Is the "big crab" in the night sky a trace left by an unusual celestial body thousands of years ago?

Author | Feng Ziyang

Review | Dong Chenhui, Zhao Jingyuan

Editor | Zhao Jingyuan

In 1054, an unusual celestial body appeared in the sky. Its brightness was similar to that of Venus. It stayed in people's sight for 642 days before disappearing into the vast starry sky. Today, people know that this is a supernova and numbered it as SN 1054, also known as the Tianguan Guest Star. The remnant it left behind is the famous Crab Nebula. The Crab Nebula is numbered as the first celestial body in the Messier star catalog, namely M1, and is located northwest of ζ Taurus (the ancient Chinese name for the star is Tianguan). With an apparent magnitude of 8.4, it is difficult to see with the naked eye, but it can be barely observed using binoculars under good conditions.

Software simulation of the explosion of SN 1054, as well as related records in ancient Chinese books, source: Stellarium, "Collection of Ancient Chinese Astronomical Records"

The Crab Nebula is located at right ascension 05h34m31.94s, declination +22°00′52.2″ (epoch J2000) on the celestial sphere. In addition to M1, it is also numbered as NGC 1952, LBN 833, etc. It is the first confirmed supernova remnant, with a pulsar at its center. The Crab Nebula is about 6,500 light-years away from the Earth, has a diameter of about 11 light-years, and is expanding at a speed of about 1,500 kilometers per second.

The Crab Nebula M1. Source: Stellarium

In the photo of the Crab Nebula, we can distinguish two areas of different colors: the outer part is composed of red filamentary structures, and its spectrum contains strong emission lines. In the supernova explosion in 1054, the progenitor star of the Crab Nebula ejected its outer layer of matter at an extremely high speed. These materials interacted with the surrounding interstellar matter to form filamentary structures; and the blue diffuse area near the core. The spectrum of this area is a continuous spectrum. The electrons inside the Crab Nebula are deflected and accelerated by the magnetic field of the pulsar. The radiation appears blue and is highly polarized.

The Crab Nebula as seen by the Hubble Space Telescope. Source: NASA

People have been observing the Crab Nebula for nearly a thousand years. On July 4, 1054, a strange sight appeared in the eastern sky in the early morning. The officials of the Northern Song Dynasty's Sitianjian faithfully recorded this event: In the fifth month of the first year of Zhihe, it appeared in the east in the morning, guarding the sky gate, and was seen during the day as if it were Taibai, with four red and white horns, and was seen for 23 days (Song Huiyao). In addition to China, there are also records of this guest star in Japan, Iraq and other countries.

"Song History·Astronomy" also recorded the Tianguan guest star, source: reference 6

In 1731, British astronomer John Bevis first observed this mysterious nebula and drew it in his star map. In 1758, French astronomer Charles Messier also independently discovered this nebula in the direction of Taurus and included it in his star catalog as object number 1 (Messier was looking for the returning Halley's Comet at the time, and he mistakenly took the nebula for Halley's Comet. It was not until some time later that he realized that this fuzzy object was not a comet. This experience prompted him to compile a list of comet-like deep sky objects, which is the Messier catalog). In the early 1840s, Irish astronomer William Parsons, 3rd Earl of Rosse, also observed this nebula and drew a sketch of it. Because the image he drew looked very much like a crab, it was named the Crab Nebula.

Sketch of the Crab Nebula by William Parsons, source: Reference 5

In 1921, American astronomer Carl Lampland discovered that the shape of the Crab Nebula had changed, which was later confirmed by American astronomer John Duncan, who emphasized that the Crab Nebula was expanding. In the same year, Swedish astronomer Knut Lundmark listed 60 suspected "novae" in ancient Chinese books (supernovae were not known at the time, and some of these "novae" were actually supernovae), and the 1054 guest star was among them. In 1928, American astronomer Edwin Hubble realized that the expansion of the Crab Nebula indicated that it might have been formed by a stellar explosion, and deduced from the expansion speed that the explosion occurred 900 years ago. He first speculated that the Crab Nebula might be related to the 1054 guest star.

In 1934, German astronomer Walter Baade and Swiss astronomer Fritz Zwicky proposed the existence of a supernova, a very energetic star explosion. In 1939, American astronomer Nicholas Mayall measured the distance and size of the Crab Nebula, deduced that the 1054 guest star might be a supernova, and preliminarily confirmed that the Crab Nebula was likely its remnant. Mayall later collaborated with Dutch astronomer Jan Oort to conduct more detailed research, and finally proved the preliminary conclusions obtained three years ago in 1942.

Radio pulses from the Crab Nebula. Credit: NRAO

In 1968, American astronomers David Staelin and Edward Reifenstein used the Green Bank radio telescope to discover two pulsating radio sources near the Crab Nebula, and pointed out that they might be related to the Crab Nebula, but due to the limited resolution, they could not accurately locate them, so they could not give a definite answer. In November of the same year, American astronomer Richard Lovelace and others used the Arecibo radio telescope to accurately locate the position of one of the pulsating radio sources, determining that it was located in the central area of ​​the Crab Nebula, and measured its pulse period to be 33 milliseconds. This is the core material left over from the supernova in 1054 - the Crab Nebula Pulsar, also numbered PSR B0531+21. The diameter of the Crab Nebula Pulsar is only about 20 kilometers, but its mass is greater than that of the sun. It is also a rare optical pulsar that also has light variations in the optical band. The variable star number is CM Tauri, and its light variation period is the same as the radio pulse period, which is 33.085 milliseconds, which is also its rotation period. The apparent magnitude of this pulsar is 16.65, which means that the absolute magnitude of this very small supernova remnant is about 5.3, which is equivalent to the absolute magnitude of the sun (the sun is slightly brighter).

The location of the Crab Nebula on the star map. Source: Stellarium

The Crab Nebula is relatively dark and is one of the more difficult objects to observe in the Messier catalog. In good conditions, you can only see a fuzzy spot with a 10×50 binoculars, and you can barely see an irregular nebula with a 5-inch telescope. Its shape is slightly obvious, and some stripe structures can be vaguely seen inside the nebula. If you use a large-aperture telescope, you can see that the main body of the Crab Nebula is S-shaped, roughly along the northwest-southeast direction, with the southeast side darker, and an extension at the end with a dark area deep into it; in the middle area of ​​the nebula, you can see two 16th-magnitude stars, and the darker one in the southwest is the Crab Nebula Pulsar.

Howard Banich, an American amateur astronomer, used a 28-inch telescope to visually observe the Crab Nebula and its pulsar and drew this sketch. Source: Reference 11

The Crab Nebula is one of the most intensively studied and famous supernova remnants. Nearly a century of research on it has profoundly influenced all aspects of astronomy, especially cutting-edge fields such as supernovae, neutron stars, and cosmic rays. On a clear night from late autumn to early spring, why not go with your friends to find this ancient and mysterious nebula and appreciate the charm of the universe.

References

(1) M1, SEDS Messier Database, http://www.messier.seds.org/m/m001.html#bolton1949

(2) Crab Nebula, wikipedia, https://en.wikipedia.org/wiki/Crab_Nebula

(3) Ronald Stoyan, ATLAS of the MESSIER OBJECTS

(4) The Death Throes of Stars, NASA, https://science.nasa.gov/mission/hubble/science/science-highlights/the-death-throes-of-stars/

(5) Lord Rosse's drawings of M1, the Crab Nebula, http://www.messier.seds.org/more/m001_rosse.html

(6) History of the Song Dynasty: Astronomy, National Digital Library of China, http://read.nlc.cn/OutOpenBook/OpenObjectBook?aid=892&bid=222852.0

(7) What is multi-band astronomy? Just read this article! | Astronomer’s Notebook, WeChat public account “Institute of High Energy Physics, Chinese Academy of Sciences”, https://mp.weixin.qq.com/s/7-y781GOT_YBxxCSmN9CfA

(8) SN 1054, wikipedia, https://en.m.wikipedia.org/wiki/SN_1054

(9) Crab Pulsar, wikipedia, https://en.m.wikipedia.org/wiki/Crab_Pulsar

(10) The Crab Pulsar and Nebula, NRAO, https://www.nrao.edu/archives/items/show/33474

(11) Howard Banich’s personal homepage, https://sites.google.com/site/howardbanichhomepage/observations/observing-notebook-scans/notebook-10-1?authuser=0