Download but restricted to nearby large stars

OUR UNIVERSE
Week 8
The Sun & the 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
Granulation
is due to
convection
cells
The Sun’s internal Structure
Convection 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 UV
Sunspots
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 cells
Differential rotation causes field
lines
to be wrapped around the Sun
Sunspots migrate to equator where they cancel 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
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
(magnetic field reversal)
Differential rotation causes field
lines
to be wrapped around the Sun
Sunspots migrate to equator where they cancel out
and eventually reverse the overall field
The Solar Corona
Visible
The Solar Corona
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.
Visible
The Solar Corona
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.
Visible
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
Neighbourhood
Transparency
circles at 5, 10, 15 ly
Transparency
Measuring Stars
Temperature T
Distance d
Luminosity L
Radius R
Mass M
Element Abundances
Measuring Stars
Distances (d)
needed for finding L
• Stellar parallax (Hipparchos satellite yielded a
revolutionary improvement)
• Proper motion studies
• Moving clusters - stars seem to get closer as the
cluster recedes.
• Comparison with standard stars of known
distance
Measuring
the Stars.
First a Reminder:
Measuring Distances
using the Earth’s
orbit around
the Sun as a
baseline
Orion
We only observe a
2-dimensional
projection of objects
in the sky.
Orion’s belt
We need
extra information to
find their position in
3-dimensions their distance d
Distance by Parallax
d
AB
2
(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.
Distance by Parallax
d
AB
2
(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.
AB
d tan  
2
Stellar Parallax
Proper 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 Motion
This 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 Motion
Barnard’s Star (at 1.82 pc, 5.98 ly)
moves over 22yrs
by 230 = 3.8´
1894
1916
Stellar motion
Vt
v
Transverse
velocity
(measured by
Vr
Proper
Radial velocity
Motion)
(measured by
Doppler
Shift)
Stellar
Motion.
An
example:
 Centauri
Measuring Stars.
Radius R
Stellar Sizes
Directly from
imaging
&
Interferometry
Indirectly from Luminosity
The Sun’s Radius
Directly from
imaging
RSun = 6.96108 m
= 109 RE
1 AU = 1.4961011 m
= 215 RE
Mercury’s orbit
= 83 Rsun
Stellar Radii
Directly from
Imaging & Interferometry
(but restricted to nearby large stars)
Indirectly from L & T:
Stellar Radii
Directly from
Imaging & Interferometry
(but restricted to nearby large stars)
Indirectly from L & T:
Stefan’s Law
Stellar Radii
Directly from
Imaging & Interferometry
(but restricted to nearby large stars)
Indirectly from L & T:
Stefan’s Law
Flux  T
4
Stellar Radii
Directly from
Imaging & Interferometry
(but restricted to nearby large stars)
Indirectly from L & T:
Flux = Power per unit area
Stellar Radii
Directly from
Imaging & Interferometry
(but restricted to nearby large stars)
Indirectly from L & T:
Luminosity = Power radiated
Stellar Radii
Directly from
Imaging & Interferometry
(but restricted to nearby large stars)
Indirectly from L & T:
Flux = Luminosity per unit area
Stellar Radii
Directly from
Imaging & Interferometry
(but restricted to nearby large stars)
Indirectly from L & T:
Lstar  4R T
2
star
4
HST Resolves a Star
The Red Giant
Betelgeuse
in the constellation
Orion
Stellar
Sizes
vary
greatly
Betelgeuse
300 R
THE END
OF LECTURE 16
OUR UNIVERSE
Lecture No. 17
Measuring Stars.
Temperature T
( i.e. Surface T )
&
Luminosity L
The
Black Body
Spectrum
Here plotted
against
wavelength 
The Sun’s
continuous
spectrum
is well
approximated
by a
Black Body
or
Planck
Spectrum
at 5800 K
A Star’s Colour
Depends on its Temperature
Planck spectrum:
& therefore
the colours of stars
only depend on T
max  T
 max  T
1
Brightness through UBV Filters
Depends on a Star’s Temperature
Brightness through UBV Filters
Depends on a Star’s Temperature
Use of filters provides a quick
and convenient method of
estimating stellar properties
Each filter samples a different part of the Planck
spectrum. The ratio of brightness in B and V filters
determines the COLOUR TEMPERATURE
B-V  log T
UBV Filters
are
supplemented
with
8 IR filters
U
360 nm
B
420 nm
V
540 nm
R
700 nm
--------------------------------------I
900 nm = 0.90 m
J
1250 nm = 1.25 m
K
2200 nm = 2.20 m
--------------------------------------L
3400 nm = 3.40 m
M
4900 nm = 4.90 m
--------------------------------------N
10200 nm = 10.20 m
Q
20000 nm = 20.00 m
The Black Body Laws
Stefan-Boltzmann Law for the Flux
Flux  T
4
Watts
-2
m
The Total Power L* emitted by a star,
of Radius R* and Area = 4 R*2 is
L*  4R T
2
*
4
Watts
Measuring Stars
Luminosity L Watts
needs distance d
&
brightness (Flux) at Earth
-2
(b Watts m )
LUMINOSITY L*
is the
Total Power emitted by a star,
2
of Radius R* and Area = 4 R*
L*  4R T
2
*
4
Watts
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
L*
b* 
2
4d*
Watts
-2
m
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*
L*  4d b
2
* *
Watts
Measuring both d* and the
apparent brightness b* as well as
T (from spectrum) gives us the
star’s Radius
L*  4R T
2
*
4
Measuring both d* and the
apparent brightness b* as well as
T (from spectrum) gives us the
star’s Radius
L*
R 
4
4T
2
*
L0
b0 
2
4d 0
L*
b* 
2
4d*
The Luminosity
is often
found in terms
of a standard
star such
as the Sun.
The Luminosity
is usually expressed
in terms of the
Solar Luminosity Lsun
For example, a Supergiant :
4
L *= 10 L s u n
If distance d not known
we must resort to other tricks
e.g. to find the distance of a cluster of
stars, compare its HR diagram with
the standard Main Sequence of Stars
with known distances
Hertzprung Russell Diagram
Recall
Spectral information from stars
• Peak  or 
T = Temperature
• Presence of Line Composition & T
• Line intensity
Composition & T
• Line width T, density, rotation…
•Doppler shift Line-of-sight velocity
Principal
Types
of Stellar
Spectra
SUN
Principal Types of Stellar Spectra
35,000 K
22,000 K
16,400 K
10,800 K
8,600 K
7,200 K
6,500 K
5,900 K
5,600 K
5,200 K
4,400 K
3,700 K
3,500 K
Spectral Classes: O B A F G K M
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 M
++++
O Be A Fine Girl Kiss Me
Girl  Guy
Strengths of Absorption lines
in Stars across the HR Diagram
(which lines dominate depends on temperature)
SUN
Principal Types of Stellar Spectra
35,000 K
22,000 K
16,400 K
10,800 K
8,600 K
7,200 K
6,500 K
5,900 K
5,600 K
5,200 K
4,400 K
3,700 K
3,500 K
Spectral Class
Hydrogen atom
Spectral Series
L L
etc
H H
etc
P P
Populations in H levels vs Temperature
1
0.9
n=1 Ground State
0.8
Population
0.7
N( 1  T )
N( 2  T )
0.6
N( 3  T ) 0.5
N( 4  T )
N( 5  T )
0.4
Part shown expanded
in the next slide
0.3
0.2
0.1
0
0
2 10
4
4 10
6 10
4
4
8 10
4
T
Temperature 3000 to 100,000 K
1 10
5
n=5
n=4
n=3
n=2
Populations showing details for excited states.
Note the expanded scale.
0
.
0
6
n=1
Population
n=5
N
(
2

T
)
N
(
3

T
)
0
.
0
4
n=4
N
(
4

T
)
N
(
5

T
)
n=3
N
(
1

T
)
0
.
0
2
0
n=2
4

2
1
0
4

4
1
0
4

6
1
0
T
4

8
1
0
Temperature 3000 to 100,000 K
5

1
1
0
Measuring Stars
Temperature T, etc
• U, B, V filters - colour Temperature
• Fit the spectrum to Planck
• Detailed modelling of the spectrum line shapes
and strengths - this also gives the surface gravity
& the elemental abundances.
Measuring Stars
Element Abundances
SAD
Solar
Abundance
Distribution
Mass fractions:
H X~ 0.73,
He Y~ 0.25
Metallicity Z ~ 0.02
Heavier elements are “Metals”
in astronomy
gSun = 274 ms-2  28 gEarth
The “Cosmic Abundance” of the
Elements determined from the Sun, Stars
and Meteorites
NB: log Abundance
CNO
Fe
“Cosmic 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
Sun
Stars in the disc
of the Galaxy
Population II:
H X~ 0.75,
He Y~ 0.25
Metallicity Z ~ 0.001
Globular Cluster Stars
in the halo
of the Galaxy
The Herzsprung-Russell Diagram
Luminosity Classes
supergiants
giants
dwarfs
Sun G2V
Vega A0V
Barnard’s Star
(Dwarf)
M4V
Betelgeuse
(Red Giant)
M2Ia
THE END
OF Week 6