Download The Ultimate Tool of Astronomy: Telescopes

The Ultimate Tool of
Astronomy: Telescopes
Announcements
q  Homework # 4 is due on Thursday
October 27th
q Grades for Quiz # 3 are available in the
Gradebook. Solutions are available on the
course website, under `Quizzes’
q Quiz # 4 will take place on Tuesday,
November 1st. Remember to bring a pencil
Today’s Goals
q  Complete discussion of Doppler Effect,
and how it is used in Astronomy
q  Discuss some of the Exam # 1 questions
q Introduce and discuss Telescopes
Doppler Shift
The first crest travels out in
circle from the original position
of the plane
Shorter wavelength
(bluer)
At a later time, a second
crest is emitted from the
planes new position,
but the old crest keeps
moving out in a circle from
the planes original
position
The same thing happens again at
a later time
Longer
wavelength
(redder)
What we actually see when we do
spectroscopy
Emission spectrum of hot gas as seen in lab
Emission spectrum of hot gas as seen in rapidly
moving object
Is this object moving towards or away from us?
What we do with the Doppler
Shift
We derive the (approaching or receding) velocity
of the source (star, galaxy, etc.).
q  The Doppler shift causes the rest wavelength, λr ,
emitted by an object to become bluer (blue-shift) or
redder (red-shift): λo- λr
q  To derive a velocity, we divide by a time T= 1/νr:
v = (λo- λr)/T = (λo- λr) νr = c (λo- λr)/ λr
v/c = (λo- λr)/ λr
This is the formula used to measured the
recession of galaxies in the Universe!
The Doppler shift
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An object shining light with restframe λr=656.3
nm is moving at V=5,000,000 m/s away from
you. What is the color, λo, of the light that you
see?
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v/c = (λo- λr)/λr
v/c =5x106/3x108 = 1.67x10-2 = (λo- λr)/λr
λo = λrx(1+1.67x10-2) = 667.3 nm
(redder!)
Survey Question
Two identical stars are observed from the Earth. Star
A’s emission lines (that are at visible wavelengths in
the rest frame) are observed to be at ultraviolet
wavelengths. The same emission lines for Star B are
observed to be at X-ray wavelengths. From these
observations you conclude that:
1) both stars are moving away from the Earth
2) Star A is moving towards the Earth faster than
Star B
3) Star B is moving towards the Earth faster than
Star A
4) Star B is moving away from the Earth while
Star A is moving towards the Earth.
Exam 1: Q. # 4
Friends tell you that they saw the constellation Orion
high in the sky at 4 a.m. this morning. You are not
interested in getting out of bed so early. How many
months will you have to wait until Orion is in the same
place of the sky at midnight?
A. One month
B. Two months
C. Three months
D. Four months
E. Ten months
Exam 1: Q. # 4
Friends tell you that they saw the constellation Orion
high in the sky at 4 a.m. this morning. You are not
interested in getting out of bed so early. How many
months will you have to wait until Orion is in the same
place of the sky at midnight?
A. One month
B. Two months
C. Three months
D. Four months
E. Ten months
Exam 1: Q. # 11
Newton concluded that some force had to act on the
Moon because
A. a force is needed to keep the Moon in motion
B. a force is needed to pull the Moon outward
C. the Moon moved at a constant velocity
D. a force is needed to pull the Moon away from a
straight-line motion
E. all of the above
Exam 1: Q. # 11
Newton concluded that some force had to act on the
Moon because
A. a force is needed to keep the Moon in motion
B. a force is needed to pull the Moon outward
C. the Moon moved at a constant velocity
D. a force is needed to pull the Moon away from a
straight-line motion
E. all of the above
Exam 1: Q. # 15
A cart of mass M is moving at constant speed v.
Suddenly a weight of mass m drops on the cart. The
momentum of the cart:
A. increases
B. decreases
C. stays the same
Exam 1: Q. # 15
A cart of mass M is moving at constant speed v.
Suddenly a weight of mass m drops on the cart. The
momentum of the cart:
A. increases
B. decreases
C. stays the same
Exam 1: Q. # 26
Due to tides raised on the Earth by the Moon, the
Moon is gradually moving FURTHER away from the
Earth. The average distance between the Earth and
the Sun remains constant with time. How will solar
eclipses be different in a distant future as a result of
the increasing distance between the Earth and the
Moon?
A. Total solar eclipses will occur only at Full Moon
B. Total solar eclipses will last considerably longer
C. Total solar eclipses will no longer occur
D. Total solar eclipses will only be visible from the
Earth’s Southern Hemisphere
E. Total solar eclipses will occur at every New Moon
Exam 1: Q. # 26
Due to tides raised on the Earth by the Moon, the
Moon is gradually moving FURTHER away from the
Earth. The average distance between the Earth and
the Sun remains constant with time. How will solar
eclipses be different in a distant future as a result of
the increasing distance between the Earth and the
Moon?
A. Total solar eclipses will occur only at Full Moon
B. Total solar eclipses will last considerably longer
C. Total solar eclipses will no longer occur
D. Total solar eclipses will only be visible from the
Earth’s Southern Hemisphere
E. Total solar eclipses will occur at every New Moon
The Ultimate Tools of
Astronomy: Telescopes!
Assigned Reading
q  For the topic of Telescopes: Units 26.1-3,
27.1-2, 28, 29.1-2, 30.1, 30.4
Telescopes
The twin 10-m Keck telescopes (Hawaii) are currently the
largest telescopes in existence.
The largest combined telescopes (4 x 8 m) are the ESO
VLT.
Plans for larger telescopes (up to 42-m) are currently
being considered (ESO, USA)
UMass
LMT
The 50-m Large Millimeter
Telescope
The largest millimeter-wavelength
telescope in the world
U Mass and Mexico
Why do you need large
telescopes?
Nasa is building
JWST, a 6.5-m
telescope, which will
be launched in space
~ 2018
Telescopes are
`light buckets’.
The bigger, the
more light they
collect, and the
more distant the
objects they can
observe.
Why do telescopes need to be big?
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The main feature of a telescope is its capacity to collect as
much light as possible
•  Like an antenna: the stronger the signal the clearer the
transmission.
•  Well, guess what: an antenna *is* a telescope (a radio
telescope, that is)
The larger the light collector, I.e. the primary mirror or
lens, the more powerful the telescope:
Collecting Area ~ 4 π D2
CAA/CAB = (DA/DB)2
•  A telescope twice as large collects four times as much
light
Reflector
Focusing Light
Refractor
Detecting Light (Instruments)
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Cameras:
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Spectrographs:
Detecting Light (Instruments)
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Cameras:
•  To obtain images at desired wavelength or
wavelengths (color images)
•  This is what yields the `pretty pictures’
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Spectrographs:
•  To study the intensity of the various
wavelengths (colors): spectra
•  This yields the physical nature (star, galaxy,
balck hole), chemical composition, physical
properties (temperature, density), dynamics
(motions, mass), distance of the sources
The Telescope’s Sharpness
To study galaxy formation both space-based sensitivity and angular resolution required!!
Note how many more details and faint objects can be observed with the Hubble Space Telescope
Subaru + SUPREME
HST + ACS
Sharpness (Angular Resolution)
• The bigger the telescope, the smaller the detail it can
discriminate
•  α = 0.02 λ(nm) / D(cm)
•  This also depends on the type of light the telescope
detects (the wavelength λ)
•  On the ground, the limitation is due to the `blurring’ of
our own atmosphere (called `seeing’).
Telescopes and Earth’s Atmosphere
Most wavelengths cannot penetrate the Earth's atmosphere!
Telescopes and Earth’s Atmosphere
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In addition to blur light (reduce sharpness), the
Earth’s atmosphere prevents some wavelengths from
getting to the Earth’s surface:
•  Ultraviolet, X-ray, infrared: telescopes working at
these wavelengths need to be placed in SPACE
•  We are grateful to the atmosphere for stopping UV
and X-ray, otherwise we would be fried to a crisp!
Wavelengths that make to the ground:
•  Optical and radio
•  We are glad radio wavelengths can travel freely,
otherwise we would not have telecommunications!
(cellphones operate at ~ 33 cm)
Different telescopes for different
types of light
LMT
Herschel
Spitzer
Hubble
Different types of telescopes
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To detect different types (wavelengths) of light,
eg. X-ray, UV, optical, infrared, radio, different
technologies are required
For example, special mirrors are necessary for
X-ray telescopes or else the radiation would
pass through them.
Hence, it is necessary to specialize telescopes
to the wavelength of light one wishes to study.
What is up there now…
HUBBLE: optical telescope
CHANDRA: X-ray telescope
SPITZER and HERSCHEL: Infrared Telescopes
Why different wavelengths are
required
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Regardless of the technology, different
wavelengths carries different information:
•  Shorter wavelengths (X-ray, UV) carry
information on very energetic phenomena
(e.g. black holes, star formation)
•  Optical wavelengths carry information on the
structures of galaxies and their motions (the
assembly of the bodies of galaxies, their size)
•  Longer wavelengths (IR, radio) carry
information on the chemical composition,
physical state (gas and dust, presence,
chemical elements; temperature)
Optical Sky
Near-infrared sky
Boldt et al.
Radio Sky
Soft X-ray Sky