A new star in heavens: how Crab Nebula was born

In 1054 A.D, a new, very bright star has appeared in Earth’s sky, in the constellation Taurus. Chinese, Japanese, Korean, and Arab astronomers observed the event and noted: “a new bright star emerged in the heavens”. The star was so bright: for nearly three weeks, it was visible even during the daytime, under the hot, shiny summer sun, and remained visible for around two years (653 days to be exact). Today, we know that that “heavenly star” was actually a supernova (SN 1054), and its remnant is what we now know as the Crab Nebula today (catalogue designations M1, NGC 1952, Taurus A).

Crab Nebula (ESO image)
This photo shows a three color composite of the well-known Crab Nebula (also known as Messier 1), as observed with the FORS2 instrument in imaging mode in the morning of November 10, 1999. It is the remnant of a supernova explosion at a distance of about 6,000 light-years. Japanese and Chinese astronomers recorded this violent event nearly 1,000 years ago in 1054, as did, almost certainly, Native Americans. It contains a neutron star near its center that spins 30 times per second around its axis (see the animation below).
In this picture, the green light is predominantly produced by hydrogen emission from material ejected by the star that exploded. The blue light is predominantly emitted by very high-energy (“relativistic”) electrons that spiral in a large-scale magnetic field (so-called synchrotron emission). It is believed that these electrons are continuously accelerated and ejected by the rapidly spinning neutron star at the centre of the nebula and which is the remnant core of the exploded star. This pulsar has been identified with the lower/right of the two close stars near the geometric center of the nebula, immediately left of the small arc-like feature, best seen in ESO Press Photo eso9948. Technical information: ESO Press Photo eso9948 is based on a composite of three images taken through three different optical filters: B (429 nm; FWHM 88 nm; 5 min; here rendered as blue), R (657 nm; FWHM 150 nm; 1 min; green) and S II (673 nm; FWHM 6 nm; 5 min; red) during periods of 0.65 arcsec (R, S II) and 0.80 (B) seeing, respectively. The field shown measures 6.8 x 6.8 arcminutes and the images were recorded in frames of 2048 x 2048 pixels, each measuring 0.2 arcseconds. North is up; East is left.
Image: ESO

The Crab Nebula was observed later by English astronomer John Bevis (1695-1771) in 1731, long before its true nature was understood and identified.It was identified as the supernova remnant of SN 1054 between 1921 and 1942, at first speculatively (1920s), with some plausibility by 1939, and beyond reasonable doubt by the Dutch astronomer Jan OortNotes 2 in 1942. With that identification, the Crab Nebula became the first astronomical object identified with a historical supernova explosion. Today, it is not visible to the naked eye but can be made out using binoculars under favorable conditions. It lies in the Perseus Arm of the Milky Way galaxy, at a distance of about 6,500 light years from Earth.

The core of the exploding star formed a pulsar, called the Crab Pulsar (or PSR B0531+21).

Time-lapse of the Crab Nebula

Amateur astronomer Detlef Hartmann created this amazing animation of the Crab Nebula expanding, using observations he made over a decade with a home-built telescope. Today, the Crab Nebula has a diameter of 3.4 parsecs (11 light years) and is expanding at a rate of about 1,500 kilometers per second (930 mi/s), or 0.5% of the speed of light. Here is the one-decade (from 2008 to 2017) time-lapse animation of the Crab Nebula below.

Several studies suggested that the expanding cloud of stellar debris is expanding much too fast to be associated with a supernova explosion in 1054. This dilemma led astronomers to the idea that the remnant’s central pulsar (the Crab Pulsar, a young, rapidly spinning and extremely dense neutron star, 28-30 kilometers or 17-19 mi across with a spin rate of 30.2 times per second) left over from the 1054 supernova, emitted such copious amounts of energy that it actually accelerated the expanding cloud of debris, making it move faster with time.

Notes

  1. The exact date of the event has been disputed, but most accounts accept the Chinese date of July 4. Between 532 B.C. and A.D. 1064, Chinese astronomers recorded at least 75 supernovas (they called them as “guest stars”, as they have no idea what they actually were). The 1054 supernova was the brightest.
  2. Jaan Oort (28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy. The Oort cloud, the Oort constants, and the Asteroid, 1691 Oort, were all named after Jan Oort.

Sources

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