Class 19 : The EM spectrum and
the discovery of compact objects
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The electromagnetic (EM) spectrum.
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Electromagnetic waves.
From radio waves to gamma rays.
Discovery of neutron stars.
Discovery of black holes.
I : Electromagnetic spectrum
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Have already come across relation of color to
temperature (e.g., for star).
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As temperature increases, go from
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REDYELLOWWHITEBLUE
What is physical difference between red & blue
light?
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Light is waves of electric & magnetic field.
Waves characterized by
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Wavelength () = distance between crests.
Frequency (f or ) = number of crests passing a given
point per second.
Speed of a crest: c = f.
Energy of light wave proportional to frequency.
Blue
Red
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Visible light is small part of spectrum:
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 = 300 nm for blue light.
 = 700 nm for red light.
But what about shorter and longer
wavelengths?
Large wavelength
Low frequency
Low energy
Small wavelength
High frequency
High energy
Visible and infrared astronomy
CFHT
IRTF
Gemini
UKIRT
Keck
Subaru
JCMT
Radio/mm astronomy
Greenbank, WV
Ultraviolet & X-ray Astronomy
XMM
II : Discovery of Neutron Stars
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Jocelyn Bell & Anthony Hewish.
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Radio astronomers at the University of
Cambridge in England.
Interested in radio-emissions from
“quasars.”
Bell found curious “scruff” in her signals…
blips that were always 1.33728 sec apart.
What was the periodic signal?
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Rule out equipment fault, Earth orbiting
satellite, Little Green Men (LGM).
Is, in fact, a spinning neutron star (pulsar).
From web site of Cambridge radio astronomy group
Sounds:
1 sec
0.1 sec
1.5 ms
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Pulsars…
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Now know of hundreds of pulsars.
Fastest known have periods of 1.5-3 ms
(rotate 300-600 times per second!).
Very active subject of research…
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What is the structure of a neutron star?
What determines how fast they spin?
How do they beam emission?
Magnetars.
Magnetars
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Spinning neutron stars with incredibly
strong magnetic fields.
Object
Strength
(Earth = 1)
Iron bar magnet
102
Sustained lab field
105
Strongest star
106
Strongest lab field
107
Typical pulsar
1012
Magnetar
1015
III : Discovery of black holes
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1960s and 1970s…
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What did they expect?
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Early days of X-ray astronomy.
Crude X-ray observatories placed on rockets (get few
minutes of data while rocket above atmosphere).
Knew that Sun was bright in X-rays.
Expected X-rays from Moon (reflection).
What did they find?
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Bright unidentified sources (Sco-X1 and Cyg-X1).
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Follow up study of Cyg-X1:
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Bright and variable X-ray emission.
Found high-mass star at approximate
location of X-ray source.
Found that star has very large velocity
shifts (40 m/s in each direction)…
… must be something very massive
swinging it around!
Cyg-X1 was the first black hole
candidate.
X-ray binaries
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X-ray binaries:
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“Normal star” orbiting around a neutronstar (NS) or black hole (BH).
Gas gets pulled off normal star and spirals
onto the NS/BH.
Gas heats up and emits X-rays as it falls
(like hydro-electric power).
Very powerful objects.
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Cyg-X1 is 105 times more powerful than the
Sun (and most energy emerges in X-ray
band).
Insert cool movie here…
Zooming in
on an X-ray
binary
Reynolds & Armitage