Section 3

Question 1

What is the total intensity?

Answer 1

The specific intensity integrated over all frequencies

Question 2

What is intensity independent of?

Answer 2

Distance as once inside the beam, radiation stays within it (it is conserved)

Question 3

What is flux of radiation?

Answer 3

The amount of energy crossing a unit area per unit time per unit frequency interval
flux does depend of distance

Question 4

What is the total flux?

Answer 4

The flux of radiation integrated over all frequencies/over radiation travelling from a surface in all outward directions

Question 5

At which wavelength ranges is the atmosphere transparent to radiation?

Answer 5

Radio and optical

Question 6

What is Planck’s function (B_v) used for?

Answer 6

To find the intensity of a blackbody radiation

Question 6

What has to be done in order to find the value of frequency at which the peak value is reached?

Answer 6

Planck’s function is set to zero

Question 6

How is the peak wavelength found?

Answer 6

Using Wien’s displacement law
Equation found by using the wavelength form of Planck function

Question 7

What happens in thermodynamic equilibrium?

Answer 7

The intensity is equal to the Planck function (I_v = B_v)

Question 8

What is the Rayleigh-Jeans law?

Answer 8

An approximation of the Planck function in the limits of low frequency and/or high temperature

Question 9

What is Wien’s Displacement Law?

Answer 9

It describes the location of the peak frequency (or wavelength) for an object emitting as a black body

Question 10

What are dark clouds (nebulae) ?

Answer 10

Molecular clouds consisting of entirely molecular hydrogen mixed with traces of molecules and small amounts of interstellar dust

Question 11

What are the dark lanes obscuring light from background stars?

Answer 11

Clouds filled with gas and dust, where stars are forming

Question 12

When does a Planck curve shift in peak position to the right?

Answer 12

When temperature is large enough

Question 13

In which type of imaging can star forming regions be seen clearly?

Answer 13

In CO where the dust appears brightest

Question 14

What is the temperature range of dust grains?

Answer 14

10 - 100K

Question 15

What wavelength does dust emit at?

Answer 15

Far infrared wavelengths (30 - 300um)
It appears brighter than stars as it radiates more strongly

Question 16

What is the amount of interstellar extinction roughly proportional to?

Answer 16

inversely proportional to wavelength

Question 17

Why can’t dust be seen at visible wavelengths?

Answer 17

too much interstellar extinction through absorption by dust that the galactic is obscured from view

Question 18

Why do protons and electron behave like a magnet?

Answer 18

They possess spin
Electric charges in motion generate magnetic fields

Question 19

What happens when the spin of the electron is in the opposite direction to the spin of the proton (spin-flip transition)?

Answer 19

A photon is emitted and in the case of hydrogen atom, the corresponding wavelength is 21cm (radio wavelength)

Question 20

What happens when the spin of the electron is the same as the spin of the proton?

Answer 20

Electron is in higher energy excited state

Question 21

Why does atomic hydrogen (most abudant element) emit radio waves?

Answer 21

Due to transition of electron

Question 22

How can the morphology of the Milky Way be mapped out using 21cm radio radiation?

Answer 22

Looking at plane, hydrogen cloud is moving at slightly different speeds relative to us when it travels along our line of sight
Doppler shifts of these radio waves help map it

Question 23

What aspects of a galaxy cannot be found using 21cm radio emission?

Answer 23

Gas moving perpendicular to our line of sight which won’t exhibit a detectable doppler shift so appear blank on mapping

Question 24

What is carbon monoxide?

Answer 24

a heteronuclear diatomic polar molecule (it has a non-zero electric dipole moment)

Question 25

What happens when a polar molecule rotates in space?

Answer 25

It changes energy between rotational states that obey the laws of QM
These rotational transitions used to infer that CO is molecular

Question 26

What is a the rotational constant B?

Answer 26

A fundamental property of every heteronuclear molecule

Question 27

What does a mm wavelength of radiation emitted due to energy difference between energy levels mean?

Answer 27

Telescope needs to be above ground but not in space

Question 28

What does a low temperature of < 10K for an energy level change show?

Answer 28

Energy level can be populated easily due to low temperature combined with 10K average temperature of molecular cloud

Question 29

How do excitations occur primarily?

Answer 29

Through collisions with ambient H2

Question 30

What does a higher temperature of gas mean?

Answer 30

More populated higher energy levels causing shorter wavelength transitions

(opposite occurs for lower temperatures: more populated lower energy levels)

Question 31

What does a jump in velocity from a spectrum indicate?

Answer 31

Objects captured more in the centre of the galaxy as it is redshifting

Question 32

What are Bok globules?

Answer 32

Small dense and almost spherical isolated clouds which are revealed using infrared surveys

They harbour young low mass stars

10 - 50 M_o

Question 33

How is molecular gas detectable?

Answer 33

By exploiting the low-lying rotational energy levels of CO that are easily excited in cold gas

Question 34

Where is most of the molecular gas in the Milky Way?

Answer 34

Giant Molecular Clouds (GMCs) confined in the spiral arms

M > 10^5 M_o