Stars Flashcards
(221 cards)
1
Q
To first order, what does the output spectrum of a star appear to be?
A
A blackbody radiator
2
Q
What equation predicts the shape of an ideal blackbody spectrum?
A
Planck’s radiation law, where B is the specific intensity or spectral radiance of the light
3
Q
What are the units of B in Planck’s radiation law?
A
4
Q
Does a hotter blackbody emit a greater proportion of its radiation at shorter or longer wavelengths than a cooler blackbody?
A
Shorter wavelengths
5
Q
Does a hotter blackbody emit more or less radiation at all frequencies than a cooler body?
A
It emits more radiation
6
Q
What is Wien’s Displacement Law and what does it describe?
A
It describes the peak wavelength in a blackbody curve
7
Q
What is the Stefan-Boltzmann Law?
A
Integrating Planck’s Radiation Law over all wavelengths and all solid angles, total power radiated from unit cross sectional area of an ideal blackbody at Temperature T:
8
Q
What is the Stefan-Boltzmann Law in terms of energy density?
A
9
Q
What is the Stefan-Boltzmann Law in terms of absolute luminosity and what are its assumptions?
A
Assumptions:
-Spherical, radius R
-Surface of uniform temperature T
10
Q
What is seen to be emitted by a blackbody?
A
Continuous spectrum - blackbody emits light at all wavelengths
11
Q
What is seen to be emitted from a cooler gas cloud after light from a blackbody has passed through it?
A
Emission line spectrum - electrons in excited atoms drop down to lower energy levels, emitting photons at the same wavelength as the light from the blackbody. They are often superimposed onto a continuum spectrum
12
Q
What is seen when light from a blackbody has passed through a cloud of cooler gas?
A
Absorption line spectrum - atoms in a gas cloud absorb photons with the correct amount of energy to excite atoms to higher energy levels, leaving dark lines in spectrum at specific wavelengths
13
Q
Where is the hydrogen alpha line seen?
A
When an electron transitions from the n = 3 state to the n = 2 state in hydrogen, corresponding to 656.3nm
14
Q
Where is the hydrogen beta line seen?
A
When an electron transitions from the n = 4 state to the n = 2 state in hydrogen, corresponding to 486.1nm
15
Q
Where is the hydrogen gamma line seen?
A
When an electron transitions from the n = 5 state to the n = 2 state in hydrogen, corresponding to 434nm
16
Q
What is the absolute luminosity of an object?
A
The total radiated power, in W
17
Q
At distance D, what is apparent luminosity?
A
18
Q
Supposing we know or can estimate absolute luminosity L and measure apparent luminosity l of an object, what can we estimate?
A
Distance D to the object
19
Q
What is apparent luminosity often defined in terms of?
A
Apparent magnitude m
20
Q
What are the details of the apparent magnitude system?
A
-It is a logarithmic scale
-Based upon ancient classification
-Brightest naked-eye stars called
first magnitude
-Faintest naked-eye stars called sixth
magnitude
-Means a brighter star has a smaller (more negative) magnitude
21
Q
If the apparent luminosities of two objects differ by a factor of x100, then the difference in apparent magnitude is what? How is this expressed mathematically?
A
5
22
Q
What does m-bol mean?
A
M-bol is a magnitude of 0, corresponding to an apparent luminosity of 2.54 x 10^-8 W.m^-2
23
Q
What is the apparent magnitude of the Sun?
A
-26.8
24
Q
What is the apparent magnitude of Sirius (brightest star)?
A
-1.4
25
What is the apparent magnitude limit of the human eye?
+6
26
What is the apparent magnitude limit of large telescopes?
+25
27
How is absolute magnitude M defined?
The absolute magnitude of an object is the apparent magnitude it would have if seen at a distance of 10pc
28
How many light years is a parsec?
3.26 light years
29
How many meters is a parsec?
3 x 10^16 m
30
What is equation for distance modulus (simplest form)?
Distance modulus m - M is given by
31
What are bolometric magnitudes?
Apparent and absolute magnitudes measured over all wavelengths of light emitted by a star
32
What are the Ultraviolet, Blue and Visual bands?
33
How is Colour Index defined?
34
Colour index depends on the distance D to a star. True of false?
False, Colour Index does not depend on distance because
35
Colour index can be related to surface temp. True of false?
True
36
What surface temperature gives a B-V colour index of 0?
10,000K which is the surface temp of Vega, the primary standard star.
37
How does the distance modulus equation look with extinction taken into account?
38
On average how many magnitudes of extinction per kpc are seen through the ISM?
1-2 magnitudes per kpc
39
What is the H-R diagram a plot of?
Absolute magnitude (=luminosity) against spectral type (=surface temperature)
40
What is the spectral class and surface temp of the Sun?
G2 at 5770K
41
Spectral classes O, B, A, F, G, K, M are in order of increasing surface temp. True of false?
False, they are in order of decreasing surface T
42
Spectral classes are further divided into 10 parts from 0 (hotter) to 9 (cooler). True or false?
True
43
What are main sequence stars?
Stars, like the Sun, in the MS are the longest stage of a stars active life. They are converting hydrogen into helium via nuclear reactions.
44
What are Red Giants?
Large stars with high luminosity and low surface temp
45
What are Supergiants?
Very massive stars of large size and very high luminosity
46
What are white dwarfs?
Small dense stars of low luminosity and high surface temp
47
What different physical processes happen in the ISM?
-Energy generated by stars is absorbed and re-emitted in the ISM (indicates physical conditions within the ISM)
-Material enriched in heavy elements (not hydrogen and helium) is expelled from stars to mix with existing gas
-This material condenses to form new stars
-Solar system is made from recycled interstellar gas and dust which has been enriched by the ashes of a previous generation of stars
48
What are the mass fractions of H, He and heavy elements?
Hydrogen X = 0.738
Helium Y = 0.250
Heavy elements Z = 0.012
49
On average, how far apart are stars in the Milky Way?
10 light years
50
What is temperature and density like in the ISM?
Temperature T ~ 10 - 10^6 K and number density n ~ 10^-3 - 10^6 cm^-3
Even dense regions are ultra-high vacuum: Lab UHV ~ 10^6 - 10^7 cm^-3; STP ~ 3 x 10^19 cm^-3
51
How has extensive information about the distribution of neutral H atoms been collected?
Through the measurement of the hydrogen 21cm line, emitted when the electron spin "flips" w.r.t. the proton spin
52
In the region of the Sun, what can be said about neutral gas clouds?
-Neutral hydrogen has an overall mean particle density of ~ 10^6 m^-3
-Most mass of the ISM (>95%) is thought to exist as neutral H and He
-Clouds with size scales of order ~3pc
-Kinetic temperatures range from 50-150K, with an average value of ~80K
-Denser, more opaque clouds exist called dark clouds which have temperatures of <10K
53
What is dust in the ISM?
-Small solid particles or grains of radii <1μm
-Grains absorb and scatter light of all wavelengths
-The colours of of stars seen through the clouds are altered
54
How is light scattered by small particles such as electrons, atoms and grains?
-Photons are redirected
-Photon energies can be changed or unchanged
-Examples are Compton/Thomson scattering or scattering by atoms
55
What is Thomson scattering?
-Thomson scattering happens at low energies (hf<
56
What is Compton scattering?
-Photons scattered by free electrons
-Energetic collision of "particle" with electron
57
How are photons scattered by atoms?
-Photon excites electron in atom
-Electron de-excites
-Photon of same of other wavelength emitted, in any direction
58
How does the size of a particle compared with the wavelength of photons affect the intensity of scattering?
59
What is reddening?
The preferential scattering of blue light to red
60
Why do some nebulae appear blue?
Clouds of dust scatter the light from nearby stars and this scattering makes the nebula appear blue
61
Why can infrared radiation penetrate dust?
infrared radiation has λ >> dust size, but large λ
62
Why do emission nebulae radiate emission lines?
Because there must be hot gas. They shine due to intense UV radiation from hot stars
63
Why is HI (neutral atomic hydrogen) in the ground state generally in an emission nebula?
Because densities are low
64
What is the consequence of UV photons with λ ≤ 91.2nm ionising the hydrogen in an emission nebula?
≥ 13.6 eV photon energy
-Form HII region
-Ions recombine into H atoms
-Electrons cascade to ground state
-Emit several visible photons
65
What does most of the radiation in an emission nebula escape as?
Hydrogen Balmer lines
66
What are the typical conditions and typical scale of an emission nebula?
T ~ 10^4K
n ~ 10^7 - 10^10 m^-3
size ~ 10 light years
67
What dictates the size of an emission Nebula?
UV intensity and gas density
68
What is emitted by emission nebulae as well as hydrogen Balmer lines?
-Continuum radio emissions
-Forbidden lines e.g. NII or OIII
69
Roughly what percent of the ISM, by mass, is in the form of photo-ionised gas?
1%
70
What are molecular clouds?
-opaque clouds are >70% molecular hydrogen
-Molecular clouds are mostly molecular hydrogen
(from interstellar molecular hydrogen absorption lines)
71
Giant HII regions are always found near what?
Molecular clouds
72
What role to giant molecular clouds play?
-Key role in star formation
-Mostly found in spiral arms of galaxy
-Majority of stars form in giant molecular clouds
73
What are the conditions and the scale of Giant molecular clouds (GMCs)?
Cold: ~ 20K
Dense: up to 10^15 m^-3
Size: many 10s L.Y.s across
Mass: 10^5 Solar masses
74
What are GMCs mostly composed of?
Composed of mostly Molecular hydrogen with small traces of
75
What is the importance of the significant fraction of dust grains in GMCs?
They shield molecules from disassociation by UV photons
76
What do various axes of rotation of molecules imply in GMCs?
-Number of discrete rotational energy levels
-Change in rotational state means a potential emission or absorption of a photon
-Typically at λ ~ 1mm (radio telescopes)
77
What do Molecular emission lines tell us?
-Large scale dynamical gas flows exist (~ a few km.s^-1)
-GMCs have numerous dense clumps or cores [from (r ~ 0.05pc, M ≤ 10 Solar masses) to (r ≥ 1pc, M ≥ 100 Solar masses)]
-Small cores eventually form individual stars such as the Sun
78
For linear molecules, what are the permitted energy levels (i.e. those allowed by QM for a rigid rotator)?
79
How many moments of inertia does a rotating object have?
3, one about each axis
80
What are the details of CO w.r.t. its rotational transitions?
B = 3.83 x 10^-23 J, such that the J = 1 to 0 spectral line has a frequency of about 115GHz
81
What is the key characteristic of HCl or CO?
It has one atom with net +ve charge and one with net -ve charge.
This means it has a permanent electric dipole moment
82
What does the permanent electric dipole moment of molecules such as HCl or CO imply?
Molecules rotate - +ve and -ve charges oscillate sinusoidally
Molecules can interact with an EM field - absorption or emission of photons
83
Why is molecular hydrogen hard to observe in molecular clouds?
It has no electric dipole moment and is not easily excited
84
Why is CO used as a tracer of molecular hydrogen?
-CO is most abundant molecule after hydrogen
-CO possesses a small dipole moment, so electric dipole transitions are permitted
-Lowest (J = 0 to 1) rotational levels are separated by an energy equivalent to ~6K so it is readily excited in cold clouds
85
Generally, what processes cause broadening of spectral lines?
-Quantum mechanical description of the line (natural broadening)
-Effects of other gas particles (pressure broadening)
-The motion of photon emitting atoms (Doppler broadening)
86
What do motions of photon-emitting atoms consist of?
The random thermal motions of the atoms - which are superimposed onto any large scale motion
87
What are the two types of large-scale motion that photon-emitting atoms may be part of?
-Highly ordered e.g. rotation
-Highly non-ordered e.g. Turbulent outflow, Jets
88
How does thermal doppler broadening have an effect on astrophysical spectral lines?
-Random velocities of the atoms - causes line broadening
-Radial velocity - Doppler shift, Δλ
89
If velocity distribution of particles is Maxwellian, what is the spectral line profile?
Gaussian
90
To first approximation, what can be estimated from the linewidth of a spectral line?
The kinetic temperature of the gas
91
What is the linewidth, full width half maximum (FWHM)?
92
What various conditions do emissions from interstellar atoms and molecules occur?
-In thermal equilibrium (i.e. emission of BB radiation)
-Atoms radiatively excited (e.g. emission nebula)
-Atoms collisionally excited
93
What determines why certain spectral lines are seen strongly at specific characteristic temperatures?
-Distribution of electrons in different energy levels
-Relative numbers of atoms in various stages of ionisation
94
What distribution do particles in thermal equilibrium follow?
Maxwell-Boltzmann distribution (range of speeds with max kT and rms 3/2kT)
95
How are number densities of atoms in level B related to that in level A in thermal equilibrium?
Boltzmann equation
96
What is the degeneracy of level n for the hydrogen atom?
97
When are states n=1 and n=2 equal for hydrogen?
At temperatures of 10^5 K
98
In general what does hot gas consist of?
Neutral atoms, ions and free electrons
99
When is a steady state of ionisation reached?
When the rate of ionisation equals the rate of recombination.
100
What is the partition function Z?
The weighted sum of the number of ways the atom/ion can arrange its electrons with the same energy amongst the possible orbitals.
101
A more energetic (less likely) configuration receives a lower weighting from the Boltzmann factor. True or false?
True
102
What is the Saha equation?
103
What conversion is needed to express the Saha equation in terms of electron pressure instead of electron number density?
104
In summary, what does the Boltzmann equation tell us?
Number of atoms in an excited state w.r.t. another state
105
In summary, what does the Saha equation tell us?
Relative populations of two adjacent stages of ionisation
106
What does the presence of 1 He atom in every 10 H atoms imply in stellar atmospheres?
The presence of ionised He provides additional electrons with which protons can re-combine... with He present, it takes a higher T to achieve the same degree of hydrogen ionisation
107
What does the change of intensity of some EM radiation from a background source passing through an absorbing medium depend on?
108
What is the coefficient called which is a combination of the scattering and absorption coefficients?
Extinction coefficient
109
What does the term absorption coefficient strictly refer to?
110
How is optical depth defined?
111
How is the width of the area enclosing a spectral line defined?
112
How is width related to optical depth?
113
How is cross section related to optical depth?
114
If the mean value of cross section is known and gas is optically thin, what does width depend linearly on?
Column density, N
115
When a gas is optically thick how does Width depend on N?
W ∝ √N
116
How can a gas/dust cloud contribute to emission in the line of sight in addition to absorbing and scattering EM radiation passing through it?
If absorption is caused by solid particles (e.g. dust), they act as BB radiators and emit EM radiation in the line-of-sight
117
What is a solid angle?
118
If energy dE flows through area dA in a time interval dt, in wavelength range λ to λ + dλ, within solid angle dΩ, what is the power dP received? (Hint: includes specific intensity)
119
How is specific intensity defined in terms of frequency range as opposed to wavelength?
120
What is specific intensity often also known as, particularly in radio astronomy?
Brightness (outside of astronomy it can be known as spectral radiance)
121
The specific intensity (brightness) of a source is dependent on distance from the source. True of false?
False. It is independent of distance, unlike flux density and flux which vary with distance
122
A passive optical system (e.g. a collection of lenses and mirrors forming a simple telescope) does not increase the specific intensity of the radiation. True of false?
True. If you look at the moon through a telescope, it will appear larger (in angular size) than with the naked eye, but not brighter.
123
What is total intensity?
The specific intensity integrated over all wavelengths (or frequencies)
124
What is flux density?
Spectral power received by a detector of unit projected area, integrated over the solid angle subtended by the source
125
What is total flux?
Integrating Flux density over all wavelengths gives total Flux F
126
What is specific energy density?
127
What is total energy density
128
Although massless, what do photons carry that allows them to exert radiation pressure?
Momentum
129
By integrating over all solid angles, what is radiation pressure shown to be exerted by photons of wavelengths in the range λ to λ + dλ?
130
What does the radiative transfer equation describe?
How specific intensity changes if radiation is absorbed or emitted along the path of travel
131
What is the definition of the absorption coefficient?
It is a measure of the absorption per unit path for photons of wavelength λ
132
How is absorption defined over a length L?
133
What is the value of optical depth when a medium is optically thin?
τ << 1
134
What is the value of optical depth when a medium is optically thick?
τ >> 1
135
How is emission coefficient defined?
136
What is the equation for radiative transfer?
137
How is the equation of radiative transfer re-written generally for a background spectrum passing through a volume of gas in local thermal equilibrium?
138
How is mean free path defined?
Average distance travelled between collisions of two atoms
139
What are the four main causes of opacity for a hot gas?
-Bound-bound transitions
-Bound-free absorption
-Free-free absorption
-Electron scattering
140
What is a Bound-bound transition (excitation and de-excitations)?
Occurs when an electron in an atom or ion makes a direct transition from one orbital to another. If an electron absorbs a photon, then returns directly back to the original orbital, a photon of the same wavelength is emitted but in a random direction.
141
What is Bound-free absorption?
Photo-ionisation. Results in an electron being freed from the atom. The resulting free electron can have any energy.
142
What is Free-free absorption?
A scattering process which takes place when a free electron in the vicinity of an ion absorbs a photon, causing its speed to increase. The electron can also lose energy when approaching the ion, causing a photon to be emitted (bremsstrahlung).
143
What is electron scattering?
A photon is scattered by a free electron or a loosely bound one (Thomson/Compton scattering)
144
What is the assumption needed for the equation below to be valid?
The processes governing the behaviour of the absorbing/emitting atoms must be thermal in nature.
145
At long wavelengths, what approximation can be made for the Planck function B(T)?
The Rayleigh-Jeans approximation
146
What is the definition of brightness temperature?
The temperature of a blackbody that would give off the same intensity I(λ) in a given frequency range
147
What is the equation that relates brightness temp at the source (T_b0), measured brightness temp (T_b) and the temp of the absorbing material (T)?
148
What condition needs to be met for an absorption line to be seen?
149
What can we tell from a clump of gas which is optically thin and emission from right through the gas is received?
Tells us about the column density of the gas/dust
150
What can we tell from a clump of gas which is optically thick and almost all of the radiation originates in a thin layer at the front of the gas?
Tells us about the temperature of the gas/dust
151
What direct evidence is there for small solid particles in the ISM?
-General extinction of light from stars
-The extinction increases with the path length through the ISM
152
What indirect evidence is there for small solid particles in the ISM by relative abundances of elements in interstellar gas?
-Elements most likely to form refractory solids of elements in the interstellar gas (atoms of these elements are locked up in refractory solids)
-Molecular hydrogen molecules need to form at a high rate compared with that from known gas-phase reactions (the required rate can be achieved if molecular hydrogen molecules are formed on grain surfaces)
153
What can be inferred by continuous extinction of starlight?
That there are small dust grains in the ISM
154
What can be inferred by optical extinction varying as 1/λ?
Particles have a size of ~0.3μm
155
What can be inferred by strong extinction in the UV?
Particles sizes of ≤0.1μm
156
What can be inferred by depletion of Mg, Fe, Si etc in gas?
Present in oxide or silicate solids
157
What can be inferred by the conversion rate of H into molecular H?
That grains are catalysing the reaction
158
What can be inferred by nebulosity around stars?
Scattering is occurring due to dust
159
What can be inferred by polarisation of starlight?
Aligned with particles with size ~λ
160
What can be inferred by IR spectra of circumstellar shells?
Absorption and emission features in spectra due to particles?
161
What are dust grains mainly composed of?
Heavy elements e.g. C, N, O, Al, Si, Fe, Mg
162
What is extinction?
The combination of absorption and scattering
163
What is a measure of the capacity of a single dust grain to remove radiation from a parallel beam of photons?
Extinction cross-section
164
How is extinction optical depth defined?
165
What is the equation for monochromatic flux received from a star with luminosity L_λ without extinction?
166
What is the equation for monochromatic flux received from a star with luminosity L_λ with extinction?
167
What is the distance modulus equation with extinction?
168
What is Colour index?
169
What is Colour excess?
Directly related to extinction
170
How is extinction defined at different wavelengths?
171
What is the general form for extinction?
172
What is the equation for extinction in magnitudes?
173
Over the UV-IR range, what is the extinction cross-section of a dust grain?
174
What is the core-mantle grain model?
In order for a grain to have the correct extinction curve as we see in the ISM, it requires:
-presence of water and other ices on the grain under certain physical conditions
-small grain cores (~0.05μm) made of graphite or silicates
-cores are coated with mantles which contain icy materials (water and other ices)
175
What are the two main sources of dust?
-Formed in outer atmospheres of cool red giants and pushed out into ISM
-Form in the ejecta from planetary nebulae, novae and supernovae.
176
What can dust grains form in the regions they form in?
-sufficiently dense (~10^19 m^-3)
-sufficiently cool (~10^3 K) for solids to condense out of gas
177
How does the nucleation process of dust grains occur?
-Starts via chemical reactions producing large molecules which are stable against disruption in ambient conditions.
-Material then condenses onto growing grains, if partial pressure of condensing species is sufficiently high and temperatures moderate.
178
What happens in the atmospheres of red giants where carbon is more abundant than oxygen?
-Most oxygen is locked up in CO molecules
-Remaining carbon condenses into graphite and hydrocarbons
179
What happens in the atmospheres of red giants where oxygen is more abundant than carbon?
-Most oxygen is locked up in CO molecules
-Remaining oxygen condenses into metal oxides
180
What pushes smaller dust grains out into the ISM, which if any larger would bind to the red giant?
Radiation pressure
181
What condition must be met for a grain to be pushed out into the ISM?
-The inward gravitational force on the dust grain must be smaller than the force due to radiation pressure.
-Drag effects this process (increase to effective mass of grain) meaning in practice the force due to radiation pressure must be greater than or equal to 10 times the force due to gravity.
182
What is the free-fall collapse timescale (dynamical timescale)?
Corresponds to the case when there is no pressure gradient to oppose the collapse.
183
What is the equation for escape velocity?
184
How is a characteristic timescale defined?
Characteristic distance divided by characteristic speed
185
Considering what situation provides the Kelvin-Helmholtz timescale?
-Collapse slowed by a pressure gradient
-Photons can not escape easily, so energy is absorbed, heating the object
-Example is a contracting protostar approaching MS
186
What is the Kelvin-Helmholtz timescale?
Characteristic rate of change of total energy determined by rate at which energy is radiated away (luminosity) of the proto-star.
187
How does the KH timescale compare to the free-fall timescale?
KH is much larger that FF but still much less than stellar lifetime
188
What holds together stars and large gas clouds?
-Self-gravitational collapse
-Internal pressure opposing collapse
189
What is hydrostatic equilibrium?
When inward grav attraction exactly balances outward pressure force at every distance r
190
What is the equation for hydrostatic equilibrium?
191
What is the full form of the Virial theorem?
U is total thermal energy of the cloud
Ω is the gravitational potential of the whole system
192
What is the Virial theorem when ignoring the surface pressure exerted by any surrounding gas?
193
What happens when
Gravity wins therefore contraction occurs
194
What happens when
There is equilibrium
195
What happens when
Pressure wins therefore expansion occurs
196
What does the Virial theorem tell us if gravitational collapse is slow (close to hydrostatic equilibrium)?
-Grav P.E. released (half as K.E., half is lost - radiated away)
-Cloud must lose energy to collapse
-Cooling mechanisms very important
197
What is the Jeans mass?
A cloud of radius R can condense if its mass exceeds the Jeans mass
198
What is the Jeans density?
A cloud of mass M0 can condense if its average density exceeds the Jeans density
199
What is M_0 and m_bar in relation to Jeans mass and density?
The mass of the cloud and the average mass of a particle in the cloud
200
What conditions make condensation easier?
Cold gas cloud and large mass
201
What is the brief overview of star formation?
-starts as free-fall collapse from molecular clouds (photons can easily pass through cloud)
-As clump becomes optically thicker, photons increasingly trapped and gas/dust heats up
-clump contracts to protostar
-As protostar continues to contract, grav P.E. of particles transformed into K.E. causing gas and dust to heat up
-protostar becomes MS star
202
What happens when a star forming clump reaches 10^-15 kg.m^-3 ?
-Fragments of solar mass size can contract independently
-Can form pre-protostars with radius ~10^15m (10^6 solar radius)
203
How is it possible for a clump to freely collapse unopposed by internal pressure?
If energy of photons is not absorbed and converted to random thermal motion of particles. Possible if:
-photons escape cloud easily (happens initially)
-energy goes to disassociating molecular hydrogen, then ionisation of H
204
What is the equation for energy needed to dissociate and ionise all hydrogen in a protostar of mass M?
205
How is the energy for dissociation and ionisation supplied?
By the gravitational collapse of the protostar from an initial radius to a final radius
206
What equation for energy per particle?
3/2 kT per particle
207
What is the total thermal KE of ions and electrons in a protostar?
208
What is the gravitational PE at the end of a period of rapid collapse?
209
What criteria defines a star?
Nuclear energy generation rate equals radiated power
210
What does the IR spectra of a Class I YSO look like and what does it mean for the object?
-Broader energy distribution than single BB object
-Radiation at far IR λ dominates spectra
-Broad peaks at 10 μm and 100 μm
-rarely exhibit detectable emission at optical λ
-protostars surrounded by large mass of material
-some material at 30K (ambient cloud temp) gives 100 μm peak
-warmer material at 300K heated by proximity to central protostar
-protostar still accreting material from surroundings
211
What does the IR spectra of a Class II YSO look like and what does it mean for the object?
-Significant emission in far IR, superimposed onto BB spectrum at shorter λ
-surrounded by less dust than class I, hotter central star
-emission line characteristics of T-Tauri stars (pre-main sequence stars of low mass)
-Far IR shows some luminous material is present
212
What does the IR spectra of a Class III YSO look like and what does it mean for the object?
-spectral shape reasonably fitted to BB curve of single object
-consistent with spectral distribution expected from photosphere of young stars, little evidence of circumstellar gas or dust
213
What does class 0 refer to?
young accreting protostar
214
What are stellar winds?
Streams of energetic protons/electrons emanating from surfaces of stars
215
What are molecular outflows?
-bipolar - two lobes of emission
-one lobe blue shifted gas, one red shifted
-outflow pushed by intense stellar wind from YSOs
216
What are the consequences of material falling onto a star having angular momentum?
-inflow is not direct
-material accumulates in circumstellar accretion disk
-it then spirals onto equator
217
What are the consequences of the weak magnetic field permeating the ISM on star formation?
-Through conservation of mag flux, collapse of material into disc is highly magnetic
-mag field lines thread vertically through disc
-disc channels outflow, physically and magnetically
218
What is essential for star forming to take place?
stellar winds carrying away angular momentum from the star
219
What is the equation for the size scale of a collapsing region within a cloud?
220
What is the equation for centripetal acceleration, what is it equal to and what does it imply?
collapse parallel to rotation axis can still occur and cloud flattens to disk
221
What is the equation for magnetic flux?
