High School

Earth Science

Week 3: April 20 - 24

Introduction To Stars

Since a typical star exists for billions of years, astronomers will never be able to observe one star throughout all of its life. Astronomers study stars at different stages of development and have used this information to develop theories about the evolution of stars.

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Lifecycle Of A Star: Nebula

A star begins as a nebula. A nebula is a dark cloud of dust and gas composed of about 70% hydrogen, 28% helium and 2% heavy elements. The particles in a nebula have a very weak gravitational attraction to one another. When a force, such as the explosion of a nearby star, compresses some of the particles, the nebula begins to contract. As the density of the particles increases, the gravitational force between them increases, and the nebula begins to shrink and spin rapidly.

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Lifecycle Of A Star: Protostar

This is the first stage in a star’s life. The shrinking, spinning nebula becomes smaller begins to flatten into a disk of matter with a central concentration. This is called a protostar. The pressure in the core increases the temperature of the protostar, but the temperature of the protostar is not yet hot enough for nuclear fusion to occur.

Lifecycle Of A Star: Main Sequence Star

The Main Sequence stage is the second, and longest stage in the life of a star. During this time, energy is created in the core of the star. Hydrogen is being fused into helium. Fusion releases enormous amounts of energy. The conversion of 1 gram of hydrogen into helium produces enough energy to keep a 100 watt light bulb lit for 3,000 years.

The energy created moves outward from the core. While in its Main Sequence stage, the outward movement of energy, which would cause the star to expand, is balanced by the force of gravity trying to compress the star. The star maintains its stable size for as long as it has an ample supply of hydrogen to fuse into helium.

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Lifecycle Of A Star: Giants & Supergiants

This is the third stage in a star’s life.

First, the star contracts: At this stage, almost all of the hydrogen atoms within the star’s core have been fused into helium atoms. Without ample hydrogen as a source of fuel, the force of gravity becomes the dominant force, and the core of the star contracts under its own gravity. This contraction increases the temperature in the core of the star.

Lifecycle Of A Star: Giants & Supergiants

Then, the star expands: These higher temperatures cause the helium nuclei to fuse into carbon. At the same time, some hydrogen fusion is taking place in a shell surrounding the helium core. The combined hydrogen and helium fusion release enough energy to cause the outer shell of the star to expand outward. (The energy out is greater than the pull of gravity in.) The star has become a Giant or Supergiant.

Lifecycle Of A Star: Later In A Star’s Life

What happens to a star after the giant phase depends on the star’s mass.

Riddle

What does a star win in a competition?

Answer

A constellation prize