Question 1

Which of the following statements about the main sequence stage in the life of a star is FALSE?

Accepted Answer

This statement is false. Main sequence stars are actually very common in the Milky Way, with estimates suggesting that they make up around 90% of all stars in the galaxy.

Question 2

As a star becomes a giant, its outer layers are expanding. Where does the energy for expanding these layers come from?

Accepted Answer

As a star becomes a giant, its outer layers expand due to the fusion of hydrogen into helium in a shell around the core. This fusion releases energy, which pushes the outer layers outward and causes the star to expand.

Question 3

If you wanted to discover the youngest stars you could find in some grouping of stars in the Galaxy, which type of star group would be the best to search?

Accepted Answer

Stellar associations are groups of young stars that formed together from the same molecular cloud. Therefore, they are the best choice to search for the youngest stars in a grouping.

Question 4

On an H-R diagram of a cluster of stars, which characteristic of the diagram do astronomers use as a good indicator of the cluster's age?

Accepted Answer

The point on the main sequence where stars begin to "turn off" -- to move toward the red giant region is known as the main sequence turnoff point. This point is a good indicator of the cluster's age, as stars of different masses evolve at different rates and stars near the turnoff point have just exhausted their core hydrogen and are beginning to evolve off the main sequence. The exact location of the turnoff point depends on the age of the cluster, with older clusters having turnoff points at lower temperatures and lower luminosities compared to younger clusters.

Question 5

A group of graduate students, bored during a cloudy night at the observatory, begin to make bets about the time different stars will take to evolve. If they have a cluster of stars which were all born at roughly the same time, and want to know which star will become a red giant first, which of the following stars should they bet on?

Accepted Answer

Type O stars, being the most massive and hottest among the main sequence stars, burn through their nuclear fuel much more rapidly than less massive stars. Therefore, a type O star would evolve into a red giant phase faster than the other stars listed.

Question 6

How long a main sequence star remains on the main sequence in the H-R diagram depends most strongly on

Accepted Answer

The length of time a main sequence star can remain on the main sequence is dependent on the amount of fuel it has to burn, which is determined by its mass. Higher mass stars burn through their fuel more quickly and therefore spend less time on the main sequence, while lower mass stars can remain on it for billions of years.

Question 7

As a cluster of stars begins to age, which type of star in the cluster will move off the main sequence of the H-R diagram first?

Accepted Answer

O and B type stars, which are the most massive and luminous, exhaust their nuclear fuel more quickly than lower mass stars. Therefore, they evolve off the main sequence first.

Question 8

When the outer layers of a star like the Sun expand, and it becomes a giant, which way does it move on the H-R diagram?

Accepted Answer

When a star like the Sun expands and becomes a giant, it moves toward the upper right on the H-R (Hertzsprung-Russell) diagram. This is because the star becomes cooler (moving right) and more luminous (moving up) as it expands.

Question 9

When a star first begins the long path toward becoming a red giant, a layer of hydrogen around the core begins to undergo fusion. If this layer was too cold to do fusion throughout the main sequence stage, why is it suddenly warm enough?

Accepted Answer

As the core collapses under its own weight during the post-main sequence stage, it heats up from the compression. This newly heated core then heats up the layer of hydrogen around it, making it warm enough for fusion to occur.

Question 10

How are globular clusters distributed in our Milky Way Galaxy?

Accepted Answer

Globular clusters are distributed mostly in a large spherical halo surrounding the flat disk of our Milky Way Galaxy. They orbit around the center of the galaxy and are not confined to the disk. The disk, on the other hand, is mostly populated by young stars and open clusters.

Question 11

A type of star cluster that contains mostly very old stars is

Accepted Answer

Globular star clusters are typically very old and contain mostly low-mass, evolved stars. This is due to the fact that they formed early in the history of the galaxy and have had little star formation since.

Question 12

An astronomy student, for her PhD, really needs to estimate the age of a cluster of stars. Which of the following would be part of the process she would follow?

Accepted Answer

The most common way to estimate the age of a cluster of stars is to plot the stars on an H-R diagram based on their spectral type and brightness. This will reveal the main sequence of stars as well as any red giants, and the location of a star on this diagram provides important information about its age and mass. Other options like measuring Doppler shifts or searching for x-rays or planets would not be as useful for estimating the age of a cluster. Counting the number of M type stars may give some information about the overall population of the cluster, but it would not directly help in estimating the age.

Question 13

The event in the life of a star that begins its expansion into a giant is

Accepted Answer

When almost all the hydrogen in the core of the star has been fused into helium, the core contracts and heats up while the outer layers of the star expand and cool, which marks the beginning of the star's expansion into a giant.

Question 14

Why do all stars spend most of their lives on the main sequence?

Accepted Answer

The main sequence is the stage where a star is burning hydrogen as fuel. Since hydrogen is the most abundant element in the universe, every star has enough of it to spend a long time burning it and remaining on the main sequence.

Question 15

An eccentric Hollywood movie producer, vacationing in Australia, goes to an observatory and offers to make a big donation if they can show him a place in the Milky Way Galaxy where there are more than a million stars seen together. Which object would satisfy his extravagant tastes?

Accepted Answer

The best choice would be the globular cluster Omega Centauri, as it is one of the largest and most massive clusters of stars in the Milky Way. It contains more than a million stars closely packed together, which would fulfill the producer's request. The other options, although visually stunning, do not contain as many stars as Omega Centauri.

Question 16

Which of the following types of stars will spend the longest time (the greatest number of years) on the main sequence?

Accepted Answer

The lifetime of a star on the main sequence is determined by its mass. The more massive the star, the faster it burns through its fuel and the shorter its life on the main sequence. K-type stars have a mass of about 0.6-0.8 solar masses, which is smaller than G, A, or O stars, and therefore they burn their fuel slower and spend the longest time on the main sequence.

Question 17

Biologists tell us that life on Earth took billions of years to evolve into astronomy students and other examples of intelligent life. If we want to search for planets with intelligent life-forms that evolved over the same period of time that we did, what sorts of stars should we not bother searching around?

Accepted Answer

O and B type stars are too short-lived to allow for the evolution of intelligent life-forms over billions of years.

Question 18

In a science fiction television show set in the far future, a starship finds itself approaching a stellar association. What types of objects would they be most likely to notice in such an association as they approach?

Accepted Answer

Stellar associations are typically groups of stars that share a common origin and are loosely bound by gravitational forces. They are often composed of young, hot, and massive stars, such as O and B type stars, which are very bright and would be noticeable from a distance.

Question 19

Which of the following statements about open clusters of stars is FALSE?

Accepted Answer

Open clusters typically contain fewer stars and less mass than globular clusters, which have the highest mass-to-light ratios of any known objects.

Question 20

A science fiction writer needs an environment for her latest story where stars are as crowded together as possible. Which of the following would be a good place to locate her story?

Accepted Answer

Globular clusters are some of the most tightly packed stellar systems in the universe. They can contain hundreds of thousands of stars within a relatively small volume of space, making them the ideal environment for a story where stars are crowded together. The other options (small open cluster, stellar association, outer regions of a spiral galaxy's disk) also contain multiple stars, but not necessarily in as high concentration as a globular cluster. Option E) is not a viable choice for a science fiction environment related to stars.

Quiz 22: Stars From Adolescence to Old Age