SN 393
Encyclopedia
SN 393 is the modern supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

 designation for an astronomical event that was reported by the Chinese in the year 393 CE
393
Year 393 was a common year starting on Saturday of the Julian calendar. At the time, it was known as the Year of the Consulship of Augustus and Augustus...

. The extracted record was translated into English as follows:

The second lunar month mentioned in the record corresponds to the period February 27 to March 28, 393 CE, while the ninth lunar month ran from October 22 to November 19, 393 CE. The bowl-shaped asterism named Wěi is formed by the tail of the modern constellation Scorpius
Scorpius
Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...

. This asterism consists of the stars in Scorpius designated ε
Epsilon Scorpii
Epsilon Scorpii is a star in the constellation Scorpius. Patrick Moore introduced the name Wei but this seems to be a misreading; Wěi , meaning the Tail, was originally the name of a Chinese asterism or Xiù comprising the stars ε, μ1-2, ζ1-2, η, θ, ι, κ,...

, μ
Mu Scorpii
The Bayer designation Mu Scorpii is shared by two star systems, in the constellation Scorpius:* μ¹ Scorpii* μ² ScorpiiThey are separated by 0.1° in the sky....

, ζ
Zeta Scorpii
The Bayer designation Zeta Scorpii is shared by two stars in the constellation Scorpius:* ζ¹ Scorpii * ζ² Scorpii...

, η
Eta Scorpii
Eta Scorpii is a star in the constellation Scorpius.Eta Scorpii is a yellow-white F-type giant with an apparent magnitude of +3.32. It is approximately 72.0 light years from Earth....

, θ
Theta Scorpii
Theta Scorpii is a star in the constellation Scorpius. It has the traditional name Sargas, of Sumerian origin....

, ι
Iota Scorpii
There are two stars with the Bayer designation Iota Scorpii:* Iota1 Scorpii* Iota2 ScorpiiThey are separated by 0.50° in the sky.All of them were member of asterism 尾 , Tail, Tail mansion....

, κ
Kappa Scorpii
Kappa Scorpii is a star in the constellation Scorpius. It has the traditional name Girtab, which is the Sumerian word for 'scorpion' and has survived through the Babylonian star catalogues....

, λ
Lambda Scorpii
Lambda Scorpii is the second brightest star system in the constellation Scorpius, and one of the brightest stars in the nighttime sky. It has the Bayer designation λ despite being the second brightest in its constellation...

 and ν
Nu Scorpii
Nu Scorpii is a star system in the constellation Scorpius.It is at least a quintuple star, probably a septuple, consisting of two close groups that are separated by 41". The brighter group, Nu Scorpii A and B, is split by 1.3" and composed of spectral type B2 subgiants...

. The guest star reached an estimated apparent magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 of −1 and was visible for about eight months before fading from sight. The duration of this event suggests the source was a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

, which is a cataclysmic explosion of a star.

When the material from a supernova explosion is ejected from the star, it sweeps up the surrounding interstellar material and creates an expanding supernova remnant
Supernova remnant
A supernova remnant 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...

 of gas and plasma. In 1975, there were only seven known supernova remnants in the region of the sky where SN 393 was observed. On the basis of the peak magnitude, supernova SN 393 was initially estimated to have occurred at a distance close to 10 kpc. This requirement ruled out all but three of the candidates. Of the remainder, the first, G350.0-1.8 has an estimated age of 8,000 years; too old to be the remnant of SN 393. The two remaining sources, G348.5+0.1 and G348.7+0.3 are located at a suitable distance of about 10 kpc and have an estimated age of 1,500 years. However, because this supernova occurred along the dusty galactic plane, it is difficult to explain how it could have been observed with the naked eye from a distance of 10 kpc for a period of eight months.

In 1996, the ROSAT
ROSAT
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...

 All Sky Survey discovered a new supernova remnant in this area of the sky: RX J1713.7-3946. Two years later, it was suggested that this might be a better match for SN 393. Observations in 1999 suggested that this remnant is associated with the H II region
H II region
An H II region is a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet light, ionizing the surrounding gas...

 G347.611 +0.204, which is at a distance of 6 kpc. However, in 2003, examination of the interaction between this cloud and a molecular cloud
Molecular cloud
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....

 resulted in a closer distance of 1 kpc. This estimate was strengthened by a 2004 study of the X-ray and neutral hydrogen absorption by matter between the remnant and the Earth. Given the remnant's angular size
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...

 of 70 arcminutes on the sky, at this distance it has a physical diameter of 20 pc.

The supernova remnant RX J1713.7-3946 is consistent with a type II
Type II supernova
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 9 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum...

 or type Ib
Type Ib and Ic supernovae
Types Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon...

 supernova explosion of a star with an initial mass at least 15 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

es. The explosion generated an energy of about , ejecting three solar masses of material into the surrounding interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

.
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