13.3: Supernovae

Supernovae

Supernovae were discovered long before astronomers realized that these spectacular cataclysms mark the death of stars. The word nova means “new” in Latin. Before telescopes, when a star too dim to be seen with the unaided eye suddenly flared up in a brilliant explosion, observers concluded it must be a brand-new star. Twentieth-century astronomers reclassified the explosions with the greatest luminosity as supernovae.

From historical records of such explosions, studies of the remnants of supernovae in our Galaxy, and analyses of supernovae in other galaxies, we estimate that, on average, one supernova explosion occurs somewhere in the Milky Way Galaxy every 25 to 100 years. Unfortunately, however, no supernova explosion has been observable in our Galaxy since the invention of the telescope. Either we have been exceptionally unlucky or, more likely, recent explosions have taken place in parts of the Galaxy where interstellar dust blocks light from reaching us.

Supernovae in History

Although many supernova explosions in our own Galaxy have gone unnoticed, a few were so spectacular that they were clearly seen and recorded by sky watchers and historians at the time. We can use these records, going back two millennia, to help us pinpoint where the exploding stars were and thus where to look for their remnants today.

The most dramatic supernova was observed in the year 1006. It appeared in May as a brilliant point of light visible during the daytime, perhaps 100 times brighter than the planet Venus. It was bright enough to cast shadows on the ground during the night and was recorded by observers all over Europe and Asia. Chinese astronomers, noting that it was a temporary spectacle, called it a guest star.

Astronomers David Clark and Richard Stephenson have scoured records from around the world to find more than 20 reports of this supernova. They found its faint remnant in the modern constellation of Lupus. Figure \(\PageIndex{1}\) includes a composite view of the remnant, called SN 1006, from the Chandra X-Ray Observatory. The X-rays coming from the remnant are blue, the visible light is white-yellow, and the radio signal is red. From the way its filaments are expanding, it appears to be about 1000 years old.

Figure \(\PageIndex{1}\) : SN 1006. A 1000 years later, the remnant of SN 1006 can be detected with X-ray and radio observations. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{1}\).

Another guest star, now known as SN 1054, was recorded in Chinese records in July 1054. The remnant of that star is one of the most famous and best-studied objects in the sky, called the Crab Nebula. When its explosion was first seen, we estimate that it was about as bright as the planet Jupiter. It was nowhere near as dazzling as the 1006 event but still quite dramatic to anyone who kept track of objects in the sky. SN 1054 is currently six-light-years wide. In Figure \(\PageIndex{2}\), the orange filaments are mostly made of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.

Figure \(\PageIndex{2}\): SN 1054. This Hubble Space Telescope image includes the remnant of SN 1054, known as the crab nebula, including hydrogen gas and light from the neutron star at its center. (Public Domain; NASA, ESA, J. Hester and A. Loll (Arizona State University) via Wikimedia Commons) Alternative description of Figure \(\PageIndex{2}\).

Another fainter supernova was seen in 1181. The next supernova became visible in November 1572 and, being brighter than the planet Venus, was quickly spotted by a number of observers, including Tycho Brahe. His careful measurements of the star over a year and a half showed that it was not a comet or something in Earth’s atmosphere since it did not move relative to the stars. Johannes Kepler, found his own supernova in 1604, now known as Kepler’s Supernova. Fainter than Tycho’s, it nevertheless remained visible for about a year.

No supernova has been spotted in our Galaxy for the past 300 years. Since the explosion of a visible supernova is a chance event, there is no way to say when the next one might occur. At their maximum brightness, the most luminous supernovae have about 10 billion times the luminosity of the Sun. For a brief time, a supernova may outshine the entire galaxy in which it appears. Figure \(\PageIndex{3}\) includes supernova 2014J, located in Messier 82 (M82), also known as the Cigar galaxy, roughly 11.5 million light-years from Earth. The supernova event is indicated by the box and the inset.

Figure \(\PageIndex{3}\) : SN 2014J. SN 2014J is a bright spot in this Hubble Space telescope image of M82, the Cigar Galaxy. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{3}\).

Supernova 1987A

In February of 1987, Ian Shelton, a Canadian astronomer working at an observatory in Chile, saw a large bright spot in photograph of the Large Magellanic Cloud, a galaxy about 168,000 light-years away. Concerned that his photograph was flawed, Shelton went outside and saw that a new object had appeared in the sky. Now known as SN 1987A, since it was the first supernova discovered in 1987. It was the first time astronomers had observed a star before it became a supernova, since the star, a blue supergiant called Sanduleak, had been included in earlier surveys of the Large Magellanic Cloud. Later, the Hubble Space Telescope would collect an image of the supernova remnant with its inner and outer red rings of material in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\) : SN 1987A. This image is a composite of several images taken in 1994, 1996, and 1997—about a decade after supernova 1987A was first observed. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{4}\).

History of Sanduleak

By combining theory and observations at many different wavelengths, astronomers have reconstructed the life story of the star that became SN 1987A. Sanduleak formed about 10 million years ago, with an initial mass of about 20 M_Sun. For 90% of its life, it was a main sequence star, converting hydrogen into helium. At this time, its luminosity was about 60,000 times that of the Sun, or 60,000 L_Sun, and its spectral type was O. When the hydrogen in the center of the star was exhausted, the core contracted and ultimately became hot enough to fuse helium. By this time, the star was a red supergiant, emitting about 100,000 times more energy than the Sun. While in this stage, the star lost some of its mass.

This lost material has actually been detected by observations with the Hubble Space Telescope. Figure \(\PageIndex{5}\) includes two images of a ring of gas expelled by Sanduleak about 30,000 years before it exploded and was observed as Supernova 1987A. The supernova, which has been artificially dimmed, is located at the center of the ring. The left-hand image was taken in 1997 and the right-hand image in 2003. The number of bright spots on the ring has increased from 1 to more than 15 over this time interval. These spots occur where high-speed gas ejected by the supernova and moving at millions of miles per hour has reached the ring and blasted into it. The collision has heated the gas in the ring and caused it to glow more brightly. By studying these bright spots, astronomers can determine the composition of the ring and learn about the nuclear processes that build heavy elements inside massive stars.

Figure \(\PageIndex{5}\) : Ring around SN 1987A. Before the star's supernova it released material during its red giant phase, forming a ring. The supernova released more material that has impacted the ring and observations show the number of impacts have increased with time. Since then, new material ejected from the (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{5}\).

Helium fusion lasted only about 1 million years. When the helium was exhausted at the center of the star, the core contracted again, the radius of the surface also decreased, and the star became a blue supergiant with a luminosity still about equal to 100,000 L_Sun. This is what it still looked like on the outside when, after brief periods of further fusion, it reached the end of iron fusion and exploded.

Some key stages of evolution of the star that became SN 1987A, including the ones following helium exhaustion, are listed in \(\PageIndex{1}\). Each stage of evolution happens more quickly than the preceding one, the temperature and pressure in the core increase, and progressively heavier elements are the source of fusion energy. Once iron was created, the collapsed within a few tenths of a second. The speed of infall in the outer portion of the iron core reached 70,000 kilometers per second, about one-fourth the speed of light.

Table \(\PageIndex{1}\): Evolution of SN 1987A

Phase	Central Temperature (K)	Central Density (g/cm^3)	Time Spent in This Phase
Hydrogen fusion	$40\times {10}^6$	5	$8\times {10}^6$ years
Helium fusion	$190\times {10}^6$	970	10^6 years
Carbon fusion	$870\times {10}^6$	170,000	2000 years
Neon fusion	$1.6\times {10}^9$	$3.0\times {10}^6$	6 months
Oxygen fusion	$2.0\times {10}^9$	$5.6\times {10}^6$	1 year
Silicon fusion	$3.3\times {10}^9$	$4.3\times {10}^7$	Days
Core collapse	$200\times {10}^9$	$2\times {10}^{14}$	Tenths of a second

In the meantime, as the core was experiencing its last catastrophe, the outer shells of neon, oxygen, carbon, helium, and hydrogen in the star did not yet know about the collapse. Information about the physical movement of different layers travels through a star at the speed of sound and cannot reach the surface in the few tenths of a second required for the core collapse to occur. The collapse of the core continued until the densities rose to several times that of an atomic nucleus. The resistance to further collapse then became so great that the core rebounded. Infalling material ran into the rebounding core and was thrown outward with a great shock wave. Neutrinos poured out of the core, helping the shock wave blow the star apart. The shock reached the surface of the star a few hours later, and the star began to brighten into a supernova.

Astronomers continue and learn from SN1987A. The James Webb Space Telescope collected the image in Figure \(\PageIndex{6}\) nearly 40 years since its discovery in February of 1987. SN 1987A has a central structure like a keyhole. This center is packed with clumpy gas and dust ejected by the supernova explosion. The dust is so dense that even near-infrared light that Webb detects can’t penetrate it. A bright, equatorial ring surrounds the inner keyhole, forming a band that connects two faint arms of hourglass-shaped outer rings. The equatorial ring is the same ring shown in Figure \(\PageIndex{5}\). Now spots are found on the exterior to the ring, with diffuse light surrounding it. These are the locations of supernova shocks hitting more exterior material.

Figure \(\PageIndex{6}\): JWST Observes SN 1987. James Webb Space Telescope (JWST) observed 1987A almost 40 years after it was first detected, identifying new structures. (Public Domain; NASA, ESA, CSA, M. Matsuura (Cardiff University), R. Arendt (NASA's Goddard Spaceflight Center & University of Maryland, Baltimore County), C. Fransson (Stockholm University), J. Larsson (KTH Royal Institute of Technology), A. Pagan (STScI) via Wikimedia Commons) Alternative description of Figure \(\PageIndex{6}\).

Radioactive Elements

The variations in the brightness of SN 1987A in the days and months after its discovery, which are depicted in Figure \(\PageIndex{7}\). Remember that magnitudes are a backward measure of brightness: the larger the magnitude, the dimmer the object looks. In a single day, the star soared in brightness by a factor of about 1000 and became just visible without a telescope. The star then continued to increase slowly in brightness until it was about the same apparent brightness as the stars in the Little Dipper. Up until about day 40 after the outburst, the energy being radiated away was produced by the explosion itself. But then SN 1987A did not continue to fade away, as we might have expected the light from the explosion to do. Instead, SN 1987A remained bright as energy from newly created radioactive elements came into play.

Figure \(\PageIndex{7}\) : SN 1987A Brightness vs. Time. SN 1987 briefly increased in brightness a few months after it was first detected, likely due to the radioactive decay of Nickel-56. (CC BY; Fraknoi, et al. via Openstax Astronomy 2nd ed.) Alternative description of Figure \(\PageIndex{7}\).

One of the elements formed in a supernova explosion is radioactive nickel, with an atomic mass of 56. Nickel-56 is unstable and changes spontaneously, with a half-life of about 6 days, to cobalt-56. A half-life is the time it takes for half the nuclei in a sample to undergo radioactive decay. Cobalt-56 in turn decays with a half-life of about 77 days to iron-56, which is stable. Energetic gamma rays are emitted when these radioactive nuclei decay. Those gamma rays then serve as a new source of energy for the expanding layers of the supernova. The gamma rays are absorbed in the overlying gas and re-emitted at visible wavelengths, keeping the remains of the star bright.

Astronomers did observe brightening due to radioactive nuclei in the first few months following the supernova’s outburst and then saw the extra light die away as more and more of the radioactive nuclei decayed to stable iron. The gamma-ray heating was responsible for virtually all of the radiation detected from SN 1987A after day 40. Some gamma rays also escaped directly without being absorbed. These were detected by Earth-orbiting telescopes at the wavelengths expected for the decay of radioactive nickel and cobalt, confirming our understanding that new elements were formed in the supernova.

Neutrinos from SN 1987A

If there had been any human observers in the Large Magellanic Cloud about 160,000 years ago, the explosion we call SN 1987A would have been a brilliant spectacle in their skies. Yet we know that less than 1/10 of 1% of the energy of the explosion appeared as visible light. About 1% of the energy was required to destroy the star, and the rest was carried away by neutrinos. In the initial second of the event, their total luminosity exceeded the luminosity of all the stars in over a billion galaxies. The supernova generated this energy in a volume less than 50 kilometers in diameter. Since the neutrinos come directly from the heart of the supernova, their energies provided a measure of the temperature of the core as the star was exploding. The central temperature was about 200 billion K.

In 1987, the neutrinos from SN 1987A were detected by two instruments almost a full day before Shelton’s observations. This is because the neutrinos get out of the exploding star more easily than light does, and also because you don’t need to wait until nightfall to detect them. Both neutrino telescopes, one in a deep mine in Japan and the other under Lake Erie, consist of several thousand tons of purified water surrounded by several hundred light-sensitive detectors. Incoming neutrinos interact with the water to produce positrons and electrons, which move rapidly through the water and emit deep blue light. Altogether, 19 neutrinos were detected. Since the neutrino telescopes were in the Northern Hemisphere and the supernova occurred in the Southern Hemisphere, the detected neutrinos had already passed through Earth and were on their way back out into space when they were captured. Only a few neutrinos were detected because the probability that they will interact with ordinary matter is very, very low. It is estimated that the supernova actually released 10⁵⁸ neutrinos.