Week 6: Classifying the stars

Introduction

Last week you started to look at what stars are, using the Sun as a template. We discussed the basic properties of the Sun (and indeed other stars), finding that they are essentially massive balls of hot gas. At their core, temperatures rise high enough to permit the fusion of hydrogen nuclei to form helium nuclei. Using Einstein’s famous equation you were able to understand how the vast number of such reactions occurring every second is sufficient to power the Sun for billions of years. Finally, we started looking at how the Sun fitted into the wider population of stars by looking at the properties of stars in clusters, comparing their colour and luminosity.

This week, you will continue to explore the relationship between the colour and luminosity of stars, finding that the colour of a star depends on its temperature and that there is a correlation between temperature and luminosity. Plotting these two properties against each other produces the Hertzsprung-Russell diagram – one of the most famous diagrams in astronomy. you will explore this diagram and find out what it tells us about the lifecycles and eventual fate of the stars, as Jo explains:

Download this video clip.Video player: boc_aot_1_video_week6_intro.mp4

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Transcript

INSTRUCTOR:

So this week, you need to think back to those beautiful, Messier object images that you took using COAST back in week four. Whether you realised it or not, the image that you took, if it was a nebula, was a beautiful cloud of gas and dust that can represent either the very beginning or the very end of a star's life.

This week, we're going to be looking through that entire process of stellar evolution from a cloud of gas and dust right at the beginning through the star's life cycle, its expansion, its contraction, and then finally, possibly, to an explosion producing yet another cloud of gas and dust at the other end. So as we study our way through this process, make sure that you keep that image that you took using COAST in mind.

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In the video, Jo also mentions the images that you have taken using COAST. As you work through the material this week, you can continue to enhance and improve your COAST images using the editing tools in telescope.org.

By the end of this week you will be able to:

• edit COAST images in telescope.org and save the resulting image
• understand the relationship between stellar temperature and luminosity and how this is used to classify stars on the Hertzsprung-Russell diagram
• identify the main features of the Hertzsprung-Russell diagram and associate these with different stages in a star’s lifetime
• understand how the temperature and luminosity of a star depend on the mass of a star and how this ultimately determines the star’s lifetime
• understand the lifecycles of stars and the processes taking place at different stages of a star’s life, including the reasons for instability and how this produces certain types of variable star.