Download Lecture Summary (11/22)

EXAM II is Nov. 18 during lecture.
The preview of lecture material 11/16 is most likely is over-optimistic. However, what is
not covered will not be on the exam.
Good luck studying for the exam. There will be a review in your discussion sections
Nov. 16 & 17. I will conduct 2 review sessions: Wed. Nov. 17 from 2:30-3:45 in CSS
2324 and from 4:15-5:30 in CSS 1113. You bring the questions. Makeup labs will be
conducted next week, so you do not have a NEW lab during exam week. Your TA
should have talked to you about makeup labs last week if you are entitled to one. Talk to
your TA about this if necessary.
Prof. Deming
Lecture Summary (11/9, 11/11, and preview of 11/16)
The interstellar medium (ISM) is comprised of gas and dust. Starlight is affected
as it passes through concentrations of gas and dust. There are advantages to using
infrared telescopes to probe the interstellar medium.
Our Sun formed in a nebula 4.6 billion years ago. Stars like the Sun are born as
protostars in regions where the ISM collapses. There are many examples of star-forming
nebulae, dusty disks, and infrared stars to support our theories of star formation. As
gravity compressed the gas, our protosun formed. Eventually the Sun began to fuse
hydrogen into helium and was able to reach equilibrium. All main sequence stars achieve
stability by fusing hydrogen to helium in their cores. The Sun maintains its stability at a
core temperature of 15 million K. Eventually the Sun will lose its ability to sustain itself
by hydrogen fusion as helium nuclei build up in the core. With a drop in energy, the
outward force cannot balance the inward force. Gravity causes collapse that heats the
interior, and in a shell surrounding the helium core hydrogen fusion begins again. The
outer layers of the Sun will expand at this time due to the great temperatures inside and
the Sun will become a red giant. Red giants have cool surface temperatures, but much
greater surface area so luminosity is greater than for a main sequence star. Stars as
massive as the Sun eventually fuse helium into carbon when a core temperature of 100
million Kelvins is achieved. Deep inside the red giant sun, a carbon core will develop
surrounding a helium burning shell and a hydrogen burning shell. Due to the increased
fusion in these shells, the outer layers of the Sun will become greatly heated and expand
outward producing a planetary nebula. Examples were shown. With no fusion in the now
exposed core, the Sun will collapse until (degenerate) electrons exert a pressure to
balance gravitational collapse. The Sun is now the size of the Earth and very hot.
However, its luminosity is low due to its greatly diminished surface area. At this point
the Sun has entered the white dwarf phase.
How long a star is able to sustain itself using hydrogen fusion depends on its
mass. The HR Diagram’s main sequence is a mass sequence with the most massive stars
at the upper left and the lowest mass stars at the lower right. A more massive star has a
greater core temperature and the hydrogen fusion reactions proceed at a greater rate. The
main sequence lifetime of the Sun and other one solar mass stars is approximately 10
billion years. More massive stars have shorter main sequence lifetimes; lower mass stars
have longer main sequence lifetimes.
Stars more massive than the Sun are born as protostars in nebulae. Due to their
great mass, they are forced to fuse hydrogen into helium at a higher core temperature and
consume their hydrogen at a rapid rate. Their main sequence lifetimes are much shorter
than our Sun. After becoming giants, these stars will be able to fuse carbon and continue
nuclear fusion up to an iron core. If the star can eject mass in sufficient quantities to
become less than 1.4 solar masses it will become a white dwarf.
If the iron core develops, the star is destined to become a supernova. Iron does not
fuse and yield energy, it takes energy away in the core. The inner regions of the star
catastrophically collapse beyond the white dwarf size to a size of about 20 km across.
The electrons and protons are forced together by gravity giving neutrons. Meanwhile the
outer regions of the star fall in as well, encounter this dense rigid core and recoil outward
with tremendous release of energy the supernova explosion.
The radio waves of pulsars are produced by fast moving electrons in a strong
magnetic field and are observable only when the magnetic pole is directed toward Earth.
Angular momentum would be conserved during the final collapse of the core leading to
the fast spinning neutron star. The magnetic axis is angled to the rotational axis. After
the supernova, if the core mass is less than 3 solar masses, gravity is stabilized by the
pressure exerted by the closely confined neutrons. This dense, small neutron star is what
was left of the star that blew up. If the core mass is greater than three solar masses after a
supernova, even the neutrons are forced together and the result is the formation of a black
hole.
Even though light can't escape from beyond the event horizon, astronomers
believe that they have detected black holes in several binary systems. Cygnus X-1 is the
best candidate and was used as an example of how black holes can produce detectable
observations. Cygnus X -1 is a spectroscopic binary. We can see one star, but the other
star isn't visible (the invisible companion). Mass studies of the system place the invisible
companion's mass at about 10 solar masses–massive enough to be a black hole. Cygnus
X-1 is an X- ray source. Astronomers theorize that the supergiant's loosely held
atmosphere is drawn toward the companion black hole's event horizon and as the gas is
accelerated, heating occurs. The gas drawn in flattens and forms an accretion disk.
Temperatures near 10 million K are reached in this accretion disk as the plasma
approaches the event horizon. Very hot gases emit X-rays—recall Wien's law. X-rays
travel at the speed of light and can escape the black hole if emitted before the gas crosses
the event horizon.
What do we mean when we say "life?" We have one example—life here on the
Earth. Much of the discussion around whether or not extraterrestrial life exists depends
on whether we are "typical" or a "chance occurrence." Life on Earth is carbon-based.
Chemically carbon can form 4 bonds with other atoms and therefore can form complex
molecules, chains and rings. Life processes are complex. Carbon chemistry with water
serving as the medium of exchange has the advantage of also being fairly abundant as
chemicals go. Organic molecules exist in meteorites and the ISM. Experiments have been
performed in laboratories to simulate the conditions on ancient Earth—remember, the
rock record indicates our atmosphere lacked significant quantities of oxygen originally.
All of the building blocks of life (proteins, amino acids, nuclei acids) have been produced
in such experiments. Given a billion years after the Earth's formation, life arose here. A
BILLION YEARS—this kind of time factor is completely out of the realm of human
association—simulations done on Earth have been taking place for roughly 50 years.
Microfossil and stromatolite evidence gives dates of about 3.5 billion years ago.
Photosynthesis gradually added oxygen into our atmosphere and an ozone layer
eventually developed about 2 billion years ago.
TERMS: interstellar medium, scattering, nebula, protostar, hydrogen fusion, main
sequence star, main sequence lifetime, red giant, hydrogen burning shell, planetary
nebula, white dwarf, supergiant, supernova, neutron star, pulsar, black hole, singularity,
event horizon, Cygnus X-1, accretion disk, organic chemistry, stromatolites, microfossils,
Miller Urey expt.,
QUESTIONS:
1. How does interstellar dust affect starlight?
2. At what stage does a protostar become a star?
3. Why is the Sun described as a main sequence star? What’s happening inside?
4. Describe the changes that will take place inside the Sun as it becomes a red
giant and then a white dwarf.
5. How does the evolution of a massive star differ from the evolution of our sun?
6. What is the link between supernovae, neutron stars and pulsars?
7. Describe and explain pulsar observations using a neutron star model. Use a
sketch.
8. What is the radius of the event horizon for a black hole of 10 solar masses?
9. Why do astronomers think Cygnus X-1 is a good black hole candidate? Use a
sketch to explain what's happening that explains the observations.
10. Why are scientists convinced that extraterrestrial life will be similar
chemically to life on Earth?
11. Why do scientists believe that life arose in the seas on Earth?
12. What was done in the Miller Urey experiment?