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Stars . Stars. What are Stars? Stars are large balls of hot gas. They look small because they are a long way away, but in fact many are bigger and brighter than the Sun. The heat of the star is made in the center by nuclear fusion reactions.

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What are Stars?

  • Stars are large balls of hot gas.
  • They look small because they are a long way away, but in fact many are bigger and brighter than the Sun.
  • The heat of the star is made in thecenterby nuclear fusion reactions.
  • There are lots of different colorsand sizes of star.

How are stars made?

  • Stars are made (or “born”) in giant clouds of dust and gas.
  • Sometimes part of the cloud shrinks because of gravity.
  • As it shrinks it becomes hotter and when it is hot enough, nuclear reactions can start in the centre…..
  • … and A Star is Born!

How does a star “die”?

  • Eventually, the hydrogen (the “fuel” for the nuclear fusion) in the centre of the star will run out.
  • No new heat is made and gravity will take over and the centre of the star will shrink.
  • This makes the very outside of the star “float up” and cool down, making the star look much bigger and redder - a Red Giant star.

Antares – a Red Giant


The end of a Sun-like star

  • For a star like the Sun, no more nuclear fusion can take place, so the centerof the star will then keep collapsing.

Simulation of the Death of the Sun

  • Eventually it can become almost as small as the Earth, but with the same mass as a whole star! This very dense object is called a White Dwarf.
  • A piece of White Dwarf the size of a mobile phone would weigh as much as an elephant on the Earth!

Birth and Death of Stars - Summary

  • Stars form in clouds of gas.
  • Heat from nuclear fusion, and gravity balance.
  • When the hydrogen fuel runs out, a Red Giant is formed.
  • For Sun-like stars, a White Dwarf and Planetary Nebula are left.
  • For massive stars, a Supernova explosion leaves behind a Supernova Remnant and a Neutron Star or perhaps even a Black Hole.
of stars
of Stars

White Dwarf and Planetary Nebula

Collapsing cloud

Sun-like stars

A new star

Supernova Remnant and Neutron Star

Red Giant

Massive stars

Birth and Death of Stars - Summary

star color
Star Color

-Blue stars are hotter because they emit short wave radiation.

-Red stars are cooler

-Yellow stars are middle temperature stars.

Constellations:Recognizable patterns in the sky
  • There are 88 Constellations. 12 are based on the Zodiac.
    • Orion, Leo, Aries, Taurus, etc.
  • Asterisms: Recognizable patterns of stars that are not one of the constellations are called Asterisms:
    • Big Dipper, Little Dipper, Summer Triangle
Are stars still present in the sky during the day time?
    • Yes, we just can’t see them because their dim light is overwhelmed by the brightness of the sun during the day.
seasonal changes in the sky
Seasonal Changes in the Sky
  • The night-time constellations change with the seasons.
  • This is due to the Earth’s orbit around the Sun.