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Why Do We Still See Stars That Have Already Exploded?

Stars are distant balls of hot gas. When people on Earth look up at them at night, they aren't seeing the stars as they are now, they are actually seeing them as they were hundreds, thousands or even millions of years ago. Light travels at the incredible speed of 186,000 miles per second but stars and other objects are so distant that the light from some of them can take billions of years to reach Earth.

The Speed of Light
- According to Albert Einstein's theory of Special Relativity, the speed of light is 186,000 miles per second. In general, scientists believe this is the ultimate speed limit in the universe. In one minute light travels about 11 million miles. Scientists use this as measure of distance called a light-minute. In one hour, light travels about 670 million miles. This is called a light- hour. Light travels nearly 483 billion miles in one month. In one year, light travels about 6 trillion miles. Scientists use this distance as a measure of distance called a light-year.
The Solar System
- At the speed of light, the moon is about 1.5 light-seconds away. At its closest, the planet Venus is about two light minutes away; at its most distant, it is about 15 light minutes away. On average, Mars is about 4 light minutes away. Jupiter is about 30 light minutes from the Sun and, depending on where it is in its orbit, between 20 and 40 light minutes away from Earth. Saturn is about one light hour from the Sun and Neptune is about four light hours away.
Stars
- The sun is about 93 million miles away from Earth. At that distance, light from the sun takes about eight minutes to reach Earth. If the sun exploded right now, people on Earth wouldn't know for eight minutes. Alpha Centauri, the next closest star, is four light-years away. Light from Alpha Centauri takes about four years to reach Earth. If Alpha Centauri exploded tonight, light from the explosion wouldn't be seen on Earth for four years. A spacecraft such as "Voyager" traveling 40,000 miles per hour would reach Alpha Centauri in 70,000 years. A truck-driver going 75 miles per hour would reach Alpha Centauri in 38 million years.
Other Objects
- On July 4, 1054, Chinese astronomers recorded what they called a "guest star." This "guest star" was what modern astronomers call the Crab Nebula, the remains of a supernova, or the explosion of a large star. However, the Crab Nebula is 6,000 light years away. When the star exploded 6,000 years earlier, human beings were just learning about agriculture. The Veil Nebula, which is part of what astronomers call the Cygnus Loop, is the visible part of what remains of a supernova that occurred 15,000 to 20,000 years ago.

Astronomy