How do they measure distance in space ??

One night i was surfing on my mobile and i came to know that nearest galaxy to us is Andromeda Galaxy which is about 2.5 million light years away from us. After reading it i was amazed and one question pooped up in my mind that 
How do scientists measure distance between objects in space ??

First of all let us understand that distance in space is measured with reference to light i.e. in light years. One light year is the distance which light travels in one year which is approx 10 trillion km (9,500,000,000,000 km) which is so huge.

1.How do we measure distance with light ??

We measure distance by using time taken by light to reach at an object and multiplied that time with speed of light.For example Suppose light takes 2 hours to reach from one object to another.
So the distance= speed*time
                        = 1,080,000,000km/hr * 2hr
                        = 2,160,000,000 km

2.But nearest galaxy is 2.3 million light years away, so do we need to wait 2.3 million years ??

This was confusing but then i found that there are other methods to measure distance with the help of light which is called trigonometric parallax.

Trigonometric parallax  is pretty simple. In this we see  an object from two or more different positions  and then calculate the distance and to a surprise these stars are so far that we cannot measure their distance even after having two different telescope at opposite end of equators.

3.So how to measure the distance from two very far position ??

Scientists found interesting solution to this and they used the revolution path of earth around sun.

They used to measure the 1st position of star at a moment and after 6 months when earth completes its half revolution then they measure the 2nd position and then calculate the final position.

4.But this method only works for the objects no more then a few thousand light years away, then how we discovered Andromeda galaxy ??

Beyond our galaxy the parallax is too small to detect with even our most sensitive instruments. So we need to use another method which is called Standard candles.
Standard candles are the objects whose intrinsic brightness or luminosity we know really well.
For example, if we know how bright your light bulb is and you walk away from it and the light which we will receive will decrease by square of distance. So by comparing the light that we receive by intrinsic light of object we can tell that how far the bulb is.

In space, our light bulb is a special type of star called CEPHEID VARIABLE.These are unstable, like continuously inflating and deflecting balloon. And because of this inflating and deflation causes their brightness to vary we can calculate their luminosity by measuring the period of this cycle.By comparing these lights we calculate the distance how far they are.

Unfortunately this method also have limitation.

We can only observe individual stars up to about 40 million light years away, after which they become too blurry to resolve.

But lucky we have a special type 1A supernova.

Supernovae are the giant explosions, away in which stars dies. These explosions are so bright that they outshine the galaxies where they occur

With the help of these supernovae we can measure the distance over several billions of light years away.

From all this we learned new methods which are used to measure distances in space, which are:
1.Speed*time
2.Trigonometric parallax
3.Standard candles

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