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Light Years
The distances in space are so incredibly vast that astronomers had to come up with a unit of measurement that was appropriate for them. The light-year, defined as the amount of distance light can travel in a perfect vacuum in one year's time, became this measurement. A light-year is the equivalent of 5,880,000,000,000 miles. A parsec is 3.26 light years.
Astronomical Units
The Sun is the closest star to Earth, at an average distance of 93,000,000 miles. At points, the earth is closer to and farther away from the Sun, as its orbit about the star is not a perfect circle. The 93 million miles turned into a measurement of distance called an astronomical unit, which allowed astronomers to have a more handy way of comparing distances within the solar system. However, the length of miles between the Earth and other stars was too much for something as small as an astronomical unit to be practical.
Nearby Stars
The nearest star to Earth other than the Sun is a star named Proxima Centauri. People in the northern hemisphere cannot view this star, as it never rises above the horizon. Proxima Centauri is in the constellation Centaurus and is 4.2 light-years from Earth. The brightest star in the evening sky, Sirius, is about eight light-years distant from Earth. It is visible in the winter sky in the United States, lying close to the horizon.
Apparent Magnitude
In ancient times the Greek astronomer Hipparchus developed a precise scale to measure how bright stars appeared in the sky. The brighter stars received a number value of one while the dimmest stars received a value of six. The system received much tweaking over the centuries but the general idea remains the same. On such a scale, the Sun has a magnitude of negative 26 while Sirius is a negative 1.46. This is the apparent magnitude---how bright a star is as seen from the perspective of an individual on Earth.
Effects
The absolute magnitude of a star differs from apparent magnitude. The absolute value is a number reflecting what the star's brightness is from a distance of ten parsecs. This value shows the effects of distance on brightness of stars. For example, our own Sun has an absolute magnitude of just 4.8. It would be quite dim to us if it were ten parsecs away. The very bright star Rigel in the constellation Orion, on the other hand, has an apparent magnitude of .12. However, this is because Rigel is an amazing 900 light-years away. If it were "only" ten parsecs away, its magnitude would increase to negative 8.1. It would be much brighter than anything else would in the night sky, including the planets.
Expert Insight
Astronomers using the Keck Telescope, located in Hawaii, discovered a group of galaxies that are an estimated 11.4 billion light years away. This means that the light that reached their eyes began its journey long before the Earth formed, some 4.5 billion years ago. This discovery highlights the colossal distances that stars are from Earth.