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Another galaxy: NGC 4414. The Milky Way roughly resembles it.
Today’s Lab: Galaxies & Cepheids
- Part I. Finding distances to galaxies
- Part 2. Shapes of Galaxies
- Part 3. Color of Galaxies
- Part 4. Size of Galaxies
Finding Distances

In the Solar System => Radar
Signal goes out and back at speed of light

Near Stars => Geometric Parallax

Farther Stars => Spectroscopic Parallax
(Classify the main sequence star by spectral type - surface temperature
and use that to find Luminosity and M, then use m-M dimming factor)

Star Clusters
Use dimming factor/modulus = m – M => distance
(we did this last week)
-
- How do we find distances to nearest galaxies
Similar to what we did for clusters
(using the difference between actual brightness M & apparent brightness
m)
but need way to find Luminosity / Absolute Magnitude
Shapley (1917) found that Sun was not at center of Milky Way
Shapley used distances to variable “RR Lyrae” stars (a kind of Horizontal
Branch star) in Globular Clusters to determine that Sun was 16 kpc from
center of Milky Way. Modern value 8 kpc.
Measuring the Milky Way
The usefulness of these stars comes from their
period–luminosity relationship.
Both RR Lyrae & Cepheids are post main-sequence
Using Cepheid to find m and M
mavg = (mmax + mmin) / 2 = (24.7 + 25.7) / 2 = 25.2
Period = 25.3 day
Once we have the period, go back and get luminosity from
graph like that on previous slide with the known period
Measuring the Milky Way
Period of Cepheid C46 is 25.3 days
What is its absolute magnitude M?
Measuring the Milky Way
We have now
expanded our cosmic
distance ladder one
more step.
Hubble’s Galaxy Classification
Hubble’s “tuning fork” is a convenient way to
remember the galaxy classifications, although it has
no deeper meaning.
Hubble’s Galaxy Classification
Type Sa has the largest central bulge, Type Sb is
smaller, and Type Sc is the smallest.
Type Sa tends to have the most tightly bound spiral
arms, with Types Sb and Sc progressively less tight,
although the correlation is not perfect.
The components of spiral galaxies are the same as in
our own Galaxy: disk, core, halo, bulge, spiral arms.
Hubble’s Galaxy Classification
Spiral galaxies are classified according to the size of
their central bulge.
Hubble’s Galaxy Classification
Similar to the spiral galaxies are the barred spirals.
Elliptical galaxies have no spiral arms and no disk.
They come in many sizes, from giant ellipticals of
trillions of stars, down to dwarf ellipticals of fewer
than a million stars.
Ellipticals also contain very little, if any, cool gas and
dust, and show no evidence of ongoing star
formation.
Many do, however, have large clouds of hot gas,
extending far beyond the visible boundaries of the
galaxy.
Ellipticals are classified according to their shape,
from E0 (almost spherical) to E7 (the most
elongated).
S0 (lenticular) and SB0 galaxies have a disk and bulge,
but no spiral arms and no interstellar gas.
The irregular galaxies have a wide variety of shapes.
These galaxies appear to be undergoing interactions with
other galaxies.
Irregular galaxies are the most common type of galaxy
Hubble’s Galaxy Classification
A summary of galaxy properties by type
Local Group of Galaxies
Here is the distribution of
galaxies within about 1
Mpc of the Milky Way.
Determining the amount of Red Light and Blue
Light coming from a Galaxy using SIPS
Watch TA demonstration
Finding angular size and actual size
Step 1: Find Angular Size in arcminutes
Angular Size = (angular size of window) x (Pixel size of object) /
(Pixel size of window)
This gives angular size in arcminutes / Pixel size-length using pixel ruler
Step 2: Find Angular Size in radians
Convert angular size in arcminutes to arcseconds and then radians
Angular size (arcseconds) = Angular size (arcminutes) x 60 arcseconds /
1 arcminute
Angular size (radians) = Angular size (arcseconds) x 1 radian/
206,000 arcseconds
Step 3: Find Actual Size
Actual Size (pc) = Angular Size (radians) x Distance (pc)