REPORT

M 1- THE CRAB NEBULA

 

TEAM Vladimir Nikolaev Nikolov

            Dimitar Angelov Lambev      

                Albena Yordanova Tsankova

         BULGARIA

[m1.jpg]

Right Ascension

05 : 34.5 (h:m)

Declination

+22 : 01 (deg:m)

Distance

6.3 (kly)

Visual Brightness

8.4 (mag)

Apparent Dimension

6x4 (arc min)

The Crab Nebula is the name of the most famous remnant of  supernova explosion, one can observe on the northeren sky. It is placed in Taurus constellation and in 1968 a pulsating radio source( pulsar) was discovered in it.

            Based on the theory of the massive stars evolution we can say how the Crab nebula appeared on our sky. Nearly 7000 years ago it was a massive and hot giant star at the end of its life.This end was signaled by formation of an iron core. Iron cannot burn and release energy and its atom is tightly bound by the nuclear force. Adding additional protons or neutrons weakens it and makes it break up. Thus nuclear fusion stops with iron. A star with an iron core has no more fuel. This causes a star's core to shrink and heat. In high-mass stars the shrinkage presses the iron nuclei so tightly together that a new reaction can occur: protons and electrons merge, neutralize their charge and became neutrons. From a sphere of iron the shrinking core is thus transformed into a sphere of neutrons. This result is catastrophic for the star. Most of the pressure that supported the core  supplied by electrons, but they have been absorbed by protons. The star's core pressure suddenly drops. There's nothing to support the star, and its interior begins to collapse. But the matter is so dense, its gravity force is so big that it crushes the core. In less than a second, from an iron ball with the size of the Earth, the core collapses to a ball of neutrons about 10 km in radius. There  is nothing to support the star's outer layers and they  fall inwards  under enormous pressure. They strike the neutron core of the star, crushing it still more while the impact heats the Infalling gas to billions of degrees. The pressure surges and lifts the outer layers away from the star in a titanic explosion-THE SUPERNOVA.

This explosion mixes the elements synthesized during the evolution of the star with the star's outer layers and blast them into space. This incandescent spray expands away from the star's collapsed core at more than 1000 km/sec. For the star , the supernova explosion is a quick and glorious death.For us, the remnant of this explosion is one of the most exciting and marvelous vews  on the night sky.

M1 was discovered in 1731 by british amateur astronomer John Bevis

The explosion of the supernova was noted on July 4, 1054 A.D. by Chinese astronomers, and was about four times brighter than Venus, or about mag -6. According to the records, it was visible in daylight for 23 days, and 653 days to the naked eye in the night sky. It was probably also recorded by Anasazi Indian artists (in present-day Arizona and New Mexico), as findings in Navaho Canyon and White Mesa (both AZ) as well as in the Chaco Canyon National Park (NM) indicate. In addition, Ralph R. Robbins of the University of Texas has found Mimbres Indian art from New Mexico, possibly depicting the supernova.

The nebulous remnant was discovered by John Bevis in 1731, who added it to his sky atlas, Uranographia Britannica . Charles Messier independently found it on August 28, 1758, when he was looking for comet Halley on its first predicted return, and first thought it was a comet. Of course, he soon recognized that it had no apparent proper motion, and catalogued it  on September 12, 1758. It was the discovery of this object which caused Charles Messier to begin with the compilation of his catalogue It was also the discovery of this object, which closely resembled a comet (1758 De la Nux, C/1758 K1) in his small refracting telescope, which brought him to the idea to search for comets with telescopes. Messier acknowledged the prior, original discovery by Bevis when he learned of it in a letter of June 10, 1771

This nebula was christened the "Crab Nebula" on the ground of a drawing made by Lord Rosse about 1844. Of the early observers, Messier, Bode and William Herschel correctly remarked that this nebula is not resolvable into stars, but William Herschel thought that it was a stellar system which should be resolvable by larger telescopes. John Herschel and Lord Rosse, erroneously, thought it is "barely resolvable" into stars. They and others, including Lassell in the 1850s, apparently mistook filamentary structures as indication for resolvability.

Early spectroscopic observations, e.g. by Winlock, revealed the gaseous nature of this object in the later 19th century. The first photo of M1 was obtained in 1892 with a 20-inch telescope. First serious investigations of its spectrum were performed in 1913-15 by Vesto Slipher; he found that the spectral emission lines were split; it was later recognised that the true reason for this is Doppler shift, as parts of the nebula are approaching us (thus their lines are blueshifted) and others receding from us (lines redshifted). Curtis, in his description of this object based on Lick Observatory photographs, tentatively classified it as a planetary nebula ,a view which was disproved only in 1933; this mis-classification can still be found in much newer handbooks.

The Supernova 1054 is one of  few historically observed supernovae in our Milky Way Galaxy. The SN 1006; SN 1572 and SN 1604.

Main Characteristics of M1

In 1921, C.O. Lampland of Lowell Observatory, when comparing excellent photographs of the nebula obtained with their 42-inch reflector, found notable motions and changes, also in brightness, of individual components of the nebula, including dramatic changes of some patches near the central pair of stars. The same year, J.C. Duncan of Mt. Wilson Observatory compared photographic plates taken 11.5 years apart, and found that the Crab Nebula was expanding at an average of about 0.2" per year; backtracing of this motion showed that this expansion must have begun about 900 years ago. Also the same year, Knut Lundmark noted the proximity of the nebula to the 1054 supernova.

In 1942, based on investigations with the 100-inch Hooker telescope on Mt. Wilson, Walter Baade computed a more acurate figure of 760 years age from the expansion, which yields a starting date around 1180; later investigations improved this value to about 1140. The actual 1054 occurrance of the supernova shows that the expansion must have been accelerated.

The nebula consists of the material ejected in the supernova explosion, which has been spread over a volume approximately 10 light years in diameter, and is still expanding at the very high velocity of about 1,800 km/sec. It emits light which consists of two major contributions, first found by Roscoe Frank Sanford in 1919 by spectroscopic investigations, see (Sanford 1919), photographically confirmed by Walter Baade and Rudolph Minkowski in 1930: First, a reddish component which forms a chaotic web of bright filaments, which has an emission line spectrum (including hydrogene lines) like that of diffuse gaseous (or planetary) nebulae. Second a blueish diffuse background which has a continuous spectrum and consists of highly polarised ` synchrotron radiation ', which is emitted by high-energy (fast moving) electrons in a strong magnetic field, an explanation first proposed by the soviet astronomer J. Shklovsky (1953) and supported by observations of Jan H. Oort and T. Walraven (1956). Synchrotron radiation is also apparent in other "explosive" processes in the cosmos, e.g. in the active core of the irregular galaxy M82 and the peculiar jet of giant elliptical galaxy M87. These striking properties of the Crab Nebula in the visible light are equally conspicuous in the Palomar images post-processed by David Malin of the Anglo Australian Observatory, and in Paul Scowen's image obtained on Mt. Palomar.

In 1948, the Crab nebula was identified as a strong source of radio radiation, named and listed as Taurus A and later as 3C 144. X-rays from this object were detected in April 1963 with a high-altitude rocket of type Aerobee with an X-ray detector developed at the Naval Research Laboratory; the X-ray source was named Taurus X-1. Measurements during lunar occultations of the Crab Nebula on July 5, 1964, and repeated in 1974 and 1975, demonstrated that the X-rays come from a region at least 2 arc minutes in size, and the energy emitted in X-rays by the Crab nebula is about 100 times more than that emitted in the visual light. Nevertheless, even the luminosity of the nebula in the visible light is enormous: At its distance of 6,300 light years (which is quite well-determined, by Virginia Trimble (1973), its apparent brightness corresponds to an absolute magnitude of about -3.2, or more than 1000 solar luminosities. Its overall luminosity in all spectral ranges was estimated at 100,000 solar luminosities or 5*10^38 erg/s !

On november 9, 1968, a pulsating radio source, the Crab Pulsar ( also catalogued as NP0532 or PSR 0531+21), was discovered in M1 by astronomers of  the Arecibo Observatory 300-meter radio telescope in Puerto Rico. This star is the right (south-western) one of the pair visible near the center of the nebula in our photo. This pulsar was the first one which was also verified in the optical part of the spectrum, when W.J. Cocke, M.J. Disney and D.J. Taylor of Steward Observatory, Tucson, Arizona found it flashing at the same period of 33.085 milliseconds as the radio pulsar with the 90-cm (36-inch) telescope on Kitt peak; this discovery happened on January 15, 1969 at 9:30 pm local time (January 16, 1969, 3:30 UT, according to Simon Mitton). This optical pulsar is sometimes also referred to by the supernova's variable star designation, CM Tauri.

It has now been established that this pulsar is a rapidly rotating neutron star: It rotates about 30 times per second! This period is very well investigated because the neutron star emits pulses in virtually every part of the electromagnetic spectrum, from a "hot spot" on its surface. The neutron star is an extremely dense object, denser than an atomic nucleus, concentrating more than one solar mass in a volume of 30 kilometers across. Its rotation is slowly decelerating by magnetic interaction with the nebula; this is now a major energy source which makes the nebula shining; as stated above, this energy source is 100,000 times more energetic than our sun.

In the visible light, the pulsar is of 16th apparent magnitude. This means that this very small star is roughly of absolute magnitude +4.5, or about the same luminosity as our sun in the visible part of the spectrum !

Jeff Hester and Paul Scowen have used the Hubble Space Telescope to investigate the Crab Nebula M1 (see also e.g. Sky & Telescope of January, 1995, p. 40). Their continuous investigations with the HST have provided new insight into the dynamic and changes of the Crab nebula and pulsar. More recently, the Heart of the Crab was investigated by HST astronomers.

This object has attracted so much interest that it was remarked that astronomers can be devided into two fractions of about same size: Those who do work related to the Crab nebula, and those who don't. There was a "Crab Nebula Symposium" in Flagstaff, Arizona in June, 1969. The IAU symposium No. 46, held at Jodrell Bank (England) in August 1970 was solely devoted to this object. Simon Mitton has written a nice book on the Crab Nebula M1 in 1978, which is still most readable and informative (it is also source for some of the informations here).

How to Observe M1, and Where We Should Look for It

The Crab Nebula can be found quite easily from Zeta Tauri (or 123 Tauri), the "Southern Horn" of the Bull, a 3rd-magnitude star which can be easily found ENE of Aldebaran (Alpha Tauri). M1 is about 1 deg N and 1 deg W of Zeta, just slightly south and about 1/2 degree west of a mag-6 star, Struve 742.

The nebula can be easily seen under clear dark skies, but can equally easily get lost in the background illumination under less favorable conditions. M1 is just visible as a dim patch in 7x50 or 10x50 binoculars. With a little more magnification, it is seen as a nebulous oval patch, surrounded by haze. In telescopes starting with 4-inch aperture, some detail in its shape becomes apparent, with some suggestion of mottled or streak structure in the inner part of the nebula; John Mallas reports that under excellent conditions, an experienced observer can see them throughout the inner portion of the nebula. The amateur can verify Messier's impression that M1 looks indeed similar to a faint comet without tail in smaller instruments. Only under excellent conditions and with larger telescopes, starting at about 16 inches aperture, suggestions of the filaments and fine structure may become visible.

M1 is situated in a nice Milky Way field. The star Zeta Tauri is remarkable as it is a Gamma Cassiopeiae type variable, a rather rapidly rotating star of spectral type B4 III pe which has ejected an expanding gas shell, and has a fainter spectroscopic companion star in an orbit of about 133 days period. Preceding M1 two minutes (or half a degree) in Right Ascension is Struve 742 or ADS 4200, another visual binary star with components A (mag 7.2, spectrum F8, of yellow color) and B (mag 7.8, white) separated by about 3.6" in position angle 272deg, and orbiting each other in about 3000 years.

Historical Observations and Descriptions of M1

Messier : M1.

1758 September 12. 1. 5h 20m 02s (80d 00' 33") +21d 45' 17"
"Nebula above the southern horn of Taurus, it doesn't contain any star; it is a whitish light, elongated in the shape of a flame of a candle, discovered while observing the comet of 1758. See the chart of that comet, Mem. Acad. of the year 1759, page 188; observed by Dr. Bevis in about 1731. It is reported on the English Celestial Atlas ."

[A note in Messier's handwriting added in the margin of his copy of the Connaissance des Temps for 1783 reads:] "This nebula reported on the great English atlas: Seen by Dr Bevis about 1731. according to his letter written to me on 10th June 1771"

[In the Mem. Acad. for 1959, the discovery announce for this nebula is described in the memoir by Delisle on the comet of 1758, Comet De la Nux; a position is given at the end of this paper, on p. 188, as follows:]
Nebulous star discovered on August 28, 1758: Observed Right Ascension: 80d 0' 33" (5h 20m 02s), observed Northern Declination: 21d 45' 27"; observed [ecliptical] Longitude: Gemini 20d 43' 30" (80d 43' 30"), observed Latitude: South 1d 23' 28".

Bode : Bode 11 ..

A nebula without stars.
On November 8 [1774], I looked up the nebula which Mr. Messier has discovered in 1758 according to the French memoirs, situated obliquely north above Zeta at the southern horn of Taurus. I found this object soon with the 7-foot telescope, and in the position listed by Mr. Messier relative to the stars situated closest to it, as the 8th figure shows.

John Herschel, General Catalogue : GC 1157 .

GC 1157 = h 357 = M1.
RA 05h 26m 3.9s, NPD 68d 5' 10.5" (1860.0) [Right Ascension and North Polar Distance]
vB; vL; E135deg +/-; vglbM, r. 12 observations by W. & J. Herschel.
Very bright, very large, extended along position angle approximately 135 deg; very gradually brightening a little toward the middle, mottled.
Remark: Figures in P.T. 33 [JH 1833], plate viii, Fig. 81; P.T. 44 [Lord Rosse, 1844], plate xix, Fig. 81; R. di. (The woodcut diagrams in Lord Rosse's paper, PT 1861); d'Arr. (D'Arrest's Inaugural dissertation and description of the Copenhagen Equatorial, 1861), plate ii, Fig. 4; Lass. (Lassell's Memoirs in vol xxiii of the Transactions of the Royal Astronomical Society), plate ii, Fig. 1.

Dreyer : NGC 1952 .

NGC 1952 = GC 1157 = h 357; Bevis 1731, M 1.
RA 05h 26m 6s, NPD 68d 5.0' (1860.0) [Right Ascension and North Polar Distance]
vB, vL, E 135deg +/-, vglbM, r; = M1
Very bright, very large, extended along position angle approximately 135 deg; very gradually brightening a little toward the middle, mottled.
Remark: Figures in P.T. 33 [JH 1833], plate XVI, fig. 81; P.T. 44 [Lord Rosse, 1844], plate XIX, fig. 81; R. di. [The woodcut diagrams in Lord Rosse's paper, PT 1861); d'A. (D'Arrest's Inaugural dissertation and description of the Copenhagen Equatorial, 1861], plate II, fig. 4; Lass. [Lassell's Memoirs in vol xxiii of the Transactions of the Royal Astronomical Society], plate II, fig. 1; Secchi [Secchi, Memorie dell' Osserv. del Collegio Romano, 1852-55], plate IV, fig. 8; Lass. 2 [Lassell, Memoirs RAS, vol xxxvi], plate II, fig. 6; Ld R [Lord Rosse, Observations of Nebulae and Clusters at Birr Castle, 1848-78 (Transactions Royal Dublin Societym vol. ii, 1880)], plate II; Tempel [Ueber Nebelflecken (Abh. d. K. Böhm. Gesell. d. Wiss. 1885)], plate I.

Flammarion

[L'Astronomie. Revue de la Societé Astronomique de France, November 1917. P. 385-400, here p. 396-397. With a photo and a drawing]
M.1. Taurus. Nebula with stars .
Messier's Description: "Nebula above the southern horn of Taurus, it doesn't contain any star; it is a whitish light, elongated in the shape of a flame of a candle, discovered while observing the comet of 1758."
Messier has added, in the margin, a writing which is still very readable: "This nebula is reported in the great english atlas. Seen by the doctor Bévis, in 1731, according to a letter to me from him, of June 10, 1771."
Seen on August 28, and the position determined on September 12 (1758).
It is on lurking on the return of the comet of Halley, of which we have talked more extensively, that Messier did this first discovery, near the star Zeta Tauri.
Observed by John Herschel, Admiral Smyth, d'Arrest, Lord Rosse, Schoenfeld, Lassell, Secchi, Dreyer, Webb, etc., it has received by Lord Rosse the name Crab Nebula . At most, it offers a little resemblance with the crabs which one finds at the shore of the English Channel, irregularly round, and much more like the elongated figure which I have published in Les Étoiles [The Stars], after Lord Rosse. The diverse drawings which have been made of it differ enormously.
This nebula has been photographed often enough at Juvisy since 1902. We have reobserved it in September and October 1917. The photograph reproduced here has been taken by M. Quénisset, on October 18 with a Voigtlender lens (exposed 1h 30m), and the drawing has been made by him on October 15.
Its shape is oval, almost quadrangular, measures 5' 1/2 in length and 3' 1/2 in width. Partly resolvable in more powerful instruments.

Practical Exercise

            Imagine that The Crab Nebula had been moved a 100 times closer to our planet.In these conditions  answer the questions:

1.What will be the apparent dimension of M1 in arc minutes?

2.What surface in sqr arc minutes it will cover on the sky?


 


At the center of the Crab is a pulsar, the neutron star that remains from the original star. Neutron stars are fantastic objects -- just a few kilometers in diameter but with the mass of the Sun and very rapidly rotating (the Crab pulsar rotates 30 times per second, some pulsars ten times faster).

The Supernova 1054 is one of  few historically observed supernovae in our Milky Way Galaxy. The others are: SN 1006; SN 1572 and SN 1604 .

There are two types of supernovae. The Type I SN can be observed in irregular , elliptical and spiral galaxies.The other supernovae- Type II –can be observed mainly in the spiral arms of the galaxies like ours.

Let us take a peek of the remnants of other supernovae explosions in our galaxy. They  do not give us so much information as Crab Nebula does. This is based on the fact that Crab Nebula, which is a richer and more interesting object. There is very little information about the SN 1006 in Lupus. It is about 3500pc away from us and a source of radio waves, more powerful than the one in M1, was discovered in the area of its explosion. This supernova exploded in an area poor of matter. It expanded and disappeared very fast. The data for the supernova of Ticho Brahe (1572) is very little, too. It is about 5kpc away from us. The ejected matter was very little – not more than 10% of the Sun’s mass. In the center of the explosion was discovered a source of  X – rays. The remnant of Kepler’s supernova(1604) is an interesting nebula with separated lighter areas. It is about 10 kpc away from us. All these supernovae are considered to be Type I. We think that  M1 is not a remnant of supernova Type I, because it differs with bigger mass of the ejected matter and slower expanding velocity. These differences create a variety of phenomenae, which have been discovered by observing the Crab Nebula.    

Some Links for M1

·   Visible and X-Ray images from the ASTRO-1 Space Shuttle mission (STS-35).

·   Ultraviolet images from the ASTRO-1 mission.

·   Chandra X-ray Observatory images of M1

·   M1 images from ESO's VLT

·   More images of M1

·   Amateur images of M1; More amateur images of M1

·   More on the Crab Pulsar

·   Bill Arnett's M1 photo page, info page.

·   M1 (G184.6-5.8) data from D.A. Green's Catalogue of Galactic Supernova Remnants

·   Crab Nebula Stuff from the Chandra X-ray Observatory Team

·   A History of the Crab Nebula from the Space Telescope Science Institute

·   Multispectral Image Collection of M1, SIRTF Multiwavelength Messier Museum

·   Optical Images of the Crab Pulsar (Michael Richmond and KPNO)

·   Jack Schmidling's Crab Nebula page

·   SIMBAD Data of M1

·   NED data for M1

·   Observing Reports for M1 (IAAC Netastrocatalog)

References :

·          Nikola Nikolov, Astronomy for the High Schools, Prosveta, Sofia, 1991

·          Nikola Nikolov and Marin Kalinkov, General Course of Astronomy for Universities, University Publishing House “ Sv. Kliment Ohridski”, Sofia, 1998

·          Nikola Nikolov, Venelina Racheva and Andrei Nikolov, “ The Birth and The Death of  The Stars”, “ Narodna Prosveta”, Sofia, 1988