Deck 10: The Interstellar Medium

Question 1

To an observer on Earth,dust in the interstellar medium will cause a star to appear

A) brighter and cooler than it really is.
B) brighter and hotter than it really is.
C) fainter and cooler than it really is.
D) fainter and hotter than it really is.
E) unchanged in brightness or apparent color.

Answer 1

fainter and cooler than it really is.

Question 2

Coronal gas radiates energy at

A) gamma-ray wavelengths.
B) x-ray wavelengths.
C) visible wavelengths.
D) infrared wavelengths.
E) radio wavelengths.

Answer 2

x-ray wavelengths.

Question 3

Radiation with a wavelength of 21-cm is produced by

A) the hot gas of the interstellar medium.
B) HI regions.
C) molecular hydrogen.
D) emission nebulae.
E) b and d

Answer 3

HI regions.

Question 4

Absorption lines produced by the interstellar medium indicate that some components of the interstellar medium are cold and of a very low density because

A) the lines are blue shifted.
B) the lines are red shifted.
C) the lines are extremely broad.
D) the lines are extremely narrow.
E) the lines are much darker than the stellar lines.

Question 5

The extinction of starlight due to the interstellar medium

A) I & III
B) II & III
C) I & IV
D) II & IV
E) only IV

Question 6

The presence of forbidden lines in the spectra of emission nebula indicates that the

A) nebula has a very high temperature.
B) nebula contains large amounts of hydrogen.
C) gas in the nebula has a very low density.
D) dust in the nebula is composed of very small grains of silicates and carbon.
E) nebula is very near a star whose temperature is less than that of the sun.

Question 7

The local bubble

A) is a high temperature low density region in the interstellar medium around the sun.
B) is a nearby emission nebula.
C) is composed of neutral hydrogen and a fairly dense concentration of dust.
D) produces large amounts of 21-cm radiation.
E) produces radio waves from CO and other molecules.

Question 8

Extinction by the interstellar medium dims starlight by about 2 magnitudes for every 1000 pc the light travels.What fraction of the photons remains from a star that is 5000 pc from an observer?

A) 1/10
B) 1/5
C) 1/25
D) 1/10,000
E) 1/100

Question 9

Absorption lines due to interstellar gas

A) are wider than the lines from stars because the gas is hotter than most stars.
B) are more narrow than the lines from stars because the gas has a lower pressure than stars.
C) indicate that the interstellar medium contains dust.
D) indicate that the interstellar medium is expanding away from the sun.
E) none of the above

Question 10

Which of the following is not evidence of the existence of an interstellar medium?

A) extinction
B) narrow calcium lines in the spectra of O and B stars
C) reddening
D) dark clouds
E) molecular bands in the spectra of cool stars

Question 11

The air inside two balloons has the same temperature,but balloon A has a twice the pressure of balloon B.What does this tell us about the density of the air inside the two balloons?

A) The air in balloons A and B have the same density.
B) The air in balloon B is more dense than the air in balloon A.
C) The air in balloon A is more dense than the air in balloon B.

Question 12

Atoms of cool hydrogen emit 21-cm radiation when the

A) electrons move from the fourth to the second energy level.
B) electrons move from the second to the fourth energy level.
C) protons change their spin.
D) electrons change their spin.
E) hydrogen atoms add a second electron to form H-.

Question 13

The term "metastable levels" refers to

A) regions within a molecular cloud that are not moving relative to the rest of the cloud.
B) the upper energy level of the 21-cm transition in the hydrogen atom.
C) the lower energy level of the 21-cm transition in the hydrogen atom.
D) the boundary layers between HI regions and the hotter intercloud medium.
E) energy levels in an atom that require a much longer time than average to decay and emit a photon.

Question 14

How can the coronal gas occupy most of the space but represent only 5% of the mass?

A) It has a much lower pressure than the other components.
B) It has a much higher pressure than the other components.
C) It has a much lower temperature than the other components.
D) It has a much higher density than the other components.
E) It has a much lower density than the other components.

Question 15

Extinction by the interstellar medium dims starlight by about 2 magnitudes for every 1,000 pc the light travels.If a star is observed to have a magnitude of 7,and it is determined to be at a distance of 2,000 pc,what would the magnitude be if there was no extinction?

A) 7
B) 1.75
C) 28
D) 11
E) 3

Question 16

The sky appears blue during the day because

A) small dust particles in the atmosphere absorb blue light less efficiently than red light.
B) small dust particles in the atmosphere scatter blue light more efficiently than red light.
C) the atmosphere contains a large amount of water vapor which,in large quantities,appears blue.
D) 70% of Earth's surface is water and its blue color is reflected off small particles in the atmosphere.
E) the ozone in the atmosphere absorbs ultraviolet light and passes more red light than blue.

Question 17

A balloon has a diameter of 50 cm,and the density of the air in the balloon is 10¹⁹ particles/cm³.You wish to expand the balloon so that its density is 1 particle/cm³ (the average density of the interstellar medium).What is the diameter will the balloon have when its density is 1 particle/cm³?

A) 1,100 km
B) 1.1*10⁸ km
C) 220 km
D) 2.2*10⁶ km
E) 4.3*10⁴ km

Question 18

____ are small dark nebulae less than 1 parsec in diameter that contain 10 to 100 solar masses.

A) HI regions
B) HII regions
C) Emission nebulae
D) Bok globules
E) Reflection nebulae

Question 19

Elements such as iron,calcium,and titanium are less abundant in the interstellar medium than in the sun.This is most likely due to these elements

A) not producing spectral lines,and hence we underestimate their abundances.
B) being destroyed by ultraviolet photons.
C) having condensed to form dust grains.
D) being created in the photospheres of stars like the sun and not in low density gas clouds.
E) forming the coronal gas that only emits x-rays.

Question 20

Molecular clouds can be observed

A) using infrared telescopes to detect ionized gas in the clouds.
B) using x-ray telescope to observe the x-ray radiation from the molecules in the cloud.
C) using radio telescopes to observe the CO emission from the clouds.
D) by looking for blue wispy regions near star clusters.
E) by looking for the 21-cm radiation from hydrogen.

Question 21

____ require(s)that a young hot star (T $\ge$ 25,000 K)be relatively nearby.

A) HII regions
B) HI regions
C) Molecular clouds
D) The hot gas of the interstellar medium
E) 21-cm radiation

Question 22

Stars are born in

A) reflection nebulae.
B) dense molecular clouds.
C) HII regions.
D) the intercloud medium.
E) the local bubble.

Question 23

Which of the following is evidence of the existence of a thin cold interstellar gas? ...

A) extinction
B) narrow calcium lines in the spectra of O and B stars
C) reddening
D) dark clouds
E) all of the above

Question 24

____ is(are)believed to be produced by supernova explosions.

A) Coronal gas
B) HI clouds
C) Molecular clouds
D) Reflection nebulae
E) Dark nebulae

Question 25

Molecular clouds are mapped using CO instead of hydrogen because CO is much more abundant than hydrogen in molecular clouds.

Question 26

Which of the following is evidence of the existence of interstellar dust?

A) extinction of visual light from more distant stars
B) infrared radiation emitted from between the stars
C) reddening of visual light from more distant stars
D) dark clouds seen againts the Milky Way
E) all of the above

Question 27

____ is(are)believed to make up 50 percent of the mass of the interstellar medium.

A) The intercloud medium
B) HI clouds
C) Coronal gas
D) Molecular clouds
E) Dark nebulae

Question 28

Coronal gas

A) causes stars to look bluer than they normally would.
B) causes the stars to twinkle.
C) emits 21-cm radiation.
D) is located near stars with surface temperatures greater than 25,000 K.
E) has a temperature of about 1 million K.

Question 29

The emission lines from O III are called forbidden because they cannot occur in laboratories on Earth due to the much greater pressure of the gas on Earth.

Question 30

What property of blue light allows it to be more readily scattered than red light?

A) Higher intensity
B) Faster speed
C) Higher temperature
D) Shorter wavelength

Question 31

What type of radiation is emitted when the electron in a hydrogen atom experiences a spin flip?

A) Visible light
B) Ultraviolet light
C) Gamma ray
D) Radio wave

Question 32

HII emission nebula regions are found around _________________.

A) high temperature stars
B) very cool stars
C) solitary planets
D) Wrong.Interstellar hydrogen only exists in non-ionized HI form.

Question 33

Interstellar dust scatters _______ light more than ____________ light..

A) blue
B) red
C) Both are scattered equal amounts
D) Neither are scattered by interstellar dust

Question 34

The composition of the interstellar dust suggests that it is

A) primarily produced by emission nebulae.
B) composed primarily of hydrogen and helium.
C) composed of particles that are a few millimeters in diameter.
D) formed as coronal gas cools.
E) formed primarily in the atmospheres of cool stars.

Question 35

X-rays come from certain regions of the interstellar medium because

A) those regions are heated to a very high temperature.
B) gas molecules in those regions are at a very high density.
C) dust and gas in those regions are moving very rapidly.
D) nuclear fusion is occurring in those regions.

Question 36

The dust in the interstellar medium can make distant stars look redder than they really are.

Question 37

Which of the following properties are not associated with HII regions?

A) High temperature
B) High density
C) Bright nebulae
D) Ionized gas

Question 38

Which of the following is not considered a component of the interstellar medium?

A) HI clouds
B) Intercloud
C) Coronal gas
D) Molecular clouds
E) Water vapor

Question 39

Bok globules are small,dark fragments of the interstellar medium.

Question 40

HI clouds can be observed using 21-cm radiation.

Question 41

Molecules emit radiation at ____________________ wavelengths.

Question 42

The thermal motions of the atoms in a gas cloud can make it collapse to form a protostar.

Question 43

The pressure of a gas depends on the temperature and density of the gas.

Question 44

____________________ are cool,with temperatures of 50 to 150 K and densities of ten to a few hundreds of atoms per cubic centimeter.

Question 45

HII regions are found near stars cooler than 25,000 K because large amounts of ultraviolet photons would destroy the hydrogen atoms in the gas.

Question 46

Longer wavelengths of light more easily scatter than short wavelengths.

Question 47

The ____________________ is the component of the interstellar medium that is highly ionized and produces X-rays.

Question 48

Coronal gas emits x-rays and has temperatures of 10⁶ K or higher.

Question 49

Reflection nebulae look blue because they are reflecting light from a very hot source.

Question 50

The intercloud medium has the highest density of the components of the interstellar medium.

Question 51

____________________ is the most abundant element in the interstellar medium.