# ASTR 217: Looking for Stellar Relations on Hertzprung-Russell (H-R) Diagrams

ASTR 101Name:

HR Diagrams: How They Work, How to Read Them, What They Tell Us

Hertzprung-Russell (HR) diagrams are a super-powerful tool for analyzing and understanding stars, how stars are “born” (come into existence), how stars “live” (exist and behave) through their stages of life…the list goes on.

Note that, for us to be able to portray the properties of a star on a Hertzprung-Russell diagram, the star has to emit radiation that gives it an intrinsic brightness which can be measured in units such as absolute magnitude.

In addition, the star needs to be at a temperature that is hotter than a planet and not as hot as a supernova, for us to portray the star on a normal Hertzprung-Russell diagram.

Therefore, black holes, neutron stars, and supernovae will not appear on normal HR diagrams.

1. On the HR grid below, the vertical axis is marked Mv.
2. What does Mv mean?
1. How is Mv different from mv?
1. Why do the numerical values of Mv units get smaller toward the top?
2. Mark the end of the vertical axis where the stars are less bright with the label “dimmer” and an arrow pointing in the direction of decreasing brightness.
In addition, mark the end of the vertical axis where the stars are brighter with the label “brighter” and an arrow pointing in the direction of increasing brightness.
1. Besides Mv, what other units are commonly used for the vertical axis of an HR diagram?
2. Along the right-hand side of the HR diagram, label it with solar luminosity units.
3. On the horizontal axis of the HR graph below, which spectral class is missing from the normal array of spectral classes?
4. Fill in the missing spectral class below the bottom left corner of the graph.
1. How many sub-types are there in each spectral class?
2. For instance, in the range of A type stars, how many A sub-types are there and what are their names?
3. Write down the spectral class of the sub-type halfway between K and M.
4. Which is the hottest type of star, O, B, A, F, G, K or M? Circle the hottest type.
5. Which is the coldest type of star, O, B, A, F, G, K or M? Circle the coldest type.
6. Which is hotter, a G2 star or a G5 star?
7. Mark the end of the horizontal axis where the stars are hottest with the label “hotter” and an arrow pointing in the direction of increasing temperature.
In addition, mark the horizontal axis end where the stars are colder with the label “colder” and an arrow pointing in the direction of lower temperature.
8. Draw the following zones on the HR diagram. To do so, draw a line around each zone on the HR diagram and write its label down inside the zone or adjacent to it.

hypergiants supergiants bright giants subgiants

main sequence subdwarfs white dwarfs

also label these two sub-zones in the main sequence: red dwarfs brown dwarfs

finally, write “blue” along the top horizontal axis, over the O and B spectral classes, and write “red” along the top horizontal axis over the L and M spectral classes

1. What are all main sequence stars doing in the cores?
1. What are (very nearly) all giant stars doing in their cores?
1. Using a Question 10 label, by far the largest population of stars is what type?
1. Using a Question 10 label, the second largest population of stars is what type?
2. When you look at the night sky with your naked eyes, can you see most of the nearest stars to earth?
3. Why or why not?

Hertzprung-Russell Diagram

In the HR diagrams below (modified from an article in Wikipedia), the axes are in log solar luminosities (vertical) and temperature in kelvins (horizontal). The tilted line approximates the main sequence.

1. Add labels and lines (mostly curves, actually) with arrows on the end of each (curvy) line, to show how a star such as the sun (a G2 star while it is on the main sequence) evolves through the following stages:
2. protostar (which may have formed in a Bok globule) to main sequence star
3. main sequence star to red giant to helium flash
4. ejecting a planetary nebula (which exposes an interior that is very hot but rapidly shrinking)
5. finishes losing mass, stops producing new heat, and enters the white dwarf stage

17. The HR diagrams below represent star clusters. As a star cluster ages, the O stars form first and then “die” first, exhausting their cores at a furious rate of thermonuclear fusion. Then the B stars “die.” Then the A stars, and so on. The farther right along the main sequence the cluster’s turnoff point is, the older the cluster. Above each cluster below, write down its approximate age, from the following list: 1 Myr, 10 Myr, 100 Myr, 1 Gyr, 10 Gyr. (*See note.)

(*Note: Myr means million years and Gyr means billion years.)

Introduction to HR Diagrams Lab1