Question

How does an astronomer calculate the distance of a star?
(A) Comparing apparent brightness of star to true brightness
(B) Comparing true brightness of star to apparent brightness
(C) Comparing true brightness of the stars to true brightness
(D) None

Answer 1

Hint:Normally, astronomers use parallax method to calculate the distance of a star. However, the main limitation of this technique is that it can be used only for those stars which are up to $400ly$ (light years) far from the earth. Therefore, astronomers use brightness measurements to measure the distance of the stars which are more than $400ly$ (light years) away from the earth.

Complete step by step answer:
In brightness measurements, astronomers use Cepheid variable stars which pulsate in and out similar to how our heart beats. This pulsation length and its true brightness have a direct relationship. Over the time, these Cepheid stars change their brightness by astronomers can find their true brightness.Apparent brightness means how bright the star looks from the earth.By comparing this apparent brightness of the star to the true brightness, astronomers calculate the distance of the star from the earth.

Hence, option A is the right choice.

Additional information:
An American astronomer Henrietta Leavitt had discovered the discussed method of calculating the distance of stars in 1912 and he used this method to determine distances of many globular clusters during the early nineteenth century.

Note:This method cannot be directly applied to calculate the distance of the star from the earth. Therefore, Astronomers have to apply the parallax method initially to get the distance to some closer Cepheid to use this brightness measurement technique effectively. By doing this, they can calibrate a Cepheid’s true brightness which can be used to determine its distance.