Download Topic E: Astrophysics

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The following notes were taken primarily from Physics for IB by
Chris Hamper and Physics Course Companion by Tim Kirk
Are they real?
 E.1.1
 Outline the general structure of the solar system.
 Students should know that the planets orbit the Sun in
ellipses and moons orbit planets. (Details of Kepler’s
laws are not required.) Students should also know the
names of the planets, their approximate comparative sizes
and comparative distances from the Sun, the nature of
comets, and the nature and position of the asteroid belt.
 Tons of interesting stuff about our solar system
 Aug. 2006 the International Astronomical Union declared
the official definition of a planet:
 A “planet” is a celestial body that:
a) is in orbit around the Sun
b) has sufficient mass for its self-gravity to overcome
rigid body forces so that is assumes a hydrostatic equilibrium
(nearly round) shape
c) has cleared the neighborhood around its orbit.
 8 Planets – name them….
 Elliptical orbits – to have a circular orbit an object must
have a very specific velocity. Any variations create and
elliptical or hyperbolic shape
 Moons
 Period - 27.3 days
 Which planets have them????
 Asteroids
 belt between Mars and Jupiter
 size – dust to hundreds kilometeres.
 Comets
 Similar to asteroids but made up of loose particles of ice and
 Tail is blown off by solar winds and melted by radiation.
 Some orbit, others only pass the sun once
 Planetoids???
 Pluto….
 Relative size video.
 E.1.3
 Define the light year.
 Light year (ly) – the distance that a beam of light will travel
in one year.
 How far is that? (3 x 108m/s = c)
 Used to measure distances outside of our solar system
Other important units.
 Astronomical unit (AU) – the average distance between the
Sun and Earth
 1AU = 1.5 x 1011m
 Used to measure distances inside our solar system
 Parsec (pc) – 1parsec = 3.26 ly
 Defined by making a triangle between the Earth, the Sun and a
distant object. If the angle at the distant object is 1 arcsec then
it would be 1 parsec away. (more later)
 E.1.2
 Distinguish between a stellar cluster and a constellation
 E.1.4
 Compare the relative distances between stars within a
galaxy and between galaxies, in terms of order of
 Stars are not evenly distributed.
 Stellar cluster – small groups of stars that gravitationally
interact with one another.
 Physically close to each other
 Closest star, besides the sun is Proxima Centauri - 4.25ly
 Galaxy – a very large number of stars bound together by
 Trillions of stars
 103 – 105 light years across
 Each star is approx. 1 ly apart
 Andromeda is about 2.5x106ly away
 Galaxy cluster – small group of galaxies that
gravitationally interact with one another
 There are about 20 other galaxies we are clustered with.
 Supercluster – bigger than a cluster
 The simplest explanation is that
 if all the gas is made into stars before the gas has time to form a
disk, then you get an elliptical galaxy.
 if the gas has time to stabalize into a disk before it is all used up,
then you get a spiral galaxy.
 Or perhaps some of the elliptical galaxies are made from
merging of other types of galaxies.
 Observations of distant galaxies indicates that spiral galaxies
were more common in the past than they are today.
 So maybe yesterday's spirals are todays ellipticals.
 This is an active research area. One problem is that if most of
the mass in galaxies is unaccounted for, we have a hard time
understanding the dynamics of galaxy formation.
 Constellations – groups of stars that are “linked” visually
 Ancient civilizations played “connect the dots”
 Located in the same general direction from Earth
 Not necessarily close to each other
 88 total
 Different ones are visible at different times during the year.
 E.1.5
 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.
 This is the basic background for stellar parallax. Other
observations, for example, seasons and the motion of
planets, are not expected
 Why do the stars move through the night sky?
Because the rotation of the Earth
It also matters where you are located on Earth
Ex. North Pole
 This rotation takes 23h and 56min every time.
 The effect is that it seams that the stars position at 12:00
changes each night.
 This means that the Earth rotates 360º in 23h and 56min.
 Which means 4min it will rotate 1º.
 Which means it only takes 360 DAYS for the constellations to
make one compete rotation.
 The Sun doesn’t make the same path through sky every
 For us, the summers are high in the sky, winters are low
on the horizon.
 This is because the axis of rotation for the Earth and the
axis in which we orbit around the sun aren’t the same
 The Earth is not a perfect sphere.
 This means that depending on it’s location in it’s orbit, it
will feel more or less pull from the Sun
This pulls on the Earths axis of rotation and makes it
This is technically called presession.
This means that the “North Star” won’t always be the
north star.
Period – 26,000 years
 The word planet comes from the Greek word for wanderer.
 Planets will shift back and forth in the night sky relative to
the constellation background.
 Apparent east/west motion comes from the Earth’s orbit
around the sun.
 See Diagram on board
 Apparent north/south motion come from the other planet’s
orbital plane being at a different angle from ours.
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