Thursday 6 March 2014

Analyzing The Universe - Blow by Blow

In my earlier post I gave a review of the course Analyzing The Universe. The 6-week course is available, free to students, via the excellent organisation Coursera, and is delivered by Dr. Terry Matilsky of Rutgers University. Here, as promised, is a blow by blow account of each of the six weeks in the course.
Week 1
In week 1 we have an introduction to the history of optical images and their nature, what it takes to make an astro-image, what information they provide and how they are formed. Light, energy and refraction. Lenses and optics.
We see how an x-ray image is formed and why this is different from optical image formation. How we perceive images when we look at them, and how this impacts the kind of images we want and the information we can get from them.
Finally we get an introduction to the free tool DS9, its features and use.
Week 2
In week 2 we begin an in-depth tour of DS9. We look at the supernova CAS A, which was the 'first light' observation from the Chandra X-Ray observatory, and the Coma cluster of galaxies. We look at how to use DS9 to compare the x-ray image with an optical image of the same object. CAS A and Coma are used to demonstrate some of the useful functionality of DS9 such as analysing the energy spectrum of an interesting area of the image.
Getting a bit more theoretical we look at how statistics can be used to add to the knowledge gained from our observations.
Next we look at atomic spectra, black body radiation and the Stephan-Boltzmann law relating luminosity, radius and temperature. We look at cosmic distances, using parallax for measurements, and how Cepheid variables can help us measure distances.
Week 3
In week 3 we look at the Hertzsprung-Russel diagram, how it helps us to classify stars and what it tells us about stellar evolution. We look in some detail at how stars evolve with time, and what different kinds of stars are out there.
We take an in-depth look at GK-Per, which is a double star in the Persesus constellation which went supernova in 1901. We use this as the basis for a discussion of white dwarves, and some periodic phenomena associated with their spectral properties.
Week 4
In week 4 we derive some of the useful formulae for describing circular motion. We go on to the Doppler shift and its detection when we observe binary stars whose orbital motion is aligned to make such observation possible.
We study the nature of pulsars using DS9 to analyse x-ray data from Cen X-3, observing and quantifying the periodicity of the luminosity in the x-ray data. We analyse the possible causes of the periodicity and what it can tell us about the nature of the pulsar. We observe the Doppler shift in the signal from Cen X-3, and together with our other conclusions this leads us to an understanding of Cen X-3 as an x-ray source orbiting a companion star.
We then go on to figure out what exactly Cen X-3 might be. Using our knowledge of the laws of circular motion we determine the mass of the object, and try to gain an insight into how big it is. We conclude that if it was a white dwarf, it is spinning so fast that it's gravity couldn't hold it together, but if it's a neutron star, its gravity would be sufficient.
We then go on to investigate what mechanism leads to the periodicity we've observed, in the process learning much more about the nature of Cen X-3, and pulsars.
We compare observations made at different periods in history to see that the behaviour of the object has changed over a period of tens of years. We analyse the data to seek an explanation for this.
In week 4 we learn a great deal about neutron stars and pulsars, and we fortify that knowledge with our own observations.
Week 5
In week 5 we look at what makes a star shine. We look at the x-ray object Cas-A and analyse optical spectra from different parts of the object. These observations help us towards a model of Cas-A as a type II (core collapse) supernova remnant in which the movement of material shows evidence of the shock waves resulting from the explosion.
Looking at the spectra from different parts of the object we also draw conclusions about what chemical elements are present and how they are distributed in the remnant.
We use DS9 to create an RGB image from the remnant in which red, green and blue each represent an energy band. By adjusting the energy bands and the bias and contrast for each colour we have considerable control over how we visualise the image. This gives us scope for a lot of experimentation in DS9.
In week 5 we learn a great deal about core collapse supernovae and supernova remnants, and we fortify that knowledge with our own observations. Week 6
In week 6 we begin with Cepheid variable stars – how and why they may be used to determine the distance to astronomical objects. With an accurate measure of distance it is possible to determine the size of an object from its angular size. We learn about Hubble's discovery that the further an object is from us the faster it is moving away from us – leading us to a determination of the Hubble constant for the expansion of the Universe and how this can be used to determine the age of the Universe. We see the relationship between red-shift and distance to an object.
We learn how this led to the discovery of quasars, which are more luminous than the brightest galaxies. We see also that quasars are only seen as distant objects, meaning they are a feature of the young Universe, and we consider why that might be.
We use DS9 to analyse data from the closest quasar, 3C 273, which is 2200 million light years from Earth. We see from our analysis that it is a trillion times more luminous than our sun, and we determine its size.
We see that some material appears to be moving at many times the speed of light, and we look at the explanation for the phenomenon.
We see the evidence of gravitational lensing, and the fact that it suggests that there is much more mass in the Universe than we can observe. We look at the evidence that the expansion of the Universe is accelerating, and we see that dark energy is one theory put forward to explain the evidence.
In week 6 we learn much about galaxy clusters and black holes.
For a review of the course, and my conclusions, please see the previous article.
I took this course from 28th January to 25th February 2014

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