Online Astronomy eText: Stellar Evolution
Planetary Nebulae and White Dwarfs
Images of Planetary Nebulae
(short captions; more detailed discussion below)

The Helix Nebula, the closest example of a planetary nebula formed by the death of a Sun-like star.
the Helix Nebula
closeup of a portion of the nebula
(NASA, WIYN, NOAO, ESA, Hubble Helix Nebula Team, M. Meixner (STScI), & T. A. Rector (NRAO), apod030510)

A false-color infrared image of the Helix Nebula shows the nebula as a pale green network surrounding a reddish central disc. The disc represents dust heated by the white dwarf in the center of the nebula. (NASA, JPL-Caltech, Kate Su (Steward Obs., U. Arizona), et al., apod091231)




HST polarized light image of the pre-planetary nebula, The Egg Nebula
The Egg Nebula, a pre-planetary nebula, imaged with polarized light (false-color image, as usual).
(Hubble Heritage Team (STScI/AURA), W. Sparks (STScI) & R. Sahai (JPL),
NASA)
Pre-planetary nebulae represent gases blown off by stars that will soon be white dwarfs
The Egg Nebula is perhaps 3000 light years away, in the constellation of Cygnus, the Swan

HST closeup of central portion of the pre-planetary nebula, The Egg Nebula
A closeup of the central portion of the Egg Nebula (Image Credits: ESA/Hubble, NASA)
A dusty disk in the plane of the star's rotation forces escaping gases toward the poles of the disk

The Red Rectangle Nebula, a 9th-magnitude proto-planetary nebula in Monoceros (RA 08 19 58, Dec -10 38 15) formed by gases ejected from an aging binary star system (HD 44179) at the center of the nebula. The stars themselves are not visible, due to the thick clouds of gas and dust which obscure them; but light reflected off the dust and emitted by gases absorbing the light of the stars creates one of the most unusual objects in the sky. The nebula wasn't discovered until 1973, and for years its odd appearance was a complete mystery. It is now believed that a thick dusty disk (similar to that of the forming Solar Nebula) surrounds the binary system, so that gases ejected by the aging stars can only move perpendicular to the plane of the disk. Shock waves created by the collision of the ejected gases and the disk create cone-like structures extending outward from the plane in various directions. In this case, the disk is edge-on to our line of sight, causing the cones to form a dramatic X-shaped nebula. The red coloration is probably due to complex hydrocarbon molecules, but most of the details of the nebula's appearance are still a mystery. As the stars at the nebula's center continue to age, one or both will eventually eject a substantial fraction of their mass, forming a more typical (although probably still unusually complex-appearing) planetary nebula. (Image credits: ESA, Hubble, NASA, apod100614


The Ring Nebula (M57) in Lyra. A planetary nebula, 2000 light years away. In the same field of view is IC 1296, a galaxy 200 million light years away. (Brian Lula, apod030516)


Another image of the Ring Nebula (H. Bond et al., Hubble Heritage Team (STScI / AURA), NASA, apod030322)

HST image of the central portion of the Cat's Eye Nebula, NGC 6543

     Three views of the Cat's Eye Nebula, a planetary nebula in Draco, also known as NGC 6543. Above, an HST image in visible light reveals the complex structure of the nebula. Below, Chandra X-ray images (in blue) are superimposed on the visible structure. At bottom, faint concentric rings surrounding the nebula are revealed in another HST image. None of the HST/Chandra images actually show the entire nebula, which extends well beyond the region shown here. Refer to NGC 6543 for more images and details.
     The Cat's Eye nebula has long been recognized as one of the most complex planetary nebulae known. The concentric rings are spherical bubbles of gas and dust emitted at about 1500 year intervals (based on their size, spacing, and expansion rate), each with a mass approximately equal to the planets of our Solar System, or about 1% of a solar mass. The complex structure of the Nebula itself is the subject of intense investigation. Image Credits: Top, J. P. Harrington (U. Maryland) & K. J. Borkowski (NCSU) HST, NASA) ; Middle, X-ray: UIUC/Y.Chu et al., Optical: HST, NASA); Bottom, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA), NASA)

Composite GALEX/HST image of the central portion of the Cat's Eye Nebula, NGC 6543
HST image of the central portion of the Cat's Eye Nebula, NGC 6543

Below, Abell 21, an "old" planetary nebula in Gemini (RA 07 29 03, Dec +13 14 48). Also known as the Medusa Nebula because of its braided structure, Abell 21 is approximately 1500 light years away and 4 light years in diameter. Ultraviolet radiation from the hot young white dwarf near the center of the crescent (the faint star below and to the right of the brighter pair of foreground stars) still causes the nebula to fluoresce, even as it dissipates into interstellar space; but being nearly 10 arcmin across and very faint (16th magnitude), it is difficult to see, save with long-time telescopic exposures, or the aid of filters which block all light save that radiated by the nebula. The image was taken with a 24-inch telescope under dark sky conditions.
Wikimedia Commons image of planetary nebula Abell 21, also known as the Medusa Nebula
(Image credit:
Joseph D. Schulman)

At the center of the image below is the variable star LL Pegasi (also known as AFGL 3068) (RA 23 19 12, Dec +17 11 35), a binary star consisting of a carbon star (that is, a star with strong absorption features due to carbon atoms in its atmosphere) and a hotter, bluer star of uncertain type. The pair are hidden by thick clouds of dust and gas ejected by the cooler star (the nebular structure is referred to as IRAS 23166+1655), as the star (presumably) nears the end of its life. In other words, the star is thought to be in a pre-planetary nebular stage. Theoretical calculations indicate that in such binary systems, gravitational interaction of the companion with gases ejected by the dying star can create spiral structures, and LL Pegasi demonstrates the most spectacular example of such a structure, consisting of nearly 5 complete arcs around the star. The two stars are thought to be about 110 AUs apart, and orbiting each other once every 800 years or so (the distance and period are dependent upon estimates of the distance of the system, which is thought to be about 3000 light years, but is not at all certain). The orbits are presumed to be relatively eccentric, so that periastron passages involve considerably greater gravitational interaction than those which exist during other parts of the orbital motion. The connection between the orbital interaction and the spiral structure is thought to be confirmed by the rate at which the gas in the spiral structure is expanding (about 30 thousand miles per hour) and the distance between the spiral arcs; these suggest a periodicity of 800 years in forming the arcs, in agreement with the probable orbital period. The temperature of the spiral structure is only a few hundred degrees, far too low to create significant visible radiation at even a fraction of the distance to the system. As a result, even in this near infrared image, most of the light by which the structure is seen is believed to be the light of the Milky Way itself, reflected off the structure. For comparison purposes, the "bright" star to the north (above) the spiral is a 13th-magnitude object, and the many stars and galaxies seen in this image are barely visible in most views of the region, as shown in the comparison image, below. Image Credit: ESA, Hubble, R. Sahai (JPL), NASA
HST infrared image of pre-planetary binary star LL Pegasi
Above, an HST image of LL Pegasi and its spiral shroud (with North to the upper right)
Below, a 3 arcmin wide region centered on the system (with North at the top)
SDSS image of region near pre-planetary binary star LL Pegasi

The Formation of Planetary Nebulae
     Toward the end of its red giant lifetime, the energy production in the Sun's hydrogen burning shell will become less and less stable. While it is a Main Sequence star, energy production is in the core of a star like the Sun, and any excess or deficit of energy production (compared to the outward flow of energy), will cause an expansion or contraction of the energy-producing layers, which almost instantly counteracts the original imbalance. As a result, Main Sequence stars are very nearly constant in brightness. But in red giants, the main energy production is not in the core, but in the hydrogen burning shell surrounding the core, and that region's density and temperature are not self-controlled, but externally controlled. As the helium/carbon core contracts, the temperature and density in the hydrogen burning shell increase, increasing its luminosity and forcing the outer layers of the star to expand. But this decreases the weight of those layers, and as the force compressing the shell decreases, the temperature, density and luminosity of the shell decrease. This causes the outer layers to move toward the center, increasing their weight, once again forcing the shell to higher temperature, density and luminosity, and the outer layers to move away from the center again.
     For quite some time, the star may be caught in this semi-stable balance, swinging between smaller fainter states, and larger brighter states, on a more or less regular basis. Stars such as Betelgeuse, which is within twenty to fifty thousand years of the end of its red giant lifetime, wax and wane in size and brightness over periods of a few hundred days, while their hydrogen burning shells suffer alternate compression and expansion. Sooner or later, however, things must go very badly awry, unless the star is much more massive than the Sun. This can happen in several ways, but a common situation is that the core of the star becomes partially degenerate and slows its contraction as it begins to act more like a liquid, and less like a gas. This reduces the rate at which the hydrogen burning shell is contracting, which should reduce its brightness, and allow the outer layers to move inwards. If, as the core's contraction slowed, the outer layers pulled back at exactly the right rate, the star could shrink to a totally degenerate state, without any untoward events. But in all such stars, the energy production in the hydrogen burning shell is extremely sensitive to temperature, and if the star contracts even a little too fast, the temperature in the shell will suddenly soar, the energy production will go through the roof, and a substantial portion of the star's outer layers will be violently ejected into space, forming a planetary nebula, such as the one shown below. And in fact, it is virtually inevitable that some such catastrophe will occur at the end the star's red giant lifetime.


A false-color image (as usual) of planetary nebula
NGC 2440 and its white dwarf.
(H. Bond (STScI), R. Ciardullo (PSU), WFPC2, HST, NASA (post-processing by Forrest Hamilton), apod060507)

White Dwarfs
     After the outer layers of the star are ejected, the mass of the remaining material is usually too small to provide enough pressure to maintain nuclear temperatures in the hydrogen burning shell, and the remnant star rapidly contracts to form a white dwarf -- a star with a mass similar to the Sun, but a diameter similar to that of the Earth -- such as the one seen nearn the center of the planetary nebula above. This particular white dwarf has one of the highest surface temperatures ever observed -- more than 200,000 degrees Celsius, or over 30 times hotter than the Sun. Old white dwarfs, which have had time to cool to more moderate temperatures, are thousands of times fainter than the Sun; but in recently formed white dwarfs, such as the one above, the high temperature more than makes up for their small size, and this particular white dwarf is actually 250 times brighter than the Sun.
     The first white dwarf discovered was Sirius B, the companion to Sirius A, the brightest star in the sky (more because at 8.6 light years distance it is one of the closest stars to the Sun, than because it is especially bright compared to other stars). Sirius A is a two-solar mass star with a brightness about 25 times that of the Sun, a diameter about 1.75 times that of the Sun (almost 200 Earth diameters), and a surface temperature of nearly 10,000 Kelvins (about 18,000 Fahrenheit degrees). Sirius B is ten thousand times fainter than Sirius A, despite being much hotter (almost 25,000 Kelvins, or 45,000 Fahrenheit degrees), which should make it nearly forty times brighter per square foot. The reason for its four-hundred-thousand times less than expected brightness is because it is much smaller -- in fact, only 90% the size of the Earth. Despite its small size, Sirius B is heavier than the Sun (!), implying that it must have an average density more than a million times greater than that of the Sun, or nearly 1.7 tonnes per cubic centimeter, or more than 25 tonnes per cubic inch. With such a mass, and such a small size, its surface gravity is nearly 400,000 times greater than that of the Earth, so its material would weight 25 tonnes per cubic inch times 400,000 Earth gravities, or about ten million tonnes per cubic inch.
     No normal material can possibly have a density this high, as it would require over ten trillion atoms to be stuffed into the space normally occupied by a single atom. Only a plasma, a gas consisting on nothing but pieces of atoms -- bare atomic nuclei and free electorns -- can even begin to approach such densities, and even in such materials, such high densities cause the electrons to seem to occupy all the free space inside the star, so that pushing them closer together would produce infinitely large repulsive forces, referred to as degeneracy forces. As a result, the material inside a white dwarf is referred to as electron-degenerate material, and we sometimes refer to white dwarfs as electron-degenerate stars.

     A NASA artwork, purporting to represent the relative size and appearance of Sirius A and B. At ten thousand Kelvins, Sirius A is white hot, while at twenty-five thousand Kelvins, Sirius B is beyond white-hot, giving off more blue light than other visible wavelengths, and far more ultraviolet radiation than visible light (even Sirius A has the peak of its radiation well into the ultraviolet region). Sirius B gives off 40 times more radiation per square foot than Sirius B, but is nearly two hundred times smaller, so that Sirius A is ten thousand times brighter. The relative sizes of the two stars are not shown to scale. If they were, Sirius B would be a dot too small to see; instead, the intensity of its radiation is simulated by showing it larger than it would actually appear, if its disk could be imaged. (G. Bacon, ESA, NASA, HubbleSite)