The Solar System
You need to know the names of the planets
Planets orbit in ellipses
An ellipse is a “flattened circle” with two foci
about which the planet orbits.
Moons orbit the planets in much the same way.
Mass: 1.99 x 1030 kg
Radius:6.96 x 108 m
Planet Picture Distance to the Sun
Diameter(km) Orbital period
Mercury 58 million 4 878 km 59 days 88 days 167 5,43 0
Venus 108 million 12 104 km -243 days 225 days 464 5,24 0
Earth 149,6 million 12 756 km 23, 93 h 365,2 days 15 5,52 1
Mars 228 million 6 794 km 24h 37min 687 days -65 3,04 2
Jupiter 778 million 142 800 km 9h 50min 30s 12 years -110 1,32 +63
Saturn 1 427 million 120 000 km 10h 14min 29,5 years -140 0,69 +56
Uranus 2 870 million 51 800 km 16h 18min 84 years -195 1,27 27
Neptune 4 497 million 49 500 km 15h 48min 164 years -200 1,77 13
5 900 million 2 400 km 6 days 248 years -225 2 1
More info at http://nssdc.gsfc.nasa.gov/planetary/factsheet/
Use the table to compare
The diameter of the Earth and
The distance to the Sun of Jupiter and
Another quantity of your choosing for 2
Nebula is an interstellar cloud of dust, hydrogen gas and
plasma. It is the first stage of a star's cycle but it can also refer
to the remains of a dying star (planetary nebula).
Originally nebula was a general name for any extended
astronomical object, including galaxies beyond the Milky Way
(some examples of the older usage survive; for example, the
Andromeda Galaxy was referred to as the Andromeda Nebula
before galaxies were discovered by Edwin Hubble).
Nebulae often form star-forming regions, such as in the Eagle
Eagle Nebula and the Cone nebula:
Cat’s Eye Nebula
Planetary nebulae are nebulae that form from the gaseous
shells that are ejected from low-mass giant stars when they
transform into white dwarfs.
A galaxy is a collection of a very large number of stars
mutually attracting each other through the gravitational force
and staying together. The number of stars varies between a
few million and hundreds of billions. There approximately
100 billion galaxies in the observable universe.
There are three types of galaxies:
- Spiral (Milky Way)
- Elliptical (M49)
- Irregular (Magellanic Clouds)
Spiral galaxies consist of a rotating disk of stars and
interstellar medium, along with a central bulge of
generally older stars. Extending outward from the
bulge are relatively bright arms.
Elliptical cross-section and no spiral arms.
They range in shape from nearly spherical to highly flattened
ellipsoids and in size from hundreds of millions to over one
In the outer regions, many stars are grouped into globular
• Irregular galaxies have no specific structure. The
Large and Small Magellanic Clouds, the nearest
galaxies, are an example of irregular galaxies.
Small Magellanic Cloud Hoag's Object, a ring galaxy.
COMETS are frozen balls of ice and dust that
can resemble a “dirty snowball”.
They orbit the Sun is highly elliptical orbits.
Their orbital periods can range from a few
years to several thousand years.
Halley's Comet is famous due to the fact that
everyone has a chance to see it in their
lifetime (Orbital Period of 77 years).
Solar System Simulation
Why were ancient civilisations so
interested in the motion of the planets?
Is imagination the best way of knowing
for gaining knowledge about the
Stars are formed by interstellar dust coming
together through mutual gravitational
The loss of potential energy is responsible
for the initial high temperature necessary for
The fusion process releases so much energy
that the pressure created prevents the star
from collapsing due to gravitational
are needed in
order to begin
usually 107 K.
H + 1
He + 25 MeV
Must overcome the coulomb (electrostatic) repulsion
between the nuclei so that they can fuse together.
In Stable Stars there is an equilibrium between
the gravitational attraction of all of the gas and dust
… the outward pressure exerted by the nuclear
This keeps a stable star from collapsing or
A star is a big ball of gas, with
fusion going on at its center,
held together by gravity!
There are variations between stars, but by and
large they’re really pretty simple things.
The SI unit for length, the metre, is a very small unit
to measure astronomical distances. There units usually
used is astronomy:
The Astronomical Unit (AU) – this is the average distance
between the Earth and the Sun. This unit is more used within
the Solar System.
1 AU = 150 000 000 km
1 AU = 1.5x1011m
The light year (ly) – this is the distance travelled by the
light in one year.
1 ly = 9.46x1015 m
c = 3x108 m/s
t = 1 year = 365.25 x 24 x 60 x 60= 3.16 x 107 s
Speed =Distance / Time
Distance = Speed x Time
= 3x108 x 3.16 x 107 = 9.46 x 1015 m
The distance across our galaxy, The
Milky Way is 80 000 light years.
Our nearest neighbouring galaxy, The
Andromeda galaxy, is 2.2 million light
The furthest galaxies that can be
detected with the Hubble Telescope are
over 10 billion light years away!!!
This marks the edge of the detectable
It is a big place!
The parsec (pc) – this is the
distance at which 1 AU subtends an
angle of 1 arcsecond.
1 pc = 3.086x1016 m
1 pc = 3.26 ly
“Parsec” is short for
1 parsec = 3.086 X 1016 metres
(206 000 times
the Earth is
from the Sun)
Bjork’s Eyes Space
Parallax, more accurately
motion parallax, is the change of
angular position of two
observations of a single object
relative to each other as seen by
an observer, caused by the
motion of the observer.
Simply put, it is the apparent
shift of an object against the
background that is caused by a
change in the observer's position.
Baseline – R
Parallax - p
We know how big the Earth’s orbit is, we measure the shift
(parallax), and then we get the distance…
For very small angles tan p ≈ p
In conventional units it means that
360 degrees (360o)
in a circle
60 arcminutes (60’)
in a degree
(60”) in an
Parallax has its limits
The farther away
an object gets,
the smaller its
Eventually, the shift
is too small to see.
The parallax angle for Barnards star
from the Earth is 0.545 arc secs. What
is its distances in ly, parsecs and AU
The parallax angle for 61 Cygni star
from the Earth is 0.333 arc secs. What
is its distances in parsecs and AU
Using d = 1/p to find parsec
1.83 pc, 376000AU, 5.96ly
What is the most important
thing about a star?
The mass of a normal star almost
completely determines its
LUMINOSITY and TEMPERATURE!
Note: “normal” star means a star that’s
fusing Hydrogen into Helium in its centre
(we say “hydrogen burning”).
The LUMINOSITY of a star is
the TOTAL ENERGY emitted per
time from the surface of the star:
The energy the Sun emits is generated
by the fusion in its core…
This light bulb has a
luminosity of 60
What does luminosity have to do
The mass of a star determines
the pressure in its core:
Gravity pulls outer layers
Gas Pressure pushes
weight of the
The more mass the star has,
the higher the central pressure!
The core pressure determines
the rate of fusion…
MASS PRESSURE &
…which in turn determines
Luminosity is an intrinsic property…
it doesn’t depend on distance!
This light bulb has a luminosity
of 60 Watts…
…no matter where it is, or
where we view it from, it will
always be a 60 Watt light bulb.
The Luminosity of a star is the energy that it releases
per second. Our Sun has a luminosity of 3.90x1026 W
(often written as L): it emits 3.90x1026 joules per
second in all directions.
The energy that arrives
at the Earth is only a
very small amount
when compared will the
total energy released by
The ancient Greeks classified stars by
their brightness using the naked eye.
They were quite good at it. Have we
lost skills because of our reliance on
technology? Is this a concern?
When the light from the Sun reaches the Earth it will
be spread out over a sphere of radius d. The energy
received per unit time per unit area is b, where:
b is called the apparent
brightness of the star
The apparent brightness is directly
proportional to the star’s luminosity and
varies as the inverse square of the stars
The Sun is a distance d=1.5 x 1011 m from the Earth.
Estimate how much energy falls on a surface of 1m2
in a year.
L= 3.90x1026 W
At a distance of d=1.5 x 1011 m, the energy is “distributed”
along the surface of a sphere of radius 1.5 x 1011 m
The sphere’s surface area is given by:
A = 4πd2 = 4 π x (1.5 x 1011)2 =
=2.83 x 1023 m2
The energy that falls on a surface area of
1m2 on Earth per second will be equal to:
b = L/A = 3.90x1026 / 2.83 x 1023 =
= 1378.1 W/m2 or 1378.1 J/s m2
In a year there are: 365.25days x 24h/day x 60min/h x
60s/min = 3.16 x 107 s
So, the energy that falls in 1 m2 in 1 year will be:
1378.1 x 3.16 x 107 = 4.35 x 1010 joules
d = 1 / p
L = 4πd2 b
L = 4πR2 σT4
Distance measured by parallax:
b = L / 4πd2
Surface temperature (T)
L = 4πR2 σT4
Distance measured by spectroscopic parallax / Cepheid variables: