Lecture XII:
Novae and Supernovae
Photograph Of A Nova - A nova is a star that suddenly increases enormously in brightness, then slowly fades back to its original luminosity. Novae are the result of explosions on the surfaces of faint white-dwarf stars, caused by matter falling onto their surfaces from the atmosphere of a larger binary companion. Shown is Nova Herculis 1934 in (a) March 1935 and (b) in May 1935, after brightening by a factor of 60,000.
Diagram Of A Nova System - A white dwarf in a semidetached binary system may be close enough to its companion that its gravitational field can tear material from the companion's surface. Compare Figure 20.21. Notice that, unlike in the earlier figure, the matter does not fall directly onto the white dwarf's surface. Instead, as discussed a little later in the text, it forms an "accretion disk" of gas spiraling down onto the dwarf.
Stages In The Explosion Of A Nova - In this artist's conception, material accumulates on a white dwarf's surface after being accreted from a companion star (a) and then ignites in hydrogen fusion as a nova outburst (b and c). Part of the surface gas is ejected into space in the form of "bubbles" of hot plasma; the rest relaxes back down onto the accretion disk (d).
Light Curve Of A Typical Nova - The light curve of a typical nova. The rapid rise and slow decline in the light received from the star, as well as the maximum brightness attained, are in good agreement with the explanation of the nova as a nuclear flash on a white dwarf's surface.
Interior Of A Highly Evolved Star - Cutaway diagram of the interior of a highly evolved star of mass greater than 8 solar masses. The interior resembles the layers of an onion, with shells of progressively heavier elements burning at smaller and smaller radii and at higher and higher temperatures.
Supernova 1987a - A supernova called SN1987A (arrow) was exploding near this nebula (30 Doradus) at the moment the photograph on the right was taken. The photograph on the left is the normal appearance of the star field.
Light Curves For Type I And Type II Supernovae - The light curves of typical Type-I and Type-II supernovae. In both cases, the maximum brightness or intensity can sometimes reach that of a billion suns, but there are characteristic differences in the fall-off of the luminosity after the initial peak. Type-I light curves somewhat resemble those of novae (Figure 21.4). Type-II curves have a characteristic bump in the declining phase.
Stages In Explosion Of Type II Supernavae - Type-I and Type-II supernovae have different causes. These sequences depict the evolutionary history of each type. A Type-I supernova usually results when a carbon-rich white dwarf pulls matter onto itself from a nearby red-giant companion.
Stages In Explosion Of Type I Supernavae - A Type-II supernova occurs when the core of a more massive star collapses, then rebounds in a catastrophic explosion.
Remnants Of The Crab Nebula - This remnant of an ancient supernova is called the Crab Nebula (or M1 in the Messier catalog). It resides about 1800 pc from the Earth and has an angular diameter about one-fifth that of the full Moon. Because its debris is scattered over a region of "only" 2 pc, the Crab is considered to be a young supernova remnant. In A.D. 1054 Chinese astronomers observed the supernova explosion itself.
Difference Image Showing Expansion Of The Crab Nebula - Positive and negative photographs of the Crab Nebula taken 14 years apart do not superimpose exactly, indicating that the gaseous filaments are still moving away from the site of the explosion.
Vela Supernova Remnant - The glowing gases of the Vela supernova remnant are spread across a large 6° of the sky. The inset shows more clearly some of the details of the nebula's filamentary structure. (The long diagonal streak was caused by the passage of an Earth-orbiting satellite while the photo exposure was being made.)
Cosmic Abundance's Of The Elements - A summary of the cosmic abundances of the elements and their isotopes, expressed relative to the abundance of hydrogen. The horizontal axis shows atomic number-the number of protons in the nucleus. Notice how many common terrestrial elements are found on "peaks" of the distribution, surrounded by elements that are tens or hundreds of times less abundant. Notice especially the large peak around the element iron.
Theoretical Vs Actual Light Curve From Supernova - (a) The light curve of a Type-I supernova, showing not only the dramatic increase and slow decrease in luminosity, but also the characteristic change in the rate of decay about 2 months after the explosion (after the time indicated by the arrow). This particular supernova occurred in the faraway galaxy IC4182 in 1938. The crosses are the actual observations of the supernova's light. (b) Theoretical calculations of the light emitted by the radioactive decay of nickel-56 and cobalt-56 produce a light curve very similar to those actually observed in real supernova explosions, lending strong support to the theory of stellar nucleosynthesis.