A solar system with a star like your own sun and a planet like Jupiter has been discovered 90 light years from us. If the star is just as bright as our sun, how does it compare in brightness to the other stars in our night sky?

Background information: Light from the sun takes 8 minutes to reach the earth. The magnitude system for stars and other astronomical objects in our sky is designed so that a difference of 5 magnitudes represents a factor of 100 in brightness, with the larger numbered magnitudes being the dimmest. The dimmest stars seen with the unaided eye in a dark sky are about magnitude 6; the brightest stars (other than the sun) about 0. The planet Venus at times is magnitude -4, the full moon -14 and the sun -27.

A star outside our Solar System is assigned a stellar or galactic absolute magnitude as if it were 10 parsecs distant.

A star within our Solar System is assigned a planetary absolute magnitude as if it were only 1 astronomical unit distant.

To convert a planetary absolute magnitude into a stellar one, 31.57 is added.  The Sun's apparent magnitude is given to be -26.74, thus the Sun's stellar or galactic absolute magnitude is calcuated to be 4.83 [-26.74 + 31.57 = 4.83].

Given the absolute magnitude of the distant star as 4.83 and its distance as 90 lightyears, its apparent magnitude, m, is:
m = 4.83 + 5*((log₁₀(90/3.26156) - 1) ~= 7.03409.

Stellar objects that are visible to the typical human eye are assigned apparent magnitudes less than 7. Those that are assigned values of 7 or greater are not typically visible, therefore the Sun-like star 90-lightyears distant, with an absolute magnitude of about 7.03, is just beyond the unassisted visual range.

Posted by Dej Mar on 2012-05-27 16:21:05