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Astronomy 2 > Stars and Their Spectra > Flashcards

Stars and Their Spectra Flashcards

1
Q

we model stars as…

A

self gravitating spheres of gas with the inward pull of gravity balanced by the outward gas pressure

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2
Q

the star’s core must be hot enough to

A

allow nuclear fusion to take place

ie around 10 million kelvin

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3
Q

the internal structure of a star and the way that energy flows through it depends

A

primarily on mass but also chemical composition

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4
Q

what does mass of star determine

A

internal structure of star and way energy flows through it

surface temperature

luminosity

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5
Q

convection dominates in

A

lower mass stars

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6
Q

radiation dominates in

A

larger mass stars

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7
Q

where do we learn about flux variations with energy or wavelength

A

spectra

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8
Q

where do we learn about flux variations with time

A

light curve

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9
Q

we learn everything we know about stars from

A

spectra
light curves
EM theory

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10
Q

primary source of information about physical nature of stars

A

their spectra

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11
Q

planck function

A

everything constant except frequency

wien’s displacement law derived from it

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12
Q

cooler stars peak wavelength

A

towards infrared

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13
Q

stellar photospheres are good approximations to a blackbody function but in reality….

A

their spectra are complicated by the presence of spectral lines and edges

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14
Q

edge

A

jumps in intensity on spectra

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15
Q

spectral sequence

A

OBAFGKM

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16
Q

stars in our neighbourhood have roughly

A

the same chemical composition

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17
Q

stars in our neighbourhood have different spectral lines due to

A

different temperatures

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18
Q

the stellar surface or atmospheric temperature determines

A

which ions or excited states are present

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19
Q

‘early’ group in the spectral sequence

A

OBA stars

7500<T<25000+

He, H lines

A stars have weaker H, stronger metals

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20
Q

‘solar type’ group in the spectral sequence

A

FG stars
5000<T<7500
very weak He, some metals

G stars have strong metal ions and neutrals

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21
Q

‘late’ group in the spectral sequence

A

KM stars
coolest with 3500-<T<5000

metal ions, neutrals , molecules at low enough temps

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22
Q

metal definition for astronomy

A

anything other than H or He

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23
Q

why do H and He lines only occur in hot stars

A

helium and hydrogen have high ionisation/ excitation energies (to move e- to upper level) so can only occur in hot stars

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24
Q

why are metal lines formed over a large temperature range?

A

many species with a range of ionisation energies depending on atomic mass

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25
Q

ionised metals occur more in

A

hotter stars because electrons can be stripped from their atoms in hot temps

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26
Q

neutral metals occur more in

A

relatively cooler stars

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27
Q

why can metals only exist in the outer parts of cool stars (or in thermally insulated parts of hotter stars like sunspots)

A

they are broken up at fairly low temperatures

100s to a few 1000s of kelvin

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28
Q

in the Harvard classification scheme, each type (OBA etc) is split into

A

10 subgroups 0-9 with zero being the hottest

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29
Q

line strengths and equivalent widths vary with

A

spectral type

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30
Q

early attempts to classify stars used

A

the strength of their hydrogen lines

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31
Q

how can singly ionised helium be observed when mean energy of an electron &laquo_space;ionisation energy of helium

A

maxwell boltzmann distribution

there exists some electrons with energy high enough to ionise helium

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32
Q

Hr-diagram, groups stars according to

A

spectral type and absolute magnitude

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33
Q

what did the first plot of a HR-diagram reveal?

A

a main band of stars

main sequence

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34
Q

problems with classifying stars using Hr-diagram

A

lack of fine resolution in the OBAFGKM scheme so stars line up in columns and data is discretised

HR classification is subjective based on ability to detect spectral features

some stars have odd spectra and don’t easily fit into any of the classes

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35
Q

because of difficulties with classifying stars using HR-diagram, often use

A

stellar colour instead as the basis of star classification

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36
Q

stellar colour

A

difference between stellar magnitudes

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37
Q

how is stellar colour measured

A

using standard filters in different wavelengths such as the Johnson UBV filters

eg B-V colour = mb-mv

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38
Q

difficulties with using stellar colour

A

requires well-calibrated filters for comparison between different instruments

can be tricky in a practical sense

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39
Q

colour is not independent of

A

distance

but spectra are

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40
Q

extinction of light due to interstellar medium along the observer’s line of sight is dependent on

A

wavelength

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41
Q

the diagrams essentially displaying the same info as HR are called

A

colour-magnitude diagrams

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42
Q

standard when plotting colour-magnitude diagrams is to use

A

UBV photometric system

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43
Q

Johnson system

A

U=m in U band (central wavelength =365nm, delta lambda=68nm)

B=m in B band (central wavelength = 440nm, delta lambda=98nm)

V=m in V band (central wavelength = 550nm, delta lambda=89nm)

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44
Q

colour index of a star

A

difference between the values of pairs of its Johnson magnitudes

eg U-B or B-V

can be extended to infrared with Cousins filters (UBVRI) (R=red, I=infrared)

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45
Q

transmission coefficient (used in Johnson-Cousins filter transmission profiles)

A

1= all light goes through
0= no light goes through

eg window approx 0.9

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46
Q

colour-colour diagram

A

eg U-B against B-V for objects of different temperatures

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47
Q

since filters produce brightness measurements in narrow frequency ranges,

A

a blackbody of a given temperature has a unique position on a CC diagram

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48
Q

colout colour diagram features

A
  1. main sequence lies below blackbody line
  2. there is a point of inflection between A5 and G0 stars
  3. the origin corresponds to a star of type A0V with (B-V)_0=0
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49
Q

the U-B axis on CC diagram increases

A

downwards for a given B-V colour so stars have U larger than B, they have a deficit of U band photons to B band photons

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50
Q

the large dip in photon flux at the blue end of the spectrum is due to

A

neutral hydrogen in the n=2 level being ionised

theis is the balmer series and the decrease in photon number shortward of 364.6nm is called the balmer jump. It is an ionisation edge

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51
Q

stars can be broadly classified according to their

A

luminosity

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52
Q

stefan boltzmann law

A

L prop to R^2T^4

for a given T, luminosity tells us about a star’s size

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53
Q

Morgan Keenan Kellman luminosity classification system

A

Class I: supergiants
Class II: intermediate giants
Class III: giants
Class IV: subgiants (for F-K stars), main sequence for B stars
Class V: main sequence

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54
Q

luminosity classification extends the classification of stars into

A

a two dimensional system of temperature and luminosity

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55
Q

classification of the sun

A

G2V

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56
Q

luminosity indicator

A

spectral feature (such as a line or a group of lines), the property of which is dependent upon the stellar luminosity or absolute magnitude, within one spectral class

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57
Q

luminosity indicator - O stars

A

width of the hydrogen lines increase with decreasing L

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58
Q

luminosity indicator - G stars

A

CaII, H, K widths increase with increasing L

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59
Q

luminosity indicator - K stars

A

CN line strength increases with increasing L

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60
Q

luminosity indicators are positive or negative depending on

A

whether the appearance of the indicator is enhanced or reduced as the luminosity increases

(enhanced =+, reduced=-)

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61
Q

photons travelling through the interstellar medium are

A

absorbed and scattered by it

this leads to extinction and reddening

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62
Q

extinction/reddening is a systematic shift of

A

a star’s position on the colour-colour or colour-magnitude diagrams

one of the disadvantages of using photometric instead of spectroscopic classifications

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63
Q

effects of extinction/reddening are dependent on

A

distance and can be corrected for

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64
Q

blue light and red light in the ISM

A

blue is scattered/absorbed by ISM gas/dust more than red light

bluer photons are removed and the star appears reddened. (interstellar reddening)

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65
Q

interstellar extinction causes the loss of photons
to quantify…

A

V - apparent measured magnitude in visual band
mv - the true apparent magnitude
Av - extinction coefficient

V=mv+Av

mv related to distance modulus

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66
Q

the amount of absorption depends on

A

the optical depth, tau, of dust in the ISM

Iout=Iine^-tau

so A=mout-min=-2.5log10e^-tau

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67
Q

optical depth is defined as

A

tau=N sigma d

N = number density
sigma = interaction cross-section
d=distance

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68
Q

for dust grains larger than lambda,

A

set sigma to be the grain’s geometrical cross-section

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69
Q

for dust grains smaller than lambda,

A

the interaction cross-section becomes dependent on wavelength and we observe reddening

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70
Q

blue light is scattered efficiently by

A

Rayleigh scattering where the scattering cross-section (probability a photon is scattered) varies as 1/lambda^4

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71
Q

due to the wavelength dependence of extinction, the B band flux

A

decreases more than V band flux

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72
Q

B-V and U-B both increase with increasing distance to star

measured B-V colour index must be

A

corrected by the colour excess to find true B-V colour of the star

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73
Q

Av is approx

A

3 E_B-V

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74
Q

bolometric correction

A

Mbol=MV+BC

BC can be determined from Pogson’s equation

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75
Q

how to find L and BC empirically

A

integrating over observations across a broad set of filters or using synthetic spectra

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76
Q

compared to solar type stars (spectral energy distribution peaks in visual band), bolometric corrections are

A

larger for early (hot, UV peak) and late (cool, IR peak) type stars

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77
Q

a light curve is produced by

A

monitoring the recorded intensity of a star over time (aka time series)

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78
Q

the light curve can be used to glean information about various aspects of the star such as

A

surface features on a rotating star such as starspots or disk hotspots.

the presence of a planet

stellar pulsations eg cepheid variables

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79
Q

eclipsing binary system: stars are so close to each other that

A

tidal forces cause mutual distortions away from sphericity

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80
Q

proportion of stars in binary systems

A

early-type stars >50%
solar-type stars approx 50%
late-type stars <50%

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81
Q

binary components not usually individually resolvable but using spectroscopy can look at

A

doppler shift of lines from spectroscopic binaries

light curves of eclipsing binaries

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82
Q

spectroscopy allows what measurements to be made

A

radial velocity

“radially towards the observer”

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83
Q

the inclination, i of a system with respect to our observation direction tells us

A

whether the system is edge on, i=90 i or face on i=0

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84
Q

edge on

A

i=90
bodies can totally occlude one another

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85
Q

face on

A

i=0
observer is above or below the system

(think plate analogy)

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86
Q

orbital period P of a star can be determined by

A

plotting the radial velocity curves of each star in the binary system

assume circular orbits, v=2pi r/P

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87
Q

a lower limit on the total on the total system mass M=m1+m2 can be obtained using

A

Kepler’s third law

assume momentum conservation

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88
Q

how to determine sin i and hence radii of stars

A

observe eclipsing stars

combine with radial velocity measurements

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89
Q

if stars are seen to totally eclipse then we may assume

A

i=90 so sini=1

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90
Q

we can infer stellar radii from

A

eclipsing binary light curves

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91
Q

realistic eclipsing binary light curves

A

must find a fit to i based on theoretical models of the light curve, including limb darkening

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92
Q

exoplanets have been discovered using three main methods

A

radial velocity measurements of the parent star

planetary transits

gravitational lensing

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93
Q

difficulties of radial velocity method

A

very high resolution spectroscopy and bright sources needed due to delta lambda approx 10^-14m

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94
Q

difficulties of transit method

A

very bright, stable sources are required so as to reduce the impact of photon counting noise and intrinsic variability in light curves

need regular long term observations of multiple transits

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95
Q

what have produced promising exoplanet candidates

A

big surveys eg kepler satellite which monitor hundreds of thousand of stars and focus on transits

separate radial velocity measurements can be done to determine mass, radius of orbit and planet density

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96
Q

when the transit light curve is used to determine the radius of the planet…

A

density and surface gravity can also be found

assuming sphericity and constant density

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97
Q

from the planet’s orbital radius, can determine

A

if a planet is in the habitable zone (region where liquid water may exist)

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98
Q

after expending some fraction of their core fuel, stars

A

evolve off the main sequence

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99
Q

as stars move towards the giant phase

A

they undergo pulsations

100
Q

cepheid variables

A

used as distance indicators

pulsates on a regular basis such that its radius changes

as a consequence, there are also changes in luminosity L=4piR^2sigmaT^4

101
Q

cepheids are used as distance indicators because

A

they have a tight correlation between oscillation period and luminosity

102
Q

cepheid variable: what forces are at play?

A

driving force and the restoring force

driving: radiative force drives outer layers of star outwards
restoring: gravity pulls them back

103
Q

we can derive the period-luminosity relationship by analogy with the case of

A

the simple pendulum

start from P=2pi sqrt(l/g)

104
Q

period-luminosity relationship derivation from simple pendulum

A

replace l with R and g=GM/R^2

assume spherical with mean density

then period mean density relationship

105
Q

period-luminosity relationship can be sued to determine

A

the absolute magnitude and hence distance to a Cepheid and its host galaxy (what makes them standard candles)

106
Q

the oscillation (oulsating cycle) only happens under the circumstances

A

particular to evolved stars

107
Q

to maintain the cycle, requires that the radiative driving term

A

decreases as the star expands and increases as it contracts

108
Q

the effectiveness of the radiative driving term depends on

A

how opaque the as is, described by the opacity kapa

109
Q

high opacity

A

large push from radiative driving

110
Q

low opacity

A

photons stream right through; no push

111
Q

cepheid cycle diagram

A

atmosphere contracts
ionisation increases (everything closer=more collisions)
opacity increases
radiation trapped
large outward radiation force
atmosphere expands
ionisation decreases
opacity decreases
radiation streams out
gravity dominates

112
Q

the baade-wesselink is used to

A

calculate the radii of pulsating stars

we require to know the variation of magnitude, stellar colour and radial velocity with time

113
Q

Baade-wesselink: pick data times t1 and t2 where

A

the colour indices are approximately equal

this means stellar temperatures are also approximately equal

114
Q

Baade-Wesselink: luminosities are different because

A

radii are different

115
Q

two equations from Baade-wesselink

A
  1. from stefan boltzmann and pogson
  2. from spectroscopy we also have radial velocity as a function of time
116
Q

Baade wesselink: factor of 3/2 arises because

A

we measure vr averaged over the stellar disk, it is the average of vrcostheta from theta=-pi/2 to pi/2

117
Q

spectral properties of hot stars are dominated by their

A

huge radiation output

they are massive stars with circumstellar winds

118
Q

circumstellar winds are driven away by

A

radiation pressure

the outflows of gas result in a characteristic spectral line shape - a P Cygni profile

119
Q

thomson cross-section, sigmaT

A

consider electron at distance d from a star of luminosity L

electron has a probability of interacting with a photon which is described by its interaction cross-section

120
Q

we can think of photon flux as a momentum flux since

A

each photon carries momentum

when a photon strikes an electron, and is absorbed or scattered momentum conserved so electron momentum increases

121
Q

thomson cross section derivation

A

start with flux and energy of a photon

number of photons per second per square meter = f/E

p=hv/c

radiation force on electron is equivalent to momentum transferred per unit time

122
Q

electron will move outwards from the star if

A

Frad > Fg

123
Q

Fg acts on everything but since mp»me

A

its relative effect comapred to Frad is bigger for protons compared to electrons

124
Q

Eddington luminosity

A

luminosity above which a star will blow away its outer layers in a wind

125
Q

mass loss rates are deduced from

A

observations and by applying the mass continuity equation so as to calculate the density and velocity of the stellar wind

126
Q

mass loss rates are conserved

A

mass leaves the shell at the same rate it enters

127
Q

all hot stars have winds driven by radiation pressure directly inferred by

A

the presence of a P Cygni profile in the star’s spectrum

128
Q

two main characteristics of P-Cygni profile

A

blue shifted absorption dip

red wing emission

129
Q

blue shifted absorption dip cause by

A

absorption of the star’s radiation by the stellar wind moving towards us, between us and the star.

130
Q

red wing emission caused by

A

light scattered towards us by particles in the wind and light that is directly emitted by the stellar wind

131
Q

wind speed increases with distance from the star up to

A

some maximum speed known as the terminal velocity

132
Q

in terms of a P Cygni profile, terminal velocity is defined as

A

the point where the absorption dip crosses the velocity axis

133
Q

velocity of stellar wind increases with

A

increasing radial distance according to stellar wind velocity law

134
Q

it is possible to measure from the P Cygni profile

A

velocity at the surface of the star is close to the local sound speed and depend only on stellar surface temperature

135
Q

main early type (OBA) stars are rapid rotators

evidence for this is seen in

A

their broad, (semi-) elliptical line profiles

136
Q

if you observe a large value for delta lambda, you may conclude that

A

the star’s rotation speed is also large

137
Q

the combination of many rotating components towards, at rest and away from the observer…

A

produces a broad profile with half-width delta lambda

total width of profile is combination of blue-shift, rest and red-shift

138
Q

all points on a chord AB, distance l from the rotation axis are travelling at the same speed and they produce

A

Doppler-shifted absorption or emission

139
Q

for each individual line component, the line Doppler shift depends on

A

the chord’s l position only

  1. l=R - delta lambda (l) = max
  2. l=0 - delta lambda (l) = 0
140
Q

the strength of the spectral line depends on

A

how much material is contributing to it

I(delta lambda) / I (lambda 0) = AB/2R

141
Q

If the star rotates with its axis at some inclination angle i then VE can be replaced by

A

VEsini

142
Q

the maximum rotation period of a star can be calculated since

A

VE>VEsin i

143
Q

Be stars

A

B stars with hydrogen balmer lines in emission

eg balmer H alpha (level 3 to 2 transition)

144
Q

the emission of Hbeta in a Be star is thought to originate from

A

a circumstellar disk of material

145
Q

Be stars are surrounded by

A

disks or have extended envelopes

146
Q

depending on the viewing angle of the star, the observer sees

A

different line shapes

147
Q

B stars exhibit strong radiation pressure. Since they are also rotating, the material on their surfaces is

A

subject to a strong centrifugal force which helps to drive material outwards from equatorial regions of the star, forming a disk

148
Q

what shows Be stars to have particularly pronounced rotation

A

broad elliptical profiles

149
Q

a particle in a stellar atmosphere rotating with the star is subject to

A

radiation, centrifugal and gravitational forces

150
Q

how to determine conditions where equilibrium breaks down

A

equating forces

eg: equation centrifugal and gravitational gives maximum rotation velocity

151
Q

centrifugal force has the biggest effect at

A

the equator where theta=pi/2

152
Q

why can material escape more easily from equator

A

lower effective surface gravity at the equator due to rotation

153
Q

centrifugal break-up limit

A

Fcen + Fg > or = 0

154
Q

maximum rotation velocity possible before the star breaks up

A

set RHS=0 and theta=90 (equator)

155
Q

thomson scattering occurs when

A

photons are scattered by electrons which remain bound during the collision

156
Q

predominant scattering process in B and Be stars

A

thomson scattering

157
Q

thomson scattering in the corona

A

photospheric radiation is Thomson scattered by electrons spiralling in the Sun’s coronal magnetic field and it is this that we observe during a solar eclipse.

158
Q

polarisation describes how

A

electric and magnetic components of the electric field behave relative to its propagation direction

159
Q

if we look at only the electric field, the act of Thomson scattering is similar to

A

that of the polarising filter

after filter, electric field oscillates in a single direction and is linearly polarised

160
Q

circular polarisation

A

special case of elliptical polarisation arising when x and y electric field vectors are of equal magnitude and out of phase by pi/2

161
Q

in stars, a polarisation signal tells us

A

about the circumstellae material because therein is what gives rise to polarisation

162
Q

polarisation signal is dependent on

A

the shape of circumstellar material

163
Q

spherically symmetric wind gives

A

zero net polarisation

164
Q

irregular envelope can give

A

polarisation signal that is dependent on the shape

165
Q

disk tends to give

A

strong linear polarisation

166
Q

studies of spectral line profiles and polarisation in Be stars show

A

linear polarisation, supporting the theory that these stars have circumstellar disks

167
Q

stellar classes F to M show evidence for

A

magnetic activity

168
Q

sunspots

A

surface magnetic field
activity cycle
rotation

169
Q

flares

A

coronal magnetism

170
Q

surface oscillations

A

interior magnetic field generation
helioseismology

171
Q

zeeman splitting

A

magnetic field strengths

172
Q

calcium k line reversal

A

average field strengths
activity cycle

173
Q

doppler imaging

A

surface starspots

174
Q

zeeman splitting occurs because

A

in a magnetic field, the energy levels of the electrons in an atom depend on their orbital angular momentum

175
Q

the level of splitting depends on

A

the strength of the magnetic field

176
Q

+/- in zeeman splitting equation

A

reflects that the line is split with one component at shorter and one at longer wavelengths (either side of original position)

177
Q

ballpark figure for zeeman splitting magnetic field

A

1T
considered very strong

178
Q

stellar magnetic field is produced in

A

the stellar interior

from zeeman splitting know starspots are locations of strong magnetic fields

179
Q

sunspots and starspots are formed when

A

loops of magnetic field beak through the stellar surface

180
Q

direction of field lines

A

direction of the magnetic field

181
Q

number of field lines

A

prop. to its strength

182
Q

starspots appear dark because

A

they are cooler than the surrounding material

a strong magnetic field will inhibit heat transport

183
Q

cooler or hotter patches on the surface of a rotating magnetised star will

A

produce photometric variations which are quasiperiodic in nature

184
Q

quasiperiodic

A

periodic just not perfect sin

185
Q

fractional area

A

tells you how much of the surface is sunspots

f=1, whole star is sunspots
f=0, no spots

186
Q

total radiation brightness from visible disk of star

A

B=photosphere + spot

187
Q

fractional area of sun

A

around 0.5%

188
Q

cool sunspots will reduce

A

the total brightness of visible disk hence reducing the flux at earth and increasing the observed magnitude

(B1=spots, B2=no spots)

189
Q

the presence of a starspot also changes the

A

rotationally broadened absorption line shape

190
Q

a spot at longitude theta will show up as what in the absorption line

A

bump or dip

feature moves through the line as the spot rotates across the disk

191
Q

a star’s observed flux changes as

A

the star rotates in the presence of starspots

192
Q

doppler imaging can be used to

A

work out the distribution of light and dark patches on the star

193
Q

if projected rotational velocity too slow

A

not enough spatial resolution for good imaging

194
Q

if projected rotational velocities are too fast

A

lines become too shallow due to spread from doppler broadening to make out effects due to spots

195
Q

doppler shift at i=0

A

no rotational doppler shift

196
Q

doppler shift for i=90

A

mirror symmetry about the equator so not possible so not possible to determine if spot is in northern or southern hemisphere

197
Q

most familiar signature of solar magnetic fields

A

sunspots (or starspots) visible in broadband white light

198
Q

evidence for solar magnetic fields observed in regions outwith sunspots and starspots

A

K line of Ca II

199
Q

because of its close association with regions of strong magnetic field in the sun the Ca II K line is used to

A

infer magnetic fields in other (solar-like) stars

can also be used as a luminosity indicator

200
Q

sunspots where the

A

strong magnetic field lines are

201
Q

calcium II K line

A

in emission with wings in absorption and then reabsorption in the cntre

202
Q

K1

A

absorption of BB radiation by Ca II ions in the photosphere gives an absorption line (K1)

203
Q

K2

A

hot chromosphere above the photosphere gives line-centre emission (K2)

204
Q

K3

A

re-absorption and scattering in the chromosphere gives a central re-reversal (K3)

205
Q

on the sun the ratio of core to wing emission in the Ca II K line varies with

A

magnetic field strength

the correlation is a straight line in a log-log plot

206
Q

for the sun the total number of sunspots varies over a cycle of

A

approx 11 years

during cycle the K2 component of the Ca II K line varies also

207
Q

K2 emission requires a

A

chromosphere (hotter,tenuous gas overlying a region of cooler, denser gas)

208
Q

wilson-bappu effect

A

describes the relationship between the Ca II K2 line width W and the magnitude of stars

K2 line width W correlates linearly with absolute visual magnitude over 15 magnitudes

209
Q

flares

A

most dramatic signs that a star has a magnetic field

sudden and transient releases of energy in the solar atmosphere

flares are strong, sudden brightness variations at shorter wavelengths

210
Q

what approx % of sun’s surface do flares occupy?

A

0.002%

211
Q

flare rating system

A

each letter (ABCMX) represents a factor of 10 increase in power

212
Q

a loop arises when

A

hot filament of charged material rises from the Sun’s surface before magnetically reconnecting

213
Q

energy transport in flares

A

loop arises
magnetic field lines extend beyond material
cool plasma moves in from sides
hot plasma flows up and down

storage of energy released as the flare

214
Q

Dwarf M stars with Balmer lines in emission release up to

A

100 times the energy of a typical solar flare

around 2% of stellar surface involved in flare

215
Q

the interior temp and density structure of a star depends on its

A

mass which in turn determines how energy is transported out into space

216
Q

energy generated in the core by

A

fusion

this is converted into EK of particles plus photons and neutrinos

217
Q

random walk

A

density of solar interior very high so photons constantly collide with particles and move in a random walk

218
Q

in solar type stars gas is transparent enough for photons to

A

carry all energy outwards until opacity of gas increases enough that convection will take over

219
Q

tachocline

A

boundary between radiative and convective zones

where sun’s spin rate changes dramatically

rapidly flowing ionised gas produces strong electrical currents and gives rise to magnetic field

220
Q

late type stars are thought to be

A

fully convective

221
Q

imaging of M giant show surface intensity is

A

not uniform

has one or more giant hotspots

interpretation is that hotsopts are large convection cells

222
Q

when the stellar gas becomes opaque to radiation…

A

energy is trapped

at layer where energy is trapped, gas would become very hot if other process did not take over to move energy

223
Q

temperature gradient

A

rate of change of temperature with distance

224
Q

convection stars when

A

temperature gradient becomes high enough

225
Q

convection is

A

the transfer of heat by bulk flows in a hot material

226
Q

for convection to happen in a fluid or gas

A

hot material must be able to rise and cool material sink

227
Q

convection occurs if an element of gas in a stellar interior continues to rise upwards when displaced. This happens when

A

the gas bubble is buoyant

228
Q

equation governing thermodynamics for convection tells us that

A

if stellar temperature gradient increases to the critical value defined by the adiabatic temperature gradient, convection starts

229
Q

the adiabatic temperature gradient can be calculated once the

A

stellar pressure gradient is known

230
Q

solar granulation

A

surface of stars with a convective layer like the sun have a surface pattern that looks like a pan of boiling water

231
Q

on the sun individual granules are how big

A

700km-2000km across

232
Q

lifetime of solar granules

A

about one hour

233
Q

centre of a granule

A

slightly brighter and therefore hotter than the average

234
Q

continuous appearance and disapperance of cells is a

A

random process and leads to a small-scale fluctuation in the solar brightness

235
Q

calculating fractional brightness increase due to any bright cell appearing

A

delta B / B

tempcell^4-tempstar^4 / tempstar^4

236
Q

red dwarfs

A

late type stars that are cool enough for molecules and dust to survive around them

show evident for CO, CN and CH4 and more complex molecules.

237
Q

simple molecules are thought to coagulate

A

into dust which can be driven by radiation pressure into the interstellar medium

238
Q

debris around a red dwarf

A

not enough radiation force from the star to clear the dust

239
Q

brown dwarfs

A

in main sequence stars, hydrogen burning is taking place

at lower temps there are cooler, ‘failed stars’ known as brown dwarfs

240
Q

brown dwarfs have never

A

gone through a main sequence stage

may have briefly undergone lithium or deuterium fusion

241
Q

brown dwarfs are identified based on

A

evidence for molecules in their atmosphere

(bottom right of HR diagram)

242
Q

decreasing temp implies

A

increasing molecular complexity

wien’s displacement law tells us that spectral bands are in IR region

243
Q

imaging at different IR wavelengths would allow us to

A

probe different layers in brown dwarfs atmosphere because weather should be observable due to cloud structure causing doppler broadening of spectral lines

244
Q

stellar formation

A

stars condense out of gas clouds which contract under their own gravity (if massive enough)

as gas cloud contracts it loses gravitational potential energy which heats up gas and ejected in bipolar flows away from the protostar

245
Q

when is a star not a star?

A

objects with mass not much more than jupiter can reach high enough T for deuterium burning - brown dwarf

more mass allows hydrogen burning for a red dwarf or main sequence star to form.

246
Q
A

Astronomy 2 (4 decks)

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