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Measuring the distance to Galaxies Henrietta Leavitt and the Cepheid Variable Star The Great Distances of Space Many of the challenges in understanding the universe have involved measuring distances How can you measure the distance to objects that are too far away to visit? Inverse Square Law brightness 100.0 90.0 80.0 70.0 brightness The brightness of the star changes according to an inverse square law When the star is twice as far away, it is 1/4 as bright 3 times as far away, it will be 1/9 as bright 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 5 10 distance 15 QuickTime™ and a BMP decompressor are needed to see this picture. Click animation to start Distance If you know the amount of light that a star emits, you can use the inverse square law to work out its distance from us by reading off the graph 100.0 90.0 80.0 70.0 brightness brightness versus distance 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 5 10 distance 15 Stars of equal brightness The brightness of stars varies both because of the luminosity and distance Luminosity is a way of measuring the light emitted by a star, the more luminous the star, the more light it emits Are the three bright stars equally distant - we cannot tell from the photo unless we know that they are equally luminous “Harvard Computer” Henrietta Leavitt Some stars vary in luminosity a bit like a lighthouse In 1912, Henrietta Leavitt discovered a type of variable star for which the more luminous the star, the longer the period of variation in brightness These stars are called Cepheid Variables Cepheid Variables 2.5 2 1.5 brightness The varying brightness of 2 Cepheid variable stars The bottom one (pink) is twice as luminous as the top one 1 0.5 0 -0.5 0 5 10 15 -1 -1.5 time 20 25 30 These stars have equal maximum brightness Which of these two Cepheid Variables is more luminous? Which of these stars is further away? Can you work out how many times further away? Answers 1.Which of these two Cepheid Variables is more luminous? 2. Which of these stars is further away? 3. Can you work out how many times further away? The star on the right has a longer period so must be more luminous The star on the right is more luminous but appears only as bright as the one on the left. It must be further away The period is doubled so it is twice as luminous but its brightness is halved. It is less than twice as far away. (It will be 1.4 times as far away). If the two Cepheid Variables were equally distant QuickTime™ and a BMP decompressor are needed to see this picture. A movie of variable stars in a globular cluster Finding the Distance to Cepheids The distance to nearby Cepheid variables can be determined by parallax (a method you will learn in this course) The inverse square law and the periodluminosity relationship of Henrietta Leavitt enables the distance of all observable Cepheid variables to be determined Measuring the distance to galaxies Cepheids are found in other galaxies Astronomers can therefore measure the distance to other galaxies. The Discovery of Galaxies In the 1920s astronomers questioned whether objects like this were small nebulae within our galaxy, or enormous galaxies great distances beyond our galaxy. Cepheid variables in the objects showed that they were huge distances from our galaxy