Astronomers refer to the brightness of a star as its magnitude. There are two ways to describe the magnitude:
- Actual brightness- how much light it produces
- Apparent brightness- the light as seen from Earth
For example, a small flashlight seen from a few feet away seems brighter than a car’s headlights seen from two or three miles away, even though the car’s headlight actually produce more light.
In the same way, the star Rigel produces thousands of times more light than the star Sirius, but Sirius appears brighter in the night sky because it is closer.
How much light a star produces is called the star’s absolute magnitude. The absolute magnitude does not depend on the star’s distance, but the amount of light it produces. But the apparent magnitude of a star depends on the amount of light it produces and its distance from the earth.
From Red to Blue
A star’s surface temperature also determines the star’s color. Relatively cool stars- those with surface temperatures of 5,500 degrees Fahrenheit or so- appear reddish-orange to the eye, like a campfire or a dim light bulb. Warmer stars, like our sun with its 10,000 degree surface, shine with a much brighter yellow hue. Hot stars shine pure white or blue-white, and the hottest of them all blaze a brilliant blue with surface temperatures at around 70,000 degrees or more.
Sometimes cooler stars, like Betelgeuse, manage to outshine smaller, hotter stars by simply being much bigger. The brightest stars are both large and hot, such as Rigel: even though it is about twice as far away than Betelgeuse, Rigel actually appears slightly brighter to the eye.
Giants and Dwarfs
Those stars who are brighter due to their larger size are called giants or supergiants. The largest supergiants are red like Betelgeuse and Antares. Some of the most powerful stars are hot supergiants like Deneb. Although much smaller than red supergiants, they are much denser and produce far more light for their size.
A few white-hot stars are much dimmer than average stars of the same temperature. Because their dimness is due to their very small size, these stars are called white dwarfs. Some white dwarfs are as small as the earth, but contain as much matter as the sun. The matter of these stars is packed so tightly that a singe teaspoon of it would weigh more than a full-grown elephant!