Star Life Cycle

star formation
Star Cluster Formation in Galaxy NGC 4214 - NASA

There are billions of stars in the Milky Way galaxy. These stars vary in their life stage or age. Astronomers classify the different life stages of stars in the following way:

  • Young Stellar Objects
  • Main Sequence Stars
  • Red Giants
  • Dying Stars (of which there are several outcomes)

Birth of a Star

A new star, or young stellar object, forms when hydrogen gas and dust in space from nebulas combine, held together by the force of gravity. Gravity causes the star to become smaller and more compact. This shrinking process causes the core of the star to heat up. Once the temperature of the star becomes high enough, nuclear fusion starts and the hydrogen within the star ignites.

Adult Stars

Once the hydrogen ignites, the star becomes a main sequence star, like our Sun. Main sequence stars that have a mass much smaller than our Sun are called red dwarfs because they emit a dull red glow. A main sequence star as massive as our Sun is called a dwarf. Stars spend the longest amount of their lifetimes at this stage.

Scientists expect our Sun to remain a main sequence star for another 5 billion years until it becomes a red giant, and eventually, a white dwarf.

star death
Dying Star - NASA/JPL-Caltech

Death of a Star

A star eventually turns all of the hydrogen in its core into helium through nuclear fusion. Once this happens the star begins to burn the hydrogen in its outer shell. The helium in the core begins to contract due to the force of gravity. This causes the temperature to heat up and the star becomes brighter and it burns the hydrogen in its shell faster. The outer shell of the star expands increasing the luminosity. The outer shell also becomes cooler, causing the star to emit a bright red glow. At this stage the star is called a red giant.

Eventually the hydrogen in the outer shell is used up and the star starts expelling its outer layers, forming the nebula (the gas expelled by a star) around the star.

The burned out star remains behind and the nebula dissipates into space. At this stage the star is called a white dwarf and has a carbon core. Over time the star becomes cooler until it has faded completely. The NASA/JPL-Caltech image of the dying star shows a star that is on its way to becoming a white dwarf.

The gases and dust from the nebula fuels the formation of new stars.

The life cycle of a star varies depending on its size. For example, stars with small masses (smaller than our Sun) begin life as young stellar objects, become main sequence stars, but are classified as red dwarfs. They typically remain red dwarfs forever because they burn hydrogen so slowly that they can go on forever.

In contrast, very high mass stars (much higher than the mass of our Sun) burn their hydrogen core more quickly than smaller stars. High mass stars don't become white dwarfs either. Instead, they usually become supernovas, neutron stars, or black holes at the end of their life cycle.

Image of Star formation: Courtesy of NASA.
Image of dying star: Courtesy of NASA/JPL-Caltech.