Physics · Space physics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Our solar system has one star, the Sun, with eight planets and the dwarf planets in orbit around it. Moons are natural satellites that orbit planets. The solar system is a small part of the Milky Way galaxy.
A star forms when gravity pulls a cloud of dust and gas, a nebula, together. It heats up until hydrogen nuclei fuse into helium. In the main sequence the star is stable: the outward force from fusion energy balances gravity pulling inwards. What happens next depends on the mass of the star.
Gravity also provides the force that keeps planets and satellites in orbit. In a circular orbit the speed is constant but the velocity keeps changing, because the direction changes.
Students say an orbiting satellite has constant velocity because its speed is constant. Velocity includes direction, which changes all the time, so the satellite is accelerating towards the planet.
Answer: The nebula contained material from an earlier massive star that exploded as a supernova.
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