The Solar System
Before we even begin to talk about the Universe or Astronomy as a whole we must build a solid foundation of how our own “neighborhood,” the Solar System, works. At the very least we should have a rickety little house hanging off a cliff, which is what I’ll scaffold for you quickly here.
Our Solar System has eight planets. It used to have nine, but Pluto was snuffed and kicked out by all the other Mean Girls movie archetype Mean Girls planets because its gravity just wasn’t strong enough to clear its orbit of space debris. Very sad. Very true.
We have four terrestrial planets— you know, the traditional type of planet, with ground and a rocky surface and solid stuff— and four gas giants. Everything in the Solar System orbits around the Sun.
Most of our planets are named after the Roman names for the Greek gods, except for Earth which is named Earth, and Uranus which keeps the name of the Greek version of the god. Good thing, too— imagine having to write that Roman name, Caelus, everywhere. Sounds like a piece of proprietary software. Scary, ikr.
You can remember the order of the planets using the following cringey mnemonic, as long as you remember that Mercury is first so you don’t confuse it with Mars:
My Very Educated Mother Just Served Us Noodles
This is the kind of science lesson you get in third grade, but to be fair I often do not feel any smarter than a third grader. I hope it will be as beneficial to you as it felt stupid to me to type out.
Relative and Distance Scales
On Earth we can pin-point our location in a lot of different ways:
- Latitude and Longitude
- Street and zip code
- Town and city
- State and country
- Continent
But when we’re considering our place in the universe, it’s not that simple. The amount of distance between houses or streets or any of the measurements we use on Earth are far too small to measure distances between planets, stars or galaxies.
This basically means that we have to identify units of measurement that are big enough to not yield ludicrously large numbers when we’re talking about things on a non-Earth scale.
Units of Measurement
Astronomical Units
A unit you’ll commonly see used when measuring inside of the Solar System is the astronomical unit, or AU. One AU is equivalent to the distance from Earth to the Sun, which if expressed in kilometers would be around 150,000,000 kilometers.
The AU is especially useful if you’re talking about the Solar System in relation to Earth. For example:
“Venus is about 108,000,000 kilometers from the Sun, which is equal to around 0.7 AU.”
Though an AU is a unit of distance, it is also a ratio. By saying that Venus is 0.7 AU from the Sun, you’re also saying that it is around 70% (0.7 times) the distance that Earth is from the sun. If you were to say that Neptune is 30.1 AU from the sun, you’re saying that it is 3010% (30.1 times) the distance that Earth is from the Sun.
But on a bigger scale than our Solar System, you begin to see the faults in the AU system. While 150,000,000 kilometers is a large distance compared to any route you’ll take on Earth, eventually using AU as your unit becomes too tedious, so you have to move onto something bigger…
Light Years
A light year is the distance that light travels in one Earth year. The speed of light is approximately 300,000,000 meters per second, so calculating what a light year is equivalent to in kilometers puts you in the ballpark of nine trillion kilometers. Let’s not even think about that one in meters.
A light year is useful when measuring anything that’s, well, big in the greater universe. If you say:
“The Andromeda Galaxy is 160,000,000,000 AU away.”
the amount of AU that we’re dealing with here is simply unimaginable.
Think about it this way: imagine 160,000,000,000 puppies. No matter how much you love puppies, you cannot even begin to imagine what that number of puppies looks like, much less what that would look like in distances from the Earth to the Sun.
No, it’s much easier to express these kinds of values in light years.
“The Andromeda Galaxy is 2,537,000 light years away.”
It just looks so much better! Over two million is still a giant number, but it’s
- Far easier to perform mathematical calculations with
- Not going to give you a hand cramp to write down
- Slightly more manageable
While it’s probably similarly hard to imagine two point five million puppies, it’s a bit easier to break down if you imagine, like, a bunch of giant stadium crowds stacked on top of each other. I guess.
Where Even Are We?
Let’s go back to identifying our location. Since we’re now kind of initiated and at least kind of understand how big distances between even the closest galaxies can be, it’s understandable that we should probably be able to find ourselves in the big mess of nonsense that is the universe.
But how are we even supposed to find ourselves if we have literally no idea what is what?
Big Stuff Sub Genres
- Universe
- “Space is big. Really 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, but that’s just peanuts to space.” - Douglas Adams
- Supercluster
- A supercluster is basically just a big, broad group of galaxies and smaller, not-so-super clusters. This is the structure in the universe— the universe doesn’t really count right now, since there’s only one. As far as we know, at least…
- Galaxy Group
- A galaxy group is a way to classify galaxies that are (relatively) close to each other— kind of like a county. A really big county. This is the most common type of galaxy congregational you’re going to see in the Universe.
- Galaxy
- A galaxy is a system that is comprised of stars, interstellar gas, dust, and a lot more confusing space stuff, bound together by gravitational force, usually from some sort of supermassive black hole to be able to have a gravitational pull of this size.
- Galaxies usually have more than 100 million stars. Even though they’re not the biggest type of celestial object, they are not playing around. They’re often between 3,000 and 300,000 light years in diameter…
- Planetary System
- A planetary system is a bunch of objects in orbit around a star or group of stars, where there are one or more planets.
- A planetary system can also have dwarf planets, asteroids and a whole bunch of space junk orbitting the same star/star group, but it’s the planet requirement that makes it a planetary system.
- Planet
- A planet is a round object in space that isn’t a star. Boom. Easy.
- Think: why is it round?
Cosmic Amazon Delivery Instructions
Okay, so now we know what types of things are out there— which ones are ours? How are you going to get your yesterday shipping from an advanced alien civilization who hasn’t even evolved yet, they’re that quick?
This is where our cosmic address comes into play. Obviously we’re on Earth. Let’s build up from there and go through the list above. Our planetary system is the Solar System, no duh, and our galaxy the Milky Way Galaxy, part of the Local Group— we humans are very good at naming things.
Our Local Group is inside of the Virgo Supercluster, but that itself is inside of the Laniakea Supercluster so we’re just going to go with the bigger latter one. I mean, they’re pretty advanced, they’ll figure it out.
So, our cosmic address is:
3931 Banana Road, Bananaville NJ, 09231
United States of America, North America
Earth, Solar System, Milky Way
Local Group, Laniakea Supercluster
The Universe
Not too shabby.
Properties of the Universe
You’ll notice I left that little universe blurb blank earlier— there’s a reason for that. You can’t really describe the universe in a few bullet points. It’s a whole thing— every whole thing, everything around us, every tiny piece of dust and every molecule right under our noses.
The Universe isn’t an object in the traditional sense because it is everything that we know about and don’t know about— yes, even stuff like dark matter is part of it.
But the Universe had to start somewhere, right?
I mean… probably?
The most widely accepted theory for the “start” of the Universe as we know it is called the Big Bang Theory, which you’ve probably heard of, and you are not allowed to make jokes about the show. Young Sheldon is actually so much better than every single episode of the mainline Big Bang Theory that it is ACTUALLY ridiculous.
Anyway. The Big Bang Theory goes like this:
- There was a lot of energy— all of the energy ever— at one tiny little point, which was kind of like a super, super, SUPER massive black hole.
- This energy was so concentrated that it exploded and created billions of trillions of galaxies and the like as described above.
The currently relevant model we use to understand the universe is called General Relativity. It also happens to be the only model that can withstand any sort of testing. General Relativity is based on Albert Einstein’s original work on the concept of the space-time continuum.
Space-Time
The concept of a space-time continuum, or that there even is a set relationship between space and time, comes from Einstein’s observations of light. Before Einstein, people kind of thought of space and time as wholly seperate matters, but after observating the constant speed of light no matter the speed or anything affecting the observer, he came to the conclusion that it had to be multiple things conspiring together that allowed this weird phenomenon to occur.
He re-defined the universe as a four dimensional system as opposed to a three-dimensional traditional system. The traditional system contains three axes; width, length, and height, or X, Y and Z. Einstein’s four dimensional system included time as a fourth axis.
Let’s say that Jimothy and Jimantha are going on a date. Jimothy says to Jimantha:
“I’ll meet you at the Restaurant at (X, Y, Z).”
(X, Y, Z) is kind of like a cosmic address for points in a three dimensional model. But Jimothy and Jimantha aren’t on a three dimensional model, they’re on Einstein’s model, and they’ve forgotten the fourth dimensional coordinate they want to meet at, so they haven’t met at the same place of (X, Y, Z, T), technically, because they’re not there at the same time.
How Long Has It Been?
That still doesn’t explain when or where the Universe started, besides in arbitrary terms. General Relativity itself assumes that t > 0 and hinges on this for its calculations. While you can get as close as you want to 0 and try to guess off of that, as anyone who has taken math up to Pre-Calculus knows, the limit doesn’t always equal the point that it approaches. Also, if they’re nothing, the limit doesn’t exist. Essential discontinuity. You’re welcome.
But if our working model suggests that as you travel in space you’re also travelling in time, considering time is a part of space and space is a part of time— is it even possible to measure when both space and time were zero?
Scientists figured this one out because of a whole different property of the Universe. The Universe is expanding constantly, but it is not the mass itself in the Universe that is expanding, it is the space between it. There’s a whole lot of theories ranging from dark matter to “some guy is pounding the Universe like mochi,” but what’s at least pretty widely accepted is that it is expanding and it is probably an after effect of the Big Bang.
If you apply the laws of physics to the expansion of the universe, you’ll see something odd. If this all was solely remnants of energy from the Big Bang, it should slow down over time, but instead this expansion is accelerating. Right now this is attributed to a mysterious force dubbed “dark energy,” which is just… yeah, no one knows, but it does the expandy stuff and is uniform throughout the universe.
If the speed of expansion is accelerating, we can try to measure this rate of acceleration— which, to remind you, is the rate of change of the velocity of expansion— and then use that reverse-engineering to approximate the age of the Universe. With what we know now, we can say to some degree of certainty that the Universe is around 13.8 billion years old.
Time Scales
To even begin to understand how long that is, let’s put how long we’ve existed into perspective. Let’s put all of “time” that we know about into a calendar, dividing 13.8 billion years into 365 days.
This brings us to a meagre 37808219.1781 years per day, which we’ll of course simplify into a cool 37800000 years to make our lives much, much easier. I’ll even put in some commas for you spoiled brats: 37,800,000 years occur every day on the cosmic calendar.
We’re going to put the Big Bang at January 1st for demonstration purposes, and not get into an ideological debate over it, okay?
So how is Earth doing? Our good friend Earth? Our buddy Earth? Not even a concept yet. Our Solar System only formed 4.6 billion years ago, so we’d have to go through 9.2 billion years or 243.3 days until it forms. August 30th.
You can try for yourself to figure out what day humanity, or even humanity’s ancestors, begin to resemble the modern day. It’s a bit of a doozy, though, I’ll warn you.
The Observable Universe
Let’s recall our definition of light years as the distance that light travels in an Earth year. We measure this not only so that we can use light years as a measure for large distances, but so that we can track how many years it takes for light to reach Earth.
If light comes from the Sun, it takes eight minutes to reach Earth. If light comes from farther away, it will take longer to reach us, even if the speed is constant and very, very, very fast.
This becomes even more apparent as the distance increases, until you reach places that are 1 million, 100 million, 1 billion and beyond light years away. Light transmits where was there at the moment to us, but it no longer shows us near-instant changes as it does on Earth— instead, it shows us how a place that was thirteen point eight billion light years away from our current location looked when the Universe was created.
Because the Universe is constantly expanding, things that were in that spot 13.8 billion years ago are no longer there anymore. If we were to somehow instantaneously travel there, we would not see the same thing we saw from Earth. Light transmits “memories,” almost like visual time travel, to us.
This also means that things that are actually at the edge of the possible Universe (if that even exists) will take billions of years for us to be able to see, reducing our ability to study the Universe to just studying what we see.