Have you ever wondered what happens to stars as they get older? Explore the evolution of stars with different masses.
An unannotated H-R diagram showing temperature (in Kelvin) on the x-axis and luminosity (relative to the Sun) on the y-axis. Credit: ESO
A Hertzprung-Russell diagram shows the temperatures of stars plotted against their luminosities. The position of a star on the diagram provides information about its information about the phase of life it is currently in and its mass. The longest phase in any star's life is the so-called main sequence during which they burn hydrogen into helium. The diagonal branch on the plot shows the main sequence. Red dwarfs are cool and faint, they lie in the bottom-right corner. Blue supergiants are hot and bright, they lie in the top-left corner.
When a star exhausts all its hydrogen, it leaves the main sequence and becomes a red giant or a supergiant, depending on its mass. Stars with the mass of the Sun which have burnt all their fuel evolve finally into a white dwarf.
Before beginning the activity, clear a table or floor space and lay out your rulers at a 90° angle, these will act as the Y-axis and X-axis for your H-R diagram. The Y-axis will represent Luminosity relative to the Sun (L☉), this will be shown logarithmically from 0.0001 to 1000000. The X-axis will show Temperature in Kelvin (K), the hottest temperature should be at least 35,000K and the lowest around 2,000K.
Create a model of the Sun for reference. Using yellow paper, draw a circle with a radius of 2.5cm and cut it out. Place this on the H-R diagram at 6,000 K and 1L⊙.
2a. Hand out the Stellar Fact Sheets, one per student.
2b. Assign each student a star. For beginners or younger students (KS3), stick to the Main Sequence stars. If you are doing the activity with older students (A-level and above) or for the second time, include the Other Stars.
3. Assign each of your students a star. Using the model Sun prepared earlier as a reference for mass and colour, the students will each create a model of their star.
4. Ask the students to collect a piece of coloured paper. They must choose the correct colour for their star based on temperature (e.g. Betelgeuse should be red)
|O||30,000 - 60,000 K||Blue|
|B||10,000 - 30,000 K||Blue-white|
|A||7,500 - 10,000 K||White|
|F||6,000 - 7,500 K||Yellow-white|
|G||5,000 - 6,000 K||Yellow|
|K||3,500 - 5,000 K||Orange|
|M||< 3,500 K||Red|
5. They will need to draw a circle of the correct size relative to the Sun model and the other stars on the Stellar Fact sheet. Note that even the largest star will be no bigger than the size of an A4 sheet.
6. Assign any remaining stars to the students so that you end up with circles representing all stars on the sheet.
7. Next, ask the students to place their star on the H-R diagram one at a time.
8. Once all of the stars have been plotted, gather the class around the H-R diagram. Does your plot look like the H-R diagrams you looked at earlier (during the Star in a Box presentation)? Do you see the Main Sequence, red giants, white dwarfs etc? Ask your students the following questions:
a. What is the approximate temperature of the Sun?
b.What colour would a star with the following surface temperature be:
c. What factor affects the colour of a star?
d. What factor affects the luminosity of a star?
9. When the students have finished the online activity, gather them around the H-R diagram again. Ask them to answer the following questions:
a. Most of the stars on the H-R Diagram are classified as which type of star?
b. What type of star has a high temperature but a low luminosity?
c. What type of star has a high temperature and a high luminosity?
d. What type of star has a low temperature but a high luminosity?
e. What type of star has a low temperature and a low luminosity?
f. Is the surface temperature of a white dwarf higher or lower than a red supergiant?
g. What property of a star uniquely determines where it will be on the Main Sequence?
h. If you increase the temperature of a star and leave it’s size the same, which way would it move on the H-R diagram?
i. How do we know a star is larger than the Sun from it’s position on the H-R diagram?
j. What do we know about stars directly below the Sun on the H-R Diagram?
8a. 6000 K
8b. Blue; Red; Blue-White
8d. Size and Temperature
9a. Main Sequence
9b. White Dwarfs
9c. Blue Giants
9d. Red Giants
9e. Red Dwarfs
9i. It is higher on the y-axis but the same place on the x-axis
9j. They are smaller