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Question 1

The left-hand edge of the main-sequence band in the H–R diagram is called the ……….. ………… ………….

We use the term …………… to mark the time when a star stops contracting, settles onto the main sequence, and begins to fuse hydrogen in its core.

Answer 1

zero-age main sequence

zero-age

Question 2

Answer 2

R 2

Question 3

The lifetime of a star in a particular stage of evolution depends on how much ……….. ………. it has and on how quickly it uses up that …………

Answer 3

nuclear fuel / fuel

In the case of stars, more massive ones use up their fuel much more quickly than stars of low mass.

Question 4

what determines how hot a star’s central regions get?

Answer 4

It is the mass of the star—the weight of the overlying layers determines how high the pressure in the core must be:

higher mass requires higher pressure to balance it.

Higher pressure, in turn, is produced by higher temperature.

Question 5

Stars devote an average of ……… of their lives to peacefully fusing hydrogen into helium.)

Answer 5

90%

Question 6

(Note that red giant stars do not actually look deep red; their colors are more like …………. or …………)

Answer 6

orange / orange-red

Question 7

The three basic types of clusters astronomers have discovered are ………………..

Answer 7

1. globular clusters,
2. open clusters, and
3. stellar associations.

Question 8

globular clusters contain only ………………. stars, whereas open clusters and associations contain …………. stars.

Answer 8

very old / young

Question 9

The most massive globular cluster in our own Galaxy is ………. …………, which is about 16,000 light-years away and contains several million stars

Answer 9

Omega Centauri

Question 10

Several open clusters are visible to the unaided eye. Most famous among them is the ………….. which appears as a tiny group of …….. stars (some people can see even more than ……….., and the Pleiades is sometimes called the ……….. Sisters).

Answer 10

Pleiades

six / six / Seven

Question 11

The ……… is another famous open cluster in the constellation of Taurus the bull. To the naked eye, it appears as a V-shaped group of faint stars marking the face of the bull. Telescopes show that …………. actually contains more than 200 stars.

Answer 11

Hyades

Question 12

An ………… is a group of extremely young stars, typically containing 5 to 50 hot, bright O and B stars scattered over a region of space some 100–500 light-years in diameter.

Answer 12

association

As an example, most of the stars in the constellation Orion form one of the nearest stellar associations.

Question 13

we indicated that open clusters are younger than globular clusters, and associations are typically even younger.

Answer 13

R 2

Question 14

we will show how we determine the ages of these star clusters. The key observation is that the stars in these different types of clusters are found in different places in the ………. ……., and we can use their locations in the diagram in combination with theoretical calculations to estimate how long they have lived.

Answer 14

H–R diagram

Question 15

There are real star clusters that fit this description. The first to be studied (in about 1950) was ……………., which is still associated with the region of gas and dust from which it was born

Answer 15

NGC 2264

Question 16

The location in the H–R diagram where the stars have begun to leave the main sequence is called the ……….. …………

Answer 16

main-sequence turnoff.

Question 17

how old are the different clusters we have been discussing?

Answer 17

In practice, astronomers use the position at the top of the main sequence (that is, the luminosity at which stars begin to move off the main sequence to become red giants) as a measure of the age of a cluster (the main-sequence turnoff we discussed previously).

Question 18

Once all of the interstellar matter surrounding a cluster has been used to form stars or has dispersed and moved away from the cluster, star formation ceases, and stars of progressively lower mass move off the main sequence,

Answer 18

R 2

Question 19

To our surprise, even the youngest of the globular clusters in our Galaxy are found to be older than the oldest open cluster. All of the globular clusters have main sequences that turn off at a luminosity less than that of the Sun.

Answer 19

r 2

Question 20

Indeed, the globular clusters are the oldest structures in our Galaxy (and in other galaxies as well). The youngest have ages of about ……… ……….. years and some appear to be even older. Since these are the oldest objects we know of, this estimate is one of the best limits we have on the age of the universe itself—it must be at least ……….. ……….. years old.

Answer 20

11 billion