Picturing The Crab Nebula
By Judith E Braffman-Miller
In 1054, Chinese astronomers stared up at the night sky in wonder, as they observed a new and mysterious object dancing in the Universe. This strange celestial object, that heralded the explosive “death” of a massive star, told the sad tale of how once there was a star that is now gone. Nevertheless, this stellar grand finale left behind a sparkling relic of the now-dead star’s former existence–a shimmering, glimmering, multicolored object that is now known as the Crab Nebula. The Crab Nebula was the first astronomical object to be identified with a historical supernova–the brilliant and fatal explosion of a doomed star. In March 2018, astronomers released a new and beautiful composite image of the Crab Nebula, using the Chandra, Hubble, and Spitzer space telescopes–showing how the Crab lights up the Universe in three different, dazzling wavelengths of the electromagnetic spectrum.
The Crab Nebula (catalogue designations M1, NGC 1952, and Taurus A) is located in the constellation Taurus (the “Bull”), and it received its current name from the Anglo-Irish astronomer William Parsons (1800-1867), who observed the object in 1840, using a 36-inch telescope. Parsons also made a sketch of this distant nebula that resembled a crab.
The Crab Nebula has an apparent magnitude of 8.4, which is similar to that of Saturn’s large hydrocarbon-slashed moon, Titan. As such, the nebula is not visible to the unaided human eye but it can be seen with the help of binoculars when conditions are favorable. The Crab is situated in the Perseus Arm of our Milky Way Galaxy, at a distance of about 6,500 light-years from Earth. It has a diameter of about 11 light-years, and is expanding at a rate of about 930 miles per second–or 0.5% of the speed of light.
At the heart of the Crab Nebula lurks the Crab Pulsar, a wildly whirling young neutron star that is approximately 17-19 miles across, with a breathtaking spin rate of 30.2 times per second. The Crab Pulsar manufactures pulses of radiation from gamma rays to radio waves. At X-ray and gamma-ray energies over 30 keV, the Crab Nebula is usually the most brilliant persistent source in the sky.
There are many reasons why the Crab Nebula is one of the most well-studied celestial objects. For example, it is one of only a small number of objects where there is strong historical evidence indicating when a doomed star blasted itself to pieces in a supernova tantrum. Having this definitive timeline is a valuable tool for astronomers to use in their quest to understand details of the explosion–as well as its dazzling aftermath.
In the case of the Crab, observers in several countries reported the appearance of what they called a “new star” in 1054 A.D. A great deal of information has since been provided to astronomers, courtesy of the Crab, following its stunning initial appearance in the sky. Currently, astronomers understand that the Crab Nebula is powered by the Crab Pulsar–a highly magnetized, rapidly spinning neutron star that was born like a Phoenix rising from the funeral pyre of its progenitor massive star. The progenitor star perished explosively after it had depleted its necessary supply of nuclear-fusing fuel and collapsed. The stellar mix of a strong magnetic field with rapid rotation in the Crab produces a powerful electromagnetic field that is responsible for creating jets of matter and anti-matter that rush screaming away from both the north and south poles of the pulsar, as well as a ferocious wind that shrieks out in the equatorial direction.
The Explosive End Of A Massive Star
The progenitor star of the Crab Pulsar perished in the merciless flames of a core-collapse–or Type II–supernova. Core-collapse supernovae herald the fatal grand finale of a massive star that has finished burning its necessary supply of fuel by way of the process of nuclear fusion. In order for a doomed, elderly heavy star to suffer this form of rapid, catastrophic collapse–followed by a dazzling, devastating blast–it must contain at least eight times, but no more than 40 to 50 times, the mass of our Sun. A supernova explosion can glare so brilliantly that, for one brief shining moment, it can even out-dazzle its entire host galaxy.
Supernovae are the most powerful of all known stellar explosions, and they can be seen all the way out to the very edge of the observable Universe. When a heavy star dies it leaves behind, within its own wreckage, either the extremely dense “oddball” called a neutron star, or a stellar mass black hole.
Stars produce energy through the process of nuclear fusion. Unlike our relatively small Star, the Sun, more massive stars boast sufficient mass to fuse atomic elements all the way up to iron. Stars perform this feat of atomic metamorphosis at increasing pressures and temperatures. The degeneracy pressure of electrons and the energy produced by fusion reactions are able to battle against the relentless force of squeezing gravity, thus preventing the star from collapsing. In this way, a still “living” star, maintains stellar equilibrium. The star fuses increasingly heavier and heavier atomic elements out of lighter ones, beginning with hydrogen and helium, and then continuing on and on and on through all of the atomic elements up to iron and nickel. As the now-elderly star approaches the end of that long stellar road, it contains a core of iron and nickel. Nuclear fusion of iron and nickel can produce no net energy output, and for this reason no further fusion can occur–thus leaving the nickel-iron core inert. Because there is no longer energy output producing outward pressure, equilibrium is broken, and the star is ready for its final farewell performance to the Universe. Within seconds, the outer core reaches an internal velocity of as much as 21% of the speed of light, while the temperature of the inner core sky-rockets to as much as 100 million Kelvin.
Core-collapse supernovae usually demolish the doomed massive star, tearing it to pieces and hurling its sparkling rainbow of multicolored gaseous layers out into interstellar space. The most massive stars in the Cosmos collapse and blast themselves into a stellar mass black hole. Very massive stars, that are not quite that massive, leave behind a dense, city-sized object called a neutron star–a young neutron star is a rapidly spinning pulsar, just like the one haunting the heart of the Crab.
Astronomers’ realization that the Crab Nebula was formed as the result of a core-collapse supernova blast dates back to 1921, when Carl Otto Lampland made the important announcement that he had observed alterations in the Crab’s structure. This ultimately led to the idea that the creation of the Crab Nebula corresponds to the bright “new star” recorded by Chinese astronomers in 1054. In addition, there is also a 13th century Japanese reference to this “guest star”.
For a long time, the supernova event that created the Crab Nebula was thought to be unrecorded in Islamic astronomy. However, in 1978 a reference to SN 1054 was found in a 13th century copy made by the Syrian Arab physician Ibu Abi Usailbia (1203-1270) of a work by Ibn Butlan (1001-1064), a Nestorian Christian physician who was living in Baghdad at the time the “New Star” made its brilliant debut in the sky.
The Crab Nebula was first identified in 1731 by the English astronomer John Bevis (1693-1771), and it was rediscovered independently in 1758 by the French astronomer Charles Messier (1730-1817) while he was in the process of hunting for a brilliant comet. In a classic example of scientific serendipity (serendipity means that when you are looking for something, you find something else), Messier spotted the Crab Nebula while he was searching (unsuccessfully) for Halley’s Comet. Initially, Messier thought that he had detected Halley’s Comet, but after some additional observations, he realized that he had found something else instead. This is because the object Messier was watching was not traveling across the sky. Since all comets streak across the sky with their flashing tails thrashing, Messier concluded that the object he was watching was not a comet. As a result, Messier realized the great value of compiling a catalogue of celestial objects that possessed cloudy attributes, but were fixed in the sky–in order to avoid incorrectly cataloguing them as comets.
In 1913, when the American astronomer Vesto Slipher (1875-1969) registered his spectroscopy study of the sky, he noted that the Crab Nebula was one of the first objects he had observed. In the first half of the 20th century, the analysis of early photographs of the nebula–that had been obtained several years apart–revealed that it was in the process of expanding. By tracing the expansion back in time, astronomers determined that the nebula must have been visible on Earth about 900 years earlier. Historical records showed the appearance of that strange “new star” in the same part of the sky as the nebula–the same one that had been bright enough to be seen in day light by Chinese astronomers in 1054.
Changes in the cloud indicated its small extent. Then, in 1928, the great American astronomer Edwin Hubble (1889-1953)–for whom the space telescope is named–proposed associating the cloud with the “new star” that had bewitched medieval sky-watchers in 1054. This proposal remained a topic of considerable debate until the nature of supernovae became known. It was the American astronomer Nicolas Mayall (1906-1993) who finally suggested that the stellar “visitor” of 1054 was the very same supernova whose blast in the past had created the Crab Nebula. The quest to find historical supernovae started at that point, and seven other historical sightings were found by astronomers comparing modern observations of supernova remnants to astronomical documents of previous centuries. Because of its great distance, the daytime “guest star”–that had dazzled Chinese astronomers–could only be explained as a supernova heralding the “death” of a massive, exploding star. The dying progenitor star had exhausted its necessary supply of energy from nuclear fusion and had collapsed in on itself–undergoing a sea-change to become the Crab Pulsar residing at the very heart of the Crab Nebula.
More recent studies of historical records indicate that the supernova that created the Crab Nebula probably appeared in April or early May, reaching its maximum brilliance in July–at which point it was brighter than everything in the night sky except for Earth’s Moon. The supernova could be seen with the unaided human eye for two years after its dazzling debut in 1054. Due to the fortunate recording of observations of the “new star”–by Far Eastern and Middle Eastern astronomers of that era–the Crab Nebula became the very first celestial object to be linked to a supernova blast.
With the discovery of pulsars back in the 1960s, the Crab Nebula again became a celestial object of great interest. It was during that decade that the Italian astrophysicist Franco Pacini predicted the existence of the Crab Pulsar for the first time–and that its presence would explain the brilliance of the cloud. The Crab Pulsar–which is the relic core of the dead massive star that had produced the stellar “visitor” of 1054–was finally observed in 1968, shortly after Pacini had made his prediction.
When observed in visible light, the Crab Nebula is composed of a broadly oval-shaped mass of filaments surrounding a diffuse blue central region. In three dimensions, the nebula appears to be shaped like a prolate spheroid. The filaments are really the remains of the progenitor star’s atmosphere, and they are primarily composed of hydrogen and ionized helium. However, the elements carbon, oxygen, nitrogen, iron, neon, and sulfur are also present.
Picturing The Crab
The most recent picture of the Crab is a composite that includes X-rays derived from Chandra (blue and white), NASA’s Hubble (HST) (purple), and NASA’s Spitzer Space Telescope (pink). The extent of the X-ray image is smaller than the two others because extremely energetic electrons emitting X-rays radiate away their energy more rapidly than the lower-energy electrons emitting optical (HST) and infrared (Spitzer) light.
The new composite contributes to a scientific legacy that spans almost two decades. Below is a sample of the numerous important insights astronomers have gained about the Crab Nebula using Chandra, Hubble, and Spitzer:
1999: Within weeks of being shot into orbit from the Space Shuttle Columbia during the summer of 1999, Chandra observed the Crab Nebula for the first time. The information contributed by Chandra showed features of the Crab that had never been seen before. The new data included a bright ring of high-energy particles surrounding the very heart of the Crab Nebula.
2002: The dynamic nature of the Crab Nebula was brilliantly displayed in 2002, when astronomers produced videos based on coordinated Chandra and HST observations conducted over a span of several months. The bright ring that had been observed for the first time in 1999 was seen to be composed of over twenty knots that form–then brighten, then fade, then jitter around, and sometimes even experience outbursts that create expanding clouds of particles, but nevertheless stay in approximately the same location.
These knots are produced by a shock wave, akin to a sonic boom, where fast-moving particles from the Crab Pulsar are banging into encircling gas. Brilliant wisps originating in this ring are traveling outward at 50% the speed of light to create a second expanding ring residing further away from the pulsar.
2006: NASA’s infrared Spitzer space telescope was launched in 2003, and it joined Chandra, HST, and the Compton Gamma-ray Observatory –thus, completing the development of NASA’s Great Observatory program. Only three years after Spitzer’s launch, the first composite image of the Crab with data from Chandra, HST, and Spitzer was released.
2008: As Chandra continued to study the Crab, the data gathered began to provide a better picture of what was happening in this dynamic nebula. In 2008, astronomers first reported a view of the faint boundary of the Crab Nebula’s pulsar wind nebula–a cocoon composed of high-energy particles encircling the pulsar. The newly obtained data unveiled structures that astronomers came to refer to as “fingers”, “bays”, and “loops”. These features suggested that the magnetic field of the nebula and filaments of cooler matter are controlling the movements of the electrons and positrons (the electron’s antimatter twin). The particles can jitter around rapidly along the magnetic field and move several light years before finally radiating away their energy. In marked contrast, they travel much more slowly perpendicular to the magnetic field, and wander only a short distance before losing their energy.
2011: Time-lapse movies of the Crab, created from data obtained from Chandra, have proven to be valuable tools for astronomers to use. This is because they show the dramatic alteratons in the X-ray emission close to the pulsar. In 2011, Chandra observations, conducted between September 2010 and April 2011, were obtained for the purpose of pinpointing the exact location of intriguing gamma-ray flares detected by NASA’s Fermi Gamma Ray Observatory and Italy’s AGILE Satellite. The gamma-ray observatories were unable to locate the origin of the remarkable flares within the Crab Nebula, but astronomers hoped that Chandra‘s high-resolution images would be able to accomplish this feat.
Two observations using Chandra were performed when powerful gamma-ray flares occurred. However, no clear evidence was seen for correlated flares in the Chandra images.
Despite this lack of correlation, the Chandra observations helped astronomers derive an explanation for the gamma-ray flares. Even though other theories remain viable, Chandra provided important new evidence that accelerated particles produced the gamma-ray flares.
2014: Several new images of supernova remnants were released to celebrate the 15th anniversary of Chandra’s launch. The images also included a “three-color” image of the Crab Nebula, where the X-ray data were split into three different energy bands. In this image, the lowest-energy X-rays that Chandra spots are red, the medium range are green, and the highest-energy X-rays from the Crab are blue. The extent of the higher-energy X-rays in the image is smaller than the two others. This is because the most energetic electrons responsible for creating the highest-energy X-rays radiate away their energy more rapidly than the lower-energy electrons.
2017: Building on the multiwavelength images of the Crab obtained in the past, a highly detailed view of the Crab Nebula was created in 2017 using data obtained from telescopes spanning nearly the entire breadth of the electromagnetic spectrum. Radio waves obtained from the Karl G. Jansky Very Large Array (red), HST data (green) and Chandra (purple) created a wonderful new image of the Crab.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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