The Deaths of Sunlike Stars

Low Mass DwarfsLow mass red dwarf stars cannot achieve any advanced fusionbecause they cannot get hot enough (Temp < 100 million K) to begin the next reaction (helium to carbon)

Hydrogen => Helium fusion ends at coreStar shrinks to form a white dwarf

Formation of a Giant Star

When H He fusion ceases, hydrostatic equilibrium is disrupted and gravity forces dominate. The core collapses. The collapse heats the inner core enough so that helium fusion may begin.

The new fusion reaction generates an even larger fusion pressure, which pushes the star’s outer layers outward.

As they expand, they cool, and turn redder in color. This change in temperature is known as adiabatic cooling.

Triple Alpha ProcessIf the core T > 100 Million K, three helium nuclei (alpha particles) fuse to make a carbon nucleus. Some carbon nuclei further combine with one more alpha particle to make oxygen. The star swells to become a giant.

The star develops a two-layered fusion: He C & O in the core, and H He in a shell above the core.

Inside a Giant

– Stars like the sun become giant stars of 10 to 100 times the Sun’s present diameter.

– The most massive stars become supergiant stars as much as 1,000 times larger than the Sun.

Giant Stars

Our Sun as a Red GiantWhen our Sun becomes a red giant (in about 7.6 billion years from now), it will expand so much that it will engulf the planets Mercury, Venus, and Earth.

The Earth’s orbit will slowly degrade and Earth will spiral farther into the Sun, if it has not already vaporized.

The atoms that were on Earth (like yours!) will be reclaimed by the star.

Giant and Supergiant Stars

By comparing star clusters of different ages, you can visualize how stars evolve—almost as if you were watching a film of a star cluster evolving over billions of years.

Star Clusters: Evidence of Evolution

Star clusters provide the evidence that the evolution is visible on the H-R Diagram.

Star Clusters: Evidence of Evolution

Giant Stars- Internal LayersMedium mass stars will die when carbon and oxygen

build up in their cores.High mass stars can form elements up to iron before fusion stops.

A dying giant can expel its outer atmosphere in repeated episodes to form a planetary nebula.

The first planetary nebulae discovered looked like the greenish-blue disk of a planet such as Uranus or Neptune.However, they have nothing to dowith planets. The colors come from the large amounts of ionized oxygen they expel.

Planetary Nebulae

Planetary Nebulae

The PN shells of gas are symmetrical because of magnetic fields.They are lit by ultraviolet light coming from the collapsing star. When UV strikes the expelled gases, they fluoresce.

A white dwarf star forms at the center of a planetary nebula. White dwarfs are very hot because they have been condensed into a very small area, about the size of the Earth. This heating process is adiabatic heating.

A typical planetary nebula will shine for 20,000 to 50,000 years, but the white dwarf formed will glow for billions of years.

Planetary Nebulae

So far, there has not been enough time in the history of the universe for any white dwarf to cool off so much that it does not glow (a black dwarf). The coolest white dwarfs in our galaxy are about the temperature of the Sun.

White Dwarfs

The planetary nebula designated NGC 2440, contains one of the hottest white dwarf stars known. The white dwarf can be seen as the bright dot near the  photo's center.

Credit: H. Bond (STScI), R. Ciardullo (PSU), WFPC2, HST, NASA 

The contraction of a white dwarf compresses thegases in its interior to such high densities that the electrons in the gas are pushed as close together as is permitted. Such a gas is termed degenerate matter.

Degenerate matter is much more dense than normal matter because all of the “empty space” has been squeezed out of the atoms. A teaspoon of “white dwarf” would weigh several tons.

Degenerate Matter

White dwarfs arecomposed primarily of crystallized carbon (the endpoint of fusion for stars like the Sun), so they can be thought of as the biggest diamonds in the universe!

White Dwarfs

V886 Cen is about 50 LY away from Earth. This white dwarf star weighs 5 million trillion trillion pounds. That would equal a diamond of 10 billion trillion trillion carats.

After it was discovered in 2004, astronomers nicknamed the star “Lucy” after the Beatles song Lucy In The Sky With Diamonds.

Giant Diamonds

Illustration of “Lucy” by an artist at the Harvard-Smithsonian Center for Astrophysics

White Dwarfs in Binary Systems

If a white dwarf is a member of a binary star system, and if the two stars are close together, they can transfer mass back and forth. This may alter the evolution of the stars.

White Dwarfs in Binary Systems

Mass transferred from one star to another forms a rapidly rotating whirlpool around the called an accretion disk.

White Dwarfs in Binary Systems

The gas temperature can exceed a million degrees, producing X rays.

In addition, the matter accumulating on the white dwarf can eventually cause a violent explosion called a nova.

Novae

A nova forms when star material falls onto the surface of a sizzling hot white dwarf.

The material can flare off, producing a temporary brightening of the star.

Nova Cygni, 1992

Type I Supernovae

Sometimes there is so much material accreted onto the white dwarf, it collapses and forms a spectacular explosion known as a Type I supernova.