Astronomy notes pt 2 - good luck PDF

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Astronomical Instruments How does your eye form an image?

Refraction

 Refraction is the bending of light when it passes from one substance into another.  Your eye uses refraction to focus light.

Example: Refraction at Sunset

 Sun appears distorted at sunset because of how light bends in Earth’s atmosphere.

Focusing Light

 Refraction can cause parallel light rays to converge to a focus.

Image Formation

 The focal plane is where light from different directions comes into focus.  The image behind a single (convex) lens is actually upside-down!

How do we record images?

Focusing Light

 A camera focuses light like an eye and captures the image with a detector.  The CCD detectors in digital cameras are similar to those used in modern telescopes. Digital cameras detect light with charge-coupled devices (CCDs).

CCDs

 Charge-Coupled Devices (CCDs). (a) This CCD is a mere 300-micrometers thick (thinner than a human hair) yet holds more than 21 million pixels. (b) This matrix of 42 CCDs serves the Kepler telescope. (credit a: modification of work by US Department of Energy; credit b: modification of work by NASA and Ball Aerospace)

What are the two most important properties of a telescope?

1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time. 2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.

Light-Collecting Area

 A telescope’s diameter tells us its light-collecting area:

2

A re a = π (d ia m e te r/ 2 ) .  The largest telescopes currently in use have a diameter of about 10 meters.

Thought Question

How does the collecting area of a 10-meter telescope compare with that of a 2-meter

telescope? a) It’s 5 times greater. b) It’s 10 times greater. c) It’s 25 times greater. Angular Resolution

• The minimum angular separation that the telescope can distinguish

Angular Resolution

• Ultimate limit to resolution comes from interference of light waves within a telescope. • Larger telescopes are capable of greater resolution because there’s less interference.

Angular Resolution

What are the two basic designs of telescopes? Refracting Telescope

• The rings in this image of a star come from interference of light wave. • This limit on angular resolution is known as the diffraction limit. Close-up of a star from the Hubble Space Telescope  Refracting telescope: focuses light with lenses  Reflecting telescope: focuses light with mirrors  Refracting telescopes need to be very long, with large, heavy lenses.

Reflecting Telescope

 Reflecting telescopes can have much greater diameters.  Most modern telescopes are reflectors.

Designs for Reflecting Telescopes

Mirrors in Reflecting Telescopes

What do astronomers do with telescopes?

 Imaging: taking pictures of the sky  Spectroscopy: breaking light into spectra  Timing: measuring how light output varies with time

Imaging

 Astronomical detectors generally record only one color of light at a time.  Several images must be combined to make full-color pictures. Imaging

 Astronomical detectors can record forms of light our eyes can’t see.  Color is sometimes used to represent different energies of non-visible light.

Spectroscopy

 A spectrograph separates the different wavelengths of light before they hit the detector.

Spectroscopy

 Graphing relative brightness of light at each wavelength shows the details in a spectrum. Timing

 A light curve represents a series of brightness measurements made over a period of time. Want to buy your own telescope?

   

How does Earth’s atmosphere affect ground-based observations?

 The best ground-based sites for astronomical observing are:  calm (not too windy)  high (less atmosphere to see through)  dark (far from city lights)  dry (few cloudy nights)

Light Pollution

 Scattering of human-made light in the atmosphere is a growing problem for astronomy.

Twinkling and Turbulence

Buy binoculars first (e.g., 735)—you get much more for the same money. Ignore magnification (sales pitch!). Notice: aperture size, optical quality, portability. Consumer research: Astronomy, Sky & Telescope, Mercury, astronomy clubs

Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle.

Adaptive Optics

Calm, High, Dark, Dry

Why do we put telescopes into space? Transmission in Atmosphere

How can we observe invisible light? Telescopes at other Wavelengths

Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence.  The best observing sites are atop remote mountains.

 Only radio and visible light pass easily through Earth’s atmosphere.  We need telescopes in space to observe other forms.

 A standard satellite dish is essentially a telescope for observing radio waves. Radio telescopes: • Similar to optical reflecting telescopes • Prime focus • Less sensitive to imperfections (due to longer wavelength); can be made very large

Radio Astronomy Largest radio telescope: 300-m dish at Arecibo

Radio Astronomy

Longer wavelength means poor angular resolution Advantages of radio astronomy: • Can observe 24 hours a day • Clouds, rain, and snow don’t interfere • Observations at an entirely different frequency; get totally different information

Radio Astronomy

Interferometry: • Combine information from several widely-spread radio telescopes as if they came from a single dish • Resolution will be that of dish whose diameter = largest separation between dishes

Interferometry

Interferometry requires preserving the phase relationship between waves over the distance between individual telescopes

Radio Astronomy

Can get radio images whose resolution is close to optical:

VLBI

 Very Long Baseline Array. This map shows the distribution of 10 antennas that constitute an array of radio telescopes stretching across the United States and its territories. Radio Astronomy

Interferometry can also be done with other wavelengths, but much harder due to shorter wavelengths:

ALMA

 Atacama Large Millimeter/Submillimeter Array (ALMA). Located in the Atacama Desert of Northern Chile.(credit: ESO/S. Guisard) Other Astronomies

Infrared radiation can image where visible radiation is blocked; generally can use optical telescope mirrors and lenses

Other Astronomies

Infrared telescopes can also be in space or flown on balloons:

Other Astronomies

Ultraviolet images. (a) The Cygnus loop supernova remnant (b) M81

Other Astronomies

X-rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques X-rays will reflect at a very shallow angle, and can therefore be focused:

Other Astronomies

X-ray image of supernova remnant Cassiopeia A:

Other Astronomies

Gamma rays cannot be focused at all; images are therefore coarse:

Different Views of Our Universe

We are familiar with the view of our surrounding universe in visible light Different Views of Our Universe

 James Webb Space Telescope (JWST). This image shows some of the mirrors of the JWST as they underwent cryogenic testing. The mirrors were exposed to extreme temperatures in order to gather accurate measurements on changes in their shape as they heated and cooled. (credit: NASA/MSFC/David Higginbotham/Emmett Given)

 Artist’s Conception of the European Extremely Large Telescope. The primary mirror in this telescope is 39.3 meters across. The telescope is under construction in the Atacama Desert in Northern Chile. (credit: ESO/L. Calçada) Chapter 7 Other Worlds: An Introduction to the Solar System Our goals for learning: Overview of Our Planetary System What does the solar system look like? What can we learn by comparing the planets to one another? What are the major features of the Sun and planets? What does the solar system look like?

 There are eight major planets with nearly circular orbits.  Pluto and Eris are smaller than the major planets and have more elliptical orbits.

 Planets all orbit in same direction and nearly in same plane.

Thought Question

How does the Earth–Sun distance compare with the Sun’s radius? a) It’s about 10 times larger. b) It’s about 50 times larger. c) It’s about 200 times larger. d) It’s about 1000 times larger.

What can we learn by comparing the planets to one another?

Comparative Planetology

• We can learn more about a world like our Earth by studying it in context with other worlds in the solar system. • Stay focused on processes common to multiple worlds instead of individual facts specific to a particular world.  Comparing the planets reveals patterns among them.  Those patterns provide insights that help us understand our own planet.

What are the major features of the Sun and planets?

Sun

• • •

Over 99.9% of solar system’s mass Made mostly of H/He gas (plasma) Converts 4 million tons of mass into energy each second

Mercury

• • • Made of metal and rock; large iron core • Desolate, cratered; long, tall, steep cliffs Very hot and very cold: 425C (day)–170C (night Venus

• • •

Nearly identical in size to Earth; surface hidden by clouds Hellish conditions due to an extreme greenhouse effect Even hotter than Mercury: 470C, day and night

Earth

Earth and Moon with sizes shown to scale • An oasis of life • The only surface liquid water in the solar system • A surprisingly large moon Mars

• • •

Looks almost Earth-like, but don’t go without a spacesuit! Giant volcanoes, a huge canyon, polar caps, more Water flowed in distant past; could there have been life?

• • • •

Much farther from Sun than inner planets Mostly H/He; no solid surface 300 times more massive than Earth Many moons, rings

Jupiter

Jupiter’s moons can be as interesting as planets themselves, especially Jupiter’s four Galilean moons. • Io (shown here): active volcanoes all over • Europa: possible subsurface ocean • Ganymede: largest moon in solar system • Callisto: a large, cratered “ice ball” Saturn

• • •

Giant and gaseous like Jupiter Spectacular rings Many moons, including cloudy Titan

Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon.

Cassini probe arrived July 2004 (launched in 1997).

Uranus

• • • •

Smaller than Jupiter/Saturn; much larger than Earth Made of H/He gas and hydrogen compounds (H2O, NH3, CH4) Extreme axis tilt Moons and rings

Neptune

•

Similar to Uranus (except for axis tilt)

•

Many moons (including Triton)

• • •

Much smaller than major planets Icy, comet-like composition Pluto’s main moon (Charon) is of similar size

Pluto (and Other Dwarf Planets)

Thought Question

What process created the elements from which the terrestrial planets were made? a) the Big Bang b) nuclear fusion in stars c) chemical processes in interstellar clouds d) their origin is unknown.

Two Major Planet Types

 Terrestrial planets are rocky, relatively small, and close to the Sun.  Jovian planets are gaseous, larger, and farther from the Sun.

Swarms of Smaller Bodies What have we learned?

 Many rocky asteroids and icy comets populate the solar system.  What does the solar system look like?  Planets orbit Sun in the same direction and in nearly the same plane.  What can we learn by comparing the planets to one another?  Comparative planetology looks for patterns among the planets.  Those patterns give us insight into the general processes that govern planets.  Studying other worlds in this way tells us about our own planet.  What are the major features of the Sun and planets?  The planets are very small compared to the distances between them.  The planets of the inner solar system are rocky and have few moons.  The planets of the outer solar system are gaseous and have many moons and rings.  Pluto is unlike either the inner or outer planets.

Dating Planetary Surfaces

Our goals for learning: How to date surfaces and planets?

Counting the Craters

 Our Cratered Moon. This composite image of the Moon’s surface was made from many smaller images taken between November 2009 and February 2011 by the Lunar Reconnaissance Orbiter (LRO) and shows craters of many different sizes. (credit: modification of work by NASA/GSFC/Arizona State University)

When did the planets form?

 We cannot find the age of a planet, but we can find the ages of the rocks that make it up.  We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.

Radioactive Decay

 Some isotopes decay into other nuclei.  A half-life is the time for half the nuclei in a substance to decay.

Dating the Solar System

Age dating of meteorites that are unchanged since they condensed and accreted tells us that the solar system is about 4.6 billion years old.

Dating the Solar System

 Radiometric dating tells us that the oldest moon rocks are 4.4 billion years old.  The oldest meteorites are 4.55 billion years old.  Planets probably formed 4.5 billion years ago.

Origins of the Solar System

Our goals for learning: Describe the Characteristics of planets used to create formation models of the solar system How are collisions important?

What features of our solar system provide clues to how it formed?

Motion of Large Bodies

 All large bodies in the solar system orbit in the same direction and in nearly the same plane.  Most also rotate in that direction. Notable Exceptions

 Several exceptions to the normal patterns need to be explained.

Evidence from Other Gas Clouds

 We can see stars forming in other interstellar gas clouds, lending support to the nebular theory.

Atlas of Planetary Nurseries

 Atlas of Planetary Nurseries. These Hubble Space Telescope photos show sections of the Orion Nebula, a relatively close-by region where stars are currently forming. Each image shows an embedded circumstellar disk orbiting a very young star. Seen from different angles, some are energized to glow by the light of a nearby star while others are dark and seen in silhouette against the bright glowing gas of the Orion Nebula. Each is a contemporary analog of our own solar nebula—a location where planets are probably being formed today. (credit: modification of work by NASA/ESA, L. Ricci (ESO))

Where did the solar system come from?

 According to the nebular theory, our solar system formed from a giant cloud of interstellar gas.

Galactic Recycling

 Elements that formed planets were made in stars and then recycled through interstellar space.

What caused the orderly patterns of motion in our solar system?

Conservation of Angular Momentum

 The rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted.

Conservation of Angular Momentum

 Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms.

Flattening

 Collisions between particles in the cloud caused it to flatten into a disk.

Flattening

 The spinning cloud flattens as it shrinks.

Disks Around Other Stars

 Observations of disks around other stars support the nebular hypothesis.

Why are there two major types of planets?

Conservation of Energy

 As gravity causes the cloud to contract, it heats up.  Inner parts of the disk are hotter than outer parts.  Rock can be solid at much higher temperatures than ice.

The Frost Line

 Inside the frost line: Too hot for hydrogen compounds to form ices  Outside the frost line: Cold enough for ices to form

Formation of Terrestrial Planets

 Small particles of rock and metal were present inside the frost line.  Planetesimals of rock and metal built up as these particles collided.  Gravity eventually assembled these planetesimals into terrestrial planets.

Accretion of Planetesimals Formation of Jovian Planets

 Many smaller objects collected into just a few large ones.

Formation of Jovian Planets

The Solar Wind

 Ice could also form small particles outside the frost line.  Larger planetesimals and planets were able to form.  The gravity of these larger planets was able to draw in surrounding H and He gases.  Moons of jovian planets form in miniature disks.

 Radiation and outflowing matter from the Sun—the solar wind— blew away the leftover gases.

Where did asteroids and comets come from?

 Leftovers from the accretion process  Rocky asteroids inside frost line  Icy comets outside frost line

Heavy Bombardment

 Leftover planetesimals bombarded other objects in the late stages of solar system formation.

Origin of Earth’s Water

 Water may have come to Earth by way of icy planetesimals from the outer solar system.

How do we explain the existence of our Moon and other exceptions to the rules?

Captured Moons

 The unusual moons of some planets may be captured planetesimals.

Giant Impact

Odd Rotation

 Giant impacts might also explain the different rotation axes of some planets.

Review of the nebular theory

Earth as a Planet Why have the planets turned out so differently, even though they formed at the same time from the same materials?

What are terrestrial planets like on the inside?

Seismic Waves

 Vibrations that travel through Earth’s interior tell us what Earth is like on the inside.

Earth’s Interior

 Core: highest density; nickel and iron (7000 km)  Mantle: moderate density 3.5-5 g/cm; silicon, oxygen, etc. (2900 km depth)  Crust: lowest density 3 g/cm; granite (45% 20-70 km), basalt (55% 6km).

Differentiation

 Gravity pulls high-density material to center.  Lower-density material rises to surface.  Material ends up separated by density.

Lithosphere

 A planet’s outer layer of cool, rigid rock is called the lithosphere.  It “floats” on the warmer, softer rock that lies beneath.

Strength of Rock

 Rock stretches when pulled slowly but breaks when pulled rapidly.  The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere.

Thought Question

What is necessary for diffe...