Swinburne Online Education Exploring the Solar System
Module 19: The Sun
Activity 1:
The Sun: Source of
Heat & Light
© Swinburne University of Technology
Summary:
In this Activity, we will investigate
(a) why the Sun is so important to astronomers ...
(b) the temperature at the surface of the Sun ...
(c) the brightness of the Sun …
and …
(d) a few basic definitions and ideas
This will include a discussion of the inverse square law, and
the definitions of luminosity and flux.
You will also be introduced to some of the units and
conventions used in astronomy:
• scientific notation
• the Kelvin temperature scale
• the astronomical unit
• the lightyear
• the parsec
And now, let’s have a look at the Sun … carefully,
because looking directly at the Sun can permanently
damage your eyesight.
Don’t look at the
Sun itself ...
… only at a filtered, lowintensity image
(a) Why the Sun Matters
The Sun is important to everything, living or non-living, in the
Solar System because:
• it is the gravitational centre around which the planetary
system moves
• it provides the planets with the heat and light
necessary for life and many other developments
“Sunrise, sunset …”
Since the earliest times, humans have realised the importance
of the Sun, venerating it as a deity or the chariot of a deity, and
sharing myths about why and how the Sun continues to rise
and set and what would happen if it didn’t.
A Model Star
Compared to most stars, the
Sun is not significant by any
means.
The Sun’s place in the
Milky Way Galaxy
However it provides a useful
model for us to study when we
seek information about stars in
general.
The Sun’s place in the
Solar System
(note: this is not actually the Milky Way,
but a similar spiral galaxy called NGC2997)
In particular,
• The Sun is much, much closer to us than any other star, and
therefore is a great deal easier to study in detail. The next
nearest star is about 250,000 times as far!
Earth to Sun:
about
15 millionths
of a light year
Earth to nearest star
(Proxima Centauri):
4.2 light years
“What’s a
lightyear?”
Earth to
Aldebaran:
60 light years
Earth to nearby
galaxies:
about 3 million light years
Also,
• The Sun has been studied by astronomers for thousands of
years, and therefore data are available for the Sun which are
not available for other stars.
Physical properties of the Sun
There are two very obvious things humans notice about the
Sun:
• it is a source of heat, and
• it is a source of light.
So the Sun’s temperature and brightness make a pretty good
place to start.
(b) Surface Temperature
“What’s
Kelvin?”
The surface temperature of the Sun is about 5780 Kelvin.
Interstellar space 3
Pluto 40
Jupiter 130
Mars 250
300
Earth
Venus
700
Boiling iron
3000
Betelgeuse
3500
The Sun
5780
Sirius A
10000
30000
Sirius B
0
5000
10000
15000
Temperature (K)
20000
25000
30000
How the Sun’s Temperature is measured
When you turn on a heater, the element will glow red until it
warms up. Then the colour will become closer to yellow, or to
white.
low energy &
temperature
high energy &
temperature
The same thing happens with iron as it heats up. It glows
orange at first, then becomes more yellow or white in colour as
it warms up. Scientists say that it emits like a “black body”. To
a good approximation, stars also emit like a “black body”.
We tend to associate blue with cold and red with heat, but
that’s only because of what our blood vessels do when the day
is cold or hot.
You have to forget all about that, in astronomy...
low energy &
temperature
The truth for the rest of the Universe is that
cooler stars are reddish;
hot stars are bluish.
high energy &
temperature
(c) Luminosity
The luminosity of a source of light is a
measure of the power it can provide: that is,
how much energy it puts out per second.
25 W
25 W = 25 joules per second
Luminosity will vary from star to
star ...
and will also vary
during the life cycle of
a star.
Luminosity
Time
The luminosity of the Sun at present
is 3.863 x
1026
“What does
1026 mean?”
Watts
equivalent to about 4,000,000,000,000,000,000,000,000
light globes.
This can be compared with the approximate luminosity of other nearby stars:
Betelgeuse
Antares
Arcturus
Procyon A
Sun = 1
0.1
1
10
100
luminosity
1000
10000
Flux
Flux is the power passing through a unit area, so the units of flux
are watts m-2 (watts per square metre), or joules s-1 m-2 (joules per
second per square metre).
1 square metre
The power of the Sun
(luminosity) is measured in
watts:
Power = 3.8 x 1026 W
Close to the Sun, the power
passing through a square metre
is high (for instance, on the
surface of Mercury)
1 square metre
Further from the Sun, the
power per square metre is
lower (for instance, on the
surface of Neptune)
The flux of energy at the surface of the Earth
depends on
• the luminosity of the Sun
• the distance between the Earth and the Sun,
according to the inverse square law
How bright?
Luminosity = 3.8 x 1026 W
What’s the
inverse
square law?
How far?
Distance = 1.5 x 108 km
Changes in luminosity
If you alter the setting on a heater in your home, you will
quickly feel the effect on your own temperature.
Similarly, the surface temperature of the Earth will vary during
the Sun’s history, as the luminosity of the Sun varies.
Changes in distance:
You get colder as you move further from the heater.
In the same way, the surface temperature of planets further
from the Sun is almost always lower than that of planets closer
to the Sun, largely because of the decreased energy flux.
* Remember that AU stands for Astronomical Units, and
1 AU is the average distance between the Earth and the Sun.
Not to Scale!
Just for interest:
Here is a graph showing the distance of the planets from the Sun (in AU) plotted against
their average surface temperature (in degrees K).
Average
surface
temperature
(K)
800
700
Venus is hotter than you’d
expect, as it is covered in thick
cloud that keeps in the heat
Mercury
600
500
400
300
200
100
0
Other than that, the further out
a planet is, the cooler it is
Earth
Jupiter
0.1
1
Pluto
10
Distance from Sun (AU)
100
This Activity focussed mainly on the temperature and
brightness of the Sun.
In the next Activity we will investigate more of the Sun’s
properties:
its mass and density, and what it is made of;
and we’ll take a first look at how it produces energy.
Image Credits
AAO: Clusters and nebulae © David Malin (reproduced with permission)
http://www.aao.gov.au/local/www/dfm/dark_frames.html
AAO: Sprial galaxy NGC2997 © David Malin (used with permission)
http://www.aao.gov.au/images/general/galaxy_frames.html
Stonehenge (reproduced with permission)
http://antwrp.gsfc.nasa.gov/apod/ap971217.html
NASA:
Solar flare
http://antwrp.gsfc.nasa.gov/apod/ap970918.html
Skylab
http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-spacecraft.html
Hubble Deep field
http://antwrp.gsfc.nasa.gov/apod/image/hst_deep_big.gif
Now return to the Module home page, and read more
about the Sun in the Textbook Readings.
Hit the Esc key (escape)
to return to the Module 19 Home Page
The Kelvin temperature scale 1
The Kelvin temperature scale is the same as the Celsius scale,
except that the definition of zero is different.
The Celsius scale specifies 0 degrees as the temperature at which
water freezes.
On the other hand, the Kelvin scale specifies 0 degrees as the
temperature of an object in which the kinetic energy of the
particles making up the object is at a minimum. This is called
absolute zero (0 K).
The Kelvin temperature scale 2
Therefore, 273.15 degrees Kelvin is the freezing point of water
and 373.15 degrees Kelvin is the boiling point of water.
The Celsius scale is 273.15 degrees “out of sync”:
Kelvin
0
Celsius -273
100
200
300
400
500
600
-173
-73
27
127
227
327
Melting
point of ice
Boiling
point of
water
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Activity
Scientific notation 1
In order to save writing heaps of zeroes, scientists and engineers use a system of
notation where very large numbers are written with the number of factors of ten as
an exponent.
For instance:
5 000 is written 5 x 103
6 000 000 000 is written 6 x 109
42 700 is written 4.27 x 104
Note that in scientific notation the aim is to present the number as a number
between 1 and 10 multiplied by a power of ten: 4.27 x 104
On the other hand, engineering notation always presents the power of ten as a
multiple of 3, e.g. 42.7 x 103
Scientific notation 2
Also, in order to save writing heaps of decimal places, scientists and engineers use
a system of notation where very small numbers are written with the number of
factors of ten as an exponent.
For instance:
0.007 is written 7 x 10-3
0.00000010436 is written 1.0346 x 10-7
0.000060001 is written 6.0001 x 10-5
Note that in scientific notation the aim is to present the number as a number
between 1 and 10 multiplied by a power of ten: 6.0001 x 10-5
On the other hand, engineering notation presents the power of ten as a multiple of 3,
e.g. 60.001 x 10-6
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Activity
Inverse square law 1
If something is being emitted with equal intensity in all
directions from a point source, it will obey the
“Inverse Square Law”.
Point source of light
Closer in, the intensity of light is
high as the light is only spread over
a small area
Further out, the intensity of
light is low as the light is
spread over a larger area
Inverse square law 2
Imagine that a star is emitting light equally in all directions.
At planet Alpha, the light is observed as
being fairly intense, as it is being shared
over a small area:
• small radius, therefore
• small area, therefore
• high light intensity.
At planet Beta, the light is being shared
over a larger area and so the intensity of
the light is far less:
• large radius, therefore
• large area, therefore
• low light intensity.
Star
Inverse square law 3
Mathematically,
Note that the flux is proportional to the
inverse square of distance, which is
where the law gets its name from!
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Activity
Units in Astronomy 1
Because astronomical distances and sizes tend to be so
large, our usual (Earthly) units of length (m, km and so on) are
clumsy. Instead you will frequently find astronomical
measurements made in one of these units:
• AU
• pc
• ly
AU = astronomical unit
= average distance
between Sun and Earth
= 1.496 x 1011 m
1 AU
Units in Astronomy 2
Another astronomical unit of measure is the parsec, which uses
angle to measure the distance to other stars, galaxies and so on.
Even though the unit arose from measuring angles, it is a
measure of distance and not angle
• AU (based on distance)
pc = parsec
• pc
= distance d at which 1 AU
perpendicular to the
observer’s line of sight
subtends an angle of 1
second of arc
= 3.086 x 1016 m
d (in parsec)
1 AU
Angle
(in seconds
of arc)
Units in Astronomy 3
A third astronomical unit of measure - the lightyear - came
from the knowledge that light takes a finite length of time to
travel through space.
The lightyear is the distance that light will travel in a year,
and 3.26 lightyears = 1 pc.
• AU (based on distance)
• pc (based on angle)
• ly
ly = lightyear
= distance that light travels
in one year
= 9.461 x 1015 m
1 ly (distance)
An event
happens
here ...
… and is
seen here a
year later
Look-back time 1
The lightyear is a particularly useful unit because it reminds us
that what we see actually happened a while back, when the light
left the star (or planet) that we are looking at.
Let’s imagine that three stars A, B and C are all “born” at about the
same time. Because the stars are at different distances from
Earth, and light coming from them travels at a finite speed, light
which arrives at our eyes simultaneously must have been emitted
from each star at a different time.
C
B
A
Look-back time 2
Light
received
from
star
was
emitted
very
recently.
Light
received
star
C Awas
emitted
long
ago.
It the
will
Light
from
star Bfrom
was
emitted
more
recently,
and
so
will see
the
star
about
looks
today.
give we
usWe
pictures
Cwill
close
to the
when
it as
was
first formed.
pictures
receive
ofofB
bejust
of
star
in it“middle
age”.
C
looks young
B
looks middle-aged
A
looks old
In this way, we can construct a series of images and ideas about the life
cycle of stars, using their distances to “travel in time” and see similar stars at
different stages of their development.
Pictures from C, then B, then A, will show us how that kind of star changes
with time.
Look-back time 3
For example ...
Distance = 1000 ly
We see this star as it
was 1000 years ago
Distance = 10 ly
We see this star as it
was 10 years ago
Distance = 100 ly
We see this star as it
was 100 years ago
... when we examine these three stars, we are looking not just out into space but back in
time: hence the term “look-back time”.
In practice, “look-back time” is most useful when studying distant galaxies.
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