Star Birth (Ch. 16)_ Guided Reading Questions
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Pennsylvania State University *
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Astronomy
Date
Apr 3, 2024
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1. (p. 523, Watch the Ch. 16 Overview Video) Are the molecular clouds in which stars
form cold or hot? (Think about why). Write down for yourself what is degenerate
pressure.
Very cold and dense.
2. (p. 523) What is the difference between the interstellar gas clouds (what color are
these?) and the interstellar medium?
Interstellar gas clouds are black patches within the Milky Way that block the view
of stars behind them while interstellar medium is the gas and dust found in
spaces between stars.
3. (p. 523) Which chemical elements did the first stars in the universe form out of? (also
see p. 530) Which elements does the interstellar medium (and the newer stars) consists
of today?
Hydrogen and helium gas is what made the first stars and an interstellar medium
consist of 70% hydrogen, 28% helium and 2% heavier elements.
4. (p. 524) What type of interstellar clouds do stars form in? What is the most abundant
molecule in these clouds?
Stars are born in what is called molecular clouds and the most abundant
molecule in the clouds is molecular hydrogen
5. (Fig. 16.5) How can astronomers 'see' through the dark dust clouds? (which
wavelengths?) What is interstellar reddening?
Astronomers use infrared photos to see through dark dust clouds because dust
is not effective at blocking infrared light so we can see behind the cloud in
infrared photos. Interstellar reddening is when the stars at the edge of the cloud
appear redder because the dust blocks blue light much more effectively than red
light.
6. (Video on p. 526) Why do molecular clouds where stars form need to be cold in order
to form new stars?
This is because a cold and dense environment helps gravity overcome the gas
pressure.
7. (p. 528) Why do stars form in clusters?
Gravity is stronger in a high mass gas cloud so stars form in clusters so it is
easier to overcome the outwards force of thermal pressure.
8. (p. 529) Thermal pressure vs. Gravity - what are the two ways that the outward push
of the pressure within a gas cloud (thermal pressure) fights against gravity? Explain how
the gas pressure can go up.
First individual star clumps move at different speeds indicating that the overall
gas motion is turbulent and this means that overcoming that motion can require a
lot more mass than is needed to fight thermal pressure alone. Secondly, magnetic
fields can help resist gravity through depending on how strong the magnetic field
is this can slow or even stop the gravitational collapse of a cloud.
9. (p. 529, Fig. 16.10) Why does a large molecular cloud form many individual stars
instead of a single extremely massive star? (part B of Fig. 16.10)
This is because motions in the cloud cause the cloud to be clumpy and if gravity
can overcome the thermal pressure in the denser regions, they collapse and form
smaller lumps of matter.
10. (p. 531, Fig. 16.14) Describe how a star is born from a cloud of gas. Sketch a
protostellar accretion disk and jets (Fig. 16.15)?
A cloud fragment starts to rotate and as it starts to rotate faster, the cloud starts
to shrink and flatten forming into a disk.
11. (p. 534) When does a protostar become a star, and when does it become a
main-sequence star?
A protostar becomes a real star when the core temperature exceeds 10 million K,
this is what we consider the birth of a star. When the star’s interior structure
stabilizes this is when this star becomes a main-sequence.
12. (p. 536) Make sure you understand what degeneracy pressure is. Electron
degeneracy pressure occurs when electrons get so close to one another that they exert
a pressure of their own. The trick is that the closer we compress the electrons, the
faster they jiggle. What is the name of objects supported by electron degeneracy
pressure?
We call them brown dwarfs.
13. What is the minimum mass of a star?
Anything smaller than 0.08Sun cannot form into a true star due to degeneracy
pressure
.
14. (p. 536) What is the maximum mass of a star, and why is there a maximum mass of
a star?
There is a maximum mass of at least 150MSun, this is because of what we call
radiation pressure and this shows that this type of pressure is very powerful and
stars with masses over 100M cannot last more than a few hundred years before
blowing itself apart.
15. (p. 537, Fig. 16.21) Which type of stars in the universe are the most numerous and
which stars are rare (based on their mass)?
Stars with smaller masses greatly outnumber those with larger masses making
those with a larger mass much more rare.
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