Section 3 Flashcards
What is the total intensity?
The specific intensity integrated over all frequencies
What is intensity independent of?
Distance as once inside the beam, radiation stays within it (it is conserved)
What is flux of radiation?
The amount of energy crossing a unit area per unit time per unit frequency interval
flux does depend of distance
What is the total flux?
The flux of radiation integrated over all frequencies/over radiation travelling from a surface in all outward directions
At which wavelength ranges is the atmosphere transparent to radiation?
Radio and optical
What is Planck’s function (B_v) used for?
To find the intensity of a blackbody radiation
What has to be done in order to find the value of frequency at which the peak value is reached?
Planck’s function is set to zero
How is the peak wavelength found?
Using Wien’s displacement law
Equation found by using the wavelength form of Planck function
What happens in thermodynamic equilibrium?
The intensity is equal to the Planck function (I_v = B_v)
What is the Rayleigh-Jeans law?
An approximation of the Planck function in the limits of low frequency and/or high temperature
What is Wien’s Displacement Law?
It describes the location of the peak frequency (or wavelength) for an object emitting as a black body
What are dark clouds (nebulae) ?
Molecular clouds consisting of entirely molecular hydrogen mixed with traces of molecules and small amounts of interstellar dust
What are the dark lanes obscuring light from background stars?
Clouds filled with gas and dust, where stars are forming
When does a Planck curve shift in peak position to the right?
When temperature is large enough
In which type of imaging can star forming regions be seen clearly?
In CO where the dust appears brightest
What is the temperature range of dust grains?
10 - 100K
What wavelength does dust emit at?
Far infrared wavelengths (30 - 300um)
It appears brighter than stars as it radiates more strongly
What is the amount of interstellar extinction roughly proportional to?
inversely proportional to wavelength
Why can’t dust be seen at visible wavelengths?
too much interstellar extinction through absorption by dust that the galactic is obscured from view
Why do protons and electron behave like a magnet?
They possess spin
Electric charges in motion generate magnetic fields
What happens when the spin of the electron is in the opposite direction to the spin of the proton (spin-flip transition)?
A photon is emitted and in the case of hydrogen atom, the corresponding wavelength is 21cm (radio wavelength)
What happens when the spin of the electron is the same as the spin of the proton?
Electron is in higher energy excited state
Why does atomic hydrogen (most abudant element) emit radio waves?
Due to transition of electron
How can the morphology of the Milky Way be mapped out using 21cm radio radiation?
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
What aspects of a galaxy cannot be found using 21cm radio emission?
Gas moving perpendicular to our line of sight which won’t exhibit a detectable doppler shift so appear blank on mapping
What is carbon monoxide?
a heteronuclear diatomic polar molecule (it has a non-zero electric dipole moment)
What happens when a polar molecule rotates in space?
It changes energy between rotational states that obey the laws of QM
These rotational transitions used to infer that CO is molecular
What is a the rotational constant B?
A fundamental property of every heteronuclear molecule
What does a mm wavelength of radiation emitted due to energy difference between energy levels mean?
Telescope needs to be above ground but not in space
What does a low temperature of < 10K for an energy level change show?
Energy level can be populated easily due to low temperature combined with 10K average temperature of molecular cloud
How do excitations occur primarily?
Through collisions with ambient H2
What does a higher temperature of gas mean?
More populated higher energy levels causing shorter wavelength transitions
(opposite occurs for lower temperatures: more populated lower energy levels)
What does a jump in velocity from a spectrum indicate?
Objects captured more in the centre of the galaxy as it is redshifting
What are Bok globules?
Small dense and almost spherical isolated clouds which are revealed using infrared surveys
They harbour young low mass stars
10 - 50 M_o
How is molecular gas detectable?
By exploiting the low-lying rotational energy levels of CO that are easily excited in cold gas
Where is most of the molecular gas in the Milky Way?
Giant Molecular Clouds (GMCs) confined in the spiral arms
M > 10^5 M_o
