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Download File - YEAR 11 EBSS PHYSICS DETAILED STUDIES
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Unit 1 Physics Detailed Study 3.2 Chapter 11: Astrophysics Section 11.3 We know the stars by their light Analysing starlight The way we know stars, if by their ‘light’ This light refers to the whole electromagnetic spectrum, this includes the whole visible spectrum, Inferred (IR), x-ray, gamma rays and UV. To observe these different forms of light, a variety of telescopes are required, such as the radio telescope in Parkes, which is used to pick up microwave and radio radiation. Some radiation, such as x-rays and some UV cannot penetrate the Earths atmosphere, so to get a clear picture, we sometimes need to put telescopes out into space. Section 11.3 We know the stars by their light Analysing starlight Though a picture of a star can tell us a fair bit about a star, we learn more about it from the spectrum it produces. A hot object will produce light of a continuous rainbow spectrum, unless it interacts with an outside medium, such as dust or gas. In this interaction, the gas cloud can absorb some light, this shows up as dark bands in the spectrum. In the absorption process, atoms of the gas are move into an excited state and then back to ground state. In the process, light is emitted in all directions. This light appears as bright lines on a dark background. Section 11.3 We know the stars by their light Analysing starlight These spectra of emission and absorption are characteristic of the each of the elements. For example, Sodium emits two particular yellow spectral lines. If these lines appear in the spectrum of a star, we can say that sodium present, whether it is as part of a compound or as a gas. These lines, known as Fraunhofer lines, lead to the discovery of Helium in the sun before its discovery here on Earth. As we know, we can tell a lot from the light emitted from a star, for example its temperature. Section 11.3 We know the stars by their light Analysing starlight The temperature of a star as we learnt earlier could be determined by the amount light emitted from ranges, for example, hotter stars emit more in the Blue and UV ranges then the visible spectrum. However, as we have just learnt, if this light were to encounter a cloud of dust or gas, light may be absorbed, leading to incorrect readings. Fortunately, the dark bands mentioned earlier can help in the determination of Temperature. Initially, stars were categorised according to the presence or absence of certain lines associated with hydrogen, labelled from A-O This was later changed to a system that involved placing stars with similar spectra adjacent to each other in a smooth pattern. The letters were then changed to OBAFGKM, which was later broken up further by adding 0-9 to each category. Section 11.3 We know the stars by their light Analysing starlight While not known at the time, this categorisation corresponded to the different temperatures, from O0 (hottest) to M9 (coolest). The reason for the changes between the classes was to do with some atoms becoming ionised at various temperature and at cooler temperature the light may not have sufficient energy to excite the atoms to create spectral lines. This meant the temperature of a star could be determined without worrying about losses as light travelled through gasses or dust clouds. We can also determine the size of a star by its spectrum. To do this we need to know its Luminosity, the amount of energy given off by each unit area, and an accurate surface temperature. Section 11.3 We know the stars by their light Types of stars- the Hertzsprung-Russell diagram Once this information was gathered, the natural thing to do was plot the information to look for a relationship between known quantities. (Surface Temperature, on the x-axis increasing to the left and Luminosity, on the yaxis with brightest at the top) There was a noticeable pattern produced, this is known as the Hertzsprung-Russell, or H-R diagram. Most of the time, hot stars are bright, though this is not always the case. Stars on the main pattern/line going from top left to bottom right are known as Main Sequence stars. This includes the sun. Section 11.3 We know the stars by their light Types of stars- the Hertzsprung-Russell diagram It is possible to have hot, dim stars, these types of stars are known as White dwarfs (located mostly in the bottom left of the H-R diagram), and are not visible to the naked eye as they are quite small. It is also possible to have a cool, bright star. These stars are known as Giants or Supergiants, depending on the size. Giants and Supergiants only make up ~1% of the stars in the sky, dwarfs only make up ~9% and main sequence stars make up ~90% of the stars in the sky. Section 11.3 We know the stars by their light Placing stars on the H-R diagram Placing stars on the H-R diagram is relatively easy when within parallax range as we can find their surface temperature and luminosity easily. However, when outside of parallax range, it get a little bit trickier. To do this we need to look at the spectrum of the light received from the star. The spectral lines can tell astrophysicists about not only the temperature, but can be interpreted in a way to give the luminosity of the star also. One final piece of information needed was to determine the mass of a given star. After some careful observation, a relationship was established between mass an luminosity. L is proportional to m3, so a star twice as heavy as the sun will be eight times as bright. Section 11.3 We know the stars by their light Interpreting the H-R diagram – stellar evolution For a long time, it was thought that Stars were permanent features of the night sky, however this is not the case. Understanding the Nuclear process going on inside the star lead to the understanding that these stars would not last forever… Billions of years, yes, but not forever. A star may have enough fuel to last it billions of years, however models have shown that a stars life may not be as long. The hydrogen-hydrogen fusion process produces helium, as the concentration of helium builds up in the core, the reaction zone increases by ~10%, this causes some instability in the sun, causing the sun to expand from a main sequence star to a giant. Section 11.3 We know the stars by their light Interpreting the H-R diagram – stellar evolution This model indicated to astrophysicists that stars are born on the main sequence and eventually move to the giant phase. To test this theory star clusters (stars thought to be born around the same time, from the same dust cloud) were examined. In this examination, it was discovered that the heavier, brighter, bluer stars were moving to the giant phase, before the lighter stars, this is due to the fact that the heavier stars burn through their fuel faster then lighter stars. Looking closely at the H-R diagrams of young and old clusters shows that stars spend most of their lives in the main sequence, before expanding to giant fairly rapidly. Section 11.3 We know the stars by their light Interpreting the H-R diagram – stellar evolution Stars start their life as a mass of gas and dust collapsing on itself under immense gravitation attraction. This collapse generates loads of heat, which eventually causes enough heat to cause fusion to begin. Once fusion has begun, the star stabilises and becomes a main sequence star. Once expanded to the giant phase, there is enough heat to cause heavier elements to fuse together, which stabilises the star. As the fuel beings to run out, the star begins to contract, increasing the temperature, however, decreasing the brightness, thus the star moves towards the white dwarf region, where they slowly fade away as a black dwarf. Section 11.3 We know the stars by their light Interpreting the H-R diagram – stellar evolution Some stars, however, are destined to go out with a bang. In stars around 4 times the mass of the sun, after the hydrogen-burning phase, begin to contract, causing heat ~600 million degrees, causing new fusion reactions, eventually leading to the burning of silicon to create iron. At this point, fusion does not produce energy, so the star beings to collapse, increasing the temperature to increase further. As the star continues to collapse, the temperature increase further to billions of degrees in a fraction of a second. It finally collapses to a point where it can collapse no further, but the rest of the star keeps pushing on the core, causing a huge ‘bounce back’. This bounce back causes the star to explode, releasing 1046 J of energy, leaving behind a neutron star.