ScienceVerseLearning path: Galaxies
The concepts to learn before Galaxies, in order. Each step links to its page and, where one exists, its quiz.
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Your path
- StarsDirect prerequisite
A massive, luminous sphere of plasma held together by its own gravity, generating energy in its core through nuclear fusion — mainly the fusion of hydrogen into helium. Stars form from collapsing clouds of gas and dust, shine for millions to billions of years depending on their mass, and eventually exhaust their nuclear fuel, ending their lives as white dwarfs, neutron stars, or black holes depending on their initial mass.
- GalaxiesYour goal
A galaxy is an enormous system of stars, gas, dust and dark matter held together by gravity. Our Sun is one of about 100 billion stars in the Milky Way, a barred spiral galaxy, and beyond it lie countless other galaxies — a fact only proved in the 1920s. How fast galaxies spin reveals that most of their mass is invisible dark matter.
Where to go next
Concepts that build directly on Galaxies:
- Cosmic expansion: Cosmic expansion is the stretching of space that carries distant galaxies away from one another, so that the farther away a galaxy is, the faster it recedes (Hubble's law). Traced back, it implies a hot, dense beginning about 13.8 billion years ago, whose afterglow we still detect. Since 1998 we have known the expansion is speeding up, driven by an unexplained 'dark energy' — and today's best measurements of how fast it is expanding do not agree, an open problem called the Hubble tension.