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A Brief History and Research of the Supernova Remnant Cassiopeia A By Michelle D. Wilbur
Cas A is classified as a Compact Central Object. A compact Central Object (CCO) is the remnants of a supernova explosion. Astronomers give the classification of CCO to objects which their identity is unclear. The outcome of a supernova has only two options, a black hole or a neutron star.
Neutron Stars A neutron star is the result after the collapse of a massive star, usually 8 or more times larger than the sun. a star shines due to the nuclear fusion produced this produces a large outward pushing force, the reason stars don’t expand is because of the massive force of gravity that balances the outward push.
Life of a Stars Fusion is the result of Hydrogen converted into heavier elements. As the star lives it’s life it becomes like an onion in the sense that it develops layers of elements, hydrogen towards the outside going towards the center of the star. Once Iron is formed in the core of a star, it spells DOOM!
Supernova Due to the properties of iron, it will not fuse to create any heavier elements. A star collapses due to the overwhelming force of gravity. During the last seconds of the collapse, the iron core changes into neutronium. This is a state of matter where all the electrons and protons of the iron atoms are fused together to form a star entirely composed of neutrons. As the massive star collapses, its outer layers fall inward and bounce back off of the neutronium core causing a supernova.
Neutron Stars Depending on the amount of mass remaining after a supernova it will form a neutron star or black hole. There are several types of neutron stars that occur: radio pulsars, x-ray pulsars, and magnetars, which are a subcategory of radio pulsars
What is a Pulsar in regards to a Neutron Star? A neutron star keeps most of its angular momentum after the collapse. Since it is only a fraction of the diameter of its progenitor star, it spins very rapidly at high velocities and emits electromagnetic waves at the poles of the star. This angular momentum is from the progenitor star, since most main sequence stars begin life spinning. A neutron star that exhibits this behavior is called a pulsar
Radio Pulsars Radio pulsars are the most common form of neutron star. Radio pulsars emit radio waves at the poles of the star A radio pulsar is the name of a typical pulsar
X-Ray Pulsars X-ray pulsars emit electromagnetic waves in the form of x-rays. These stars are powered by extremely hot inflowing matter instead of by their own rotation. X-ray pulsars are commonly found as part of a binary system and steal matter from their companion star. However, if enough matter falls into the x-ray pulsar, it may collapse into a black hole. It is frequently found in pulsars that the x-ray pulses are in phase with the radio pulses. This implies that the electrical current producing the radio waves is also creating the x-rays.
Magnetars Magnetars are produced from massive stars that spin rapidly. If the rotation is fast enough, the speed will match the inner convective currents of the massive star. The magnetic strength of a magnetar is in the order of ten gigateslas. An average neutron star has a magnetic field of 10^8 teslas and a main sequence star like our sun has a magnetic field of .0001 teslas
Nebula Compact objects tend to lie in the center of nebulae. Nebulae are the gas and matter of the star that was hurdled into space as a result of the explosion. Stars have several types of elements contained inside due to nuclear fusion. These gases are expelled during the supernova explosion and compose the nebula. Since each element has different properties, each refracts light differently.
X-Ray Telescope (Chandra) When scientists search for compact objects, they use telescopes that pick up certain wavelengths, commonly it is x-rays. The leading x-ray telescope in the field is called Chandra; it was launched July 23, 1999. Astronomers and Astrophysicists focus satellite telescopes like Chandra on a certain region of space where a neutron star or other compact object is at, and collect data.
Data Analysis Through calculations and observations, scientists have developed models for neutron stars as well as for other compact objects. These models for neutron stars include blackbody radiation and x-ray spectral fitting. Blackbody radiation is electromagnetic radiation that is emitted from a blackbody, an object that absorbs all electromagnetic radiation, at a given temperature. X-ray spectral fitting is a process in which scientists make a model of a neutron star by using data such as the mass, the radius, and the composition of the atmosphere. Scientists make models of stars using computer code to mathematically represent the physical features.
Why do we care about these models? Using the data gathered from telescopes, we can compare it to the given models and try to determine the identification or certain properties of the neutron star. Comparing the wavelengths emitted by the neutron star, we can also determine the elements that are present.
Cassiopeia A An object of interest in terms of compact objects is the supernova remnant in the constellation Cassiopeia. Its given name is Cassiopeia A or Cas A for short. It is believed that Cas A is the product of a supernova that occurred in 1680 and observed by John Flamsteed, an English astronomer. Cas A is the youngest known supernova remnant in the Milky Way Galaxy. This makes Cas A avaluable object to observe in expanding on the field of compact objects.
What is it? For many years the identity of Cas A eluded scientists. From the observations that we have gathered from Chandra, we know that Cas A is a neutron star. However Cas A is not a typical neutron star in the sense that it does not behave like a pulsar. Using typical methods of retrieving data on Cas A produces results that do not fit our basic models of a neutron star.
A Radio Quiet Neutron Star Cas A is a radio silent neutron star because hardly any radio waves have been detected. This is uncommon for a young neutron star that should be emitting radio waves as a pulsar. Cas A does emit x-rays but, it does not pulsate like a traditional pulsar. neutrons stars have strong magnetic fields due to their progenitor stars, however the magnetic field of Cas A is rather weak.
Elemental Atmosphere on the surface of Neutron Stars Neutron stars will commonly obtain a thin atmosphere of an abundant element that lingers in the nebula after the supernova. the physical processes happening in relatively compact (up to 10 - 100 grams per c.c.) plasma in Teragauss (100 Megagauss) magnetic fields, where non-ionized atoms provide the main contribution to the opacity. Using the opacities of strongly magnetized plasmas, scientists can construct models of the neutron stars’ atmospheres.
Carbon Atmosphere of Cas A After trying to calculate the effects of different elements as atmospheres, along with blackbody radiation, scientists noticed that with a carbon atmosphere, Cas A fit the model for a neutron star Using a blackbody model with a carbon atmosphere, Cas A is determined to be an average sized neutron star, with a diameter of 12-15 km and a mass of around 1.4 solar masses. Accounting for the carbon atmosphere also shows that Cas A emits radiation over the entire surface of the star instead of at the poles like a classic pulsar.
Why Do We Care about Cas A? Since Cas A is the youngest observed neutron star in the Milky Way Galaxy, there is much to learn on the early stages of neutron stars from Cas A Although Cas A demonstrates many abnormalities from average neutron stars, it could just be an anomaly in it behavior, but either way it is valuable to scientists in furthering the field of neutron stars.
Dig a Little Deeper If you want to know more about Cas A check out my senior project at http://digitalcommons.calpoly.edu/physsp/25