Study Guide for Test #4

Dr. J. R. Webb

Stellar Astronomy



Chapter 23. Our Galaxy







The Milky Way Galaxy is a flattened disk with the Sun 25,000 ly from the center and Globular star clusters “clustered” around the galactic center.  We live in the disk which contains gas and dust that obscures our view of the galactic center. See Figure 25-8 in the book.



      T = 4.7 m d   km/sec where d is the distance in parsecs.

       Radial Velocity –The speed along the line-of-sight measured using the   

       Doppler shift of spectral lines R = Dl/l0.



The Mass of the Galaxy

Once we determine the Sun’s distance from the galactic center rSun  and the orbital speed, we can calculate the orbital period.  Period (P) = 2 p r / vLSR, where vLSR is the velocity of the LSR, and  rSun = 25,000 ly.  The period T works out to be around 200,000 years.  

Now use Kepler’s third law (yes again!!) to estimate the mass.

P2 = 4p2a3/G(M)  or M = 4p2a3/GP2

The Mass of the Galaxy is:  Mgal ~ 9.4x1010 MSun


Galactic Center

The galactic center is highly obscured by gas and dust which absorbs visible light.  However, gamma-rays and radio waves penetrate the gas and dust and we are able to study the galactic center. We see two expanding arms of gas, one on this side of the center expanding toward us, the other on the far side expanding away.  We believe there is at least 2,000,000 MSun within 2 light years of the galactic center.  A massive black hole????


CHAPTER 24. The Universe of Galaxies.


·        Immanual Kant speculated about the diffuse nebula astronomers saw as being “Island Universes” in 1755.

·        1845 Lord Rosse – observed spiral structure in some nebula.

·        1900’s -  Herschel and son catalogued more than 10,000 nebula

·        1920  The Great Debate – features Harlow Shapley as the champion for the spiral nebula being members of our galaxy, while Heber Curtis argued they were isolated star systems, i.e. external galaxies.

·        Edwin Hubble – used the new 100-inch telescope to observe Cepheid variables in the Andromeda spiral nebula.  He determined Andromeda was about 2.25 million light years away, i.e. an external galaxy.


Types of Galaxies

Spiral Galaxies


Tightly wound arms

Prominent nuclei

Gas/dust in disk



Moderately wound arms

Moderate nucleus




Loosely wound arms

Small nucleus


Barred Spirals


Tightly wound arms

Prominent nuclei

Gas/dust in disk



Moderately wound arms

Moderate nucleus

Gas/dust in disk



Loosely wound arms

Small nucleus

Gas/dust in disk


EO – E7



Very little gas and dust





Gas and dust


·        Hubble Tuning Fork Diagram –  a way of classifying galaxies, but it is not an evolutionary sequence.


·        Irregulars have no location on the tuning fork diagram.


·        Spiral Arms – Gravitational density waves caused by gravitational instabilities.  As the stars, gas and dust move through a spiral shock location, they get compressed and induce further star formation.  We understand how the shocks persist, but not t=how they form originally!


·        Rotational Curves of Galaxies – a plot of rotational speed as a function of distance from the center of the galaxy.  In Keplerian orbits, the rotational speed decreases as you go out, but galaxies rotational curves flatten out suggesting “missing mass” (see missing mass problem #1 above).


Masses and Sizes of Galaxies





109 to 4x1011

20,000 to 50,000


5x105 to 1013

2,000 to 500,000


106 to 3x1010

2,000 to 30,000



·        Clusters of Galaxies – galaxies tend to be found in clusters.  Rich clusters have 1000’s of members while poor clusters have a few dozen members.  Member galaxies are gravitationally bound to each other.

·        Local Group – Out Galaxy the Milky Way is a member of the “Local Group”, out local galactic cluster.


Distances to Galaxies and Hubble’s Law




V = Ho D

Where V is the recessional velocity (V = cDl/lo) in units of km/sec, D is the distance in megaparsecs (106parsecs) and Ho is Hubbles constant in units of km/sec-Mpc.



Chaper 26. The Big Bang Theory and the History and Future of the Universe



T =0  Big Bang

Universe, space, time, matter and energy are all contained in an infinitesimally small point.

0 to 10-43 sec

Planck Era – need quantized gravity to know what happened here.

10-43 to 10-35 sec

Grand Unified Field Era, no normal atoms, strong force, weak force and electromagnetic force are all one force here!

10-35 to 10-10sec

Electro-weak era – The strong force splits off, so there are now 3 fundamental forces: gravity, strong and electro-weak.  Elementary particles form.

10-10 to 0.001

Electromagnetic and weak forces separate.   Particle era.

0.001 to 3 minutes

Era of nucleosynthesis –protons, neutrons form first atoms and H forms helium during the early part of this era.

3 min – 300,000 years

Atoms now form and radiation (photons) become free from the matter, they decouple.  The radiation cools as the universe expands, and the atoms are free to pick up electrons, form complete atoms, and start forming stars and galaxies.

300,000 years to 1 billion years

First galaxies from during this period.

1 billion years –present

Stars, galaxies evolve and life evolves on Earth!