20-10 A supernova remnant can be detected at many wavelengths for centuries after the explosion

Many supernovae in our Galaxy are hidden from us by the obscuring interstellar medium

Astronomers find the debris of supernova explosions, called supernova remnants, scattered across the sky. A beautiful example of a supernova remnant is the Veil Nebula, shown in Figure 20-21. The doomed star’s outer layers were blasted into space with such violence that they are still traveling through the interstellar medium at supersonic speeds 8000 years later. As this expanding shell of gas plows through space, it collides with atoms in the interstellar medium, exciting the gas and making it glow. We saw in Section 18-8 that the passage of a supernova remnant through the interstellar medium can trigger the formation of new stars, so the death of a single massive star (in a core-collapse supernova) or white dwarf (in a thermonuclear supernova) can cause a host of new stars to be born.

Figure 20-22: R I V U X G
The Vela Nebula—A Supernova Remnant The Vela Nebula is embedded in an even larger remnant called the Gum Nebula. The Vela supernova explosion occurred about 11,000 years ago, and the remnant now has a diameter of about 700 pc (2300 ly).
(© Axel Mellinger)
Figure 20-21: R I V U X G
The Veil Nebula—A Supernova Remnant The Cygnus Loop is a roughly spherical remnant of a supernova that exploded about 8,000 years ago. The distance to the nebula is about 1,500 ly and the overall diameter of the loop is about 100,000 ly. This supernova remnant is so large that it covers an area of about 45 full moons! The image contains more than 1.6 Gb of data and is one of the largest astronomical images ever made.
(National Optical Astronomy Observatory [NOAO] and WIYN partners)

A few nearby supernova remnants cover sizable areas of the sky. The largest is the Gum Nebula, named after the astronomer Colin Gum, who first noticed its faint glowing wisps on photographs of the southern sky. Its astonishingly wide 40° angular diameter is centered on the constellation Vela (the Ship’s Sail).

The Gum Nebula looks big because it is quite close to us and has had a long time to expand—its center is only about 1000 ly from Earth, and it originated from a supernova explosion about a million years ago. Embedded in the Gum Nebula is another large supernova remnant called the Vela Nebula (Figure 20-22), which is about 16° wide. Studies of the nebula’s expansion rate suggest that this supernova exploded around 9000 b.c.e. At maximum brilliance, the exploding star probably was as bright as the Moon at first quarter. Like the first quarter moon, it would have been visible in the daytime!

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Figure 20-23: R I V U X G
Cassiopeia A—A Supernova Remnant This false-color radio image of Cassiopeia A was produced by the Very Large Array (see Figure 6-23). The impact of supernova material on the interstellar medium causes ionization, and the liberated electrons generate radio waves as they move. Cassiopeia A is roughly 3300 pc (11,000 ly) from Earth.
(NRAO/AUI)

Many supernova remnants are nearly invisible at optical wavelengths. However, when the expanding gases collide with the interstellar medium, they radiate energy at a wide range of wavelengths, from X-rays through radio waves. For example, Figure 20-23 shows a radio image of the supernova remnant Cassiopeia A. (Compare to Figure 18-26, which is a composite of observations of Cassiopeia A at X-ray, visible, and infrared wavelengths.) As a rule, radio searches for supernova remnants are more fruitful than optical searches. Only two dozen supernova remnants have been found in visible-light images, but more than 100 remnants have been discovered by radio astronomers.

From the expansion rate of Cassiopeia A, astronomers conclude that this supernova explosion occurred about 300 years ago. Although telescopes were in wide use by the late 1600s, no one saw the outburst (and no one today knows why). The last supernova seen in our Galaxy, which occurred in 1604, was observed by Johannes Kepler. In 1572, Tycho Brahe also recorded the sudden appearance of an exceptionally bright star in the sky. To find any other accounts of nearby bright supernovae, we must delve into astronomical records that are almost 1000 years old.

At first glance, this apparent lack of nearby supernovae may seem puzzling. From the frequency with which supernovae occur in distant galaxies, it is expected that we should have about two supernovae per century. Where have they been?

As we will learn when we study galaxies in Chapters 22 and 23, the plane of our Galaxy is where massive stars are born and supernovae explode. This disklike region is so rich in interstellar dust, however, that we simply cannot see very far into space when looking in the plane of the disk, which is in the directions occupied by the Milky Way (see Section 20-2). In other words, supernovae probably do in fact erupt every few decades in remote parts of our Galaxy, but their detonations are hidden from our view by intervening interstellar matter.

Relics of the Fall: White Dwarfs, Neutron Stars, and Black Holes

A supernova remnant may be all that is left after some supernovae explode. But for core-collapse supernovae of Types II, Ib, and Ic, the core itself may also remain. If there is a relic of the core, it may be either a neutron star or a black hole, depending on the mass of the core and the conditions within it during the collapse. Neutron stars, as the name suggests, are made primarily of neutrons. Wholly unlike anything we have studied so far, these exotic objects are the subject of the next section. We will study black holes, which are far stranger even than neutron stars, in Chapter 21.

CAUTION!

Although neutron stars and black holes can be part of the debris from a supernova explosion, they are not called “supernova remnants.” That term is applied exclusively to the gas and dust that spreads away from the site of the supernova explosion.

We have seen in this chapter that only the most massive stars end their lives as core-collapse supernovae. We learned earlier that mass plays a central role in determining the speed with which a star forms and joins the main sequence (see Figure 18-10), the star’s luminosity and surface temperature while on the main sequence (see Figure 17-22 and the Cosmic Connections figure in Chapter 17), and how long a star can remain on the main sequence (see Table 19-1). Now we see that a star’s initial mass also determines its eventual fate (Figure 20-24a). We also see that a star’s fate is connected to the formation of future stars and the matter from which even we are made of (Figure 20-24b).

Figure 20-24: A Summary of Stellar Evolution (a) The evolution of an isolated star (one that is not part of a close multiple-star system) depends on the star’s mass. The more massive the star, the more rapid its evolution. If the star’s initial mass is less than about 0.4 M, it evolves slowly over the eons into an inert ball of helium. If the initial mass is in the range from about 0.4 M to about 8 M, it ejects enough mass over its lifetime so that what remains is a white dwarf with a mass less than the Chandrasekhar limit of 1.4 M. If the star’s initial mass is more than about 8 M, it ends as a core-collapse supernova, leaving behind a neutron star or black hole. (b) These images summarize the key stages in the cycle of stellar evolution.
(b: top: Infrared Space Observatory, NASA; right: Australian Astronomical Observatory; bottom: NASA; left: NASA; middle: Australian Astronomical Observatory/David Malin Images)

CONCEPT CHECK 20-16

How can astronomers estimate how long ago a supernova exploded by its remnant?