Space and Scale

One thing about space...it's big.

Really, really, REALLY big!

Naturally, 'space' by definition is nothingness (save for an average 0.25 atoms per cubic meter of interstellar space, and radiation). So when I say 'big', I'm referring to the distances between heavenly bodies.

The heavenly body nearest to the Earth is the Moon, some 300,000 km away. Yet, this is a trifle compared to the distances to other planets/stars/galaxies.

None of the illustrations or pictures you've seen of space truly express the sheer distances between heavenly bodies. This, of course, is a practical matter. There simply isn't enough room on a page/chart/screen to portray these distances at true scale.

So to get an appreciation of the true distances, let's scale heavenly bodies down to the size of everyday objects.

The Solar System

If our Sun was the size of a basketball, then the earth would be the size of a small pellet, a quarter the size of a pea.

If we placed the Sun-basketball under the hoop of a basketball court, our Earth-pellet would be under the other hoop, at the other end of the court.

Naturally, the Earth-pellet is too small to be visible here.

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The farthest major planet, Neptune, would be a pea...almost a kilometre away.

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(NOTE: Pluto is now classified as a dwarf planet. It would be a grain of sugar, a hundred meters further than the pea-Neptune.)

All of the above is so far just within our solar system. What happens when we go interstellar? Let's find out.

Interstellar

Our nearest star is the aptly-titled 'Proxima Centauri'. Compared to our Sun-basketball, it would be the size of a golf ball.

You cannot see Proxima Centauri in the night sky without a telescope, only because it's not bright enough. You can however, see other stars that are much farther away (because they are bright enough). For example, Polaris, the familiar north star, is easily visible via the naked eye despite being 77 times further away than Proxima Centauri.

So at our reduced scale, how far away would you estimate the Proxima Centauri-golf-ball to be from the Sun-basketball? Take your best guess.

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Would it surprise you to learn that if the Sun-basketball was placed in the middle of New York City, then the PC golf-ball would in Warsaw, Poland? 6,850 kilometres away! 

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Between them, (aside from a handful of planets/asteroids/comets/dust, together amounting to less then 0.5% of the combined mass of the two stars), complete nothingness!

Travelling in our fastest spacecraft, it would take us 164,000 years to reach there.

And that's our nearest star, a mere (in astronomical terms) 4.3 light years away. For comparison, Polaris, easily visible each clear night, is 323 light years away. It's clear that at  such distances, even our scaled-down model isn't useful.

It seems counterintuitive that something (stars) we can see so easily just by looking up are actually so terrifically far away. The reason we can still see stars despite the colossal distances separating them to us is not mainly because stars are so large (that's part of it), but chiefly because of their relative brightness. Compared to our Sun-basketball, Polaris would be the size of a house, and shining with the brightness of 130,000 blowtorches glowing all together. That's why we can see it even though it's so far away.

In reality, even as the pin pricks that we see them, they actually appear a lot larger than they truly are...once again, due to their relative brightness. Similar to how a really bright bulb in the middle of a dark field would appear like a glob of light much bigger than the bulb that emanates that light...a phenomenon called 'atrefacts' in photography.

Now, we have a somewhat realistic appreciation of the distances between stars.

So how big is a galaxy?

The Galaxy

Nearly every star visible to us when we look up at the night sky is within 1,000 light years of us. By comparison, our Milky Way galaxy is 100,000 light years in diameter.

Light Years: Expressing astronomical distance in light years is practical, but has the tendency to inadvertently underplay just how large those distances actually are.

A light year, of course, is the distance that light with it's blistering speed travels in a whole year...9.4 Trillion kilometres.

To appreciate that, consider what happens when you speak to someone over the phone at the other side of the world. The electric signal containing their voice travels from their phone to yours at the speed of light (electricity is transmitted at light speed), and you hear it instantaneously despite being separated by 20,000  or more kilometres. In fact, that signal could travel around the whole world 7 and a half times before you would perceive a 1 second delay. That's that distance light can travel in just 1 second. 1 light second. Now, think...4.3 light years to the nearest star! And close to 100,000 light years to the edge of our galaxy.

As discussed earlier, stars due to their relative brightness appear a lot larger than they actually are. So although most of the individual points of light in the above picture represent individual stars, they're much smaller at true scale in proportion to the galaxy. 

To appreciate this and the size of the galaxy, consider the following rescaling. If you enlarged the above picture of the Milky Way so that it was as large as the entire Earth, you would still need a microscope to see our Sun!

With just a single galaxy being that large, dare we go intergalactic? We can try.

Intergalactic

Our nearest galaxy is the venerable Andromeda Way. It's around twice as large as our Milky Way, but contains nearly 5 times more stars. Despite being the 'nearest' galaxy, it's located 2.5 Million light years away (remember earlier in the post, when it seemed that the 4 light years to the nearest star was far?). 

Incredibly, despite being so unimaginably far away, the Andromeda is visible via the naked eye. It appears as a somewhat fuzzy looking star.

What we can see of the Andromeda is only its very brightest part, i.e. the galactic core. The galactic core is the very center of the galaxy, containing million of stars packed together densely in a relatively small volume. That's what makes it so bright, and also fuzzy (since that light is the collection of light from millions of individual stars).

If all the stars of Andromeda were bright enough, the galaxy would appear 6 times larger to us than a full moon does.

Further

As mentioned, Andromeda is our nearest galaxy. There are at least 2 Trillion other galaxies, most being much, much further away. The Hubble Deep Field photograph displays galaxies hundreds of millions of light years away. The furthest galaxy observed so far is an incomprehensible 13 Billion light years away.

Space...it's big!

Epilogue

Returning to our own galaxy, the Milky Way. As we saw, every star we can see without a telescope is no more than 1,000 light years away. Whereas the galaxy itself is a hundred times bigger. With our nearest star itself being so far that it takes light 4.3 years to reach there, imagine how wonderous and exotic planets so far as the other side of the galaxy might be. To say nothing of the planets in the Andromeda Galaxy.

We know there are planets which have 3 suns and multiple moons. Planets whose night sky contains thousands of times more stars than ours, such that their night sky is nearly as bright as their day. Planets that have atmospheres so thick, you could literally fly in it just by flapping your arms. Planets where it rains diamonds! 

If there is life on any of these planets, imagine how unimaginably different it would be. What are the chances we would ever come in contact with alien life? Might we ever 'hear' from them via radio? Hold a conversation? Even more tantalizingly, could they ever visit us? Or we visit them? 

I have some thoughts about this, looking purely from the perspective of physics and statistics. Thoughts that I will expound upon in a future post.