Please tell me if there are mistakes, however small, in the answers I have provided to these questions. Thank you in advance. 1. Why are pulsars not always associated with supernova remnants? Most pulsars are created by Type II supernovae, but out of all the pulsars detected yet (around two thousand), only a few are associated with supernova remnants. There are three main reasons for this. Firstly, some pulsars move at very high velocities through space and leave their supernovae remnants behind rather quickly (in several tens of thousands of years). Those pulsars have high velocities because they were given a strong kick at birth. This strong kick may be violent turbulence in the core of a massive star during an assymetrical explosion or the flinging of two massive stars apart during a supernova explosion in a binary system. Secondly, the beams of some pulsars within supernova remnants never sweep over Earth and are not detected by astronomers. Thus, from Earth, the supernova remnant does not seem to be associated with a pulsar. Thirdly, pulsars live for more time than their supernova remnants. Pulsar remain detectable for around 10 million years, while their supernova remnants dissipate into the interstellar medium after around 50,000 years. It is not surprising that pulsars over 50,000 years old have already lost their supernova remnants. 2. Provide a brief argument for what you think is the most compelling evidence for the Big Bang. The most compelling observational evidence for the Big Bang is the expansion of the universe as shown by the Hubble Diagram. The Big Bang is defined as the initial moment of expansion of the universe. This expansion continued after the “moment” of the Big Bang and is still going on. Galaxies and quasars in all directions are receding from astronomers' observation point, i.e are redshifting. By plotting the redshift of different galaxies on the Hubble Diagram, it has been found that the farther a galaxy is from us, the faster it is receding from us (i.e its redshift is higher). A lineal relation exists between the velocity of redshift and distance. The galaxies themselves are not moving, but the space between them is expanding. The light from the light-source and the observer gets stretched into longer wavelengths as it travels through the expanding space. As time goes, the space separating the galaxies becomes larger and larger. The second most compelling evidence of the Big Bang, the uniform presence of cosmic microwave background radiation (CMBR), is dependent on the expansion of the universe. The original blackbody radiation from the Big Bang has expanded with the universe to uniformly fill it. The wavelength of that radiation has also been redshifted with the expansion; while it originally had a temperature of 3000 K, it now appears to have a temperature of 2.7 K. 3. Provide a summary of the argument first used to prove that the Solar System is not at the centre of out Galaxy. Early in the 20th century, the American astronomer Harlow Shapley mapped the distances and direction of Cepheid variable stars in globular clusters. Prior to mapping globular clusters, Shapley had noticed that those are concentrated in the direction of the constellation Sagittarius and Scorpius on the celestial sphere. He had guessed (assumed) that this concentration of globular clusters are controlled by the combined gravitational field of the entire larger star system. He realized that if his assumption was correct, he could know about the size and extent of the entire star system by studying globular clusters. He studied the distances and distribution to globular clusters using H.S Leavitt's period-luminosity law and a statistical method he had devised to replace the apparent magnitude on the period-luminosity diagram by absolute magnitudes. He searched catalogs for the proper motions of some Cepheid variable stars, and used his statistical method to find their average distance and average absolute magnitude from those proper motions. Then, by comparing the stars' absolute magnitudes with their apparent magnitudes using the formula mv - Mvb = 5 + log10 (d), he was able to find the distances to the globular clusters they were in. He plotted the distances and directions of those globular clusters and discovered that their distribution are not centered around the solar system. The concentrated center of the globular clusters was indeed situated near Sagittarius and Scorpius, many thousands of light-years away from the solar system. The solar system is only in the 'suburbs' of the galactic star system, about two-thirds of the way out from the center.