A star forms when gravity compresses a cold cloud of gas until the core grows hot enough for nuclear fusion. In the main sequence stage, hydrogen fuses into helium and the outward pressure of energy balances the inward pull of gravity. The star’s mass acts like a script: small stars burn fuel slowly and can shine for tens of billions of years, while massive stars live fast and die young. When the hydrogen thins, the core contracts and the outer layers swell into a red giant or supergiant. The ending depends on mass—some stars cast off their outer shells to reveal white dwarfs, and the most massive collapse and explode as supernovae, seeding space with heavy elements that future planets and life can use.
Comprehension Check
What keeps a main sequence star in balance?
Why do massive stars have shorter lifetimes than small stars?
Planets and Dwarf Planets
Planets orbit stars and are massive enough for their own gravity to pull them into nearly round shapes. A key difference between a planet and a dwarf planet is orbital neighborhood: a planet has cleared most nearby debris, while a dwarf planet shares its region with many similar bodies. In our solar system, rocky inner planets formed close to the Sun where volatile materials evaporated, and gas giants assembled farther out where ices could survive and attract thick atmospheres. Beyond Neptune, the Kuiper Belt holds icy worlds—including Pluto—that record early solar system conditions. Comparing planets and dwarf planets shows how location, composition, and history shape the diversity of worlds.
Comprehension Check
Which criterion distinguishes a planet from a dwarf planet?
Why did gas giants form far from the Sun?
Small Bodies: Comets, Asteroids, Meteors
Small bodies are the solar system’s leftovers. Asteroids are rocky or metallic fragments that mostly orbit between Mars and Jupiter; their diverse shapes and compositions hint at violent beginnings. Comets are icy bodies from the Kuiper Belt or Oort Cloud that grow glowing comas and tails when sunlight heats their surfaces. When tiny pieces of rock enter Earth’s atmosphere, friction makes them burn as meteors; if any fragments reach the ground, we call them meteorites. Studying these travelers gives direct clues to early solar chemistry and the delivery of water and organic molecules to young planets, including Earth.
Comprehension Check
What causes a comet’s tail to appear?
Which term describes a space rock that reaches the ground?
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