Question

Which star is closest, A or B? Star A has a parallax of 0.005 arcseconds and Star B has a distance of 500 parsecs.

Answer 1

A is closer, it is only 200 parsecs away whereas B is 500 parsecs away.

Explanation:

A parsec, about 3.26 light-years, is the distance where a star appears to shift by one second of arc as the Earth moves from one point in its orbit to the opposite point. This shift due to our changing of vantage point is one second of parallax.

If a star is farther away, the parallax will be less; it varies inversely with distance. Thus if the star is 10 parsecs or 32.6 light years away, we see a parallax of 1/10 or 0.1 second. For 0.005 secobd that distance would be 200 parsecs.