Final answer:
A star's absolute brightness, or luminosity, depends on its size and temperature, not its color or distance. Cepheid variables are used to measure distances to galaxies by comparing observed brightness with known absolute brightness. Apparent brightness changes with distance, but absolute brightness does not.
Step-by-step explanation:
A Star's Absolute Brightness
The absolute brightness of a star, also known as its luminosity, depends on the star's size and temperature. While the color of a star, which is an indicator of its temperature, does not change with distance, the apparent brightness or magnitude does. This is because the color of a star is a measure of its surface temperature and is not affected by how far the star is from the observer.
Cepheid variables are a key tool for measuring distances to local galaxies because they have well-known absolute brightnesses at a standard distance. When measuring the distance to these stars, astronomers compare the measured brightness, which decreases as the square of the distance from the observer, to the absolute brightness. By doing this, they can calculate how far away the star, and therefore the galaxy, is from Earth.
In conclusion, a star's absolute brightness depends primarily on its size and temperature, which together determine its luminosity. Distance affects the apparent brightness but not the absolute brightness.