1. The Properties of Stars

2. Stars at Night  When the sun sets, and the Moon is not out we quickly discover that our sky is filled with many thousands of stars.

3. Stars at Night  Constellations, such as the Pleiades, are seen throughout the sky.

4. Stars at Night  Upon closer examination we discover that these stars emit different levels and colours of light.  Take the stars in the constellation Orion for example.

5. Light  We are able to see these colourful stars because of the light they emit.  But what is light?

6. Light  Light is a form of energy.  It travels in a straight line unless affected by extreme gravity.  Like water, it travels in wavelengths.

7. The Electromagnetic Spectrum  There are many forms of light.  All of them, including visible light, are a form of electromagnetic radiation.

8. The Electromagnetic Spectrum  All of the different forms of electromagnetic radiation (light) can be placed on a chart or spectrum.

9. The Electromagnetic Spectrum  Stars give off all these different forms of light.  Our eyes, and light collecting telescopes, can only see a very small portion of the spectrum called visible light.

10. Star Brightness  Star brightness is dependant on three things: ProximityProximity TemperatureTemperature Size/MassSize/Mass

11. Star Brightness-Proximity  Imagine a car was parked on a hill one kilometer from you.  At the same time a friend was standing next to you with a small pocket flashlight.  Each light was shone in your eyes.  Which would appear brighter?  Which is really brighter?  Star brightness works the same way.

12. Star Brightness-Proximity  The actual brightness of a star is dependant on how much energy it emits.  Its apparent brightness is determined by this factor but also how far or close it is to Earth.

13. Star Brightness-Temperature  Temperature can affect brightness.  Which has the highest temperature: A light bulb filament.A light bulb filament. A heating element.A heating element.

14. Star Brightness-Temperature  Different temperature stars have different brightness.  The hotter the star the brighter the star.

15. Star Brightness-Colour  Temperature also affects the colour of a star.  The cooler the star the closer it is to red in colour.  The hotter the star the closer it is to blue-white in colour.

16. Star Brightness-Colour  Betelgeuse, a red giant, has a lower temperature (and therefore brightness) than our sun.

17. Star Brightness-Colour  Rigel, a large white supergiant, is not only large but extremely hot.  It is therefore extremely bright.

18. Hang on…  Let’s not get ahead of ourselves.  Turn to page 462 and …

19. Star Temperature-Colour  As we have seen star temperature can affect brightness.  Different star types also have different temperatures.  They can be linked to colour.

20. Star Temperature-Colour  Generally speaking red stars have the lowest temperatures while blue-white stars are the hottest.

21. Star Colour and Composition  While colour can tell us about brightness and temperature it can also tell us what a star is made of.  To do this they use something called a spectroscope.

22. Star Colour and Composition  A spectroscope acts like a prism, dividing light into the different spectral colours.

23. Star Colour and Composition  When light leaves a star it passes through the elements in the star.  This leaves darkened lines on the spectrum of light as some of the elements block out that light.

24. Activity  Turn to page 465 in your text and read the instructions with your teacher, then complete the analysis questions.

25. Star Size and Mass  Stars come in many different sizes.  Star size is measured once we know the distance to the star from Earth. (This is done using something called parallax.)  Combining this knowledge with the brightness and temperature of the star astronomers can deduce the size of the star.

26. Star Size  Our sun is the baseline.  Other stars can be as small as 0.1 the radius of our sun or as large as 1000 times as large.

27. Star Size  Arcturus is a red giant and the third brightest star in the sky.

28. Star Size  VY Canis Majoris is the largest known star in the universe.

29. Star Mass  For many years there was no way to calculate the mass of any star, even our sun.  In order to calculate the mass of a large object (like a star) you need to see how gravity affects it.  Stars are so big that they are not affected by the gravity of planetary objects.

30. Star Mass  What was needed was another large object close to a star, such as another star.

31. Binary Stars  A binary star is a star system where two stars orbit one another.  Their orbits allow astronomers to calculate the mass by observing how the gravity of one affects the gravity of the other.

32. Solar Mass  Star mass is expressed as solar mass.  The Sun has a solar mass of 1.  All the others are compared to the sun.

33. Putting It All Together The Hertzsprung-Russell Diagram  To make sense of all this two astronomers, Enjar Hertzsprung and Henry Norris Russell devised a diagram to better explain it.

34. The Hertzsprung-Russell Diagram

35.  Scientists began to notice something about the diagram.  Most stars fit in the center. They were referred to as main sequence stars.

36. The Hertzsprung-Russell Diagram  Other stars were outside this main sequence.  Why?  Were they special in some way?  Exceptions to the rule?

37. Homework  Please read all of section 14.1, from pages 462 to 467.  Answer questions 1-6 on p.467  Vocabulary Electromagnetic SpectrumElectromagnetic Spectrum Binary starsBinary stars Solar MassSolar Mass

38. Homework to Hand In Page 466-467  Please complete a hand drawn and coloured version of the Hertzprung-Russell Diagram.  Your diagram should be neatly presented on unlined paper. Don’t forget to clearly label each axis and have the diagram coloured.  With your diagram you need to include an explanation (approximately 1-2 paragraphs), in your own words of how the diagram works.