Welcome everyone to our new topic in the blog section. We are going to talk about The Violent Universe. We start today talking about a star we have been observing on tour recently, Sirius. Be warned there is a lot of science in here but hopefully I have kept the geeky math bits to a minimum. In the early 19th century, Bessell used the world’s best telescope (designed by Fraunhofer) to discover that the star Sirius was wobbling backwards and forwards every 50 years. He concluded that it must have a massive but invisible companion. He worked out that the mass of this companion was roughly 1 solar mass. 50 years later, the companion (known as Sirius B) was seen for the first time. Curiously, it is around 1000 times fainter than the main star Sirius (now called Sirius A), despite having half its mass. It was initially assumed that its faintness must be due to its cool temperature, but early in the 20th century, spectroscopy showed that Sirius B was extremely hot, 2.5 times hotter than Sirius A. If it was so hot, why was it so small? The only possibility was if it was extremely small. Using the Stefan-Boltzman equation we were able to work out its size from the luminosities, which comes out as around 6000 km for Sirius B, i.e. slightly smaller than the Earth! Despite weighing as much as the Sun. Sirius B is so small and so heavy that the force of gravity near its surface must be enormous. Using Newtons law of gravity it was possible to estimate the pressure in the centre of a star using a bit of calculus, if we assume that the density of the star is constant. In this case, we found the pressure gradient by balancing forces on a cylinder of star material (the pressure on the inside must exceed the pressure on the outside by enough to balance gravity), using the calculations, we could see that the pressure in the centre of Sirius B is vastly greater than that in the middle of the Sun. All of which leaves us with two problems. Firstly, given that Sirius B is much denser than the Sun, and hotter two, it should be doing nuclear fusion at a much faster rate than the Sun. But it is clearly not – or it would be more luminous than the Sun. So why isn’t it undergoing fusion? Secondly, the gravity on Sirius B is so intense that no material could withstand it. So why doesn’t its surface collapse inwards? In the Sun, heat from fusion stops this collapse. But there is no such heat in Sirius B – so what is holding its surface up?
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