PRS questions
(before exam 1)
Note: PRS questions from the
last lecture before the exam will
be posted on Tuesday, Sept. 30)
There are at least 100,000,000,000
galaxies in the universe. They exhibit two
odd behaviors:
1. The vast majority of galaxies are
moving away from us.
2. The more distant the galaxy, the
faster it is moving away from us!
PRS question. True or false: this means
that there was a huge explosion, and since
all galaxies are moving away, we are close
to the center of the explosion.
1. TRUE
2. FALSE
1
The Milky Way: a disk-like object
Left side
Right side
From this picture, we
can conclude that the
inner galaxy is on
1. The right side
2. The middle
3. The left side
The Milky Way
• Our Galaxy is shaped like a disk.
• Our solar system is in that disk.
• When we look at the Milky Way in the sky, we are
looking along that disk, i.e., at the disk edge-on
2
The Milky Way: a disk-like object
But what are those dark
patches and streaks?
Answer: dust and gas
in space
Also, why is it bulged in one direction?
Answer: because the disk is bulged in that direction.
The current model of our galaxy…
Scientific model: a concept/representation/tool used
to think about how something works.
3
Constellations.
PRS question. True or false:
Constellations are important in
modern astronomy.
1. TRUE
2. FALSE
A Constellation is just a region of the sky
•
•
Most official constellation names come from antiquity. Some southern
hemisphere constellations were named by European explorers in the 17th
& 18th centuries.
The patterns of stars have no physical significance! Stars that appear
close together may lie at very different distances.
Constellations were useful
in prehistoric times to keep
track of the year, e.g., when
to plant or harvest the crops.
Modern astronomers
mainly use constellations
as landmarks, but their
usefulness is pretty limited.
4
PRS Question:
Modern astronomers use
constellations for which of the
following purposes?
1. For specifying the locations of objects in the sky
2. For naming celestial objects Yes, occasionally…
(not very often
3. For studying the ways that stars interact
and
though)
evolve
4. For determining when to plant crops and when
to harvest
For the most part,
5. All of the above
constellations are not
used in most modern
6. None of the above
astronomy
PRS question:
If you travel north from the
US into Canada, how will
Polaris (the North Star)
change?
1. It will be brighter.
2. It will be dimmer.
3.It will be higher in the sky.
4. It will be lower in the sky.
5. It will be exactly the same.
5
Polaris, the “North Star”
Straight up
at “A”
Suppose that “A”
is the North Pole.
The Earth rotates
around the North
Pole. At this place,
Polaris would be
straight up. It would
be as high in the sky
as it can be.
At “B”, Polaris would be
just barely above the
horizon. The angle
between “straight up”
and Polaris would be
large.
Straight up
at position “B”
PRS Question:
Most stars rise and set, but
some never rise or set (the
circumpolar stars).
Is this statement universally
true for any observer on
Earth?
1. Yes. At any place, some stars will be
circumpolar, and some will rise/set.
2. No.
6
Consider an observer at
the North or South Pole:
At midnight, the
star is here.
The observer
can still easily
see the star.
At noon, a star is
here.
The observer
can easily see
it at noon.
At the North or South Pole, all stars are circumpolar.
Nothing rises or sets!! (We would not say “most stars
rise and set” if we lived at the North Pole.)
Imagine
axis of
rotation instead.
Seasons are due to the tilt
of thethis
earth’s
axis.
PRS QUESTION: Suppose that the tilt of the Earth’s axis of
rotation were 90 degrees instead of 23.5 degrees, and the North
Pole faces the Sun in June. HOW MANY HOURS OF SUNLIGHT
WOULD WE HAVE IN AMHERST IN DECEMBER?
7
Day length
= 24 hours
Day length
= 0 hours
Day length
= 12 hours
In this imagined Solar System, the amount of daylight in
Amherst at different times of year would be as indicated…
PRS question. We are made from a variety of
elements; carbon is a particularly important
example.
True or false: most of the atoms from which
we are made were once deep inside the Sun.
1. TRUE
2. FALSE
Yes, the Sun creates heavier elements in its interior.
However, those elements remain stuck deep inside.
Something must happen to the Sun to get the stuff out!
8
PRS Question
At aphelion, does a planet move faster,
slower, or the same speed as it does at
perihelion?
1. Faster
2. Slower
3. The same
speed
Kepler’s Third Law
3. The ratio of the cube of a planet’s average
distance from the Sun to the square of its
orbital period is the same for each planet.
a3 / P2 = 1
a3 = P2
(a must be in AU and
P in years)
First correct mathematical formulas in astronomy!
9
Most asteroids are located between
Mars and Jupiter. If an asteroid has an
average distance of 3AU from the
Sun, how long does that asteroid take
to orbit the Sun (approximately)?
1.
2.
3.
4.
5.
0.5 years
1 year
2 years
5 years
50 years
Kepler’s 3rd law: P2 = a3
•
•
•
•
•
a = 3 AU
a3 = 3 x 3 x 3 = 27
P2 = 27, so P = square root (27) = √27
√27 is roughly equal to √25
√25 = 5 years
10
Phases of the Moon PRS question.
This month, the new moon was on September
29th. When will the next full moon occur?
1.
2.
3.
4.
5.
September 29th
October 1st
October 7th
October 15th
Halloween
PRS Question
If the Moon is relatively far from the Earth so
that its umbra does not reach the Earth’s
surface, someone directly below the umbra will
observe:
1. a penumbral lunar eclipse
2. a partial lunar eclipse
3. a partial solar eclipse
4. an annular eclipse
5. no eclipse
11