How do the scientist the measure the distance of an object which are light years away?

Question

I came to know about J1470B which is 430 ly away from earth. How do scientists measure the distance. It can not be by light I guess because light itself takes 430 years to come to earth. So what is it? How do they measure?

In: 12

Answer 1

There are different methods that work differently well depending on distance. Mainly it is either about parallax, comparing true brightness to appearent brightness, or measuring redshift.

Parallax: you measure the difference of relative position of an object with 6 months time difference, just like when you look your thumb with one and then the other eye the thumb appears to be in a different position. You know how far earth travels in half a year, therefore you can calculate the distance after you measured the difference. A parces is defined by that effect (PARallax-SECond) works well for closer things.

If you know the difference between true and appearent brightness it is easy to calculate distance – doubling the distance makes things 4 times dimmer.
Knowing the true brightness is hard. But there are different so called standard candles. Two examples are cepheid variables and type 1A supernovae.

Cepheid variables: stars that oscilate regularly in brightness, the frequency depends on their true brightness. They were the first standard candles astronomers found, but when they are too far away they are too dim to spot.

Type 1A supernovae: stars that die under very specific conditions. Way brighter than stars, thus easier to spot at a greater distance. They don’t happen continuosly, but when we see one we know how far away it happened.

Redshift is a hole other perspective. The further something is away, the faster it moves away from us, and because space expanded during the trip the light takes to reach us the wavelength gets longer, i.e. more red. Each element gives us specific fingerprints in the spectrum of light, and we can measure how far it shifted. And due to comparing this method to our theories and other observations we can calculate how far the light has treveled. This works better on very large distances than anything else, but it depends heavily on models and theory.

Now to get an accurate measurement you want to combine different methods, which isn’t always that simple. But because of our continuous observations and models, etc. we are getting more and more accurate measurements for distances.