The Betelgeuse in the constellation of Orion is a fascinating star. It’s a red supergiant more than six hundred times the diameter of the Sun, making it one of the largest stars known to astronomers and one of the brightest stars in the night sky. Despite being only 10 million years old, Betelgeuse has already exhausted the hydrogen fuel in its core and is now in the final stage of its life. Eventually, gravity will cause the core to collapse, resulting in a spectacular supernova explosion. This explosion will be so massive and violent that, even though Betelgeuse is 650 light-years away from Earth, it will illuminate the night sky as brightly as the full moon for up to three months. However, we will not witness this dramatic event, as astronomers predict it won’t occur for another 100,000 years.
A mesmerizing supernova remnant called G299 located 16,000 light years from Earth in the Milky Way galaxy.
Astronomers have observed supernovae long before they have understood them. Often, these dying stars were too dim to be seen with the naked eye, but when they suddenly flared up in a brilliant explosion, observers concluded it must be a brand-new star, hence the term “nova,” meaning “new” in Latin.
On average, a supernova explosion occurs somewhere in the Milky Way Galaxy every 50 years. Given the rarity of these events and the fact that we can observe only a tiny fraction of our galaxy, it is really fortuitous that we have been able to observe several such such events with the naked eye.
SN 185
Composite images in the infrared and X-ray spectrum of RCW 86, the supernova remnant of SN 185.
The first written evidence of a supernova comes from China in the year 185 CE. Record of this “guest star”—a term that Chinese astronomers used to denote a star which suddenly appeared in a place where no star had previously been observed and becomes invisible again after some time—first appeared in The Book of Later Han, where the author described the star to be “as large as half a yan, with scintillating, variegated colors”, and that it was visible in the sky for eight months. The word “yan” has been interpreted as “bamboo mat”, which might have a size of about 6 feet. But it could also mean a yan-chuang, which is very low stool about 2 feet across. Astronomers from the Chinese Academy of Sciences have argued in a paper published in 2006 that “as large as half a yan” is only an imaginary figuration of the ancient observer and might not reflect the true scale of the spectacle. However, they concede that a bright source of light can appear larger than they actually is due to the effect of intense refraction and dispersion by the atmosphere.
With the aid of modern telescopes, astronomers have identified the exploding star as RCW 86 in the southern constellation Circinus. The remnant of this supernova has a diameter of 85 light years and is located 9,000 light years away. When viewed from Earth, it appears wider than the full moon. Of course, the gases have cooled down to a very low temperature, so it is no longer visible to the unaided eye. But you can imagine why someone might have described it as “as large as half a yan” when it was shining in its full glory.
Also read: Halton Arp’s Atlas of Peculiar Galaxies
SN 386
In 386, Chinese astronomers reported another “guest star” in the constellation Sagittarius that remained visible for three months. Due to the given duration of the appearance, it has been suggested that the object was a supernova. Since 1976, several supernova remnants have been identified in the region as possible candidates of the year 386 explosion.
SNR G11.2−0.3 was initially regarded as the prime candidate remnant of the guest star, but was recently ruled out as it cannot have been visible to the naked eye, given its distance and large extinction. Another possible candidate is SN G7.7−3.7, which some astronomers believe fits the age (less than 2000 years old) and position of the reported guest star.
SN 393
Just seven years later, yet another guest star was spotted by Chinese astronomers in the modern constellation of Scorpius. The guest star, which was as bright at Sirius—the brightest star in the night sky, remained visible for about eight months before fading from sight. Given the duration of the event, it has been suggested that it was a supernova.
The quest to identify SN 393 proved to be difficult. There were seven supernova remnants near where SN 393 was observed. Four were immediately ruled out for not matching the reported magnitude of SN 393. Another was discounted for being too old. Of the remaining two, both had the correct age—about 1,500 years—and identical distance of 33,000 light years, but they are located close to a particularly dusty part of the galactic plane and would not have been visible to the naked eye over eight months.
Another possible candidate was discovered in 1996, which proved to be a better match for SN 393. This supernova remnant, RX J1713.7-3946, was initially observed to be at 20,000 light year, but in 2003 it was re-calculated to be much closer, at only 3,000 light year. This star which exploded to form RX J1713.7-3946 was at least 15 times larger than the sun, and upon destruction, about three solar masses of material was ejected into the surrounding interstellar medium.
SN 1006
False-color X-ray image of SN 1006 supernova remnant.
The brightest supernova ever observed from earth appeared in the year 1006 CE in the southern constellation of Lupus. It was bright enough to cast shadows on the ground during the night and was recorded with awe and fear by observers all over Europe and Asia. Egyptian astrologer and astronomer Ali ibn Ridwan stated that the "spectacle was a large circular body, 21⁄2 to 3 times as large as Venus” and as bright as a quarter of the moon. According to reported observations, there appears to have been two distinct phases in the early evolution of this supernova. There was first a three-month period at which it was at its brightest. After this period it diminished, and then returned after some time. The supernova then glowed for another eighteen months.
Modern astronomers have discovered the faint remnant of this explosion at a distance of 7,100 light-years from the Earth. It is believed that the supernova was produced either by mass accretion from an unevolved star, similar to or less massive than the Sun, or by merging with another white dwarf.
SN 1054
The famous Crab Nebula, the remnants of SN 1054, taken by the Hubble Space Telescope in visible light.
Another spectacular supernova observation was made in the year 1054 CE in the constellation of Taurus. The event was recorded extensively in Chinese astronomy, as well as in Japanese documents and in the Islamic world. Furthermore, there are a number of proposed references from European sources, as well as a pictograph associated with the Ancestral Puebloan culture in New Mexico, United States.
At its peak, the luminosity of SN 1054 may have been four times as bright as Venus, and it remained visible in daylight for 23 days and more than 650 days in the night sky.
The remnant of SN 1054 is the famous Crab Nebula, one of the most studied astronomical objects outside the Solar System. The Crab Nebula is easily observed by amateur astronomers thanks to its brightness, and was also catalogued early on by professional astronomers, long before its true nature was understood and identified. It is one of the few galactic supernovae where the date of the explosion is well known and is one of the best known supernovae in the history of astronomy.
SN 1181
Records of a “guest star” that appeared in the constellation Cassiopeia in year 1181 CE can be found in Chinese and Japanese text. It remained in the sky for 185 days, immovable against the fixed stars in the sky, which can only be explained as a supernova.
Before 2013, the pulsar 3C 58 was considered as the most likely stellar relic from this event. However, this association was problematic because the pulsar rotates too fast to have been formed only 800 years ago. Direct measurements of the expanding shell of 3C 58, as well as measurements of the pulsar’s offset from the center of 3C 58, the pulsar’s rate of loss of rotational energy, and the rate of cooling, all yields an age between 3,700 years to 5,300 years, which is much too old to be a remnant of SN 1181.
The most likely candidate is Pa 30, a nebula with a hot central star (the hottest star known) and an intense stellar wind expanding at a very high velocity of 16,000 km/s. Such a speed could only arise from a supernova or an event of similar magnitude. It has been suggested that Pa 30 is the remnant of a rare class of supernovae known as "sub-luminous Type Iax Supernova" and that a merger of a carbon and oxygen rich white dwarf and an oxygen and neon rich white dwarf produced the remnant shell along with its supermassive white dwarf remnant.
SN 1572
Remnant of Tycho's Supernova as seen in X-ray light.
The supernova of 1572 was described in great detail by Danish astronomer Tycho Brahe because of which it is often referred to as Tycho's Supernova.
The supernova appeared in early November 1572 in the constellation Cassiopeia. Within a week it was already brighter than Jupiter, and two weeks later achieved peak brightness at about magnitude −4.0, with some descriptions giving it as equal to Venus when that planet was at its brightest. Contrarily, Brahe described the supernova as "brighter than Venus". The supernova remained visible to the naked eye into early 1574, gradually fading until it disappeared from view.
During this period in Europe, a common belief was the Aristotelian idea that the cosmos beyond the Moon and planets was unchanging. Consequently, observers argued that any unusual phenomenon must be occurring within the Earth's atmosphere. However, Tycho Brahe noted that the object remained stationary night after night, without changing its parallax, indicating that it was located far away. He published his observations in a small book titled De nova et nullius aevi memoria prius visa stella (Latin for "Concerning the new and previously unseen star") in 1573. The modern term “nova” for cataclysmic variable stars is derived from the title of this book.
The star that went supernova was a white dwarf star that had accreted matter from a companion star until it approached the Chandrasekhar limit and exploded. The supernova remnant now consist of a shell of gas expanding from its center at about 5,000 km/s.
SN 1604
A false-color composite image of the supernova remnant nebula from SN 1604.
The last visible supernova in the Milky Way Galaxy was SN 1604, which was observed on October 9, 1604. Several people, including Johannes van Heeck, noted the sudden appearance of this star, but it was Johannes Kepler who became noted for his systematic study of the object itself. Thus the supernova is also known as Kepler's Supernova.
Kepler's Supernova was brighter at its peak than any other star in the night sky, with an apparent magnitude of −2.5. It was visible during the day for over three weeks, and almost a year during night.
The supernova remnant of SN 1604 is a dim nebula invisible without a telescope. Only filaments can be seen in visible light, but it is a strong radio and X-ray source. It is located 16,000 light years away from earth.
Telescopic Observations
The true nature of the supernova eluded astronomers until 1866, when English astronomer William Huggins made the first spectroscopic observations of a nova, discovering lines of hydrogen in the unusual spectrum of the recurrent nova T Coronae Borealis. Huggins proposed a cataclysmic explosion as the underlying mechanism, and his efforts drew interest from other astronomers.
The first telescopic observation of a supernova, and the first one to be seen outside the Milky Way was SN 1885A. The supernova was initially seen on August 17, 1885, by French astronomer Ludovic Gully during a public stargazing event. But Gully thought it was scattered moonlight in his telescope and did not follow up on this observation. It was Estonian astronomer Ernst Hartwig who first identified the object, leading to a flurry of observations across the globe.
SN 1885A reached a peak magnitude of 5.85, or about as bright as the planet Uranus, briefly outshining the entire nucleus of the Andromeda Galaxy, where it occurred, before fading away six months later.
The term supernova wasn’t used until the 1930s when Walter Baade and Fritz Zwicky began studying this new phenomenon. They postulated that the energy generated from a supernova was due to the gravitational collapse of ordinary stars into neutron stars. The fact that many heavy elements in the universe are produced in supernova by thermonuclear reactions was proposed by Fred Hoyle in 1946. Together with William Fowler, Hoyle proposed the concept that rapid nuclear fusion was the source of energy for a supernova explosion.
Supernova 1987A as captured by the Hubble Space Telescope.
The Supernova 1987A was the first supernova that modern astronomers were able to study in great detail. It occurred in 1987 in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way, about 168,000 light-years from Earth. Light from the explosion reached earth on February 23, 1987 and on May of that year, its brightness peaked becoming brighter than the Andromeda Galaxy. The relative proximity of this supernova has allowed detailed observation, and it provided the first opportunity for modern theories of supernova formation to be tested against observations.
The rate of supernova discovery steadily increased throughout the twentieth century. In the 1990s, several automated supernova search programs were initiated. One of the most successful was the Lick Observatory Supernova Search (LOSS), which discovered 96 supernovae by year 2000. Thanks to better equipment, the number of supernovae discovery have exploded exponentially in the past two decades with more than 20,000 discoveries made in the year 2023 alone. The Vera C. Rubin Observatory in Chile, which is still under construction at the time of this writing, is predicted to discover three to four million supernovae during its ten-year survey.
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