Star Colors and Magnitudes
The surface temperature of a star has direct correlation to its color, where decreasing temperature is represented as increasing redness.
Magnitudes can be used to describe the brightness of objects in the sky. Magnitudes are built on a logarithmic scale.
- Apparent magnitude = measures how bright or faint something appears from our perspective on earth
- objects in the sky that appear fainter than the star Vega have positive apparent magnitudes; objects that appear brighter than Vega have negative apparent magnitudes
- Absolute magnitude = used to quantify how bright an object would appear if it were at a standard distance of 10 parsecs away.
The physical property that magnitudes measure is flux (the amount of light that arrives in a given area on Earth in a given amount of time)
Apparent magnitude (m) related to flux (f) =
Distance from Earth in parsecs (d), apparent magnitude (m) and absolute magnitude (M) are related with the distance modulus equation =
Luminosity represents the same physical property as absolute magnitude, which is the amount of energy released in a given amount of time.
Star Clusters
An open star cluster is a relatively young group of stars (between a million and a few billion years old) which has formed from a travelling cloud of dust when it entered a galactic disk.
Because of the way they formed, we know that stars in clusters are in the same small region of space, so we approximate that each one is at the exact same distance from Earth.
Color-magnitude diagram (CMD)
When stars in the cluster first form, most of them lie in the main sequence. Their exact position is determined by their mass, with the more massive stars sitting in the brighter bluer area. The most massive stars often evolve into red giants, staying at about the same luminosity but redder, producing the red giant branch. The corner or bend where the main sequence leads into the red giant branch is called the main sequence turn-off.
From a CMD, we can estimate two key physical properties of the cluster:
- age = based on the lifetime of the bluest and most massive stars still on the main sequence.
- distance from earth = using the distance modulus equation and magnitudes at the main sequence turn-off point