Light-year

A distance, not a time

The name sounds like a length of time, but a light-year measures distance. It is how far light travels through empty space in one year.

Light covers about 186,000 miles every second. Multiply that by the number of seconds in a year and you get about 5.88 trillion miles. The International Astronomical Union defines the year for this purpose as a Julian year of exactly 365.25 days.

Numbers like trillions of miles are hard to picture and awkward to write. Saying a star is 4 light-years away is shorter, and it carries a bonus: it also tells you the light you see left that star about 4 years ago.

Three yardsticks: AU, light-year and parsec

Astronomers pick the unit that fits the job, the way you might use inches for a book and miles for a road trip.

The astronomical unit, or AU, is the average distance between Earth and the Sun, about 93 million miles. It works well inside the Solar System. Jupiter sits about 5.2 AU from the Sun and Neptune about 30 AU. One light-year equals about 63,000 AU.

The parsec is the unit professional astronomers often prefer for stars and galaxies. It equals about 3.26 light-years. The name comes from parallax second, because it is tied directly to the parallax method described below. Big distances are often given in kiloparsecs (thousands) or megaparsecs (millions).

How far are the stars and galaxies?

The closest star to the Sun is Proxima Centauri, a small red dwarf about 4.24 light-years away. It is part of the Alpha Centauri system, whose two brighter stars lie at about 4.37 light-years. Even at the speed of our fastest spacecraft, a trip there would take thousands of years.

Sirius, the brightest star in the night sky, is about 8.6 light-years away. Many of the stars you can see without a telescope lie within a few hundred to a few thousand light-years.

Our own galaxy, the Milky Way, is more than 100,000 light-years across, and the Sun sits roughly 26,000 light-years from its center. The Andromeda Galaxy, our large neighbor, is about 2.5 million light-years away. On a dark, moonless night it shows up as a faint smudge, which makes it one of the most distant things the human eye can see unaided.

Measuring distance with parallax

Hold a finger in front of your face and close one eye, then the other. Your finger seems to jump against the background. That shift is parallax, and the closer the finger, the bigger the jump.

Astronomers do the same thing using Earth's orbit. They photograph a nearby star, then photograph it again six months later, when Earth is on the opposite side of the Sun. The star shifts very slightly against much more distant stars. Measuring that tiny angle and knowing the size of Earth's orbit gives the distance with simple geometry.

The angles are extremely small, far less than the width of a human hair seen from across a football field. The European Space Agency's Gaia mission measured parallaxes for well over a billion stars, building the most detailed 3D map of our galaxy so far.

Standard candles for the far universe

Parallax stops working once stars are too far away for the shift to be measured. For greater distances astronomers use standard candles, which are objects whose true brightness is known.

The idea is the same as with a streetlight. If you know how bright a bulb really is, how dim it looks tells you how far away it is. Cepheid variable stars are one example: they pulse in brightness, and the length of the pulse reveals their true brightness. Henrietta Leavitt discovered this relationship in the early 1900s.

Even farther out, a certain kind of exploding star, called a Type Ia supernova, serves as a very bright standard candle. These can be seen across billions of light-years. Each method is checked against the one before it, so astronomers call the whole chain the cosmic distance ladder.

Why it matters

Distance is one of the hardest things to measure in astronomy, yet almost everything depends on it. Without knowing how far a star is, we cannot work out its true brightness, size or age.

The light-year also reminds us that seeing is never instant. When you look at Andromeda, you see light that left it before modern humans existed. Distance and history are woven together in every view of the night sky.