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~17h —30

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Possible supernova

light curves, there are at least two types of supernovae, with differing initial chemical compositions. Type II have the somewhat greater abundance of metals.40 They are the end points of evolution of very massive stars whose inevitable internal collapse is brought about by the exhaustion of exothermic nuclear fuel. With sudden collapse, the fresh fuel in the outer layers falls into much higher temperature regions, resulting in a catastrophic explosion that can either shatter the entire star, leave only a very high density core remnant,41 or compress the inner core into a black hole.42 Supernovae are rare events—occurring at a typical frequency of a few per galaxy per century. The Milky Way galaxy is overdue for another occurrence—the last recorded supernova was that described by Kepler (and other astronomers around the world) in the constellation of Ophi-uchus in October 1604. Table 5.12 lists the known and strongly suspected cases of supernovae in our galaxy (and in the Magellanic Clouds). The latter are separate galaxies but are close enough for the supernova discovered by Ian Shelton in 1987 to be visible to the naked eye.

40 Astrophysically, in this context, the term 'metals' refers to all elements heavier than Helium. Types I and II supernovae belong to star populations II and I, respectively. Population II stars are older, less confined to the galactic plane, and have less metal content than do Population I stars.

41 A "neutron star" is so-called because its density is like that of the nuclear component itself—electrons and protons so closely packed together that they are forced into neutron mergers! The mass of such a star («1 solar mass) is compressed into a sphere only a few 10's of kilometers in diameter. A spinning neutron star can be detected as a radio (and sometimes also an optical or x-ray) pulsar. A pulsar at the location of SN1987a was detected in February 1989.

42 This refers to an object with so strong a gravitational field that light cannot escape from it. First described by Laplace as a "corps obscure," several candidates are contained in binary star systems, where their masses, although not directly seen, exert gravitational force on their visible companions. There is strong evidence for very massive black holes at the centers of at least some galaxies, and for a modest-sized black hole at the center of the Milky Way galaxy.

Ancient observations of supernovae can aid modern astrophysics! First, the identification of the moment of explosion helps to determine the distance as well as the age of the object. We can make use of the basic relationship between arcs and angles (§2.2.4): The angular size (a) of the object in the sky depends on both the physical size, (say, diameter, D) and the distance (r), D = axr. Second, the rate of expansion as measured by Doppler shifts of spectral lines can be compared with the rate of nebular expansion to reveal the rate of deceleration of the nebulosity (the rate at which it slows as it leaves the star due to the gravitational pull of the remnant). This provides information about the environment of the supernova and the mass of the remnant core, if any. Finally, estimates of the observed brightness of the supernova and the rate at which it fades provide its luminosity. At present, there are long-standing questions about the reliability of supernovae as standard candles; therefore, each one that can be calibrated provides more evidence for the potentially brightest of all standard candles. The distance scale of the universe is at stake in the correct resolution of this question.

Table 5.12 lists candidates for supernovae that are recorded in ancient records. Most of the data in Table 5.12 are taken from Hsi Tse-Tsung (1958), Clark and Stephenson (1977), or Stephenson and Clark (1978). These include the date and location in the sky with western identification and modern right ascension and declination, the maximum brightness in visual magnitudes, and the historical sources of the data. The peak brightness was determined by Stephen-son and Clark from the records or on the basis of the duration, to which the peak brightness is related. The brightest supernova ever recorded (as far as we know) was that of 1006, which was visible in broad daylight. See §15.2.2 for a discussion of possible supernova sightings in connection with the Star of Bethlehem.

Tycho's "Star" of 1572 in Cassiopeia was discovered just prior to Kepler's "Nova." This was an important observation because Tycho demonstrated that the lack of parallax implied a distance greater than that of the Moon. This was indeed a sign of the mutability of the heavens, and a blow

Table 5.12. Possible supernovae in ancient records.

Dates

Name/location

Source/remnant evidence

(14h29m, -60°) Nantou (Sag.) (18 30, -25) Wei (Sco) (17 10, -40) Tienti or Ti ~ Lupus (15 10, -40)

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