1Two things to know about stars and light
Stars are everywhere — and their light takes time to get here
You only need two ideas. Watch each one:
Stars are in every direction
Space is full of stars, near and far. Pick any direction and keep going — there are stars scattered out that way, some close, some unimaginably far.
Light is fast, but not instant
Light travels at a speed. A faraway star's light has to cross all that space to reach you — and that takes time. Light from the most distant stars may still be on its way.
2Two ways the sky could be
A sky that's seen everything vs a sky that hasn't yet
Imagine the same star-packed space two different ways. They make very different skies:
Light from everywhere has arrived
Imagine stars extending forever through static space, shining forever. In that thought experiment, every direction would eventually meet a star.
Only nearby light has arrived
A finite history limits how much starlight can have arrived. The dots here illustrate that limit; they do not show every real dark direction.
3Your turn — point and look
Aim a line out into space and see what it hits
Drag your finger across the sky to aim a sightline out into deep space. Some directions stop on a near star. Where there's no near one, your line keeps going, deeper and deeper — hunting for a star far out there.
4The big test
More stars — or more directions?
Try two new toy skies. Each has eight stars and eight numbered viewing directions. All stars switch on together. Light travels one distance unit per clock tick.
After six ticks, which sky lights more directions?
Sky A: four stars line up in direction 1 and four in direction 5. In each direction their distances are 2, 3, 4 and 5 units.
Sky B: two stars line up in each of directions 2, 4, 6 and 8. Their distances are 5 and 7 units.