our universe week 8. the sun & the stars. visible uv
TRANSCRIPT
OUR UNIVERSEOUR UNIVERSEWeek 8
The Sun & the The Sun & the
Stars.Stars.
Visible UV
Sun’s Structure
Sun’s Structure
The ultra-hot core extends outward from the star's center to about 20% of its radius.
The temperature at the centre of the core is around 15 million kelvin, and it gradually decreases further from the center.
The core is the location within the Sun in which hydrogen fusion occurs.
Above the core is the radiation zone.
It extends from the top of the core outward to about 70% of the Sun's radius.
Temperatures varies from 10 to 5 million kelvin.
Energy is carried through the radiation zone via electromagnetic radiation (photons).
Above the radiation zone, and extending all the way to the "surface" of the Sun, is the convection zone.
This part of the Sun is relatively "cool", with temperatures ranging downward from a peak of around 2 million kelvin.
Energy flows upward through this area in a different manner than in the underlying radiation zone.
Gigantic blobs of matter, heated by the radiation zone below, rise to the Sun's surface, carrying heat with them.
As these blobs of plasma emit their energy into space at the Sun's surface, they cool somewhat;
enough so that their densities increase and they sink back down.
This convective motion is akin to that seen in a lava lamp.
Solar Solar
Granulation Granulation
is due tois due to
convection convection
cellscells
The Sun’s internal StructureThe Sun’s internal Structure
Convection cellsConvection cells
At the topmost boundary of the convection zone lies the photosphere.
The photosphere is often referred to as the "surface" of the Sun.
The photosphere marks an abrupt transition in the optical properties of the material that makes up the Sun.
Below the photosphere, the photons bounce around so much that they don't travel direct paths to viewers on Earth.
Hence we cannot see deeper into the Sun than the photosphere.
So the photosphere is the "visible surface" of the Sun.
The temperature of the photosphere is about 5800 K.
Above the photosphere the Sun's vast “atmosphere” extends outward into interplanetary space.
In the case of the Sun, the density of material in the solar atmosphere is much less than is the case below the photosphere within the Sun's "interior".
Also, the physical properties that control motions of material and the temperatures encountered are far different in the Sun's atmosphere than in the layers of the Sun beneath the photosphere.
There are two major regions within the Sun's atmosphere:
the lower and much smaller chromosphere,
and the upper and much larger corona.
The relatively thin chromosphere is just a few thousand kilometers deep, less than Earth's diameter.
Although temperatures within the Sun gradually decrease as one moves outward,
(from 15 million kelvin in the core to 5,800 kelvin at the photosphere)
they begin to climb once again as we rise through the Sun's atmosphere.
The temperature of the chromosphere increases from 4,300 kelvin (slightly above the photosphere) to around 50,000 kelvin (near the corona).
Powerful magnetic fields in the Sun's atmosphere accelerate the plasma as they transfer energy to it, heating the material in ways that scientists still don't fully understand.
Until relatively recent times, when special filters and space-based telescopes became available, the Sun's atmosphere was only visible during total solar eclipses.
During an eclipse, the chromosphere could be seen as a colorful reddish zone around the edge of the occluded solar disk, thus earning the region its name (Greek "chromos" = "color").
Chromosphere
The Sun's much larger upper atmosphere, the corona, extends unevenly for millions of kilometers into space.
The temperature of the solar atmosphere climbs sharply in a narrow transition region between the chromosphere and the corona.
The temperatures in the corona range from around 800,0000 K to 3 million K
corona,
Matter is continuously flung outward by the Sun.
An electrically charged "soup" of protons, electrons, and lesser numbers of heavier atomic nuclei flows outward into space.
This extremely tenuous plasma is called the solar wind.
In a sense, the solar wind is a vast extension of the Sun's atmosphere.
The solar wind flows past Earth and beyond.
All of the planets are within the gigantic "bubble" of the solar wind.
Eventually, on the far edge of our solar system, the solar wind merges with the outpourings of other stars, and the extended solar atmosphere ends.
The gigantic region within this solar wind "bubble" is called the heliosphere.
The boundary of the heliosphere, where the extended atmosphere of the Sun finally gives way to interstellar space, is called the heliopause.
The location of the heliopause is something like 70AU from the Sun.
Solar Prominence in UVSolar Prominence in UV
SunspotsSunspots
It appears that sunspots are the visible counterparts of magnetic flux tubes in the sun's convective zone that get "wound up" by differential rotation.
Convection cellsConvection cells
Differential rotation causes field Differential rotation causes field
lineslines
to be wrapped around the Sunto be wrapped around the Sun
Sunspots migrate to equator where they cancel Sunspots migrate to equator where they cancel
out and eventually reverse the overall field out and eventually reverse the overall field
If the stress on the tubes reaches a certain limit, they curl up like a rubber band and puncture the sun's surface.
Convection is inhibited at the puncture points; the energy flux from the sun's interior decreases; and with it surface temperature.
Sunspots are depressions on the sun's surface.
Sunspots come in pairs with opposite magnetic polarity.
From cycle to cycle, the polarities of leading and trailing with respect to the solar rotation] sunspots change from north/south to south/north and back.
Sunspots usually appear in groups.
Sunspot Sunspot
Structure Structure
• The sunspot itself can be divided into two parts:
• The central umbra, – which is the darkest part, where the magnetic field
is approximately vertical.
• The surrounding penumbra, – which is lighter, where the magnetic field lines are
more inclined.
• Sunspot activity cycles about every eleven years.
• The point of highest sunspot activity during this cycle is known as Solar Maximum, and the point of lowest activity is Solar Minimum.
• Early in the cycle, sunspots appear in the higher latitudes and then move towards the equator as the cycle approaches maximum.
Sun spots have a 11 year cycle Sun spots have a 11 year cycle
(magnetic field reversal)(magnetic field reversal)
Differential rotation causes field Differential rotation causes field
lineslines
to be wrapped around the Sunto be wrapped around the Sun
Sunspots migrate to equator where they cancel Sunspots migrate to equator where they cancel
out and eventually reverse the overall field out and eventually reverse the overall field
The Solar CoronaThe Solar Corona
Visible
The Solar CoronaThe Solar Corona
Visible
A corona is a type of plasma "atmosphere" of the Sun.
It extends millions of kilometers into space
Most easily seen during a total solar eclipse,
but also observable in a coronagraph.
The Solar CoronaThe Solar Corona
Visible
A corona is a type of plasma "atmosphere" of the Sun.
It extends millions of kilometers into space
Most easily seen during a total solar eclipse,
but also observable in a coronagraph.
Light from the corona comes from three primary sources (called K, F and E), which are called by different names although all of them share the same volume of space.
The K-corona (K for kontinuierlich, "continuous" in German).
Created by sunlight scattering off free electrons;
Doppler broadening of the reflected photospheric absorption lines completely obscures them, giving the spectrum the appearance of a continuum with no absorption lines.
The F-corona (F for Fraunhofer).
Created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight;
the F-corona extends to very high elongation angles from the Sun, where it is called the Zodiacal light.
The E-corona (E for emission).
Result from spectral emission lines produced by ions that are present in the coronal plasma;
it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.
The Solar The Solar
NeighbourhoodNeighbourhoodTransparency
Transparency
circles at 5, 10, 15 ly
Measuring Measuring
StarsStarsTemperature T Distance d
Luminosity LRadius RMass M
Element Abundances
Measuring StarsMeasuring Stars
Distances (Distances (dd) )
needed for finding needed for finding LL• Stellar parallax (Hipparchos satellite yielded a Stellar parallax (Hipparchos satellite yielded a
revolutionary improvement)revolutionary improvement)• Proper motion studiesProper motion studies• Moving clusters - stars seem to get closer as the Moving clusters - stars seem to get closer as the
cluster recedes.cluster recedes.• Comparison with standard stars of known Comparison with standard stars of known
distancedistance
Measuring Measuring
the Stars.the Stars.
First a Reminder:First a Reminder:
Measuring DistancesMeasuring Distances
using the Earth’susing the Earth’s
orbit around orbit around
the Sun as a the Sun as a
baselinebaseline
Orion’s belt
We only observe a2-dimensional
projection of objects in the sky.
We needWe need
extra information toextra information to
find their position find their position
inin
3-dimensions -3-dimensions -
their distance dtheir distance d
Orion
Distance by Distance by
ParallaxParallax(a) Planet observed from A & B against background of distant stars.(b) Photos taken from A & B
show the planet’s image has moved against the background stars.
dAB2
Distance by Distance by
ParallaxParallax(a) Planet observed from A & B against background of distant stars.(b) Photos taken from A & B
show the planet’s image has moved against the background stars.
dAB2
2tan
ABd
Stellar ParallaxStellar Parallax
Proper MotionProper Motion
The proper motion of a star is its angular change in position over time as seen from the Sun.
It is measured in seconds of arc per year.
Proper MotionProper MotionThis contrasts with radial velocity, which is the time-rate of change in distance toward or away from the viewer.
(usually measured by the Doppler shift)
Proper MotionProper MotionBarnard’s Star (at 1.82 pc, 5.98 ly) Barnard’s Star (at 1.82 pc, 5.98 ly)
moves over 22yrsmoves over 22yrs
by 230by 230 = 3.8´ = 3.8´
1894
1916
Stellar motionStellar motion
vVt
Transverse velocity
(measured by Proper Motion)
Vr
Radial velocity(measured by
DopplerShift)
Stellar Stellar
Motion.Motion.
AnAn
example:example: CentauriCentauri
Measuring Stars.Measuring Stars.
Radius Radius R
Stellar SizesStellar Sizes
Directly from imaging
&Interferometry
Indirectly from Luminosity
The Sun’s RadiusThe Sun’s RadiusDirectly from
imaging
RSun = 6.96108 m
= 109 RE
1 AU = 1.4961011 m
= 215 RE Mercury’s orbit
= 83 Rsun
Directly from Imaging & Interferometry
(but restricted to nearby large stars)(but restricted to nearby large stars)
Stellar RadiiStellar Radii
Indirectly from L & T:
Directly from Imaging & Interferometry
(but restricted to nearby large stars)(but restricted to nearby large stars)
Stellar RadiiStellar Radii
Indirectly from L & T:
Stefan’s Law
Directly from Imaging & Interferometry
(but restricted to nearby large stars)(but restricted to nearby large stars)
Stellar RadiiStellar Radii
Indirectly from L & T:
4TFlux Stefan’s Law
Directly from Imaging & Interferometry
(but restricted to nearby large stars)(but restricted to nearby large stars)
Stellar RadiiStellar Radii
Indirectly from L & T:
Flux = Power per unit area
Directly from Imaging & Interferometry
(but restricted to nearby large stars)(but restricted to nearby large stars)
Stellar RadiiStellar Radii
Indirectly from L & T:
Luminosity = Power radiated
Directly from Imaging & Interferometry
(but restricted to nearby large stars)(but restricted to nearby large stars)
Stellar RadiiStellar Radii
Indirectly from L & T:
Flux = Luminosity per unit area
Directly from Imaging & Interferometry
(but restricted to nearby large stars)(but restricted to nearby large stars)
Stellar RadiiStellar Radii
Indirectly from L & T:
424 TRL starstar
HST Resolves a StarHST Resolves a Star
The Red GiantThe Red GiantBetelgeuseBetelgeuse
in the constellation in the constellation
OrionOrion
Stellar Stellar
SizesSizes
vary vary
greatlygreatly
BetelgeuseBetelgeuse
300 300 R⊙
THETHE END END OF LECTURE 16OF LECTURE 16
OUR UNIVERSEOUR UNIVERSELecture No. 17
Measuring StarsMeasuring Stars..
Temperature Temperature T
( i.e. ( i.e. SurfaceSurface T )
&&
LuminosityLuminosity L
The The
Black BodyBlack Body
SpectrumSpectrum
Here plotted Here plotted
againstagainstwavelength
The Sun’s The Sun’s
continuous continuous
spectrum spectrum
is well is well
approximated approximated
by a by a
Black BodyBlack Body
or or
Planck Planck
SpectrumSpectrum
at 5800 K
A Star’s Colour A Star’s Colour
Depends on its TemperatureDepends on its Temperature
Planck spectrum:Planck spectrum:
& therefore& therefore
the colours of starsthe colours of stars
only depend on only depend on T
T
T
max
1max
Brightness through UBV FiltersBrightness through UBV Filters
Depends on a Star’s TemperatureDepends on a Star’s Temperature
Brightness through UBV FiltersBrightness through UBV Filters
Depends on a Star’s TemperatureDepends on a Star’s Temperature
Use of filters provides a quickUse of filters provides a quick
and convenient method ofand convenient method of
estimating stellar propertiesestimating stellar properties
Each filter samples a different part of the Planckspectrum. The ratio of brightness in B and V filtersdetermines the COLOUR TEMPERATURE
B-V log T
UBV FiltersUBV Filters
are are
supplemented supplemented
with with
8 IR filters8 IR filters
U 360 nmB 420 nmV 540 nmR 700 nm---------------------------------------I 900 nm = 0.90 mJ 1250 nm = 1.25 mK 2200 nm = 2.20 m---------------------------------------L 3400 nm = 3.40 mM 4900 nm = 4.90 m---------------------------------------N 10200 nm = 10.20 mQ 20000 nm = 20.00 m
The Black Body LawsThe Black Body Laws
Stefan-Boltzmann Law for the Flux
Watts m-2
The Total Power L* emitted by a star,
of Radius R* and Area = 4 R*2 is
Watts
4TFlux
42** 4 TRL
Luminosity Luminosity L Watts
needs distanceneeds distance d
&&
brightness (Flux) at Earth brightness (Flux) at Earth (b Watts m-2)
Measuring StarsMeasuring Stars
LUMINOSITY L*
is the Total Power emitted by a star,
of Radius R* and Area = 4 R*2
Watts
42** 4 TRL
The Measured apparent brightness b* is the Flux reaching the Earth at
distance d* from the Star
The Measured apparent brightness b* is the Flux reaching the Earth at
distance d* from the Star
Watts m-22*
** 4 d
Lb
If we Measure both
apparent brightness b*
& distance d*
we obtain Luminosity L*
If we Measure both
apparent brightness b*
& distance d*
we obtain Luminosity L*
Watts*2** 4 bdL
Measuring both d* and the
apparent brightness b* as well as
T (from spectrum) gives us the star’s Radius
42** 4 TRL
Measuring both d* and the
apparent brightness b* as well as
T (from spectrum) gives us the star’s Radius
4*2
* 4 T
LR
The Luminosity is often
found in terms of a standard
star such as the Sun.2
*
** 4 d
Lb
20
00 4 d
Lb
The Luminosityis usually expressed
in terms of the
Solar Luminosity Lsun
For example, a Supergiant :
L*=10 4Lsun
If If distancedistance d not known not known
we must resort to other trickswe must resort to other tricks
e.g. to find the distance of a cluster of e.g. to find the distance of a cluster of
stars, compare its HR diagram with stars, compare its HR diagram with
the standard Main Sequence of Stars the standard Main Sequence of Stars
with known distanceswith known distances
Hertzprung Russell DiagramHertzprung Russell Diagram
RecallRecall
Spectral information from starsSpectral information from stars• Peak Peak or or T = TemperatureT = Temperature• Presence of LinePresence of Line Composition & TComposition & T• Line intensity Composition & TLine intensity Composition & T• Line width T, density, rotation…Line width T, density, rotation…•Doppler shiftDoppler shift Line-of-sight velocity Line-of-sight velocity
Principal Principal
TypesTypes
of Stellar of Stellar
SpectraSpectra
SUN
Principal Types of Stellar SpectraPrincipal Types of Stellar Spectra
35,000 K
3,500 K
3,700 K
5,200 K4,400 K
5,600 K
5,900 K
8,600 K
7,200 K6,500 K
10,800 K
22,000 K
16,400 K
Spectral Classes: O B A F G K Spectral Classes: O B A F G K
MM
Stellar Classification
• Astronomers classify of stars based on their spectral characteristics.
• Based on which atomic excitations are most prominent in the light,
• giving an objective measure of the temperature in this chromosphere.
• Most stars are currently classified using the letters
• O, B, A, F, G, K and M,
• O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class.
• According to an informal tradition:
• O stars are "blue"
• B "blue-white"
• A stars "white"
• F stars "yellow-white"
• G stars "yellow"
• K stars "orange"
• M stars "red“
• Spectral letter is enhanced by a number from 0 to 9 indicating tenths of the range between two star classes.
• E.g., A5 is five tenths between A0 and F0, but A2 is two tenths of the full range from A0 to F0.
• Aditionally, the luminosity class expressed by the Roman numbers I, II, III, IV and V.
• It expressed the width of certain absorption lines in the star's spectrum.
• It has been shown that this feature is a general measure of the size of the star, and thus of the total luminosity output from the star.
• Class I are generally called supergiants,
• class III simply giants and class
• V either dwarfs or more properly main sequence stars.
• For example our Sun has the spectral type G2V,
• (which might be interpreted as "a 'yellow' two tenths towards 'orange' main sequence star“)
• The apparently brightest star Sirius has type A1V.
Spectral Classes: O B A F G K MSpectral Classes: O B A F G K M
++++++++
O BO Be e A FA Fineine G Girlirl K Kississ M Mee
GGirl irl GGuyuy
Strengths of Absorption linesStrengths of Absorption lines
in Stars across the HR Diagramin Stars across the HR Diagram
(which lines dominate depends on temperature)(which lines dominate depends on temperature)
SUN
Principal Types of Stellar SpectraPrincipal Types of Stellar Spectra
35,000 K
3,500 K
3,700 K
5,200 K
4,400 K
5,600 K
5,900 K
8,600 K
7,200 K
6,500 K
10,800 K
22,000 K
16,400 K
Spectral Class
L L etc
P P etc
H H etc
Hydrogen atom Spectral Series
0 2 104
4 104
6 104
8 104
1 105
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
N 1 T( )
N 2 T( )
N 3 T( )
N 4 T( )
N 5 T( )
T
Populations in H levels vs TemperaturePopulations in H levels vs Temperature
Temperature 3000 to 100,000 K
Populationn=1 Ground State
n=3n=4
n=2
n=5
Part shown expandedin the next slide
2 104
4 104
6 104
8 104
1 105
0
0.02
0.04
0.06
N 2 T( )
N 3 T( )
N 4 T( )
N 5 T( )
N 1 T( )
T
Populations showing details for excited states. Populations showing details for excited states.
Temperature 3000 to 100,000 K
Populationn=1
n=3
n=4
n=2
n=5
Note the expanded scale.Note the expanded scale.
• U, B, V filters - colour TemperatureU, B, V filters - colour Temperature• Fit the spectrum to PlanckFit the spectrum to Planck• Detailed modelling of the spectrum line shapes Detailed modelling of the spectrum line shapes
and strengths - this also gives the surface gravity and strengths - this also gives the surface gravity
& the elemental abundances.& the elemental abundances.
Measuring StarsMeasuring Stars
Temperature Temperature T, etc
Element Abundances
Mass fractions: H X~ 0.73, He Y~ 0.25 Metallicity Z ~ 0.02Heavier elements are “Metals”in astronomy
SADSolarAbundanceDistribution
gSun = 274 ms-2 28 gEarth
Measuring StarsMeasuring Stars
The “Cosmic Abundance” of theThe “Cosmic Abundance” of the
Elements determined from the Sun, Stars Elements determined from the Sun, Stars
and Meteoritesand MeteoritesNB: log Abundance
FeCNO
““Cosmic Abundance” of the ElementsCosmic Abundance” of the Elements
NB: log Abundance
Fe
Stars are classified into 2 broad categories depending on
Element Abundances
Population I:H X~ 0.73, He Y~ 0.25 Metallicity Z ~ 0.02
Population II:H X~ 0.75, He Y~ 0.25 Metallicity Z ~ 0.001
SunStars in the disc
of the Galaxy
Globular Cluster Starsin the halo
of the Galaxy
The Herzsprung-Russell DiagramThe Herzsprung-Russell Diagram
Luminosity ClassesLuminosity Classes Sun G2VSun G2V
Vega A0VVega A0V
Barnard’s StarBarnard’s Star
(Dwarf)(Dwarf)
M4VM4V
Betelgeuse Betelgeuse
(Red Giant)(Red Giant)
M2IaM2Ia
supergiants
giants
dwarfs
THETHE ENDEND
OF Week 6OF Week 6