume of matter contained in one location or is it the container of that matter? In the case of air bubbles we would say the former, whereas for soap bubbles we would say the latter. [return]
- The speed of light is a cosmic speed limit only as far as bodies local to each other are concerned. Parts of the universe sufficiently distant from Earth are travelling away from Earth faster than the speed of light, relative to Earth. [return]
- Take that, Copernicus! [return]