IYA

November - The Crab Nebula

Finding the Crab Nebula in the Sky

You will need a modest-sized telescope and clear, dark skies to observe the Crab Nebula above the east to northeast horizon in November. Located near the southern "horn" of the Taurus constellation, the Crab Nebula forms a rough triangle with the bright stars Aldeberan (in Taurus) and Betelguese (in Orion). You can find observing tips and resources in the November 2009 IYA Discovery Guide, and in the Exploring Further section of this article.

 

The Crab Nebula is the only supernova remnant – the leftovers from the spectacular end of a massive star – in the famous Messier Catalog of celestial objects viewable in backyard telescopes. In the mid-20th century, astronomers established that it was related to an extraordinarily bright “guest star” recorded in 1054 AD by the Chinese and by Native Americans in the southwest. Apparently brighter than Venus, this guest star remained easily visible to the naked eye during daytime for nearly three weeks and in the night sky for about two years. Today, the Crab Nebula is one of the best-studied objects beyond our Solar System, and provides insights into a wide variety of astronomical questions.

A Complex, Interconnected System of Small and Big

Like Sherlock Holmes solving cases, astronomers have deduced a great deal about the Crab Nebula using telescopic observations, logic and astrophysical concepts. Inhabiting our Milky Way galaxy, the Crab Nebula is about 6,500 light years from the Sun. So the brilliance of the "guest star," even this far away, indicates that the explosion must have been a supernova that instantaneously unleashed the energy equivalent of a 100 billion stars.

A Hubble Space Telescope image captures the bedlam of the event more than a millennium later. Some 10 light years across, the scalloped Crab shows filaments of helium, carbon, oxygen, etc., which are thought to be the shattered remnants of the progenitor star's atmosphere. The ejecta are plowing into the surrounding medium at speeds greater than 1,000 miles per second, and are expected to be generating shock waves that will shake up the surrounding gas and dust. Astronomers, still searching for the shock boundary, deduce by its faintness that the nebula is expanding into space in which the density of the matter is too low to show any spectacular effects of the shocks.

The filamentary web surrounds an inner bluish part that almost seems to hum. The polarized, wispy blue light is also seen in other chaotic, high-energy processes such as jets from active galactic nuclei. This filled region also shines bright at radio, X-ray (as seen by the Chandra X-Ray Observatory, for example) and gamma-ray wavelengths (imaged by the Fermi Gamma-ray Space Telescope, for example). All these clues divulge the Crab Nebula as being a cosmic particle accelerator where electrons, moving at about half the speed of light, are gyrating in a strong magnetic field and giving off intense radiation.

So, what energizes the fast-moving electrons? In the heart of the nebula is its powerhouse: a pulsar, a rapidly whirling, radiation beaming, neutron star (the compact and incredibly dense, collapsed remnant of the massive parent star that exploded as a supernova). The pulsar flashes with a very regular repetition rate of 33 times per second, and is detected in virtually every part of the electromagnetic spectrum including visible light and X-rays. The Crab Pulsar's rotation has been very gradually slowing down; in about 10,000 years, the pulsar will rotate at half its current spin rate.

Pulsars are highly magnetized, shooting off radiation and matter-antimatter beams from "hot spots" on their surface. The Crab Pulsar shows some remarkably dynamic features such as an inconstant wind from its equator that crashes into the surrounding nebula (part of which it is creating!) and forms energetic shock waves. The energy lost from the pulsar's rotation – tens of thousands of times more than the Sun's luminosity – and the energetic wind are what keep the nebula shining and expanding.

Now, a neutron star is the fossil remains of a massive star, which, having exhausted all its core nuclear fuel, collapses in on itself and explodes. The discovery of a pulsar in the Crab Nebula was the first evidence that supernovae, neutron stars and pulsars are all interconnected phenomena. Furthermore, a neutron star is an extreme objects: denser than an atomic nucleus, it packs more than the mass of our Sun into a volume of a medium-sized city! Astronomers studying the Crab Pulsar thus have a unique window into how matter behaves at mind-boggling densities that could never be achieved in our laboratories.

Summing up

The Crab is just one of about 100 million supernova explosions that have occurred over the 13 billion years of our Milky Way’s lifetime. Each supernova would have released vast amounts of energy (about 1028 megatons of TNT), and enriched the Galaxy with heavy elements like carbon, oxygen, calcium, silicon, iron – vital components of planets like the Earth, and of living beings including ourselves. While thousands of supernovae have been discovered in other, distant galaxies (including SN 1987A in our galactic neighbor, the Large Magellanic Cloud), only the explosive death of a star closer than about a hundred light years from us could have noticeable effects on the Earth’s atmosphere and life. In the last millennium, humanity has observed with the unaided eye three supernova explosions in our Galaxy (none so close as to affect the Earth): two during Galileo’s time, and the “guest star” of 1054 AD that generated the Crab Nebula. The next supernova in our Galaxy is overdue … Both professional and amateur astronomers are scouring the skies with numerous telescopes to be the first to spot it whenever it may occur – next week or next year or sometime in the next few hundred years.

 

The Crab Nebula.

In this Hubble Space Telescope image, the Crab Nebula shows a colorful network of filaments blasted into space during the supernova explosion and an inner, blue region of high-speed electrons curving in an intense magnetic field.

Crab Nebula, radio continuum image.

The Crab Nebula shines bright in a radio continuum image made by the Very Large Array. The image captures several solar mass of material originally thrown off by the star in the supernova explosion, into which slams a highly energetic wind from the pulsar.

Crab Nebula's X-ray-emitting pulsar wind nebula.

A Chandra X-ray view of the Crab Pulsar’s equatorial wind made of matter-antimatter particles.

Combined X-Ray and Optical Images of the Crab Nebula.

A remarkable view of the Crab’s dynamic interior, showing complex knots and wisps around the pulsar.

Venus transit

Crab Nebula

In 1054 AD, Chinese astronomers were startled by the appearance of a new star, so bright it could be seen in broad daylight for several weeks. Today, the Crab Nebula is what's left of the supernova explosion they witnessed.