In the 1920s, a scientist named Edwin Hubble was studying galaxies. He looked at light from these galaxies and noticed something interesting called redshifts. This helped him figure out how fast the galaxies were moving. Hubble also discovered a type of star called Cepheid variables in these galaxies. Cepheid variables are special because they change in brightness over time.
Hubble found out that bigger, brighter Cepheid stars have longer periods of brightness changes, while smaller, dimmer Cepheids have shorter periods. By watching how fast these stars pulsed, he could estimate how bright they were and how far away the galaxies were.
When Hubble made a graph showing the speed of the galaxies compared to their distance, he noticed two important things:
Most galaxies (except a few close ones) are moving away from us. This means the universe is getting bigger.
The farther away a galaxy is, the faster it is moving away. This idea is known as Hubble's Law.
It's important to note that when we say galaxies are moving away, we really mean that space itself is expanding. This expansion creates more space between us and distant galaxies, pushing them farther away than closer ones.
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