Stellar Temperature Measurement  

How hot is that Star?

Interpreting the HR Diagram

Look carefully at the diagram. Are all the stars located in one group? Explain you results.

Look at Star A. Describe its temperature: 

Describe its luminosity: 

You described the star as being and

Now look at Stars B through D. Describe each of them, and see if their brightness and temperature meet the expectation.

Star B:  and 

Star C:  and 

Star D:  and 

Are there any stars that seem out of place?. How might you explain them? Get with your group or neighbors and see if you can propose any solutions. How, for example, might a star be very very hot, but not very bright? What other factors might affect the overall brightness?

Relating Position on Diagram to Characteristics of Star

Astronomers reasoned that if a star were hotter, it should have a higher luminosity, and a cooler star would be dimmer. As it turns out, most stars fit this pattern. They can be found on the HR Diagram in the large group that stretches across the middle of the diagram. These are called the Main Sequence

Our sun is a main sequence star, as are many stars close to our solar system in our galaxy.

Other stars, however, have characteristics that place them i other groups on the chart. Stars that are cool but very luminous must be very large. These we call red giants or super giants.
Finally, there are stars that seem very hot, but dim. These are known as white dwarf stars. They shine with great intensity, but are so small that the total amount of light given off is very small. Imagine a very small flashlight.

If you look at it directly, it looks "bright", but could it light up you entire classroom?

Compare the flashlight to the classroom lights.
You can easily look directly at them, so they don't seem as bright, but are able to easily light up the entire room. This is why some very large, cooler stars are among the brightest in the night sky.

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