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• 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.

Before correcting for atmosphericblurring

After correction

Light Pollution

• 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

North America at night

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)

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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– Interferometry improves resolution

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Resolution: De-blend Stars

Low resolving power telescope. High resolving power telescope.

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 VenusThe 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.

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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 (proto-planetary disks) in our Galaxy.

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

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The Inner or Terrestrial Planets

• Solid, rocky, small-sized, 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, low-density, similar composition as the Sun including the relative amounts of hydrogen and helium

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

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Planet Hunting 2: Doppler Shift

Red-shifted absorption spectrum

Lamp-emission spectrum at rest

Lamp-emission spectrum at rest

Blue-shifted absorption spectrum

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.

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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% CO2.

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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

Astronomy Picture of the Day

Volcanic ash on Mars