R136a1 is over 8 million times brighter than the sun in terms of absolute magnitude, but its great distance means it appears dimmer than the sun from Earth. This difference between absolute and apparent brightness is due to distance. Apparent magnitude ranks how bright stars appear from Earth, while absolute magnitude ranks brightness independent of distance. Hertzsprung-Russell diagrams classify stars based on two properties: luminosity, which relates to size, and temperature, which relates to color from hottest blue to coolest red.

1. Motion and Brightness
1

2. Motion
Terms involved in planetary motion:
Orbit - the path of one object revolving
around another
Revolution - the time it takes for an
object to orbit another object
i.e. Earth’s revolution is 365.24 days
Rotation - the turning of an object
around an imaginary axis running
through it
i.e. Earth’s rotation is approximately 24 hours

3. Motion
We see the moon, planets and stars moving in the sky:

4. Motion
From the earth, it looks as though the sun and moon
orbit around us. But is this their real motion path?

5. Motion
Motion is referred to as either
Real or Apparent.

6. Motion
Does these pictures show
real motion or apparent motion?

7. Motion
Apparent motion – what we see in the night sky.
Does not always represent how the observed
object is actually moving.
Real motion – How an object is actually moving.

8. Motion
Does these pictures show
real motion or apparent motion?

9. Motion: Retrograde motion of Mars
Retrograde motion - the movement of a planet from east
to west in the sky, rather than its normal motion from
west to east

10. Motion
Does these pictures show
real motion or apparent motion?

11. Motion
Does these pictures show
real motion or apparent motion?

12. Motion: Polaris
Polaris (the North Star) lines-up almost perfectly with the
Earth's axis over the north pole.
For this reason, it does not appear to move in the night sky
as the Earth rotates, unlike other stars.

13. Motion: Polaris
Stars appear to spin around Polaris, although they
don’t. It’s actually the earth spinning on its axis.
This is apparent motion.

14. Motion: Polaris

15. Motion: Polaris
INTERESTING FACT:
Over many years the Earth's axis slowly changes
alignment as it rotates and revolves. This "wobble"
is called Precession (much like a top or dreidel).

16. Motion
SUMMARY
Mars appears to change its direction, although it
doesn’t actually do this. This retrograde motion is
apparent motion.
The plants do orbit around the sun; thus this is real
motion.
Stars don’t spin around Polaris. It is the earth
rotating on its axis. This is apparent motion.

17. Brightness
The magnitude* of an object's brightness can
be referred to as either
Absolute or Apparent.
*brightness is often referred to as “magnitude”

18. Brightness
Which star is brighter?
The sun R136a1

19. Brightness
R136a1 is 8 700 000 times brighter than the sun
But its distance from the earth means that the sun
looks brighter in comparison

20. Brightness
The distance of R136a1 to the earth is so great that it does
not look as bright as the sun in comparison from earth
R136a1 earth sun

21. Brightness
Apparent magnitude ranks stars by how bright
they look from Earth.
The brightest star (Sol, otherwise known as the sun)
is -27, and the dimmest star is 36.

22. Brightness
Absolute magnitude ranks stars by how bright
they would look if they were all equal distances
(33 ly) away from Earth.
The brightest star is -13, and the dimmest star is 15.
Our sun is +5.
(Temperature)

23. Brightness
The sun is
around
this area
(Temperature)
Hertzsprung-Russel Diagram

24. Star Luminosity and
Temperature

25. Hertzsprung-Russel Diagrams
Stars are classified using 2 properties:
Luminosity – Magnitude of brightness from dim to
very bright. Directly related to size.
Temperature – How hot the star is. Directly related
to colour (blue is hottest, red is coolest).

26. Hertzsprung-Russel Diagrams

27. Hertzsprung-Russel Diagrams

28. Hertzsprung-Russel Diagrams