1. Module : Activities: From H-R Diagrams Spectral Types Swinburne Online Education Exploring Stars and the Milky Way © Swinburne University of Technology

2. The seven classes The seven spectral classes are listed below. Traditionally, astronomers remember the sequence by the mnemonic “Oh, be a fine girl, kiss me!”. If you don’t like this, see if you can make up your own!

3. Some examples We’ll now browse the skies for samples of each classification. Type O This is Sirius (“scorching”), the Dog Star, one of the brightest stars in the sky. It is the jewel on the collar of Canis Major (the larger of Orion’s two hunting dogs). Sirius A is a type A star, but it has a dwarf companion (Sirius B) which is a type O but is too small to see here. Other examples include Meissa, or Orionis lambda in the constellation Orion (actually, in Orion’s helmet!).

4. Type B The sky over the Canary Islands includes the constellation of Orion the Hunter. Rigel is a type B star clearly visible to the naked eye below Orion’s “belt” and his “sword”. Type B

5. Type A Sirius is the brightest star in the constellation Canis Major (“Big Dog”). It is also the brightest star seen from Earth. One of the two stars that make up what we call “Sirius”,Sirius A is a type A star.

6. Type F Procyon I, a type F star, is the brightest star in the constellation Canis Minor (“Little Dog”).

7. Type G Our own Sun is a G- type star. This means that it is yellow in colour, and not very hot at all in stellar terms.

8. Type K Arcturus is the bright star under and to the left of Comet Hyakutake in this photo, which was taken in 1996 in New South Wales. Aracturus is the brightest star in the constellation Bootes, the Bear Driver.

9. Type M Betelgeuse is one of the best-known red giant stars. If Betelgeuse were to swallow the solar system, it would do so out as far as Jupiter! This terrific photo was taken by the Hubble Space Telescope.

10. This activity In this Activity we have had a look at the Balmer series, and how its occurrence in the photospheres of stars will vary with temperature. The temperature, and hence the colour and spectral line strength characteristics of stars, is used to classify them into types O, B, A, F, G, K and M-type stars.

11. Module : Activity: From H-R Diagrams Swinburne Online Education Exploring Stars and the Milky Way © Swinburne University of Technology

12. Summary: In this Activity the Herzsprung-Russell (H-R) diagram is introduced: how H-R diagrams came into use patterns and trends in H-R diagrams and a bit of history about classification of stars

13. O B A F G K M lowluminosityhigh White dwarfs Red dwarfs Main sequence Super- giants Giants H-R diagrams and spectral classes We’ll use this version of an H-R diagram to show how spectral classes appear in that format.

14. Looking for patterns hightemperaturelow lowluminosityhigh Huge, cool stars appear in the top right, and small, hot stars tend to gather in the bottom left. But the rest of the stars lie somewhere along the main sequence. Astronomers suspected that there might be some kind of laws or mathematical relationships between L and T and mass. T increasing L increasing Mass increasing

15. Mass and luminosity It turns out that there is a very nice relationship indeed. Luminosity (compared to the Sun) Mass (compared to the Sun) If you’ve got a line like this, then you can predict luminosity from mass, and vice versa We do more on how luminosity and mass are related in the next Activities Most of the stars on the main sequence lie in the band shown on the right. It means that these stars all “work” the same way, obeying the same laws.

16. What are they doing? hightemperaturelow lowluminosityhigh What are those laws? What could all these stars be up to that is the same, and means that there is a very clear link between their size and their energy output? The answer is: they are all producing almost all of their energy by hydrogen fusion in the core, and the end result is helium. Big stars producing lots of energy making helium Medium-sized stars producing energy making helium Teensy stars producing a wee bit of energy making helium

17. Luminosity classes The various families of bright stars form luminosity classes. (We won’t be discussing the white dwarves here.) Luminosity (compared to the Sun) Temperature (decreasing) Ia = most luminous supergiants Ib = less luminous supergiants II = luminous giants III = normal giants IV = subgiants V = main sequence stars

18. The evolution of a star is often drawn on a similar diagram, which can show how its temperature and luminosity change with time (just like Jim’s hair colour and hair length). Giants White Dwarfs Main Sequence

19. H-R diagrams are often used to compare the properties of stars of different luminosities and temperatures. For instance, a star to the right will be cooler and redder. A star higher up will be brighter, and often larger. Summary Cooler and redder To keep the picture simple, we’ve left off the “reverse” arrows (such as “Hotter and bluer”). Brighter (usually larger) Brighter & cooler (usually larger) Brighter & hotter (bigger/more mass)

20. In this Activity In this Activity we had a look at how stellar classification came about, how H-R diagrams were developed, and why the temperature scale goes “the wrong way”. In the next Activity, we will study a real example by continuing the story of how a huge, cool molecular cloud can become a blazing star, and we’ll use H-R diagrams to help us.

21. Image Credits Hubble Space Telescope picture of globular cluster G1 circling galaxy M31: http://oposite.stsci.edu/pubinfo/pr/96/11/A.jpg Star trails in the Southern Cross, © David Malin http://www.aao.gov.au/local/www/dfm/image/s_cross_trails.jpg

22. Image Credits Orion - star trail step focus photograph © David Malin http://antwrp.gsfc.nasa.gov/apod/ap980829.html Sirius, the Dog Star. Credit: ROSAT, MPE, NASA, Courtesy Skyview © Oniversity of Leicester http://antwrp.gsfc.nasa.gov/apod/ap960902.html Sky over the Canary Islands, showing Rigel. Credit and copyright A. Vannini, G. Li Causi, A. Ricciardi, A. Garatti http://antwrp.gsfc.nasa.gov/apod/ap980305.html Betelgeuse Credit A. Dupree (CfA), R. Gilliland (STScI), NASA http://antwrp.gsfc.nasa.gov/apod/ap970216.html Solar Disk in broadband K Ca Data. © J. Harvey, National Solar Ovservatory (Tucson/Kitt Peak, AZ). http://www.hao.ucar.edu/public/slides/slide2.html