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Chapter 24
Galaxies
Nick Devereux 2006
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Birr Telescope - Ireland
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Built in 1845 by the Third Earl of Rosse
In terms of size, it is as large as the Hubble Space TelescopeMirror size is 72 inches ~ 6 feet ~ 2 m.
Most important discovery was spiral structure in theMessier nebula, M51;
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Determining Distances to Galaxies
(In approximately the order that they were introduced);
1. Cepheids2. Velocities3. Supernovae
Nick Devereux 2006
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The Distance Ladder
Nick Devereux 2006
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Cepheids
• Henrietta Leavitt (1868 – 1921), working on the stellar spectra project at Harvard, discovered variable stars by inspecting photographic plates.
• She found 2400 Cepheids.
Nick Devereux 2006
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Cepheids undergo periodic changes in brightness
Nick Devereux 2006
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Which in turn is correlated with the stars luminosity – extremely useful as a distance indicator
Nick Devereux 2006
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The luminosity is a measure of absolutebrightness. The fact that the apparent brightness is less than the absolute brightness is a consequence of distance.Hence the distance, D.
Nick Devereux 2006
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With a distance D = 0.7 Mpc, Andromeda is the nearest spiral galaxy to us
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The Cepheids yielded distances to the Andromeda Galaxy, and other galaxies,which enabled Hubble to calibrate his new distance determination technique based on the recession velocities of galaxies.
Nick Devereux 2006
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Velocities
Nick Devereux 2006
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Redshifts are converted into recession velocities using the Doppler equation
Doppler Equation
v/c =
Nick Devereux 2006
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The Hubble Law
v = Ho D
Nick Devereux 2006
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The Hubble Constant
• Ho = 50 kms-1/Mpc according to Sandage
• Ho = 100 kms-1/Mpc according to DeVaucouleurs
• Ho = 75 kms-1/Mpc according to modern measurements.
The way to use the equation is to measure the
recession velocity, and divide by Ho to get D in Mpc. Nick Devereux 2006
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Supernova
Nick Devereux 2006
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Type 1a Supernova attain approximately the same peak brightness
Nick Devereux 2006
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Different types of Supernovae,but the Type Ia’s are the best as they reach the same maximum brightness
Nick Devereux 2006
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Type Ib
Nick Devereux 2006
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Type Ic
Nick Devereux 2006
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Type II
Nick Devereux 2006
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Supernova light curves
Nick Devereux 2006
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One Major Complication is Dust
• The major problem with all distance determinations is the extinction of starlight due to dust in our Galaxy and other galaxies.
Nick Devereux 2006
Maximum brightness for a Type 1a supernova is MB = -19.6 +/- 0.2 mag
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The amount of dust extinction depends on which direction you look
Nick Devereux 2006
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The consequence of dust extinction is to make the “star” appear further away than it actually is, so we have to correct for this effect.
Nick Devereux 2006
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Because the extinction changes the color of the object as well as making it fainter.
Nick Devereux 2006
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Nick Devereux 2006
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So, if you know what the intrinsic color of the object is you can figure out what the extinction coefficient is from the observed color.
Nick Devereux 2006
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Not all Galaxies are Spirals however….
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Galaxies collide and merge
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Distant galaxies look chaoticHow did they become the beautiful symmetric objects we
see today? - an active area of current research