Physics, Space

Our place in the universe!

Where are we in the universe? 

When I was younger, whenever I was tasked with addressing Christmas letters or thank yous, I would always be overly specific- house, street, town, county, country, earth… ending with The Universe! But where in the universe actually are we?  

Starting off with the basics: Earth is the 3rd planet from the sun, 150 million km (also known as 1 astronomical unit -AU-, what astronomers use to measure distances within our solar system) from our source of energy. We are slotted between Earth’s so-called deadly twin planet, Venus, and what could be humanity’s first interplanetary destination- Mars! These inner planets are actually quite cramped together: Venus has less than 0.3 AU on either side, whereas the asteroid belt is a whole AU across! In the outer solar system, the gas and ice giants are separated by even vaster distances. 

Beyond Neptune lies the Kuiper belt, which contains many dwarf planets, including Pluto. These objects cross each other’s orbits and interact with each other, which sometimes sends chunks of rock and ice towards the planets- the asteroids and comets that can tell us so much about our past! The Kuiper belt extends from just beyond the orbit of Neptune, all the way out to 50 AU. All of this is within the sun’s heliosphere, the area where the sun’s solar wind (ionised particles that flow from the sun) can reach. These particles travel at up to 900km/s when they are emitted from the sun, but as they travel through the solar system, they slow down, and at around 75 AU, they have decelerated to below the speed of sound. And like when a plane goes supersonic, when the solar wind goes subsonic, it creates a shock wave that distorts the magnetic field and heats the surrounding gas, amongst other effects. This is called the Termination Shock and was passed by the Voyager probes in 2007/8, although they did not agree on the distance (Voyager 1 crossed at 94 AU but Voyager 2 crossed at 76, suggesting it isn’t a perfectly spherical boundary).

The Solar System and surroundings on a logarithmic scale

Beyond this might be the Oort Cloud. This is a ‘bubble’ of icy material that seems to be the source of some long-period comets, and is thought to stretch from 2000 and 100,000 AU! That’s nearly halfway to our nearest stellar neighbour- Proxima Centauri.

Proxima Centauri is a red dwarf, part of a 3 star system, 4.2 light-years (265,000 AU) from our sun. Not only is it our closest stellar neighbour, but it is also home to our nearest exoplanet! Proxima B is probably a rocky planet, a little more massive than earth. Technically, it is in the star’s habitable zone, but as I have discussed previously, red dwarves tend not to be the best nurseries for astrobiology, and its orbit is so tight that it is almost certainly tidally locked. 

Most of the other stars in our local bubble of stars are red dwarfs too! When looking at the stars around us, we notice something else as well- the sun is actually a bit of an oddity! Over 80% of the stars in the night sky are actually in binary (or multi-star, like Alpha/Proxima Centauri) systems, including well-known stars like Mizar (and Alcor) in the Big Dipper (83 light-years away), and Sirius (the brightest star in the night sky, 8 light-years away). All of this occurs within our little corner of the Milky Way: the Orion arm. 

Our galaxy, the Milky Way, is a spiral galaxy- the most common type of galaxy. It has four major spiral arms, each with many irregularities and offshoots/secondary arms, including ours. Scientists think that the milky way also has a bar in the centre, but they aren’t certain. What we are certain of though, is that at the very centre, in the galactic bulge, is a supermassive black hole- Sagittarius A*. The black hole, which was discovered by tracing unknown radio signals and Xray flares, is around 4.6 million solar masses- and would fit within the orbit of Mercury! Its the only supermassive black hole that we can study in great detail, mainly by tracking the stars and gas around it- although last year saw the Event Horizon Telescope team take a picture of the black hole in an elliptical galaxy called M87, so maybe soon we will be able to investigate other supermassive black holes in more detail!? 

Ok, so far we have… Earth, Solar System, Local Bubble, Orion Arm, Sagittarius Arm, Milky Way… but that’s only one galaxy, in a very big universe!

The Milky Way is about 150,000 light-years across and is part of the local group- a dumbell shaped group of galaxies, containing the Milky Way (and its satellite galaxies) at one end, and Andromeda (and its satellites), as well as about 50 dwarf galaxies. This covers just under 10 million light-years, although the distance to Andromeda is shortening as it is travelling towards our galaxy! This makes it one of the few galaxies that are blue-shifted, as the majority of galaxies are moving away from us and each other due to dark energy. Increasing the field of view even more, we notice that we are on the edge of an even larger group of galaxies… the Virgo Supercluster! This contains over 100 groups of galaxies, all centred on the massive Virgo Cluster, which contains 2000 galaxies on its own! This is 100 MILLION light-years across, and contains 10^15 solar masses (so has a mass of 10,000,000,000,000,000 suns- I don’t even know what that number is called it’s so big! Maybe 10 quadrillion?).

This is really earth-centric, as the centre of the supercluster is actually the Virgo Group/Cluster

Get ready to have your mind blown even more

The Virgo supercluster is thought to be part of an even larger structure called the Laniakea Super-duper cluster (ok I made that up, it’s just called the Laniakea Supercluster, but I think my name represents the size better), which is over 520 million light-years across and could contain 100,000 galaxies! On this scale, you start to see what could be the cosmic web- the filaments of dark matter where galaxies have tended to form. 

The cosmic web- check out for an interactive version

And yet there are many of these galaxy superclusters in the universe because there is so much space for them to form in: the observable universe is everything we can see. And seeing as light travels… at the speed of light, you would expect to be able to observe a universe that with 13.8 billion light-year radius. But our universe isn’t that simple! Dark energy, which makes up ¾ of the universe, causes the universe to expand. So since that light was emitted 13.8 billion years ago, the universe has more than tripled in size: now, the diameter of the universe is a whopping 96 billion light-years across!

I don’t know about you, but that blows my mind- its fascinating how even at such large distances, we are able to do science and study our universe. It also makes me more hopeful that we might find life elsewhere.. maybe even intelligent life because there is just so much space.. and time! But that’s a story for another blog post. 

I hope that helped you get a picture of our place in the universe, and remember: even though we seem small compared to the universe, we are not insignificant! Stay Curious and Stay Safe! 

This is a really neat animation demonstrating the scale of the universe, it might help you get your head around the sizes mentioned… and/or completely blow your mind!

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