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Brightness and Distance Help

By — McGraw-Hill Professional
Updated on Sep 17, 2011

Brightness and Distance

Have you ever heard that stars are “distant suns”? This is true, but it does not tell the whole story. There are plenty of stars that resemble our Sun, but many are larger or smaller, brighter or dimmer, or hotter or cooler. Some stars radiate more energy at shorter wavelengths than does our Sun; other stars favor the longer wavelengths. Stars are as diverse as snowflakes.

Have you ever been told that interstellar space is a “vacuum”? This is true in a relative sense. We could not breathe in space without special equipment, and the pressure is low enough to be considered a vacuum by most laboratory technicians on Earth. However, interstellar space contains matter. We can see some of it directly, whereas other space matter is visible only because it blocks light from stars behind it.

How Bright? How Distant?

You’ve learned about visual magnitudes of objects in the sky. A difference of one magnitude is the equivalent of a brightness change of 250 percent, that is, 2½ times. As the brightness increases, the magnitude number goes down. However, when you look at a dim star in the sky and then you look at a much brighter one, how do you know which of the two really emits more light? After all, if Star X is 2½ times brighter than Star Y but is only 1/100 as far away, Star Y is more brilliant in an absolute sense.

Absolute Visual Magnitude

The observed magnitude of a star or other celestial object is called its apparent visual magnitude , or simply the apparent magnitude . The actual brightness is called the absolute visual magnitude , or simply the absolute magnitude . Absolute versus apparent: Things are not always as they appear. By definition, the absolute magnitude of a star is the apparent magnitude we would give it if it were 3.09 × 10 14 km (1.92 × 10 14 mi) away. This is the distance that light travels in 32.6 years. By Earthly standards, or even when measured against the scale of the Solar System, this is a huge distance. Compared with the size of the Milky Way, though, it is not far at all. Compared with the size of the known Universe, it is microscopic.

One of the brightest known stars, in absolute terms, is Canopus , which is best observed from the southern hemisphere. People who live north of the thirty-eighth parallel (roughly the latitude of San Francisco) cannot see Canopus. This star has an absolute visual magnitude of –4.4. By comparison, our Sun has an absolute magnitude of +4.8; thus Canopus is approximately 5,000 times as brilliant as our parent star. If the Sun were to increase in brightness suddenly to the same absolute magnitude as Canopus, we would all have to wear dark glasses to get around. However, that wouldn’t be the only problem. Canopus radiates more energy than the Sun at other wavelengths, too. Its ultraviolet rays would sunburn us in seconds; the heat would incinerate Earth and end all life here in a matter of days, if not hours.

Because of its distance, Canopus is not a remarkable object in the sky. People who live north of the latitude of San Francisco aren’t missing much just because they can’t see Canopus. Stars like our Sun, when observed from the same distance as we see Canopus, are so faint that they cannot be seen except by people who live far away from the skyglow caused by city lights and who have keen eyesight.

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