The Space Between

Space captivates me in a way few other things can. I’ll happily fritter away a Sunday learning everything I can about a new cosmic-related subject. Absorbing mind-bending facts that feel like fiction to a brain accustomed to things here on Earth. I always leave wanting to know more.

• Like, how can a teaspoon (5 mg) of neutronium, the hypothetical substance made purely of neutrons that we believe is inside of every neutron star, weight 5.5 billion (5,500,000,000) metric tons?^{1: Go to reference 1 at the end of the page}
• And how can a red dwarf, the smallest possible star, live for trillions of years^{2: Go to reference 2 at the end of the page} when we believe the universe itself is only 13.8 billion (13,800,000,000) years old?^{3: Go to reference 3 at the end of the page}
• Or how does the Sun convert 600 million (600,000,000) tons of hydrogen into 596 million (596,000,000) tons of helium every second?^{4: Go to reference 4 at the end of the page}

See what I mean about mind-bending? The science of space takes the numbers and measurements we are used to in everyday life and blows them up in spectacular fashion.

Now let’s look at our closest cosmic neighbor. The Moon’s orbit is elliptical, so the distance between us isn’t always the same, but at its closest it’s about 360,000 km^{5: Go to reference 5 at the end of the page} from us. That’s hundreds of thousands of kilometers from us! The Earth’s circumference at the equator, the longest possible journey around our planet, is just over 40,000 km, 1/9^th the distance. If we had a highway between the moon and us and drove it at 120 km/h, it would take about 125 days^{6: Go to reference 6 at the end of the page} of non-stop driving to get there. That’s a hell of a long way and this is the closest thing to us out there.

But hundreds of thousands is still a number we are used to seeing, right? If I told you I had $360,000 (I don’t), you would be happy for me (I assume), but you wouldn’t be in awe. It falls in the realm of numbers we come across in our Earth-shackled lives. So, let’s go a little farther out.

When we look at the planets closest to us, things really start to get interesting. Venus, our sister planet,^{7: Go to reference 7 at the end of the page} comes within 41,000,000 km^{8: Go to reference 8 at the end of the page} from us. MILLIONS! We are a bit beyond comparing to car rides here because a drive to Venus would take about 39 years.^{9: Go to reference 9 at the end of the page} I hope you brought a snack! It gets even more intense with Mars. The red planet’s closest approach to Earth is about 56,000,000 km.^{10: Go to reference 10 at the end of the page} When NASA delivered its most recent Mars rover, Perseverance, the journey took more than 29 weeks, with the spacecraft hurtling through space at nearly 40,000 km/h.^{11: Go to reference 11 at the end of the page} The trip isn’t a straight line and is significantly longer than 56,000,000 km (because space) but you get the idea. Getting anywhere off-planet, even our cosmic backyard, is really hard and takes a lot of time.

Astronomers believe the Scattered Disc to be the spawn point of most short-period comets.^{27: Go to reference 27 at the end of the page} Sometimes referred to as dirty snowballs,^{28: Go to reference 28 at the end of the page} they zip around the Sun with an orbital period under 200 years. Up to and over 100 AU from us, these objects are far but not as far as the Oort Cloud, the hypothetical home of long-range comets.^{29: Go to reference 29 at the end of the page} Although only theorized, we believe there are icy remnants from the solar formation that surrounds the Sun at a distance of 2,000 to 200,000 AU.

200,000 AU is just shy of 30 trillion (30,000,000,000,000) km. That number is so massive, it’s hard to even wrap our heads around it. But let’s try anyway. If a person got 1$ every second, they would be a millionaire in under 12 days, but it would take almost 32 years to become a billionaire. Pretty big difference, right? Well, it would take over 950,000 years to hit 30 trillion.^{30: Go to reference 30 at the end of the page}🤯 Astronomical!!

Stars are so widely spaced, unless gravitationally bound, that even a galactic collision, like the one expected between the Milky Way and Andromeda in 4 to 5 billion years, likely won’t cause stars to crash into each other.^{32: Go to reference 32 at the end of the page} Two clouds with billions of stars colliding into each other and none will make contact? Seems implausible at first but only because our minds can’t visualize that much space. Our sun’s diameter is about 1,390,000 km but the distance between Proxima Centauri and us is about 4.2 light-years, which is approximately 29 million times bigger.^{33: Go to reference 33 at the end of the page} You can fit a whole lot of other suns in there.

The Parker Solar Probe, a nifty little spacecraft NASA launched in 2018 to make observations on the Sun’s corona, is the current titleholder for fastest man-made object in space. It has reached speeds of 690,000 km/hour^{34: Go to reference 34 at the end of the page} and it would still take over 6,600 years^{35: Go to reference 35 at the end of the page} to reach Proxima Centauri, the star closest to our sun. A craft travelling almost 700,000 km/hour would still need over six and a half millennia to reach the closest star to us? I love trying to find relative ways like this to consider cosmic distances. It paints a picture in my brain and this picture is big!

Now let’s take a big step back and look at the Milky Way. Our galaxy is over 90,000 ly in length and Sagittarius A* (pronounced Sagittarius Eh Star), the black hole at the center of it, is about 26,000 ly from us.^{38: Go to reference 38 at the end of the page}

90,000 ly is about eight hundred fifty quadrillion (850,000,000,000,000,000) km. I’ve got nothing to compare that to. We aren’t driving that, probes aren’t flying that, and even the oligarchs who own everything don’t have that kind of money. It’s hard not to lose all perspective at this point. Might as well say the galaxy is ten million gazillion km in diameter because that makes about as much sense to my brain.

With galaxies needing so much space, there must be like what? 7 or 8 of them out there? Well… no. There are somewhere between 200 billion (200,000,000,000) and 2 trillion (2,000,000,000,000) galaxies in the observable universe,^{39: Go to reference 39 at the end of the page} and they are all absurdly far from one another. In the mid 1990s, NASA pointed the Hubble Space Telescope to a tiny piece of space no bigger than a person’s thumbnail would appear on their outstretched arm. About one 24-millionth of the whole sky. They left the telescope pointed to that one location for days to collect as much light and see as far into space as possible. The image they got revealed a sea of galaxies gradually fading into oblivion.^{40: Go to reference 40 at the end of the page}

A parsec is not that much bigger than a light-year, 1 parsec = 3.26 light-years, but it’s handy in defining big distances because, like meters, you can add prefixes to it and multiply its value exponentially.

• 1 parsec (pc) = 3.26 light-years (ly)
• 1 kiloparsec (kpc) = 3,261.56 light-years (ly)
• 1 megaparsec (Mpc) = 3,261,563.78 light-years (ly)

So, although the Milky Way is over 90,000 ly in diameter, that is only about 27.5 kpc. Every time the values start to get unmanageable, our old friend science kicks the door in with a new unit of measurement. Huzzah for science! 🎉🔭🎉 Why do we use megaparsecs instead of light-centuries or light-millennia? No idea.

I mentioned earlier how the Milky Way would one day collide with the Andromeda galaxy. That’s because we are both part of a local group of galaxies known as the Local Group (again, brilliant!). So far, we’ve counted about 80 galaxies, covering roughly 3 Mpc (10,000,000 ly).^{41: Go to reference 41 at the end of the page} With gravity’s help, every star in this group will eventually be in one giant galaxy. I’ve heard the name Milkdromeda used to refer to it. Yeesh! At least we’ll all be dead, amiright everyone? *waits for high five*

The Local Group is part of a cluster of galaxy groups known as the Virgo Supercluster, which has an estimated diameter of 33 Mpc (110,000,000 ly) and contains at least 100 galaxy groups and clusters.^{42: Go to reference 42 at the end of the page} It’s like Russian nesting dolls, you keep pulling back and you get bigger and bigger structures made of stars.

The Virgo Supercluster is in an even bigger supercluster named the Laniakea Supercluster. At around 160 Mpc (520 million light-years) in diameter, this giant supercluster is estimated to contain over 100,000 galaxies,^{43: Go to reference 43 at the end of the page} each of which able to produce billions or even trillions of stars.

Once we go back far enough, we start to see that the universe’s galaxies occupy what looks like giant filaments of matter. Often compared to a spider web or the neurons in a person’s brain, these enormous structures are separated from one another by even bigger voids and supervoids of mostly empty space.

The biggest structure of named galaxies we are part of (that I could find) is the Pisces–Cetus Supercluster Complex. The estimates I found put its diameter at around 300 Mpc, almost 1 billion light-years.^{44: Go to reference 44 at the end of the page} No references left. Just look at the awesome illustration by Richard Powell. Can you see your house?

How big does it all get? Can the universe just keep going on forever? Well, it might. Truth is, we aren’t all that sure. Our best science says there was a start to the universe (Big Bang)^{46: Go to reference 46 at the end of the page} about 13.8 billion years ago and it has been growing bigger ever since.^{47: Go to reference 47 at the end of the page} So, with that, we could say there is a border or rim around it all, but again it’s just too big for us to know for sure.

The observable universe is the part of the Universe that we can actually see, and it’s nowhere near the whole thing. The particle horizon (or cosmological horizon) defines how far light could have traveled to us from the most distant particles since the universe’s origin.