This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Planetary Nebulas and White Dwarfs

by Dr Jamie Love Creative Commons Licence 1997 - 2011

When a red giant dies it does so in a massive explosion but it leaves behind two things - a white dwarf and a planetary nebula.

The planetary nebula is made of all the materials that were in the envelope of the red giant, plus anything that was in the way - such as planets! A red giant of one solar mass (the mass of the Sun) will lose about 40% of its materials in the planetary nebula. More massive stars will lose a greater percentage to the nebula.

These materials are usually pushed away from the star as an expanding sphere, moving away in all direction, but when we look at it we tend to see it as ring around the white dwarf because of the line-of-sight effect. A view along the edge of the sphere displays its densest portion, from our perspective, and that is the part we can see best. This material was called a "planetary nebula" because, when they were first discovered, astronomers thought they were nebula that go on to form planets. They were wrong (sort of, see below) but the name has stuck. You should understand that planetary nebulas have nothing to do with planets (directly). You should also understand that these special nebulas are the result of a dying star and have nothing to do with the star-making nebula you learned about last month. (At least, not directly.)
Sometimes the material is jetted away in two opposite directions, probably due to the star's magnetic field. Other planetary nebulas are irregular in shape for reasons we don't quite understand.

When it is first ejected from the dying star, the material consists of extremely hot atomic and molecular gases but these soon cool and coalesce to form warm dust particles. The nebula continues to move away at speeds of around 20 kilometers per second! It's temporarily illuminated by the white dwarf if leaves behind.

At first this illumination is very bright because it's re-emitted light. Materials in the nebula are excited and ionized by the energy from the white dwarf. These excited dust particles quickly release that energy, some of it at visible wavelengths (like a fluorescent light) but they can also radiate at lower energies (radio and infrared) or higher energies (x-rays). This is an emission planetary nebula and analysis of its emission spectrum allows us to determine its composition.

As time goes by the nebula drifts further away from its star and the illumination is only by reflected light. Therefore, it is dimmer and often has a bluish color because blue light is scattered, and therefore reflected, more than red.

Eventually, after about 10,000 or 100,000 years, the planetary nebula has moved so far away, say half a light-year, that it can no longer be illuminated by its white dwarf so the planetary nebula simply disappears.
However, some of this material will lie along our line-of-sight and obscure our view of any stars behind the nebula. These dark nebula are scattered throughout our galaxy (and probably other galaxies too). They are rich in fairly heavy elements, such as oxygen and carbon, made inside the red giant before it blew up.

Speaking of things you can't see, I should mention protoplanetary nebulas. These occur BEFORE the red giant goes "bang". During the last 10,000 years or so of a red giant's life it undergoes a great deal of pulsing. (Remember?) Some of these pulses can be strong enough to push materials off the star's tenuous surface and out into space. The red giant has a very cool surface temperature so it's unable to excite these materials. If these nebulas are seen at all it is by reflected light so they are not very bright. Indeed, the protoplanetary nebula can be so thick that its dust will obscure the light of the dying red giant. That means the dimming of a red giant might be an indication that it is about to blow in a few thousand years.
Fortunately, this material is warmed by the red giant so it will emit some infrared energy which can be detected by orbiting telescopes. Since the 1970s over 2000 "mid-infrared stars" have been identified in our galaxy.

Over many years planetary nebula loose their ring or sphere shape (assuming they had that shape to begin with) and may even merge with other nebula, produced by other red giants. Gravity causes these nebulas to coalesce into spheres and rotating disks which will go on to eventually form new stars and planets.

Hey, I thought you said planets don't come from planetary nebula!

I know what I said. (Gimme a break, will ya'! )
Planetary nebulas, the rings seen around white dwarfs, do not DIRECTLY go on to form planets - as astronomers once thought. However, long after the planetary nebula has disappeared and mingled with other nebula, these materials can contribute to the formation of a new star and even help to create new planets orbiting that new star. I think it's pretty cool that most of the carbon and oxygen that makes most of my body was created in a red giant that blew up billions of years ago! As Carl Sagan once said, "We are made of star-stuff."
We'll come back to the topic of planet formation in August.

The "corpse" that remains behind is very small and dim with a very hot surface. When the "heart" of the red giant is first exposed it has a temperature as high as 70,000oC (or oK) but it cools down over time. Most of the energy is given off in the ultraviolet (and higher energy) parts of the spectrum that we cannot see. The overall effect of this large amount of energy is that we tend to see this core as white in color (see below). Because it's white and dim this star is called a white dwarf and it will be found in the lower left portion of the H-R diagram. The H-R diagram places them at a temperature of about 11,000oC but that's only the energy (temperature) of the white portion of the spectrum that we can see. Think back to your lesson about colors and the Maxwell-Boltzmann distribution. Then you will understand that the white-hot color of a white giant is only a small portion of its total energy output.

White dwarfs make up about 9% of the stars on the H-R diagram.

All that remains of the star is its core of carbon (and a few other light elements like oxygen, but mostly it's a core of carbon). There is no hydrogen left - it has all been "burned" in nuclear fusion or drifted away with the planetary nebula. Now that its envelope and shell have gone, the core is not under any pressure from the surface. Therefore, this star has no nuclear fusion.

But if there's no fusion, what causes a white dwarf to shine (and illuminate the planetary nebula)?

Good question.

All of the light and heat are the energy left over from the star's earlier fusion history. This isn't too difficult to imagine. The core has been heated by nuclear fusion for billions of years and it retained most of that heat because it was covered with a shell and layers. The core of a star is always much hotter than its surface. Once it has shaken off its covering, the naked core radiates its old heat into space.
By analogue, once a human dies the body no longer produces heat but the body will remain warm for several hours. This analogue breaks down when you understand that the "dead" white dwarf is actually much hotter than the "live" red giant. That's because the red giant's many layers were insulating its center and once that insulation is gone the white dwarf left behind shows its true temperature - and color!

Recall that most of the energy released from a white dwarf is in the ultraviolet, so it's hotter than a blue star. We can't see ultraviolet light so you might think white dwarfs would be invisible. Fortunately they are not. Some of the ultraviolet light is absorbed and cooled by the star's local environment and re-emitted as white light. Also, some of the energy sneaks into our visible part of the spectrum due to the redistribution of energy. (Remember the Maxwell-Boltzmann distribution?) That's why white dwarfs are white.

White dwarfs are dwarfs because of their luminosity as well as their size. They are very hot but they are also very dim. One reason white dwarfs appear so dim is because most of the energy is radiated in the ultraviolet, so we see only a tiny portion of the true energy output. (Right? It bears repeating. ) Another reason white dwarfs are so dim is because they are so small. A star's size is normally a compromise between its energy output (trying to increase its size) and its gravity (trying to make it small). Now that the nuclear furnace has died, a white dwarf is stuck with its own gravity and that causes the star to shrink to a very tiny size.

Like all stars, the exact size of a white dwarf depends upon its original mass. (Which also determines its position on the Main Sequence and the size of its red giant, right? It bears repeating. ) Once our Sun has passed through the red giant stage it will shed about 40% of its mass producing its planetary nebula. The remaining 60% of the Sun's mass will be left as the white dwarf mass but then it will have only its own gravity to determine its size. (While nuclear fusion was going on the upwelling of energy and materials caused the star to puff up into a big giant.) The tremendous force of gravity will crush all the remaining material to the size of the Earth! That's right - more than half the mass of the Sun squeezed into a sphere the size of the Earth. That's a lot of material to squeeze into a tiny sphere and all white dwarfs are about Earth-size.
Therefore, all white dwarfs are VERY dense. A teaspoon of white dwarf material weighs a ton - literally! (That's about a thousand kilograms per cubic centimeter.) It's impossible to squeeze atoms that tightly without destroying their atomic structure. Every atom in the white dwarf is transformed into an unusual state of matter called degenerate matter - a tight mass of nuclei and electrons with no real atomic structure. There's nothing on Earth like it!

That small size gives the white dwarf special features. The heat inside the star has only a limited surface area from which the heat can escape. That helps to give the star a very hot surface. Also, the small surface area means there is less opportunity for the heat to escape, so it takes a long time to cool. The small size and high heat in an average white dwarf cause it to last about a billion years - maybe longer. By then it has cooled down so much that it stops emitting any light at all and it becomes a black dwarf. It's still made of degenerate matter and has a powerful gravitational field, but it's dark. You won't find black dwarfs on the H-R Diagram for the same reason you won't find protostars - they don't give off any light. They are black like a nebula.

Cosmologists (people who study the origin, size and age of the universe) believe that white dwarfs will vary little in size and luminosity and hope to use white dwarfs to measure the distances to far off galaxies. (This has to do with a strategy called "standard candles" and we will come back to this topic in November.)

This and the previous lesson have taught you about the aging and death of stars. Let's review this important part of astronomy and put things in perspective.

You should understand that red giants and white dwarfs no longer fuse hydrogen in their core and that's why they are off of the Main Sequence. Red giants are cool (large, fluffy) and bright so they are in the upper right of the H-R diagram. They fuse hydrogen nuclei in the layers outside the core, and any nuclear fusion in the core of a red giant involves helium, not hydrogen. White dwarfs have no nuclear fusion at all - they are "dead". We still call them "stars" because they shine, but they are just shedding "old energy". In fact, they are so hot that white dwarfs are to the left of the Main Sequence but they are also dim so that they are positioned below the Main Sequence. That puts white dwarfs in the lower left side of the H-R diagram. You should also remember that white dwarfs are made of degenerate matter. Next month I will teach you about some other exotic objects made of degenerate matter.

All stars start "life" on the Main sequence as ZAMS stars.
Because 90% of stars are on the Main Sequence, astronomers figure that 90% of a star's life is spent on the Main Sequence. That same sort of logic causes them to conclude that a star is a red giant for about 1% of its life and then it becomes a white dwarf for the remaining 9% of its life. The H-R diagram is a chart of stellar evolution and the distribution of the stars among the three groups (Main Sequence, red giants and white dwarfs) reflects their aging time.

Our Sun is of average size and has a place near the middle of the Main Sequence. It started as a ZAMS star about 5 billion years ago, point 0 on the H-R diagram below, and has been on the Main Sequence ever since. We are now at point 1.

The Sun will continue to be a member of the Main Sequence for another 5 billion years (2). That prediction is based upon calculations of how fast the Sun is currently using its fuel and how much it has left. During that time (from point 0 to point 2) the Sun's core will fuse hydrogen into helium. Then it will contract causing hydrogen fusion to occur in the shell around the core and increase the size of its envelope as it grows into a red giant (point 3). Eventually, a tremendous helium flash will occur (point 4) as helium fusion begins in the core. The products of helium core fusion - oxygen and especially carbon - will build up in the core and the Sun's energy output will decrease due to the ever worsening inefficiency of helium fusion in its core. So the Sun will drift to point 5. Eventually helium (and remaining hydrogen) fusion will move into the shell, surrounding the core, and produce bursts of energy as helium shell flashes increase the luminosity of the Sun.

Finally, at point 6, a superwind will blow away the Sun's envelope as a bright, hot gas (7). When the Sun's planetary nebula dissipates its hot core, made of degenerate matter (mostly degenerate carbon), will be revealed and our Sun will retire into the white dwarf population (8) where it will cool off for a long time before becoming a black dwarf (9).
Our Sun's 10 billion years on the Main Sequence is very long compared to the 1% of time (100 million years) it will be red giant.

Small stars, like Wolf 339, will go through the same history but at a much slower rate because their smaller mass causes them to use their fuel more slowly. It may take a trillion years for Wolf 339 to become a white dwarf. Regardless, we assume that 90% of its lifetime will be spent on the Main Sequence, 1% will be spent as a very small red giant and it will then end up a very small white dwarf.

Big stars burn their fuel quicker, leave the Main Sequence quicker and they become red supergiants. If you think red supergiants become big white dwarfs you are wrong (!) but don't feel bad. Large stars not only live fast and die young, but they also have an amazing "exit" that you will learn about in next month's lesson.

You can continue on and learn the objects currently in the eastern evening sky, including where to find a planetary nebula! Otherwise, take a break and come back after you're refreshed.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.