SN 1987A

SN 1987A was a peculiar type II supernova in the Large Magellanic Cloud, a dwarf galaxy satellite of the Milky Way. It occurred approximately 51.4 kiloparsecs (168,000 light-years) from Earth and was the closest observed supernova since Kepler's Supernova, visible from earth in 1604. 1987A's light reached Earth on February 23, 1987,[4] and as the first supernova discovered that year, was labeled "1987A". Its brightness peaked in May, with an apparent magnitude of about 3.

It was the first opportunity for modern astronomers to study the development of a supernova in great detail, and its observations have provided much insight into core-collapse supernovae.

SN 1987A provided the first chance to confirm by direct observation the radioactive source of the energy for visible light emissions, by detecting predicted gamma-ray line radiation from two of its abundant radioactive nuclei. This proved the radioactive nature of the long-duration post-explosion glow of supernovae.

Discovery

Large.mc.arp.750pix
SN 1987A within the Large Magellanic Cloud

SN 1987A was discovered independently by Ian Shelton and Oscar Duhalde at the Las Campanas Observatory in Chile on February 24, 1987, and within the same 24 hours by Albert Jones in New Zealand.[2] On March 4–12, 1987, it was observed from space by Astron, the largest ultraviolet space telescope of that time.[5]

Progenitor

SN 1987A HST
The remnant of SN 1987A[6]

Four days after the event was recorded, the progenitor star was tentatively identified as Sanduleak −69 202 (Sk -69 202), a blue supergiant.[7] After the supernova faded, that identification was definitely confirmed by Sk −69 202 having disappeared. This was an unexpected identification, because models of high mass stellar evolution at the time did not predict that blue supergiants are susceptible to a supernova event.

Some models of the progenitor attributed the color to its chemical composition rather than its evolutionary state, particularly the low levels of heavy elements, among other factors.[8] There was some speculation that the star might have merged with a companion star before the supernova.[9] However, it is now widely understood that blue supergiants are natural progenitors of some supernovae, although there is still speculation that the evolution of such stars could require mass loss involving a binary companion.[10]

Neutrino emissions

Composite image of Supernova 1987A
Remnant of SN 1987A seen in light overlays of different spectra. ALMA data (radio, in red) shows newly formed dust in the center of the remnant. Hubble (visible, in green) and Chandra (X-ray, in blue) data show the expanding shock wave.

Approximately two to three hours before the visible light from SN 1987A reached Earth, a burst of neutrinos was observed at three neutrino observatories. This was likely due to neutrino emission, which occurs simultaneously with core collapse, but before visible light was emitted. Visible light is transmitted only after the shock wave reaches the stellar surface.[11] At 07:35 UT, Kamiokande II detected 12 antineutrinos; IMB, 8 antineutrinos; and Baksan, 5 antineutrinos; in a burst lasting less than 13 seconds. Approximately three hours earlier, the Mont Blanc liquid scintillator detected a five-neutrino burst, but this is generally not believed to be associated with SN 1987A.[8]

The Kamiokande II detection, which at 12 neutrinos had the largest sample population, showed the neutrinos arriving in two distinct pulses. The first pulse started at 07:35:35 and comprised 9 neutrinos, all of which arrived over a period of 1.915 seconds. A second pulse of three neutrinos arrived between 9.219 and 12.439 seconds after the first neutrino was detected, for a pulse duration of 3.220 seconds.

Although only 25 neutrinos were detected during the event, it was a significant increase from the previously observed background level. This was the first time neutrinos known to be emitted from a supernova had been observed directly, which marked the beginning of neutrino astronomy. The observations were consistent with theoretical supernova models in which 99% of the energy of the collapse is radiated away in the form of neutrinos.[12] The observations are also consistent with the models' estimates of a total neutrino count of 1058 with a total energy of 1046 joules, i.e. a mean value of some dozens of MeV per neutrino.[13]

The neutrino measurements allowed upper bounds on neutrino mass and charge, as well as the number of flavors of neutrinos and other properties.[8] For example, the data show that within 5% confidence, the rest mass of the electron neutrino is at most 16 eV/c2, 1/30,000 the mass of an electron. The data suggest that the total number of neutrino flavors is at most 8 but other observations and experiments give tighter estimates. Many of these results have since been confirmed or tightened by other neutrino experiments such as more careful analysis of solar neutrinos and atmospheric neutrinos as well as experiments with artificial neutrino sources.[14][15][16]

Missing neutron star

New image of SN 1987A
The bright ring around the central region of the exploded star is composed of ejected material.[17]

SN 1987A appears to be a core-collapse supernova, which should result in a neutron star given the size of the original star.[8] The neutrino data indicate that a compact object did form at the star's core. However, since the supernova first became visible, astronomers have been searching for the collapsed core but have not detected it. The Hubble Space Telescope has taken images of the supernova regularly since August 1990, but, so far, the images have shown no evidence of a neutron star. A number of possibilities for the 'missing' neutron star are being considered.[18] The first is that the neutron star is enshrouded in dense dust clouds so that it cannot be seen. Another is that a pulsar was formed, but with either an unusually large or small magnetic field. It is also possible that large amounts of material fell back on the neutron star, so that it further collapsed into a black hole. Neutron stars and black holes often give off light as material falls onto them. If there is a compact object in the supernova remnant, but no material to fall onto it, it would be very dim and could therefore avoid detection. Other scenarios have also been considered, such as whether the collapsed core became a quark star.[19][20]

Light curve

Much of the light curve, or graph of luminosity as a function of time, after the explosion of a type II supernova such as SN 1987A is provided its energy by radioactive decay. Although the luminous emission consists of optical photons, it is the radioactive power absorbed that keeps the remnant hot enough to radiate light. Without radioactive heat it would quickly dim. The radioactive decay of 56Ni through its daughters 56Co to 56Fe produces gamma-ray photons that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times (several weeks) to late times (several months).[21] Energy for the peak of the light curve of SN1987A was provided by the decay of 56Ni to 56Co (half life of 6 days) while energy for the later light curve in particular fit very closely with the 77.3-day half-life of 56Co decaying to 56Fe. Later measurements by space gamma-ray telescopes of the small fraction of the 56Co and 57Co gamma rays that escaped the SN1987A remnant without absorption[22][23] confirmed earlier predictions that those two radioactive nuclei were the power source.[24]

Because the 56Co in SN1987A has now completely decayed, it no longer supports the luminosity of the SN 1987A ejecta. That is currently powered by the radioactive decay of 44Ti with a half life of about 60 years. With this change, X-rays produced by the ring interactions of the ejecta began to contribute significantly to the total light curve. This was noticed by the Hubble Space Telescope as a steady increase in luminosity 10,000 days after the event in the blue and red spectral bands.[25] X-ray lines 44Ti observed by the INTEGRAL space X-ray telescope showed that the total mass of radioactive 44Ti synthesized during the explosion was 3.1 ± 0.8×10−4 M.[26]

Observations of the radioactive power from their decays in the 1987A light curve have measured accurate total masses of the 56Ni, 57Ni, and 44Ti created in the explosion, which agree with the masses measured by gamma-ray line space telescopes and provides nucleosynthesis constraints on the computed supernova model.[27]

Interaction with circumstellar material

Sn87a
The expanding ring-shaped remnant of SN 1987A and its interaction with its surroundings, seen in X-ray and visible light.
SN1987a debris evolution animation time scaled
Sequence of HST images from 1994 to 2009, showing the collision of the expanding remnant with a ring of material ejected by the progenitor 20,000 years before the supernova[28]

The three bright rings around SN 1987A that were visible after a few months in images by the Hubble Space Telescope are material from the stellar wind of the progenitor. These rings were ionized by the ultraviolet flash from the supernova explosion, and consequently began emitting in various emission lines. These rings did not "turn on" until several months after the supernova; the turn-on process can be very accurately studied through spectroscopy. The rings are large enough that their angular size can be measured accurately: the inner ring is 0.808 arcseconds in radius. The time light traveled to light up the inner ring gives its radius of 0.66 (ly) light years. Using this as the base of a right angle triangle and the angular size as seen from the Earth for the local angle, one can use basic trigonometry to calculate the distance to SN 1987A, which is about 168,000 light-years.[29] The material from the explosion is catching up with the material expelled during both its red and blue supergiant phases and heating it, so we observe ring structures about the star.

Around 2001, the expanding (>7000 km/s) supernova ejecta collided with the inner ring. This caused its heating and the generation of x-rays—the x-ray flux from the ring increased by a factor of three between 2001 and 2009. A part of the x-ray radiation, which is absorbed by the dense ejecta close to the center, is responsible for a comparable increase in the optical flux from the supernova remnant in 2001–2009. This increase of the brightness of the remnant reversed the trend observed before 2001, when the optical flux was decreasing due to the decaying of 44Ti isotope.[28]

A study reported in June 2015,[30] using images from the Hubble Space Telescope and the Very Large Telescope taken between 1994 and 2014, shows that the emissions from the clumps of matter making up the rings are fading as the clumps are destroyed by the shock wave. It is predicted the ring will fade away between 2020 and 2030. These findings are also supported by the results of a three-dimensional hydrodynamic model which describes the interaction of the blast wave with the circumstellar nebula.[31] The model also shows that X-ray emission from ejecta heated up by the shock will be dominant very soon, after the ring will fade away. As the shock wave passes the circumstellar ring it will trace the history of mass loss of the supernova's progenitor and provide useful information for discriminating among various models for the progenitor of SN 1987A.[32]

In 2018, radio observations from the interaction between the circumstellar ring of dust and the shockwave has confirmed the shockwave has now left the circumstellar material. It also shows that the speed of the shockwave, which slowed down to 2,300 km/s while interacting with the dust in the ring, has now re-accelerated to 3,600 km/s.[33]

Condensation of warm dust in the ejecta

Images of the Warm Dust in the SN 1987A debris
Images of the SN 1987A debris obtained with the instruments T-ReCS at the 8-m Gemini telescope and VISIR at one of the four VLT. Dates are indicated. An HST image is inserted at the bottom right (credits Patrice Bouchet, CEA-Saclay)

Soon after the SN 1987A outburst, three major groups embarked in a photometric monitoring of the supernova: SAAO,[34][35] CTIO,[36][37] and ESO.[38][39] In particular, the ESO team reported an infrared excess which became apparent beginning less than one month after the explosion (March 11, 1987). Three possible interpretations for it were discussed in this work: the infrared echo hypothesis was discarded, and thermal emission from dust that could have condensed in the ejecta was favoured (in which case the estimated temperature at that epoch was ~ 1250 K, and the dust mass was approximately 6.6×10−7 M). The possibility that the IR excess could be produced by optically thick free-free emission seemed unlikely because the luminosity in UV photons needed to keep the envelope ionized was much larger than what was available, but it was not ruled out in view of the eventuality of electron scattering, which had not been considered.

However, none of these three groups had sufficiently convincing proofs to claim for a dusty ejecta on the basis of an IR excess alone.

Model of the dust distribution
Distribution of the dust inside the SN 1987A ejecta, as from the Lucy et al.'s model built at ESO[40]

An independent Australian team advanced several argument in favour of an echo interpretation.[41] This seemingly straightforward interpretation of the nature of the IR emission was challenged by the ESO group[42] and definitively ruled out after presenting optical evidence for the presence of dust in the SN ejecta.[43] To discriminate between the two interpretations, they considered the implication of the presence of an echoing dust cloud on the optical light curve, and on the existence of diffuse optical emission around the SN.[44] They concluded that the expected optical echo from the cloud should be resolvable, and could be very bright with an integrated visual brightness of magnitude 10.3 around day 650. However, further optical observations, as expressed in SN light curve, showed no inflection in the light curve at the predicted level. Finally, the ESO team presented a convincing clumpy model for dust condensation in the ejecta.[40][45]

Although it had been thought more than 50 years ago that dust could form in the ejecta of a core-collapse supernova,[46] which in particular could explain the origin of the dust seen in far galaxies,[47] that was the first time that such a condensation was observed. If SN 1987A is a typical representative of its class then the derived mass of the warm dust formed in the debris of core collapse supernovae is not sufficient to account for all the dust observed in the early universe. However, a much larger reservoir of ~0.25 solar mass of colder dust (at ~26 K) in the ejecta of SN 1987A was found[48] with the Hershel infrared space telescope in 2011 and confirmed by ALMA[49] later on (in 2014).

ALMA Observations

Following the confirmation of a large amount of cold dust in the ejecta,[49] ALMA has continued observing SN 1987A. A synchrotron radiation due to shock interaction in the equatorial ring has been measured. Cold (20–100K) carbon monoxide (CO) and silicate molecules (SiO) were observed. The data show that CO and SiO distributions are clumpy, and that different nucleosynthesis products (C, O and Si) are located in different places of the ejecta, indicating the footprints of the stellar interior at the time of the explosion.[50][51][52]

See also

References

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Further reading

External links

Astron (spacecraft)

Astron was a Soviet spacecraft launched on 23 March 1983 at 12:45:06 UTC, using Proton launcher, which was designed to fulfill an astrophysics mission. It was based on the 4MV spacecraft design and was operational for six years as the largest ultraviolet space telescope during its lifetime. The project was headed by Alexandr Boyarchuk.The spacecraft was designed and constructed by the Crimean Astrophysical Observatory and NPO Lavochkin. A group of scientists from these institutions was awarded the USSR State Prize for their work on Astron.Astron's payload consisted of an 80 cm ultraviolet telescope which was designed jointly by the USSR and France, and an X-ray spectroscope on board.

It could take UV spectra 150-350 nm.Placed into an orbit with an apogee of 185,000 kilometres (115,000 mi) it could make observations outside the Earth's umbra and radiation belt.

Among the most important observations by Astron were those of the SN 1987A supernova on March 4–12, 1987 and of Halley's Comet in December, 1985, that allowed a group of Soviet scientists to develop a model of the coma surrounding Halley's Comet.

Blue supergiant star

Blue supergiant stars are hot luminous stars, referred to scientifically as OB supergiants. They have luminosity class I and spectral class B9 or earlier.Blue supergiants (BSGs) are found towards the top left of the Hertzsprung–Russell diagram to the right of the main sequence. They are larger than the Sun but smaller than a red supergiant, with surface temperatures of 10,000–50,000 K and luminosities from about 10,000 to a million times the Sun.

Convective overturn

The convective overturn model of supernovae was proposed by Bethe and Wilson in 1985, and received a dramatic test with SN 1987A, and the detection of neutrinos from the explosion. The model is for type II supernovae, which take place in stars more massive than 8 solar masses.

When the iron core of a super massive star becomes heavier than electron degeneracy pressure can support, the core of the star collapses, and the iron core is compressed by gravity until nuclear densities are reached when a strong rebound sends a shock wave throughout the rest of the star and tears it apart in a large supernova explosion. The remains of this core will eventually become a neutron star. The collapse produces two reactions: one breaks apart iron nuclei into 13 helium atoms and 4 neutrons, absorbing energy; and the second produces a wave of neutrinos that form a shock wave. While all models agree that there is a convective shock, there is disagreement as to how important that shock is to the supernova explosion.

In the convective overturn model, the core collapses faster and faster, exceeding the speed of sound inside the star, and producing a supersonic shock wave. This shock wave explodes outward until it stalls when it reaches the neutrinosphere, where the pressure of the star collapsing inward exceeds the pressure of the neutrinos radiating outwards. This point produces heavier elements as the neutrinos are absorbed.

The stalling of the shock wave represents the supernova problem, because once stalled, the shock wave should not be "reenergized". The prompt convection model states that the shock wave will increase the luminosity of the neutrinos produced by the core collapse, and this increase in energy will start the shock wave going again. The neutron fingers model has instability near the core expel another wave of energized neutrinos which reenergizes the shock wave. The entropy convection model has matter falling inward from above the shock layer down to the gain radius, which would not increase neutrino luminosity, but would allow the shock wave to continue outwards.

All of these models exhibit convective overturn in that they rely on a convection mechanism to re-energize the stalled shock wave and complete the supernova explosion.

There are still open issues in both the convective models and in the more general core collapse model, which include not taking into account flavor mixing and mass of neutrinos, and the inability to model large explosions. Current models indicate that the collapse may occur more slowly than thought before, which would mean the shock wave would penetrate farther into the upper layers of the star. The proto-neutron star boosts neutrino luminosities, and the additional neutrinos emitted help re-energize the shock wave. These changes remove some, but not all, of the supernova problem, and strengthen the idea of convection being an important factor in supernova explosions.

Ian Shelton

Ian Keith Shelton (born 30 March 1957) is a Canadian astronomer who discovered SN 1987A, the first modern supernova close and bright enough to be visible to the naked eye.

Born in Winnipeg, Manitoba, Canada, Shelton received his B.Sc. in 1979 from the University of Manitoba and in 1981 began his professional career working as Resident Astronomer at the University of Toronto Southern Observatory at Las Campanas, Chile.

International Ultraviolet Explorer

The International Ultraviolet Explorer (IUE) was an astronomical observatory satellite primarily designed to take ultraviolet spectra. The satellite was a collaborative project between NASA, the UK Science Research Council and the European Space Agency (ESA). The mission was first proposed in early 1964, by a group of scientists in the United Kingdom, and was launched on January 26, 1978 aboard a NASA Delta rocket. The mission lifetime was initially set for 3 years, but in the end it lasted almost 18 years, with the satellite being shut down in 1996. The switch-off occurred for financial reasons, while the telescope was still functioning at near original efficiency.

It was the first space observatory to be operated in real time by astronomers who visited the groundstations in the United States and Europe. Astronomers made over 104,000 observations using the IUE, of objects ranging from solar system bodies to distant quasars. Among the significant scientific results from IUE data were the first large scale studies of stellar winds, accurate measurements of the way interstellar dust absorbs light, and measurements of the supernova SN1987A which showed that it defied stellar evolution theories as they then stood. When the mission ended, it was considered the most successful astronomical satellite ever.

Irvine–Michigan–Brookhaven (detector)

IMB, the Irvine-Michigan-Brookhaven detector, was a nucleon decay experiment and neutrino observatory located in a Morton Salt company's Fairport mine on the shore of Lake Erie in the United States 600 meters underground. It was a joint venture of the University of California, Irvine, the University of Michigan, and the Brookhaven National Laboratory. Like several other particle detectors (see Kamiokande II), it was built primarily with the goal of observing proton decay, but it achieved greater fame through neutrino observation, particularly those from Supernova SN 1987A.

Kvant-1

Kvant-1 (Russian: Квант-1; English: Quantum-I/1) (37KE) was the first module to be attached in 1987 to the Mir Core Module, which formed the core of the Soviet space station Mir. It remained attached to Mir until the entire space station was deorbited in 2001.The Kvant-1 module contained scientific instruments for astrophysical observations and materials science experiments.

It was used to conduct research into the physics of active galaxies, quasars and neutron stars and it was uniquely positioned for studies of the Supernova SN 1987A. Furthermore, it supported biotechnology experiments in anti-viral preparations and fractions.

Some additions to Kvant-1 during its lifetime were solar arrays and the Sofora and Rapana girders.

The Kvant-1 module was based on the TKS spacecraft and was the first, experimental version of a planned series of '37K' type modules. The 37K modules featured a jettisonable TKS-E type propulsion module, also called the Functional Service Module (FSM).

The control system of Kvant-1 had been developed by NPO "Electropribor" (Kharkiv, Ukraine).After previous engineering tests with the Salyut 6 and Salyut 7 space stations (and temporarily attached TKS derived space station modules like Kosmos 1267, Kosmos 1443 and Kosmos 1686) it became the first space station module to be attached semipermanently to the first modular space station in the history of space flight.

Kvant-1 was originally planned to be docked to the Salyut 7 space station, the plans however evolved to launch to Mir, initially considered on board the Soviet Buran space shuttle, which finally changed to a launch to Mir by the Proton-K rocket.

Mark M. Phillips

Mark M. Phillips (born March 31, 1951) is an American astronomer who works on the observational studies of all classes of supernovae. He has worked on SN 1986G, SN 1987A, the Calán/Tololo Supernova Survey, the High-Z Supernova Search Team, and the Phillips relationship. This relationship has allowed the use of Type Ia supernovae as standard candles, leading to the precise measurements of the Hubble constant H0 and the deceleration parameter q0, the latter implying the existence of dark energy or a cosmological constant in the Universe.

He is the past director of Cerro Tololo Inter-American Observatory of the National Optical Astronomy Observatory and is the Associate Director and Carnegie Staff Member at Las Campanas Observatory in Chile, part of the Observatories of the Carnegie Institution for Science.

He received his undergraduate degree in Astronomy from San Diego State University in 1973, and his Ph.D., also in Astronomy & Astrophysics in 1977, from the University of California, Santa Cruz and Lick Observatory where he was a student of Professor Donald Osterbrock. After graduate school, he was a postdoc at CTIO, then at Anglo-Australian Observatory, moving back to Chile in 1982 to become a staff astronomer at CTIO.

In addition to his work on supernovae, he has also worked extensively on the spectroscopic studies of Active Galactic Nuclei.

Multi-messenger astronomy

Multi-messenger astronomy is astronomy based on the coordinated observation and interpretation of disparate "messenger" signals. Interplanetary probes can visit objects within the Solar System, but beyond that, information must rely on "extrasolar messengers". The four extrasolar messengers are electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays. They are created by different astrophysical processes, and thus reveal different information about their sources.

The main multi-messenger sources outside the heliosphere are expected to be compact binary pairs (black holes and neutron stars), supernovae, irregular neutron stars, gamma-ray bursts, active galactic nuclei, and relativistic jets. The table below lists several types of events and expected messengers.

Detection from one messenger and non-detection from a different messenger can also be informative.

NGC 150

NGC 150 (also known as PGC 2052) is a barred spiral galaxy in the constellation Sculptor. It is about 70 million light years away from the solar system, and it has a diameter of about 55,000 light years. It was discovered on 20 November 1886, by Lewis A. Swift. The Type II supernova SN 1990K was detected in NGC 150, and was reported to be similar to SN 1987A.

NGC 4921

NGC 4921 is a barred spiral galaxy in the Coma Cluster, located in the constellation Coma Berenices. It is about 320 million light-years from Earth. The galaxy has a nucleus with a bar structure that is surrounded by a distinct ring of dust that contains recently formed, hot blue stars. The outer part consists of unusually smooth, poorly distinguished spiral arms.In 1976, Canadian astronomer Sidney Van den Bergh categorized this galaxy as "anemic" because of the low rate at which stars are being formed. He noted that it has "an unusually low surface brightness and exhibits remarkably diffuse spiral arms". Nonetheless, it is the brightest spiral galaxy in the Coma Cluster. This galaxy is located near the center of the cluster and has a high relative velocity (7,560 km/s) compared to the mean cluster velocity. When examined at the 21 cm wavelength Hydrogen line, NGC 4921 was found to be strongly H I deficient, indicating it is low in hydrogen. The distribution of hydrogen has also been deeply perturbed toward the SE spiral arm and is less extended than the optical disk of the galaxy. This may have been caused by interaction with the intergalactic medium, which is stripping off the gas.On May 4, 1959, a supernova explosion was observed in this galaxy by M. L. Humason using a Schmidt telescope at the Palomar Observatory. It appeared "quite far from the center" of the galaxy, and reached an estimated peak magnitude of 18.5. The light curve proved similar to supernova SN 1987a in the Large Magellanic Cloud, and it displayed "unusual photometric behavior".

Neil Gehrels

Cornelis A. "Neil" Gehrels (October 3, 1952 – February 6, 2017) was an American astrophysicist specializing in the field of gamma-ray astronomy. He was Chief of the Astroparticle Physics Laboratory at NASA's Goddard Space Flight Center from 1995 until his death, and was best known for his work developing the field from early balloon instruments to today's space observatories such as the NASA Swift mission, for which he was the Principal Investigator. He was leading the WFIRST wide-field infrared telescope forward toward a launch in the mid-2020s. He was a member of the National Academy of Sciences and the American Academy of Arts and Sciences.

Gehrels died on February 6, 2017, at the age of 64. On January 10th, 2018, NASA announced that Swift had been renamed the Neil Gehrels Swift Observatory, in his honor..

Patrice Bouchet

Patrice Jean Emmanuel Bouchet de Puyraimond is a French astrophysicist best known for his discovery of the Rings of Neptune, his infrared observations of supernova SN 1987A in the Large Magellanic Cloud, and the dust extinction law in the Small Magellanic Cloud.

Red Square Nebula

The Red Square Nebula is a celestial object located in the area of the sky occupied by star MWC 922 in the constellation Serpens. The first images of this bipolar nebula, taken using the Mt. Palomar Hale telescope in California, were released in April 2007. It is notable for its square shape, which according to Sydney University astrophysicist Peter Tuthill, makes it one of the most symmetrical celestial objects ever imaged.The explanation proposed by Tuthill and his collaborator James Lloyd of Cornell University is that the square shape arises from two cone shapes placed tip-to-tip, as seen from the side. This also explains the "double-ring" structure seen in SN 1987A.A series of faint spokes radiate from the center of the structure. One possible explanation is that these spokes are shadows cast by periodic ripples or waves on the surface of an inner disk close to the central star.There is no clear explanation of how the central star could produce the nebula's shape:

Towards the end of their lives, many low-mass stars, like the Sun, slough off their outer layers to produce striking 'planetary' nebulae. But the hot star at the heart of the Red Square nebula, called MWC 922, appears to be relatively massive, suggesting another process formed its signature shape. "How did all this beautiful, crisp structure form?" asks Peter Tuthill of the University of Sydney in Australia. "This is the million dollar question."

SN 1972E

SN1972E was a supernova in the galaxy NGC 5253 that was discovered 13 May 1972 with an apparent B magnitude of about 8.5, shortly after it had reached its maximum brightness. In terms of apparent brightness, it was the second-brightest supernova of any kind (fainter only than SN 1987A) of the 20th century. It was observed for nearly 700 days, and it became the prototype object for the development of theoretical understanding of Type Ia supernovae.

SN 1993J

SN 1993J is a supernova observed in the galaxy M81. It was discovered on 28 March 1993 by F. Garcia in Spain. At the time, it was the second-brightest type II supernova observed in the twentieth century behind SN 1987A.The spectral characteristics of the supernova changed over time. Initially, it looked more like a type II supernova (a supernova formed by the explosion of a giant star) with strong hydrogen spectral line emission, but later the hydrogen lines faded and strong helium spectral lines appeared, making the supernova look more like a type Ib. Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other type II supernovae but did resemble the variations observed in type Ib supernovae. Hence, the supernova has been classified as a type IIb supernova, an intermediate class between type II and type Ib. The scientific results from this supernova suggested that type Ib and Ic supernovae were actually formed through the explosions of giant stars through processes similar to what takes place in type II supernovae. The supernova was also used to estimate a distance of 8.5 ± 1.3 Mly (2.6 ± 0.4 Mpc) to Messier 81.Light echoes from the explosion have subsequently been detected.

The progenitor of SN 1993J was identified in pre-explosion ground-based images. The progenitor was observed to be a K-type supergiant star, with an excess in the ultraviolet possibly due to surrounding hot stars or a hot binary companion. While the supernova is located in a region populated by young massive stars, late-time photometry with the Hubble Space Telescope and spectroscopy with the Keck 10m-telescope presented by Maund and collaborators revealed the presence of the long-suspected B-supergiant companion star.

SN 2004dj

SN 2004dj was the brightest supernova since SN 1987A at the time of its discovery.

This Type II-P supernova was discovered by Koichi Itagaki, a Japanese astronomer on July 31, 2004. At the time of its discovery, its apparent brightness was 11.2 visual magnitude; the discovery occurred after the supernova had reached its peak magnitude. The supernova's progenitor is a star in a young, compact star cluster in the galaxy NGC 2403, in Camelopardalis. The cluster had been cataloged as the 96th object in a list of luminous stars and clusters by Allan Sandage in 1984; the progenitor is therefore commonly referred to as Sandage 96. This cluster is easily visible in a Kitt Peak National Observatory image and appears starlike.

Supernova remnant

A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way.

There are two common routes to a supernova: either a massive star may run out of fuel, ceasing to generate fusion energy in its core, and collapsing inward under the force of its own gravity to form a neutron star or a black hole; or a white dwarf star may accrete material from a companion star until it reaches a critical mass and undergoes a thermonuclear explosion.

In either case, the resulting supernova explosion expels much or all of the stellar material with velocities as much as 10% the speed of light (or approximately 30,000 km/s). These ejecta are highly supersonic: assuming a typical temperature of the interstellar medium of 10,000 K, the Mach number can initially be > 1000. Therefore, a strong shock wave forms ahead of the ejecta, that heats the upstream plasma up to temperatures well above millions of K. The shock continuously slows down over time as it sweeps up the ambient medium, but it can expand over hundreds or thousands of years and over tens of parsecs before its speed falls below the local sound speed.

One of the best observed young supernova remnants was formed by SN 1987A, a supernova in the Large Magellanic Cloud that was observed in February 1987. Other well-known supernova remnants include the Crab Nebula; Tycho, the remnant of SN 1572, named after Tycho Brahe who recorded the brightness of its original explosion; and Kepler, the remnant of SN 1604, named after Johannes Kepler. The youngest known remnant in our galaxy is G1.9+0.3, discovered in the galactic center.

University of Toronto Southern Observatory

The University of Toronto Southern Observatory (UTSO) was an astronomical observatory built by the University of Toronto at the Las Campanas Observatory in Chile. It hosted a single 60 cm Cassegrain telescope and a small cottage for the operators, located amongst the instruments funded by other organizations. The first observational runs started in 1971, and like many smaller instruments, it was later shut down in favor of a partial share in a much larger telescope in 1997. Although small by modern standards, the Southern Observatory nevertheless became famous for its role in the discovery of SN 1987A when UofT astronomer Ian Shelton spotted the supernova while attempting to fix another disused telescope at the site.

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