M1 (The Crab Nebula)

Distance: 6300 light years

Right Ascension: 05 : 34.5 (hours : minutes)
Declination: +22 : 01 (degrees : minutes)

The Crab supernova remnant represents the remains of a shattered supergiant star that met its explosive end in the year 1054. Witnessed and documented by Chinese astronomers the star was observed to be four times brighter than Venus and visible in the daytime sky for 23 days after the initial event. It is the most famous and most studied supernova remnant and one of only a few observed by humans within our own galaxy. It is estimated that a supernova should occur about once every three decades in our galaxy but dense interstellar matter obscures large parts of the galaxy making observations of supernova within our Milky Way much less frequent. Due to the enormous number of observable galaxies supernovas are frequently observed in other galaxies.

The Crab nebula is comprised of gaseous and filamentary material ejected at the time of the supernova. The nebula has steadily expanded since that time at a rate of about 1800 kilometers per second and now occupies a volume of about 10 light years. Optically the nebula consists of two components. The web-like infrastructure of filamentary material glows from emission of the shocked ambient gases of the interstellar medium. The second optical component is the diffuse blue glow permeating the nebula. The blue emission is the result of "synchrotron radiation", a form of radiation released when fast moving electrons generated by the supernovas expanding shock front encounters the strong magnetic field produced by the Crab's pulsar.

In 1968 a pulsating radio source was identified at the heart of M1 which we now know to be the crab pulsar, a neutron star rotating at 30 revolutions per second. A neutron star is a remarkably dense object of about 1.4 solar masses squeezed into a space about 10 kilometers wide. The incredible density approaches 50 trillion times that of lead and represents the endpoint of collapse of a massive star which began its life on the main sequence with at least 15 to 30 solar masses. Above 30 solar masses collapse is predicted to produce a black hole and under 15 masses a white dwarf and its transient planetary nebula phase. A supergiant star shines for only a few million years, a short life compared to lower mass stars. Close to the end of its life the star will shed prodigious amounts of mass and gradually deplete its hydrogen fuel. Without hydrogen the nuclear furnace fuses progressively heavier elements within its core. When the endpoint of nuclear fusion reaches iron the end of the nuclear cascade is reached. Iron cannot be fused further as thermodynamics will not allow it. When the total core accumulation reaches 1.4 solar masses, a critical mass is achieved known as the "Chandrasekhar limit". At this stage things happen quickly as the extreme conditions leading to the supernova event are reached. In an instant the core collapses producing a titanic blast which rips through the surrounding nuclear fusion shells and the remaining stellar envelope. The explosion tears apart the star and expels its envelope into space producing a nebula we call a supernova remnant. The explosive force and energy release is so intense that for a short while the visible light of the supernova can match the light output of an entire galaxy. This explains why supernova events can be detected in very remote galaxies even billions of light years away.

The inner part of the crab surrounding the rotating pulsar is an extraordinary dynamic environment. High resolution images taken by the HST and X-ray observations show a series of elliptical features at a distance of about 1 light year from the pulsar, which change shape every few days. These features are faintly seen in the center of the accompanying image. The elliptical wisps represent the shock fronts of high energy particles being accelerated out by the pulsar's magnetic field at velocities up to 70% the speed of light.

The matter ejected by the supernova collides into the surrounding stationary dust and gas of the interstellar medium creating shock fronts of multimillion degree gas and high energy particles. The heated matter produces intense radio and x-ray emission for thousands of years before it gradually dissipates into space. Supernovas enrich the surrounding interstellar medium with heavier elements like carbon, nitrogen, oxygen, silicon, sulfur, and iron which can only be created by the intense heat of the nuclear furnaces within stars. Supernovas play a significant role in the dynamics of the galaxy as the expanding shock fronts trigger the collapse of nearby molecular clouds which lead to new generations of stars. Our sun contains telltale heavier elements which allude to its origin from an ancient supernova some 5 billion years ago.