5.3The Milky Way and Other

Galaxies

How Many Galaxies Exist? When astronomers chose a relatively empty spot

on the sky near the Big Dipper and used the Hubble Space Telescope to record and extremely long exposure of 10 days duration, they found thousands of galaxies crowded into the image. Known as the Hubble Deep Field.

Other deep fields have been imaged since. The GOODS (Great Observatories Origins Deep Survey)

has used the Hubble Telescope with other space telescopes such as the Spitzer Space Telescope and the Chandra X-ray Observatory to image selected deep fields.

Types of Galaxies

3 main types of galaxies:

1. Spiral Disk-shaped usually consisting

of spiral arms, containing gas and dust.

2. Elliptical No disk, no spiral arms, and

almost no gas and dust.

3. Irregular Generally shapeless and tend to

be rich in gas and dust.

Sa

Sb

Sc

E0 = Spherical

Sc = Small nucleus; loosely

wound arms

E1

E6

Sa = Large nucleus; tightly

wound arms

E7 = Highly elliptical

Elliptical: Spiral:

Galaxy Classification

Galaxy Classification

Roughly 2/3 of all spiral galaxies are barred-spiral galaxies, classified SBa, SBb, and SBc.

Show a pronounced bar structure in the center.

Our own Milky Way galaxy is an example of this.

Neighboring Irregular Galaxies

Some smaller galaxies (“dwarfs”) orbit larger galaxies.

Example: The Large Magellanic Cloud and Small Magellanic Cloud are 2 of 11 known dwarf galaxies orbiting our Milky Way galaxy.

Large Magellanic Cloud

Small Magellanic Cloud

Distance to Galaxies

The distances to galaxies are so large that it is not convenient to measure them in light-years, parsecs, or even kiloparsecs. Instead, astronomers use the unit Megaparsec

(Mpc), or 1 million pc.

To find the distance to a galaxy, astronomers must search among its stars, nebulae, and stars clusters for familiar objects whose luminosity or diameter they know. Such objects are called distance indicators.

▪ Most are objects whose brightness is known, an astronomers refer to them as standard candles.

The Hubble Law

In the 1920’s, Edwin Hubble and Milton Humason measured the distances to a number of galaxies using Cepheid variable stars, stars changing their brightness with a period of 1-60 days where the period of variation is directly related to its luminosity. Published a graph in 1929 that plotted

the apparent velocity of recession versus distance for their galaxies.

The straight-line relation from Hubble’s diagram can be written as a simple equation.

This relation between red-shift and distance is known as the Hubble Law.

Hubble LawFormula

Vr = HdVr = apparent velocity of

recession, in km/s

H = Hubble constant ~ 70 km/s/Mpc

d = distance, in Mpc

The Hubble Law

The Hubble Law

Example 1: A galaxy is calculated to be 5 Mpc away.

What is its apparent recessional velocity?

Example 2: A galaxy has an apparent recessional

velocity of 700 km/s. What is the distance to this galaxy?10 Mpc

350 km/s

Distance Pyramid

Hubble’s Law

Standard Candle/

Tully-FisherVariable

Stars (Cepheids)

Spectroscopic Parallax

Stellar Parallax

Radar Ranging