Astronomers see most stars as single points of light even in the most powerful telescopes. And stars are of all different brightness levels, so how do they know how far away the most distant stars and galaxies are? The answer is surprising and complex.
The theory goes that the universe is expanding, and the farther away something is, the faster it is moving away from us. They measure how fast something is moving, by looking at how red it is. Just like how the sound of an ambulance changes when it is moving away, the colour of stars become more red the faster they are moving away.
But some stars are red anyway - so how do they know which ones are naturally red, and which ones just look red due to motion? They look for 'standard candles' - stars in distant galaxies that look like they are of a particular stable type, which we know the colour of by examining them in our galaxy. Then they examine how red these stars look in other galaxies and compute how fast they are moving, then they use that to figure out how far away they are (assuming that fast = far away). Complex.
But what if something else, some unknown process, makes some stars shift red even if they aren't quite so far away as it seems?
This is important because we keep finding impossible events when we look deep into space. There is an example in the press today:
An impossibly bright supernova - brighter than 570 billion stars - twenty times brighter than our whole galaxy? The astronomers admit, physics says such a bright star is impossible. But how do they calculate how bright it is? Its all based on the red-shift of nearby 'standard candles' and an assumption that fast = far away.
On the other hand, if the red-shift theory is flawed, and this supernova is not as far away as it seems, then it is smaller and less bright, and all the physics works out neatly.