Transcript August 29 - Astronomy

From the Big Bang to Planets a
brief history of the Universe
Background material – Chapter 1 in text
Things I expect you to know, but will not be
tested on
Intro to some useful physics – Chapters 4 & 5
Things you will be tested on
The Hot Big Bang Theory
The Universe started from a very hot, dense
state around 14 billion years ago and is
expanding and cooling down with time.
How do we know?
1. Hubble’s law – distant galaxies are all
moving away from us, the farther away
they are, the faster they are moving
2. The Cosmic microwave background
radiation – things that are hot emit light
and the universe today is filled with this
light emitted when it was much hotter
Hubble’s Law
Raison bread analogy
As dough expands all
raisons move away
from all other raisons
Cosmological Redshift
Light, which is an electromagnetic wave, gets
stretched as it travels though the expanding
space to reach us from far away galaxies.
Because red light has longer wavelengths than
blue light, we say the light from these galaxies is
redshifted, meaning shifted to longer
wavelengths.
The cosmological redshift
Misconception
The cosmological redshift is not a Doppler shift.
Doppler shifts are caused by the motion through
space, cosmological redshifts are caused by the
expansion of space.
But there is no way to tell the difference from
observations
1st rule to memorize
Cosmological redshifts depend on the distance
Doppler shifts do not depend on the distance
See Chapter 5
Discussion
If all of the galaxies are today getting farther
away from each other, what does this tell you
about the galaxies in the past?
Hubble time
If we know how fast the universe is expanding,
we can trace it back to find a time when all the
galaxies we see were at the same place.
This happened about 14 billion years ago.
Discussion
What does this tell you about the size of the
universe in past?
How big is the Universe?
Universe – size unknown, may be infinite
Observable universe – 14 billion light years in
all directions
Speed of light and Distances
Light travels through space at of fixed speed.
3.00 X 108 m/sec
We can use this as a unit for measuring
distances. A light year is the distance light
travels in a year.
The Cosmic Horizon
If the universe is only 14 billion years old, then
the only light that we can see is the light that
has had time to reach us.
The farthest we can see is therefore 14 billion
light years away, anything farther the light has
not had enough time to reach us.
Cosmic Horizon
1st Law of Astronomy 201
If you squish something it gets hotter
Discussion
Can you think of any examples from everyday
life of something becoming hot as it is
compressed?
Diesel vs. gasoline engines
Gasoline – air fuel mixture ignited by sparkplug
Diesel – air fuel mixture ignited by compression
Discussion
Diesel engines are about twice as efficient as
gasoline engines, so why are the vast majority
of cars gasoline cars?
Think about how the engines need to be
different.
What is temperature?
Temperature is average random speed of
atoms or molecules that make up an object.
Higher average random motions = higher
temperatures
Temperature
Air molecules at 0 C have an average velocity of
about 485 m/s or 1090 miles/hour.
Discussion
If temperature is a measure of how fast the
atoms are moving, how can we have a cold
wind? Shouldn’t all winds by definition be
warmer than still air?
Why do compressed objects heat
up?
Consider a super ball bouncing on a table.
What happens to the bouncing ball if you
drop a book on it?
Electrostatic charges
Like charges repel (+,+) or (-,-)
Opposite charges attract (+,-)
Discussion
If like charges repel each other, how came the
nucleus of an atom can contain more than
one positively charged protons? Why don’t all
atoms heavy than hydrogen fly apart?
Protons and Neutrons
You can consider a neutron to be a proton + an
electron.
A neutron decays into a proton and an electron.
A proton smashed together with an electron will
turn into a neutron.
Elements and isotopes
The element is defined by the number of
protons in the nucleus.
The isotope is defined by the number of
protons + neutrons in the nucleus.
1st Law of astronomy 201
If you squish something, it heats up
The opposite is also true, if something
expands it cools.
Example – a refrigerator
Space is very cold, now
As the universe expands, it cools. Currently
its temperature is 2.73 K or –455 degrees F.
But, as we run time back, the universe was
much denser and much hotter.
Hydrogen
The early universe consisted of entirely of a
plasma of hydrogen nuclei (protons), neutrons,
electrons and photons.
More complex nuclei would be busted up by
high speed collisions
Nucleosynthesis
As the universe cooled
Proton – neutron collisions produced
deuterium
deuterium – deuterium collisions produced
3He
deuterium – 3He collisions produced 4He.
The first 5 minutes
At the end of nucleosynthesis the universe
consisted of 75% hydrogen and 25% helium
with a trace of lithium and beryllium.
Also there were left over traces of deuterium
and helium-3
Neutral atoms
After about 380,000 years of expansion
the universe cooled to about 3000 K and
electrons could remain bound to protons
forming the first neutral atoms.
Discussion
What happens when a cloud of hydrogen and
helium contracts because of gravity?
Cosmic Microwave Background
The early universe contained no
life
Without carbon, life could not exist in the early
Universe.
Planets like the Earth could not form without
iron, silicon and oxygen.
Andromeda Galaxy
What are stars?
Stars are enormous balls of mostly hydrogen and
helium gas.
Internal pressure halts
gravitational collapse
The core of the star gets so hot that hydrogen
can fuse into helium. The heat released by
this reaction stops the star from contracting.
The struggle to avoid collapse
When the hydrogen in the core runs out, the
star starts to collapse again and becomes
even hotter.
If the star is massive enough it will become
hot enough to fuse helium into carbon.
Ring nebula
How do the elements got out of
the stars?
Once enough iron builds up in the core of a
massive star, it suddenly collapses. The
outer layers of the star “bounce” off the core
and are blasted into space in a supernova
explosion.
Supernovae
Supernovae explosions are the most
energetic events in the Universe. A single star
can outshine 100 billion stars.
The explosion of the outer layers is so violent
that all the elements heavier than iron can be
produced, including many unstable
radioactive isotopes.
Supernova 1987a
The Crab nebula
Supernovae can trigger star
formation
The expanding shock wave of a supernova
explosion, can cause a nearby cloud of gas to
collapse and form new stars.
Thus, its heavy nuclei get incorporated into
the next generation of stars.
Summary
Squishing things makes them hot
Temperature is a measure of the average
random speed of atoms
The universe started out with mostly hydrogen
and helium with all other elements were
created during supernovae explosions of stars
more massive than the Sun