In 1054 AD Chinese astronomers recorded the appearance of a “new star” in the sky. It appeared in the constellation Taurus and remained visible, even in the daytime, for nearly a month. What they were observing was the supernova explosion of a large star some 6500 light years from Earth. Interestingly, that means the star actually exploded in 6446 BC, but it took 6500 years for the flash of light to reach the Earth.
In 1731 it was “rediscovered” by English astronomer John Bevis. It’s easily visible even in a small telescope, shining at magnitude 8.4. The image above is a false color mosaic assembled from 24 individual exposures taken by the HST. It has come to be known as the Crab Nebula because of its apparent shape when seen through less powerful telescopes.
Later, in 1781, Charles Messier published his famous list of celestial objects that might be mistaken for comets — those that are not point-like but are spread out and diffused. The Crab Nebula was first on Messier’s list, and so today is often referred to as simply M1. Messier’s original catalog went up to M103. In the 20th century seven more objects were added, taking the list up to M110.
As it appears today, the nebula’s structure suggests a violent explosive origin. In fact, the explosion is ongoing. Vast amounts of material ejected by the explosion are still spreading outward into space at speeds up to 1500 km/s (930 miles/sec). M1 is currently about 11 light years in diameter, but when observed by Bevis it was around 9.8 light years in diameter. Over millennia it will continue to expand and eventually dissipate.
Perhaps the most amazing thing about M1 is what was left behind after the star exploded. Its largely iron core collapsed into a neutron star with a diameter of around 20 km, a mass of 8–12 times the mass of the Sun, a magnetic field 1 trillion times stronger than Earth’s, and a spin of 30 rotations/second. This spin causes the neutron star at the heart of M1 to act as a pulsar. As its intense magnetic field rotates, it send out pulses of radio waves that can be detected at a frequency of 30 Hz.
When you look at M1 through a telescope, as I have often done, you see a static frame from an evolving structure. Recent projects by NASA have unveiled the dynamic aspect of M1. These are years-long time-lapse videos that show how the internal structure of M1 is driven by the pulsar at its heart.
This first video shows dynamic rings, wisps and jets of matter and antimatter around the pulsar in the Crab Nebula as observed in optical light by the HST. The movie was made from 24 exposures taken between August 2000 and April 2001. The image sequence was looped several times, so the video repeats. Credit: NASA/HST/ASU/J.Hester et al.
The second video shows the same dynamics as observed in X-ray light by the Chandra X-ray Observatory. The movie was made from seven still images taken by Chandra between November 2000 and April 2001. Again, the image sequence was looped several times. The inner ring is about one light year across. Credit: NASA/CXC/ASU/J.Hester et al.
Dynamic processes in Nature are sometimes visualized best through time-lapse. I’ve used the method often, capturing such phenomena as cloud buildup, thermal expansion of seawater, and nyctinasty in flowers. Time-lapse speeds up processes that would be otherwise invisible to our eyes. Astronomers learned a lot about how pulsars interact with their nebula when they analyzed these videos.
Next Week in Sky Lights ⇒ Persistence of Vision