The Crab Nebula is the brightest and most famous and of the supernovae remnants (SNR) whose largest extent covers an area of 6′× 4′arcmin. Lying in eastern Taurus and some 1.2° NW from Zeta Taurri, this relatively easy object can be found in apertures as small as 5.0cm. Although not a true planetary, I have included it for completeness and it key interest among many amateurs. At 9th magnitude, NGC 1952 appears as a small grey oval-like haze along with a few faint stars, and the nebula bears magnification well. This Supernova Type I was mainly reported by the Chinese to have been first noticed on 04th July 1054AD. The star was so bright that it was seen for several months and unbelievably stayed for some twenty-three days visible during the daylight hours. Some believe it may have reaching the maximum brightness between -5 and -7 visual magnitude.
EARLY TELESCOPIC DESCRIPTIONS
of the CRAB NEBULA
Its remnant was accidentally re-discovered by the Englishman John Bevis in 1731. Although Bevis did not record a catalogue as such, he placed the object among sixteen others he labelled as ‘nebula’, in the Star Map of Taurus within his beautifully detail and expressive star atlas, Uranographia Britannica, which later published in London in 1750. This particular reference went unnoticed by all of the astronomical elite of the time, so Bevis’ discovery remained just an obscure oddity.
In September 1758, Charles Messier also again accidentally found this object in the year of the first predicted return of Halley’s comet. Messier discovery occurred while he was making nearby observations of the now obscure Comet De la Nux. When Messier eventually publish his famous deep-sky catalogue until 1771, Bevis wrote to Messier on the 10th June to inform him that he had discovered the small nebulosity in 1731. Messier examined his claim and agreed, and then in the subsequent versions of the catalogue acknowledged Bevis’ discovery.
Messier described NGC 1952 appearance as;
“Nebula above the southern horn of Taurus, contains no star; it is a whitish light, elongated like a flame of a taper, discovered while observing the Comet of 1758. See Chart of that comet, Mèm. Acad., p.188, (1759): observed by Doctor Bevis in about 1731. It is reported on the English Celestial Atlas.”
He was so impressed with this little nebula and its resemblance to some comet, that it became listed as the first object in Messier’s Deep-Sky Catalogue (M1).
John Bode was the next person to record this object in 1777, listing NGC 1952 as No.11 in his deep-sky catalogue of seventy-seven objects. He simply comments it appears as;
“A nebulous patch without stars”
Bode later said;
“I found this object as with the 7-foot telescope, and in the position listed by Mr. Messier relative to the stars situated closest to it…”
Later observations by other noted observers continued, but all with the misconception that it was a collection of unresolved stars — possibly similar to some of the globulars that they also had seen.
For example, William Herschel made many observations using various sizes of telescopes. He was notably tempted in his believe of resolution of the nebulosity, and this faith in some unresolvable cluster affected many others who followed him. Using his first 7-foot telescope at 287×, he observed the same thing as Messier did; “…light without stars.” His second observation in 1784, which set contentions for his following observations, clearly states;
“M1 [with] my 7, 10, and 20-feet reflectors shewed a mottled kind of nebulosity, which I shall call resolvable; so that I expect my present telescope will, perhaps, render the stars visible of which I suppose them to be composed…”
NOTE: I have merged this comment from three different sources who say very similar things.
It is interesting to comment, as this is supposed to be a PNe dissertation, that Herschel believed that both the Dumbbell Nebula (M27) and the Ring Nebula (M57) showed the characteristics of a some resolution. At first I thought, in the Crab Nebula’s case, the starry field that occupies the field influenced Herschel decision of near resolution. However, he applies a similar description to M3 and M79, both globular star cluster, and the bright galaxies like M33, M81 and M101. William Herschel’s uncertainty remained.
Then using the larger 20-foot telescope, after 1783-1784, he expressed his observation and opinion say;
“…Very bright, of an irregular figure; full 5 minutes in longest direction. I suspect it to consist of stars.”
In 1805, using the 10-foot telescope at 220x;
“…the diameter is 4′ 0″ with this power and light it is what must be called resolvable [mottled].”
In 1809, and using the same telescope, he says;
“It is resolvable [mottled]. There does not seem any milky nebulosity minced with what I take to be small lucid points.”
And at a similar time;
“As all the observations of the large telescopes agree to call this object resolvable, it is properly a cluster of stars at no very great distance beyond their gaging powers.[and] …is near the Milky Way.”
William Herschel’s last comments, says among the seven objects within the first of his “class systems” he remained;
“…undecided into which class they should be placed.”
Until his death, it is certain that he was convinced these presumed stars were just beyond his telescope’s grasp. John Herschel observations later in 1831, and continuing his father’s work, followed next. He listed M1 as h.357 in his catalogue while conducting his many sweeps of the northern skies. he described M1 in his abbreviated terminology as;
“vL; E; vglbM; r; 4′ l, 3′ br; pos of longer axis n p to s f. A fine object.”
John Herschel says in a drawn figure that appears in Fig. 81 of his first major work “Catalogue of 1833.”, and holding similar beliefs to his father, (though it is possible this comment might be independent); his caption says;
“…are clusters of stars, beginning with a barely resolved one, and ascending by successive degrees…”
Admiral Smyth observation in the last days of December 1836 that is contained within the famed Cycle of Celestial Objects CCXII. [212], describes M1 as;
“A large nebula, pearly white, about a degree north-west of the star Zeta on the tip of the Bull’s southern horn, and on the outskirts of the galaxy. It is of an oval form, with its axis-major trending np to sf, and the brightest portion toward the south.”
Then commenting on John Herschel’s observation, he concludes;
“All this shows the difficulty of what, to my view, is rather a milky nebulosity than a cluster.”
Of all the mammoth telescopes of the 19th Century, the construction of Earl of Rosse’s 1.0-metre 40-foot telescope was certainly made to answer the problems by attempting to resolving some of the nebulosities. Although the telescope turned out to be disappointing because of its bulk and the near impossibility of moving and tracking objects, Rosse did observe and draw several of the bright and questionable nebulosities in the sky.
Earl Rosse’s most famous quote and drawing, is taken from the Philosophic Transactions of the Royal Society of 1844, (Phil.Trans. R.S.; p.321-324 (1844))
“Fig. 81 is also a cluster; we perceive in this, however, a considerable change of appearance; it is no longer an oval resolvable Nebula; we see resolvable filaments singularly disposed, springing principally from its southern extremity, and not, as is usual in clusters, irregularly in all directions. Probably greater power would bring out other filaments, and it would then assume the ordinary form of a cluster. It is stubbed with stars, mixed however with a nebulosity probably consisting of stars too minute to be recognized. It is an easy object, and I have shown it to many, and all have been at once struck with its remarkable aspect. Everything in the sketch can be seen under moderately favourable circumstances.”
It is the representation of Figure 81 that is the most stunning. He clearly Earl Rosse is see the filaments of the supernova ejecta. Rosse presented the drawing as-is, and made no comment on its presumed appearance. In England, the distribution of this drawing through the astronomical community caused a sensation. It is uncertain who first described the shape as looking like a crab, but it certainly was not Rosse as so often quoted in the general astronomical literature. Of all the things that Rosse did, one of his biggest legacy is that the name the “Crab Nebula” was brought into common usage.
However to me the biggest problem with his and other previous observations is the persistent firm belief that the nebulosity was composed of stars. Nearly all of the material written about the Crab Nebula never asks about why these observers saw stars. This is especially true when telescopically it is clearly nebulose and similar to many of the intriguing PNe. Admiral Smyth makes an interesting comment in his Cycle of Celestial Objects, CCXII [212] about Rosse’s observations and the belief about nearly all other deep-sky objects;
“The powerful telescope constructed by Lord Rosse, however, not only displays the component stars distinctly, but also shows several fringy appendages around, and a deep bifurcation to the south. So do siderial wonders increase with our means of optical practice?”
Looking at the final question here is interesting. The only problem with
One of the first observation of the true nature of the nebulosity was made by William Lassell in December 1852. Observing with 0.7m (24-inch) speculum mirrored reflector from the Mediterranean island of Malta. In a climate more suitable than from Great Britain, he says;
“With 160x it is a very bright nebula, with two or three stars in it, but with 565x…long filaments run out from all sides and there seems to be a number of minute and faint stars scattered over it.”
On the 6th January of the following year Lassell reobserved the Crab with 565×:
“The brightest parts are about 2′ in length, while the outlying claws are only just circumscribed by the edge of the field of 6′ in diameter.”
Later Observational Descriptions
The sheer number of deep-sky descriptions for this object are that many I thought it more useful to give a general précis by increasing aperture than list them all.
Rev. T.W. Webb (1962) describes;
“ oblong; pale; 1° np. [Zeta Tauri] … inlock, gaseous spectrum.”
This later comment about Winlock incidentally comes from Epsin. It is not often commented, by this is likely the first comment on observations of M1’s spectrum. The first significant spectroscopic observation with a sizable telescope was by V.M. Slipher between 1913 and 1915 at Lowell Observatory. Slipher found an incandescent gas source with much of the light concentrated into a few colours and bright lines spanning the nebulosity. He further found using photography that these lines were strangely curved or bowed and that some were strangely duplicated - and something no amateur has observed. (See Below Section on ‘Spectra of NGC 1952’.)
AOST1 wrongly describes the Crab Nebula as a PNe, but this words are a little misgiven, as the other PNe NGC 1514, and Hind’s Variable Nebula; NGC 1554-1555 surely rates much higher than this. David Frew correctly deletes the following statement of Hartung saying;
“Only one of the planetary nebulae repays observations, and this is the interesting M1.”
AOST2 describes NGC 1952 as;
“…a bright elliptical haze about 4′×2.5′ in PA 125° in beautiful contrast with a starry field and shows a single prism image, though with some continuous spectrum. The image is clear with 7.5cm and the object itself is quite easy. Hartung saw no sign of structure beyond the irregular edges and a faint, curved extension southwest. D is about 2000pc.”
The only real difference between these two sources is the distance. AOST1 gives 1 200pc, AOST2 gives 2 000pc.
AOST2 is also notes;
“Some 26′ east is the elegant pair Σ742 (7.2 7.8 3.5″ 269°).”
In Deep-Sky Walter Scott “Dissecting the Crab” he describes that it is only a “sort hop from the heart of Hyades” and says M1 been carefully studied by radio and X-ray astronomers. He goes on to state;
“It is known to be the remains of a supernova…, and the pulsar at its center is suspected to be a rotating neutron star.”
Houston also notes;
“…it was also in the search region for the 1835 return of Halley’s Comet.”
In December 1852, the English telescope maker and observer William Lassell used a 24-inch speculum-metal reflector to view M1 from Malta. He noted,
“With 160x it is a very bright nebula, with two or three stars in it, but with 565× …long filaments run out from all sides and there seems to be a number of minute and faint stars scattered over it.”
On January 6th of the following year Lassell reobserved the Crab with 565x:
“The brightest parts are about 2′
in length, while the outlying claws are only just circumscribed by
the edge of the field of 6′ in diameter.’ …the Crab can be seen in 2-inch
finders. Small telescopes reveal only a shapeless 8th-magnitude blur
variously sketched as oval, rectangular, or more often something in
between. The Crab Nebula usually shows in small telescopes as a
featureless gray ghost. My 4-inch Clark refractor has revealed hints
of the nebula’s ragged edge that
appears so prominently in photographs. These edge serrations are
usually apparent in a 12-inch telescope and easy in a 17-inch.
Increased magnification does not seem to change the appearance much.
Telescopes of 12-inch aperture and larger often reveal delicate
filamentary structure in the nebula.”
“My impression is that large amateur
reflectors do not show much more of the Crab Nebula than a 6-inch
does, though of course they show it better. Amateurs with access to a
16-inch or larger telescope, perhaps a club telescope, can perform
some interesting experiments on how the appearance of M1 varies with
different magnifications and telescope f/ratios. The latter can be
changed by making aperture masks of varying diameters.”
“Mike Mattei of Littleton,
Massachusetts, has suggested using a nebula filter in the ‘flicker’ mode.
By rapidly moving it in and out of the space between the eyepiece and
your eye, it is easy to note the effect of the filter. I tried this
technique using a 15 x 65 monocular at my home in East Haddam. The
Crab was easily detected with the flicker method, but I was unable to
hold the nebula steadily when viewing either without the filter or
with it fixed in place.”
Celcia Payne-Gaposchkin, C (1964) says of the spectrum;
“The spectrum of the nebulous remnant, observed by Minkowski (1943), shows bright lines of [O I], [O III], [N II], [S II] and H I. The intensity ratio of hydrogen to nitrogen is similar to that shown by the spectrum of the Crab Nebula; both spectra suggest poverty in hydrogen. The relative spectral intensities are affected by interstellar reddening, and Minkowski estimates a color excess of 2.1 mag. The observed radial velocity of the nebulous fleck, -200 km/sec, affected by an unknown projection factor, is not incompatible with expansion like that of the Crab Nebula. The bright-line spectrum is not unlike that of the Crab Nebula, but is without the strong continuum of the latter.”
Supernovae of 1054 AD (Crab Nebula) : Prior to 1968
One of the most eloquent piece of writing I have found on the Crab Nebula, or in fact all nebulae, was the introduction to Nicholas Mayall leaflet paper of January 1939, entitled “The Crab Nebula, A probable Supernova.” Here he eloquently says;
’IN THE YEAR of our Lord 1054, when Omar Khayyam was a small boy, and the Battle of Hastings still twelve years into the future, an unknown Chinese astronomer, perhaps weary and sleepy after working all night, was astonished to see a strange and brilliant new star in the greying eastern sky just before sunrise. The object was located in the Chinese stellar division of Peih, which we know as the constellation of Taurus (The Bull), a little less than halfway from the centre of Peth, the Hyades, towards Pih Ho, the Twins, Castor and Pollux. Although this noteworthy astronomical event occurred on the Fourth of July, and the Chinese probably had a plentiful supply of firecrackers, it was not appropriately celebrated, so far as we know. Instead, the astronomer carefully noted the new star’s approximate position with respect to a familiar star, Teen Kwan, known to us as the third magnitude Zeta Tauri, and during the next six months watched it fade to invisibility.’
Edwin Hubble in 1928 was the first to suggest that the Crab Nebula was associated with the “star” seen by the Chinese in 1054AD. This remained an curiosity until renewed observations of the nebula's structure between about 1938 and 1942, when it became obvious this was an example of catastrophic stellar demolition - even though the mechanism to cause such flagrant amounts of energy to be expelled was not fully understood. At the time this was thought to be another example of the nova phenomena, even though this was clearly obviously that this was no ordinary nova. Suspicion of the power of these events was seen with the other three known galactic supernovae at this time. I.e. The supernova in Lupus in 1006AD, Tycho’s Star in 1572 and Kepler’s Star in 1604. This was the first of five well known galactic supernovae to have been observed in the last one thousand years. Only two recent visual events have occurred. In 1885, S Andromedae was first known extragalactic event. Appearing on 17th August 1885 in the Andromeda Galaxy, NGC 224, the star reached the maximum of 5.4 magnitude close to the central hub of the galaxy. Five months later, in February 1886, S And dropped below naked-eye visibility, and was observed for several years until fading from view. (Type II)
Most recent was the Supernova 1987A in the Large Magellanic Cloud. In 1987, this event reached its 3rd magnitude maximum shining for several months before dropping below naked-eye visibility. It is only the second known SNR with a bright expanding surrounding shell of gas that has been visually prominent since about mid-1998. Southern astronomers have been studied the expansion of the gas shell and established some new understanding in how supernova work.
Looking at the modern historical interpretations by Oort and Mayall (PASP, 54, 95 (1942))
Note: I do suggest readers that would like to know these historical details, might like to read the entire article which can be found through the Astrophysical Data Base (ADS)
The Crab Nebula in images shows delicate structures and in some ways it is a very unique structure and has a well established with a very bright remnant. The outer areas show filamentary structure that in the main part of the familiar S-shaped structure.
The Spectrum of NGC 1952
The prism image shows the nebulae as a stellar continuous spectrum. Within this spectrum is the bright-line spectra as seen in Wolf-Rayets and PNe's, with the lines of H , [N II], [O II], [O III], [Ne III] and [S II]. All these lines can be used to determine velocities in the expanding nebulosity. These line intensities also show the central star is hydrogen depleted, suggesting the progenitor was a white dwarf, and producing the Supernova Type I.
The size of the nebulosity producing 80% of the light and the continuous spectra is about 3.2′×5.9′ in size, with the outer filamentary structures being more like 4.0′×6.0′.
M1 is also a powerful radio source that was properly identified by three Australian astronomers J.G. Bolton, G.J. Stanley and O.B. Slee (1949). They also found three discrete inner sources. In 1954, Balwin showed that the amount of radio energy was twice that of the optical source. The origin of the radio energy is mainly from the filaments whose distortions are caused by synchrotron radiation in the strong magnetic fields.
Supernova of 1054 AD (Crab Nebula) : After 1968
In the last fifty years or so our knowledge has expanded enormously regarding the Crab Nebula and the events that created it. In areas of importance, the discovery of the neutron star and pulsar has had the greatest influence regarding the progression of our understanding of stellar evolution, the final stages of stellar life and the consequences of supernovae. The latter is likely the most important aspect as many of the most important elements used by humanity have origins in the discarding of the outer shells of red giants and the explosion of supernovae. It is humbling to think that the heavy elements like gold, silver, platinum, and uranium, for example, could have only been manufactured within the creation of these titanic explosions. Some of these elements, like iodine and selenium, are even very essential for our human body’s well-being.
Yet the Crab Nebula is only a single example of the passing eons of untold supernovae events that have happened since after the Big Bang. Understanding how these events really happen is not just philosophical or humanistic. Such implications have driven study into the avenues that the astronomers in the first half of the 20th Century could only dream. For example, discovery and theoretical studies of black holes, for example, which have captured so much of the public’s imagination, and have produced ramifications that stretch all the way back to the origin of the Universe. It was just over eighty years ago that astronomers were still questioning if galaxies were indeed extragalactic systems, and only just over sixty years ago that we found the mechanism of how the stars produce their energy and the chemical elements. How wonderful to live in an age where we have some understanding of the way the Universe works and have some insight of the lifestyles of the stars abd their eventual demise!
The Crab Pulsar
In retrospect, most of these theoretical ideas about supernovae and neutron stars remained incomplete until the defining discovery in 1968 by the Englishwoman Joslin Bell of the Crab Nebula's pulsar. So unexpected was her discovery, that at first it was thought the received signals were some intermittent radio telescope problem, or that the regular pulses were sent from an intelligent race beyond the solar system. Through careful elimination and deduction she and her supervisor realised they had stumbled onto something new.
The pulsar is labelled NP 0531+21 and has a period, as when discovered, of 0.0330976 seconds that in turn gives the rotation rate of the pulsar as the dizzying velocity of 30.21367 times per second. Its pulses continue very gradually to decreasing by around (•P) 36.520 nanoseconds each day, and this is sometimes expressed as the value ‘T’ of 0.0025( .P/P) being the time period lost over one million years (106). This shows that pulsar periods change very slightly.
Using the slight periodic changes observed because pulsar position in the Earth’s orbit finds the distance as about 1.70 kpc. The pulsar has also been identified visually and is known as the variable CM Tauri.
REFERENCES
1. Baade (1942)
2. Duyvendak (1942)
3. Duncan, John C.; “The Crab
Nebula”, S&T., 1, 3 (Dec) (1942)
4. Jones, K.G.; “The Search for
Nebulae”, Alpha Academic (1975)
5. Minkowski, R.; “The Crab
Nebula.”, A.J., 96, 199 (1942)
6. Minkowski, R.; “The continuous
spectrum of the Crab Nebula”, Annales d’Astrophysique (AnAp), 9, 97
(1946)
7. Mayall, N. U.; “The Crab Nebula, a
Probable Supernova.”, Astron. Soc. Pac. Leaflets (ASPL),
3, 145 (1939)
8. Mayall, N. U.; “The Spectrum of the
Crab Nebula in Taurus”; PASP., 49, 101 (1937)
9. Oort & Mayall “Further Data
Bearing on the Identification of the Crab Nebula : Pt II The
Astronomical Aspects”, PASP, 54, 95 (1942)
10. Lampland, C. O.; ”Observed Changes in the Structure of the
Crab” Nebula (N.G.C. 1952)”; PASP, 33, 79
(1921)
11. Payne-Gaposchkin, C.; “The Galactic
Novae”; Chap. 9 “The
Supernova”, p.276-278 (1964)
12. Sanford, R. F.; “Spectrum of the
Crab Nebula”, PASP., 31, 108 (1919)
13. Tauri, M. I., Roberts, I.; “Photograph of the crab nebula”,
MNRAS, 55, 399 (1895)
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nebula”, MNRAS, 52, 502 (1892)
15. Webb, Rev. T.W.; “Celestial Objects
for Common Telescopes.” pg.237, Dover Press (1962)