Crab nebula
The Crab Nebula is a prominent interstellar cloud consisting of glowing gas and dust, formed from the remnants of a supernova explosion that occurred approximately 1,000 years ago. Located in the constellation of Taurus, it is visible with binoculars or small telescopes and is recognized for its resemblance to a crab. The explosion that created the nebula was recorded by Chinese astronomers in 1054, who described it as a "guest star," noting its brightness and visibility during the day for nearly a month.
At the heart of the Crab Nebula lies a neutron star, the highly dense remnant of the original star, which spins rapidly and emits beams of radiation, creating a pulsing effect observable from Earth. This neutron star, known as the Crab Pulsar, demonstrates the complex processes involved in stellar evolution, including the transition from a massive star to a supernova and the formation of such dense astronomical objects. The Crab Nebula's expansion continues today, with gas moving outward at an impressive speed, and its composition includes elements like hydrogen, helium, carbon, and oxygen, surrounding the central pulsar.
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Crab nebula
The Crab Nebula is a large interstellar cloud of glowing gas and dust left over from the explosive death of a star thousands of years ago. The nebula, which can easily be seen with binoculars or a small telescope, is located in the constellation of Taurus and was named after its apparent resemblance to a crab. The explosion that gave birth to the nebula was first observed on Earth one thousand years ago and “rediscovered” by astronomers centuries later as a fuzzy patch of light in the sky. In the twentieth century, astronomers connected the object to the historical accounts of a “guest star,” realizing the nebula was a remnant of an ancient stellar explosion. Later observations revealed the center of the Crab Nebula is home to a super-dense object known as a neutron star, the rapidly spinning “corpse” of the original star.
Background
Stars are giant, hot, glowing balls of gas powered by a nuclear reaction in their cores. The fuel for this reaction is mainly hydrogen atoms, which are fused together by the immense heat and pressure found within the star. The process of joining two hydrogen atoms creates one helium atom and produces a tremendous amount of energy. It is this energy that makes the star “shine.”
Stars live their lives in a state of equilibrium between the nuclear fusion at their cores that pushes outward and the force of gravity that is constantly pushing inward. As long as a star has enough hydrogen fuel to power its nuclear engine, it will remain stable and continue to shine. However, after millions or billions of years, stars use up their hydrogen fuel and begin to die. In stars the size of our sun, gravity begins compressing the core, further heating it up, and fusing helium atoms into carbon atoms. The outer layers of the star heat up and expand, turning the star into a red giant. Eventually, the star runs out of helium fuel, its outer layers are dispersed into space, and its core becomes a small white dwarf star.
If a star is more than eight times the size of the sun, its massive gravity will continue the collapse of the core to the extent that the fusion process will turn carbon into oxygen and other heavier elements. At a certain point, the core cannot fuse further elements and gravity takes over, compressing the core into a super-dense object and triggering a violent explosion that blows the outer layers of the star apart. This explosion, known as a supernova, happens in seconds and produces a tremendous amount of energy. Supernova explosions are so powerful that they can outshine entire galaxies for a brief time.
Overview
The stellar explosion that formed the Crab Nebula occurred about 6,500 light years away, meaning light from the supernova took 6,500 years to reach Earth. Chinese astronomers recorded the phenomenon on July 4, 1054, noting the sudden appearance of what they called a “guest star” in the heavens. The supernova was the second brightest object in the night sky—behind only the moon—and was visible in daylight for almost a month. Astronomers in China and in other parts of the world observed the object for more than 650 days before it faded from view of the naked eye.
British astronomer John Bevis was the first to observe the nebula in 1731 when he noted the existence of a small, fuzzy blob in the constellation of Taurus, the Bull. In 1758, French astronomer Charles Messier also saw the nebula through a telescope and at first mistook it for a comet. After realizing it was a fixed object and not a comet, Messier began to catalog other fixed stellar objects and designated the nebula as M1 for Messier Object 1. After British astronomer William Parsons observed and sketched M1 in 1844, he called the object the Crab Nebula because he thought it was shaped like the shelled crustacean. Using more advanced technology, twentieth-century astronomers were able to determine that the gases in the nebula were expanding outward at a rate of about 932 miles per second (1,500 kilometers per second). By tracing the rate of expansion backwards, they concluded that its origins and location matched reports of the stellar explosion from 1054.
The Crab Nebula is located near the southern “horn” of Taurus, a constellation that is more prominent in winter and early spring. The nebula has an apparent magnitude of 8.4, a value that places it below the visible threshold of the naked eye. Lower magnitudes correspond to brighter stellar objects, with the unaided eye capable of seeing objects of magnitude 6 or less. The Crab Nebula can be seen as a faint smudge with binoculars and can be viewed in more detail with a small telescope.
The gaseous tendrils of the nebula are the remains of the original star's outer layers and are spread out over a distance of about 10 light years wide. These tendrils are mostly made of hydrogen and helium but also include elements such as carbon, oxygen, nitrogen, iron, sulfur, and neon. The temperatures of the expanding gases range from about 19,300 degrees to 32,000 degrees Fahrenheit (10,700 to 17,760 degrees Celsius).
The super-dense collapsed core of the star remains at the center of the nebula. Objects such as this are known as neutron stars because, as they collapse, they are compressed by gravity to such an extent that the protons and electrons in their cores are squeezed together to form neutrons. The neutron star at the center of the Crab Nebula is so dense that while its mass is about 1.4 to 2 times that of our sun, it has been compressed to a size of about 17 to 19 miles (27.3 to 30.6 kilometers) wide. This stellar corpse spins at such an incredibly fast rate that it whips around twin beams of radiation like the rotating light from a lighthouse. Seen from Earth, the energy seems to produce a flickering pulse that turns “on” and “off” at a rate of about thirty times a second. Astronomers first detected this phenomenon in the 1960s and labeled the objects “pulsars.” The neutron star at the center of the Crab Nebula is also known as the Crab Pulsar.
Bibliography
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