Transcript No Slide Title

Class Meets:- Tuesdays 12.00 in LTB
Thursdays 10.00 – 12.00 in TB13.
Lecturer:- W.Gelletly
Office 13BB03
e-mail:- [email protected]
All diagrams and pictures on slides
All notes and pictures etc on Physics Intranet – 1IASS-08
Books:- An Introduction to Modern Astrophysics by Carroll and Ostlie
Second edition-this has SI units.
Universe by Freedman and Kaufmann – Sixth edition.
Assessment:- 70% written examination in summer
30% multiple choice test on Thursday 21st,February at 11.00
Milky Way-over Alps with lunar eclipse
M31-Andromeda Galaxy-2.2Mly from Earth, Part of our Local
cluster of galaxies. It will collie with the Milky Way in about
6 billion years or so.
Photo-mosaic picture of the Sombrero Galaxy taken with the
Hubble Space Telescope over several orbits.Glowing central
bulge of stars surrounded by pancake shaped disc.
M74 Gemini – Spiral galaxy in Pisces
- It is about 30Mly away and has  1011 stars
Taken with Gemini North Telescope – Mauna Keau
Helix Nebula-The glowing gas consists of N,O and H ejected by a Solar
Mass star in its death throes. Radiation from the remaining central star
causes the gas to glow.This star will become a White Dwarf. Dwarf.
Photograph by D. Malin
Cometary knots in the
Helix Nebula.
Gaseous objects seen with the HST in the
Helix Nebula. The head is about twice the
size of the solar system and the tail is of
length 1011 km.
They may be formed when hot, low density
gas emitted by the dying star collides with
cool, higher density gas ejected some 104
years earlier.
Picture taken Aug 1994 with wideField planetary camera 2 - HST
Crab-UV
Crab-vis
CRAB-IR
Crab-X-ray
Supernova seen by Chinese
in 1054 A.D.
Bright clumps moving outwards
at v ~ 0.5c
Filaments have lower mass and
higher velocity than models
would predict.
Crab Nebula at X-ray wavelengths
There is a pulsar ( = neutron star) at the centre which rotates at
about 30 times per second.
It is about 7 kly away in Taurus
Crab Nebula(7kly away in Taurus)-several exposures with HST.
Clear changes in central regions-wisp like structures moving outwards
with v=0.5c and halo which is stationary but brightens and weakens.
Coma Cluster - 20Mly across containing thousands of galaxies(300 seen
here). It is about 270 Mly away.Two supergiant galaxies seen in centre.
Hubble Deep Field-A very narrow sample of the sky looking as far
back as 10 10 years in some cases.Data taken over 10 consecutive days.
Globular Cluster M10 – 16,000 ly away in ORPHIUCUM
Diameter is  70 ly. Mainly post-Main sequence stars
Aurora seen over Edmonton on 4/11/2003. We see the Clover Bar
Power station on the Saskatchewan river. The auroral light is seen
reflected in a small pond.
Isabel Terra (category 5) hurricane passing east of the Bahamas
NASA LONDON
Ngc6240 merging and infrared bright galaxy in Orpiuchus
SN1987A- Supernova in large Magellanic cloud recorded in October
1987. We see the region before and after the supernova.
Supernova 1998bw- scales are different on right (before)
and left (after)
Eagle Nebula- 7000ly away
Pillars of H and dust. In pillars
gas is contracting to form new
stars. Radiation boils
away low density
material at ends of
pillars.
Lagoon nebula  5000ly away, 100 ly across. It is in Sagittarius
Slides 1-12
1.The milky Way seen over the Alps during a lunar eclipse.
2.&3.the Andromeda Galaxy-M31.It lies 2.5Mly away and is orbitted by two small elliptical
galaxies M32 and M110.It will collide with the Milky Way in about 6 billion years.
4.Sombrero Galaxy about 50 Mly away in Virgo.We see it edge on.We can see the large central
bulge and the distinct dust lanes in the edge.This obscures stars behind them but is a region of
star formation and there are many bright stars.In the bulge there are many globular clusters.
5.M74-a photogenic spiral galaxy in Pisces.It is about 30 Mly away and has about 1011 stars.
This is more or less how the Milky Way must look from outside.Picture taken with the
Gemini North telescope on Mauna Keau.
6.Helix Nebula-the end of a Sun-like star.The It is a planetary nebula with a central
White Dwarf Star.Radiation from this star is fluorescing the clouds of H,O and N thrown off
in the final unstable stages of the star’s life. Slide which follows shows some detail.
7-11.Crab Nebula-In 1054 AD the chinese recorded a bright new star seen in daylight for quite
a long time.It was a Supernova and we see the remnants in these pictures.The first four show
it as seen in four different regions of the electromagnetic spectrum[UV,visible,IR,X-ray].The
final picture shows detail.The bright clumps of light are moving outwards at v = 0.5c.The
filaments have lower mass and higher velocity than models would predict.There is a pulsar
= neutron star at the centre,the remnant of the original star.It rotates at 30 times per sec.
12.Coma cluster-One of the most dense galactic clusters known with about 10,000 galaxies.
Each of these galaxies contains more than 1 billion stars.This is a regular cluster about
270 Mly awayand 20 Mly across.Most of the galaxies are elliptical.Two giant ellipticals
dominate in the centre of picture and we see a star local to us top right.
Slides 13-17
• 13.Hubble Deep Field-a narrow region of the sky viewed by the HST
4 days.We see more and more galaxies no matter how far we go
back in time.
14.M10 globular cluster with a few hundred thousand stars.It is
16,000ly away in Ophiucum.Diameter is 70 ly.Stars are highly
evolved and are mainly Red and Blue Giants.These are post-Main
Sequence stars.
15.Aurora seen over Edmonton on 4/11/2003.Clover Bar Power station
photographed by auroral light from North Saskatchewan River.Small
pond reflects green auroral light.
16.Isabel Terra-a hurricane,almost category five,passing east of the
Bahamas.Huge swirling storms,called typhoons in East,get their
energy from warm evaporated ocean water.As water vapour vapour
cools and condenses it heats air,lowers pressure and causes cooler air
to rush in.Winds can be up to 250 km per hour.
17.London at night from orbit.The M25,Heathrow and Gatwick are
clearly visible.
Slides 18-22
• 18.NGC6240-I optical and X-ray regions.
19.SN1987A-A supernova in the Large Magellanic Cloud in October
1987.We see before and after.This marks the death of a massive star.
20.Similar picture of another ,much more distant supernova.
21.Eagle Nebula.This region of star formation is 7000ly away.
Evaporating gaseous globules emerging from pillars of H and dust.
In interior of pillars gas is contracting to form new stars.At the ends
of the pillars intense radiation from young stars boils away low
density material.
22.Lagoon Nebula[M8].5000ly away,100ly across.It can be seen in
Sagittarius with the naked eye.In the detail shown with HST we see
two funnel shaped regions where stars are forming.The vast walls
of dust hide other hot young stars.
The nature of Astronomy
 Astronomy relies almost entirely on observation.
 We rely on radiation and particles emitted by astronomical objects.
 We can measure radiation intensities, spectra etc
- We deduce
compositions of stars
temperatures of their surfaces
total luminosities
atmospheres of planets etc.
- We can also obtain information on material between us and the
emitting object (gas and dust) from absorption and scattering.
 How does Astronomy work?
Observations
Models of processes
Predictions
Test by further observations
 To do this we must make a series of assumptions—next slide
Assumptions
 The Laws of Physics apply. Even although the conditions in
the astrophysical objects may be well beyond anything we can
explore on Earth.
 In particular we assume the Laws are invariant w.r.t. place and time.
Caveats
The evidence to support these assumptions is very limited.
 It is also of quite different quality for each of the four forces we know.
 There may also be forces or other Laws of which we are unaware.
Then our models would be quite inadequate.
Astrophysics is an application of Physics - we must understand Physics.
Electromagnetic Wave-Schematic picture
• Here we see a plane wave propagating in vacuum in the
z-direction with velocity v = c where c = 3 x 10 8 m/sec.
• The wave is linearly polarised and the E field
oscillates along the y-axis.
• The magnetic field(B) is along
the x-axis and is in phase
with the E field
• E and B are always in the same
proportion
• , and c depend on mode
of production and medium
• In medium it can be scattered,
reflected,refracted and slowed down.
Consequences of the Finite Velocity of Light
• Velocity c =  with c = 3 x 108 ms-1 in vacuum.
Typically visible = c/vis = 3 x 108 / 500 x 10-9 = 6 x 1014 Hz
Velocity is reduced by  1% in gas and tens of percent in solids
• Finite velocity means time delay. Thus
Time(Sun-Earth) = 1.496 x 1011/ 3 x 108 = 0.5 x 103  8 mins.
Time(Moon-Earth) = 4 x 108 / 3 x 108  1.3 s
• From Alpha Centauri  4 years
From nearby galaxies  105 - 106 years
Across a “typical” galaxy  105 years
•This leads to the definition of the LIGHT YEAR (ly) as the
distance travelled by light in vacuum in 1 year.
1 ly = 3 x 108 x 3.15 x 107 m = 9.45 x 1012 km  1013 km
= 9.461 x 1012 km
The age of the Earth is  5 x 109 years
So if we observe a galaxy 1010 ly away the light was emitted before
the Earth was formed.
ANALOGY (commonly quoted in textbooks)
If the Universe began at midnight.
Earth formed in mid-afternoon.
Plants began to produce oxygen in early evening.
Humans began 2 mins. from Midnight
Magellan circumnavigated globe 0.003 secs. from Midnight
Each of us lives for < 0.001 secs.
Electromagnetic Spectrum-Radio waves to gamma rays.
Shown as a function of wavelength() and frequency().
All such waves have velocity c = .
The Story so far
The Nature of Astronomy
- based on Observation not experiment
- very different from Physics generally
- relies on assumptions, particularly that the laws of Physics are invariant in space and time
and can be applied in the very different conditions which may prevail in the astronomical
objects we observe.
To proceed we need to be reminded of some of the simple physics we will use
to explain what we see.
The Electromagnetic Spectrum
• The figure shows the electromagnetic
spectrum over all wavelengths.
• EM radiation exists at all wavelengths
and has the same basic properties.In
particular they always propagate with
velocity v = c in vacuum and can be
refracted, reflected, scattered etc
• The various parts of the spectrum are
named by the method of production
and not by energy or wavelength. For
example gamma rays arise from
transitions between levels in atomic
nuclei.
Blackbody Radiation
• General question:-What is the spectrum of EM radiation emitted by
an object of arbitrary temperature T in thermal equilibrum.
We assume that this “blackbody” reflects no radiation at any 
•Max Planck showed that the spectrum is given by
u d  =
8hc -5.d
[exp(hc/kT) - 1]
where ud  is the energy
density =energy/unit volume
• Although no perfect blackbody exists solids and stars follow
Planck’s Law very closely.Note that picture is on log-log scale.
Blackbody Radiation
Sun-Yellow
Red
• Spectrum of Blackbody Radiation as a function of wavelength.
• Energy emitted by four blackbodies with equal surface areas.
• Note that they are plotted on log-log scale.
• Area is proportional to total power per unit surface area (PA)
• Stefan-Boltzmann Law- PA = .T4
• PA is in Wm-2 and  = 5.67 x 10 -8 Wm-2 K-4
Wien’s Displacement Law
• Doubling T increases P by 16 since
PA = .T4
Luminosity = Surface area x PA
• Note that maximum wavelength max
shifts with .This can be quantified in
Wien’s Displacement Law.
• This quantifies the observation
max.T = const.
that an object changes colour
with Temperature e.g.At room
= 2.9 x 10-3 mK
temp. spectrum peaks in infra-red.
• Very important since it allows us to obtain a measure of the SURFACE
TEMPERATURE of a star from max.For the Sun max is in blue with but
a lot of radiation in red so it looks yellow.For stars with T = 3000k
max is in infrared but significant amount in red.Red Giants are at this T.
Betelgeuse
Bellatrix
Orion nebula
Alnitak, Alnilam and Mintaka
Rigel
Saiph
Photons
So far everything I have said assumes that EM Radiation is a wave.
Planck’s Law was deduced assuming it is emitted by oscillators with
discrete energies. Einstein introduced the idea that it consists of
particles called PHOTONS each with E = h = hc/
where h = 6.6 x 10-34 Js is Planck’s constant
h is a very small number so number of visible photons needed for us to
see is very large.
Peak of blackbody spectrum gives surface temperature only. Photons
emitted in the centre are scattered and absorbed before they go very far.
They heat the layers outside them. The surface is heated by conduction
and convection of the gas.
Sun appears to be a blackbody at 5850 K. In centre it is  5 x 107 K
Stellar Spectra and Kirchhoff’s Laws
• Late 18th/early 19th C William Wollaston and others saw dark lines
imposed on Sun’s blackbody spectrum. Light was absorbed at these s.
• 1814-Fraunhofer had catalogued 500 lines and noted Na line.
• Kirchhoff showed that the dark lines are due to absorption of light at
that  by atoms of a particular element.His results are summarised in
Kirchhoff’s Laws:1)Hot, dense gas or solid emits a continuous spectrum.
2)A hot,diffuse gas produces bright spectral lines.
3)A cool,diffuse gas in front of a source with a cts. Spectrum produces
dark spectral lines.
• Full explanation had to wait for the Bohr-Rutherford theory of the atom.
Summary of Kirchhoff’s Laws in pictorial form
Stellar Spectra – A digression
• Main features of the Bohr-Rutherford Model
- Central atomic nucleus containing Z protons and
N neutrons
-A= N + Z
- Neutral atom has Z electrons
- Electrons are held in place by electrostatic force
• - Two main assumptions namely
1) The only orbits are those where the electron’s angular momentum
is an integral multiple of h/2 (h = Planck’s constant ) = n h/2
2) Electrons emit no radiation so long as they remain in an allowed
orbit. Radiation is emitted or absorbed when an electron makes a
transition from a higher(lower) to a lower(higher) state.
- States are characterised by Quantum numbers n,l where n is the
Principal quantum number and l is the orbital ang. Mom. Quantum no.
Stellar Spectra - 2
• Main results from Bohr-Rutherford Model
-The radii of the orbits are given by R = 0.h2.n2
,where me is the
.me.Ze2
is the electron mass and 0 is the electrical permittivity of free space.
-The energies of the orbits are given by
En = - me.e4.Z2
80.h2.n2
• In this simple model n dictates the level energies hence Principal Q.N.
-For H(Z = 1) we find that En = - 13.6/ n2 eV
-Note that the zero of energy is at infinity.
• Note:-the elements in increasing Z are Helium(He),Lithium(Li),
Beryillium(Be),Boron(B),Carbon(C),Nitrogen(N),Oxygen(O),
Fluorine(F),Neon(Ne),Sodium(Na),Magnesium(Mg),Aluminium(Al),
Silicon(Si),Phosphorus(P),Sulphur(S),Chlorine(Cl),Argon(Ar),
Potassium(K),Calcium(Ca),Scandium(Sc),------------------------------
Stellar Spectra-3
• Pauli Principle:-No two electrons can have the same quantum numbers.
As a result each level can hold two electrons.The Periodic Table is
built up by placing successive electrons in levels two at a time.Noble
gases represent atoms with last electron particularly tightly bound.
• If we think of light in terms of photons then E = h.Photons emitted or
absorbed in atoms have discrete energies given by
h = me.e4.Z2[1/n12 - 1/n22]
802.h2.c
from our expression En = - me.e4.Z2
for the level energies.
80.h2.n2
• In general we see series of spectral lines with 1/ = R[1/m2 - 1/n2],where
R is a constant and m,n are integers.
In H, Balmer series has m = 2,n = 3,4,5---Paschen series has m = 3,n = 4,5,6,------
Levels and Transitions in Hydrogen
The figure shows the levels
in the hydrogen atom.
The zero of energy is at infinity.
The levels are labelled by the
Principal Quantum Number n
and by the energy[on the left]
The series of spectral lines that
were found empirically by various
experimenters are shown.
Each series ends on a particular
level.
Stellar spectra-4
Here we see the atomic spectra
for white light,sunlight and a
series of elements.
Note that the last spectra are for
Na in emission and absorption.
These spectra provide clear
fingerprints for the chemical
elements.
Molecular Spectra
• Molecular spectra are more complex than those of atoms-in general.
In addition to discrete levels molecules may rotate and vibrate as well.
The resulting sets of levels mean that a “chunk” of the spectrum is
absorbed.
• When molecules join together to form a dust particle the absorption
bands broaden until they approach solid matter and blot out the light.
Absorption by interstellar dust is a problem-T is low so molecules
stay intact.
• Earth’s atmosphere is an intermediate case.T is low enough for CO2,
H2O and O3 to hold together.They absorb bites out of the spectrum in
the UV and IR regions of the spectrum due to the excitation of
vibrational and rotational bands.Stellar atmospheres are much hotter
than that of a planet and so molecules generally break up.Thus the Sun
has a series of dark absorption lines due to H,Ca,Fe,Na,etc.
Information on Surface Temperature from Spectral Features.
• Stars contain 75% H and 25% He plus small amounts of other elements.
• At T = 6000K we have 3/2 kT  eVs. This is similar to the binding
energies of molecules.At this T they are broken up in collisions.As T
rises spectral features related to molecules will disappear.
• At low T atoms are neutral.As T increases collisions cause them to be
ionised.At even higher T they become doubly then triply ionised etc.
The spectra of ions differ from those of atoms.
• We get a progression as T increases.At low T we have neutral atoms
and molecules.The former disappear and the latter fade as T increases.
We then get spectra from singly charged ions. At still higher T we get
doubly charged ions.
• H line is n = 2 to n =3 absorption line. The n = 2 level is first excited
state in H.It is in middle of red part of spectrum.
At low T very few H atoms are thermally excited so H is weak.
As T increases so does occupation of n = 2 level and H becomes
stronger. This absorption line reaches a peak at 10,000K. Beyond this
many of the atoms are ionised and it fades again.
UV
Proportion of light
which arrives at
sea level
Altitude at which atmos.
reduces intensity of radn
by one-half.
• Picture shows absorption of radiation by Earth’s atmosphere.
There is strong absorption by N and O in the X-ray and gamma-ray
regions,strong absorption by ozone in the UV,absorption of H2O in
the infrared.Free electrons in the ionosphere reflect very long 
radio waves.
Doppler Shift
Observer A
Observer B
Source moving relative to observer.
If the source is moving towards you each successive wave is
emitted closer to you. Wavelength appears shorter—blue shifted
 Opposite is true if it is moving away from you.—red shifted.
Doppler Shift
• If v  c then the shift in wavelength due to the motion of an object
relative to an observer is  = v/c.
• If v is +ve then  increases and we have a redshift, object looks colder.
If v is -ve then  decreases and we have a blueshift, object looks hotter.
• If 0 is the wavelength emitted by a stationary source then
 = (  - 0 )
• We now define the Redshift Z as
Z = (  - 0 )/ 0 = / 0 = v/c
• We can measure  spectroscopically. As a result if we can identify a
spectral line and measure  then we can measure the relative velocity.
• If v is not much less than c then we must use the full expression
Z = [(c + v)/(c - v)]1/2 - 1 = / 0
The Story so far.
The Nature of Astronomy—based on observation
Our information largely comes from electromagnetic radiation emitted
- EM radiation has const. vel. in vacuum c = λν
- all λ exist
- can be polarised
Black Body radiation
-Stefan-Boltzmann Law PA = σT4
-Wien’s Law λMAX.T = 2.9 x 10-3 m.K
Atoms can only exist in discrete energy levels. Consequently transitions
between levels have discrete energies. The spectrum of lines is
then characteristic of the chemical element.
Kirchhoff,s Laws:- Summary of observations about BB spectrum
and both emission and absorption spectra
Doppler shift:Z = [(c + v)/(c - v)]1/2 - 1 = / 0
If v  c then we can write
 = v/c.
Stellar spectra-4
Here we see the atomic spectra
for white light,sunlight and a
series of elements.
Note that the last spectra are for
Na in emission and absorption.
These spectra provide clear
fingerprints for the chemical
elements.
Molecular Spectra
Molecule can rotate and vibrate so in
addition to the discrete levels we have
levels built on them with energies
associated with them.
We end up with closely spaced bands
of levels built on each intrinsic level.
[Rotations and vibrations]
Vibrations:- Levels equally spaced
Rotations:- LevelsE = (h/2)2.[I(I + 1)]
2ζ
where ζ is the moment-of inertia
Information on Surface Temperature from Spectral Features.
• Stars contain 75% H and 25% He plus small amounts of other elements.
• At T = 6000K we have 3/2 kT  eVs. This is similar to the binding
energies of molecules.At this T they are broken up in collisions.As T
rises spectral features related to molecules will disappear.
• At low T atoms are neutral.As T increases collisions cause them to be
ionised.At even higher T they become doubly then triply ionised etc.
The spectra of ions differ from those of atoms.
• We get a progression as T increases.At low T we have neutral atoms
and molecules.The former disappear and the latter fade as T increases.
We then get spectra from singly charged ions. At still higher T we get
doubly charged ions.
• H line is n = 2 to n =3 absorption line. The n = 2 level is first excited
state in H.It is in middle of red part of spectrum.
At low T very few H atoms are thermally excited so H is weak.
As T increases so does occupation of n = 2 level and H becomes
stronger. This absorption line reaches a peak at 10,000K. Beyond this
many of the atoms are ionised and it fades again.
Summary of Stellar classification
[Metals?]
Temperature
Remember-each of the classes is further subdivided 0-9
In the last few years there has been an attempt to introduce two more groups on the
low temperature end. These are faint stars at low temperature, more and more of which are
being classified. They are L and T. L stars have T between 1300 and 2500K. One sees a lot
of metal hydride molecules such as CrH and FeH. T dwarves are even cooler and show a lot
of methane. They are, in general, failed stars like Jupiter.
Classification of Stellar Spectra – Harvard Scheme
TYPE Colour
O
B
A
F
G
K
M
Approx T
Blue
>25000K
Blue
11-25000K
Blue
7.5-11000K
Blue/white 6-7500K
White/yellow 5-6000K
Orange/Red 3.5-5000K
Red
<3500K
Main Characteristics
Singly ionised He in emission/absorption
Neutral He in absorption
H lines at max. strength for A0, decreasing thereafter
metallic lines become noticeable
Solar-type, Absorption lines of metallic atoms/ions grow
Metallic lines dominate
Molecular bands of TiO noticeable
Examples
10 LACERTA
RIGEL/SPICA
SIRIUS/VEGA
CANOPUS
SUN
ARCTURUS
BETELGEUSE
Temp.
Within each of these broad categories Annie Jump Cannon assigned sub-categories 0 – 9 with 0 being at the high T end.
This is known as the Harvard scheme. It was funded by the wife of a wealthy doctor-Henry Draper and was carried out
by a team composed largely of women [see photo of them in Universe, 6 th Ed. Fig 19.10]. They included Williamina
Fleming, Antonia Maury and Annie Jump Cannon.
Later Cecilia Payne and Meghnad Saha showed that the catalogue the Harvard group created and classified was an
indicator of surface temperature.
Note:- The spectra reflect the temperature and composition of the surface and essentially the composition of the Star prior
to formation since no nuclear reactions occur in the surface and there is little mixing with the interior
A full classification should include Luminosity - See YERKES or MMK classification scheme.
UV
N and O
O3
Proportion of light
which arrives at
sea level
H2O
Ionosphere
Altitude at which atmos.
reduces intensity of radn
by one-half.
• Picture shows absorption of radiation by Earth’s atmosphere.
There is strong absorption by N and O in the X-ray and gamma-ray
regions, strong absorption by ozone in the UV,absorption of H2O in
the infrared. Free electrons in the ionosphere reflect very long 
radio waves.