The Andromeda Galaxy is a fairly typical spiral galaxy. Located some 2.5 million light years from Earth, it is part of the Local Group of which our own Milky Way Galaxy and the Triangulum Galaxy (located 3.2 million light years away) are also a part of. Since we are unable to take a nice profile image of our own galaxy (imagine taking a picture of the outside of your house while you and your camera were in the bathroom), Andromeda is often used as a stand in for the Milky Way. In the far distant future, Andromeda may merge with our galaxy because it appears to be moving towards us at nearly 200 miles per second. Anyone still alive in 3 billion years will be witness to one hell of a show!
Hey, Dan, how does science really know how far away the Andromeda Galaxy is?
There are a few methods which can be used to determine distances between galaxies – The Standard Candle method, Type Ia Supernovae observations and Brightest Cluster Galaxy observations are a few examples. For the sake of brevity and simplicity I will talk about the simplest method, the Standard Candle.
The Standard Candle method works when you know exactly how bright something is (like a street lamp or candle). Once you know how bright something is you can use the Inverse Square Law to determine how far away it is.
Sound complicated? Actually, you can do this experiment at home with the following items:
- small maglite flashlight or LED light source
- piece of cardboard
- file card
- graph paper
Cut a 1/2 inch square hole in the file card. Next mount the light source behind the file card. Now position your graph paper so that the light shining through the hole in the file card is being reflected on the squares of the graph paper. Move the graph paper towards the light source until only 1 square is being lit up and measure that distance. Now move the graph paper away from the light source so that it is twice as far away as in your first measurement. Count how many squares are being lit up. Move the graph paper again equal to the same distance you moved it the first time and once more count the squares being lit up. Notice anything interesting?
What you just observed was that (for example) when the card was 1 inch away only 1 square was lit up and when it was 2 inches away 4 squares were lit up and when it was three inches away 9 squares were lit up. If you moved the paper 1 more inch away then 16 squares will be lit up. What this means is that the area being illuminated is equal to the square root of the distance being measured (the square root of 4 is 2, the square root of 9 is 3, etc…)
Astronomers can use this method to determine how far away something is. For example let’s say we observe a star similar to our sun in the Andromeda Galaxy. Since we know about how bright that star is (just like we knew how bright our maglite was) we can observe the intensity of that light to determine how far away it is. Simple!
I must warn you that this method only works with objects up to a certain distance away because at really great distances the data becomes unreliable. I’m not going to go into how those methods work since what I am wanting to prove can be done with the above method.
Alright, Dan, you’ve proven to me how we know how far away something is but you also said Andromeda was speeding towards us. How do you know that?
Simple. Ever hear a police siren speeding past you? What do you notice about the sound the siren makes? As it is coming towards you the siren is high pitched and as it moves away from you it is lower pitched. The reason for this is because since the police car is speeding towards you it is, in effect, compressing the sound waves blaring out of the siren. When the police passes you the sound waves get stretched out. What is true for sound is also true for light. When a galaxy moves away from us at a high speed the light from that galaxy appears red because red has a long wavelength. When the galaxy speeds towards us at high speed it’s light gets shifted towards the blue end of the spectrum because blue has a shorter wavelength.
Fine, Dan, you’ve proven how far away Andromeda is and that it is speeding towards us. Who cares?
Since we know how far away the Andromeda galaxy is we can deduce a very important fact. You see light always travels at 186,282.397 miles per second. Always. It’s a constant – meaning it never changes. Ever!
Since my post today is already getting to be epic in length I’ll leave it up to you to understand why the speed of light never changes.
With the speed of light as a constant and the distance known about how far away Andromeda is (taking into account its motion towards us) we know that it is roughly 2.5 million light years from Earth. That means that even if the Andromeda galaxy popped into existence because God said “Let there be light” about 10,000 years ago then we would not even be able to see the Andromeda galaxy for another 2.49 million years. Basically we would not even see it coming until it was already nearing our doorstep but not a moment before.