Star Colors and Temperatures
Temperature. The Hertzsprung–Russell diagram shows the relationship between a stars luminousity and it's temperature. The difference between luminosity and apparent brightness depends on distance . relationship that relates these three quantities–apparent brightness. Some extremely large and hot stars blaze away with the luminosity of a million suns! But other stars look Color is a telltale sign of surface temperature. Astronomers call the true, intrinsic brightness of a star its luminosity.
When it runs out of hydrogen to burn, the star becomes unbalanced, and its size and temperature can change. For some stars, particularly those that start off very hot, the temperature at this point in the star's life will tend to decrease and therefore the star will become red.
So that is one way in which older stars are redder than young stars.
Is there a relationship between a star's temperature and its true brightness
Often, though, when we talk about stars' ages being related to their color we are talking about stars which are still on the "main sequence", since that's where they spend most of their lives. Basically, it turns out that the hotter and therefore bluer a star is while it's on the main sequence, the faster it burns its hydrogen and the quicker it dies.
- Brightest Stars: Luminosity & Magnitude
- Luminosity and Apparent Brightness
- Prime-Time Stars: The HR Diagram
So if you see a blue star on the main sequence, you know it must be relatively young - otherewise it would have burnt out already. But the converse isn't true - that is, just because you see a red star doesn't mean that it's old!
Where this becomes useful is when you look at a bunch of stars for example, all the stars in a faraway galaxy. If you see that all the stars are red, then you can infer that there hasn't been much star formation going on in this galaxy recently - otherwise, you'd expect to see some young blue stars just by chance. So you can figure out that the stars in the galaxy must be relatively old. All the above has to do with the color of light that a star emits.
Brightest Stars: Luminosity & Magnitude
When I say apparent brightness, I mean how bright the star appears to a detector here on Earth. The luminosity of a star, on the other hand, is the amount of light it emits from its surface.
The difference between luminosity and apparent brightness depends on distance. Another way to look at these quantities is that the luminosity is an intrinsic property of the star, which means that everyone who has some means of measuring the luminosity of a star should find the same value.
However, apparent brightness is not an intrinsic property of the star; it depends on your location.
What is the relationship between star color and true brightness for stars in the main sequence?
So, everyone will measure a different apparent brightness for the same star if they are all different distances away from that star. For an analogy with which you are familiar, consider again the headlights of a car. When the car is far away, even if its high beams are on, the lights will not appear too bright. However, when the car passes you within 10 feet, its lights may appear blindingly bright.
To think of this another way, given two light sources with the same luminosity, the closer light source will appear brighter. However, not all light bulbs are the same luminosity. If you put an automobile headlight 10 feet away and a flashlight 10 feet away, the flashlight will appear fainter because its luminosity is smaller.
Stars have a wide range of apparent brightness measured here on Earth.