Download Astrophysics - Student Reference Packet

IB Physics Option
Option E – Astrophysics
E1
Introduction to the Universe
E1.1
Outline the general structure of the solar system
Our solar system consists of a number of objects that orbit around a medium sized yellow star
commonly called the Sun. These orbits are elliptical in shape with the Sun located at one of
the foci of the orbital path. An ellipse . . .

is an egg like shape that has two foci. The
path of the ellipse is traced out such the sum
of the measured distances from a point on
the edge of the ellipse to each focus is
constant. a + b = constant

Eccentricity describes how flat an ellipse is. A circle would have an eccentricity of zero
and as eccentricity approaches one, the ellipse gets progressively flatter.
Objects that orbit the Sun include planets, moons, asteroids and comets.
What is a Planet? (essay from http://www.teachersdomain.org/resources/hew06/sci/ess/eiu/planetdefine/index.html)
Astronomers have attempted to develop a uniform standard of classification for the variety of
astronomical objects that have been, and continue to be, discovered. The International
Astronomical Union (IAU), founded in 1919 and composed of professional astronomers from
around the world, serves as the authority for naming celestial bodies and the surface features
found on them.
One of the most hotly debated issues for the IAU to resolve was the scientific definition of a
planet. For centuries, the common understanding was that a planet was a large object orbiting a
star. However, with the continual advancement of technology and astronomy, new objects were
being discovered that called upon the need for an official definition. This debate was fully ignited
in 2005 with the discovery of a new object in our solar system larger than Pluto. Originally
known as 2003 UB313, this object was eventually named Eris in 2006.
In August 2006, members of the IAU passed a resolution that defined a planet as
 a celestial body that is in orbit around the Sun;
 has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a
hydrostatic equilibrium (nearly round) shape (i.e., it assumes a nearly round shape due to
its own gravity);
IB Physics Option
Option E – Astrophysics

and has cleared the neighborhood around its orbit (i.e., it is the dominant mass in its
orbit).
According to this definition, our solar system has eight planets: Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, and Neptune. The IAU determined that Pluto has not cleared its
neighborhood because it orbits among the objects of the Kuiper Belt. As such, Pluto is no longer
classified as a planet, but rather as a dwarf planet. Eris has also been designated as a dwarf
planet. It is expected that the list of dwarf planets will increase while the number of planets will
remain at eight.
Significant controversy surrounds this definition of a planet. For the general public, it was
difficult to unlearn what they had been taught about the number of planets in the solar system
and to lose Pluto, often a sentimental favorite. However, in addition to the media frenzy over the
demotion of Pluto, there was also protest within the scientific community.
Among astronomers, the objection was not over the loss of Pluto as a planet but over the
wording of the definition, which is ambiguous. For example, what defines a "cleared
neighborhood," and how round is "nearly round"? In addition, the definition applies only to our
solar system, so there is no universal definition for a planet. Within one week of the resolution's
passage, more than 300 scientists signed a petition stating that they did not agree with the IAU's
definition of a planet and that a better definition was needed. As of 2006, the debate is not over.
The definition put in place by the members of the IAU who voted (only about 5 percent of the
world's astronomers) may yet be redefined.
The “eight” planets in our solar system
Planet Name
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Diameter
× 106 m
Size
relative
to Earth
4.88
12.1
12.8
6.79
143
120
51.8
49.5
0.4
0.9
1.0
0.5
11.2
9.4
4.0
3.9
Mean
distance to
Sun
× 108 m
58
107.5
149.6
228
778
1427
2870
4497
Distance to Sun
compared to
Earth
0.4
0.7
1.0
1.5
5.2
9.5
19.2
30.1
Number
of
known
moons
0
0
1
2
63
52
27
13
Moons are bodies that have been captured by a planet’s gravity and pulled into a regular orbit.
As planets orbit the Sun, their moons orbit along with them.
IB Physics Option
Option E – Astrophysics
Along with planets and moons there are large numbers of rocks orbiting the Sun called
asteroids. Asteroids vary in size from the large (Ceres at 900 km diameter) to small (only 1
m). Most asteroids are found in the appropriately named asteroid belt (shown below).
Comets are composed of ice and dust and sometimes referred to as dirty snowballs. The
orbits of comets are highly elliptical (a high eccentricity value) and pass very close to the Sun
to out beyond Pluto. Comets are small (less than 10 km in diameter) and are difficult to
detect until their orbital path brings them closer to the Sun than Jupiter. As the comet gets
closer to the Sun, heat vaporizes the ice liberating vapor and dust creating a massive (106 km)
glowing ball with a distinctive and long (108 km tail).
Haley’s comet is perhaps the most famous visible comet and has an orbital period of 76 years.
This is quite short compared to another recent visible comet, Hyakutake which has a period of
over 30, 000 years.
E1.2


Distinguish between a stellar cluster and a constellation
Stellar clusters are large groups of stars that were created at about the same time.
Gravitational force holds stellar clusters together.
A constellation is a collection of stars (or galaxies) that form a visible pattern on Earth.
A well known example is Ursa Major, the Big Dipper. Galaxies are large collections of
stars. The nearest galaxy to Earth is Andromeda (1022 km away) named for the
constellation it is part of.
IB Physics Option
Option E – Astrophysics
E1.3 Define the light year
A light year is the distance light travels in a year. Show through calculation, this value to be
close to 9.46 × 1015 m.
E1.4
Compare the relative distances between stars within a galaxy and between galaxies, in
terms of order of magnitude.
A galaxy is a large cluster of stars. Our Sun is part of the Milky Way galaxy, a spiral shaped
collection of over 200 billion stars (quite a large galaxy) over 100,000 light years in diameter.
From the “side”, the Milky
Way is shaped like the
diagram shown. The stars
in the Milky Way galaxy
orbit a common center of
mass called a galactic centre.
This diagram shows a “polar” view
of the Milky Way. The average
distance between stars in a galaxy is
in the order of 1017 m. This is the
approximate distance between our
Sun and Alpha Centauri (the next
nearest star). The Sun is
approximately 1020 m from the
galactic centre of the Milky Way.
The next nearest galaxy (called
Andromeda) is about 1022 m away.
This may not appear to be much but
compare it to the diameter of the
Milky Way.
IB Physics Option
Option E – Astrophysics
E1.5
Describe the apparent motion of the stars/constellations over a period of a night and
over a period of a year, and explain these observations in terms of the rotation and
revolution of the Earth.
Motion of Stars
When looking at the night sky in the northern hemisphere, the North Star (or Polaris) always
stays in the same place and the other stars appear to move in circular paths around this
“celestial pole”. They complete one rotation every 24 hrs (called diurnal motion) although we
can only observe part of their path (we can’t see them during the day because the scattered
blue light of our Sun is too bright). In ancient times, sky watchers believed the stars were
fixed to a giant rotating sphere that formed the outer boundary of the universe. We now
know that diurnal motion is not produced by the stars moving but by the Earth’s daily
rotation. The daily rotation affects the position of all objects in the sky but does not change
their relative positions.
Key Concept: the observed motion of stars at night is the path of an
arc that circles around the celestial pole (Polaris in the northern
hemisphere)
Although we know that there is no giant, rotating celestial sphere, it is
useful to think that way to create a reference system for locating and
studying the movement of stars and planets.
Imagine looking up at the “starry vault” or “celestial sphere” slowly
rotating around the pole star. The celestial equator is a projection of
the Earth’s equator on the celestial sphere. Directly overhead is an
imaginary point called the zenith (this would also be Polaris if you
were at the North pole).
Polaris (North celestial pole)
IB Physics Option
Option E – Astrophysics
Motion of the Sun
Due to the rotation of the Earth, the Sun rises in the East and sets in the West each day. The
East to West path of the Sun across the daytime sky varies depending on the time of year and
the latitude of the observer’s location.
Key Concept 1: An observer in the northern hemisphere sees the Sun pass to the South at
noon as it moves from East to West. An observer in the southern hemisphere sees the Sun
pass to the North at noon.
Regardless of the location of the observer, the Sun reaches its highest point in the sky at noon.
The measurement of the Sun’s elevation above the horizon is called altitude.
Key Concept 2: Over the course of the year, the noontime altitude of the Sun changes. In
the northern hemisphere:
 Maximum altitude occurs on June 21st (the summer solstice)
 Minimum altitude occurs on December 21st (the winter solstice)
In the southern hemisphere, the opposite occurs.
The maximum altitude of the Sun
corresponds to the longest day of the
year (in terms of daylight hours). In
summer, the Sun's path is longer, and
so are the days.
Similarly, the minimum altitude
corresponds to the shortest day of the
year and in winter, the shorter path of
the Sun results in shorter days.
The diagram shows the apparent path of the Sun across the sky at different times of the year
for an observer in the northern hemisphere. March 21st (the vernal equinox) and September
22nd (the autumnal equinox) indicate the approximate dates where the day length equals the
night length (12 hours) and the Sun’s position at noon lies on the celestial equator (directly
overhead if you stood at the Earth’s equator). The altitude of the celestial equator for any
latitude on the Earth can be simply calculated using the expression: CEaltitude = 90° - Latitude
In the northern hemisphere, this calculation would give the equinox altitude of the Sun above
the southern horizon at noon. The summer solstice altitude would be +23½  while the
winter solstice altitude would be -23½ .
Key Concept: the Sun’s altitude varies yearly over a range of 57
IB Physics Option
Option E – Astrophysics
The diagrams below shows the relative altitude of the Sun above the horizon for an observer
located at 4 different latitudes. The three different positions of the Sun are for different times
of the year.
50 North
23.5 North
90 (North pole)
0 (Equator)
At the North Pole, the Sun is not visible for 6 months of the year and varies in altitude
between the horizon and 23.5 for the other 6 months.
At the Equator, the Sun would be seen to pass across the sky to the South and to the North
depending on the time of year.
To see all of this in one great simulation (read the instructions first) go to
http://www.astro.uiuc.edu/projects/data/Seasons/index.html
IB Physics Option
Option E – Astrophysics
Motion of the Planets
Planets orbit the Sun in the plane of the ecliptic (just like Earth). This implies that our solar
system is relatively flat. Each planet has a unique orbital period that increases with distance
from the Sun.
If a series of lines where drawn from the Sun through the Earth and out into space, they
would trace out the path of the ecliptic on the celestial sphere.
See a beautiful picture (http://antwrp.gsfc.nasa.gov/apod/ap960921.html) of the moon, the Sun's
corona, Saturn, Mars, and Mercury lined up in the plane of the ecliptic, from the "Astronomy
Picture of the Day" Site.
Early astronomers observed the planets to move relative to the background stars on the
celestial sphere. The typical motion of the planets was eastward along the ecliptic but
occasionally planets were observed moving westward.
Key Concept: Planets move through the celestial sphere along the ecliptic. Normal motion
of the planets is eastward. Retrograde motion describes a temporary westward
movement before moving eastward again.
Observations of Mars in the diagram below show retrograde motion of the period of a few
weeks relative to the background stars. Normal motion is from west to east but Mars
occasionally changes direction for a brief period.
IB Physics Option
Option E – Astrophysics
For a java based simulation of retrograde motion check out this link
http://www.astro.uiuc.edu/projects/data/Seasons/index.html
Motion of the Moon
As the Earth orbits around the Sun on a yearly
basis, the Moon orbits the Earth. The Lunar
Phase cycle is 29.5 days long (called a synodic
month). This refers to the time required for the
Moon to circle the Earth and return to the same
position with respect to the Sun. This is actually a
bit more than one complete orbit of Earth (that
only takes 27.3 days)
Earth
Moon
Sun
The Moon is observed from Earth because it is
illuminated by the Sun. The different
appearances of the Moon are called lunar phases.
Lunar phases depend on the relative orientation
between the Sun, the Earth and the Moon.
One synodic
month
(29.5 days) later
The lunar phases are illustrated in the diagram below.
A = Noon : B = 6 PM : C = Midnight :
D = 6 AM
(Earth is rotating clock-wise)
The Moon will rise in the East and
set in the West.
At 6 PM (position B)
 a full moon would begin rising
 a first quarter moon would be
at its peak
 a waxing crescent moon would
be beginning to set
 a waning moon would not be
visible
Confused? Check out this link http://www.astro.uiuc.edu/projects/data/MoonPhases/index.html