Chapters 6 and 7

Question 1

Name the two spectral windows through which electromagnetic radiation easily reaches the surface of Earth

Answer 1

Visible light and radio-waves are spectral windows allow electromagnetic radiation through the atmosphere

Question 2

When astronomers discuss the apertures of their telescopes, they say bigger is better. Explain why.

Answer 2

Aperture can increase resolution and light gathering

Question 3

The Hale telescope at Palomar Observatory has a diameter of 5 m, and the Keck I telescope has a diameter of 10 m. How much more light can the Keck telescope collect than the Hale telescope in the same amount of time?

Answer 3

4 times more light
pi(10/2)^2/ pi(5/2)^2
= 10^2/5^2
= 100/25
= 4

Question 4

What is meant by “reflecting” and “refracting” telescopes?

Answer 4

A “reflecting telescope” uses mirrors to focus light and create an image, while a “refracting telescope” uses lenses to bend light and focus it

Question 5

Why are the largest visible-light telescopes in the world made with mirrors rather than lenses?

Answer 5

Mirrors are easier to produce, lighter, can have flaws inside the mirror, and are easier to support structurally.

Question 6

Why is it difficult to observe at infrared wavelengths? What do astronomers do to address this difficulty?

Answer 6

Peoples eyes cannot see infrared and a lot of it is blocked in Earth’s atmosphere. So astronomers have to observe higher up in the atmosphere.

Question 7

Radio and radar observations are often made with the same antenna, but otherwise they are very different techniques. Compare and contrast radio and radar astronomy in terms of the equipment needed, the methods used, and the kind of results obtained.

Answer 7

Radio:
- Needs: Antenna, telescope, and a receiver
The radio signals can reach far lengths, billions of LYs

Radar
- Needs: Antenna, receiver, and transmitter
The radar signals are only good for about 1 AU

Question 8

Why do astronomers place telescopes in Earth’s orbit?

What are the advantages for the different regions of the spectrum?

Answer 8

To avoid having the atmosphere absorb or significantly scatter radiations that allow us to get a higher resolution of cosmic objects. Also avoid weather on planet earth.

Question 9

Describe the techniques radio astronomers use to obtain a resolution comparable to what astronomers working with visible light can achieve.

Answer 9

They use a technique called interferometry, where 2 or more telescopes are linked together to increase resolution

Question 10

What happens to the image produced by a lens if the lens is “stopped down” (the aperture reduced, thereby reducing the amount of light passing through the lens) with an iris diaphragm—a device that covers its periphery?

Answer 10

Smaller aperture, higher f-stop, higher focus, more dim
High aperture, lower f-stop, lower focus, more light

Question 11

In broad daylight, the size of your pupil is typically 3 mm. In dark situations, it expands to about 7 mm. How much more light can it gather?

Answer 11

pi x r^2
pi(7/2)^2/ pi(3/2)^2
= 7/3^2
= 5.4 times brighter

Question 12

How much more light can be gathered by a telescope that is 8 m in diameter than by your fully dark-adapted eye at 7 mm?

Answer 12

8m = 8000mm
pi(8000/2)^2 / pi(7/2)^2
= (8000/7)^2
= 1,306,122

Question 13

Venus rotates backward and Uranus and Pluto spin about an axis tipped nearly on its side. Based on what you learned about the motion of small bodies in the solar system and the surfaces of the planets, what might be the cause of these strange rotations?

Answer 13

Something collided or impacted them, changing it’s angle

Question 14

The primary mirror (the one at the bottom in Figure 6.6) of the Gemini North telescope is 8 m in diameter. The secondary mirror at the top is about 1 m in diameter. Use the formula for the area of a circle to estimate what fraction of the light is blocked by the secondary mirror.

Answer 14

pi(1/2)^2/ pi(8/2)^2
= (0.5/4)^2
= 0.0156
0.0156 x 100 = 1.56%

Question 15

What does a planet need in order to retain an atmosphere? How does an atmosphere affect the surface of a planet and the ability of life to exist?

Answer 15

Gravity is needed for an atmosphere
The atmosphere holds water, deflects rays, holds oxygen, and heat to allow life.

Question 16

What is the difference between a meteor and a meteorite?

Answer 16

Meteor: In the sky
Meteorite: On the ground
Meteoroid: In space

Question 17

1. the least-dense planet
2. the densest planet
3. the largest moon in the solar system
4. excluding the jovian planets, the planet where you would weigh the most on its surface (Hint: Weight is directly proportional to surface gravity.)
5. the smallest planet
6. the planet that takes the longest time to rotate
7. the planet that takes the shortest time to rotate
8. the planet with a diameter closest to Earth’s
9. the moon with the thickest atmosphere
10. the densest moon
11. the most massive moon

Answer 17

1. Saturn
2. Earth
3. Ganymede
4. Earth
5. Mercury
6. Venus
7. Jupiter
8. Venus
9. Titan
10. Io
11. Ganymede

Question 18

What characteristics do the worlds in our solar system have in common that lead astronomers to believe that they all formed from the same “mother cloud” (solar nebula)?

Answer 18

All the planets orbit around the Sun the same and rotate on their axis the same direction.

Question 19

Why are there so many craters on the Moon and so few on Earth?

Answer 19

The Earth’s atmosphere burns or breaks up meteors, it’s weather also causes craters overtime to disappear.

Question 20

How do asteroids and comets differ?

Answer 20

Asteroids are primarily made of rock and metal, while comets are composed of ice, dust, and rock

Question 21

Describe how we use radioactive elements and their decay products to find the age of a rock sample. Is this necessarily the age of the entire world from which the sample comes? Explain.

Answer 21

We use them to measure age by calculating the rock’s half life. We can find the age of an entire world through a sample by how many half lives have passed, how much the sample’s radioactive element remains, and how much the decay product.

Question 22

Calculate the density of Saturn. Show your work. How does it compare with the density of water? Explain how this can be.

Answer 22

Density: mass/ vol
mass of Saturn: 5.7 x 10^26kg
volume of sphere: 4/3(pi)r^3
radius of Saturn = 58.23 x 10^7m

5.7 x 10^26/ 4/3(pi)(5.82 x 10^7)^3
= 690kg/m^3
Saturn is less dense then water, and would float on water, b/c it’s mainly composed of gases.

Question 23

A radioactive nucleus has a half-life of 5×10^ 8years. Assuming that a sample of rock (say, in an asteroid) solidified right after the solar system formed, approximately what fraction of the radioactive element should be left in the rock today?

Answer 23

Fraction = (1/2) ^ t/t(half life)
1/2^(4.5 x 10^9/ 5 x 10^8)
= 0.002

Solar system formed: 4.5 x 10^9 years ago