When we talk about the enormity of the cosmos, it’s easy to toss out big numbers – but far more difficult to wrap our minds around just how large, how far, and how numerous celestial bodies really are. How big is our Milky Way Galaxy and how far away are exoplanets, the planets beyond our solar system?
The Milky Way
gets its name from a Greek myth about the goddess Hera who sprayed milk
across the sky. In other parts of the world, our galaxy goes by other
names. In China it’s called the “Silver River,” and in the Kalahari
Desert in Southern Africa, it’s called the “Backbone of Night.”
A light-year is a measure of astronomical
distance: Light travels through a vacuum at precisely 983,571,056 feet
(299,792,458 meters) per second, making a light-year approximately 6
trillion miles (9.7 trillion kilometers).
A light-year is a measurement of distance and not time (as the name might imply). A light-year is the distance a beam of light travels in a single Earth year, which equates to approximately 6 trillion miles (9.7 trillion kilometers).
On the scale of the universe,
measuring distances in miles or kilometers is cumbersome given the
exceedingly large numbers being discussed. It is much simpler for astronomers to measure the distances of stars from us in the time it takes for light to travel that expanse. For example, the nearest star to our sun, Proxima Centauri, is 4.2 light-years away, meaning the light we see from the star takes a little over four years to reach us.
How far is a light-year?
The speed of light
is constant throughout the universe and is known to high precision. In a
vacuum, light travels at 670,616,629 mph (1,079,252,849 km/h). To find
the distance of a light-year, you multiply this speed by the number of
hours in a year (8,766). The result: One light-year equals
5,878,625,370,000 miles (9.5 trillion km). At first glance, this may
seem like an extreme distance, but the enormous scale of the universe
dwarfs this length. One estimate puts the diameter of the known universe
at 28 billion light-years in diameter.
Why use light-years?
Measuring
in miles or kilometers at an astronomical scale is impractical given
the scale of figures being used. Starting in our cosmic neighborhood,
the closest star-forming region to us, the Orion Nebula, is a short 7,861,000,000,000,000 miles away, or expressed in light-years, 1,300 light-years away. The center of our galaxy is about 27,000 light-years away. The nearest spiral galaxy to ours, the Andromeda galaxy, is 2.5 million light-years away. Some of the most distant galaxies we can see are billions of light-years from us. The galaxy GN-z11 is thought to be the farthest detectable galaxy from Earth at 13.4 billion light-years away.
Like
degrees, the light-year can also be broken down into smaller units of
light-hours, light-minutes or light-seconds. For instance, the sun is
more than 8 light-minutes from Earth, while the moon is just over a light-second away. Scientists use these terms when talking about communications with deep-space satellites or rovers. Because of the finite speed of light, it can take more than 20 minutes to send a signal to the Curiosity rover on Mars.
Measuring
in light-years also allows astronomers to determine how far back in
time they are viewing. Because light takes time to travel to our eyes,
everything we view in the night sky has already happened. In other
words, when you observe something 1 light-year away, you see it as it
appeared exactly one year ago. We see the Andromeda galaxy as it
appeared 2.5 million years ago. The most distant object we can see, the cosmic microwave background, is also our oldest view of the universe, occurring just after the Big Bang some 13.8 billion years ago.
Alternatives to light-years
Astronomers also use parsecs
as an alternative to the light-year. Short for parallax-second, a
parsec comes from the use of triangulation to determine the distance of
stars. To be more specific, it is the distance to a star whose apparent
position shifts by 1 arcsecond (1/3,600 of a degree) in the sky after
Earth orbits halfway around the sun. One arcsecond is equal to 3.26
light-years.
Whether
it's light-years or parsecs, astronomers will continue to use both to
measure distances in our expansive and grand universe.
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