Astronomy: H-R Diagram and Stellar Life Cycles Worksheet

Question 1

On an H-R diagram, what two main stellar properties are usually plotted on the horizontal and vertical axes?

Accepted Answer

An H-R diagram usually plots surface temperature or spectral class on the horizontal axis and luminosity or absolute magnitude on the vertical axis. Temperature usually decreases from left to right, while luminosity increases upward.

Question 2

A star is very hot and very luminous. On what general region of the H-R diagram would it appear, and what type of star could it be?

Accepted Answer

A very hot and very luminous star would appear in the upper left region of the H-R diagram. It could be a massive blue main sequence star or a blue supergiant.

Question 3

A star has a low surface temperature but very high luminosity. Explain why this star is likely a giant or supergiant rather than a main sequence star.

Accepted Answer

A cool star with very high luminosity must have a very large surface area, because cool surfaces do not emit as much energy per unit area as hot surfaces. Its large size makes it likely to be a red giant or red supergiant.

Question 4

The Sun is a G-type main sequence star with a luminosity of 1 solar luminosity. Where is the Sun located on the H-R diagram?

Accepted Answer

The Sun is located on the main sequence near the middle of the H-R diagram. It has a moderate temperature and a luminosity of 1 solar luminosity.

Question 5

Compare the lifetime of a massive blue main sequence star with the lifetime of a low-mass red main sequence star. Explain the reason for the difference.

Accepted Answer

A massive blue main sequence star has a much shorter lifetime than a low-mass red main sequence star. Although massive stars have more fuel, they burn that fuel much faster because their cores are hotter and nuclear fusion occurs at a much higher rate.

Question 6

A star is plotted in the lower left region of an H-R diagram. It is hot but dim. What type of star is it most likely to be, and why?

Accepted Answer

The star is most likely a white dwarf. White dwarfs are hot but dim because they have very small radii and therefore emit little total light despite their high surface temperatures.

Question 7

Explain why the main sequence is called a sequence rather than a random scatter of stars.

Accepted Answer

The main sequence is called a sequence because stars fall along a clear diagonal band where temperature, luminosity, and mass are related. Main sequence stars are all fusing hydrogen into helium in their cores, so they follow a predictable pattern.

Question 8

List the main life cycle stages of a low-mass star similar to the Sun, from birth to final remnant.

Accepted Answer

A low-mass star forms in a nebula, becomes a protostar, enters the main sequence, expands into a red giant, sheds its outer layers as a planetary nebula, and ends as a white dwarf.

Question 9

List the main life cycle stages of a high-mass star, from birth to final remnant.

Accepted Answer

A high-mass star forms in a nebula, becomes a protostar, enters the main sequence, expands into a red supergiant, explodes as a supernova, and leaves behind either a neutron star or a black hole.

Question 10

A star has a luminosity of 100 solar luminosities and a surface temperature similar to the Sun. Compared with the Sun, what must be true about its radius?

Accepted Answer

The star must have a larger radius than the Sun. If two stars have similar surface temperatures, the one with greater luminosity must have a larger surface area, which means a larger radius.

Question 11

Use this simplified main sequence lifetime relationship: lifetime is approximately 10 billion years divided by mass squared, where mass is measured in solar masses. Estimate the main sequence lifetime of a star with 2 solar masses.

Accepted Answer

The estimated lifetime is 10 billion years divided by 2 squared, which is 10 billion years divided by 4. The star's main sequence lifetime is about 2.5 billion years.

Question 12

A star cluster contains many blue main sequence stars and no red giants. Is this cluster likely to be young or old? Explain your reasoning.

Accepted Answer

The cluster is likely to be young. Blue main sequence stars are massive and have short lifetimes, so they are still present only in relatively young clusters, and the lack of red giants also suggests little stellar evolution has occurred.

Question 13

A different star cluster has no blue main sequence stars, but it has many red giants and white dwarfs. Is this cluster likely to be young or old? Explain your reasoning.

Accepted Answer

The cluster is likely to be old. Its massive blue stars have already left the main sequence, and the presence of many red giants and white dwarfs shows that many stars have evolved into later life cycle stages.

Question 14

Explain how astronomers can use the main sequence turnoff point of a star cluster to estimate the cluster's age.

Accepted Answer

Astronomers find the point where stars are leaving the main sequence. The mass and lifetime of stars at that turnoff point indicate the cluster's age, because those stars are just finishing their main sequence phase.

Question 15

A student says, "White dwarfs are dim because they are cold." Correct this statement using evidence from the H-R diagram.

Accepted Answer

The statement is not correct because white dwarfs are often very hot and appear on the left side of the H-R diagram. They are dim because they are extremely small, so their total luminosity is low despite their high temperature.

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Astronomy: H-R Diagram and Stellar Life Cycles

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