Question 1

The main sequence lifetime of an O star compared with an M star is

Accepted Answer

O stars are much more massive than M stars and therefore the gravitational pressure is much higher. This results in a much faster burning rate of fuel and a shorter main sequence lifetime for O stars compared to M stars.

Question 2

Estimate the main sequence lifetime for a star that is twice as massive as the Sun and 8 times as luminous.

Accepted Answer

B

Question 3

A two solar mass star has about 2 times the Sun's supply of nuclear energy.Its luminosity is 10 times that of the Sun.How does the lifetime of the star compare with that of the Sun?

Accepted Answer

The lifetime of a star is roughly proportional to its fuel supply divided by its luminosity. Since this star has 2 times the Sun's fuel supply but is 10 times as luminous, its lifetime is 1/5th that of the Sun.

Question 4

The least massive stars in which nuclear fusion occurs have masses of about 8% that of the Sun.Why don't less massive objects ever experience nuclear fusion?

Accepted Answer

Nuclear fusion requires extremely high temperatures and pressures to overcome the repulsion between atomic nuclei. Less massive objects do not generate enough heat and pressure to achieve these conditions, preventing nuclear fusion from occurring.

Question 5

Massive stars on the upper end of the main sequence generate energy by means of the

Accepted Answer

Massive stars on the upper end of the main sequence generate energy by means of the CNO cycle, where carbon, nitrogen and oxygen act as catalysts for converting hydrogen into helium. This is different from smaller stars, which primarily use the proton-proton cycle to generate energy. The triple alpha reaction is primarily important in the later stages of massive star evolution, when helium nuclei fuse to form heavier elements in the core. Radiation pressure is a result of the energy produced by nuclear fusion reacting against the gravitational pressure of the star.

Question 6

Which of the following cannot be assumed about the stars in a given cluster?

Accepted Answer

The stars in a given cluster do not necessarily have the same mass, as the initial mass of a star depends on the amount of material available in its formation region. Therefore, different stars within a cluster can have different masses, despite having the same age and initial chemical composition.

Question 7

The main sequence turnoff of a cluster of stars is a good indicator of which of the following properties of the cluster?

Accepted Answer

The main sequence turnoff point is the point on the Hertzsprung-Russell diagram where stars are evolving away from the main sequence and towards becoming red giants. The location of the turnoff point is determined by the mass of the stars in the cluster, which in turn is determined by the age of the cluster. Therefore, the main sequence turnoff point can be used to estimate the age of the cluster.

Question 8

When a star is just evolving off the main sequence,which element is most common in the core?

Accepted Answer

When a star evolves off the main sequence, its core is primarily composed of helium. This is because the star has spent the majority of its main sequence life fusing hydrogen into helium in its core.

Question 9

Carbon,hydrogen,and helium can all be used a fuels during the career of a massive star.Which of the following gives these elements in the correct order in which they will be consumed in the star?

Accepted Answer

In the life cycle of a massive star, nuclear fusion begins with hydrogen (the most abundant element in stars) fusing into helium. Once the hydrogen is depleted, the star begins to fuse helium into heavier elements, including carbon. Therefore, the correct order in which these fuels are consumed is hydrogen, helium, then carbon.

Question 10

An evolutionary track describes the career of a star by showing changes in its location in

Accepted Answer

An H-R (Hertzsprung-Russell) diagram plots a star's luminosity (brightness) against its temperature, and is commonly used to display a star's evolution.

Question 11

If M is the mass of a star and L is its luminosity,which of the following is proportional to the main sequence lifetime of a star?

Accepted Answer

The main sequence lifetime of a star is proportional to its mass divided by its luminosity (M / L), because the lifetime depends on how much fuel (mass) the star has and how quickly it uses it (luminosity).

Question 12

Which of the following occurs just after a star like the Sun exhausts the hydrogen in its core?

Accepted Answer

When a star like the Sun exhausts the hydrogen in its core, the core contracts and heats up. As a result, the outer layers of the star expand, becoming cooler and redder, turning into a red giant.

Question 13

Cluster A has no stars of spectral type O,B,and A on its main sequence,whereas cluster B has stars of all types but O on its main sequence.Which of the following correctly describes the relative ages of clusters A and B?

Accepted Answer

The absence of stars of spectral type O, B and A on the main sequence of cluster A suggests that all stars of these types have evolved off the main sequence - either as giants, supergiants, or white dwarfs. This indicates a relatively advanced age for cluster A. On the other hand, cluster B has stars of all types except O on its main sequence, which suggests that O stars have evolved off the main sequence but the other types are still there. This implies that cluster B is younger than cluster A, but still old enough to have formed stars of spectral type B and A on its main sequence. Therefore, the relative ages of clusters A and B can be deduced from the presence or absence of certain spectral types on their main sequences, and it appears that cluster A is older than cluster B.

Question 14

What do all main sequence stars have in common?

Accepted Answer

All main sequence stars are consuming hydrogen in their cores, which causes nuclear fusion and releases energy. This is what keeps them stable and shining. Some other characteristics of main sequence stars can vary, such as their mass and age, but hydrogen fusion is a defining feature.

Question 15

What is the general name for a star that is converting hydrogen into helium in its core?

Accepted Answer

A star that is converting hydrogen into helium in its core is known as a Main sequence star. This is the stage where a star spends the majority of its life cycle, burning hydrogen to produce energy and balancing the force of gravity.

Question 16

The age of a cluster can be estimated by

Accepted Answer

Plotting the stars on an HR diagram and observing the lower turnoff point (where the stars start to depart from the main sequence) provides an estimate of the age of the cluster. The lower the turnoff point, the older the cluster is thought to be. Option A and B are contradictory, and options C and E are not the most accurate methods for estimating the age of a cluster.

Question 17

Based on your knowledge of the H-R diagram,which of the descriptions of a star cluster is astrophysical nonsense?

Accepted Answer

A star cluster with no red giants but a large number of white dwarfs is unlikely because white dwarfs typically form from red giants after they have exhausted their nuclear fuel.

Question 18

All the stars in a cluster are usually assumed to have the same

Accepted Answer

Since all the stars in a cluster are formed from the same molecular cloud at roughly the same time, they should have the same age. However, the mass, temperature, and main sequence lifetime can vary within a cluster depending on individual variations in the formation process.

Question 19

As soon as its core hydrogen is exhausted,a star like the Sun begins to evolve

Accepted Answer

When a star like the Sun exhausts its core hydrogen, it begins to evolve off the main sequence towards the upper right in the Hertzsprung-Russell (H-R) diagram, becoming a red giant. This is because the star expands and cools, which places it higher (brighter) and to the right (cooler) on the diagram.

Question 20

The most important factor affecting a star's main-sequence lifetime is

Accepted Answer

The mass of a star is the most crucial factor in determining its main-sequence lifetime. Stars with greater mass burn through their nuclear fuel more quickly, leading to shorter lifespans, while less massive stars burn their fuel more slowly, resulting in longer lifespans.

Quiz 19: The Evolution of Stars