1.)
What is a molecular cloud?
Answer: A molecular cloud is a cloud of interstellar gas that is
cold (T ~ 10 to 30K) and
dense (density = ~ 300 molecules per cm3)
and in which many of the atoms have bonded
together
to form molecules (such as H2 and CO).
2.) At the beginning of the star formation process, the
material that stars eventually form from is:
a.) interstellar gas
b.) relatively dense (density greater than or equal to
300 particles/cm3)
c.) cold (T < 50 K)
d.) all of the above
e.) none of the above
Answer: d
3.) During the star formation process, what happens to the material
from which the star eventually forms:
a.) gravity tries to make the clump of material contract
b.) thermal pressure tries to make the clump of material expand
c.) sometimes the clump of material cools down by radiating photons
d.) all of the above
e.) none of the above
Answer: d
4.) Compare the number of initial star forming cloud(s) with the
number of star(s) formed:
a.) usually, multiple stars are formed from a single cloud
b.) usually, a single star is formed from a single cloud
c.) usually, multiple clouds combine to form a single star
Answer: a
5.) Consider the dense cloud core that will evolve into
a star. While it is collapsing, what
what happens to its rotation speed?
Answer:
It speeds up. It has constant
angular momentum (m v x r).
So, in order to have a constant angular momentum
as the size (r) decreases,
the velocity (v) must increase.
6.) What is a protostellar disk?
a.) a wind of material that flows out from a protostar
b.) one of many lumps of material that prevent radiation from leaving the
protostar
c.) the portion of a star forming cloud where most of the protostars
reside
d.) none of the above
Answer: d
7.) When do stars like the Sun begin to fuse hydrogen into helium:
a.) about the time that the birth cloud begins to contract
b.) about the time that the protostar fires a jet
c.) about the time that the protostar's surface temperature reaches
3000 K
d.) none of the above
Answer: d
8.) Brown dwarfs:
a.) are white dwarfs that have cooled down
b.) are very low mass stars that only started nuclear burning recently
c.) will never fuse hydrogen into helium
d.) all of the above
e.) none of the above
Answer: c
9.) Why is it impossible to make a star with a mass over
200 x MSun in the modern universe?
a.) electron degeneracy pressure prevents the protostar from
collapsing enough
b.) nuclear burning is not possible when the temperature in the
protostar's center is too high
c.) the pressure of photons leaving the star is great enough to
blow off some of the material
d.) all of the above
e.) none of the above
Answer: c
10.) (Problem adapted from textbook:)
"Briefly describe
the process by which a protostar and protostellar disk form from
gas in a molecular cloud".
Answer: Like much of the universe, the gas in molecular clouds has lumps in it
where the density is greater.
Gravity pulls nearby cloud material toward the
lumps. As material "accretes" onto the lumps,
gravitation potential energy is converted into
kinetic energy and then thermal energy.
i.e. the cloud heats up.
Some of this heat is then converted into radiative energy
(light, often infrared light) and radiated away from the cloud.
Eventually, the lump grows
(by adding material) to be too opaque
to effectively radiate away the excess heat.
As a result, the
thermal pressure (related to the temperature) becomes large enough to
prevent gravity from further compressing the lump.
By this time, the lump, now called
a protostar, has "spun up".
Earlier, when the lump (and the surrounding gas that
accreted onto it)
was contracting, angular momentum was conserved. As a
result, the rotational velocity increased.
The "spinning up" of the lump changed its shape into a disk.
Some of the angular momentum (and mass) of the disk are
ejected into space,
via jets and winds.
The remaining material forms the star (and its planets).
The protostar
officially becomes a star when it's center begins
to fuse hydrogen into helium.
This happens when the center gets hot enough (10 million K).