As the stove gets hotter, the light from the coils goes from red to yellow to whitish.

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As the stove gets hotter, the light from the
coils goes from red to yellow to whitish.
What are some other examples?
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Blackbody Radiation
Temperature is measured on the absolute (Kelvin)
scale.
0O K -273O C D1O K D1O C 0O K -460O F
D1O K D1.8O F
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Holding a poker over fire black-red-white color
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• Properties of Electromagnetic Radiation
• Light as a transverse wave (polarizer
  experiments)
• Light has wavelength (diffraction experiment)
• Light in a thermal environment-- Blackbody
  spectrum
• Kirchoff (1824-1897) An opaque body heated to a
  temperature T emits continuous radiation, known
  as Thermal radiation, with a distribution as a
  function of Wavelength ? characterized by T only.
  
• 2. Stefan (1835-1893) and Boltzmann
  (1844-1906)
• Total radiation emitted/Area/sec (flux)
  proportional to T4,
• F ? T4,
• (? 5.67 10-8 Watt/(m2 K4) is known as
  Stefan-Boltzman constant)
• 3. Wien (1864-1928) displacement law
• Peak of radiation ?max 0.29 cm/ T (
  in Kelvin)
• (temperature of Universe today has T2.7
  K, ?max 1mm)
• (temperature of Sun approx 6000K, ?max
  5 10-4 cm 500 nm)

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Why do we humans (and most other animals on
earth) detect only visible radiation when it is
such a small part of the electromagnetic spectrum?
Where does the Sun give off most of its radiation?

• 0.293/T 0.293/5800 5.05 x 10-5 cm
• This is the wavelength of green light.

There is a big evolutionary advantage for
animals that can detect light where the Sun puts
most of it out.
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Thermal Radiation from Objects
There are also a variety of non-thermal
processes (often involving magnetic fields) which
produce radiation at all wavelengths (and can
produce VERY high energy radiation) all the way
up through gamma rays. They are often associated
with violent phenomena (explosions, black holes,
etc.).
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• Doppler effect is used to all types of
  measurements- from Doppler Radar to measure speed
  of cars, planets, asteroids, to Doppler
  measurement of spectral lines to measure speed of
  astronomical objects (more on this later).
• Joseph Fraunhofer (1787-1826) observed solar
  spectrum
• Robert Bunsen (1811-1899) Bunsen burner
• Gustav Kirchoff (1824-1897) codiscovered cesium,
  rubidium, invented flame spectroscopy

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The Doppler Shift how we use it
Atomic energy transitions leave features in the
spectrum whose rest wavelengths are known from
laboratory work. We can measure observed shifts
in these wavelengths from astronomical objects,
and see how fast they are moving (you only get
the line-of-sight motion towards or away from
you).
More subtle analysis can also yield other
motions, like rotation or turbulent motions.
These are all direct uses of the Doppler shift.
It doesnt matter how far away the source is,
either.
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• Kirchoff's three laws of radiation
• 1. A body heated to incandesence emits a
  continuous spectrum
• 2. A gas heated to incandescence emits a bright
  spectrum of lines (emission lines)
• 3. A cold gas placed before a hot solid produces
  a bright continuum superimposed by dark lines.
  (absorption lines)

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By the end of the 19th century, spectroscopists
had measured "flame" spectrum of all common
elements. Why did the elements all have
characteristic lines at which they would emit and
absorb radiation? (Answer would have to await
20th century invention of quantum mechanics).
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Unique Atomic Signatures
Each atom has a specific set of energy levels,
and thus a unique set of photon wavelengths with
which it can interact.
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Spectrum of hydrogen
Note the absorption when H cloud is illuminated
from behind.
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• The Doppler Effect
• Speed of light is c to all observers, but
  wavelength of light is dependent on motion of
  observer relative to source. (effect applies to
  sound as well as to light)
• ?observed/ ?emitted 1 v/c
• (where v is velocity of observer relative to
  source, and c is speed of light (or sound))
  
• Crests of wave pile up in forward direction,
  stretch out in backward direction (blueshift and
  redshift)

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The Doppler Shift how it works
When a source is moving, an observer gets the
waves either stretched out or crunched together,
depending on their relative motion with the
source. In the case of light, longer wavelengths
look redder and shorter wavelengths look bluer.
This is given by the Doppler formula
Where v is the velocity of the moving object. v
is negative for an approaching source if the
distance is shrinking, the wavelength is too
To get an appreciable change, you have to be
moving with an appreciable fraction of the speed
of the wave
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Searching for SNe in nearby galaxies

• SNe are as bright as entire galaxy!
• Finding them in the Universe is a probe of
  cosmology (later)

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Two broad categories of supernovae

• Type I (no hydrogen lines in spectrum) - found in
  all types of galaxy
• --White dwarf supernova
• Type II (strong hydrogen lines in spectrum) found
  only in star-forming galaxies (formed from
  massive, short-lived stars)
• Total energy of SN exceeds that released by
  nuclear fusion over full lifetime of star.
• Where does all this energy come from?
• -Answer gravitational potential of newly formed
  neutron star ? 2GM/(Rc2) ? 0.1
• -This implies that if you construct a neutron
  star out of X neutrons, and each neutron has mass
  m in isolation, that the total mass of the
  neutron star will not be Xm, but will be
  (1-?)Xm ? 0.9Xm
• -Gravitational binding energy represents 10 of
  rest mass of neutrons, which has been radiated
  away.
• -If you were to land on surface of neutron star,
  your rocket would have to accelerate to vescape
  c ?? 95,000 km/s for you to escape (Warning
  Keep your distance!!)

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Fractional Binding energy of a star

• ? 2GM/(Rc2) 10-9 (for gas cloud before Sun,
  with v10 km/s)
• (vesc/c)2 ?
• ? 2GM/(Rc2) 4 x 10-6 (for solar mass MS
  star)
• - .7 x 10-3 available when H fuses into H
• - Escape velocity 600 km/s
• ? 2GM/(Rc2) 5 x 10-4 (for solar mass WD)
• -More than 100x that of Sun on main sequence.
• -Escape velocity from a W.D. exceeds 6000 km/s !!
• -Average density 3 x 106 gm/cm3
• ? 2GM/(Rc2) .2 (for solar mass NS)
• vesc c ?1/2 130,000 km/s
• ? 2GM/(Rc2) 1 (for Black Hole)
• vesc c

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Supernova Light Curves
(Type II)
(Type I)
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Neutron Stars

• are the leftover cores from supernova
  explosions.
• If the core be held up by neutron degeneracy pressure.
• Neutron stars are very dense (1012 g/cm3 )
• 1.5 Msun with a diameter of 10 to 20 km
• They rotate very rapidly Period 0.03 to 4 sec
• Their magnetic fields are 1013 times stronger
  than Earths.

Chandra X-ray image of the neutron star left
behind by a supernova observed in A.D. 386. The
remnant is known as G11.2?0.3.
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The Iron (Fe) Problem

• The supergiant has an inert Fe core which
  collapses heats
• Fe can not fuse
• It has the lowest mass per nuclear particle of
  any element
• It can not fuse into another element without
  creating mass

So the Fe core continues to collapse until it is
stopped by electron degeneracy. (like a White
Dwarf)
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Supernova

• BUT the force of gravity increases as the mass
  of the Fe core increases
• Gravity overcomes electron degeneracy
• Electrons are smashed into protons, making
  neutrons
• (at room temp. and pressure, neutrons decay to e-
  p ?)

• Without e-, the core collapses until it becomes a
  neutron star!
• The neutron core collapses until abruptly stopped
  by neutron degeneracy
• this takes only seconds
• The core recoils and sends the rest of the star
  flying into space

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Supernova
The amount of energy released is so great, that
most of the elements heavier than Fe are
instantly created In the last millennium, four
supernovae have been observed in our part of the
Milky Way Galaxy in 1006, 1054, 1572, 1604
Crab Nebula in Taurus supernova exploded in 1054
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Supernovae
Tychos Supernova (X-rays) exploded in 1572
Veil Nebula
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Products of Supernova-- Elements of all types!

• Note that elements made in star while a Red giant
  are more abundant than neighboring elements
• Note that elements heavier than Fe are produced
• -Free neutrons are available

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Crab Nebula, in X-ray and optical
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Pulsars

• In 1967, graduate student Jocelyn Bell and her
  advisor Anthony Hewish accidentally discovered a
  radio source in Vulpecula.
• It was a sharp pulse which recurred every 1.3
  sec.
• They determined it was 300 pc away.
• They called it a pulsar, but what was it?

Jocelyn Bell
Light Curve of Jocelyn Bells Pulsar
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The mystery was solved when a pulsar was
discovered in the heart of the Crab Nebula.
The Crab pulsar also pulses in visual light.
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What is a pulsar?

• Could it be a WD spinning very fast?
• Suppose it has a hot spot that accounts for the
  light, and that it is size of Earth. Let it orbit
  at vc
• C2?R 40,000 km. C v ? (?time to spin
  once)
• ? C/v (4 104 )/(3 105 ) .13 sec
• Clearly, star must be 100-1000 times smaller than
  WD! Neutron star is small enough.
• Neutron star
• Suggested as stable state of condensed star by
  Oppenheimer in late 1930s, while he worked at
  Berkeley (go bears!)
• Entire mass of a star in 10 km radius!
• Black Hole
• No way to anchor beacon, cannot see a black hole
  directly

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Pulsars and Neutron Stars
Pulsars are the lighthouses of Galaxy!
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Pulsars and Neutron Stars

• All pulsars are neutron stars, but all neutron
  stars are not pulsars!!
• Synchotron emission --- non-thermal process where
  light is emitted by charged particles moving
  close to the speed of light around magnetic
  fields.
• Emission (mostly radio) is concentrated at the
  magnetic poles and focused into a beam.
• Whether we see a pulsar depends on the geometry.
• if the polar beam sweeps by Earths direction
  once each rotation, the neutron star appears to
  be a pulsar
• if the polar beam is always pointing toward or
  always pointing away from Earth, we do not see a
  pulsar

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Rotation Periods of Neutron Stars

• As a neutron star ages, it spins down.
• The youngest pulsars have the shortest periods.
• Sometimes a pulsar will suddenly speed up.
• This is called a glitch!
• There are some pulsars that have periods of
  several milliseconds.
• they tend to be in binaries.

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Birth of a Millisecond Pulsar

• Mass transfer onto a neutron star in a binary
  system will spin the pulsar up faster.
• to almost 1,000 times per sec
• Like white dwarf binaries, an accretion disk will
  form around the neutron star.
• the disk gets much hotter
• hot enough to emit X-rays
• We refer to these objects as X-ray binaries.