# P7.3

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• Created by: amy_mair
• Created on: 23-06-16 17:00
• P7.3
• Astronomial Distances
• Parallax
• 1. Parallax is an apparent change in position of an object against a distant background
• It makes stars appear to move relative to distant ones over the course of the year
• The parallax angle is half the angle moved against the distant backgrounds stars over 6 months. The nearer an object is to you, the greater the angle
• 2. The angle is often measured in arcseconds rather than degrees
• 1 arcsecond = 1- (1 / 3600)
• 3. Parallax is useful for calculating the distance to nearby stars. It is a similar magnitude to a light year
• Astronomers usually use a distance called parsec
• You can calculate the distance to a star using the equation
• Distance = (1/ angle)
• Intensity
• 1. The luminosity or intrinsic brightness of a star depends on the time and the temperature.. The bigger and hotter it is, the more energy it gives out, so the brighter it is
• 2. As you move away from a star, it looks dimmer- because the energy reaching you get less. So the observed intensity of the light seen on earth depends on luminosity and how far it is away from earth
• 3. So if you looked at 2 stars with the same luminosity but one was further away than the other, the more distant star will look dimmer
• Cehied Variable
• 1. A group of stars called the Cephied Variable pulse in brightness- they get brighter and then dimmer over a period of several days
• 2. How quickly they pulse is directly linked to their luminosity. The greater the luminosity the longer time between the pulse
• 3. So, if you see 2 Cepheid variable stars with the same observed brightness that pulse at different rates, you know that the star with the longer pulse period must have the higher luminosity
• The scale of the universe
• Telescopes
• 1. If you went out in the countryside on a clear night you could probably see about 1500 stars. If you looked with a small telescope, you could probably see half a million
• 2. The more stars you can see, the more you notice that they are not evenly dotted about the sky.
• Most of the stars appear to be concentrated in a bright ***** across the sky - the milky way galaxy
• 3. Away from the *****, the number of visible stars is much smaller
• 4. Our sun is jut one of the approximately 10 to the 11th star in the galaxy
• 5. The milky way is actually a spiral galaxy. But because we are part of its disk, we can see it edge on
• The Curtis Shapley Debate
• Shapleys argument
• 1. Shapley believed the universe was just one gigantic galaxy about 100,000 parsecs across
• 2. He reckoned our sun and solar system were far from the centre of the galaxy
• 3. He believed that nebulae were huge clouds of gas and dust. These clouds were relatively nearby and actually a part of the milky way
• Structure of the universe
• Curtis's argument
• 1. Curtis thought the universe was made up of many galaxys
• 2. He thought our galaxy was smaller than shapley suggested
• 3. The spiral nebulae were other very distant galaxies, completly separate from the milky way
• Both of them were right
• 1. Shapley was right that the solar system is far from the centre of our galaxy
• 2. Curtis was also right about the spiral nebulae
• The scale of the Universe
• Hubble
• 1. Hubble helped solve the Curtis-Shapley debate with the observations of the Andromeda Nebula
• 2. Using images taken using the largest telescope at the time, he found that this spiral shaped fuzzy blob, some of whihc were Chepeid Variables
• 3. Hubble calculated the distance to the Andromeda Nebula by working out the distance to the Cepheid Variables within it, using the relationship between their brightness and pulse frequency
• He studied other spiral nebulae and found a similar result
• Red Shift
• Distant galaxies are moving away from us
• 1. When a galaxy is moving away from us the wavelength of the light from it changes- the light becomes redder. This is called red shift
• By seeing how much the light has been redshifted you can work out the recession velocity of the galaxy
• Using red shift Hubble found this... The more distant a galaxy the faster it is moving away from us
• This suggests that the universe is expanding
• Rd shift is fairly to measure, so a galaxy recession velopsicy can be calculated easily enough
• Speed of recession = hubble's constant x distance
• Using data on Cephied variables stars from distant galaxies has givven us better values of Hubbles constant