Tuesday October 21
As the core of a star heats up during the M-S phase, it can actually ignite hydrogen burning in a shell surrounding the core. When the core has used up its hydrogen, the only energy generation occuring in the star is in this hydrogen burning shell.
Now, as the core isn't generating energy anymore, it is no longer supported against gravitational collapse. So it collapses.
As it collapses, it's density and pressure increase, so the core gets hotter. Thus it actually releases MORE energy than it did when it was burning hydrogen. But it is gravitational energy, rather than nuclear energy.
This increased energy output from the collapsing core plus the H-burning shell causes the envelope of the star to expand. It expands, decreasing its pressure and density. So it cools off.
The star becomes cooler and more luminous than it was on the main sequence. That is, it turns into a Red Giant.
Red Giant atmospheres are much less dense than those of Main Sequence stars, and are much more tenuously bound to the star. This means the mass-loss rates for Red Giants are a lot higher than for M-S stars.
Their cores are nearly pure Helium, very dense, very hot, and continuing to collapse due to their self-gravity. The core will continue to heat up until it becomes hot enough for helium fusion to occur. The details of this were first worked out by Fred Hoyle in the 1950s. Because the net reaction involves fusing three helium nuclei (also known as alpha particles), this set of reactions is also known as the "triple alpha process":
Once there is a significant amount of Carbon, the core also begins generating Oxygen:
When core helium burning begins, the core has a new source of internal pressure to balance its self gravity, so the core stops collapsing. Thus the star stabilizes again. The star becomes hotter and less luminous than a Red Giant, and thus moves across the H-R diagram to the Horizontal Branch.
There's more to this story, but I'm going to put it off for the moment. Instead I shall illustrate all the above by looking at some models and observations of star clusters. One can use stellar evolution theory to generate model CMDs for star clusters from birth through the phase when most stars are on the Main Sequence to the point that the cluster is dominated by Red Giants.
Models of evolving star clusters compare well with the observations of actual clusters, ranging from very young clusters such as NGC 2264 that only have their massive stars on the Main-Sequence to clusters like the Pleiades that have fully populated Main Sequences, and evolving massive stars, to old open clusters such as M67, to very old globular clusters such as M13 , where only stars less massive than the Sun remain on the Main Sequence.
Recall that stars in a cluster are all the same age and all at the same distance. This means one can use color-magnitude diagrams to study their ages, even if you don't know their actual distances. The key observable for age-dating a cluster is its Main Sequence Turn Off. Stars at the Turn-Off are currently exhausting their core hydrogen supply. The oldest clusters (the globular clusters) have turn-offs at about 0.8 Solar masses. This indicates ages of about 12 Gyr.
Now, let's step back from this for a moment to review the evolution of stars in various mass-ranges, starting with low-mass stars (less than 0.8 Solar Masses or so). Such stars consume their fuel very slowly, due to their relatively low core temperatures. They are also fully convective, so the entire stellar mass is available for hydrogen fusion. As a result, they will have VERY long lives on the main sequence. A trillion years or so, in rough numbers. This is well in excess of the age of the Universe, so we don't expect (NOR do we SEE) any stars of this mass that have evolved off the main sequence.
By "Intermediate-Mass" I mean in the range 0.8 up to about 8 Solar masses. Such stars will follow the evolutionary course outlined above, from the Main Sequence (core Hydrogen burning) through the Red-Giant Branch (Hydrogen shell burning), the Horizontal Branch (core Helium burning), and the Asymptotic Giant Branch (Helium shell burning). That's as far as they go. They are not massive enough for their cores to ever reach the tempertures and densities required for Carbon/Oxygen fusion. For such stars, the HB phase lasts about 1% of the M-S lifetime. So a star like the Sun will have an HB lifetime of about 100 million years. The HB phase ends when the core has converted all its He into C and O.
At this point, the core begins collapsing again, and the envelope expands and cools. The same basic physics applies here as when the star initially turned into a Red Giant. And the basic result is also the same. Now the star can become even larger and more luminous than on its first ascent. It is now called an Asymptotic Giant Branch (or AGB) star. The internal structure of such stars is a series of shells of different elements, inside an enormous Hydrogen envelope.
In intermediate-mass stars, the core continues to collapse and heat up until the onset of electron degeneracy pressure. This is a quantum mechanical effect that we won't dwell on the details of. Essentially, the electron density becomes so high that the electrons are packed together as tightly as they can be. We shall consider such cores in more detail shortly. The key point at present is that electron degeneracy pressure will support the core of the star against self-gravity before the core becomes hot enough for the carbon and oxygen to fuse to heavier elements.
Thursday October 23
The envelopes of both RGB and AGB stars are mostly neutral, even quite deep in the star. This means that the star is convective throughout (recall that only the outer 30% or so of the Sun is convective). A result of this is that material that has been processed via nuclear burning is convected up to the surface. This process is known as Dredge Up.
This process alters the amounts of CNO relative to one another, and generally increases them relative to Hydrogen. In AGB stars, dredge-up mainly increases the amount of carbon in the photosphere. Combined with the low surface temperatures of AGB stars, this leads to the formation of carbon-based molecules such as CO, CH, CN. The lines of these molecules are very pronounced in the red part of the spectrum of such Carbon Stars.
The atmospheres of AGB stars are only weakly bound to the star, and this leads to very high mass-loss rates:
Thus the dredged-up material gets dumped back into the ISM, increasing the relative abundance of carbon. This material can either be in the gas phase, or, more commonly, in the form of dust grains. Basically, soot. This is the main source of dust grains in the Galaxy.
This surface mass-loss isn't really a gradual, steady thing. It occurs in a series of thermal pulses. These pulses are due to runaway bursts of helium burning in the stellar interior. When the interior pulses, the energy eventually lifts a layer of the surface off the star, and ejects it into the ISM. This will continue until the entire envelope is ejected. A 1 Solar mass M-S star will end up ejecting something like 0.4 Solar masses of material in this manner.
At this point, the exposed surface of the star is, essentially, the stellar core. The surface is still extremely hot:
And the envelope is now diffuse extended gas, surrounding the star. Thus the gas becomes an emission-line nebula. Such objects are called Planetary Nebulae.
They were called "planetary nebulae" in the 1700s because they were resolved in the telescopes of the time, so they looked like disks, and they are generally green or blue to the eye, like Neptune or Uranus.
Although many planetary nebulae are ring-like, lots of other morphologies exist also.
The gas is expanding, and the central star is also cooling off rapidly. As a result, the Planetary Nebulae (or PN) phase is fairly short:
We know the gas expansion rate from the Doppler shifts of the emission lines. It is roughly 10-30 km/sec.
Most stars are less than 8 Solar masses, so most stars will go through the PN phase. This ends up putting a lot of recycled material back into the ISM. PN appear to account for about 15% of the material recycled into the ISM. This is a major source for elements like Carbon and Nitrogen.
Now, what about the Central Star? As mentioned above, the central star is now electron degenerate, so it is no longer collapsing, nor is it generating energy from fusion. So it simply cools off. Such an object is called a White Dwarf Star. These stars form a sequence in the H-R diagram that is roughly parallel to the Main Sequence, but shifted to much lower lumionosity at a given temperature. A Solar mass White Dwarf is about the same size as the Earth. This means a teaspoon of a White Dwarf would weigh several tons on the surface of the Earth.
These stars are very faint, so we can only see them fairly nearby. Given the number we see, there are an awful lot of them in the Galaxy.
Observations of Binary Stars with a WD as one of the components are the fundamental source for data on WD masses and radii.
All measured masses for White Dwarfs are less than 1.4 Solar masses. This is because a stellar core that is more massive than 1.4 Solar masses will overcome electron degeneracy pressure. This maximum mass was calculated by Subrahmanyan Chandrasekhar, and is thus called The Chandrasekhar Mass:
Stars with M-S masses greater than about 8 Solar masses.
Evolution to core helium burning is pretty much the same as for low-mass stars. But from that point, there are major differences. The key issue here is this:
So electron degeneracy will not halt the core collapse for such stars. Instead, they continue to undergo nuclear fusion, building up heavier and heavier elements in their core. As the core goes through this series of alternating fusion and collapse episodes, the envelope goes through stages of shell burning, and the star wanders around the high-luminosity part of the H-R diagram, as either a red or a blue Supergiant. The star is very extended, and therefore loses mass rapidly.
As a result of all the shell-burning, the star's interior takes on a sort of onion-like quality. This complex structure has an ending point, and that is reached when the core fuses into Fe.
Iron is a special element. It is the most stable nucleus. This means that it takes energy to either break iron down to lighter elements, or to fuse with iron to make heavier elements. In other words, once the core is Fe, no more energy can be generated by fusion.
That's all she wrote, folks.
The core collapses catastrophically, in a fraction of a second. When the core reaches nuclear density, the collapse is halted by Neutron Degeneracy Pressure. At this point, the stellar core is about 10-20 miles in diameter.
Material falling on the core from above runs into the now stable core, and bounces. This bounce creates a pressure wave that travels out through the envelope and blows the star apart. This event is called a Supernova
Supernovae eject about 90% of the stellar mass back into the ISM. The explosion is so energetic (the photon and mechanical energy released is comparable to the total energy the Sun will generate over its entire main-sequence lifetime) that nuclear reactions occur, creating all the elements from Fe up to U.
Supernovae are observed to occur. Several are included in both European and Chinese astronomical records over the last 1000 years. A more recent example is SN 1987A, which occured in the Large Magellanic Cloud (LMC). The LMC is a satellite of our Galaxy. SN 1987A was the most recent naked-eye SNe to occur (only visible from the Southern hemisphere). Supernovae are very luminous,
They are also fairly rare events. About one occurs in the Galaxy every century. Note that the last Galactic SNe was seen in 1604. But extinction due to dust limits how far we can see in the plane of the Galaxy.
These explosions remain bright for a fairly long time -- months to years. This is due both to the slow cooling of the hot gas, and to additional energy input through the decay of radioactive nuclei synthesized in the explosion itself.
Tuesday October 28
The relic blast waves from SNe can be seen for some tens of thousands of years after the explosions. These " Supernova Remnants" (SNRs) radiate across the E-M spectrum, from radio waves through x-rays.
The age of the SNRs can be estimated from the Doppler shifts (and proper motions) of the emission filaments. The oldest recognizable SNRs are tens of thousands of years old.
Everything I've had to say about stellar evolution has been in the context of single stars. But, as you may recall, most stars are in multiple star systems (binaries, triples, and so forth). Can this effect their evolution? Yes it can, and sometimes does so dramatically. The closer the two stars of a binary are to one another, the greater the effects on their evolution. If one considers the masses and seperations of the stars, one can construct a map of the gravitational potential of the system. The contours in the plot are lines of equal gravitational potential. If matter from either star gets to the point labelled L1, it can be transferred to the other star. In Close Binaries, the stars are near enough to one another than the more massive star (and thus the first to begin evolving into a Red Giant) can shed material onto the less massive star. The technical term is that the evolved star overflows its Roche lobes.
The transfer of mass between evolving stars in binary systems can substantially alter the evolutionary histories of both stars, and can lead to some really dramatic final results.
Novae are much less luminous than Supernovae:
Like Supernovae, novae brighten quickly, and fade slowly. The remnants of the nova outburst can be seen for months to years afterward.
Novae occur in binary systems in which one star is a White Dwarf. The WD accretes matter (hydrogen) from the companion onto its surface. The accreted matter is heated by falling onto the WD, and the hydrogen "flash-fuses" into helium.
In Novae, mass is accreting onto the WD at a fairly high rate. Because of this, the material cannot get rid of its heat efficiently, reaches fusion temperatures on the surface, and largely blown off into space. Given the mechanism, it turns out that a system can go Nova several times. The typical time between nova outbursts in such systems is at least many decades, and can be thousands of years.
But what if the mass accretion rate is slow enough for the heat to escape without a nova eruption? Then the mass of the WD will slowly increase until it reaches the Chandrasekhar limit. If the WD accretes enough mass to drive it over the Chandrasekhar limit (1.4 Solar masses), the star undergoes runaway Carbon burning, and explodes.
In other words, there are (at least) two types of Supernovae
The classification of the two types of Supernovae was originally based on optical spectra:
It was later realized that the two classes of spectra corresponded to two different mechanisms of supernova explosion. Exploding massive stars create Type II supernovae.The Type I SNe are further divided into Types Ia, Ib, and Ic. Types Ib and Ic appear to be due to exploding massive stars, like Type II SNe, but the progenitors of Types Ib and Ic are stars that managed to shed their entire Hydrogen envelope before exploding.
Type Ia SNe appear to be different beasts altogether. While Type II (and Ib and Ic) SNe are always associated with regions of recent star formation, Type Ia SNe can happen in any environment.
The current understanding is that Type Ia SNe are due to accrection onto WD stars in close binaries. If the WD accretes enough mass to drive it over the Chandrasekhar limit (1.4 Solar masses), the star undergoes runaway Carbon burning, and explodes.
Because there is no collapse to nuclear densities in Type Ia SNe, there is no neutrino burst from them. Thus, although the photon luminosities of Type Ia's is comparable to that of Type II's, the Total energy released (including neutrinos) is much less in Type Ia's.
It is possible to distinguish between Type Ia and Type II SNe just from their light curves. This means they can be distinguised at large distances, even if they are too faint for good spectroscopy.
A last comment about stellar evolution. This process, by which hydrogen is converted into heavier elements in stars, and then returned to the ISM via stellar mass loss (stellar winds, planetary nebula ejection, supernovae) is the means by which the heavy elements in our bodies were produced. The carbon, oxygen, and calcium in our bodies were made in stellar interiors. And it is via the process of stellar evolution that this material found its way back out into space to form our planet.
