The Lifecycle Of A Star

Explore the lifecycle of stars and learn about Hertzprung-Russell diagrams, supernovae, pulsars, red giants, white dwarfs, and black holes.

When we look up at the sky with the naked eye, all we see are some twinkling dots of light. If we look really hard, we may be able to determine that they are different colored twinkling dots but not much else. In reality, different stars are very different from each other. They are different colors, different distances away, different temperatures, and they emit different types of light. A specific star changes over time, evolving throughout its lifetime.

The basic properties and evolution paths of stars are graphed on diagrams called Hertzprung-Russell diagrams. The horizontal axis of the graph is the star's temperature or spectral classification (a letter system based on temperature). The vertical axis is the luminosity of the star, graphed relative to the sun. Named for two early 20th Century astronomers, Ejnar Hertzprung and Henry Norris Russell, who first plotted this information, H-R diagrams (as they are typically called) effectively map out the lifecycle of stars.

The Regions of a Hertzprung-Russell Diagram

When plotted on the H-R diagram most stars fall in one of three regions. The majority of stars fall in a long narrow band from the bottom right to the top left of the diagram. This is called the main sequence and is where stars spend most of their life. Another pocket of stars is found in the upper right corner of the diagram. Stars there are called giants and supergiants. These are stars that are reaching the end of their lives and have started to expand and blow off extra mass. The third dense region on the graph is in the lower left corner. Stars there are called white dwarfs and are the cores of low mass stars that no longer sustain nuclear fusion and have effectively ended their life.

The Lifecycle of a Star

Stars are formed in gigantic, cold, globular clouds containing tens of thousands or even millions of solar masses worth of dust that condense into individual stars. First, they form protostars, a clump of gas held together by gravity but always wanting to fly apart from the force of thermonuclear reactions. Protostars are surrounded by gas and dust, which prevent them from being seen. Eventually, the protostar settles down into a T-Tauri, which is the first stage we can see. Nuclear fusion reactions start, and the star enters the Main Sequence. Stars spend most of their life, approximately 90%, in the Main Sequence. As stars start reaching the end of their lives, they turn into giants or supergiants and begin synthesizing heavy elements. When these heavier fusion reactions start to burn out, the stars either eject a planetary nebula or turn into supernovas, depending on their mass. Stars of about 5 stellar masses or less turn to nebulas, and stars of greater than 5 solar masses turn nova. The final resting stage of stars is also dependent on mass. Stars of less than 1.4 solar masses become white dwarfs, stars of 1.4-8 solar masses become neutron stars, and stars with more than 8 solar masses end their lives as black holes.

Main Sequence Stars

Main Sequence stars have reached hydrostatic equilibrium and are stable in terms of their size, mass, and luminosity. In this life stage, the stellar core turned hydrogen into helium via fusion reactions. These reactions power the star and cause it to emit radiation. The luminosity of a Main Sequence star is directly proportional to its temperature: cool stars are dim, and hot stars are bright.

Giants and Supergiants

As stars end their lives, they run out of hydrogen to synthesize into helium. The core gets hotter and the stars begin to synthesize helium into higher elements. The stars begin to expand and start throwing off much of their mass. This causes the star to get brighter. At the same time, because the surface is larger, the average temperature at the surface cools. The star moves into the upper right (cool and bright) region of the H-R diagram and is classified as a giant or supergiant depending on the ratio of luminosity to temperature.



Planetary Nebulae

At the end of its tenure as a giant or supergiant, a low mass star sheds the bulk of its mass in a gas pocket that envelops the star. Eventually, this gas breaks free from the star and becomes a separate entity, an expanding pocket of gas. This new entity is called a planetary nebula even though it has absolutely nothing to do with planets.

Supernovae

More massive stars get rid of this extra mass with explosions rather than the gentle separation of planetary nebulae. The core of these stars has turned into a very rapidly spinning neutron star, called a pulsar, and the explosion is a reaction to some of the mass getting drawn into the core and reacting to its mass.

White Dwarfs

Stars with less that 1.4 solar masses end their lives as white dwarfs. White dwarf stars are the remnants of giants that eject planetary nebulae. All fusion reactions have ceased, and all that remains is a very dense iron core. Because the core is so dense, its surface temperature is hotter than most stars. Thus, white dwarfs are very hot stars that give off no energy and are found in the lower left corner of an H-R diagram.

Neutron Stars and Pulsars

After more dense stars become supernovae, the core neutron star or pulsar remains. Neutron stars form when the core of a star is compressed so much that the electrons and protons fuse together to form neutrons. They typically measure a mere 30-50 kilometers in diameter with a mass greater than the mass of Earth. Pulsars are neutron stars that rotate rapidly and have very large magnetic fields. Most stars with 1.4-8 solar masses turn into pulsars.

Black Holes

Very massive stars with masses greater than 8 solar masses end their lives as black holes. Typically, these stars have supernova explosions, but the mass of the core and force of the explosion is so extreme that the matter turns in on itself causing a point singularity.

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