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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 Telescope
Mirror size is 72 inches ~ 6 feet ~ 2 m.
Most important discovery was spiral structure in the
Messier nebula, M51;
Determining Distances to Galaxies
(In approximately the order that they were introduced);
1. Cepheids
2. Velocities
3. 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
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Which in turn is correlated with the stars luminosity –
extremely useful as a distance indicator
 Nick Devereux 2006
The luminosity is a measure of absolute
brightness. 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 = /
 Nick Devereux 2006
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
 Nick Devereux 2006
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
 Nick Devereux 2006
Maximum brightness for a Type 1a
supernova is MB = -19.6 +/- 0.2 mag
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
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 chaotic
How did they become the beautiful symmetric objects we
see today? - an active area of current research