Crab Nebula & Pulsar

Palomar Observatory, STScI, NASA -- larger image
(Shockwaves from the pulsar's polar jets crash into the surrounding nebula -- more).

According to Robert Burnham, Jr. (1931-93), the Crab Nebula was discovered by English physician and amateur astronomer John Bevis (1695-1771) in 1731, who added it to his sky atlas, Uranographia Britannica. Some 27 years later, however, Charles Messier (1730-1817) independently found a "nebulosity above the southern horn of Taurus ...." on August 28, 1758, when he was looking for Comet Halley on its first predicted return. At first, Messier thought that it was a comet but recognized that it had no apparent proper motion. It was the discovery of this object which led Messier to compile his famous catalogue of nebulae and star clusters so that others would not confuse them with comets "just beginning to shine" -- beginning with the Crab Nebula as Messier object "Number 1." Moreover, since the discovery of the Nebula was made with his small refracting telescope, Messier was encouraged to search for more comets with telescopes. Messier later acknowledged the prior, original discovery by Bevis after learning of it in a letter of June 10, 1771.

STScI, AURA, NASA



Larger image.




The center of the Crab Nebula
shows ragged shreds of gas
that are expanding away from
the explosion site at over
three 3 million miles per
hour (more at STScI).

As the nebula is found in a bubble of the local interstellar medium (that was probably created by an earlier supernova), all the nebular material must have been ejected by its progenitor star. Its network of filaments originates from the material from the outer layers of the star that was expelled during the explosion and is now expanding outward at high speed. Furthermore, the spinning "neutron star" at the center of the nebula is also sending shockwaves of high-energy electrons that crash into the older nebular material like waves onto a beach.

S. Lee, CXC, NASA

Larger illustration.


Core implosion rebounds into a
supernova explosion that blasts
its outer layers into interstellar
space (more).

If the Crab Supernova had occurred within 26 ly of Sol, most living things on Earth's surface would probably have been destroyed as its intense radiation (particularly gamma and cosmic rays) breakdown the ozone layer (Gehrels et al, forthcoming). Fortunately, since the Crab Supernova in 1054, astronomers on Earth have observed only two stellar explosions of comparable visibility within the Milky Way Galaxy, "Tycho's Star" in 1572 and "Kepler's Star" in 1604, although "Flamsteed's star" of 1680 (or 1667) -- now known as Cassiopeia A -- was apparently too obscured by interstellar gas and dust to be noticed by anyone else. In 1987, however, a supernova was observed in the Large Magellanic Cloud about 179,000 ly away.

Hester et al, 2003;
CXC, HST, NASA



Larger composite optical and
x-ray image.




At the center of the Crab Nebula is a city-sized,
magnetized neutron star that spins 30 times a
second, where ring-like structures emit x-rays
as high-energy particles slam into the nebular
material (more at Astronomy Picture of the Day
and CXC).

The progenitor star of the Crab Nebula was once a main sequence dwarf of spectral type O or B (Mdzinarishvili and Dzigvashvili, 2001). Although it evolved into a supergiant that still held eight to 12 Solar-masses, most of its substance was expelled during the subsequent type-II supernova explosion to form the Crab Nebula. The remaining two Solar-masses of its core, however, was crushed into a sphere with a diameter of only six miles (about 10 km) to form a rapidly rotating neutron "star" known as a pulsar. This occurred just seconds before the Crab's supernova explosion when cessation of core fusion allowed the star's tremendous gravity to crush its core to densities as much as 50 trillion times that of lead. Being relatively young, the Crab Pulsar was the first known example of a neutron star which was located at the site of an optically visible object.

NASA Observatorium

The Crab's progenitor star was
once a supergiant with an
interior of onion-like layers
fusing ever heavier elements
inwards, until an inert iron
core developed.

See a discussion of the
"Burning of Elements
Heavier than Helium" and
"Supernova Explosions"
as part of stellar
evolution and death.

Because the spin of a stellar object was concentrated into a city-sized object during the collapse of the progenitor star's core, the neutron star created by the supernova is spinning very rapidly, completing one full revolution every 33 milliseconds. Human ice skaters obtain the same effect while twirling by pulling their arms to spin faster. In the case of the Crab pulsar, however, a point on its equator now moves at roughly four million miles (6.4 million km) per hour. The rapid spin of the Crab Pulsar also created a highly magnetized object, so that the neutron star acts like a gigantic rotating magnet that generates 10 quadrillion volts of electricity (30 million times that of a typical lightning bolt on Earth. Generating the equivalent of 100,000 times the power output of Sol, gradually slows down the pulsar so that its period of rotation is growing by 15 microseconds per year.

Chandra X-Ray Observatory (CXC),
SAO, NASA





Larger image.






The inner part of the ring
surrounding the Crab Pulsar
spans a light-year, hiding the
neutron star (more at Astronomy
Picture of the Day and CXC).

The Crab Nebula emits radiation from the motion of high-speed electrons being accelerated or decelerated in its magnetic field by its the pulsar. Due to its similarity to the process in a cyclotron, the creation of high-energy electrons that spiral in a large-scale magnetic field has been dubbed "sync[h]rotron radiation" (I.S. Shklovsky, 1953; J.H. Oort; and T. Walraven). As a result, the Crab Pulsar generates enough energy to keep the entire nebula radiating over almost the entire electromagnetic spectrum, with enough power to make the nebula shine brighter than 75,000 suns.

STScI, NASA




Larger image.





The environment of the Crab
Nebula around the rapidly
spinning neutron star is
filled with high-energy
radiation (more at STScI).

The strong magnetic fields of many neutron stars can focus the light, radio waves, and other forms of the radiation that they emit into two narrow beams pointing in the direction of the magnetic field axis (one in the direction of the north magnetic pole, the other in the direction of the south magnetic pole. If the polar field axis of such a neutron star is not aligned with its rotation axis, then its bi-polar beams sweep out two swaths of the sky (as it rotates) -- somewhat like the rotating light beam of a lighthouse. If an observer is located in the direction of one of the swaths (e.g., on Earth), the observer would see pulses of radiation each time the beam crosses the observer's line of sight to the remnant. Such a neutron star is known as a pulsar, which is short for "pulsating radio star."

NASA Observatorium



The magnetic poles
of a pulsar are not
aligned with its
axis of rotation.



See a discussion
of neutron stars as
part of stellar
evolution and death.

Astronomers are now aware of about a thousand pulsars, but this number grows almost daily through discoveries from radio telescopes. The Crab, however, is one still one of the youngest and most energetic pulsars known. While a few dozen pulsars have been observed to pulse in X-rays and six pulse in gamma-rays, the Crab seems to pulse in almost every wavelength, in radio, optical, X-rays, and gamma-rays, and its nebula is also visible over that broad range of wavelengths. (For a view of what the Crab Nebula and Pulsar might be like after 10,000 more years, see the Vela supernova remnant at Chandra X-Ray Observatory and Astronomy Picture of the Day).

On January 8, 2007, a team of astronomers announced supporting evidence that the neutron star inside the Crab Nebula may have four magnetic poles, rather than the usual two. The extra set of poles may have somehow been frozen into the neutron star during its formation in a supernova explosion. Although the pulsar beam from only one pole is usually observed, sometimes a second, weaker signal can be detected if the beam from the other pole points roughly in Earth's direction when it comes into view. The Crab pulsar has long been known to display weaker, secondary pulses. The new observations indicate that the primary and secondary pulses are radically different, which would be difficult to explain as coming from opposite magnetic poles. Some astronomers now speculate that the secondary pulses are related to an additional pair of magnetic poles (more).