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 galaxiesSimilar to what we did for clusters (using the difference between actual brightness M & apparent brightness m)

but need way to findLuminosity / 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.

The usefulness of these stars comes from their period–luminosity relationship.

Both RR Lyrae & Cepheids are post main-sequence

Measuring the Milky Way

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

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.

Measuring the Milky Way

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.

Hubble’s Galaxy Classification

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

A summary of galaxy properties by type

Hubble’s Galaxy Classification

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)