Stars – Classification and Life Cycles

Stars form when giant clouds of hydrogen gas collapse under their own gravity. As the cloud contracts, it heats up until the core reaches temperatures high enough (~15 million K) for nuclear fusion to begin.

In fusion, hydrogen nuclei combine to form helium, releasing enormous amounts of energy as heat and light. This is how stars shine—converting mass into energy according to Einstein's E = mc².

Surface temperature determines a star's color: cool stars (~3,000 K) appear red, medium stars like our Sun (~5,800 K) appear yellow, and hot stars (~10,000+ K) appear blue-white.

The H-R diagram plots luminosity (vertical axis) against temperature (horizontal axis, decreasing left to right). Stars cluster into distinct regions:

• Main Sequence: Diagonal band where stars spend most of their lives fusing hydrogen
• Red Giants: Cool but luminous—large expanded stars in later life stages
• White Dwarfs: Hot but dim—small, dense stellar remnants
• Supergiants: Extremely luminous massive stars

Stars are classified by spectral type: O, B, A, F, G, K, M (hottest to coolest). Our Sun is a G-type star, about 4.6 billion years old and halfway through its main sequence life.