SUPERNOVA REMNANT A IMAGE A1 – taken in 2019 IMAGE A2 – taken in 2020 SUPERNOVA REMNANT B IMAGE B1 – taken in 2018 IMAGE B2 – taken in 2020 SUPERNOVA REMNANT C IMAGE C1 – taken in 2015 IMAGE C2 – taken in 2020 SUPERNOVA REMNANT D IMAGE D1 – taken in 2017 IMAGE D2 – taken in 2020 SUPERNOVA REMNANT E IMAGE E1 – taken in 1983 IMAGE E2 – taken in 2020 Te Crab in 1956 21 Te Crab in 1999 22 INTRODUCTION TO SUPERNOVAE According to the Annals of pearance of Kepler’s star, no others have been seen in the Sung Dynasty (the Sung-shih), on the Galaxy. the frst day of the chi-ho reign period, during the 5th month, on the chi-chou, a “guest star” ap- Tis does not mean, however, that no additional peared to the south east of Tian-kuan. Te guest similar objects have been observed. In 1885 a new star star was so bright that it could be seen during the appeared in the center of the Milky Way’s companion daytime, and it remained so for 23 days. Afer that, galaxy M31, in the constellation of Andromeda. it gradually dimmed, fnally fading from visibility It reached a peak brightness of 6-7th , making it easily visible in small telescopes against the star. background glow of the galaxy itself. Te object is Tis impressive important for historical reasons: it was used to argue, object may have incorrectly, that the great spiral nebula of Andromeda also been record- was not a galaxy in its own right, but instead a much ed in disparate smaller object inside the Milky Way. Te astronomer cultures around Harlow Shapley, in a famous 1920 debate with Heber the globe, includ- Curtis on the nature of the spiral nebulae, assumed ing Europe, Asia, the “new star” was a relatively low-energy event, and possibly even and that meant it was close by as such things go. North America. His argument was later shown to be wrong. Edwin However, the date Hubble measured the distance to the Andromeda Crab nebula given in the Chinese nebula and proved it was well outside our Galaxy, and annals, by our modern reckoning, would have been was, in fact, an independent system of stars – a galaxy on a par with the Milky Way. Te great distance of throes of the Dark Ages, and the Norman Invasion Andromeda meant that the star seen in 1885 was very was just a few years away. Perhaps they were too oc- energetic indeed. cupied with worldly concerns to mark down the Tough these guest stars are rare events in any given appearance of a celestial visitor (though the Bayeux galaxy, the universe contains many, many galaxies. Tapestry, created just a few years later, has a clear With the advent of large telescopes in the 1920s and depiction of Hally’s comet). Perhaps whatever re- 30s it was soon noticed that guest stars could be seen cord existed has been lost. In any case, no defnitive quite ofen if one looked at many galaxies. Te fact European record of the event has ever been found. that the guest stars were nearly as bright as the galax- Since the appearance nearly a millennium ago of ies in which they occurred meant that they were enor- the Sung “guest star” there have been only two oth- mously energetic. Teir great brightness and release er similar objects seen in our . One occurred in 1572 in the constellation Cassiopeia. Tis was dub them , because they appeared simi- observed by the Danish astronomer Tycho Brahe lar to, but far brighter than, the “novae” seen in our and bears his name. It became bright enough to galaxy. Supernova is the name by which we still call be visible in full daylight. Te other star ap- them today, though we now know they have nothing peared in the constellation Ophiuchus in in common with novae except a name: supernovae 1604 and was studied by Tycho’s student are exploding stars, whereas a nova is the much smaller explosion of the atmosphere of a it was seen earlier by several other peo- star that is acquiring ple. Kepler’s star, while not as bright matter from a nearby binary as Tycho’s, was still as bright as companion star. - 6 Te Crab Nebula is located just above the star marking the tip of the lower horn of Taurus, the Bull. Recent observations of supernovae similar to the Crab pulsar would weigh more than a billion one seen in Andromeda in 1885 have allowed us to tons. measure the vast size and expansion rate of the uni- In the remainder of this education unit, you verse. To our great surprise, these extremely distant will explore the amazing properties of super- supernovae indicate that the expansion is accelerat- novae and neutron stars. You will also begin ing, rather than slowing down. Tese observations to learn about some of the tools scientists indicate that approximately 70% of the energy in the use to understand them. universe is something never before observed, with properties heretofore only imagined in the most - ing that the universe is small, as Shapley argued, su- pernovae have shown us that the universe is not only Te activities vast, but much stranger than we had imagined. and explanations in this unit all deal If you point a telescope toward the patch of sky with the explosions of described in the Chinese records from 1054, just a massive stars. Tese are few degrees north and east of Aldebaran, the “eye” called Type II superno- of Taurus, the bull, you will fnd a faintly glowing vae. Tere is another cloud. Tis is the Crab . It is the remains type of supernova, called of a star that exploded some 7000 years ago. Te Type I, which involves explosion was seen on Earth only 1000 years ago the explosion of a because it was so distant that its light required 6000 small, dense core of a years to reach us; the Sung inhabitants were seeing more sunlike star (in the explosion 6000 years afer it happened. Te fact, the supernova of Crab Nebula is a , the 1885 was just this type debris from an exploded star. It is still of explosion). While expanding today at more than 1000 fascinating in their own km/s. Inside the nebula is the Crab right, Type I superno- pulsar, the compact remnant of the vae are not covered in core of the exploded star. Te pulsar this unit. is a highly magnetized, rapidly spinning , a class of object that is among the most bizarre found in nature. A mere teaspoon of the WHY STARS EXPLODE Te stars in the sky seem eternal and unchanging. But that’s an illusion. Like all Initially, the star fuses hydrogen into helium. Like ash things, stars are born, live out their lives, in a fre, the helium builds up in the core, but it does and eventually die, doomed to fade away. Stars not fuse because helium takes a lot more pressure and like the Sun, which have a relatively low mass, age heat than hydrogen does to fuse. If the star is massive gracefully and die quietly afer billions of years. But enough, though, it can ignite helium fusion in its core. massive stars, with more than ten or so times the mass Te helium fuses into carbon, which then starts to pile of the Sun, “do not go gently into that good up in the core. In very massive stars this process night, but instead rage, rage against the repeats again and again, fusing lighter ele- dying of the light”. Tey explode in H ments into heavier ones: hydrogen to a catastrophic detonation, send- He helium, helium to carbon, carbon to ing their outer layers scream- C neon, neon to oxygen, oxygen to ing outwards at a few per- silicon, silicon to iron. Te star’s Ne cent of the speed of light: core starts to look like an onion, O with layers nested inside one what astronomers call a Si supernova. another. Fe Te seeds of a star’s ul- At every step, the process gen- timate destruction are erates more heat, and the fu- planted deep in its core, sion goes ever faster. A star may where its energy is generat- fuse hydrogen into helium for ed. Stars are giant balls of gas, millions or billions of years, but and when a gas is compressed it by the time it starts to fuse silicon heats up. Because stars are so big into iron, it may take mere days. As they have a lot of gravity, so at iron piles up in the core, the the core of a star the pressure Near the end of a massive star’s life, the fusion occurs in shells star is headed for disaster. is intense. Tis means they around the core, like the layers of an onion. Why? Because up until iron, get very hot, hot enough to all the fusion reactions have smash together . And when nuclei col- produced energy in the form of heat. However, there is lide, they can stick together in a process called fusion. not enough heat and pressure to fuse the iron nuclei, so Tis process releases a lot of energy (in fact, it’s what once iron builds up in the core, the star’s source of energy makes hydrogen bombs explode), which heats up the shuts of. Worse, the electrons in the core combine with core. In a stable star like the Sun, the inward crush of the protons in the iron nuclei to form neutrons - and gravity is balanced by outward pressure caused by the the electrons were crucial to give the star support as heat.