How do we measure distances to a star or galaxy by just sitting here on Earth?

Humans are pretty smart. Think about it. Scientists can calculate the distances to stars and galaxies without taking a ruler. They don’t have to leave Earth. In fact, they don’t even have to leave their chair. The combination of some fundamental rules of physics lets us do that.

So how do we measure distances to a star or galaxy? When you look up at the sky, have you ever wondered about that? The sky may seem 2-dimensional to us with all stars the same distance away, just different in their shine and intensities. This was the belief for many a millennium.

But that is not the case. We now know that some of the stars are very close to us in space, and some are incredibly far away. But they are so huge and bright that we mistake them for being close. And we are not even talking about the galaxies that we cannot even see with the naked eye!

Hubble Deep Field

Hubble Deep Field. Source: Wikipedia

Think like a scientist now. You look up at the sky and you suddenly want to calculate the distances to the stars. You want to travel to them and so you want to know how much time it may take you to get there. So how will you do that?

This problem may seem trivial now, but it is indeed an incredible problem. We just never think about it because the stars are always available in the night sky and we get used to them.

To solve this problem, you need the knowledge about many different things. Stop and sit and think about this problem to really get an idea of why we need to have knowledge of different concepts.

What you need to know

Here is a list of things that you will need to know that will help you calculate the distances to a star. And if you can do that for one star, then you can do it for any star and with some modifications, any galaxy.

You need to know trigonometry. Next, you need to know the concept of parallax.

You first use this and calculate the distance to the Sun.

Then you calculate the distance to the nearby stars (but you have to wait for 6 months to get the measurements required for parallax).

Just using this method you can calculate the distances to any stars within 10,000 lights years from Earth.

Then it becomes a little complicated for more distant objects and we have no one accurate method to calculate the distances.

For distant galaxies, you need to know the standard brightness type Ia supernovae. This is a complicated and advanced concept but explore it.


Bonus curiosity:

Now the speed of light is constant throughout space. With this and the distances we get from above stars, we find the startling discovery that the light that left the stars and galaxies have been travelling for hundred or thousands or billions of years. And we are just now receiving the light that left those stars and galaxies.

So every time you look up at the sky, you are gazing into the past! Amazing, isn’t it?

If you know about Doppler effect then you will again find that every distance galaxy is moving away from us. The universe is expanding!

Stay curious.


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