Two Cepheid stars are observed to have the same apparent magnitude.

Question

Cepheid 1 has a period of 3 days and Cepheid 2 has a period of 8 days. Which of these two stars is the nearest and why?

Answer 1

a Cepheid is a variable star, that is its luminosity changes over time. a cepheid's luminosity can be measured on the basis of its period, a 3 day cepheid will always be 800 times as luminous as the sun, an 8 day cepheid 2,500 times and so on. the more luminous a star is the easier it is to measure from a distance, If Cepheid two has a longer period it has a higher luminosity so it is the most distant because it appears to be as bright as Cepheid one,from Earth veiw, even though Cepheid 1 is 1,300 times less luminous in terms of its period. brighter objects appear fainter if they are far away, therefore Cepheid 2 is the most distant and Cepheid 1 is nearest.