Astrophysics is a branch of astronomy that applies the
theories and techniques of physics to the study of celestial
objects. For the large and massive objects in our universe,
the most important force is gravity, and the base of our
present theoretical understanding is Einstein's theory of
gravitation and Einstein's theory of general relativity.
Other ideas of physics are also used in astrophysics, such
as the use of emission spectrum from stars to determine
their composition, or doppler shifts in the emission
spectrum to determine the speed of stars with respect
to the Earth.
Cosmology is the study of the organization and structure
of the universe and its evolution. Cosmology studies the
universe as a whole and seeks to find answers to questions
such as how the universe was formed, why does it look the
way it does, and what will happen to it in the future.
It looks for answers to what seems to be impossible
questions, such as "Has the universe always existed,
or did it have a beginning in time?" "If it had a
beginning, what came before that?" "Is it finite or
infinite?" "If it is finite, what is out there beyond it?"
On a clear and moonless night, thousands of stars of varying
degrees of brightness can be seen from Earth, as well as the
elongated cloudy stripe known as the Milky Way. The elongated
cloudy stripe is actually composed of countless individual
stars that are part of our Galaxy. Our Galaxy is a
spiral-armed galaxy that is flat like a disc, and has a
central bulging nucleus as depicted on page 2. Our Galaxy
is about 100,000ly in diameter and 2000ly thick, with the
central bulge about 6000ly thick. Our Galaxy contains about
100 billion stars and our Sun is just one of them.
Our Sun is a little more than half way out from the galactic
center and orbits the galactic center once every 200 million
years.
Shown above are two views of our Milky Way Galaxy, one from above and
one from the side.
Using powerful telescopes such as the Hubble space
telescope, which is in orbit about the Earth, billions and
billions of other galaxies can be seen beyond our Milky Way
Galaxy. The nearest one is the Andromeda galaxy, which is
over 2 million light years away. When we look up and see
the Andromeda galaxy, what we see is the light that left
the Andromeda galaxy over 2 million years ago. What does
the Andromeda galaxy look like today? We won't know for
over 2 million years, since light leaving the Andromeda
galaxy will not arrive on Earth for over 2 million years.
In this way, astronomers are able to look back into the past.
The farther away a star or galaxy is, the 'older' the light
is that reaches the Earth. The orbiting Hubble space
telescope can see more clearly galaxies farther away than
other telescopes because it is above the Earth's atmosphere
and its images are free from distortions that are caused by
the Earth's atmosphere. The Hubble space telescope has seen
galaxies as far away as 14 billion light years, which means
it has looked back into the past 14 billion years.
As nuclear fusion begins in the core of the star,
the star continues its gravitational collapse.
What keeps a star from continuing to collapse due
to gravity at this point in its evolution? The answer
is radiation pressure. The process of nuclear fusion
in the core, in which hydrogen is 'burned' to become
helium, releases large amount of energy in the form of
electromagnetic radiation. As this radiation moves out
from the core of the star, it exerts an outward pressure
on the particles of the star. When enough nuclear
fusion takes place to create a radiation pressure that
balances the gravitational collapse, the protostar
becomes a star and its size becomes stable. Our Sun and
other stars with similar masses take about 30 million
years to stabilize and become a star. They have enough
hydrogen to remain stable through hydrogen 'burning'
for about 10 billion years, assuming our theories about
stellar evolution are correct. Although most stars are
billions of years old, there is evidence (and some
beautiful pictures from the Hubble space telescope,
http://hubble.stsci.edu) that stars are being born
right this moment.
When the supply of hydrogen in the core is sufficiently
depleted, the core contracts and heats up. This results
in an increase in hydrogen fusion, which is an increase
in radiation pressure, and the star expands into a red
giant. A star entering the red giant phase can be
thought of as having two regions, a core and an outer
shell. In the core, a buildup of helium from the
hydrogen 'burning' has reduced the radiation pressure,
so gravitational collapse gets ahead and contracts the
core. But as the core collapses, it heats up and the
process of hydrogen 'burning' goes faster, creating
more radiation pressure. The core will become stable
again, but more dense and at a temperature ten to a
hundred times hotter. The outer shell doesn't have
the helium from the hydrogen burning, but it does see
the increased radiation pressure from the core, so it
expands until radiation pressure and gravitational
collapse are balanced. Our Sun and the planets of
our solar system are about 5 billion years old.
Our Sun will enter the red giant phase in about
another 5 billion years. When it does, it should
expand until its radius is about equal to the distance
from the Sun to the Earth. In the core of a red giant
the temperature is ten to a hundred times hotter than
it is in a star. At this temperature, not only does
hydrogen 'burning' go faster, but there is also enough
kinetic energy for other types of nuclear fusion.
Helium can be fused into beryllium, and if the star
is massive enough, elements all the way up to iron
can be made.
What is the fate of a red giant? That depends on the
size of the star. Stars with a mass a little larger
than our Sun or smaller will run out of stuff to 'burn'
and cool, becoming white dwarfs. The white dwarf will
continue radiating away energy until it eventually
becomes a black dwarf, a dark, cold piece of ash.
More massive stars can continue to contract, due to
their larger gravitational forces, until the atoms
are crushed and the atomic nuclei are mashed up
against each other. Under these high pressures,
the electrons combine with the protons to form
neutrons and the result may be a neutron star.
Neutron stars are thought to be so dense as to have
diameters of only a few kilometers (and masses greater
than our Sun!!!) It is thought that one way in which
a supernovae can be created happens when a neutron
star undergoes its final collapse. When the atoms
give way and it crushes to nuclear density, tremendous
energy can be released in a short period of time.
This cataclysmic event has enough energy to create
virtually all of the elements of the period table.
Stars with masses much greater than our sun will
continue to collapse even beyond the density of a
neutron star, crushing not only the atoms, but the
neutrons as well, pushing the quarks together.
When this happens, a black hole is formed, an
object whose gravitational attraction is so great
that no matter or light can escape it. If our Sun
had the same density as a black hole, it would be
about the size of a golf ball. It is believed
that there are black holes at the center of
galaxies, and that our own Milky Way Galaxy has
a black hole at its center with a mass estimated
to be several million times that of our Sun.
So we can calculate that 14 billion years ago the
Earth and everything around it were in roughly the
same spot, but surely the Earth isn't at the center
of the universe? Heck, it's not even at the center
of our Galaxy (which is a good thing since there is
probably a massive black hole there). So an
assumption must be made that there is nothing special
about our little corner of the universe. This
assumption is called the cosmological principle,
and it states that the universe looks the same no
matter which way we look or where we are. This
assumption is only valid on a large scale, which
for the universe is a size much bigger than our Galaxy.
On a local scale (for Cosmology), this clearly isn't
true; our Galaxy is a slowly rotating flat disc
appearing different when we look at it from the side
or from the top. The Cosmological principle is only
assumed for really big things. So what does that
imply for the Earth and everything around it?
In the 14 billion years it took for everything around
us to expand to where it is today, the Earth moved
from the center of the universe to where it is today.
We've certainly come a long way from the ideas of
Aristotle with the Earth at the center of the universe.
A view of the universe that was held until only a
few hundred years ago!
In order to produce a universe as large as the
one we observe today, both in size and mass,
the Big Bang must have been huge. So huge in fact,
that much of the very interesting features of the
Big Bang happened very quickly, as the universe
was rapidly expanding away from a single point
and rapidly cooling. The two variables we will
keep track of when we talk about the evolution
of the universe are time and temperature.
The standard cosmological model can only make
predictions about the universe back until about
10^-43 seconds, when the temperature of the
universe was about 1032K. We simply do not know
enough about what might have happened before that
because we have no good theory to predict the
behavior of gravity at such a high temperature.
It is believed that gravity must be quantized,
but no good quantum mechanical theory for
gravity has yet to be developed and
experimentally verified. That is not to say
that no theory exist, lots of theories exist,
but none of them can be experimentally verified.
Such is the nature of science.
From 10^-43 seconds to 10^-35 seconds, the standard
cosmological model predicts a temperature drop
from 1032K to 1027K. At these temperatures,
there is enough kinetic energy for the quarks
to freely exist with the leptons. The quarks
are going so fast that they cannot be bound
into things like neutrons and protons.
We call this era the Grand Unified Era where
quarks and leptons are whizzing about in a dense soup.
From 10^-35 seconds to 10^-6 seconds (one millionth
of a second), the standard cosmological model
predicts a temperature drop from 1027K to 1013K.
At these temperatures, there is not enough kinetic
energy for the quarks to exist freely and they bind
together to form hadrons (hadrons is the name for
the class of particles that contains neutron,
protons, and things like them).
We call this era the Hadron Era.
From 10^-6 seconds to 10 seconds the standard
cosmological model predicts a temperature drop
from 1013K to 1010K. At these temperatures,
there is not enough kinetic energy to form hadrons,
but there is still enough energy to form leptons,
such as electrons, which are lighter than neutrons
and protons. We call this era the Lepton Era.
From 10 seconds to about 1 million years,
the standard cosmological model predicts
a temperature drop from 1010K to 3000K.
At these temperatures, there is not enough
kinetic energy to form even leptons, and by
now most matter had disappeared through
particle-anti-particle annihilations.
The universe is still quite hot however
and is dominated by radiation.
We call this era the Radiation Era.
It is believed that towards the end of the
Radiation era the temperature of the universe
had become low enough for electrons to combine
with nuclei to form stable atoms.
From 1 million years to 14 billion years
(the present), the standard cosmological
model predicts a temperature drop from
3000K to 3K. As the universe expands and
cools, the density of radiation is dropping.
During this era in the evolution of the universe,
the radiation density has dropped low enough
that it is below the density of matter in the
universe. We call this era the Matter-dominated Era.
It is believed to be in this era that stars and
galaxies formed, probably from self-gravitation
around mass concentrations.
____________________________
Dr. Todd B. Smith
Physics Department
Science Center - Room 3
(in basement near KU)
229-2435
todd.smith@notes.udayton.edu