Supernovae ... The Fate of the Largest Stars

I know exactly why I chose this color to be the background for supernovae. I need to save the color black for Neutron Stars and Black Holes, which is the obvious choice there. I personally dislike any color remotely associated with the ViQueen football team, but Green and Gold seemed just right for the supernovae chapter. Afterall the Green Bay Packers are full of superstars :)

I have waited a long time to finally arrive at this point in this Astronomy course. As long as I have been teaching, there are few subjects that arouse more curiosity and excitement than the explosions of massive stars and the fate of their unusual remnant cores. You have waded through chapters on main sequence dwarf stars whose end is a beautiful planetary nebula and dimunitive White Dwarf. You have read about the very common Red Dwarf whose demise is a boring collapse into a Black Dwarf stellar corpse, although not in anyone's immediate future since none of these have perished yet in the Universe. More boring still are star wannabes like the Brown Dwarf who never even get the chance to become a star in terms of generating gamma ray energy, or smaller Jupiter-sized planets who are truly pretty with their colored weather bands, but don't even make the grade as a group of stars. You have looked at the hottest stars, the White Dwarf remnants of Asymptotic Giant Branch stars whose strong stellar winds have created spectacular planetary nebulae, and whose remnant Carbon/Oxygen cores are well over 100,000 K. You took a nice long look at the brightest stars, the O class supergiant whose luminosity makes individual stars of their nature visible in distant galaxies. You looked looked at the coolest stars that are not Red Dwarfs, the Red Giants, the larger Asymptotic Giant Branch stars, the larger still Red Supergiants, and the humungous Red Hypergiants. Perhaps you found the connection between the Supergiant Luminous Blue Variables and the Red Supergiants and Red Hypergiants to be interesting, but now the real fun begins. After racing through its lifecycle at speeds 1000 times faster than that of our Sun, the Red Supergiant or Red Hypergiant has exhausted its internal core supply of Hydrogen. Through a repeated series of core collapse and reignition of increasingly heavier elements, the star has taken on an onion-like internal structure. Layers of nuclear fusion and the nucleosynthesis of heavier elements has generated pulsating piston-like waves outward against the inward pressure of gravity, and lifted the outermost Hydrogen envelope of these massive stars to immense proportions where diameters are measures in Astronomical Units instead of mere kilometers. The diagrams below indicate what is happening toward the end of the life of a massive star

 

 

Supernovae are extremely rare events, and are not to be confused with novae. The term "nova" is derived from Latin word "new" and here refers to a "new star." While the star itself is certainly no new, people who first witnessed them saw the object appearing in the sky where no star had previously been. A nova is the result of a White Dwarf accreting (adding) material onto its surface and suddenly brightening. These events are periodic in nature, and many novae are therefore witnessed on a regular or semiregular basis. Indeed, the star SS Cygni is such a repeating dwarf nova, and when it brightened in 1996, astronomers were prepared with 3 satellites pointed exactly onto its location to study the effects of the infalling matter of a companion onto the surface of the white dwarf that brightened. Dwarf novae continue to accrete matter and demonstrate episodic brightening. Supernovae are events that result in the detonation of the star and the occasional disappearance of the star itself.

We learned about the difference between the two types of "new stars" after Edwin Hubble measured the distances to galaxies where they were occuring. Tycho Brahe in 1572 and Johannes Kepler in 1604 witnessed "supernovae," as did the Chinese in 1054 AD. After careful study of ancient records that were tabulated by the Chinese, we have discovered that supernovae are so rare that they occur in our Galaxy only about once every 200 years. Supernova 1987A happened in our neighbor galaxy, the Large Magellanic Cloud in 1987, but this companion galaxy is 150,000 light years away, making study of it a bit harder. However, this well-documented supernova was photographed and is the subject of an interesting lab exercise. If you want to look at it now, click on SN 1987A now.

Observations have revealed two very different kinds of supernova events, and knowing the difference is a key to determining the distance to far-distant galaxies. Type I supernovae have Hydrogen emission lines in their spectrum, brighten to an Absolute Visual Magnitude of -19, and follow a distinct pattern of brightening and dimming relative to time that is consistent for all the Type I events witnessed. Type II supernovae do not have Hydrogen emission lines, brighten to an Absolute Visual Magnitude maximum of -16, and follow a different pattern of brightening and dimming relative to time. A very simplified diagram of the difference between the light curves of Type I and Type II supernova events is seen below.

Even more interesting has been the discovery that these two kinds of supernova events come from two different classes of stars. The Type II supernovae appear to come exclusively from Population I stars and ONLY explode in the disks of galaxies with spiral structures. Population I stars are youthful, with a chemical composition similar to that of our Sun ... meaning that they are formed from material previously existing in an exploded star. Type I supernovae are found everywhere, including the Population II stars of the galactic halo and cores. Population II stars are more ancient, and have few heavy elements. A technical, and very detailed paper on the difference in supernovae is worth the visit of those not faint of heart when looking at hardcore science.

Type Ia and Type II Supernovae

As seen in the diagram and words above, Type Ia and Type II supernovae are different in their light curves, as well as their progenitor stars. This section will explore the differences between them in less detail than the link above, and focus on the two types that are also most relevant to the course.

Type Ia Supernova arise when a White Dwarf star that is below the Chandrasekhar Limit of 1.4 solar masses suddenly exceeds that limit. The White Dwarf is a compact, Earth-sized stellar core which has shed most of its outer layers. Nuclear fusion has stopped and gravity has collapsed the core down to a 10,000 km radius. Degenerate electron pressure is preventing further collapse. If the White Dwarf is a single star, like our Sun, then its fate is probably to remain as a White Dwarf and slowly cool down to become a Black Dwarf. However, if the White Dwarf is a companion to a Larger Star, it may draw stellar matter onto its surface. Accretion disks of stellar gas and dust form around the White Dwarf and may give rise to a special object known as a Dwarf Nova. When a small "clump" of gas suddenly strikes the surface of the White Dwarf, a brief eruption of Hydrogen --> Fusion occurs, and the star flares temporarily in brightness.

Now, if the White Dwarf is close to 1.4 solar masses, the result is not very "dawrf-like" at all. If the White Dwarf is the remnant of a highly-evolved high mass star, its neon-manganese-oxygen core would collapse and form a Neutron Star. However, if the White Dwarf is more typically like the Sun, then its helium-carbon will heat up. It is believed that just prior to surpassing the Chandrasekhar Limit of 1.4 solar masses, the core will ignite Carbon into Oxygen. Since the outward pressure of degenerate electrons is not affected by temperature, there is no control on the burning. In exceedingly rapid succession, fusion becomes a run-away process as heavier elements are fused at temperatures soaring into the billions of Kelvins, and all within a matter of a few seconds. The outward pressure of fusion is so great that the star literally comes apart. The kinetic energy of the stellar atoms reaches values of 2x10^44 joules! That is the same as 2x10^44 watts (a trillion, trillion, trillion, million 200 watt light bulbs). At these energies, the star blows itself apart, and nothing is left behind at all because the matter/energy rushes out at very high velocities. The energy is so great that the object achieves an Absolute Magnitude (Mv) of -19. Since these objects appear to erupt the same way everytime, and reach the same maximum brightness, they make excellent distance indicators called "standard candles." And at such high magntidues, they are visible in even the most remote galaxies. See the Wikipedia Site for more details on Type Ia Supernovae

Type II Supernovae arise when a massive Giant Star reaches the end of its lifecycle. The interior of these stars is depicted a few clicks above this page. When Iron is fused from Silicon and Sulfur, no other nuclear fusions will be sustainable. This is because every other element in the Periodic Table heavier than Iron absorbs energy upon its fusion. Thus, nuclear burning in the giant star stops and gravity begins a run-away process. Much of the core will collapse at velocities approaching 1/4 the speed of light. Electron degeneracy is defeated owing to the high mass of the collapsing core and either a Neutron Star or a Black Hole will form, depending of whether the end mass amount of the core is 1.5 - 3 solar masses or greater than that value.

Meanwhile, the sudden collapse triggers an outward shockwave that spreads quickly through the outer layers of the giant star. The energy level of this outward flow is 1x10^44 watts, and Absolute Magnitude levels approach Mv = -16. There is so much mass outside the collapsing core, and it is over 100 billion K of energy, that it is like at "over-kill" of excess energy. With so much energy available, and so much mass close by, every other element in the Periodic Table can suddenly be formed. However, owing to the rapid expansion, there is not much time for the formation of heavy elements, and this is why they may be so rare. See the Wikipedia Site for more details on Type 2 Supernovae.

More to the Story

While it may be confusing here, Population II stars are probably stars formed from the earlier moments of the Big Bang or when galaxies first coalesced into their various structures. The galactic halo clusters and the central bulge of spiral galaxies are almost exclusively Population II stars. Indeed, eliptical galaxies appear to be composed entirely of Population II stars. On the other hand, Population I stars are composed of recycled stellar material ... the result of stars that have exploded in the past, and whose ejected matter had recollapsed to form new star with heavy elements not original to the Big Bang event or early galactic raw materials. That the Population I stars are found ONLY in the spiral arms of large galaxies is indication that star birthing is confined to those regions, and not found in the galactic bulge or halo clusters. The Andromeda galaxy is seen in the image below and left for a reference to which we can compare our own Galaxy ... the Milky Way below and right. Spiral arms are pictured in the Milky Way representation in a blue color, indicating the presence of new star formation and a predominance of Population I stars, while the reddish-orange central bulge is depicted that way to emphasize the presence of ancient Population II stars. Incredibly, the pictures of galaxies conform to the color representation below because the original high mass bulge stars have long ago evolved and died, leaving behind only those stars yet to conclude their lifecycles ... and these are almost exclusively M Class stars.

 

Below is a comparison of spiral galaxies (top left is an HST image of M74, and top right is another HST image of NGC 4622), elliptical galacxy (lower left NOAO/AURA/NSF image of Centaurus A), and an irregular galaxy (lower right NOAO/AURA/NSF image of the Small Magellanic Cloud). It is readily apparent from the top two images that the spiral arms have the bright, massive, blue-hot OB class stars, while the cores of both spirals, as well as the majority of stars in the elliptical and irregular galaxies have more of the M class stars.

The Cause of a Supernova

The current theory for supernova explosions is not entirely satisfactory, but no better model yet exists to explain these events. Elements light than Iron are fused in successive shells around an ever-collapsing core of heavier materials. In the stars final stage, the Silicon/Sulfur mixture is quickly fused into and Iron/Nickle mixture. As mentioned in the previous chapter, Iron is the most stable of the Periodic Table of Elements. Energy cannot be derived from fusing it into anything else, so thermonuclear fusion in the core of a highly evolved supergiant star suddenly stops. The core survives for a few milliseconds as an Iron White Dwarf, but then gravity takes over. With no more outward pressure from thermonuclear fusion, the inward pressure of gravity is no longer balanced in Hydrostatic Equilibrium (present force balance working within our Sun). Gravity compresses the Iron core at ever-increasing speeds that approach light speed. In a fraction of a second, the Earth-sized Iron core is crushed to a diameter of a small city. The Iron is broken down into its constituent particles of protons, neutrons, and electrons. Degenerate electrons are forced into free-protons, producing immense numbers of neutrons. With no degenerate electron pressure, the core shrinks to a tiny ball of super-dense neutrons ... becoming a "neutron star." It may even defeat "degenerate neutron" pressure, if the core mass exceeds about 3 solar masses, collapsing down to sizes less than that of an atomic nuclei, becoming a "black hole." We will look at the neutron stars and black holes in the next chapter on smallest stars, but here we first look at what happens after the inward collapse of the Iron core.

At densities that approach what is found in an atomic nucleus (10^14 g/cm^3), the core of the star that was Earth-sized becomes an object with radius of 10-15 km. This is akin to squeezing the entire Earth into a ball only 200 meters across. The high-velocity inward crush of the core suddenly stops when the neutrons cannot be packed any more tightly (this is termed "neutron degenerate pressure") and a powerful shock wave tries to rebound outward through the massive stellar envelope of the onion star. All of those layers of various elemental nuclear burning prevent the outbound shock wave from propagating outward, and it temporarily stalls. Meanwhile, the creation of neutrons from the forcible union of electrons and protons in the shrinking core releases large numbers of tiny particles called Neutrinos. These particles have either an immeasuralbe mass, or no mass at all. Our Sun is releasing trillions upon trillions every second, and they move at or near light speed. Owing to their tiny size, almost everyone passes directly through the Earth without a trace. In a collapsing Iron core however, so many are released that the crowd of neutrinos generates sufficient outward pressure to assist the shock wave in its journey outward through the shells of nuclear burning, as well as thr outermost Hydrogen envelope. Within a few hours, the shock wave of stalled gas is pushed outward in a roaring explosion, and the star lights up in spectacular detonation. If Betelgeuse were to explode in this fashion, it would shine in our sky as brightly as a Full Moon, and the supernova witnessed by the Chinese in 1006 came close to this brightness. The event witnessed by the Chinese in 1054 today is known as the Crab Nebula (M1). This supernova brightened to the level of Venus (mv = -4), even though it was 6000 light years away. The Crab Nebula is seen in the NOAO/AURA/NSF image below. Inside all of that mess is a tiny neutron star spinning so rapidly, your could not pervceive it.

Here is where the real fun begins. Supernovae, at least those with Hydrogen in their specta, which means stars of high mass, result from the collapse of iron cores that develop within supergiants. At the peak of their detonation, the release of energy from a Type II supernova is 10^48 watts (our Sun produces 3.52x10^26 watts), and this incredible number is greater than the light produced by all of the stars in the known Universe! Type I supernovae are even brighter events, exceeding the Type II forms by almost 3 levels of magnitude (almost 16 times brighter). These Type I supernovae do not have Hydrogen lines in their spectra, and apparently come in two different forms Type Ia and Type Ib, depending on the presence of Silicon. Silicon lines are absent on Type Ib events, which are the fainter of the two supernovae forms, and like the Type II explosions occur in the galactic disks where new stars are being formed. It is the Type Ia events that are seen in galactic halos. Therefore, scientists conclude that the Type Ib variety are derived from Wolf-
Rayet
stars (more on Wolf-Rayet stars) whose Hydrogen envelope has been stripped by strong stellar winds earlier in their lifecycles.

Where Does Everything Come From?

In the monumental explosion of these Supergiants and Hypergiants, the products of millions of years of nucleosynthesis are blasted into space. Huge quantities of Helium, Carbon, Oxygen, and others are flung away from the explosion's center. The Iron core collapses, so the only things being blasted into space are elements lighter than Iron. Iron occupies a spot on the Periodic Table of Elements closer to Hydrogen than to Uranium. With only 26 protons, there are 66 natrually-occuring elements heavier that we find on our planet. I wear a gold wedding ring. My wife likes gold and silver jewlery. Some kids at Hopkins High School wear heavy Platinum ornaments from their necks. Where does this stuff come from if stars cannot make stuff heavier than Iron? Well, it turns out that the expanding shock wave within the supernova is at such a high temperature (in excess of 100 billion K) that fierce explosive burning takes place within the Oxygen, Carbon, Neon, and Silicon shells. The result is an almost instantaneous creation of all of the remaining elements observed in the Universe, including a large fraction of radioactive Nickel. It is the decay of this Nickel through Cobalt into Iron that is largely responsible for the extreme brightness of the supernova's light, and then this Iron is released into space ... independent of the Iron core.

Here's where things get a little more technical in terms of the nuclear physics of this supposed high school level Astronomy course. Inside giant stars, neutrons can be captured by atomic nuclei in a slow process (the "s-process") to produce elements up to Bismuth. Inside the turmoil of a supernova, neutrons can be captured in rapid speeds, allowing the almost instantaneous creation of very heavy isotopes that then decay into the heaviest known elements like Uranium, Plutonium, and even beyond. This "rapid neutron capture" (also known as the "r-process") can make all of the elements heavier than Iron ... such as Gold, Silver, and Platinum.

The expanding cloud of dust grains composed of these newly forged elements will blow outward into space, and may, in time, coalesce into a new star with planets. It is this very process that possibly gave birth to our own Solar System. In the distant past, a supernova blasted its materials outward, and from that debris, our Sun formed, complete with its list of heavy metals. The ligher elements in the swirling cloud spun outward from the collapsing baby star, to form the gas giant planets. The heavier elements accreted more close to the Sun, where chunks became the inner rocky worlds. Gold is so rare on Earth because it is so rare in space. In only a few moments of time, as a supergiant star erupts, can jewelry elements be forged, and even then, there is a scant supply. Earth has a large Iron/Nickel supply because it is readily formed during the explosion itself. Therefore, the elements of which your body is made ... Iron in the hemoglobin molecule, Zinc in many cofactors of enzymes, and Copper in other enzyme cofactors come from materials forged in some distantly past star.

Why Do Some Stars Blow Up and Others Do Not?

We have now seen that Red Supergiants can explode into Supernovae, while others may end as more gentle planetary nebulae. Some Blue Supergiants erupt as supernovae while others evolve to the right on the HR Diagram and become Red Supergiants that may either go supernovae or planetary nebulae. Other supernovae do not even occur until the massive star reaches the Wolf-Rayet life stage. Apparently a great deal of uncertainty surrounds the eventual fate of massive stars, and one reason for this uncertainty may be the rate of mass loss due to stellar winds. No matter how great the initial mass, if the winds blow out all but 1.4 solar masses, the remains of the star can become little more than a White Dwarf. The best candidates for an upcoming supernova show are Wolf-Rayet stars like Gamma-2 Velorum, White Dwarfs in the center of Ring Nebulae that are accreting mass onto their surfaces, LBV's like Eta Carinae, and perhaps Betelgeuse. It is certainly possible that one of these stars has already exploded, and we are merely awaiting the light from that fateful event to reach our planet. Who knows, maybe tonight is the night we see one!

Supernovae Remnants

One thing we do know ... the theory of the shock wave propagation outward, assisted by extreme numbers of outflowing neutrinos has been modeled on computers repeatedly. In every instance, the Iron core collapses into a Neutron Star or Black Hole, but the star never explodes. The explosion of the star against the inward pressure of gravity remains a mystery, but the fact that these stars explode is evidenced all over space. Below and in the upper left and right images is the Crab Nebula ... the remains of a star that the Chinese witnessed explode in 1054 AD. The upper right image is a close-up of the interior of the nebula, and is an HST Heritage image, while the left image is courtesy of the NOAN/AURA/NSF facility. The bottom pair of images are of the Veil Nebula, the remains of a supernova that detonated far longer in the past, in an unknown year. The left image is again from the NOAO/AURA/NSF facility, while the close-up look into the Veil is from the HST Wide Field Planetary Camera 2.

 

 

From this page, you have seen that the really massive stars who do not blow most of themselves out into space with powerful stellar winds will die a spectacular death as a Supernova. This fate is reserved only for the most massive of stars ... those with an initial mass in excess of 10 solar masses. You have also seen that ALL of the metals and elements heavier than Iron are formed when these massive stars explode, and without supernovae, life would be impossible. Our own Sun is the offspring from the debris left over from a supernova event.

Before we take a look into the other remains of a supernova besides the expanding cloud of dust and gas, I would like you to move to the assignments page and check out Supernova 1987A. This star is located in the Large Magellanic Cloud, and is the closest supernova event witnessed in a very long time. The assignment takes you through a series of photographs that chronicled this event, and lets you determine whether this was a Type I or Type II supernova. This assignment is not required, but presents you with an opportunity to earn extra credit points. Below are two images from the Hubble Heritage Collection, with the left image showing the area within the LMC where SN 1987A exists, and the right image showing a close-up of the explosion remnant.

The search for supernovae is an amateur's passion, and it requires a good telescope, clear night skies, and a familiarity with the deep sky objects where these events occur. The professional astronomers rely on the amateurs to locate new stars shining brightly in a galaxy, announcing the supernova event. Below are two photographs of galaxies from the NOAO/AURA/NSF facility showing a suddenly bright star in an otherwise non-descript galaxy. One particular amateur astronomer, Tim Puckett, devotes 40-60 hours of night observing each week, hoping to catch a supernova in progress at its earliest stages in some distant galaxy. To date, he and his team members have spotted 54 supernovae. Tim also holds a day job in addition to his supernova hunt passion. You can connect to his website at the Tim Puckett Observatory and take a look at his telescopes, team members, and discoveries. If you are ever interested in doing this type of search, you may want to join the International Supernova Network and see what you need to do in order to make significant contributions to the Astronomy field.

Finally, I have included several interesting links to Supernova sites where you can learn more about these exciting celestial events.

HEASARC site ... High Energy Astrophysics Site

NASA site

Supernova and Remnant Links

CHANDRA Supernova site ... X-Ray telescope website

There are four possible endpoints for a star ...

1) A White Dwarf, which is the demise of the vast majority of stars, since high mass stars are not often formed

2) A dense ball of neutrons, called the Neutron Star.

3) A Black Hole, which is an impossibly small star of significant solar mass, whose gravity is so great that light cannot escape.

4) Complete annihilation where nothing remains, and that seems to be the endpoint of Type Ia Supernovae. These Type Ia Supernovae are almost certainly related to White Dwarfs, and will be discussed in the next chapter, The Smallest Stars. Indeed, the November issue of Scientific American has a cover page showing a White Dwarf colliding with our Sun.

Now ... let's look at what happens to the remaining core of the supernova, if anything is remaining. Please move forward to the page entitled Smallest Stars where we will learn about Neutron Stars and Black Holes, or return to the Largest Stars page, the Star Introduction, or go to the Syllabus .


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