From night to night and year to year the stars, including the Sun, appear constant and unchanging. However, the stars do change with time, only on time scales much, much longer than days and years. Over the past two centuries advances in telescopes and detectors have revealed details in the properties of stars such as size, temperature, mass, luminosity, and composition. These properties have been observed to vary in regular patterns and tell a clear story about how stars change with time. With this story has come a new understanding about the origin of the chemical elements and our connection to the stars.

A Star is Born

The interstellar medium is the gas and dust that fills the enormous void between the stars. It is mostly hydrogen gas plus trace amounts of heavier elements with an average density of one atom per cubic centimeter. Over millions of years, due to the action of gravity, slightly denser regions in it contract, and usually fragment into clusters of smaller collapsing clouds. As the fragments become denser, their cores become hotter. When a core gets so hot and dense that hydrogen nuclei are able to fuse into heavier helium nuclei, a star is born.

Adulthood

The story of a star’s life is basically the story of a balance between the force of gravity pulling matter together and an outward pressure resisting gravity. For most of a star’s active lifetime, nuclear fusion in its core generates an enormous amount of energy. This energy influences the star’s temperature and results in an outward pressure that balances the inward pull of gravity.

Stars like our Sun can live in the state of balance for around 10 billion years, calmly fusing hydrogen into helium inside their cores. More massive stars are hotter, and have higher pressure that balances their higher gravity. They use up their fuel faster and live shorter lives (tens of millions of years). Stars with less mass than the Sun are not so hot and use their fuel much more slowly and thus live longer lives (hundreds of billions of years).

Red Giant Phase

When a star runs out of hydrogen fuel in its core, the core begins to collapse under gravity's pull and heats up more. This energy causes the outer envelope of the star to expand, which leads to the overall star becoming bigger, brighter, cooler and redder in color. This time of a star's life is called the Red Giant phase.

Within the core, temperatures rise high enough to fuse helium into carbon and oxygen. The red giant phase is very short compared to the overall star's lifetime, only a few million years. Eventually, the star runs out of helium to fuse in the core.

What happens next to the star again depends on whether the star has relatively low mass or high mass.

Death of a Low Mass Star

The Sun and stars less than about 5 times the mass of the Sun will never be able to fuse the carbon and oxygen in their core into heavier elements.

As an aging, lower-mass red giant becomes more and more luminous, the outer layers puff out more and more. It reaches a point where the star loses gravitational hold on its outer layers and they get pushed away by the pressure exerted by the stellar wind and by the light leaving the core. (Yes, light can exert a pressure!) These shed outer layers are called a planetary nebula. As this planetary nebula continues to drift away and gets more dilute, it becomes a part of the interstellar medium, enriching the interstellar medium with heavier elements for future generations of stars and planets.

After only some tens of thousands of years, what is left behind is a super dense, white-hot core made of carbon and oxygen. This hot ball of carbon and oxygen is called a white dwarf. Here, gravity is balanced by the pressure of electrons that resist being squeezed too tightly, called degenerate pressure.

White dwarfs are only about the size of Earth but contain the mass of a star, so they are very dense. One cubic centimeter of white dwarf material would weigh a ton on Earth. A white dwarf will cool and dim over a period of about ten billion years into a cold dark hulk.

Death of a High Mass Star

If a star starts out with more mass than about 5 times the mass of the Sun it can end its life in a much more dramatic fashion than lower mass stars. Massive red giants can generate high enough density and temperature in their cores to fuse elements even heavier than carbon and oxygen. Near the end of the red giant phase, a high mass star's core will develop several shells of heavy elements fusing into heavier elements. However, stars cannot fuse elements heavier than iron. Fusing iron does not release energy. It uses up energy. Thus a core of iron builds up in the centers of massive stars. This core is held up against gravity by the pressure of electrons resisting being forced too tightly together, like in white dwarfs.

When the growing iron core reaches about 1.4 times the mass of the Sun, not even this pressure can hold it up against gravity. The core collapses and, in the mayhem, the star's exterior explodes. This exploding star is called a Core Collapse Supernova. Supernovae are about ten billion times as luminous as the Sun and can rival entire galaxies in brightness for weeks.

During the supernova, a tremendous amount of energy is released. Some of that energy is used to fuse elements even heavier than iron. This is where such heavy elements like gold, silver, zinc, and uranium come from! The enriched material ejected into space as a result of the supernova becomes part of the interstellar medium.

Stellar Graveyard

If the mass of the core that remains after the supernova is less than 2 or 3 times the mass of the Sun, it becomes a neutron star, so named because it is made almost entirely of neutrons. During the collapse of the core, electrons are forced into the nuclei of the atoms and combine with protons to create neutrons. Neutron stars are unbelievably dense. They have between 1.4 and 3 times as much mass as the Sun, but are compressed into a ball with a radius of about 10 km. A teaspoon of neutron star matter would weigh as much as a mountain. A neutron star fights gravity with the pressure of neutrons resisting being pushed more tightly.

Sometimes not all the matter outside the core is blown away in the supernova explosion, and some of the matter falls back down onto the collapsed core. If the core grows to more than about 3 times the mass of the Sun, it will be unable to hold itself up against gravity, so it collapses even more. This probably happens for most stars that originally started out with more than 20 times the mass of the Sun.

Once started, there is no force that can halt this collapse, and a dense, compact black hole forms. Nothing can escape from the black hole's surface -- not even light! Space and time are warped beyond recognition, and gravity has won the battle.

Dust to Dust, Completing the Cycle

In the end, the quest for balance between the inward pull of gravity and outward pressure produces dark compact objects like white dwarfs, neutrons stars, or black holes. It also recycles matter, as the interstellar medium is enriched with heavier elements from previous generations of stars. Star death can also trigger the production of new stars. The shock waves produced by stellar winds and explosions such as supernovae can compress the interstellar medium and set off new rounds of star formation.

We owe our existence to previous generations of stars. The Universe started out with mostly hydrogen, a little bit of helium and not much else. It took a few generations of stars to produce the iron that is in our blood, the silicon that makes up the rock of our world, the carbon that is the primary building block of our bodies, and the oxygen that we breathe. We are made of stardust!

 

The Orion Nebula. Credit: STScI/NASA

The Sun in extreme ultraviolet light (EUV), imaged by the SOHO spacecraft. Image is color-coded to help us see the invisible EUV. Credit: SOHO/NASA

The central star of this beautiful planetary nebula is hidden by dust. Credit: STScI/NASA

The Crab Nebula, the remnant of a core collapse supernova explosion that was observed worldwide in 1054 A.D. Credit: STScI/NASA

An artist’s concept of a black hole with an accretion disk and magnetic field. Credit: XMM-Newton/ESA/NASA