After stars have exhausted their fuel, gravity pulls them into further collapse leading to a variety of types of peculiar object.


"Roll on, ye stars! exult in youthful prime,

Mark with bright curves the printless steps of time...

Flowers of the sky! ye too to age must yield,

Frail as your silken sisters of the field."

-- Erasmus Darwin (grandfather of the naturalist Charles Darwin)

Key points: Nature of white dwarfs, neutron stars; what planetary nebulae and pulsars are

Why should we concern ourselves with the deaths of stars?

Some stellar deaths are spectacular -- the role of "guest" stars in Chinese history is a good example. We now know that many of these suddenly-appearing stars were supernovae, where stars blew up as they died. Dead stars leave extreme and interesting objects behind.

When stars die, a lot of their material is ejected back into the interstellar medium through the following mechanisms:

1) Winds -- all stars lose a small amount of mass due to material that "boils" off the surface in a manner analogous to the solar wind. As a star becomes a red giant, the wind may strengthen. Winds usually remove only a small fraction ( less than a percent) of a star's mass.

2) Planetary Nebula -- the material ejected by a moderate mass red giant as its core collapses into a white dwarf.

3.) Supernova Explosion -- the violent ejection of nearly the entire material in a massive star when its core collapses and the outer layers implode onto the core. The remnant can be a white dwarf, neutron star, or black hole.

All of these events, but mostly supernova explosions, eject material into space that has been processed from hydrogen and helium up to heavier elements. When this material is incorporated into new stars, it also provides the ingredients to make planets like the earth and all the things on their surfaces -- like you!

Deaths of stars can also influence the formation of new stars, such as when shock waves from supernovae cause interstellar clouds to collapse.

What does a dead star look like?

hrdeath.gif (88224 bytes) What a star becomes when it dies depends on the mass left when all possible nuclear fuels are exhausted and the star has lost some of its original mass by ejecting it:

M <= 1.4 M -----> white dwarf/planetary nebulae (PN to the left)

1.4 M <M < ~3 M --> neutron stars/pulsars

M > ~ 3 M ----> supernovae/black holes

White Dwarfs and Planetary Nebulae

Comparison of sizes of sun and white dwarf After a star like the sun exhausts its nuclear fuels, it loses its outer layers as a "planetary nebula" and leaves behind the remnant "white dwarf" core. The white dwarfs are extremely small stars -- they are the bare remnant cores of stars after they have gone through all of their lifetimes.(from Imagine the Universe,
We show the evolution of a star like the sun on the H-R diagram (to left) and as it might appear if we were watching (to right) (from Jake Simon and Charles Hansen, It is pretty boring because the main sequence lifetime is so long (a good thing for us!). Eventually the star becomes a red giant - its luminosity goes way up, it swells, and its temperature drops. That does not last long - quickly it expels its outer layers and its core shrinks to a white dwarf. We see it briefly as a planetary nebula, but the gas dissipates and we end with an isolated white dwarf that cools as it loses its stored energy. The bar to the lower left shows the age of the star.

An early step in the mass loss and transformation to planetary nebula is when tiny dust grains form near the surface of the star and the pressure of the light photons on them expels them, carrying some of the gas along. Here is a simulation of how it would look - the diameter of the field is about 10 times the size of the orbit of the earthen00500_1.jpg (18578 bytes).(from Peter Woitke,

When only the white dwarf core is left of the star, it lights up the material that was expelled because the white dwarf is very hot. Here is a very young planetary nebula, where a lot of the structure of the just-ejected material is still present. The optical image to the left shows shadowing by a disk, waves from pulsed mass loss, and "searchlights" of light. The infrared image on the right shows molecular hydrogen (red) and the denser material that shapes the visible searchlights. Egg Nebula, a very young planetary nebula
Older planetary nebulae take many wonderful and beautiful forms. However, they all are created through similar processes.  This one is called the Cat's Eye, (Vicent Peris, HST, STScI)
From Doppler shift velocities, most planetary nebulae have ages around ~20,000 years. A star may lose 0.1-0.2 M this way. The hot star in the center is the former red giant core, now a white dwarf. Here is the "Helix", from HST.
HST image of "hourglass" planetary nebula Planetary nebulae were misnamed because of their round shapes and greenish colors (caused by an oxygen emission line) -- as in the bright central part of the one to the left.. They actually have nothing to do with planets.

They are among the most intriguing objects on the sky

And the most beautiful. This one is the Butterfly Nebula, (from Astronomy Picture of the Day, NASA, ESA, and SM4 ERO team.


helix1.gif (3083291 bytes) This animation is a kind of review. It shows the overall collapse of the red giant accompanied by the ejection of material that expends into the "Helix" (From NASA, STScI).

The Nature of White Dwarfs

Once nuclear reactions cease, the stellar remnant has no means of counteracting the force of gravity, and the interior of the star collapses. It doesn't collapse forever because a new force develops that can resist gravity. This force is electron pressure. The material in a white dwarf has been compressed so much by gravity that all the electrons have been stripped away from all of the atomic nuclei. The electrons form a gas. The electrons are squeezed together by gravity, but as described by quantum mechanics, the electrons eventually resist being squeezed together any further. Thus happens when they become degenerate:

Degenerate matter: No two electrons can have exactly the same energy, spin, position (according to quantum mechanics) so when electrons are compressed enough, they fill up all of the available energy states. Such dense matter is called degenerate.

A white dwarf has a diameter similar to the Earth's and a density such that a teaspoonful weighs a ton!

Models of white dwarfs can be calculated using the laws of quantum mechanics --

animation of pushing a white dwarf over the Chandrasekhar limit The radii of white dwarfs DECREASE with INCREASING mass because of the increasing strength of gravity.

When the mass exceeds 1.4 M, electron degeneracy is no longer strong enough to resist the pull of gravity and the white dwarf abruptly collapses into a neutron star. (animation by G. Rieke)

1.4 M is called the Chandrasekar limit in honor of the astronomer who first explained the nature of white dwarfs (and won the Nobel prize for his work). Amazingly, he worked it out to avoid being bored on the long boat ride from India to graduate school in England!

mass-radius relation for white dwarfs shows more mass = smaller size This behavior is summarized a bit more formally in the mass-radius relation for white dwarfs

The sun will end its life as a white dwarf.

Neutron Stars: The Fate of Stars with M>1.4M

Massive stars can lose mass even more dramatically than for planetary nebulae:

etacarina.jpg (37339 bytes)
Eta Carina is a very luminous massive star (roughly 100 times the mass of the sun) that appears to be virtually at the end of its life. It had a violent outburst 150 years ago, but has somehow survived. However, the material it has lost has built up this impressive nebula.  J. Morse, K. Davidson, STScI

Stellar remnants that remain larger than 1.4M cannot evolve into white dwarfs ---- their gravitational field is so strong that the electrons and protons in the remnant are squeezed together to form neutrons:

e- + p+ ----> no

Further collapse is prevented by pressure from the "neutron gas," which behaves as degenerate matter in a fashion analogous to the behavior of electrons in white dwarfs.

Size comparison of a white dwarf and a neutron star neutronstar.jpg (43614 bytes)
Neutrons can be shoved much closer together than electrons so even though neutron stars are more massive than white dwarfs, they have smaller radii (tens of km rather thousands of km in diameter) (to left, from U Tenn, With great artistic license (because the gravity of the neutron star would tear the earth apart), the picture to the right shows the size of a neutron star by imagining it were set down on top of New York City. (from


In the late 1960s, radio astronomers discovered a type of radio source that they dubbed "pulsars" ribbon.jpg (3557 bytes)


1) typically do not have visible counterparts

2) their radio output varies in a precise, repetitive pattern

3) they are found in or near supernova remnants

Artist's concept of a neutron star Pulsars appear to be spinning neutron stars with rotation axes tilted to their magnetic fields. Energetic electrons and light pour out the magnetic poles, and as the star spins the beam of light is swept across the sky like a lighthouse beacon. (From Chaisson & McMillan, Astronomy Today)
animation of spinning flashlight Their behavior is like a spinning flashlight in a darkened room. (animation by G. Rieke)

Pulsars were considered extremely remarkable when first discovered because of the extremely short periods of their variations (ranging from a fractions of a second to a few seconds; vastly different from the 10-1000 day periods for red giant/supergiant variables), and because of the extreme constancy of their periods --- they are extremely accurate clocks. A normal star could not vary so quickly and regularly, because even at the speed of light it could not communicate across its diameter fast enough! Neutron stars were the only objects predicted to exist that could have the behavior of pulsars.The concept is strengthened by noting that if you imagine shrinking a normal star with its ~20 day rotation period, it would speed up to pulsar-like time scale if shrunk to a size only 10 km in diameter.

Later it was discovered that all pulsars are slowing down, but slowing down so gradually that the change can be detected only be using the most accurate atomic clocks. Further confirmation comes from the equivalence of the amount of energy in the escaping  beam of particles and light and the energy loss indicated by the rate of slowing down.

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Evolution of the Sun, from interstellar cloud (back) to red giant (front). from

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"Embedding" diagram, showing how a black hole distorts space/time, from C. Pickover,

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hypertext copyright.jpg (1684 bytes) G. H. Rieke

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