Lecture 23: The Post-Main-Sequence Life of Stars

Don't Panic

``There's husbandry in heaven; Their candles are all out.''
Banquo in MacBeth, Act II, Scene 1 (William Shakespeare)

Sections

1. A Brief Introduction to Galaxies
2. The History of the Discovery of Galaxies before Hubble
3. Hubble

---

Introduction

---

---

The Fate of the Sun

---

When the hydrogen in the core of the Sun becomes exhausted, the main sequence life ends in about 6 Gyr (FK-493). Then there is a rather complex evolution---to say the least.
1. A hydrogen burning shell will still exist around the core.

2. The Sun's core continue contracting because of thermal energy loss, but it will get hotter because of the contraction.

3. The energy generation rate will actually increase because of the hotter core.

4. The Sun's will then expand into a RED GIANT with a radius of order 1 AU because of the extra heating (Se-249; FK-470).
   [Red giantism is something we understand in a computer modeling sense. We put the right ingredients into a computer model and they show that after the core hydrogen burning is over stars should expand into red giants. But there is no short explanation of the process.]

5. The Sun will then be much brighter than it is now, but its photosphere temperature will be lower eventually reaching about 3000 K (Ze2002-341).

6. By Wien's law, the peak wavelength of it's emission will then be

   
         wavelength_max = about 3000 micron-K / 3000 K  = 1 micron
   
         and thus in the infrared.
        

7. The Sun in the visible will be red: hence the name RED GIANT.

8. When the helium core's temperature reaches about 100*10**6 K, the HELIUM will suddenly start burning to CARBON and OXYGEN pretty much everywhere in the core within perhaps a few minutes (Ni-177; Ze2002-343).

9. This is the HELIUM FLASH. We never see this event because it is always buried in the core and lasts only seconds---but theoretically it must happens it seems (FK-473).

   HELIUM FLASHES happen only to stars in the 0.4--2 or 3 solar mass range (Se-251; FK-472).

10. Post-helium flash, the Sun will then contract and become a bit less bright oddly enough. Essentially this is because there is again nuclear burning in the core, but now its helium burning.

11. At this stage the Sun is called a HORIZONTAL BRANCH STAR. (Shu-152;. Shu-252).

    The Sun from Main Sequence to the Horizontal Branch.

12. But the helium burns much more quickly than hydrogen.

    So the helium exhausts in the core and then there is an non-burning carbon-oxygen core, surrounded by a helium-burning shell surrounded by a hydrogen-burning shell.

13. The Sun then puffs up to be a RED GIANT again---but only for awhile.
    [This second red giant phase is called the asymptotic giant branch (AGB) phase (Shu-151), but I'm trying to resist loading you down with astro-jargon---but I still might use the asymptotic giant branch (AGB) phase on a test.]

14. The helium burning is unstable and of order every 100,000 years has a runaway burst of burning called a THERMAL PULSE (FK-493).

    Each THERMAL PULSE causes a superwind that blows off much of the the Sun's envelope.

15. The escaping material forms a complex expanding nebula called a PLANETARY NEBULA---the name is a total misnomer since these nebulae have nothing to do with planets.
    [The name planetary nebula originated in the 18th because the closest, most obvious ones are large enough to have a finite disk in a telescope and there greenish color made them look sort of like Uranus (FK-494; CK-329).]

    The Sun from the 2nd red giant phase to white dwarfdom.

    Planetary nebula atoms are excited by UV radiation from the remnant star---which is very hot on the surface---and emit copious LINE SPECTRA.

    Green lines from doubly ionized oxygen are often particularly noticeable (Se-268).

    PLANETARY NEBULAE have huge range of behavior---which we won't go into here, but we will show some pictures.

    ---

    The Ring Nebula from HST. The best known of all planetaries.

    The Ring Nebula is about 2000 lyr away in constellation Lyra. The ring is about 1 lyr in diameter. The green color is due to green forbidden lines of O III (i.e., twice ionized oxygen). The blue is from helium and the red color is from ionized nitrogen.

    Credit: NASA: Imagine the Universe.

    ---

    Planetary nebula Cat's Eye from HST.

    The Cat's Eye is about 3000 lyr away in constellation Draco. It is one of the most complex of planetary nebula and is estimated to be only about 1000 years old. The green color is due to green forbidden lines of O III (i.e., twice ionized oxygen).

    Credit: NASA: Imagine the Universe.

    ---

    The PLANETARY NEBULAE disperse in the interstellar medium after a few thousands to tens of thousands of years.

    After some thermal pulses, the remnant Sun will have lost most of its hydrogen and helium and perhaps about half its mass and shrunk to a size of order the size of the EARTH.

16. The Sun is then a WHITE DWARF.

    The time from becoming a 2nd red giant (AGB star) to being a WHITE DWARF is about 0.7 Myr (FK-493).

The SOLAR WHITE DWARF will be mostly CARBON and OXYGEN from helium burning. The Sun cannot burn carbon and oxygen: it's core never gets hot and dense enough.

Stars that start out more massive than about 4 solar masses can burn carbon and oxygen (FK-497), but if they too end as white dwarfs if they arn't massive enough to become supernovae: i.e., have masses greater than about 8 solar masses.

But although the SOLAR WHITE DWARF will be mostly carbon and oxygen, the lighter helium and hydrogen atoms float on the top and form a skin.

WHITE DWARFS are not held up by ordinary gas and EMR pressure, but by a quantum mechanical pressure of degenerate electrons that is NOT very temperature dependent.

WHITE DWARFS are initially very hot with a surface temperature of order 30,000 to 100,000 K (Ni-178).

Initially, a WHITE DWARF'S EMR peaks in the UV and in the visible they appear white: hence the ``white'' in WHITE DWARF.

But they arn't very bright because of their small size.

But WHITE DWARFS have no energy sources.

They can only lose thermal energy and cool off forever.

Eventually they will drop to such low temperatures that they only radiate in the microwave or radio. They will then be called BLACK DWARFS.

But their cooling time is very long---many billions of years. Since the Big Bang, no WHITE DWARF has cooled off to become a BLACK DWARF it is thought.

The coldest existing WHITE DWARFS have surface temperatures of about 6000 K????.

---

Caveat: the post-main-sequence evolution of stars is fairly well understood in qualitatively from a combination of observations and theory.

But quantitative predictions are difficult. The three-dimensional hydrodynamic computations required test the limits of our modern computer codes.

We can only tell the best story we can given present understanding.

---

The Fate of the Earth

---

What happens to the EARTH when the SUN dies?

Well its a bit uncertain. It depends on how large Sun gets in its red giant phases.

If the Sun's radius is always less than 1 AU, the Earth might survive as a cold rock for many billions of years after the Sun becomes a white dwarf. It's orbit will have to change somehow as the Sun loses mass.

If the Sun exceeds 1 AU as some sources claim (Ze2002-344), then the Earth will be enveloped by the Sun.

Once inside the Sun's atmosphere, there will be drag force that will cause the Earth to spiral into deeper layers and there it will be evaporated. The time scale after envelopment is about 200 years (Ze2002-344).

`` So the glory of this world passes away: sic transit gloria mundi.''