Introduction to Astronomy

Q1: SunQ2: windsQ3: ionization

Stellar Death Match I: Planetary Nebulae


Now, all along, I've been saying that there are two ways for stars to die. The first is by supernova (a huge explosion!), which happens to stars with masses greater than 8 times the mass of the sun. Stars less massive than 8 solar masses die in another way entirely. These stars spend a long time in the upper half of the H-R diagram, and a lot happens to them while they are there. Once the star finishes all of the hydrogen in it's core, it cools, and swells a little bit, moving up to the Red Giant part of the H-R Diagram.

As the core collapses, the helium 'ash' left behind from burning hydrogen eventually ignites. At about the same time, a shell of hydrogen also ignites. This is the "core helium burning phase", because it's the phase during which helium is burning in the core.

Here's a picture of a star in this stage. Notice how much simpler it is than the high mass star---there are only three layers here because the pressure and the density inside never get high enough to fuse carbon or oxygen into other elements.

These stars move around quite a bit at the top of the HR diagram, as you can see:

Eventually, the star runs out of He in the core. It moves back across the Horizontal Branch as the temperature falls, and back into the Red Giant part of the H-R diagram.

  1. Now you have a lump of carbon, left over from burning helium in the core, surrounded by a burning hydrogen shell.
  2. He builds up in a ring around the (mostly) Carbon core, until there's enough of it at high enough pressure to ignite. It burns, puffing up the star (two sources of energy inside), then stops burning as the pressure drops.
  3. He builds, then burns, then builds, then burns, in a series of thermal pulses. So the star doesn't smoothly increase in luminosity, but instead, takes two steps forward and one step back. In massive stars, these pulses happen deep inside, and don't have much effect on the external luminosity. However, in lower-mass stars (of order 1 MSun), the luminosity will increase by about a factor of 10 for 100 years every 100,000 years. (did you get that? better read it again).
  4. The mass of the core increases as this goes on.
  5. A wind develops, and the mass losing phase begins: the star begins to lose mass at a phenomenal rate (as much as 10-5 solar masses per year. At this rate, the entire sun would disappear in only 10,000 years. (an astronomically short period of time!) What drives the mass loss? i.e. what powers the wind? We don't know, but I'm working on it.
  6. This mass lost moves away from the star, and forms a shell around it. Theory predicts that the shell should be spherical, but usually it's not. We don't know why, but I'm working on it!
  7. As the mass leaves the star, it exposes hotter and hotter depths. The central star gets bluer and bluer, until it is emitting in mostly the UV. This UV light ionizes the dust and gas that was lost, which then glows.
  8. The star winds up as a "planetary nebula" surrounding a white dwarf.
  9. The nebula gradually spreads out, getting thinner and thinner until you can't tell where it stops and where the ordinary stuff between the stars begins.
One of the outstanding questions in planetary nebula astronomy today is 'How do they get their freaky shapes?', which is a great excuse to show some pretty pictures!

Abell 39 looks like something that might have come from a round star:

And for a long time, we thought this was what they all looked like. Several close one, like the Dumbbell Nebula:

made us think they were all round-ish.

But then, after Hubble Space Telescope was launched, we began to get detailed pictures of nebulae that looked round from the ground. For example, the Eskimo nebula:

and the Stingray Nebula:

while not quite exactly round, are sort of round. These can be explained away by small deviations of the central star---maybe it had spots or planets or something that broke the symmetry of the mass loss. But then, you see pictures of the Egg nebula, M2-9, or craziest of all, Mycn18:

and you suddenly realize that we HAVE NO IDEA what's going on! At the moment, the top two explanations are

a) Planets around these stars make the mass loss 'funny'.

b) Magnetic fields around these stars make a disk around the equator of the star that in turn, makes the mass loss 'funny'.

Recent discoveries of massive, close-in extra-solar planets made (a) the favorite for a while, in spite of the fact that the models are not as good at creating REALLY wacky shapes as one might like. Even more recently, magnetic fields have been discovered in mycn18, lending support to explanation (b). The jury is still out though, and astronomers meet every couple of years to discuss the problem, and try to figure out the answer. Eventually, we'll get it figured out!

Concept Question 1:
The sun will die as a planetary nebula because

  1. it has only one solar mass.
  2. it is small enough.
  3. it is faint enough.
  4. it won't! The Sun will die as a supernova.
Concept Question 2:
The winds from evolved stars are started by

  1. Radiation pressure.
  2. Gravitational contraction of the core.
  3. Line-driving.
  4. We don't really know yet.
Concept Question 3:
Planetary nebulae suddenly become visible when they are exposed to lots of UV radiation from the core. This happens suddenly because

  1. the star gains mass, and thus becomes hotter.
  2. deeper and hotter layers of the star are exposed as mass is lost.
  3. the star begins burning carbon, and thus becomes hotter.
  4. We don't really know yet.