A star is a large, hot ball of gases that gives off its own light and is held together by gravity.
Constellation - a group of stars that appear to form a pattern as viewed from Earth.
Some constellations can only be seen on certain seasons.
The distance of stars from Earth is measured in lightyears (9.5 trillion km)

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Stars

​Actual Distance of the Orion Constellation

​A Constellation Chart

Some stars look brighter than others, but their brightness is affected by their distance from Earth.
Absolute magnitude is the star's actual brightness.
Apparent magnitude is a star's brightness as seen from Earth.

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Properties of Stars

A star's color tell us about its surface temperature.
Red and orange color mean the stars are cooler. Yellow stars are hotter than red, and blue-white are the hottest stars.
Stars also have different sizes. The Sun is an average star. Red super giants are the largest stars. White dwarfs are the smallest stars.

Properties of Stars

Protostars - starts as a nebula (cloud of dust and gas). Gravity contracts and heats up the nebula into a protostar.
Main-sequence stars - the center of a protostar becomes very hot and causes nuclear reactions. Hydrogen atoms are fused into helium. It is now a main-sequence star.

Development of Stars

Red Giants and Super Giants - the star starts to run out of hydrogen and so it expands. It becomes a red giant or super giant star. It now uses helium for nuclear fusions.

Development of Stars

Final Stages - the end of a star depends on its mass.
Stars with up to 10x the mass of the sun - nuclear reactions release energy and form a planetary nebula. The surface of the star becomes white-hot, and it becomes a white dwarf. When it completely cools off, it becomes a black dwarf.

Development of Stars

Stars with more than 10x the mass of the sun - when massive stars are no longer able to produce energy to counter their gravity, the star collapses and explodes into a supernova.
After that, a supernova may become a neutron star. Sometimes, neutron stars spin so fast that they appear to blink. These are called pulsars.
The most massive stars collapse into themselves and turn into black holes.

Development of Stars