This photo from the Washington Post is a depiction of a Coronal Mass Ejection. What's more interesting about this picture is that it outlines one of the toughest concepts people encounter when learning about the Universe, namely, measuring it. Space is so vast and filled with objects so enormous that common measurements here on Earth involving size and distance become impractical. Therefore, we will begin our exploration of the Universe by talking a little about the size and scale of it, and some of the tricks we use to grasp those enormous numbers.
The Size of Planets and Stars.
It is difficult to imagine just how big the Sun really is, or to understand just how wide our own galaxy is. When we attempt to measure things in space, conventional measurements like Miles and Kilometers simply do not cut it. Therefore, we've replaced them with more exotic (less intuitive) measurements like Light years, Astronomical Units, and Plank Lengths. There is no easy way to comprehend these relatively new units of measurement, and so the best we can do is to look at scale pictures, and make analogies. When we talk about the size of stars or planets, we will often use a comparative form of volume, e.g Jupiter could fit more than 1000 Earths inside it!
However even using the entire Earth as a measuring stick has its limits. For instance, can you tell by looking at the picture on the left that the Sun is about the size of 1 million Earths? When we make that claim, we are not accounting for factors like density. Rather, we're just saying that the Sun could fit over 1 million Earth sized objects inside of it.
Now that you know the Sun is roughly the size of 1 million Earths, can you imagine how big a star like Eta Carinae is? Eta Carinae is 400 times the size of the Sun! Perhaps you can't imagine something that big, but now you know that Eta Carinae, the biggest known star, can fit over 400 million Earths inside it!
We use the Earth for comparisons because though we can have trouble imagining its size, it is still the most intuitive celestial object we have at our disposal. Try to imagine 2 Earths - that the Earth was twice as big as it is - and then try to imagine 1 million Earths. It's mind boggling, and stars can be even bigger than that! In order to further illustrate this, lets do a little thought experiment:
Stand with your shoes just 1 inch apart from each other:
Now if we somehow inflated the Earth beneath you to the size of, say, Jupiter, your shoes would spread out until they were about 83 feet apart. If we could inflate the Earth to the size of the Sun, one shoe would be over 15 miles away from the other one!
However, there are limiting factors built into the laws of physics that define how big and how small some objects can be. On the small end, an asteroid that is less than 300 miles across will not generate enough gravity to crush itself into a sphere. Anything smaller will maintain its natural jagged and lumpy shape. Anything with sufficient density that is wider than about 300 miles across will crush itself into a spherical shape and turn into a planet or star, or a Dwarf Planet like Pluto. The smallest stars have to be much bigger than the biggest planet in our solar system, Jupiter, or they will not turn into stars at all.
We will discuss how the different sizes of stars effect everything from their chemical make-up to their color and, also, how they die. For now, though, it is enough to just try and imagine the Earth relative to the stars in our galaxy. Sitting the Earth down next a mammoth star like Betelgeuse would render it too small to be seen. And though even the smallest stars are incredibly big, they are tiny compared to the biggest things in the Universe like nebulae, galaxies and galaxy clusters. If you look back at the Hubble Deep Field, almost every little dot and speck of light we can see in the picture contains billions of planets and stars of all shapes and sizes. The objects in the picture above are all from our own cosmic neck of the woods, our own galaxy, the Milky Way.
Now that we have at least some idea about how big things really are, we're prepared to start a discussion on the distances between things in space using the Sun-Basketball Model. If we could shrink the Sun down to the size of a basketball, and shrink the rest of the Universe down with it, how big would the planets be, and how far away from the basketball would they rest?
Note: when you think about this analogy, keep in mind that the Sun is over a million times the size of the Earth. Take that huge star in your mind, and shrink it down to the point where it would not only fit on the Earth's surface, but would be the size of a basketball, rendering Earth roughly the size of a very small pebble. Now lets take the basketball and put it on a court, under one of the goal lines.
However even using the entire Earth as a measuring stick has its limits. For instance, can you tell by looking at the picture on the left that the Sun is about the size of 1 million Earths? When we make that claim, we are not accounting for factors like density. Rather, we're just saying that the Sun could fit over 1 million Earth sized objects inside of it.
Now that you know the Sun is roughly the size of 1 million Earths, can you imagine how big a star like Eta Carinae is? Eta Carinae is 400 times the size of the Sun! Perhaps you can't imagine something that big, but now you know that Eta Carinae, the biggest known star, can fit over 400 million Earths inside it!
We use the Earth for comparisons because though we can have trouble imagining its size, it is still the most intuitive celestial object we have at our disposal. Try to imagine 2 Earths - that the Earth was twice as big as it is - and then try to imagine 1 million Earths. It's mind boggling, and stars can be even bigger than that! In order to further illustrate this, lets do a little thought experiment:
Stand with your shoes just 1 inch apart from each other:
Now if we somehow inflated the Earth beneath you to the size of, say, Jupiter, your shoes would spread out until they were about 83 feet apart. If we could inflate the Earth to the size of the Sun, one shoe would be over 15 miles away from the other one!
However, there are limiting factors built into the laws of physics that define how big and how small some objects can be. On the small end, an asteroid that is less than 300 miles across will not generate enough gravity to crush itself into a sphere. Anything smaller will maintain its natural jagged and lumpy shape. Anything with sufficient density that is wider than about 300 miles across will crush itself into a spherical shape and turn into a planet or star, or a Dwarf Planet like Pluto. The smallest stars have to be much bigger than the biggest planet in our solar system, Jupiter, or they will not turn into stars at all.
We will discuss how the different sizes of stars effect everything from their chemical make-up to their color and, also, how they die. For now, though, it is enough to just try and imagine the Earth relative to the stars in our galaxy. Sitting the Earth down next a mammoth star like Betelgeuse would render it too small to be seen. And though even the smallest stars are incredibly big, they are tiny compared to the biggest things in the Universe like nebulae, galaxies and galaxy clusters. If you look back at the Hubble Deep Field, almost every little dot and speck of light we can see in the picture contains billions of planets and stars of all shapes and sizes. The objects in the picture above are all from our own cosmic neck of the woods, our own galaxy, the Milky Way.
Now that we have at least some idea about how big things really are, we're prepared to start a discussion on the distances between things in space using the Sun-Basketball Model. If we could shrink the Sun down to the size of a basketball, and shrink the rest of the Universe down with it, how big would the planets be, and how far away from the basketball would they rest?
Note: when you think about this analogy, keep in mind that the Sun is over a million times the size of the Earth. Take that huge star in your mind, and shrink it down to the point where it would not only fit on the Earth's surface, but would be the size of a basketball, rendering Earth roughly the size of a very small pebble. Now lets take the basketball and put it on a court, under one of the goal lines.
The Sun-Basketball Model.
If the Sun were shrunk to the size of a basketball and places under the goal on a regulation basketball court:
Mercury would be smaller than a BB, and rest about 33 feet away. In other words, take a bb and put it past the the Three-Point Line.
Venus would be a very small pebble resting 62 feet away. To represent Venus, you could put a pebble close to the Three-Point Line on the other side of the court.
Earth would be a very small pebble resting 86 feet away. To represent the Earth, you could put a pebble under the opposite goal.
Mars would be an even smaller pebble resting 130 feet away, somewhere in the stands.
Let's take a moment to reflect on this. If the Sun were a basketball under the goal on a regulation court, Mercury, the closest planet to our Sun, would be smaller than a bb and resting passed the Three-Point Line. You'd probably have trouble seeing it if you stood next to the basketball. Mars would be a little bit bigger, and resting somewhere in the stands on the opposite side of stadium. The reason we should reflect here is because after Mars, none of the other planets would even be in the stadium with the basketball. That's how big our solar system is.
Jupiter would be the size of a golf ball resting 445 feet away.
Saturn would be the size of a ping pong ball resting about 1050 feet away
Uranus would be the size of a marble about 2133 feet away
Neptune would be the size of a marble about 3281 feet away
Pluto would be a grain of sand 3280 feet away! That's right. If the Sun were the size of a basketball under the goal of a regulation court, Pluto would be a grain of sand over half a mile away!
Mercury would be smaller than a BB, and rest about 33 feet away. In other words, take a bb and put it past the the Three-Point Line.
Venus would be a very small pebble resting 62 feet away. To represent Venus, you could put a pebble close to the Three-Point Line on the other side of the court.
Earth would be a very small pebble resting 86 feet away. To represent the Earth, you could put a pebble under the opposite goal.
Mars would be an even smaller pebble resting 130 feet away, somewhere in the stands.
Let's take a moment to reflect on this. If the Sun were a basketball under the goal on a regulation court, Mercury, the closest planet to our Sun, would be smaller than a bb and resting passed the Three-Point Line. You'd probably have trouble seeing it if you stood next to the basketball. Mars would be a little bit bigger, and resting somewhere in the stands on the opposite side of stadium. The reason we should reflect here is because after Mars, none of the other planets would even be in the stadium with the basketball. That's how big our solar system is.
Jupiter would be the size of a golf ball resting 445 feet away.
Saturn would be the size of a ping pong ball resting about 1050 feet away
Uranus would be the size of a marble about 2133 feet away
Neptune would be the size of a marble about 3281 feet away
Pluto would be a grain of sand 3280 feet away! That's right. If the Sun were the size of a basketball under the goal of a regulation court, Pluto would be a grain of sand over half a mile away!
A Little Bit Further.
You can already see the distance between celestial objects, even in our own solar system, is huge. But, let's take this one step further and see how far Proxima Centauri, the star nearest to our Sun, would be if the Sun were the size of a basketball.
If the Sun were the size of a basketball:
Proxima Centauri would be another basketball over 4000 miles away!
In other words, if our Sun were a basketball in Kansas City, Missouri, Proxima Centauri would be another basketball in the streets of London! Inflate the Sun back to its actual size, and you can see that the distances in space are overwhelming. Talking about distances in space using miles or kilometers will give us numbers with way too many zeros at the end of them. So, in order to clear up some confusion and make these huge numbers more manageable, scientists invented a new unit of measurement.
If the Sun were the size of a basketball:
Proxima Centauri would be another basketball over 4000 miles away!
In other words, if our Sun were a basketball in Kansas City, Missouri, Proxima Centauri would be another basketball in the streets of London! Inflate the Sun back to its actual size, and you can see that the distances in space are overwhelming. Talking about distances in space using miles or kilometers will give us numbers with way too many zeros at the end of them. So, in order to clear up some confusion and make these huge numbers more manageable, scientists invented a new unit of measurement.
The Light Year.
Before we jump right in to light years, let's consider some manageable distances: namely, the distance between our Sun and the Earth, and the distance between our Sun and Proxima Centauri. But let's look at these distances in miles.
Distances between celestial bodies in Miles:
Distance from Earth to the Sun: 93,000,000 miles. (ninety three million miles)
Distance from Sun to the nearest star: 25,000,000,000,000 miles. (twenty five trillion miles)
As you can see, the zeros stack up quickly when we talk about objects in space relative to miles. Measuring from the Sun to Proxima Centuri requires a fifteen digit number. In order to make the numbers more manageable, scientists adopted a different unit of measurement to replace miles and kilometers called the light year.
Distance From Sun to nearest star in miles: 25,000,000,000,000 (twenty five trillion miles)
Distance from Sun to nearest star in light years: 4.2 light years
Distance from Earth to the Sun in miles: 93,000,00 miles (ninety three million miles)
Distance from Earth to the Sun in light years: (.0002 light years) OR about 8 light minutes.
You may be feeling a little overwhelmed at this point, but there is good news. Once you grasp the concept of a light year, you can talk about any point in space in the observable Universe, and you'll only have to raise the number up to the billions to do it! So, what exactly is a light year?
The definition of a light year is the distance light travels in one year. Simple right? Well, not really. We’re used to dealing with light here on Earth in relatively small spaces like our living room, or out to the horizon, so the distance light actually travels in a year may be hard for us to understand. Over the relatively short distances we're used to dealing with on Earth, the movement of light appears instantaneous. When we click on the lights in our room, we don’t have to wait for the room to get bright; it seems to happen instantly the very moment we flip the switch. But light does not travel instantly. It has a set speed, and we've measured it!
Put simply, it's fast. A photon of light can travel around the entire Earth, a distance of 25,000 miles, about 7 and a half times per second. Put a different way, a photon of light can travel around the entire Earth multiple times in the time it takes you to blink your eyes. Try blinking your eyes right now, and image a photon of light whipping around the entire Earth and right by your head multiple times before you can open them. That's how fast light is. Numerically, the speed of light is 186,282 miles per second. This may seem a little hard to digest, but now we can put it into perspective using numbers we already know. Grab your calculators!
Distance Between Earth and the Sun: 93,000,000 miles
Speed of Light: 186,282 miles per second
If we divide these two numbers, distance and speed, we come up with an estimate of how many seconds (time) it takes light from the Sun to reach the Earth. Dividing, we see that it takes light from the Sun roughly 499 seconds to reach the Earth. If we then divide 499 seconds by 60 (seconds in a minute), we see that it takes light about 8 minutes to get from the Sun to the Earth. This might set off a bell in your mind if you've been reading carefully. It takes light about 8 minutes to get from the Sun to the Earth, and in the above paragraph we said that the the distance between the Earth and the Sun is about 8 light minutes.The values are the same. To use light years to measure distances in space is to use exactly the speed of light to measure the time it takes light to get from one place to another. If it takes light a year to get from one place to another, those places are exactly 1 light year apart. For one more example consider that It takes light 1.3 seconds to get from the Earth to the Moon. Therefore you can say that the moon is 1.3 light seconds away from Earth.
All of this may not seem very impressive until you imagine some more tangible consequences of the speed of light. Understanding light isn't as simple as understanding how fast it moves. Its important to realize that nothing in the universe can move faster, not stars, not galaxies, not electrons - not even information itself, which can be quite terrifying to contemplate.
Consider what would happen if the Sun suddenly vanishes right now at the exact moment you read this:
The first thing you should do is look at your clock. Mine says 4:11 pm. Now, look outside. Even if the sun had vanished when you read the above sentence, you would still see normal daylight outside for around 8 more minutes before the Earth goes dark. In my case, I'd have until about 4:19 pm to get my affairs in order. But therein lies the problem. Because not even information can travel faster than light, it is physically impossible for us to know that the Sun has vanished, or exploded, until we see it disappear from the sky, 8 minutes after it actually happened, and by then, its too late.
If it's night-time where you are right now, you would still see the moon shining for around the same amount of time before it went dark. If you want to get technical, the moon will shine for about 1.3 seconds longer than the Sun in this scenario. The extra second is the result of the time it takes the light to bounce off the moon and hit Earth. So, if the Sun suddenly vanished, for around 8 minutes everything would appear normal in our sky, even through our telescopes.
An even scarier scenario is a Gamma Ray Burst, and unlike entire stars mysteriously vanishing, gamma ray bursts happen all the time. When huge stars die they sometimes release two focused beams of high energy particles (Not exactly lasers, but for now you can think about it like a huge destructive laser beam that gets shot out of both sides of a star when it explodes). We'll talk more about what they are and how they work later, but gamma ray bursts are extremely powerful, and they travel at the speed of light.
Consider: A star that is 1 light year away explodes today, as you read this sentence, and releases a gamma ray burst aimed directly at Earth. The star will look normal to us through telescopes for an entire year hence. If we take a direct hit, it could kill everything on Earth within a few seconds. If that isn't scary enough, consider that up until the very moment we are hit, everything will look completely normal because the gamma ray burst travels at the speed of light. We are physically incapable of getting a warning. Even if we have a satellite far away from Earth that was designed specifically to detect these bursts, the information from that satellite would travel at the same speed as the gamma ray burst and our warning would still come at the exact same time as the burst itself. Again, it is impossible for us to have an advance warning because of the nature of the speed of light.
So now that we have done a little work with the implications of the speed of light, we want to look at how far light travels in a year. We’re going to have to stretch our imaginations a little bit because light travels about 6 trillion miles in one year.
Distance Light Travels in a Year in miles: 6,000,000,000 miles.
6,000,000,000 miles: 1 light year.
It is alright if this is still a little confusing. The good news is you don't have to know how many miles are in a light year to use them to measure distances in space. However, just this little bit of background on light years has hopefully impressed upon you how enormous the distances are between planetary bodies, stars, and galaxies. After all of this, it should come as no surprise that the observable Universe is about 93 billion light years across! That's about 558,000,000,000,000,000,000,000 miles.
Distances between celestial bodies in Miles:
Distance from Earth to the Sun: 93,000,000 miles. (ninety three million miles)
Distance from Sun to the nearest star: 25,000,000,000,000 miles. (twenty five trillion miles)
As you can see, the zeros stack up quickly when we talk about objects in space relative to miles. Measuring from the Sun to Proxima Centuri requires a fifteen digit number. In order to make the numbers more manageable, scientists adopted a different unit of measurement to replace miles and kilometers called the light year.
Distance From Sun to nearest star in miles: 25,000,000,000,000 (twenty five trillion miles)
Distance from Sun to nearest star in light years: 4.2 light years
Distance from Earth to the Sun in miles: 93,000,00 miles (ninety three million miles)
Distance from Earth to the Sun in light years: (.0002 light years) OR about 8 light minutes.
You may be feeling a little overwhelmed at this point, but there is good news. Once you grasp the concept of a light year, you can talk about any point in space in the observable Universe, and you'll only have to raise the number up to the billions to do it! So, what exactly is a light year?
The definition of a light year is the distance light travels in one year. Simple right? Well, not really. We’re used to dealing with light here on Earth in relatively small spaces like our living room, or out to the horizon, so the distance light actually travels in a year may be hard for us to understand. Over the relatively short distances we're used to dealing with on Earth, the movement of light appears instantaneous. When we click on the lights in our room, we don’t have to wait for the room to get bright; it seems to happen instantly the very moment we flip the switch. But light does not travel instantly. It has a set speed, and we've measured it!
Put simply, it's fast. A photon of light can travel around the entire Earth, a distance of 25,000 miles, about 7 and a half times per second. Put a different way, a photon of light can travel around the entire Earth multiple times in the time it takes you to blink your eyes. Try blinking your eyes right now, and image a photon of light whipping around the entire Earth and right by your head multiple times before you can open them. That's how fast light is. Numerically, the speed of light is 186,282 miles per second. This may seem a little hard to digest, but now we can put it into perspective using numbers we already know. Grab your calculators!
Distance Between Earth and the Sun: 93,000,000 miles
Speed of Light: 186,282 miles per second
If we divide these two numbers, distance and speed, we come up with an estimate of how many seconds (time) it takes light from the Sun to reach the Earth. Dividing, we see that it takes light from the Sun roughly 499 seconds to reach the Earth. If we then divide 499 seconds by 60 (seconds in a minute), we see that it takes light about 8 minutes to get from the Sun to the Earth. This might set off a bell in your mind if you've been reading carefully. It takes light about 8 minutes to get from the Sun to the Earth, and in the above paragraph we said that the the distance between the Earth and the Sun is about 8 light minutes.The values are the same. To use light years to measure distances in space is to use exactly the speed of light to measure the time it takes light to get from one place to another. If it takes light a year to get from one place to another, those places are exactly 1 light year apart. For one more example consider that It takes light 1.3 seconds to get from the Earth to the Moon. Therefore you can say that the moon is 1.3 light seconds away from Earth.
All of this may not seem very impressive until you imagine some more tangible consequences of the speed of light. Understanding light isn't as simple as understanding how fast it moves. Its important to realize that nothing in the universe can move faster, not stars, not galaxies, not electrons - not even information itself, which can be quite terrifying to contemplate.
Consider what would happen if the Sun suddenly vanishes right now at the exact moment you read this:
The first thing you should do is look at your clock. Mine says 4:11 pm. Now, look outside. Even if the sun had vanished when you read the above sentence, you would still see normal daylight outside for around 8 more minutes before the Earth goes dark. In my case, I'd have until about 4:19 pm to get my affairs in order. But therein lies the problem. Because not even information can travel faster than light, it is physically impossible for us to know that the Sun has vanished, or exploded, until we see it disappear from the sky, 8 minutes after it actually happened, and by then, its too late.
If it's night-time where you are right now, you would still see the moon shining for around the same amount of time before it went dark. If you want to get technical, the moon will shine for about 1.3 seconds longer than the Sun in this scenario. The extra second is the result of the time it takes the light to bounce off the moon and hit Earth. So, if the Sun suddenly vanished, for around 8 minutes everything would appear normal in our sky, even through our telescopes.
An even scarier scenario is a Gamma Ray Burst, and unlike entire stars mysteriously vanishing, gamma ray bursts happen all the time. When huge stars die they sometimes release two focused beams of high energy particles (Not exactly lasers, but for now you can think about it like a huge destructive laser beam that gets shot out of both sides of a star when it explodes). We'll talk more about what they are and how they work later, but gamma ray bursts are extremely powerful, and they travel at the speed of light.
Consider: A star that is 1 light year away explodes today, as you read this sentence, and releases a gamma ray burst aimed directly at Earth. The star will look normal to us through telescopes for an entire year hence. If we take a direct hit, it could kill everything on Earth within a few seconds. If that isn't scary enough, consider that up until the very moment we are hit, everything will look completely normal because the gamma ray burst travels at the speed of light. We are physically incapable of getting a warning. Even if we have a satellite far away from Earth that was designed specifically to detect these bursts, the information from that satellite would travel at the same speed as the gamma ray burst and our warning would still come at the exact same time as the burst itself. Again, it is impossible for us to have an advance warning because of the nature of the speed of light.
So now that we have done a little work with the implications of the speed of light, we want to look at how far light travels in a year. We’re going to have to stretch our imaginations a little bit because light travels about 6 trillion miles in one year.
Distance Light Travels in a Year in miles: 6,000,000,000 miles.
6,000,000,000 miles: 1 light year.
It is alright if this is still a little confusing. The good news is you don't have to know how many miles are in a light year to use them to measure distances in space. However, just this little bit of background on light years has hopefully impressed upon you how enormous the distances are between planetary bodies, stars, and galaxies. After all of this, it should come as no surprise that the observable Universe is about 93 billion light years across! That's about 558,000,000,000,000,000,000,000 miles.