our universe

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.

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:





Stefan’s Law





4TFlux Stefan’s Law





Flux = Power per unit area





Luminosity = Power radiated





Flux = Luminosity per unit area





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)


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.


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


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

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