Final answer:
Galaxy B is farther from Earth than galaxy A, being 1500 million light years away compared to 800 million. Star A appears brighter than Star B because it is closer. The Hubble Telescope can resolve individual stars in distant galaxies like Andromeda, giving us a view into the past.
Step-by-step explanation:
According to Hubble's Law, the farther a galaxy is from us, the faster it is moving away. Since galaxy B is 1500 million light years away and galaxy A is 800 million light years away, galaxy B is farther from Earth. The apparent brightness of a star is inversely proportional to the square of its distance from us. Therefore, Star A, being closer at 20 light-years away, appears brighter than Star B, which is 40 light-years away.
The intensity or brightness of a star diminishes with the square of the distance. So if Star B is twice as far away, it would appear 2^2=4 times less bright than Star A. This is also why the incredible Hubble Space Telescope can provide us with stunning images of distant galaxies and celestial objects, resolving individual stars within them despite their immense distances. Lastly, when considering the light coming from these distant stars and galaxies, we are looking into the past since the light has taken so many years to reach us. For instance, the light from the Andromeda galaxy we see today actually left it 2 million years ago.