Last month you learned how stars like our Sun will eventually become red giants and finally white dwarfs. However very big stars do not end up as white dwarfs. They have a much more exciting "death" and the "corpses" they leave behind are some of the most amazing things in the universe!
The big stars go through similar core/shell changes as the small stars but their extra gravity causes them to have higher temperatures and pressures. This causes big stars to undergo nuclear fusion more rapidly and that's why big stars are very luminous and age quickly. More importantly, the extra gravity and temperature allow big stars to fuse heavy elements into heavier elements. Remember, red giants produced from small or average mass stars cannot fuse atoms of more than a few protons in size. That's why "normal" red giants have cores of carbon (perhaps with a small amount of oxygen and nitrogen). Big stars, on the other hand can continue fusing carbon, oxygen and similar sized atoms to produce atoms as large as iron! (Iron has 26 protons and at least as many neutrons.) As big stars fuse larger atoms they radiate the extra energy and continue to shine.
Like any nuclear powered star, the fusion in the core is surrounded by thick layers of non-fusing materials. You will recall that these layers make up the radiative and convection zones. The final energy is released at the surface through the photosphere. I thought I'd remind you about these overlayers of the star because we spend so much time discussing the core that students often forget about the rest of the star! Remember, the core is only the center of the star. That's where all the "action" is, so it is easy to forget the rest. In my drawings I tend to only show the core, but try to remember that there is a lot of non-fusing material surrounding it.
Supergiants have a core made of a series of fusion-burning shells and, like all giants, they have a thick envelop above all that fusion.
You learned about the complex structure of red giants last
month and will recall that the deeper you go inside a star the hotter
it gets, the more pressure it has and the more fusion it can perform.
This causes red giants to have layered structures but the layering
is not as complex or as interesting as that of a red supergiant.
Supergiants' cores have many shells each one involved in a different kind of nuclear fusion. The outermost shell is the coolest and is where hydrogen fusion can continue as normal. The helium produced by the outer shell falls down to the shell below it. In this second shell the helium is fused into the other elements like carbon, neon and oxygen. These products fall into the next lower shell where they are fused into heavier elements like silicon and sulfur. These fall further towards the star's center and are fused into iron atoms. Iron falls to the core where it is stored away, not fusing and not contributing to the star's output. Supergiants accumulate a core of iron because iron is the largest atom that can be made that still produces energy from the fusion. | ![]() |
In many ways the core of a supergiant is a lot like a white dwarf.
They are both about the size of the Earth and one teaspoon of
material weighs a ton. A white dwarf and a supergiant's core do
not undergo fusion. Also, both a white dwarf and the core of a
supergiant are held up by degenerate electron pressure - that
weird quantum mechanical effect that can counter the squeezing
caused by the immense gravity. The only difference is that a white
dwarf has a degenerate core of carbon (and oxygen, etc.) while a supergiant
core is made of degenerate iron. It is this degenerate matter
that allows such a high density to be achieved. Normally a teaspoon
of iron would weigh a few grams but, because there are no electron
shells in degenerate matter, these iron nuclei can be squeezed closer
together.
[Remember, "normal" matter is made of "normal"
atoms that have electron shells surrounding each nucleus. The
electron shells give each atom a bigger volume so each atom takes
up more space due to the shells. The electrons and nuclei make
up a very small part of the atom's entire volume. Most of the
atom is really just empty space. Degenerate matter doesn't waste
space!]
As time goes by a supergiant accumulates a lot of degenerate
iron in its core and its shells run low on the smaller atoms.
With less and less fuel in the shells to "burn", a supergiant
starts to "flicker" because it can no longer maintain
a steady flow of nuclear energy from the various shells. This big star is about to die.
Each "flicker" causes the core to contract and as it contracts it heats up. This causes something very strange to happen. The iron in the core actually fuses to create elements heavier than iron BUT, instead of releasing energy, iron fusion consumes energy! In spite of this energy consumption, the core remains hot (from contraction and the load of energy produced by the nuclear fusing shells above the iron). Indeed, the fusion of the iron causes the core to contract more and that produces more heat and that is used to fuse more iron. As iron atoms fuse they suck energy away from the iron core and the layers above. This causes the core to compress and heat up and fuse more iron. It's madness! The iron core keeps wasting energy as it fuses the degenerate iron nuclei. This cannot last forever and, indeed, this iron-fusing stage lasts only a fraction of a second. A series of rapid contractions causes the core to collapse into a new kind of matter!
What kind of matter?
Well, that's another matter and we'll, come back to that matter in a moment.
The upper layers collapse onto this special core producing tremendous pressure and heat. This huge energy output causes the upper layer materials to rebound off the core in a powerful explosion. As it explodes the heat and pressure cause further fusion reactions to occur! All this energy causes the star's luminosity to increase by over a BILLION fold and the star will continue to shine brightly for several years.
This is called a Type II supernova. (That's "Type two supernova".)
What's a "Type I supernova"? For that matter, what's a "nova"?!
A nova is a star that over a period of a few days increases its brightness by a factor of only a thousand or so. Supernovas are much brighter than novas and last longer. That's why they are "super".
The word "nova" is Latin for "new". The Ancients
would occasionally see a "new star" appear (only to
disappear a few days later) so they thought it was a "new
star" - a nova. In fact, novas are not new stars. They are
just a star that had gone unnoticed until it suddenly brightened.
Novas have NOTHING to do with the aging of supergiants. The sudden
brightening that produces a nova or a type I supernova is caused by the interaction of two stars. I will teach
you about these "binary stars" next month, but here
I want to stick with the topic of supergiant aging and that means
we will only discuss Type II supernovas. These stars still have a great deal of hydrogen in their outer shell(s) so, when they explode the spectrum they produce shows signs of hydrogen. By definition, a Type II supernova shows hydrogen in its spectrum (and a Type I supernova does not).
During the explosion of a supergiant its temperature will increase to about 100 billion degrees (Celsius or Kelvin - it doesn't matter). These high temperatures, along with the tremendous pressure produced from the rebound of the star's materials off the core, cause big atoms like iron to fuse into some very heavy elements. Some of these new elements are so big that they are unstable and disintegrate immediately. The heaviest fairly "stable" element produced by a Type II supernova is uranium. (It has 92 protons and over a hundred neutrons.)
A cloud, rich in all the elements, is blown into space at a
speed of around 1000 kilometers per second! This is like a massive
planetary nebula but moving a lot faster (planetary nebula move at only 20 kilometers per second) and full of heavy elements with which it seeds
the universe. Eventually these elements, all the product of stellar
evolution, may condense into a nebula - an element rich nebula - and give
rise to new stars, planets and people!
The iron in your blood
(hemoglobin) was once in a star that went supernova.
The carbon and sulfur that make your proteins were made in the
shell of a supergiant. The elements that make minerals and cause
our enzymes to work also came from the shells of supergiants. (Cool, huh? | ![]() |
Interesting, but what about that special matter left behind?
![]() | Well, during the collapse of a supergiant, atoms are fused together in the shells but in the core the electrons are fused with protons to form neutrons! You should understand that protons have a positive charge, electrons a negative charge and a neutron has no charge at all. So this fusion of an electron and a proton creates a particle with no charge - a neutron. When a proton and electron are fused into a neutron, a peculiar particle called a "neutrino" is released. |
Neutrinos have no charge, very little mass and they travel at very close to the speed of light.
If all that sounds strange, well, it is!
It never ceases to amaze me that to understand the largest objects in the universe - stars - you also have to understand the smallest objects in the universe - subatomic particles. But this is neither the time nor place to teach you the details of quantum mechanics or subatomic theory.
Neutrinos are produced by a variety of interactions between subatomic particles. Indeed, our Sun produces some neutrinos and they can be detected using sophisticated nuclear chemistry and sensitive detectors. However the amount of neutrinos produced by a Type II supernova is outstanding. These neutrinos pass through the expanding shell of material like a fast-moving ghost. When astronomers detect a "neutrino burst" it's a signal that somewhere in the universe a star has gone supernova.
OK. So all these newly created neutrinos leave. What's left behind?
That depends upon the leftover mass in the core. If the core is above a certain mass (as in our story here), most or all of it will be crushed into neutrons and the core will be held up by degenerate neutron pressure. This core becomes a neutron star - a star made mostly of neutrons.
An astronomer from India named Chandrasekar developed an equation to determine the mass needed to cause this
collapse. Chandrasekar
asked, "What's the maximum possible mass for a stable white dwarf?" knowing that anything more massive would collapse into a neutron star. He was really asking, "How strong is the degenerate electron pressure that holds up the surface of a white dwarf?".
Notice that he thought in terms of a "dead star" not a "live star". A supergiant is NOT a dead star - it's a live
star undergoing fusion and the fusion keeps the star expanded
in size. Chandrasekar knew that a neutron star could NOT form from
a "burning" star because burning stars are all puffed
up by the heat they produce. To become a neutron star it must
first "burn out".
[The point I am trying to make is that matter undergoing fusion is "puffed up" by its own energy and expands like a gas (or, to be more correct, "a plasma"). Matter undergoing fusion (in a "live star") cannot experience degenerate pressures so it cannot form a neutron star held up by neutron degenerate pressure or a white dwarf held up by electron degenerate pressure.]
Chandrasekar figured out that the largest, stable white dwarf
would have a mass of about 1.44 times the mass of the Sun - that's 1.44 solar masses. (One solar mass is the mass of our Sun and is a common unit to use when comparing stars.)
Any non-fusing object with a mass greater than 1.44 solar masses, Chandrasekar's
limit, would become a neutron star. Therefore any star that
has "burned out" but retained at least as much mass as
1.44 that of the Sun will become a neutron star.
Our Sun will never become a neutron star because the Sun does not exceed Chandrasekar's limit, so the core it will leave behind will be far too small. Instead, the Sun will simply produce a white dwarf and a simple planetary nebula when it dies.
Degenerate neutron pressure allows the matter in a neutron star to be packed very tight. Neutron stars are only 10 kilometers in diameter but over 1.44 times as massive as the Sun so they are amazingly dense! Nothing is denser than a neutron star. [Although one could argue about another special from of matter but I'll leave that for the last part of this month's lessons.]
All we know about neutron stars is based upon our understanding of the physical events we expect to occur. We expect the outer "crust" of a neutron star to be a mix of atomic nuclei (mostly iron) and electrons that escaped compression. Below the crust, and making up most of the mass of a neutron star, should be nothing but neutrons. Most of the neutron star is an amazingly dense material with unusual properties. There are no normal atoms, just subatomic particles.
I think a review is in order.
Before the supergiant's core collapsed it was made of degenerate
iron and had a density like that of a white dwarf - a teaspoon
of it would weigh a ton. The iron core of a supergiant is held
up by degenerate electron pressure. Eventually the star flickered and in a spasm of energy it produced a supernova explosion and transformed the core into a much denser object - a neutron star. A teaspoon of neutron star
"stuff" should weigh millions of tons! This high density
has do to with the supergiant's core (exceeding a mass of 1.44 that of the Sun's) getting compressed
from the size of the Earth to only 10 kilometers in diameter.
| ![]() |
Like a white dwarf, a neutron star does not produce any heat by
nuclear fusion. Instead it glows from all the leftover heat it
has acquired over its "lifetime" at the center of a
star. A neutron star's residual heat is much greater than that
of a white dwarf's. You'll recall that a white dwarf is dim and
white because most of its energy is emitted in the ultraviolet (invisible) part
of the spectrum. Only a tiny portion of its energy output is
visible and that we see as a white color. So, in the visible range of light, white dwarfs are
dim and white. White dwarfs have a surface temperature less than 100,000o.
A neutron star has a surface temperature over a
MILLION degrees and radiates most of its energy as X-rays! These
rays are so powerful that we don't normally get a glimmer of visible
light from a neutron star so they are invisible. Therefore, we can't see
a neutron star because it radiates in a powerful part of the
spectrum that we cannot see.
If you can't see them, how do you know they are there?
That's a very good question. I will tell you how we detect them (and more) in our next lesson. If you want to take a break this would be a good time to do so. Otherwise continue on to learn all about pulsars!