Astronomy Homework Solutions: Star Formation and Evolution | Assignments Astronomy

Physics 101 Astronomy

Fourth Homework Solutions

14.4 Young stars (i.e. blue stars) are usually associated with giant

molecular clouds – that is, they are located inside or next to the

clouds. When observing those clouds using infrared telescopes, we

can see hot compact sources inside the dark clouds, which are

consistent with protostars forming out of the collapse of the cloud. We

also usually see star formation propagating through a cloud from one

end to the other. For example, in the Orion nebula, we see young hot

stars in the Trapezium cluster, and in the molecular cloud right next

to them are compact infrared sources, which is consistent with the

sequential model of star formation we discussed in class.

14.6 The star forms inside a molecular cloud and that hides it from

view. The molecular cloud does not dissipate until the star begins

hydrogen fusion. Then, the radiation that the star is producing will

blow away the surrounding cloud, revealing the star. By the time the

cloud dissipates, the star is already on the main sequence.

14.7 In two ways. First, the radiation and material being expelled

from the newly formed star pushes away the cloud immediately

surrounding the star. Second, the shock wave generated by this

rapidly moving material can trigger a second round of star formation

further along in the cloud (refer to the sequential model of star

formation).

14.11 The star formation cascade is perpetuated by the shock wave

formed when rapidly expanding hot gas, heated by young stars,

crashes into the much cooler gas in the rest of the molecular cloud.

This forms a shock wave where the hot gas pushes the cool gas. That

shock wave compresses the cool gas to the point where it can begin to

collapse gravitationally and form new stars.

15.3 Refer to figure 15.15, which shows HR diagrams for several

clusters. Refer to those for the Pleiades and the Hyades and ignore

the rest. Notice how the hottest and most massive stars at the top of

the main sequence are beginning to evolve toward the red giant stage

in both clusters. You can tell this because the top of the line for each

cluster is peeling away from the main sequence, bending toward the

giant region. Since this turnoff point is occurring for lower mass stars

in the Hyades cluster (it is lower on the main sequence), and since we

know that lower mass stars sit on the main sequence for a longer time

before they become giants, the Hyades have to be older than the

Pleiades. Medium size stars in the Hyades are leaving the main

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Physics 101 Astronomy Fourth Homework Solutions 14.4 Young stars (i.e. blue stars) are usually associated with giant molecular clouds – that is, they are located inside or next to the clouds. When observing those clouds using infrared telescopes, we can see hot compact sources inside the dark clouds, which are consistent with protostars forming out of the collapse of the cloud. We also usually see star formation propagating through a cloud from one end to the other. For example, in the Orion nebula, we see young hot stars in the Trapezium cluster, and in the molecular cloud right next to them are compact infrared sources, which is consistent with the sequential model of star formation we discussed in class. 14.6 The star forms inside a molecular cloud and that hides it from view. The molecular cloud does not dissipate until the star begins hydrogen fusion. Then, the radiation that the star is producing will blow away the surrounding cloud, revealing the star. By the time the cloud dissipates, the star is already on the main sequence. 14.7 In two ways. First, the radiation and material being expelled from the newly formed star pushes away the cloud immediately surrounding the star. Second, the shock wave generated by this rapidly moving material can trigger a second round of star formation further along in the cloud (refer to the sequential model of star formation). 14.11 The star formation cascade is perpetuated by the shock wave formed when rapidly expanding hot gas, heated by young stars, crashes into the much cooler gas in the rest of the molecular cloud. This forms a shock wave where the hot gas pushes the cool gas. That shock wave compresses the cool gas to the point where it can begin to collapse gravitationally and form new stars. 15.3 Refer to figure 15.15, which shows HR diagrams for several clusters. Refer to those for the Pleiades and the Hyades and ignore the rest. Notice how the hottest and most massive stars at the top of the main sequence are beginning to evolve toward the red giant stage in both clusters. You can tell this because the top of the line for each cluster is peeling away from the main sequence, bending toward the giant region. Since this turnoff point is occurring for lower mass stars in the Hyades cluster (it is lower on the main sequence), and since we know that lower mass stars sit on the main sequence for a longer time before they become giants, the Hyades have to be older than the Pleiades. Medium size stars in the Hyades are leaving the main

sequence for the giant stage but they are still on the main sequence in the Pleiades. 15.5 The evidence comes from planetary nebulas. The theory of stellar evolution says that when a red giant blows off its outer layers, it leaves behind a hot dense core that will become a white dwarf. If that is the case, then we would expect the central stars of planetary nebulas, when plotted on an HR diagram, to lie between the red giant and white dwarf regions. If you refer to the HR diagram figure 15.21, that is exactly where they do lie. 15.7 The two groups of stars compare in a couple of related ways. First, we can tell that stars in open clusters are younger than those in globular clusters by several lines of evidence – for example, open clusters are dominated by hot blue stars, which we know don’t live very long, whereas globular clusters are dominated by yellow and red stars and contain few, if any blue stars. This indicates that overall the population of stars in globular clusters is much older than in open clusters since the hot blue stars have had time to evolve off the main sequence. As a consequence of this difference in age, globular clusters contain stars whose composition is almost entirely hydrogen and helium whereas open clusters contains stars whose composition is more like the Sun, with a much larger percentage of heavier elements. This follows from the fact that the globular cluster stars are older, and formed at a time when there were no heavier elements available, whereas open cluster formed later out of material that had been enriched in heavier elements produced by nuclear fusion in the older stars. 15.9 The five solar mass stars reach the main sequence first and the one solar mass stars later. The five solar mass stars also leave the main sequence first because they have shorter lifetimes. In both cases, the stars will eventually become red giants but the five solar mass stars will do so first so there will be a time when the cluster is half red giant and half main sequence. Some time after that, the stars will produce planetary nebulas and become white dwarfs but again the five solar mass stars will do so first so we will have a cluster of half red giants and half planetary nebulas/white dwarfs. Finally, all the stars reach the white dwarf stage and eventually fade away. 16.3 White dwarfs can be observed directly (e.g. Sirius B). We can calculate their mass from their orbital motion when they are in binary systems as Sirius is. We can calculate their size from their brightness and temperature. We find that white dwarfs are the size of the Earth but have the mass of the Sun, a highly compressed state of matter that is consistent with what is predicted by the theory of stellar

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