The Formation of Galaxies in the Universe
Much of this page on galaxy formation was taken directly from the Zebu
Astronomy website, with the intent of giving you a simply version of the
process. To be sure, there is a great deal of mathematics involved in a deeper
understanding of the formation of galaxies which is beyond the intended scope
of this course, but I hope you will find these words helpful. Should the Zebu
site ever cease to exist, this course will have this page within it for your
If you want, there is a tremendous
article from Sky & Telescope Magazine (October, 2004) that deals with
this topic, has great pictures, is easy to understand, and is the most current
information out there (as of the date of this page writing ... November, 2004).
Galaxy Formation :
Galaxies are the basic unit of cosmology. They contain stars, gas, dust and
alot of dark matter. They are the only `signposts' from here to the edge of
the Universe and contain the fossil clues to this earlier time. They come is
a variety of sizes, shapes, and star numbers, and most amazingly, they are everywhere
you look in the night sky.
The physics of galaxy formation is complicated because it deals with the dynamics
of stars (gravitational interaction), thermodynamics of gas and energy production
of stars. For example, stars form from gas clouds, but new stars heat these
clouds, which dissipates them and stops the formation of other stars.
After recombination, density enhancements either grew or dispersed. According
to our hybrid top-down/bottom-up scenario, an assortment of enhancements formed
of various sizes. Small, dense ones collapsed first, large ones formed slower
and fragmented as they collapsed.
The first lumps that broke free of the Universe's expansion were mostly dark
matter and some neutral hydrogen with a dash of helium. Once this object begins
to collapse under its own gravity, it is called a protogalaxy. The first protogalaxies
appeared about 14 billion years ago.
Note that dark matter and ordinary matter (in the form of hydrogen and helium
gas at this time) separate at this time. Gas can dissipate its energy through
collisions. The atoms in the gas collide and heat up, the heat is radiated in
the infrared (light) and the result is the gas loses energy, moves slowly =
collapses to the center. Dark matter does not interact this way and continues
to orbit in the halo.
Even though there are no stars yet, protogalaxies should be detectable by
their infrared emission (i.e. their heat). However, they are very faint and
very far away (long time ago), so our technology has not been successful in
discovering any at this time.
Formation of the First Stars :
As the gas in the protogalaxy loses energy, its density goes up. Gas clouds
form and move around in the protogalaxy on orbits. When two clouds collide,
the gas is compressed into a shock front.
The first stars in a galaxy form in this manner. With the production of its
first photons by thermonuclear fusion, the galaxy becomes a primeval galaxy.
Star formation sites in primeval galaxies are similar to star forming regions
in present-day galaxies. A grouping of young stars embedded in a cloud of heated
gas. The gas will eventually be pushed away from the stars to leave a star cluster.
The first stars in our Galaxy are the globular star clusters orbiting outside
the stellar disk which contains the spiral arms. Most galaxies with current
star formation have an underlying distribution of old stars from the first epoch
of star formation 14 billion years ago.
Stellar Death :
The most massive stars end their lives as supernova, the explosive destruction
of a star. Supernova's occur when a star uses up its interior fuel of hydrogen
and collapses under its own weight. The infalling hydrogen from the star's outer
envelope hits the core and ignites explosively.
During the explosion, runaway fusion occurs and all the elements in the periodic
table past lithium are produced. This is the only method of producing the heavy
elements and is the origin to all the elements in your body.
This shell of enriched gas is ejected into the galaxy's gas supply. Thus,
the older a galaxy, the more rich its gas is in heavy elements, a process called
Ellipticals vs. Spirals :
The two most distinct galaxy types are ellipticals and spirals. Ellipticals
have no ongoing star formation today, spirals have alot. Assuming that ellipticals
and spirals are made from the same density enhancements at the time of recombination,
why did they evolve into very difference appearances and star formation rates?
The answer is how rapid their initial star formation was when they formed.
If star formation proceeds slowly, the gas undergoes collisions and conservation
of angular momentum forms a disk (a spiral). If star formation is rapid and
all the gas is used up in an initial burst, the galaxy forms as a smooth round
shape, an elliptical.
Gas falling into a spiral disk is slowed by collisions and star formation
continues till today. The spiral arms and patterns are due to ongoing star formation,
whereas ellipticals used all their gas supplies in an initial burst 14 billion
years ago and now have no ongoing star formation.
Galaxy Mergers/Interactions :
After their formation, galaxies can still change their appearance and star
formation rates by interactions with other galaxies. Galaxies orbit each on
in clusters. Those orbits can sometimes cause two galaxies to pass quite close
to each other to produce interesting results.
Solid objects, like planets, can pass near each other with no visible effects.
However, galaxies are not solid, and can undergo inelastic collisions, which
means some of the energy of the collision is transfered internally to the stars
and gas in each galaxy.
The tidal forces will often induce star formation and distort the spiral pattern
in both galaxies.
If enough energy is transfered internally to the stars, then galaxies may
merge. Galaxy mergers are most frequent in dense environments, such as galaxy
clusters. Don't forget about the Sky & Telescope Magazine
article on Galaxy Formation ... it's pretty
I think you might enjoy learning about the farthest objects that we can see,
as well as those that existed in the earliest times of the Universe: Quasi-Stellar
Objects, better known as Quasars. Go
here BEFORE you move on to the next phase of this course.
Now it is time to move ahead to Galaxy
Structure , or your could go back to the Milky
Way, or go to the Syllabus
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