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Long way to the chemist’s: a rough guide to distances in the universe

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” Douglas Adams…

Stars are immense, but the space between them is truly phenomenal. chefranden

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

Douglas Adams, The Hitchhikers Guide to the Galaxy

We all know the universe is large, very large, but is it possible to really comprehend just how large it really is? Sit down, take a deep breath, and we can give it a go.

In my previous scale article, we considered the sizes of stars, and finished by imagining the sun being the size of an orange. On this scale, the nearest star to the sun, also the size of an orange, would be 2,300 kilometres away.

Even through stars can be immense on human scales, they are dwarfed by the distances between them.

Let’s continue our journey outwards and consider larger distances in the universe. The first stop is our cosmic home, the Milky Way galaxy. From our vantage point, buried deep within, the Milky Way appears as a broad band of stars encircling the sky.

An artist’s impression of the Milky Way. NASA

On a clear night, away from the lights of civilisation, we may be able to pick out a few thousand individual stars as mere points of light.

The smooth swathe of light that accompanies them, however, is the combined light of many more distant stars. How many? It turns out the Milky Way is home to more than 200 billion stars, lots of stars like the sun, a few spectacular giants, and many, many faint dwarfs.

To get a handle on the size of the Milky Way, let’s pretend the distance across it is 3,000km, roughly the distance between Sydney and Perth.

On this scale, the separation between the sun and its nearest neighbour would be about 100 metres, whereas the diameter of the sun itself would be about a tenth the thickness of a human hair. Other than a bit of tenuous gas, there’s a lot of empty space in the Milky Way.

For much of human history, we have prided ourselves on being at the centre of the universe, but as Douglas Adams pointed out, we live in the “unfashionable end of the Western Spiral arm of the Galaxy”.

If the small town of Ceduna in South Australia, sitting roughly midway between Sydney and Perth, was the centre of the Milky Way, our sun would be orbiting 850km away, somewhere beyond Mildura in north-western Victoria (and, no, I’m not suggesting Mildura is unfashionable!)

So the Milky Way is huge, and light, traveling at 300,000 kilometres a second, takes 100,000 years to cross from side to side.

But we know that we share the universe with many other galaxies, one of the nearest being a sister galaxy to our own, the large spiral galaxy in Andromeda.

The Andromeda galaxy. NASA

I am writing this in the dome of the 4-metre Mayall Telescope at Kitt Peak in Arizona, during a night where we are observing the Andromeda galaxy.

As the light falls on our electronic detectors, it’s always startling to think it has taken more than two million years to travel from there to here, and we are seeing Andromeda as it was before our ancestors, homo ergaster, walked Earth.

Andromeda and the Milky Way inhabit a small patch of the universe known as the Local Group. While these two galaxies are by far the largest members, there are another 70 galaxies that are considerably smaller.

To think about the scale of the Local Group, imagine that the Milky Way is a large dinner plate, with a diameter of roughly 25cm.

With this, the Local Group would occupy the volume of a five storey building, one that is as wide and deep as it is tall, and if the Milky Way sits on a table on the second floor, Andromeda would be a plate on a table on the fourth floor.

Spread throughout the rest of the building would be the 70 other Local Group galaxies. While some will be scattered almost randomly, many will be closer to the larger galaxies, but as dwarfs, most would be only a centimetre or less in size.

While dwarfs represent the smallest of galaxies, we know we share the universe with some absolute galactic monsters.

The largest yet discovered goes by the unassuming name of IC 1101, located a billion light years away (a single light year being equivalent to slightly less than 10 trillion kilometres) from the Milky Way.

It truly dwarfs the Milky Way, containing more than a trillion stars, and would easily fill our five storey building.

4-metre Mayall Telescope Astro Guy

So, we approach the ultimate distance scale for astronomers, the size of observable universe.

This is the volume from which we can have received light in the 13.7 billion year history since the Big Bang. Due to the expansion of the universe, the most distant objects are a mind-boggling 46 billion light years away from us. Can we hope to put this on some sort of understandable scale?

The answer is yes! Let’s think of the entire Milky Way as a 10c coin, roughly one centimetre across. Andromeda would be another 10c coin just quarter of a metre away, and the Local Group could easily be held in your arms.

The edge of the Observable Universe would be 5km away, and the universe would be awash with 300 billion large galaxies, such as our own Milky Way, living in groups and clusters, accompanied by an estimated ten trillion dwarf galaxies.

This is a total of 30 billion trillion individual stars. And yet most of the universe is almost completely empty.

At the edge of the Observable Universe, we have almost reached the end of our journey. We are left with the question of what is beyond the Observable Universe? Just how much more is out there?

If we combine all of our observations of the universe, with our theoretical understanding of just how it works, we are left with a somewhat uncomfortable fact.

The universe appears to be infinite in all directions, containing a infinite number of galaxies and stars. And that really is a lot to think about.

Read part one of the Cosmic Scale Series by Geraint Lewis – They might be giants: a mind-blowing sense of stellar scale.

Join the conversation

7 Comments sorted by

  1. Phillip Jones

    Mr

    on this subject i suggest people look up the song 'far' by george hrab (i'm sure its on youtube somewhere). it's the theme song he wrote for the 365 days of astronomy podcast.

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  2. Roger Powell

    logged in via Facebook

    Geraint,

    You said in the article:

    (1) "Due to the expansion of the universe, the most distant objects are a mind-boggling 46 billion light years away from us."

    - but you concluded with:

    (2) "The universe appears to be infinite in all directions, containing an infinite number of galaxies and stars."

    If the Universe really is infinite, then on what basis can the most distant objects be confined to a 'mere' 46 billion light years?

    Regards

    Roger Powell (MAS)

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    1. Geraint Lewis

      Professor of Astrophysics at University of Sydney

      In reply to Roger Powell

      Hi Roger - It's important to separate the whole Universe and the Observable Universe. The Observable Universe is the part of the Universe from which we can have received light from since the Big Bang, 13.7 billion years ago. If you think of an atom just after the Big Bang emitting a photon of light, that light travels towards us, taking the age of the Universe to get here. In that time, the expansion of the Universe has carried the atom to 46 billion years from us.

      Our Observable Universe is embedded in the infinitely large whole Universe, which goes on for ever. But every second that passes, the Observable Universe grows and more of the whole Universe is revealed.

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  3. wilma western

    logged in via email @bigpond.com

    It's great to spend a while contemplating the wonders of the universe . How about some sketches to accompany the word pictures?

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    1. Geraint Lewis

      Professor of Astrophysics at University of Sydney

      In reply to wilma western

      An excellent question, and one which may not make sense to start with. We know from special relativity, nothing can travel faster than light (recent neutrino claims excepted). But in reality, special relativity says that nothing can go faster than the speed of light **locally**, so in a small box, if I try and race an electron and a photon across the box, the photon will win.

      With the expanding universe, the question we are asking is a little different. We are asking how fast something is moving "over there", rather than locally. It turns out that, if you crank the handle of general relativity, that something over there can be moving faster than the speed of light here. But anyone in the universe who does the local test on the speed of light, by racing photons and electrons, will always find that the photons will win.

      If you think about it, what it means that, relative to the speed of light here, light out there is moving faster than the speed of light.

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  4. Rudi Schoebel

    logged in via Facebook

    Hi Geraint,

    Great article but it's left my very basic knowledge of cosmology in tatters! I can't fathom how that atom in your reply to Roger is now 46 billion light-years from us.

    If the photon emitted just after the Big Bang took 13.7 billion years to reach us (photon A), imagine a photon emitted at the same time but in the opposite direction (photon B).

    In the 13.7 billion years it's taken photon A to reach us photon B has travelled the same distance in the opposite direction, so shouldn't it "now" be 13.7+13.7=27.4 billion light-years from us? Even if the atom is moving away from us at near light-speed, how has it managed to get more than 27.4 billion light-years from us?

    It's been a long weekend, and I think I'm missing something very obvious, so please be easy on me!

    Thanks again

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    1. Geraint Lewis

      Professor of Astrophysics at University of Sydney

      In reply to Rudi Schoebel

      Ooops - hit wrong reply.

      An excellent question, and one which may not make sense to start with. We know from special relativity, nothing can travel faster than light (recent neutrino claims excepted). But in reality, special relativity says that nothing can go faster than the speed of light **locally**, so in a small box, if I try and race an electron and a photon across the box, the photon will win.

      With the expanding universe, the question we are asking is a little different. We are asking how fast something is moving "over there", rather than locally. It turns out that, if you crank the handle of general relativity, that something over there can be moving faster than the speed of light here. But anyone in the universe who does the local test on the speed of light, by racing photons and electrons, will always find that the photons will win.

      If you think about it, what it means that, relative to the speed of light here, light out there is moving faster than the speed of light.

      report