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Object of the Week, December 28, 2015 - Messier 1, the Crab Nebula
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"What caused me to undertake the catalog was the nebula I discovered above the southern horn of Taurus on September 12, 1758, whilst observing the comet of that year. This nebula had such a resemblance to a comet in its form and brightness that I endeavored to find others, so that astronomers would no more confuse these same nebulae with comets just beginning to appear. I observed further with suitable refractors for the discovery of comets, and this is the purpose I had in mind in compiling the catalog.
After me, the celebrated Herschel published a catalog of 2000 which he has observed. This unveiling of the heavens, made with instruments of great aperture, does not help in the perusal of the sky for faint comets. Thus my object is different from his, and I need only nebulae visible in a telescope of two feet [focal length].”
Charles Messier
Although Messier found M1 in 1758 it had actually been discovered 27 years earlier by an English physician and amateur astronomer, John Bevis, and it wasn’t until 1771 that Messier became aware of the priority of Bevis’ discovery.
Interestingly, it seems he came across M1 on August 28, but measured its position on September 12th, which he regarded as its discovery date.
Messier’s terse description of what he saw doesn’t give a novice observer much to go on:
“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.”
But what an amazing object M1 turned out to be – the only supernova remnant in his catalog and one of the most intensely studied objects in the sky for the past 90 years.
Physical Properties
The supernova left behind M1’s expanding nebulosity and a fast spinning neutron star at its center. Spinning almost exactly 30 times per second, this 1.4 to 2.0 solar mass object is only about 18 miles in diameter. Its powerful magnetic field focuses its electromagnetic radiation into narrow beams, which happen to be directed toward Earth.
The neutron star is very gradually slowing its spin, a process that puts out 75,000 times the energy of our sun, and powers the synchrotron radiation that makes the main body of M1’s nebulosity glow. Synchrotron radiation is produced by electrons curving their way through a strong magnetic field at up to half the speed of light.
This extreme energy produces an astronomically wild place in the central area of M1. The pulsars equatorial wind slams into the nebula and forms a shock front that changes shape daily.
The filaments are likely the remains of the progenitor star’s atmosphere and are composed of ionized helium and hydrogen with carbon, oxygen, nitrogen, iron, neon and sulfur mixed in.
M1 is approximately 6300 light years away, and since its explosion has expanded to about 13 light years along its greatest length.
Visual Observations
Messier didn’t make a sketch of his first nebula, but many 19th century astronomers did. As was too often the case, sketches of the same object bore little resemblance to each other, sparking a lively debate on their scientific usefulness. The German astronomer Wilhelm Tempel complied a few of the more prominent sketches of M1 to make his point that a standardized methodology needed to be followed when making astronomical drawings. It’s a fascinating example of what 19th century astronomy was faced with until the invention of practical astrophotography. Clockwise from top left the sketches were made by Tempel, John Herschel, Heinrich d’Arrest, William Lassell, Pietro Secchi, and Lord Rosse. Yes, these sketches are all of M1.
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By the way, it was Lord Rosse who gave M1 its Crab Nebula nickname when he discovered its filaments in 1844. His 1855 “bumblebee” sketch is shown above (center left) but his first drawing looked much like Secchi’s in the lower left corner.
Into this mix I’ll toss two sketches I made in early December, 2011. I sketched M1 without filters and then with an OIII filter to show how the appearance of the nebula changes.
Without a filter the nebula has a broad, soft “S” shape that gently feathers along both ends. The central area is brightest and with a 16 inch or larger scope the 16th magnitude pulsar can be seen when the seeing is steady under a dark sky. A star just slightly brighter than pulsar, but not associated with it, forms an optical double with the pulsar and both can be seen near the center of the nebula. The pulsar is the slightly dimmer of the two.
The overall scene at 253x through my 28 inch scope is quite nice as M1 sits in a lovely star field that compliments its soft nebulosity.
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With the OIII filter in place the nebula takes on a very different appearance. Now strongly oval in shape, several of the famous filaments appear, with the two brightest being quite easy to see. I’ve seen vague hints of other filaments but not enough to mark them on my sketch. However, the entire nebula has a mottled look and the perimeter of the nebula has a feathered appearance. Filaments have been detected with 12 inch scopes, so a huge instrument isn’t needed to get a good view.
But a really big scope comes in handy if you want to see the pulsar in action. A few years ago Dan Gray (Sidereal Technology) made a rotating shutter especially to “blink” M1’s pulsar. He set the rotation speed just slightly off from the rotational speed of the pulsar so its image would slowly fade and brighten over about 15 seconds. Attempts with our respective 28 inch scopes failed, but he met with success with 60 inch and 90 inch professional scopes in Arizona. I was part of the observing group that saw the pulsar dim and brighten using Dan’s shutter when hooked up to the Bok 90 inch scope at Kitt Peak, and that experience ranks as one of my top observing highlights.
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All Messier knew in 1758 was that he hadn’t found a comet. Fortunately M1 turned out to be one of the most intriguing astrophysical objects in the sky that’s easily accessible to amateur telescopes.
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