Astronomy 162: Professor Barbara Ryden

Monday, February 3


``Do not go gentle into that good night.
Rage, rage against the dying of the light.'' - Dylan Thomas

Key Concepts

All you need to know about the periodic table (for the purposes of this lecture): Hydrogen is the most abundant element in our galaxy. Helium comes in second. Carbon and oxygen are neck and neck for third place.

(1) The ultimate fate of a star depends on its initial mass.

Toward the end of its life, a massive supergiant star has a central iron core, surrounded by a shell where silicon is being fused to iron, surrounded by a shell where oxygen is being fused, surrounded by a shell where carbon is being fused, surrounded by a shell where helium is being fused, surrounded by a shell where hydrogen is being fused.

The energy generated in the center of the supergiant, where the temperature is extremely high, is carried away primarily by neutrinos, which can zip freely through the star. Although neutrinos carry energy away at a tremendous rate, the iron core is unable to replenish the energy by fusion reactions. The iron core, its energy bleeding away, undergoes a rapid collapse. Ordinary pressure can't save it from collapse. Degenerate-electron pressure can't save it from collapse. (The iron core is more massive than 1.4 Msun, the upper mass limit for a white dwarf.)

(2) A massive star ends with a violent explosion called a supernova.

In the absence of effective pressure support, the iron core collapses in less than a second. When the core reaches the density of an atomic nucleus (an amazing 400 million tons per cubic centimeter), it resists further compression and bounces back. The rebounding core sends a shock wave through the outer layers of the star, heating them up. (The heating process is helped by neutrinos, a few of which are actually absorbed at these high densities, and by turbulent convection.)

The shock-heated gas starts to expand outward at high speed (roughly 5 percent the speed of light, at first). Thus, the implosion (or rapid collapse) of the core ultimately triggers the explosion (or rapid outflow) of the star's outer layers. This explosion is what astronomers call a supernova. [The word ``nova'' is Latin for ``new''. When a star explodes, its luminosity shoots up, and may cause a previously invisible star to look like a ``new star'' (or ``nova stella'') in the sky. As long as I'm giving a Latin lesson, I should note that the plural of ``supernova'' is ``supernovae''.]

Supernovae are rare, luminous, and relatively brief events.

On July 5, AD 1054, Chinese astronomers noted the appearance of a ``guest star'' (as they called it) in the constellation Taurus. The ``guest star'' was visible in broad daylight for three weeks, and was visible at night for two years before it faded into invisibility.

Flash forward: when modern astronomers turn their telescopes to the position of the guest star, they see the Crab Nebula (pictured below):

The Crab Nebula is located 2000 parsecs (6500 light years) from Earth, and is 1.5 parsecs (5 light years) in radius. It consists of an expanding filamentary shell of gas. The currently measured expansion velocity of 1500 km/second is sufficient to have carried the shell outward by 1.5 parsecs in the past 949 years, since the ``guest star'' was first noted.

Plausible hypothesis: What the Chinese astronomers saw 949 years ago was a giant explosion. What we see today is the expanding debris from that explosion.

(3) The matter ejected in a supernova becomes a glowing supernova remnant.

The Crab Nebula is an example of a supernova remnant. The gas ejected in the supernova sweeps up the gas that was earlier lost in the supergiant's stellar wind, as well as scooping up interstellar gas. It is interesting to note that the emission lines of a supernova remnant, in addition to lines from H, He, C, O, and Fe, also show lines from elements heavier than Fe (that is, containing more than 26 protons in their nuclei).

Making elements heavier than iron is difficult because it requires the addition of energy. (Going from wood to ashes is easy, because burning wood releases energy; going from ashes to wood is difficult, because it requires the addition of energy.) In fact, it seems that the high-energy shockwaves in supernovae are the only places in the universe where heavy elements are made in bulk. Thus, gold, lead, silver, copper, uranium, and other heavy elements are forged in supernova explosions, and spread by expanding supernova remnants throughout the galaxy.

Supernova remnants, you will recall, also play a major role in star formation, by triggering the collapse of dark nebulae. As they expand outward, supernova remnants empty out huge low-density ``bubbles'' in the interstellar medium. We appear to be inside such a bubble, 100 parsecs across, carved out by a supernova which went off 300,000 years ago.

The scarcity of supernova explosions is frustrating to astronomers. No supernova has been seen in our galaxy since AD 1604 (this supernova was seen by Johannes Kepler, and hence is generally called Kepler's Supernova). The most recent naked-eye supernova was SN1987A, which appeared in the Large Magellanic Cloud, a satellite galaxy which orbits our own galaxy.

``After'' [left] and ``Before'' [right] pictures of the supernova SN1987A in the Large Magellanic Cloud. In the right panel, an arrow is pointing to the supergiant star SK-69 202, which was the progenitor of the supernova. In the left panel, the supernova is shining brightly as it explodes.
(Image credit: Anglo-Australian Observatory)

The supergiant Betelgeuse is only 160 parsecs away (only 1/12 the distance of the Crab Nebula). When it becomes a supernova, its apparent brightness will be (for a short time) brighter than the full Moon.

Prof. Barbara Ryden (

Updated: 2003 Feb 3

Copyright 2003, Barbara Ryden