The Crab Nebula (M1), Katherine Strong
The Crab Nebula is a supernova remnant found in the constellation Taurus. It consists of gas and dust that was left behind after the explosion of a massive star. Ancient Chinese and Japanese records describe the appearance of what they call a “guest star,” occurring on July 4, 1054AD (Duyvendak, 1942). This star is now identified as the progenitor of the Crab Nebula. The explosion of the supernova gave off so much energy that its light was visible during the daytime for three weeks, and it took another twenty-two months for the star’s light to fade completely. Given this information, the supernova must have had an apparent magnitude between -4 and -6, about three times brighter than Venus (Brecher, et al., 1983).
Certain rock art found in North America also seems to depict the supernova explosion of 1054AD. At the time of the explosion, the crescent moon would have passed within a few degrees of the supernova as observed by present-day western United States. Anasazi cave art that has been found in Arizona portrays a crescent moon in close proximity to a very bright star (a feature uncommon to the art of this tribe). Although the conclusion may not be drawn with certainty, this artistic feature has lead to the supposition that Native American tribes of the western United States also observed the astronomical event of 1054AD (Brecher, et al., 1983).
The Crab Nebula was first observed in modern times by John Bevis in 1731 and was subsequently rediscovered in 1758 by Charles Messier, becoming the very first Messier object. The origin of its name is ascribed to William Parsons, the third Earl of Rosse (Hester & Sankrit, 1997). While testing his 36-inch Newtonian reflecting telescope, the Earl observed and drew the following image of Messier 1:
Noting its resemblance to a crab, he so named the nebula. Four years later, after making new observations with a 72-inch telescope, the Earl revised his previous sketch of the nebula and asserted that it did not, in fact, resemble a crab, though the name stuck (Dewhirst, 1983).
Dewhirst, D. (1983)
The Science Behind The Nebula
The Crab Nebula began as an extremely massive star, more than ten times the mass of the sun. Due to this mass, once the star used up all of its energy, it collapsed, leaving behind only its central core and a cloud of dust and gas surrounding that core. Hester et al. 1990 have created a map of the nebula that illustrates the location of the dust and filaments of the nebula:
Hester et al. 1990
The central star of the nebula consists of mostly neutrons and is known as a neutron star. It is fairly small, about 15km in radius, and is rotating extremely quickly, thirty times a second. The magnetic field around the star is about 1 billion times stronger than the magnetic field of the earth (University of Tennessee). Electrons that are accelerated through this magnetic field emit radiation known as synchrotron radiation.
The strong magnetic field concentrates this radiation at the poles of the rotating star, so we observe only a single beam of synchrotron radiation. As the star rotates around, this radiation reaches the earth in "pulses." For this reason, the star at the center of the nebula is called a pulsar, specifically the Crab Pulsar. An X-ray image of the nebula reveals the nature of this radiation:
The radiation emitted by the Crab Pulsar creates a wind that generates a shock wave moving outward from the central neutron star. Images taken with NASA's Chandra X-ray Observatory and the Hubble Space Telescope demonstrate the motion of this shock wave:
NASA/CXC/ASU/J.Hester et al.
The Crab Nebula in true color
The above image shows the nebula as it appears in true color. That is, this image combines data taken from red, blue, and visual (green) filters to reveal what the nebula actually looks like in space. The following images were taken using specialty filters, and the colors have been enhanced in order to demonstrate certain features of the nebula.
Triply-ionized oxygen emission from the Crab Nebula
Using an [OIII] filter, we were able to take an image of the triply-ionized oxygen emission. The brackets around the OIII mark this emission line as what is called a "forbidden line." Forbidden lines are simply spectral lines that are extremely rare. The electrons of the oxygen are excited to a metastable state, an excited state but one whose nature does not allow the electrons to return to the ground energy level. In atomically dense areas, such as on earth, the energy from these excited electrons is simply absorbed by other atoms that collide with the excited atoms; however, in space where there may be only one atom per cubic centimeter, these collisions do not occur, so there is nothing to absorb the energy of the electrons. Eventually, the excited electrons will spontaneously emit photons, releasing energy via this "forbidden" mechanism. In the case of triply ionized oxygen, the photon is emitted with the energy of green light. For this reason, in our data, we have colored the [OIII] emission green.
The [OIII] emission line is extremely faint in our data, so the above image has been greatly enhanced in order to illustrate more clearly the oxygen emission. As shown in the image, the oxygen is concentrated along the outer edge of the nebula. It appears almost like a skin surrounding the inner synchrotron nebula.
The reasons for the actual structure of the Crab Nebula are somewhat of a mystery. The nebula clearly has an [OIII] skin, which surrounds it, separating the synchrotron nebula and the ejecta expelled by the supernova explosion, but this [OIII] skin is unstable, and all observations suggest that it should have dissolved (Hester & Sankrit, 1997). One theory that accounts for the persistence of this membrane is the hypothesis that a yet-unobserved substance (an extended remnant) lies beyond the nebula and serves as a barrier keeping the atoms of the skin from escaping. Sanskrit and Hester 1997 have proposed an alternative model, a shock model, that considers the skin “as a cooling region behind a radiative shock that is driven into an extended remnant by the pressure of the synchrotron nebula.” They hypothesize that it is this radiative shock that keeps the [OIII] skin from dissolving.
H-alpha and NH-alpha emission from the Crab Nebula
Above is an image colored to reveal the inner synchrotron nebula and the outer filamentary structure. The outer filaments are rich with hydrogen atoms that are at the right temperature to emit the H-alpha spectral line. This line is visible in the red spectrum, and so we have colored this emission red. Due to the intense temperatures of the inner synchrotron nebula arising from the radiation of the pulsar, hydrogen is completely ionized in the inner part of the nebula and does not emit H-alpha. Therefore, when we take an image with an NH-alpha ("not H-alpha") filter (a filter that transmits red synchrotron emission but not the H-alpha emission) we are able to observe and color the inner part of the nebula. We have chosen to color this emission blue for the sake of contrast. Notice that the stars in the background also appear blue in color. This is due to the fact that stars do not emit the H-alpha spectral line, so they are indicated in our data by the NH-alpha emission.
Our image shows that the synchrotron nebula is surrounded by a web of filaments. These filaments are accelerating outward due to the pressure exerted by the shock wave coming from the synchrotron nebula (Hester & Sankrit, 1997). They reach velocities on the order of 15,000 km/s (Charlebois, et al., 2010). Charlebois, et al. 2010 have been able to measure the velocities at which the filaments of the nebula are traveling, and from this data, they have produced the following velocity map:
Charlebois, M., Drissen, L., Bernier, A., Grandmont, F., & Binette, L. (2010)
They have also been able to use this data to create a three-dimensional image of the outer filamentary structure of the nebula:
Charlebois, M., Drissen, L., Bernier, A., Grandmont, F., & Binette, L. (2010)
One image of each B, V, and R filters was taken in Rehoboth in order to compare new data with archive data taken of the Crab Nebula in 2004. No observable expansion seemed to have occurred in the filaments of the nebula between the two data sets, so we chose to make use of the archive data in our own analysis.
The true color image was created by combining the calibrated archive data, using the clear filter as a luminance and the B, V, and R filters to color the image accordingly. An unsharp mask was then applied to sharpen the image.
A more complicated process was undertaken to create the images of the [OIII] and the H-alpha emission. The data was first dark subtracted and flat-fielded to calibrate the images. These images were then combined to create a single image in each of the H-alpha, NH-alpha, OIII, NOIII and U filters.
To process the images further, the OIII and the NOIII images were first resized to have the same dimensions as the data taken in Rehoboth. Then all of the data, including the archive data was aligned using a manual two-star align process. The NOIII image was then subtracted from the OIII image so that the OIII image consisted of only the [OIII] emission. Similarly, the NH-alpha was subtracted from the H-alpha. We took the additional step of subtracting the H-alpha from the NH-alpha in order to create an image that revealed all emission that was not H-alpha. The images were subtracted from one another taking into consideration the varying magnitudes of stars, dependent upon the filter used. The subtraction ratios were chosen so that the background stars in the subtracted image would essentially be erased.
To create the colored image of the [OIII] emission, the R filter from the archive data was used as a luminance and the H-alpha, U, and OIII filters were used to color the image. The H-alpha was colored red, the U was colored blue, and the OIII was colored green. In order to more clearly illustrate the [OIII] emission, this filter was boosted to be about 60 times stronger than the H-alpha filter.
To create the colored image of the H-alpha and the NH-alpha image, a two color combine process was used. This process simply combined the subtracted images of the H-alpha and the NH-alpha. The H-alpha was colored red and the NH-alpha was colored blue. Some green was added to each of these images as well, simply to tone down the intensity of the red and blue colors.
Brecher, K., Fesen, R., Maran, S., & Brandt, J. (1983). Ancient Records and the Crab Nebula Supernova. The Observatory, 103, 106-113.
Chandra: X-ray Observatory. (2010, February 24). Crab Nebula. Retrieved April 29, 2010.
Dewhirst, D. (1983). Early Drawings of Messier 1: Pineapple or Crab? The Observatory, 103, 114-116.
Duyvendak, J. (1942). Further Data Bearing on the Identification of the Crab Nebula with the Supernova of 1054AD. Part 1. The Ancient Oriental Chronicles. Astronomical Society of the Pacific, 54, 91-94.
Fesen, R., & Kirshner, R. (1982). The Crab Nebula. I. Spectrophotometry of the Filaments. Astrophysical Journal, 258, 1-10.
Hester, J., Graham, J., Beichman, C., & Gautier, T. (1990). Infrared and Optical Imagery of the Crab Nebula. Astrophysical Journal, 357, 539-547.
Hester, J., & Sankrit, R. (1997). The Shock and Extended Remnant Around the Crab Nebula. Astrophysical Journal, 491, 796-807.
University of Tennessee, Dept. of Physics and Astronomy. Pulsars in Astronomy 162: Stars, Galaxies, and Cosmology. Retrieved May 6, 2010.
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