The Speed of Light
Light travels faster than anything else in the universe. In the almost completely empty
vacuum of space, light travels at a speed of approximately 300,000,000 meters per second
(670,000,000 miles per hour). To give you an idea of how fast that is, a beam of light could
travel from New York to Los Angeles and back again nearly 40 times in just one second.
Even though light travels extremely fast, objects in space are so far away that it takes a
significant amount of time for light from those objects to reach us. For example, light from
the Sun takes about 8 minutes to reach Earth.
Light-Years
Because astronomical distances are so large, it helps to have a unit of measurement that is
good for expressing those large distances. Alight-yearis a unit of distance that is defined
as the distance that light travels in one year. One light-year is approximately equal to
9,500,000,000,000 (9.5 trillion) kilometers, or 5,900,000,000,000 (5.9 trillion) miles. That’s a
long way! By astronomical standards, it’s actually a pretty short distance.
Proxima Centauri, the closest star to us after the Sun, is 4.22 light-years away. That means
the light from Proxima Centauri takes 4.22 years to reach us. The galaxy we live in, the
Milky Way Galaxy, is about 100,000 light-years across. So, how long does it take light to
travel from one side of the galaxy to the other? 100,000 years! Even 100,000 light years
is a short distance on the scale of the whole universe. The most distant galaxies we have
detected so far are more than 13 billion light-years away. That’s over a hundred-billion-
trillion (100,000,000,000,000,000,000,000) kilometers!
Looking Back in Time
When we look at astronomical objects such as stars and galaxies, we are not just seeing over
great distances—we are also seeing back in time. Because light takes time to travel, the
image we see of a distant galaxy is an image of how the galaxy used to look. For example,
the Andromeda Galaxy, shown inFigure23.1, is about 2.5 million light years from Earth.
If you look at the Andromeda Galaxy in a telescope, you will see the galaxy as it was 2.5
million years ago. If you want to see the galaxy as it is now, you will have to wait and look
again 2.5 million years into the future!
Electromagnetic Waves
Earlier, we said that light is one type of electromagnetic (EM) radiation. That means light is
energy that travels in the form of anelectromagnetic wave.Figure23.2shows a diagram of
an electromagnetic wave. An EM wave has two components: an electric field and a magnetic