The Cosmic Distance Ladder
Distance measurements in the Universe rely on having objects of a known brightness. Essentially all Type Ia Supernovae reach the same peak brightness before fading away; this makes them excellent examples of what we call “standard candles”. We know how a candle of a particular brightness grows fainter the further it moves away from us. So if we know the true brightness of the candle, and we measure its observed brightness, we can calculate its distance. But how do we measure the true brightness of that “standard candle” in the first place? Astronomers use another kind of “standard candle” called Cepheid Variable stars: they pulsate at a frequency that is tied to its absolute brightness.
These and other techniques are being used to measure distances in the Universe. Each method provides a rung on what is called the “Cosmic Distance Ladder”.