A light-year measures distance, not time. It is how far light travels in one Earth year. NASA puts that at about 6 trillion miles, or 9 trillion kilometers. That is a long way.
Light is fast, but space is huge
Light travels about 186,000 miles, or 300,000 kilometers, every second. That sounds instant. Yet stars and galaxies sit so far off that their light needs years, or far longer, to get here.
The year here is one Earth year. For how our calendar keeps pace with that year, see why we have leap years.
Every look is a look back
Light takes time to arrive. So we always see things as they were when the light left them. The farther away something is, the further back in time we see it.
Our Sun, 8 minutes ago
The Sun is our closest star, about 93 million miles away. Sunlight needs roughly 8.3 minutes for that trip. The Sun in our sky is always the Sun of 8.3 minutes earlier.
The next star over
The next nearest star lies about 4.3 light-years off. Tonight’s view of it shows light that set out 4.3 years ago. Every other star visible without a telescope is farther, some by thousands of light-years.
A galaxy next door
Stars gather in huge groups called galaxies. The numbers grow fast. One galaxy can hold millions, even billions, of stars. Andromeda, our nearest large neighbor, sits 2.5 million light-years from us. Its light shows us Andromeda from 2.5 million years back.
Light from the early universe
In 2016, Hubble, NASA’s space telescope, studied a galaxy named GN-z11. Its light traveled for 13.4 billion years, so we see this galaxy from 13.4 billion years back. By NASA’s count, that is just 400 million years after the big bang.
NASA’s page, last updated in 2020, calls GN-z11 one of the first galaxies to form. Studying such early galaxies helps scientists learn what the young universe was like. Light also explains why the sky looks blue, as our sky post shows.
