Life Cycle of a Star

There are seven stages in the life cycle of a star. The mass of a star determines how radiantly it shines and how it dies. Let’s look at the Sun’s life cycle, a medium-sized star.

  • Nebula – Stars are born from enormous clouds of gas and dust called nebulas, which consist primarily of hydrogen.
  • Protostar – The gas and dust clump together to form a dense core known as a protostar. 
  • Main Sequence Star – Nuclear fusion begins in the core of the star. Hydrogen atoms fuse to form Helium, releasing massive energy in the process. Most of a star’s life happens at this phase.
  • Red Giant – When the Hydrogen in the core gets used up, the core shrinks, and fusion starts at the outer layers. The star expands into a Red Giant.
  • Planetary Nebula – The outer layers drift away, leaving behind a glowing shell of gas called a planetary nebula.
  • White Dwarf: The remaining core becomes a white dwarf, slowly cooling over billions of years.
  • Black Dwarf: Over billions of years, a white dwarf will cool and fade, eventually becoming a cold, dark remnant. This is just a theory, as the universe has not witnessed a black dwarf yet. 

Star Sizes

Our Sun is more than 1.4 million kilometers wide. But not all stars are the same size. Some stars are as small as a city, while others are big enough to swallow half of our Solar System!

  • Red Dwarfs: These are the smallest and coolest types of stars, with a diameter and mass less than half that of the Sun.
  • Yellow Dwarfs: Our Sun is an example of a yellow dwarf. These stars have a diameter and mass similar to the Sun.
  • Blue Giants: These stars are much larger and hotter than the Sun, with a diameter up to 10 times greater.
  • Supergiants: Supergiants are among the largest stars in the universe, with diameters hundreds or even thousands of times greater than the Sun.
  • Hypergiants: Even larger than supergiants, hypergiants can be up to thousands of times the Sun’s diameter and have a mass dozens of times greater.

Fun Facts

Here are some fun facts about the stars:

  • Stars appear to twinkle because their light passes through Earth’s atmosphere, which causes them to bend and shimmer.
  • The color of a star tells us its temperature. Blue stars are the hottest, while red stars are the coolest.
  • Some stars end their lives in a spectacular explosion called a supernova. 
  • When a massive star dies, it can become an incredibly dense neutron star or a black hole.


Let’s quickly recap what we learned about the life cycle of a star:

  • What is the birthplace of a star? Nebula
  • What process powers a main sequence star? Nuclear Fusion
  • What phase does a star enter after using up its hydrogen fuel? Red Giant
  • What do small to medium stars become at the end of their life? White Dwarf
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