Download 1. How old is our sun now? How does its present luminosity

1. How old is our sun now? How does its present luminosity compare to the luminosity it had
when the Solar System first formed? How much longer will it remain on the main sequence?
What will happen to the sun after it leaves the main sequence?
(a) Our sun is now 4.6 Byrs old.
(b) When it first formed its luminosity was about 70% of its present luminosity.
(c) It takes a star like the sun about 10 Byrs to go from birth to the red giant stage.
Thus it still has about 10 - 4.6 = 5.4 Byrs on the main sequence.
(d) After the sun leaves the main sequence it will become a red giant. After further
adventures, it will end as a white dwarf.
2. How did the Earth’s atmosphere 4 billion years ago differ from our atmosphere today? What
did the Miller-Urey experiment demonstrate?
(a) Most important, the early atmosphere had almost no oxygen. There was plenty of
CO2 and H2 O, and probably some CH4 . It was probably mildly reducing. Today
the atmosphere is mostly N2 and O2 and is highly oxidizing.
(b) The Miller-Urey experiment showed that putting electrical sparks into a strongly
reducing atmosphere produced lots of organic molecules like amino acids.
3. A nearby star has a parallax angle of 0.20 00 and a luminosity of 10 L . If this star were
moved to a distance such that its parallax were 0.10 00 , how would its apparent brightness
change? (Give a numerical answer.) Would this affect its luminosity?
(a) Distance (in parsecs) is 1 over the parallax angle (in seconds of arc): d = 1/p. Thus
for p=0.20 00 , d = 5 pc, while for p=0.10 00 , d = 10 pc. So we doubled the distance
to the star. The apparent brightness goes as the inverse square of the distance: flux
∝ 1/d2 . Thus doubling the distance will cause the apparent brightness to decrease
by 1/22 = 1/4.
(b) The luminosity of the star is an absolute measure of the amount of energy the star
radiates and does not depend on distance. It will be a 10 L star regardless of
where you put it.
4. What is the final remnant left behind at the end of the life of a 15 M star? What event
precedes the formation of this object? Why are these (rare) massive stars both important for
life and perhaps dangerous to it?
(a) A neutron star (in the form of a pulsar). (A star must be over 25 M to form a
black hole.)
(b) The neutron star is formed when the star explodes as a supernova (Type II).
(c) Supernovae are important for the heavy elements they return to the interstellar
medium, which are needed to make planets in the next generations of stars. But
if a planetary system is too close to a supernova and finds itself within the gaseous
remnant, the cosmic rays could be deadly to life on those planets.
5. Why have astronomers introduced the concept of dark matter? Give two cases where dark
matter has been invoked to explain astronomical observations.
There are cases where motions seem to require more gravitational force than can be
provided by the observed (luminous) matter. Some examples are:
(a) The rotation curves of spiral galaxies remain flat in the outer regions instead of
dropping off, as would be expected based on the visible starlight.
(b) In clusters of galaxies, the galaxies move with high velocities, which indicates a
stronger gravitational field that predicted by the mass of the galaxies and gas seen
in the cluster.
There are other possible answers: cosmological models indicate a high density of matter
during the big bang, formation of large scale structure in the universe couldn’t occur in
the time available without dark matter, etc.
6. In discussing the growth of planets in the early solar nebula, the ice line is an important concept. Where was this ice line located? How was the type of planet which formed influenced
by its location relative to the ice line?
(a) It was located just inside the orbit of Jupiter, in the (outer) asteroid belt.
(b) Inside the ice line, only iron, silicates, etc. could condense, growing to form the
terrestrial planets. Outside the ice line, ices could condense as well, leading to much
larger bodies (15 Earth masses), which then could capture gaseous H and He from
the nebula, growing to the giant planets.
7. Define the concept of convergent evolution. Explain why each of the following is or is not
an example of convergent evolution:
(a) the evolution of eukaryotes from a symbiosis of archaea and bacteria
(b) the evolution of birds and bats
Convergent evolution is the independent evolution of similar features or adaptations by
organisms whose common ancestor did not have those features.
Case (a) is not convergent evolution as this is just a single line of development. Case (b)
is a good example: the common ancestor of birds and mammals could not fly, but each
evolved independently to transform their fore-legs into wings for this purpose.
9.
8. About how many different amino acids are used in living organisms? What happens to amino
acids in the ribosomes?
(a) 20 amino acids are used (out of many more that exist).
(b) In the ribosomes the amino acids are linked to make proteins.
10. What are cyanobacteria and why are they important for the development of life on Earth?
When did they flourish and what evidence of their existence has been found?
(a) Cyanobacteria are single-celled prokaryotes that live by photosynthesis. They consume CO2 and release O2 . They are important because it is believed that they are
responsible for generating the Earth’s oxygen rich atmosphere.
(b) They were probably present over 3 B yrs ago, but were very common 2 to 2.5 B yrs
ago, where microfossils are found and also stromatolites, which are layered rocks
formed from sediments trapped by growing mats of cyanobacteria.
11. The scales below indicate the sweep of time before the present in three intervals: 0-20 billion
yrs, 0-5 billion yrs, and 0-1/2 billion yrs (0-500 million yrs). On the appropriate scale, mark
the time of:
(a) the big bang
(b) the formation of our Moon
(c) the (hypothetical) period of chemical evolution
(d) appearance of an oxygen rich atmosphere
(e) plants first colonize the land
(f) the extinction of the dinosaurs
(a) 13.7 billion yrs
(b) 4.5 billion yrs
(c) 3.8 - 4.4 billion yrs
(d) 2.0 - 2.5 billion yrs
(e) 400 million yrs
(f) 65 million yrs