Transcript Quiz #3 Review

Quiz #3 Recap
20111103
Earth’s Atmosphere &
Telescopes
• Whether light is
absorbed by the
atmosphere or not
depends greatly on
its wavelength.
• Earth’s
atmosphere can
absorb certain
wavelengths of
light so much that
astronomers can
not observe them
from the ground.
• UV light is strongly
absorbed by ozone
molecules.
Atmospheric Effects
Before correcting
for atmospheric
blurring
After correction
• The temperature
variations in the
atmosphere cause
light from objects to be
bent like passing
through a lens
• Random bending of
light causes stars to
“twinkle” and blurs
images in telescopes.
• These effects can be
reduced but not
eliminated by placing
telescopes on
mountains.
Light Pollution
North America at night
• Light from cities
has greatly
reduced
everyone’s ability
to enjoy the night
sky.
• The atmosphere
scatters light from
cities over long
distances
• Astronomers must
go to remote
locations to make
observations
Why Put Telescopes in Space?
• Avoid image blurring due to atmospheric
turbulence
• Avoid scattered city light
– Also, can observe around the clock in space
• Some electromagnetic waves cannot
penetrate the Earth’s atmosphere:
– Gamma-rays, X-rays, most UV light, most
infrared light, microwave light, submillimeter
and millimeter wavelength light
• If we want to observe these regions of the
electromagnetic spectrum, we must put telescopes
above the Earth’s atmosphere  in space
Why Put Telescopes in Space?
• Keep in mind:
– Sending probes to other planets to
• take pictures from orbit around planet
• land on planet
• plunge into planet’s atmosphere
– helps to get a better picture of a planet
because the camera (on the satellite) is closer
to the planet
Why Put Telescopes in Space?
• but
– Putting a telescope in orbit around Earth (like
the Hubble Space Telescope) gives no
distance advantage
• the change in distance between Earth’s surface
and Earth orbit is miniscule compared to cosmic
distances
• the advantages in this case are those that relate
to getting the telescope above the Earth’s
atmosphere
Refracting Telescopes
• Light enters a refractor through the Primary Lens.
There is a secondary lens at the other end of the
tube at the eyepiece.
• You see the image by looking through the
eyepiece.
Reflecting Telescopes
• In a reflector the light enters the tube and is
reflected off of the Primary Mirror mounted at
the back.
• The light is then focused towards a secondary
mirror which then sends the light to the
eyepiece.
Reflector Advantages
• Mirrors can be made lighter, larger, and cheaper
– Largest practical diameter for a lens is ~1 meter.
– Mirrors may reach sizes greater than 8 meters in
diameter
– Several large mirrors can be combined to reflect light
to a common focus, effectively making telescopes
with even larger diameters (10 meters in diameter)
• Since large mirrors are lighter than large lenses, the
telescope support structure will also be lighter, cheaper
to build, and easier to maintain
• Lenses
– sag over time (change focus)
– absorb some light
– may act as prisms (chromatic aberration)
Sensitivity and Resolution
• Sensitivity is the light collecting power of a
telescope
– The larger the telescope diameter, the greater the
sensitivity
– Fainter objects can be detected using a telescope
with greater sensitivity
• faint objects may be nearby, or faint objects may be
intrinsically bright, but far away
• Resolution is the ability discern fine detail
(clarity/sharpness)
– Imaging: de-blend stars that are close together
– Spectroscopy: de-blend lines that are close together
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– Interferometry improves resolution
Resolution: De-blend Stars
Low resolving power telescope.
High resolving power telescope.
12
No Atmosphere on Mercury
• Mercury’s proximity to the Sun makes its sunlit side very hot, ~700K, and
• Mercury’s low mass means low gravity and low escape velocity
• if Mercury had an atmosphere the (hot) gas molecule speed would be
greater than Mercury’s escape velocity
Surface of Venus
The surface of Venus is
perpetually shrouded by
sulfuric acid clouds.
The Magellan spacecraft
used radar ranging to map
the elevation of Venus’
surface.
Re-projected images (from panoramic cameras)
of Venus from the Venera 13 (top) and
Venera 13 (bottom) landers.
Solar System Formation from
Collapsed Rotating Cloud
• All the planets orbit
the Sun in the same
direction.
• All the planets orbit
within nearly the
same plane. Like a
disk.
• Observations of
other solar systems
in formation (protoplanetary disks) in
our Galaxy.
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Collapsing Gas Clouds
• As the cloud collapsed the
original slow spin began to
speed up. This caused the
cloud to flatten into a disk
shape.
– Ordered rotation leads to
flattening
• The gravitational pull of the
cloud caused it to shrink
further and caused most of
the material to fall towards
the core forming a large
bulge.
– Infall
The Inner or Terrestrial
Planets
• Solid, rocky, smallsized, low-mass,
high-density,
similar composition
as the Sun except
for the relative
amounts of
hydrogen and
helium
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The Outer or Jovian Planets
• Gaseous+liquid
with small rocky
core, large-sized,
high-mass, lowdensity, similar
composition as the
Sun including the
relative amounts of
hydrogen and
helium
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Heating and Condensation of the
Solar Nebula
•
•
•
•
The heat from the Sun
prevents ices from reforming
on the dust grains in the
region near the Sun.
Ices (water, ethane, methane,
ammonia [all contain
hydrogen]) condensed only in
the outer parts of the Solar
nebula.
In the inner portion of the disk
only materials like iron and
silicates (rock) can condense
into solids. Slowly they form
clumps of material.
In the outer portion of the disk
much more material can
condense as solids including
ice. This extra material allows
clumps to grow larger, more
massive, and faster.
Planet Hunting 1:
Direct Imaging
Planet Hunting 2:
Doppler Shift
Planet Hunting 2:
Doppler Shift
Lamp-emission spectrum at rest
Blue-shifted absorption spectrum
Lamp-emission spectrum at rest
Red-shifted absorption spectrum
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Planet Hunting 3:
Gravitational Lensing
Planet Hunting 4:
Transit Light Curves
Inner Planets’ Atmospheres
• Those inner planets that have
atmospheres (Venus, Earth, and Mars)
acquired the gases via
– interior gases venting to surface
– captured gases from vaporized comets
• recall that comets contain water; they orbit the Sun
on very eccentric orbits that extend to the outer
solar system where it is cold and therefore they
retain their water/ice
• asteroids that may have collided with inner planets
during the solar system formation likely had little if
any water/ice, since they orbited the Sun within the
warm inner solar system.
The Greenhouse Effect
• When the gases in an
atmosphere allow sunlight
to strike the surface the
surface heats up and gives
off infrared radiation.
• If the atmosphere however
prevents the infrared
radiation from radiating
back out to space the
temperature of the planet
can increase, this is the
Greenhouse Effect.
• Carbon Dioxide CO2
behaves this way and is an
important greenhouse gas.
Venus’ atmosphere is 95%
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CO .
Why is Earth’s Atmosphere so
Different from Mars’ and Venus’?
• Water + CO2 makes carbonic acid = soda water
• Rain on Earth removes CO2 from the
atmosphere and locks it into the rocky ground
• Venus’ atmosphere is too hot for water to
condense out  no water rain to remove CO2
• Mars’ atmosphere is too thin and cold for water
rain
(may have fog)
– Mars does have CO2 snow at poles
– Mars currently has very little water in its atmosphere
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Why is Earth’s Atmosphere so
Different from Mars’ and Venus’?
• Role of Biology on Earth
• Plants use carbon-dioxide to make cellulose
• Sea creatures use carbon-dioxide runoff (from
rain) to make shells (calcium carbonate).
• Plants break down water and carbon dioxide by
photosynthesis, releasing oxygen into the
atmosphere
• Geological processes melt rock in the hot mantle
re-releasing carbon-dioxide into the atmosphere
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Astronomy Picture of the
Day
Volcanic ash on Mars