Date: July 30, 2010

Title: A Brief Introduction to Galaxy Dynamics


Podcaster: Edgardo Molina

Description: Galaxy Dynamics is the study of the motions of the stars, gas and dark matter in order to explain the main morphological and kinematic features of galaxies. It is one of Astronomyʼs greatest challenges to understand. Galaxy Dynamics is a very changing and mind blowing subject. New studies and discoveries are constantly shaping the equations that describe it.

Organization: Pleiades. Research and Astronomical Studies A.C. (web site soon to be presented also in English)

Bio: Edgardo is a very kind gentleman man who loves astronomy and wishes to share his joy for this wonderful science. 🙂

The 365 Days of Astronomy team would like to wish Edgardo a very happy birthday today!

Edgardo Molina has a B.S. in Mechanical Engineering from the Anahuac University in Mexico City. Post graduate studies in IT Engineering and a Masters Degree in IT Engineering. Working for IPTEL, an IT firm delivering solutions to enterprises since 1998. Space exploration enthusiast who participated in several Mexican space related activities. Licensed amateur radio operator with call sign XE1XUS. Amateur astronomer since childhood and actual founder and president of the Pleiades. Research and Astronomical Studies A.C. in Mexico City, Mexico. Avid visual observer and astrophotography fan. Public reach through education in exact sciences, engineering and astronomy. Lectures and teaching in several universities since 1993.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the Ann Arbor Science & Skeptics group in Ann Arbor, Michigan. You can find them on Facebook and


Galaxy Dynamics

Hi! This is Edgardo Molina. From Pleiades. Research and Astronomical Studies in Mexico City, Mexico. I am your host today for this episode of the 365 Days of Astronomy Podcast. I am grateful for the patience and help you continuously show, for my frequent participation in this magnificent project. I thank you all for your kind efforts and support. Long live this project.

Galaxy Dynamics is the study of the motions of the stars, gas and dark matter in order to explain the main morphological and kinematic features of galaxies. It is one of Astronomyʼs greatest challenges to understand. Galaxy Dynamics is a very changing an mind blowing subject. New studies and discoveries are constantly shaping the equations that describe it.

In a big effort to explain what Galaxy Dynamics is all about without the need to use numbers and their associated equations, I will try to introduce you to this fascinating subject with words only. Easy task? Certainly not. I hope you enjoy it. If not, my apologies beforehand, try instead to Google “Galactic or Galaxy Dynamics” and enjoy the heavy mathematics that abound there and press your brains to ischemia through the process of understanding the information. Elegant solutions full of equations leading to exquisite results. Even me, I must confess I am not sharply dressed for such an elegant occasion.

Now, to the point. The main components of the Universe are the galaxies. Galaxies are collections of stars, gas, dust and dark matter bound together by their mutual gravity, typically spanning a mass range from 100 million to 10 billion times that of the Sun.

Galaxies comprise the basic building blocks of the Universe and can be seen a good fraction of the way across it . When galaxies were born and how they evolved is currently a very active topic. At present, the Universe is thought to have been created in the Big Bang, in which space and time were in a simple hot energetic, state, about 14 billion years ago.

Since the Big Bang, the evolution of galaxies and their structures is briefed in the following parragraphs:

At 0.01 seconds, electrons and positrons form as the temperature drops to 1011 Kelvin. After 1 second, the Universe becomes transparent to neutrinos, which from now on hardly interact further with matter.

During the first 10-40 seconds after the Big Bang, the four fundamental forces are unified (although no complete physical description of this era yet exists). Temperature 1032 Kelvin. 10-40 seconds defines the time when gravity splits from the other forces.

Then, up to 10-35 seconds, quarks and anti-quarks dominate the Universe. The strong force separates from the weak and electromagnetic forces. Temperature drops to 1027 Kelvin. At 10-12 seconds the four forces become distinct.

At three minutes after the big bang, the temperature has reached 109 K, protons and neutrons combine to form what will become the nuclei of elements mostly H and He). After 300,000 years the temperature has dropped to 3000 K and the electrons are captured by nuclei to form neutral atoms.  The Universe becomes transparent to light (photons stop interacting with free electrons) resulting in the formation of the Cosmic Background Radiation.

After 1 billion years, the temperature is 20 K and galaxies and stars have begun to form via gravitational contraction of over-densities in the initial Universe. After a few billion years our Galaxy forms, at about 10 billion years after the Big Bang the Sun and Earth form. After 15 billion years we reach the present and a background temperature of about 3 K.

About 300,000 years after the Big Bang, there was the era of recombination in which protons and electrons combined to form neutral Hydrogen.  At this point, baryonic matter in the Universe consisted of about 75% Hydrogen and 25% Helium (by mass), with some small amounts of heavy elements (elements starting from Lithium). The distribution of this material was very close to, but not quite, uniform. These slight over- and under densities were observed for the first time by the COBE satellite (launched in 1989) and amount to only a few parts in 100,000. The variations were mapped out over the whole sky, on scales of about 7 degrees. The successor mission, WMAP, mapped the sky to much better resolution (about 0.25 degree), and has confirmed that a new element in the basic mix: “dark energy”.

After recombination, the Universe entered a period called the “Dark Ages”, until gravitational attraction had operated on very slight over densities in the matter distribution, leading to the formation of light emitting stars and galaxies. The Universe was optically observable again.

Exactly how stars and galaxies formed, when the process started and how long it took is currently a major area of research.  A simple picture runs like this: about 1 billion years after the big bang the first star forming regions, conglomerates of perhaps 106 to 109 solar masses began to develop. Over the next several billion years, most of these merge to form larger units or are partially destroyed by the energetic supernovae which develop as a natural part of star formation. Within a few billion years most of these have developed into stable configurations of stars and gas and are recognizable as “galaxies”. 

Probably most of the “mature” galaxies we see around us now originate from this epoch 10-12 billion years ago.  Classically (20 years ago) it was considered that most or all galaxies had formed at this time. However, many studies now show that galaxy formation is an on-going process, but at a much reduced rate in the present universe compared to about 10 billion years ago.

The distribution of galaxies in space around us in the nearby Universe is currently a very active research area. Surveys have revealed a very detailed foamy structure in which galaxies are found along “walls” surrounding large “voids” which are relatively galaxy free. The precise form of this distribution places interesting constraints on the type of Universe we inhabit — its total mass, expansion rate and ultimate fate.

The observable Universe has a diameter of order 10 Gpc. To find and observe galaxies at large distances (and therefore in the early Universe) requires large or special telescopes. The Hubble Space Telescope has obtained one such view in a very narrow “pencil beam” (unlike the survey above which covers a large part of the sky) view of the Hubble Deep Field. Galaxies are visible in this image more than halfway across the visible Universe. At these early times in the evolution of the Universe galaxies look quite different to what they do today.

The way Galaxies look is studied by Galaxy Morphology, the most well known classification of galaxies is due to Hubble. The main types are elliptical galaxies (E), normal spiral galaxies (S), barred spiral galaxies (SB) and irregular galaxies.
• SPIRAL galaxies have fast rotating, flattened disks, and contain some gas and dust.

• ELLIPTICAL galaxies are slowly rotating ellipsoids, usually containing little gas and dust.

• IRREGULAR galaxies have no simple shape, and usually contain a lot of gas and dust.

The elliptical galaxies have various degrees of ellipticity from 0 (E0) to 0.7 (E7), and they are slowly rotating. On the other hand the spiral and barred spiral galaxies are rotating fast and they contain spiral arms. There are tight spirals (Sa, SBa), intermediate (Sb, SBb) and open spirals (Sc, SBc). They are flat systems, containing also gas and dust, out of which new stars are formed continuously.

The irregular galaxies are relatively small systems that accompany large spiral galaxies. The shapes of the galaxies are governed by their stars, that form their main bodies. The gas forms a relatively small proportion of the matter of the galaxies, while the dark matter extends to large distances.

The motions of the stars are due to the gravitational forces. The gravitational potential of a galaxy is composed of a mean field, due to the average distribution of the galactic matter, and fluctuations due to the approaches (encounters) between individual stars. An estimate of the effects of these encounters is provided by the “relaxation time” of the system, which is the time required by a star to change its average direction of motion by 90º due to the cumulative effect of encounters. This time is very long, of the order of 10E+12 years, while the periods of the motions of the stars around the center of the galaxy are of the order of 10E+8 years (“dynamical time”). Thus, in a first approximation, we may consider the orbits of the stars as due to the general distribution of the galactic matter.

There are numerous efforts today to use super-computers and computer clusters to simulate Galaxy Dynamics and Galaxy Interaction. The software used for those simulations encompass all the math and theories required to visualize the “dance of the heavens” as anybody could call the mating act of the Galaxies. It is quite shocking to see how our galaxy neighbor M31 (Andromeda galaxy) will collapse with our Milky Way. I just hope there are still humans on this galaxy to witness such an event.

From here you have either to order from the menu or leave with a half empty stomach. If possible I would like to approach this subject again in future podcasts to continue explaining the facts and figures of Galaxy Dynamics.

In the mean time please stop worrying about the day to day problems your head might be dealing with, do a clean restart and imagine how insignificant your problems and us as human kind are in relation to all the Universe that surrounds us.

For the 365 Days of Astronomy Podcast, this is Edgardo Molina, wishing you open your minds to the greatness of our extragalactic surroundings!

End of podcast:

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