Sometimes the Final Stages of the Life of a Star Result in Violent Explosions

Key points: What causes novae, and type I and type II supernovae; origin of elements heavier than helium

Novae

A nova is a star that brightens suddenly by 100 to 10,000 times in a single day.

Novae are frequently (perhaps always) members of binary systems where the more massive star evolves more quickly and becomes a white dwarf. The less massive star loses some of its hydrogen-rich outer atmosphere to the white dwarf where it collects. (adapted by G. Rieke from J. Blondin, http://wonka.physics.ncsu.edu/~blondin/AAS/)

The high gravity and intense heat of the white dwarf make conditions suitable for hydrogen fusion into helium. When enough hydrogen collects, it explodes in a fusion reaction, causing a nova. The gas ejected in the explosion moves outwards at 1000-2000 km/sec. (adapted by G. Rieke from HEASARC) Here is a close-up of the explosion (from NASA/GALEXhttp://www.nasa.gov/mov/170967main_GalexNova_01-30-07_HalfRes.mov)

Supernovae

are stars that end their lifetimes in a violent nuclear explosion during which a single star can become as bright as an entire galaxy like the Milky Way. When one explodes nearby, the appearance of a bright new star is an awesome sight

• supernovae explosions are the only source of elements heavier than iron, such as nickel, copper, gold, silver, etc.

Two Types of Supernovae       By examining both the light curves and spectra of supernovae, we have learned that there are two very different causes for a supernova explosion.   from J. Kaler via http://www.chapman.edu/oca/benet/intro_sn.htm

Type I:

The "Type I" supernovae arise when the white dwarf star member of a binary system accretes so much matter from its companion that it is tipped over the 1.4M Chandrasekar limit. The white dwarf collapses very rapidly (just a few seconds) until the infalling layers hit the very hard surface of the neutron star. The resulting shock goes back through the collapsing layers, and they explode by hydrogen fusion. (we will show this explosion process when we discuss Type II supernovae.) (from Australian National University, Astronomy and Astrophysics, http://wwwmaths.anu.edu.au/research.programs/aap/)

Type II:

The "Type II" supernovae are the result of a massive star consuming all of its nuclear fuel and then exploding. The black line traces the evolution of a massive star schematically, until it reaches the supernova stage. (from G. Smith, http://casswww.ucsd.edu/public/tutorial/SN.html)

Recall the structure of a massive star with an iron-rich core, silicon, oxygen,  and carbon burning shells surrounding the core. There is no fusion reaction involving iron that produces energy (it has the strongest binding energy in its nucleus of any element, thus there is no way to add protons and get energy out).  (From Chaisson & McMillan, Astronomy Today)

This figure shows how the greatest binding energy per nuclear particle (nucleon) occurs for iron. (from Bodner Research Group, Purdue University,  http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch23/modes.html.) Thus, when the stellar core becomes solid iron, there is no fusion reaction available to produce energy to keep the core hot and maintain the pressure that resists gravity; the iron core collapses in just a few seconds to a neutron star (or black hole). Nuclear energy can be produced by elements with heavier nuclei than iron, but only by fission, where the nucleus splits into lighter ones and releases the energy that way. This only works well for nuclei much more massive than iron, and stars have no way to build such elements in their core reaction cycles.

Core Collapse and Supernovae

With the support from the core removed, the upper layers of the star collapse. When they hit the new, very hard neutron-star core, they bounce and send material crashing back up through the still-falling layers (see simulation below). The result is a rapid increase in temperature to several billions of degrees.

Core collapse: We zoom in on a tiny region (300 km across) with the stellar core at the lower left corner and then watch the star fall in onto the neutron star core. During the first 0.4 sec of the core collapse, many neutrinos are formed leading to a burst of neutrino emission. Gravitational waves are also likely to be emitted from the rapidly collapsing core but have never been observed. Eventually the surrounding material becomes so hot it begins to boil and starts the explosive outwards shock that ignites the hydrogen in the outer layers, causing the huge energy release in the supernova.(Animation adapted by G. Rieke from A. Burrows, Univ. of Arizona, full movie at )(reload page to restart lecture animations)

If we look in high time resolution the first fraction of a second in the stellar core is very chaotic and leads to lots of structure - the colors code the level of local organization of the material, which reflect the turbulence. This behavior can be reflected by non-spherical ejection of material in the explosion.   (from NASA Visualization Explorer https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=11735, Christian Ott)

This simulation shows what happens in the upper layers of the star as the shock passes through them. The neutron star core is at the lower left corner. The huge release of energy causes much of the hydrogen to fuse into helium, releasing huge amounts of energy that create the visible supernova. So much energy is released that the material in the star can react and produce all the elements heavier than iron, none of which can be produced during the previous life of the star. Animation from Tetsuya Shimizu.

Here is a more distant overview of the process . (from Hayden Planetarium Audiovisual Archive)

And how it might look from a desolate planet orbiting the star, with the optimistic assumption that the observer would live long enough to see the pulsar emerge! Animation from NASA/HEASARC

After collapse, the stellar brightness increases dramatically (by a least at factor of 10,000)

In the end, all the outer layers of the star are blown away, leaving only a black hole or neutron star -- the latter may send out searchlight beams of light that sweep the sky as it spins and can appear to us as a pulsar.

The properties of a Type I explosion differ from a Type II because the white dwarf never synthesized any elements heavier than carbon in its core.

What becomes of any core material left from a supernova explosion?

1) If the core that remains has a mass less than 3M, it will become a neutron star/pulsar like the Crab pulsar.

2) If the core is too massive, it will become a black hole because not even neutron degeneracy pressure will be able to resist the force of gravity.

Test your understanding before going on

"Embedding" diagram, showing how a black hole distorts space/time, from C. Pickover, http://sprott.physics.wisc.edu/pickover/graphcp.html		Painting of Crab supernova, Chaco Canyon  http://www.colorado.edu/Conferences/chaco/tour/blanco.htm
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