AS102OC JOURNEY THROUGH THE COSMOS, Final Exam Notes
Module Five: Our Star, the Sun, and other Planetary Systems
5-1: The Sun
The Sun has been generating light (across the entire spectrum) and its solar wind for about
5 billion years and is expected to continue, unchanged for another 5 (or so) billion years.
The Special Theory of Relativity is based on two, seemingly harmless, postulates:
o The laws of nature (physics) are the same for everyone and are the same
everywhere in the universe.
o The speed of light is the same for everyone.
Einsteins theory resulted from his thoughts about what it would be like to ride on a beam of
light. If the speed of light is the same for everyone then it must be independent of the speed
of the light source, a counter-intuitive outcome.
The STR makes the following predictions:
o The speed of light is the upper speed limit nothing can travel as fast as light.
o Moving objects shrink.
o Moving clocks run slow.
o The famous E=mc equation energy and mass are equivalent (mass is energy and
energy is mass).
The Sun is in a stable state because the inward gravitational pull is balanced by the outward
thermal pressure (push) caused by the energy released during the fusion process.
By watching sunspots (through a filter, of course) it becomes clear that the Sun is rotating.
In fact, it rotates faster at the equator (once every 27 days) than it does closer to either pole
(once every 31 days). We call this differential rotation.
As you start your journey into the Suns surface you will notice a sort of headwind called the
solar wind consisting of energetic photons, ions, and subatomic particles.
The region above the Suns surface is called the corona where the temperature is a
surprising 1 million K and is the source of many of the Suns X-rays.
5-2: The Cosmic Crucible
In fusion, radiation energy is produced when two lightweight atoms (such as hydrogen or
oxygen) are joined together.
In fission, energy is released when one heavyweight atom (such as uranium or plutonium) is
split forming two daughter atoms. Nuclear power plants produce energy by fission. Power
generation by fusion has been a goal of scientists for years but no one has yet discovered a
commercially viable way to do this.
In nature, there are four fundamental forces that run the universe:
o Gravity Inverse-square law with distance
o Electromagnetic Coulombs law describes this force, also an inverse-square law
o Strong nuclear Responsible for holding nuclei together and key in understanding
fusion; operates over a very short range
o Weak nuclear Important in some elementary particles
It is estimated that the Suns luminosity has increased in this time by about 30% meaning
that the Earth is now bathed in 30% more sunlight than 4 billion years ago. So, why wasnt
the Earth much hotter in the past?
The Earth has its own thermostat allowing it to maintain a fairly constant temperature over
the years, despite receiving an increasing level of solar radiation. The nature of this
thermostatic is likely related to the greenhouse effect which could have been stronger in
years past than it is today.
A stronger greenhouse effect would have resulted in a warmer surface temperature than
dictated by a smaller luminosity, although not warmer than today. Thus, it appears that the level of greenhouse warming has exactly balanced the increasing solar luminosity,
maintaining a fairly constant surface temperature on Earth.
The energy generated from the fusion reactions then starts its journey out toward the surface
of the Sun but, because of the circuitous route it must follow, it takes hundreds of thousands
of years to get there.
We are fairly certain that weve got most of it (solar) information correct by three main
techniques: mathematical modelling, studying solar surface vibrations, and observations of
Sunspots are irregularly-shaped dark regions on the Suns surface, or photosphere, where
the temperature is about 1800K cooler than the normal 5800K temperature elsewhere on the
Sunspots vary in size but typical ones are comparable to the Earths diameter, and they
appear dark because they are cooler and, therefore, emit less radiation.
It has been speculated that sunspots are sources of magnetic field lines which poke nearly
straight out from the interior; sunspots nearly always occur in pairs connected by magnetic
field lines which often soar high above the photosphere referred to as solar prominences,
rising to heights of more than 100,000 km above the surface.
As the magnetic field lines grow in strength and twist (and shout) solar flares often develop,
spewing out immense qualities of X-rays and charged particles which may, when they reach
Earth, affect the ionosphere (affecting radio communication), disrupt electrical and
communications on the Earths surface, pose a health hazard to astronauts in orbit, and
permanently affect the electronics of satellite operations.
Solar scientists now believe the Suns strong magnetic fields arising from sunspots carries
energy from the surface upward to the various layers above the surface into the Suns
o This energy appears as heat resulting in an increased temperature. The 1,000,000K
temperature in the corona appears to be the major source of X-rays from the Sun.
One particularly striking observation from this 400-year record is the very low level of solar
activity from about 1650 to 1720. This period, known as the Maunder Minimum, was
accompanied by unusually cool temperatures over much of the Earth. This period is also
referred to as the Little Ice Age.
Evidence points to other periods of quiet solar activity (from around 1475 until 1625) but
also stronger solar activity over a 200-year period centered around 1200. No one knows
why these longer-term variations happen.
5-4: Exoplanets: The Study of Distant Worlds
1. Stars orbit at the centre of the mass of the star-planet system so if we had sensitive
enough telescopes we should be able to detect the star moving from side-to-side indicating
the existence of an orbiting planet.
Because of the difficulty in measuring the small sideways displacement of the star and the
long observing time (years) needed to make the measurement, this simple technique has
seen limited use. Detection of such bodies by simply observing them through telescopes has
proved impossible and so we had to reply on a more indirect technique.
2. When a planet orbits a star it exerts a slight gravitational tug on the star, alternately
moving it toward us and away from us once each orbit. Thus, the light coming from the star is
alternatively blue-shifted, as the star moves towards us, and red-shifted, as it moves away
(the Doppler Effect).
Using the Doppler technique we can determine some planetary orbital properties such as
period, distance, and shapes as well as some bulk planetary properties (size, mass). Manydiscoveries of extra-solar planets have been made using this technique which has become
quite refined over the past decade although there are limitations.
3. If we observe the intensity of the light from the star and are able to measure the dip in
starlight intensity as the planet crosses in front of it, then we can infer the data a reasonable
approximation of the planets size.
A planet seen by a transit will most likely also experience an eclipse. Of course, the eclipse
occurs when the exoplanet goes behind the star resulting in the light from the planet being
blocked out so again the total light from the star/exoplanet combination will suffer a slight dip
in intensity during the eclipse period.
The mere existence of planets around other stars has changed our perception of our
place in the universe because it shows that our planetary system is not unique.
Module Six: Stars Properties and Life Cycles
Stars are classified according to their luminosity and their surface temperature.
A stars luminosity is the total amount of power it radiates into space. Luminosity cannot be
measured directly because its brightness depends on its distance from us as well as its true
In Grecian days, Hipparchus developed a magnitude system to classify stars according to
their brightness. This system is still in use today. The brightest stars are classified as first
magnitude, the next brightest as second magnitude, and so on to sixth or seventh magnitude
for the faintest stars.
A stars surface temperature determines the colour of light it emits. To understand this we
need to discuss some elementary thermodynamics.
o All bodies (even you) lose heat by three main processes conduction (through
physical contact with some cooler object), convection (by moving cooler air past you)