Long-period comets, on the other hand, can have periods on the order of millions of years and may have orbits that extend halfway to the nearest stars. They are powered by thermonuclear fusion which is the process of combining two light elements to form a heavier element plus large amounts of energy. The brightness of a star depends upon its size and temperature. Astronomers refer to the brightness of a star as its magnitude. Magnitude is defined in two ways, the apparent magnitude and the absolute magnitude.
Apparent magnitude refers to how bright the star appears from Earth .
Stars can range in size from one tenth the size of our sun to more than 100 times as large. Giants and supergiants are generally yellow or red in color. These are very large stars that radiate huge amounts of energy but are generally cooler than our sun. Main sequence stars make up the majority of stars that we observe. These stars tend to radiate energy that is proportional to their surface temperature.