Chap 19 & 20

Question 1

Explain how parallax measurements can be used to determine distances to stars.
Why can we not make accurate measurements of parallax beyond a certain distance?

Answer 1

Triangulation is used to measure the distance
The angle becomes so small that it isn’t measurable anymore.

Question 2

Suppose you have discovered a new cepheid variable star. What steps would you take to determine its distance?

Answer 2

Measure it’s period and then use it to find luminosity and apparent brightness.

Question 3

Explain how you would use the spectrum of a star to estimate its distance.

Answer 3

Spectral lines and luminosity class can find luminosity and absolute magnitude.

Question 4

Which method would you use to obtain the distance to each of the following?
1. An asteroid crossing Earth’s orbit
2. A star astronomers believe to be no more than 50 light-years from the Sun
3. A tight group of stars in the Milky Way Galaxy that includes a significant number of variable stars
4. A star that is not variable but for which you can obtain a clearly defined spectrum

Answer 4

1. Parallax / Radar
2. Parallax
3. Period luminosity relationship
4. HR Diagram, luminosity class, and spectral class

Question 5

Most distances in the Galaxy are measured in light-years instead of meters. Why do you think this is the case?

Answer 5

Meters would be too small of a measurement for the grand numbers

Question 6

The AU is defined as the average distance between Earth and the Sun, not the distance between Earth and the Sun. Why does this need to be the case?

Answer 6

The distance between Earth and the Sun changes due to Earth’s elliptical orbit.

Question 7

What would be the advantage of making parallax measurements from Pluto rather than from Earth? Would there be a disadvantage?

Answer 7

It would allow for more precise measurements and has a bigger angle for parallax. A disadvantage would be that Pluto takes many years to orbit.

Question 8

For centuries, astronomers wondered whether comets were true celestial objects, like the planets and stars, or a phenomenon that occurred in the atmosphere of Earth. Describe an experiment to determine which of these two possibilities is correct.

Answer 8

Parallax can determine whether in space or atmosphere, as a smaller parallax means a bigger distance.

Question 9

A G2 star has a luminosity 100 times that of the Sun. What kind of star is it? How does its radius compare with that of the Sun?

Answer 9

It would be a giant type star and it would have a bigger radius then the Sun.

Question 10

A star has a temperature of 10,000 K and a luminosity of 10–2 LSun. What kind of star is it?

Answer 10

White dwarf

Question 11

The New Horizons probe flew past Pluto in July 2015. At the time, Pluto was about 32 AU from Earth. How long did it take for communication from the probe to reach Earth, given that the speed of light in km/hr is 1.08×109?

Answer 11

32 AU
1AU= 1.49 x 10^11m= 1496 x 10^8km
speed light: 1.08 x 10^9km/hr
time= distance/speed
32(1.49 x10^8)/ (1.08 x 10^9)
= 4.43 hr

Question 12

The most recently discovered system close to Earth is a pair of brown dwarfs known as Luhman 16. It has a distance of 6.5 light-years. How many parsecs is this?

Answer 12

1pc = 3.262 ly
6.5 ly
P= 1/D
6.5/ 3.262 = 1.99 pc

Question 13

distance modulus
calculation of distance from apparent and absolute magnitude

Answer 13

d= 10 x 10 ((m - M)/ s)
m: apparent mag
M: absolute mag
10 x 10 ( (12-20)/ 5) = 0.251 pc

Question 14

Why do nebulae near hot stars look red?
Why do dust clouds near stars usually look blue?

Answer 14

The star releases UV light that ionizes the hydrogen, as the ionized hydrogen then combines with electrons it creates a reddish glow. Emission nebula.
When starlight passes through dust, shorter wavelengths of light (like blue) are scattered more than longer wavelengths (like red). Creating the blue in a reflecting nebula.

Question 15

Describe the characteristics of the various kinds of interstellar gas (HII regions, neutral hydrogen clouds, ultra-hot gas clouds, and molecular clouds).

Answer 15

• HII regions: UV radiation from nearby stars ionizes hydrogen, releases red light, around 10,000K
• Neutral H Clouds (HI regions): Cold, most common form of H, no visible light but emits radio radiation at a 21cm wavelength.
• Ultra hot: Very hot, found around supernova remnants that heat up gases.
• Molecular clouds: Colder then other types, great for star formation, contains lots of molecules.

Question 16

Describe how the 21-cm line of hydrogen is formed. Why is this line such an important tool for understanding the interstellar medium?

Answer 16

The line happens when neutral hydrogen spin flips, which emits a photon at a 21cm wavelength that radio signals can pass. With neutral hydrogen being everywhere, the line can be used to map out the galaxy.

Question 17

Why is it difficult to determine where cosmic rays come from?

Answer 17

They don’t travel in straight lines due to magnetic fields.

Question 18

What causes reddening of starlight? Explain how the reddish color of the Sun’s disk at sunset is caused by the same process.

Answer 18

The dust and gas surrounding the star scatters the shorter wavelengths, which are blue, leaving the longer wavelengths, that are red.
When the Sun sets, it’s near the horizon, so it’s light travels through the thicker Earth atmosphere that has more dust and particles that scatter the blue wavelengths, and leave red wavelengths.

Question 19

Why do molecules, including H2 and more complex organic molecules, only form inside dark clouds? Why don’t they fill all interstellar space?

Answer 19

The molecules aren’t stable enough around high temperatures and ultra violet light. Dark clouds are one of the only environments that shield them.

Question 20

Why can’t we use visible light telescopes to study molecular clouds where stars and planets form? Why do infrared or radio telescopes work better?

Answer 20

Molecular clouds dust and density scatters or absorbs visible light, so infrared or radio telescopes are better as they can pass through the blocking.

Question 21

Figure 20.2 shows a reddish glow around the star Antares, and yet the caption says that is a dust cloud. What observations would you make to determine whether the red glow is actually produced by dust or whether it is produced by an H II region?

Answer 21

Look at spectrum of colors and lines.

Question 22

Even though neutral hydrogen is the most abundant element in interstellar matter, it was detected first with a radio telescope, not a visible light telescope. Explain why. (The explanation given in Analyzing Starlight for the fact that hydrogen lines are not strong in stars of all temperatures may be helpful.)

Answer 22

It isn’t hot enough to be on the visible spectrum.

Question 23

The terms H II and H2 are both pronounced “H two.” What is the difference in meaning of those two terms? Can there be such a thing as H III?

Answer 23

H II is just ionized hydrogen, where the hydrogen atom lost an electron
H2 is molecular hydrogen, which consists of two hydrogen atoms bonded together.
H III is not possible because hydrogen only has one electron to lose, so it cannot be “doubly ionized

Question 24

Describe the spectrum of each of the following:
1. starlight reflected by dust,
2. a star behind invisible interstellar gas, and
3. an emission nebula.

Answer 24

1. Mostly blue
2. Absorption lines make gas invisible
3. Red due to ionized hydrogen

Question 25

Suppose that, instead of being inside the Local Bubble, the Sun were deep inside a giant molecular cloud. What would the night sky look like as seen from Earth at various wavelengths?

Answer 25

At visible, ultraviolet, and X-ray wavelengths, the night sky would be completely black. You would only be able to see through radio or infrared.

Question 26

Suppose that, instead of being inside the Local Bubble, the Sun were inside an H II region. What would the night sky look like at various wavelengths?

Answer 26

Other wavelengths would look brighter but at visible, the sky would look like its glowing red.

Question 27

At the average density of the interstellar medium, 1 atom per cm3, how big a volume of material must be used to make a star with the mass of the Sun? What is the radius of a sphere this size? Express your answer in light-years.

Answer 27

34

Question 28

Consider a grain of sand that contains 1 mg of oxygen (a typical amount for a medium-sized sand grain, since sand is mostly SiO2). How many oxygen atoms does the grain contain? What is the radius of the sphere you would have to spread them out over if you wanted them to have the same density as the interstellar medium, about 1 atom per cm3? You can look up the mass of an oxygen atom.

Answer 28

35