Chapter 72, Gas and Dust in the Milky Way Video Solutions, Pathways to Astronomy

01:19

Problem 1

A Pop II star has a highly eccentric orbit 12 kpe in diameter. What is its orbital period?

Suhas Katkar

Numerade Educator

06:08

Conservation of angular momentum (Unit 20) requires that the product of the speed at which an object is orbiting times the distance from the point around which it is orbiting remain constant. If the Sun is currently orbiting at $220 \mathrm{km} / \mathrm{sec}, 8 \mathrm{kpc}$ from the center of the Milky Way, how fast did the material that formed the Sun rotate if the gas began 400 kpe distant (about halfway to the next large galaxy)?

Matthew Miranda

Numerade Educator

01:50

Problem 2

If a gas cloud shrinks to half its size, how much change is there in its density? in its temperature?

Bridger Johnston

Numerade Educator

02:06

Problem 3

A $10^{9}-M_{\odot}$ galaxy merges with a $10^{10}-M_{\odot}$ galaxy. If $30 \%$ of their combined mass is lost in the collision, what is the total mass of the newly formed galaxy?

Zulfiqar Ali

Numerade Educator

02:22

Problem 3

Convert the following radio wavelengths to frequencies (see Unit 22 ):
a. $21-\mathrm{cm}$ line of atomic hydrogen
b. $2.6-\mathrm{mm}$ line of carbon monoxide

Mahendra K

Numerade Educator

02:06

Problem 4

Suppose that after $x$ billion years, the Milky Way converted half of its gas into stars. In the next $x$ billion years, it converted half of the remaining gas into stars-and so on, each $x$ billion years. The Milky Way is estimated to be about 13 billion years old. and only about $15 \%$ of its disk remains today in the form of
gas.a. What value of $x$ would give the Milky Way's current gas fraction? (Hint: You can calculate this mathematically, or you can draw an approximate graph of the declining gas fraction and estimate $x$.
b. In how many billion years will the gas fraction be just $1 \% ?$

Sarah Mccrumb

Numerade Educator

04:12

Problem 4

Find the ratio of the energy of a 21 -cm photon to the energy of an electron shifting from the second energy level of hydrogen to the ground state, which produces a photon of 122 -nm wavelength.

Shalini Tyagi

Numerade Educator

02:15

Problem 5

Suppose we found a nearby Pop II star that is on a highly elliptical orbit, but it is currently near its most distant point from the center of the Milky Way in its orbit. To stars like the Sun in orbit in the disk, what would be the approximate relative speed of this Pop II star? In what direction?

Donald Albin

Numerade Educator

01:07

Problem 5

On average, a hydrogen atom emits a 21 -cm photon once every 15 million years. How many 21 -cm photons does a $100-M_{\odot}$ gas cloud emit per second?

Raj Bala

Numerade Educator

04:40

Problem 6

The dust in an interstellar cloud blocks blue light in the following way: For every 1 pc light travels through the cloud, only $90 \%$ of the light continues. Thus, after $2 \mathrm{pc}, 90 \%$ of the remain$\operatorname{ing} 90 \%,$ or $81 \%(0.9 \times 0.9 \times 0.81),$ remains.
a. You might initially expect that after $10 \mathrm{pc}$, the ten $10 \%$ reductions would have removed all of the blue light. How much blue light actually remains?
b. The same cloud removes about $7 \%$ of the red light every parsec, so after $1 \mathrm{pc}$, the ratio of blue to red has dropped to $90 / 93=97 \%$ of its unreddened value. What is the ratio of blue to red after 5 pc? after 10 pc?

Samantha Baker

Numerade Educator