Schematic view of the Pulsar-Planet system PSR J1719-1438 showing the pulsar with 5.7 ms rotation period in the centre, and the orbit of the planet in comparison to the size of the sun (marked in yellow). Credit: Swinburne Astronomy Productions, Swinburne University of Technology
[/caption]
“Remember when you were young… You shone like the sun.” Four thousand light years away in the constellation of Serpens, a millisecond pulsar binary is pounding out its heartbeat. Meanwhile an international research team of scientists from Australia, Germany, Italy, the UK and the USA, including Prof. Michael Kramer from Max Planck Institute for Radio Astronomy in Bonn, German are listening in. Utilizing the 64-m radio telescope in Parkes, Australia, the team made a rather amazing discovery. The companion star could very well be an ultra-low mass carbon white dwarf… one that’s transformed itself into a planet made of pure diamond.
“The density of the planet is at least that of platinum and provides a clue to its origin”, said the research team leader, Prof. Matthew Bailes of Swinburne University of Technology in Australia. Bailes leads the “Dynamic Universe” theme in a new wide-field astronomy initiative, the Centre of Excellence in All-sky Astrophysics (CAASTRO). He is presently on scientific leave at Max Planck Institute for Radio Astronomy.
Like a lighthouse, PSR J1719-1438 emits radio signals which sweep around methodically. When researchers noticed a specific modulation every 130 minutes, they realized they were picking up a signature of planetary proportions. Given the distance of its orbit, the companion could very well be the core of a once massive star whose material was consumed by pulsar’s gravity.
“We know of a few other systems, called ultra-compact low-mass X-ray binaries, that are likely to be evolving according to the scenario above and may likely represent the progenitors of a pulsar like J1719-1438” said Dr. Andrea Possenti, of INAF-Osservatorio Astronomicodi Cagliari.
With almost all of its original mass gone, very little of the companion could be left save for carbon and oxygen… and stars still rich in lighter elements like hydrogen and helium won’t fit the equation. This leaves a density which could very well be crystalline – and a composition which closely resembles diamond.
“The ultimate fate of the binary is determined by the mass and orbital period of the donor star at the time of mass transfer. The rarity of millisecond pulsars with planet-mass companions means that producing such ‘exotic planets’ is the exception and not the rule, and requires special circumstances”, said Dr. Benjamin Stappers from the University of Manchester.
“The new discovery came as a surprise for us. But there is certainly a lot more we’ll find out about pulsars and fundamental physics in the following years”, concludes Michael Kramer.
Pulsars are noted as being some of the universe’s best clocks. Their highly magnetized nature gives rise to beams of high energy radiation that sweep out across the universe. If these beams pass Earth, they can rival atomic clocks in their precision. So precise are these timings, that the first extrasolar planet was discovered through the effects it had on this heartbeat. But in September of 2007, pulsar J1718-3719 appears to have had a seizure.
These disjunctions aren’t unprecedented. While not exactly frequent, such “glitches” have been noted previously in other pulsars and magnetars. These glitches are often displayed as a sudden change in the period of the pulsar suddenly drops and then slowly relaxes back to the pre-glitch value at a characteristic rate dependent on the previous value as well as how large the jump was. Behavior like this has been seen in other pulsars including PSR B2334+61 and PSR 1048-5397.
The size of a glitch is measured as a ratio of the change in speed due to the glitch as compared to that of the pre-glitch speed. For past glitches, these have generally been changes that are around a hundredth of a percent. While this may not sound like a large change, the stars on which they act are exceptionally dense neutron stars. As such, even a small change in rotational energy means a large amount of energy involved.
Previously, the largest known glitch was 20.5 x 10-6 for PSR B2334+61. The new glitch in PSR J1718-3718 beats this record with a frequency change of 33.25 x 10-6. Aside from being a record setter, this new glitch does not appear to be following the trend of returning to previous values. The changed period persisted for the 700 days astronomers at the Australia Telescope National Facility observed it. Pulsars tend to have a slow braking applied to them due to a difference between their rotational axes and their magnetic ones. This too generally returns to a standard value for a given pulsar following a glitch, but PSR J1718-3718 defied expectations here as well, having a persistently higher braking effect which has continued to increase.
Currently, astronomers know precious little about the effects which may cause these glitches. There is no evidence to suggest that the phenomenon is something external to the body itself. Instead, astronomers suspect that there are occasional alignments of the stars internal superfluid core which rotates more quickly, with the star’s crust that cause the two to occasionally lock together. Models of neutron stars have had some success at reproducing this odd behavior, but none have suggested an event like PSR J1718-3718. Instead, the authors of the recent study suggest that this may have been caused by a fracturing of the crust of the neutron star or some yet unknown internal reaction. The possibilities currently are not well constrained but studying future events like these will help astronomers refine their models.
A new detector at the Westerbork Synthesis Radio Telescope (WSRT) allows for a much wider view of the sky in the radio spectrum. In this image, the two pulsars are separated by over 3.5 degrees of arc in the sky. Image Credit: ASTRON
[/caption]
To aid in the digestion of a new era in radio astronomy, a new technique for improving the is unfolding at the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands. By adding a plate of detectors to the focal plane of just one of the 14 radio antennas at the WSRT, astronomers at the Netherlands Institute for Radio Astronomy (ASTRON) have been able to image two pulsars separated by over 3.5 degrees of arc, which is about 7 times the size of the full Moon as seen from Earth.
The new project – called Apertif – uses an array of detectors in the focal plane of the radio telescope. This ‘phased array feed’ – made of 121 separate detectors – increases the field of view of the radio telescope by over 30 times. In doing so, astronomers are able to see a larger portion of the sky in the radio spectrum. Why is this important? Well, in keeping with our food course analogy, imagine trying to eat a bowl of soup with a thimble – you can only get a small portion of the soup into your mouth at a time. Then imagine trying to eat it with a ladle.
This same analogy of surveying and observing the sky for radio sources holds true. Dr. Tom Oosterloo, the Principle Investigator of the Apertif project, explains the meat of the new technique:
“The phased array feed consists of 121 small antennas, closely packed together. This matrix covers about 1 square meter. Each WSRT will have such a antenna matrix in its focus. This matrix fully samples the radiation field in the focal plane. By combining the signals of all 121 elements, a ‘compound beams'[sic] can be formed which can be steered to be pointing at any location inside a region of 3×3 degrees on the sky. By combining the signals of all 121 elements, the response of the telescope can be optimised, i.e. all optical distortions can be removed (because the radiation field is fully measured). This process is done in parallel 37 times, i.e. 37 compound beams are formed. Each compound beam basically functions as a separate telescope. If we do this in all WSRT dishes, we have 37 WSRTs in parallel. By steering all the beams to different locations within the 3×3 degree region, we can observe this region entirely.”
In other words, traditional radio telescopes use only a single detector in the focal plane of the telescope (where all of the radiation is focused by the telescope). The new detectors are somewhat like the CCD chip in your camera, or those in use in modern optical telescopes like Hubble. Each separate detector in the array receives data, and by combining the data into a composite image a high-quality image can be captured.
The new array will also widen the field of view of the radio telescope, which allowed for this most recent observation of widely separated pulsars in the sky, a milestone test for the project. As an added bonus, the new detector will increase the efficiency of the “aperture” to around 75%, up from 55% with the traditional antennas.
Dr. Oosterloo explained, “The aperture efficiency is higher because we have much more control over the radiation field in the focal plane. With the classic single antenna systems (as in the old WSRT or as in the eVLA), one measures the radiation field in a single point only. By measuring the radiation field over the entire focal plane, and by cleverly combining the signals of all elements, optical distortion effects can be minimised and a larger fraction of the incoming radiation can be used to image the sky.” This image illustrates the larger field of view afforded by the new instrument. Image Credit: ASTRON
For now, there is only one of the 14 radio antennas equipped with Apertif. Dr. Joeri Van Leeuwen, a researcher at ASTRON, said in an email interview that in 2011, 12 of the antennas will be outfitted with the new detector array.
Sky surveys have been a boon for astronomers in recent years. By taking enormous amounts of data and making it available to the scientific community, astronomers have been able to make many more discoveries than they would have been able to by applying for time on disparate instruments.
Though there are some sky surveys in the radio spectrum that have been completed so far – the VLA FIRST Survey being the most prominent – the field has a long way to go. Apertif is the first step in the direction of surveying the whole sky in the radio spectrum with great detail, and many discoveries are expected to be made by using the new technique.
Apertif is expected to discover over 1,000 pulsars, based on current modeling of the Galactic pulsar population. It will also be a useful tool in studying neutral hydrogen in the Universe on large scales.
Dr. Oosterloo et. al. wrote in a paper published on Arxiv in July, 2010, “One of the main scientific applications of wide-field radio telescopes operating at GHz frequencies is to observe large volumes of space in order to make an inventory of the neutral hydrogen in the Universe. With such information, the properties of the neutral hydrogen in galaxies as function of mass, type and environment can be studied in great detail, and, importantly, for the first time the evolution of these properties with redshift can be addressed.”
Adding the radio spectrum to the visible and infrared sky surveys would help to fine-tune current theories about the Universe, as well as make new discoveries. The more eyes on the sky we have in different spectra, the better.
Though Apertif is the first such detector in use, there are plans to update other radio telescopes with the technology. Dr. Oosterloo said of other such projects, “Phased array feeds are also being built by ASKAP, the Australia SKA Pathfinder. This is an instrument of similar characteristics as Apertif. It is our main competitor, although we also collaborate on many things. I am also aware of a prototype being tested at Arecibo currently. In Canada, DRAO [Dominion Radio Astrophysical Observatory] is doing work on phased array feed development. However, only Apertif and ASKAP will construct an actual radio telescope with working phased array feeds in the short term.”
On November 22nd and 23rd, a science coordination meeting was held about the Apertif project in Dwingeloo, Drenthe, Netherlands. Dr. Oosterloo said that the meeting was attended by 40 astronomers, from Europe, the US, Australia and South Africa to discuss the future of the project, and that there has been much interest in the potential of the technique.
The white dwarf in the AE Aquarii system is the first star of its type known to give off pulsar-like pulsations that are powered by its rotation and particle acceleration.
[/caption]
Some satellites get all the glory. While Hubble, Chandra, and Spitzer frequently make headlines with their stunning images, many other space based observatories silently toil away. One of them, known as the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) has been in orbit since 2006, but rarely receives media attention although a stunning discovery has led to the publication of over 300 papers within a single year. A new paper in that onslaught has proposed an interesting new object: pulsars powered by white dwarfs.
PAMELA isn’t a satellite in its own right. It piggybacks on another satellite. Its mission is to observe high energy cosmic rays. Cosmic rays are particles, whether they be protons, electrons, nuclei of entire atoms, or other pieces, that are accelerated to high velocities, often from exotic sources and cosmological distances.
Among the types of particles PAMELA detects is the elusive positron. This anti-particle of the electron is quite rare due to the scarcity of anti-matter in general in our universe. However, much to the surprise of astronomers, in the range of 10 – 100 GeV, PAMELA has reported an abundance of positrons. In even higher ranges (100 GeV – 1 TeV) astronomers have found that there is a rise in both electrons and positrons. The conclusion from this is that something is able to actually create these particles in these energy ranges.
A flurry of papers went to publication to explain this unexpected finding. Explanations ranged from showers of particles created by even higher energy cosmic rays striking the interstellar medium, to the decay of dark matter, to neutron stars, pulsars, supernovae, and gamma ray bursts. Indeed, many events that produce high energies are sufficient to spontaneously produce matter from energy through the process of pair production. However, the range of these ejected particles would be limited. Effects, such as synchrotron and inverse Compton emission would drain their energy over large distances and as such, by the time they reached PAMELA’s detectors would be too low energy to account for the excesses in the observed energy ranges. From this, astronomers are presuming the culprits are in the local universe.
Joining the long list of candidates, a new paper has proposed a mundane object could be responsible for the high energy necessary to create these energetic particles, albeit with an unusual twist. Neutron stars, one of the potential objects formed in a supernova, are known to release large amounts of energies when spinning quickly while creating a strong magnetic field in the form of pulsars, but the authors propose that white dwarfs, the products of the slow death from stars not massive enough to result in a supernova, may be able to do the same thing. The difficulty in creating such a white dwarf pulsar is that, since white dwarfs don’t collapse to such a small size, they don’t “spin up” as much as they conserve angular momentum and shouldn’t have the sufficient angular velocity necessary.
The authors, led by Kazumi Kashiyama at Kyoto University propose that a white dwarf may reach the necessary rotational speed if they undergo a merger or accrete a sufficient amount of mass. This idea is not unheard of since white dwarf mergers and accretion are already implicated in Type Ia Supernovae. The combination of this with the expectation that around 10% of white dwarfs are expected to have magnetic fields of 106 Gauss, the steps necessary to produce a pulsar from a white dwarf seem to be in place. They note that since white dwarfs tend to have weaker magnetic fields, they shed their angular momentum more slowly and would last longer. Although this duration is still far longer than humans can possibly watch, this may indicate that many of the pulsars observed in our own galaxy are white dwarfs.
Next, the authors hope to conclusively identify such a star. The creation of each of these types of pulsars may provide a clue: Since neutron stars form from supernovae, they are surrounded by a shell of gas that contains a shock front from the supernova itself, which is more dense than the interstellar medium in general. As particles pass through this shock front, some of them would be lost. The same would not be said for white dwarfs which formed from a more gentle release and aren’t impeded by the relatively high density area. This shift in energy distributions may be one distinguishing characteristic.
Some stars have even been tentatively proposed as candidates for white dwarf pulsars. AE Aquarii was seen to give off some pulsar-like signals. EUVE J0317-855 is another white dwarf that appears to meet the qualifications, although no signals have been detected from this star. This new class of stars would be able to explain the excess signal in the higher energy range detected by PAMELA and will likely be the target of further observational searches in the future.
Screenshot of Einstein@Home. Image courtesty of B. Knispel of Albert Einstein Institute
[/caption]
Hooray for citizen scientists! The Einstein@Home project has discovered a unusual pulsar approximately 17,000 light-years away in the constellation Vulpecula. The project works by people “donating” idle time on their home computers. This is the first deep-space discovery by Einstein@Home, and the finding is credited to Chris and Helen Colvin, from Ames, Iowa in the US, and Daniel Gebhardt of Universitat Mainz, Musikinformatik,Germany.
The newly discovered pulsar, PSR J2007+2722, is an isolated neutron star that rotates 41 times per second and has an unusually low magnetic field.
Jim Cordes, Cornell professor of astronomy, said the object is particularly interesting because it is likely a recycled pulsar: a neutron star that once had a companion star from which it acquired mass; but whose companion exploded, kicking it free.
Unlike most pulsars that spin as quickly and steadily, PSR J2007+2722 sits alone in space, and has no orbiting companion star. However, the scientists say they can not rule out that it may be a young pulsar born with an lower-than-usual magnetic field.
“We think there should be more of these disrupted binary pulsars, but there haven’t been that many found,” said Cordes. “No matter what else we find out about it, this pulsar is bound to be extremely interesting for understanding the basic physics of neutron stars and how they form.”
The discovery demonstrates the power of the network used to collect and sort through vast amounts of data, Cordes said.
Einstein@Home was originally organized to find gravitational waves — ripples in space-time — using the Laser Interferometer Gravitational Wave Observatory (LIGO). In 2009, data from the Arecibo Observatory were included in the processing.
Chris and Helen Colvin who were credited with discovering a new pulsar. Image courtesy Chris Colvin.
Einstein@Home is based at the Center for Gravitation and Cosmology at the University of Wisconsin-Milwaukee and at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, or AEI) in
Hannover, Germany. About one-third of Einstein@Home’s computing capacity is used to search Arecibo data.
Einstein@Home volunteer Daniel Gebhardt from Germany. Image couresty of Gebhardt.
“This is a thrilling moment for Einstein@Home and our volunteers. It proves that public participation can discover new things in our universe,” said Bruce Allen, leader of the Einstein@Home project, AEI director and adjunct professor of physics at the University of Wisconsin-Milwaukee. “I hope it inspires more people to join us to help find other secrets hidden in the data.”
Gebhardt and the Colvins will receive plaques noting their discovery, and all plan to stay involved.
Artist's impression of an anomalous X-ray pulsar. Credit: ESA
Observational data from nine pulsars, including the Crab pulsar, suggest these rapidly spinning neutron stars emit the electromagnetic equivalent of a sonic boom, and a model created to understand this phenomenon shows that the source of the emissions could be traveling faster than the speed of light. Researchers say as the polarization currents in these emissions are whipped around with a mechanism likened to a synchrotron, the sources could be traveling up to six times light speed, or 1.8 million km per second. However, although the source of the radiation exceeds the speed of light, the emitted radiation travels at normal light speed once it leaves the source. “This is not science fiction, and no laws of physics were broken in this model,” said John Singleton of Los Alamos National Laboratory at a press briefing at the American Astronomical Society meeting in Washington, DC. “And Einstein’s theory of Special Relativity is not violated.”
This model, called the superluminal model of pulsars, was described by Singleton and colleague Andrea Schmidt as solving many unanswered issues about pulsars.”We can account for a number of probabilities with this model,” said Singleton, “and there is a huge amount of observational data available, so there will be ample opportunities to verify this.”
Pulsars emit amazingly regular, short bursts of radio waves. Within the emissions from the pulses, the circulating polarization currents move in a circular orbit, and its emitted radiation is analogous to that of electron synchrotron facilities used to produce radiation from the far-infrared to X-ray for experiments in biology and other subjects. In other words, the pulsar is a very broadband source of radiation.
However, Singleton said, the fact that the source moves faster than the speed of light results in a flux that oscillates as a function of frequency. “Despite the large speed of the polarization current itself, the small displacements of the charged particles that make it up means that their velocities remain slower than light,” he said.
These superluminal polarization currents are disturbances in the pulsar’s plasma atmosphere in which oppositely-charged particles are displaced by small amounts in opposite directions; they are induced by the neutron star’s rotating magnetic field. This creates the electromagnetic equivalent of a sonic boom from accelerating supersonic aircraft. Just as the “boom” can be very loud a long way from the aircraft, the analogous signals from the pulsar remain intense over very long distances. Rapid condensation of water vapor due to a sonic shock produced at sub-sonic speed creates a vapor cone (known as a Prandtl–Glauert singularity), which can be seen with the naked eye.
Back in the 1980s, Nobel laureate Vitaly Ginzburg and colleagues showed that such faster than light polarization currents will act as sources of electromagnetic radiation. Since then, the theory has been developed by Houshang Ardavan of Cambridge University, UK, and several ground-based demonstrations of the principle have been carried out in the United Kingdom, Russia and the USA. So far, polarization currents traveling at up to six times the speed of light have been demonstrated to emit tightly-focused bursts of radiation by the ground-based experiments.
Although Singleton and Schmidt’s highly technical presentation was admittedly over the heads of many in attendance (and watching online), LANL researchers said the superluminal model fits data from the Crab pulsar and eight other pulsars, spanning electromagnetic frequencies from the radio to X-rays. In each case, the superluminal model accounted for the entire data set over 16 orders of magnitude of frequency with essentially only two adjustable parameters. In contrast to previous attempts, where several disparate models have been used to fit small frequency ranges of pulsar spectra, Schmidt said that a single emission process can account for the whole of the pulsar’s spectrum.
“We think we can explain all observational data using this method,” Singleton said.
When asked, Singleton said they have received some hostile reactions to their model from the pulsar community, but that many others have been “charitably disposed because it explains a lot of their data.”
Lead image caption: Artist’s impression of an anomalous X-ray pulsar. Credit: ESA
This illustration shows a pulsar’s magnetic field (blue) creates narrow beams of radiation (magenta). Image credit: NASA
How do you detect a ripple in space-time itself? Well, you need hundreds of precision clocks distributed throughout the galaxy, and the Fermi gamma ray telescope has given astronomers a new way to find them.
The “clocks” in question are actually millisecond pulsars – city-sized, sun-massed stars of ultradense matter that spin hundreds of times per second. Due to their powerful magnetic fields, pulsars emit most of their radiation in tightly focused beams, much like a lighthouse. Each spin of the pulsar corresponds to a “pulse” of radiation detectable from Earth. The rate at which millisecond pulsars pulse is extremely stable, so they serve as some of the most reliable clocks in the universe.
Astronomers watch for the slightest variations in the timing of millisecond pulsars which might suggest that space-time near the pulsar is being distorted by the passage of a gravitational wave. The problem is, to make a reliable measurement requires hundreds of pulsars, and until recently they have been extremely difficult to find.
“We’ve probably found far less than one percent of the millisecond pulsars in the Milky Way Galaxy,” said Scott Ransom of the National Radio Astronomy Observatory (NRAO).
Data from the Fermi gamma-ray space telescope, which started collecting data in 2008, have changed the way millisecond pulsars are detected. The Fermi telescope has identified hundreds of gamma-ray sources in the Milky Way. Gamma rays are high-energy photons, and they are produced near exotic objects, including millisecond pulsars.
“The data from Fermi were like a buried-treasure map,” Ransom said. “Using our radio telescopes to study the objects located by Fermi, we found 17 millisecond pulsars in three months. Large-scale searches had taken 10-15 years to find that many.”
Ransom and collaborator Mallory Roberts of Eureka Scientific used the National Science Foundation’s Robert C. Byrd Green Bank Telescope (GBT) to find eight of the 17 new pulsars.
Right now astronomers have only barely enough millisecond pulsars to make a convincing gravitational wave detection, but with Fermi to help identify more pulsars, the odds of detecting these ripples in space-time are steadily increasing.
Ransom and Roberts announced their discoveries today at the American Astronomical Society’s meeting in Washington, DC.
[/caption]
An innovative project that provides high school students in Australia the opportunity to work with the famous Parkes radio telescope will soon make the data available to schools around the world. The PULSE@Parkes project allows for hands-on remote observing of pulsars producing real-time data, which then becomes part of a growing database used by professional astronomers. “Students can help monitor pulsars and identify unusual ones or detect sudden glitches in their rotation,” said Rob Hollow from the Australia Telescope National Facility, and coordinator for the PULSE@Parkes project. “They can also help determine the distance to existing pulsars.”
Initially, the project was only available to schools in Australia, but PULSE@Parkes hopes to expand globally, allowing students to collaborate on monitoring pulsar data. The first international session will be held on Dec. 7, 2009 at Cardiff University in the UK.
“We had the challenge to develop and implement simulation radio astronomy activities for high school students, providing the opportunity for them to actually use a radio telescope facility and engage with professional scientists,” said Hollow, speaking at the .Astronomy (dot Astronomy) conference this week in Leiden, The Netherlands. “We also wanted to have students doing science that is appropriate for them and useful for professional astronomers.”
Students in Sydney controlling the Parkes radio telescope. Credit: R. Hollow, CSIRO
Hollow said that even though radio astronomy data consists of squiggly lines, students are still engaged by the results, even without the pretty pictures produced by other astronomical instruments. “It works surprisingly well, and the visuals haven’t been as big an issue and we thought,” Hollow said. “But in looking at pulsars, the students do get the pulse profiles and they get immediate feedback.”
Plus, when the dish actually moves in response to the students’ inputs, they really become engaged. “There’s a real ‘wow’ factor in being able to control the telescope,” Hollow said. “The students pick it up quickly, and they really like that they are contributing to science.”
The program is done remotely, and students view webcams of the telescope and control room. They control the telescope directly via the internet, monitor the data in real time, and use Skype to communicate with astronomers at Parkes.
So far, Hollow said, they have done 25 sessions, with 28 schools, working with about 450 students. “This project is not just for gift and talented students,” he said, “and any school can apply.”
The Parkes Radio Antenna. Credit: R. Hollow, CSIRO
Parkes is a 64 m diameter radio antenna that was built in 1961. Hollow said the dish has received regular updates and is still on the cutting edge of science. Most famously, Parkes was to receive video from the Apollo mission to the Moon.
Hollow said he sees PULSE@Parkes as just the beginning of working with students. The Australian Square Kilometre Array Pathfinder (ASKAP) will be coming online in just a couple of years, with thirty-six 12-meter dishes. “This will provide for very fast surveys that will increase the area of coverage and increase the capability for sensitivity,” Hollow said. “From ASKAP, we’ll be getting massive data sets, which will provide more opportunity for student and public involvement.
For more information, including an audio of what a pulsar “sounds” like, as well as info for schools and teachers, requirements, and how to apply visit the PULSE@Parkes website
Imagine an object with the mass of the Sun, crushed down to the size of Manhattan. Now set that object spinning hundreds of times a second, blasting out powerful beams of radiation like a lighthouse. That’s a pulsar, one of the most exotic objects in the Universe.
The Crab Nebula; at its core is a long dead star. Did early massive stars die in supernova explosions like this? Image credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)
The Crab Nebula, or M1 (the first object in Messier’s famous catalog), is a supernova remnant and pulsar wind nebula. The name – Crab Nebula – is due to the Earl of Rosse, who thought it looked like a crab; it’s not in the constellation Cancer (the Crab), rather Taurus (the Bull).
The supernova which gave rise to the Crab Nebula was seen widely here on Earth in 1054 (and so it’s called SN 1054 by astronomers); it is perhaps the most famous of the historical supernovae. It is certainly one of the brightest (estimated to be –7 at peak), partly because it is so close (only 6,300 light-years away), and partly because it’s not hidden by dust clouds. The expansion of the nebula – as in seen-to-be-getting-bigger, rather than the-gas-is-moving-very-fast – was first confirmed in 1930.
As it was a core collapse supernova (a massive star which ran out of fuel), it left behind a neutron star; by chance, we are in line with its ‘lighthouse beam’, so we see it as a pulsar (all young neutron stars are pulsars, but not all of them have beams which point to us in one part of the cycle). It’s a pretty fast pulsar; the neutron star rotates once every 33 milliseconds. Because it’s so young and so close, the Crab Nebula pulsar was the first to be detected in the visual waveband, and also in x-rays and gamma rays. Being the source of the tremendous output of energy, from both the pulsar wind nebula and the pulsar itself, and as energy is conserved, the pulsar is slowing down, at a rate of 15 microseconds per year.
The inner part of the Crab Nebula, the pulsar wind nebula, contains lots of really hot (‘relativistic’) electrons spiraling around magnetic fields; this creates the eerie blue glow … synchrotron radiation. This makes the Crab Nebula one of the brightest objects in the x-ray and gamma ray region of the electromagnetic spectrum, and as it is a relatively steady source (unlike most high energy objects) it has given its name to a new astronomical unit, the Crab. For example, a new x-ray source may be 2 mCrab (milli-Crab), meaning 0.002 times as strong an x-ray source as the Crab Nebula.
This SEDS page has a lot more information on the Crab Nebula, both historical and contemporary.
