chapter 24 galaxies nick devereux 2006. birr telescope - ireland
TRANSCRIPT
Chapter 24
Galaxies
Nick Devereux 2006
Birr Telescope - Ireland
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;
Determining Distances to Galaxies
(In approximately the order that they were introduced);
1. Cepheids2. Velocities3. Supernovae
Nick Devereux 2006
The Distance Ladder
Nick Devereux 2006
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
Cepheids undergo periodic changes in brightness
Nick Devereux 2006
Which in turn is correlated with the stars luminosity – extremely useful as a distance indicator
Nick Devereux 2006
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
With a distance D = 0.7 Mpc, Andromeda is the nearest spiral galaxy to us
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
Velocities
Nick Devereux 2006
Redshifts are converted into recession velocities using the Doppler equation
Doppler Equation
v/c =
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The Hubble Law
v = Ho D
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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
Supernova
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Type 1a Supernova attain approximately the same peak brightness
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Different types of Supernovae,but the Type Ia’s are the best as they reach the same maximum brightness
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Type Ib
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Type Ic
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Type II
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Supernova light curves
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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
The amount of dust extinction depends on which direction you look
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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.
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Because the extinction changes the color of the object as well as making it fainter.
Nick Devereux 2006
Nick Devereux 2006
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
Not all Galaxies are Spirals however….
Galaxies collide and merge
Distant galaxies look chaoticHow did they become the beautiful symmetric objects we
see today? - an active area of current research