Astronomy 1 – Fall 2014 Lecture 6: October 21, 2014.

Astronomy 1 – Fall 2014

Lecture 6: October 21, 2014

Previously on Astro 1• Light can have particle-like properties. • Particle energy: E = h = hc/• Electronic energy levels are quantized (i.e., discrete)• Every element (even every ion) has a unique spectral

fingerprint.

• Spectroscopy reveals the composition of distant objects

• Geometrical optics• Reflection & Refraction• Focus, Spherical aberration, Chromatic aberration

• Telescopes • Light gathering power• Magnification• Resolving power

The Secondary Mirror Does Not Cause a Hole in the Image

This illustration shows how even a small portion of the primary (objective) mirror of a reflecting telescope can make a complete image of the Moon. Thus, the secondary mirror does not cause a black spot or hole in the image. (It does, however, make the image a bit dimmer by reducing the total amount of light that reaches the primary mirror.)

Telescopes:Light gathering power

Light gathering power depends on size of objective lens or primary mirror

Reflecting Telescopes

This view of the Gemini North telescope shows its 8.1-meter objective mirror (1). Light incident on this mirror is reflected toward the 1.0-meter secondary mirror (2), then through the hole in the objective mirror (3) to the Cassegrain focus

How Do You Make a Lightweight 10m Diameter Mirror?

How much more light gathering power does a 10m telescope have

than an 0.5 m telescope?

Answer: The light gathering power is proportional to the square of the mirror’s diameter.(10m)2/(0.5m)2 = 100m / 0.25m = 400

So you can see objects about 400 times fainter with the 10m telescope in the same amount of time.

Angular Resolution

Angular resolution of the telescope

Limited by:

• Blurring effects of the atmosphere (“seeing”), i.e. the twinking of stars

• The quality of the optics and detector on the telescope.

• The size of the telescope – the “diffraction limit.”

The diffraction limitθ= diffraction-limited angular resolution of the telescope, in arcsecondsλ= wavelength of light, in metersD = diameter of telescope objective, in meters

Example: What is the diffraction limit for red light (640nm=6.4×10-7m) for a telescope with with a 0.5m objective/primary.

So even if you had a perfect atmosphere and perfect optics, you couldn’t resolve details finer than 0.32” with a 0.5m telescope.

Today astronomers build telescopes at the best sites in the world, then travel to the telescope to observe, or have someone else on-site observe for them, or observe remotely over the internet.

Mauna Kea, an extinct volcano in Hawaii that reaches 13,400 feet, is the best site in the world for optical and infrared telescopes. It has mostly clear, dark skies, little atmospheric turbulence, and is above most of the water vapor in the Earth’s atmosphere. Notice the snow and lack of vegetation.

Adaptive Optics Help Telescopes on Earth Remove the Blurring Caused by the Atmosphere

Laser Beacon Makes an Artificial Star

Adaptive Optics System

The percentage of radiation that can penetrate the Earth’s atmosphere at different wavelengths. Regions in which the curve is high are called“windows,” because the atmosphere is relatively transparent at thosewavelengths. There are also three wavelength ranges in which theatmosphere is opaque and the curve is near zero: at wavelengths less than about 290 nm, which are absorbed by atmospheric oxygen and nitrogen; between the optical and radio windows, due to absorption by water vapor and carbon dioxide; and at wavelengths longer than about 20 m, which are reflected back into space by ionized gases in the upper atmosphere.

Astronomy Uses the Entire EM Spectrum

Hubble Space Telescope

James Webb Space Telescope(see movie on class website)

Orion

The Milky Way Galaxy

Discovery Enabled by YearThe heavens are not perfect and unchanging; (ultimately) the Earth is not the center of the universe.

The telescope and Galileo’s observations. ~1609

The sun and stars are giant balls of hydrogen undergoing fusion.

Fraunhofer’s invention of the spectrograph. 1814

Our galaxy is not the center of the universe, and the universe is expanding.

Edwin Hubble and the giant Palomar 200-inch and large-format photographic plates.

1929

The universe started as a hot “Big Bang”

Penzias and Wilson using a radio “telescope,” confirmed by satellites.

1965

Planets are common in the universe. Modern charge-couple-device detectors (CCD); Iodine cell for spectrograph.

1995

Dark Energy dominates the universe.

Large-format CCD detectors; 10m Keck telescope.

1998

And there are many more involving infrared, x-ray, ultraviolet and gamma-ray discoveries.

Today on Astro1

• Our Solar System

• Terrestrial vs. Jovian Planets

• Why do some planets have atmospheres?

• Smaller chunks of rocks and ice also orbit the Sun.

• What do craters tell us about the geological history of planets?

• What do magnetic fields tell us about planetary interiors?

• Any model for the origin of the solar system must explain this!

All the planets orbit the Sun in the same direction, in nearly the same plane, and most also rotate in the direction of the orbit.

Terrestrial planets• Closest to Sun• Small, high density, rocky

Jovian planets• Furthest from Sun• Large, low density, gaseous

Our Solar System Has Two Broad Categories of Planets

This Diversity Results From Its Origin and Evolution

Orbital Radii of Terrestrial and Jovian Planets

Measuring Density• Distance: From period via Kepler's Third Law (P2 = a3)• Size: Observed angular size and distance (small angle formula)• Mass: satellite’s period & Newton’s Form of Kepler's Third Law• Density: mass/volume (r(H2O) = 1000 kg/m3)

Average Density of a Planet:A Clue About Its Composition

Rocky!

Density of Jovian Planets

• Gaseous• Visible surfaces show cloud formations

- Seven large satellites almost as big as terrestrial planets- Comparable in size to Mercury- Only Titan has an atmosphere- Remaining satellites (>140 known today!) much smaller

Giant Satellites

Classification of Extrasolar Planets (iclickers Question)

• Suppose that in the near future a series of extrasolar planets are discovered with the following characteristics: spherical solid surfaces; mean densities about four times that of water; radii about 4000 km; low density atmospheres. How would these planets be classified in terms of our solar system

•A) Jovian Planets•B) Cometary nuclei•C) Asteroids•D) Terrestrial Planets

Spectroscopy of Titan(iclickers Question)

• A ground based telescope is pointed at the atmosphere of Titan and a spectrum is obtained. The spectral lines observed in this spectrum:•A) Can only be features of Titan•B) can be characteristic of the Earth’s atmosphere as well as Titan’s atmosphere•C) Can be characteristic of the cooler outer layers of the Sun’s atmosphere as well as of Titan’s atmosphere •D) can be characteristic of the atmosphere of Titan and the Earth and also of the cooler outer layers of the Sun’s atmosphere.

- Dips: due to absorption by hydrogen atoms (H), oxygen molecules (O2), and methane molecules (CH4)

- Only methane actually present in Titan’s atmosphere

Spectroscopy:Chemical Composition of Atmosphere

Europa:- No atmosphere- Sun light reflected from surface

What are ices?

• Hydrogen and Helium are gaseous except at extremely low temperature and high pressure.

• Rock forming substances such as iron and silicon are solids except at temperatures well above 1000 K.

• Between these two extremes are substances which solidify at low temperatures (from below 100 to 300 K) to solids called ices. H20, CO2, CH4, NH3

They are liquids or gases at somewhat higher T.

Mars: - Composed mostly of heavy elements (iron, oxygen, silicon, magnesium, nickel, sulfur) → red surface- Atmosphere thin, nearly cloudless - Olympus Mons = extinct volcano, nearly 3 times height of Mount Everest

Jupiter: - Composed mostly of lightest elements (hydrogen, helium), colorless - Colors: trace amounts of other substances- Giant storm = Great Red Spot, >300 years old

What Determines Whether a Planet Has an Atmosphere?

• Escape Speed• Vesc = (2GM/R)1/2

• Jupiter’s escape velocity is about 5.3x that on Earth (11.2 km/s).

• The higher surface temperatures of terrestrial planets help to explain the absence of H and He.• Average speed of a gas atom or molecule

V = (3kT/m)1/2

• A planet can retain a gas if the escape speed is at least 6 times greater than the average speed of the molecules in the gas.

Particle Velocities in an Atmosphere

Required Escape Velocity

Most ProbableVelocity

Average Speed of Molecules in an Atmosphere(iclicker Question)

The temperature of Earth’s atmosphere is roughly 20o C. Jupiter’s atmosphere is colder at -148o C. Compare the speed of nitrogen molecules in Earth’s atmosphere to that of He atoms in Jupiter’s atmosphere.

A. Nitrogen molecules move faster because Earth’s atmosphere is hotter.

B. Nitrogen molecules move slower because Earth’s atmosphere is colder on the Kelvin temperature scale.

C. Nitrogen molecules move slower because Nitrogen (atomic number 7) is heavier than Helium (atomic number 2).

D. They move at roughly the same speed.

E. Both B & C

Small Chunks of Rock & Ice Also Orbit the Sun

• Asteroids• An extension of planets to lower masses

• Found in asteroid belt between Mars and Jupiter

• Trans-Neptunian Objects• Pluto and Eris are the most massive

• Over 900 identified at much lower masses

• Orbits cross Neptune’s orbit

• Comets

Asteroids

- Rocky objects between Mars and Jupiter in “asteroid belt”- Left-overs that did not form a planet- Combined mass < Moon

Asteroids

433 Eros: - 33 km (21 mi) long and 13 km (8 mi) wide- Gravity too weak to have pulled it into a spherical shape- Image taken March 2000 by NEAR Shoemaker, first spacecraft to orbit around and land on an asteroid.

Trans-Neptunian Objects (TNOs) = Kuiper Belt Objects (KBOs)

Pluto and Eris (2003 UB313):- Two largest Trans-Neptunian Objects- Orbits steeply inclined to the ecliptic

Trans-Neptunian Objects (TNOs) = Kuiper Belt Objects (KBOs)

- Rocky & icy objects beyond Neptune (> 900 known; maybe up to 35,000?)- High eccentricities- Pluto is first discovered TNO (1930) and second biggest- Reside in Kuiper belt (30-50 AU from sun)- Debris left over from formation of solar system

Comets (“Dirty Snowballs in Space”)

- Rocky & icy objects on eccentric orbits that come close to sun.- Few tens of km in diameter- From Kuiper Belt or even further out (Oort Cloud; 50,000 AU)- e.g. if collision of two KBOs, a fragment can be knocked off and diverted into elongated object, brings it close to sun

Comets

Hale-Bopp: (April 1997)- Near Sun: solar radiation vaporizes some icy material- Bluish tail of gas, white tail of dust- Tails can extend for tens of millions of kilometers

• Asteroids/Comets on elongated orbits, can collide with planet/satellite- Jovian planets: swallowed by atmosphere- Terrestrial planets: impact crater (central peak!)

Craters Reveal Geological History

Earth: - Only < 200 craters- Manicouagan Reservoir in Quebec- Relic of a crater formed >200 million years ago; eroded by glaciers

Mars:- Lowell Crater in the southern highlands, 201 km (125 miles) across- Craters on top of craters- Light-colored frost: condensation of carbon dioxide from atmosphere

Observations:• LARGER worlds have more geological activity (volcanoes, faults, etc.)

• SMALLER worlds (the Moon, Mercury, Mars) have more craters than larger worlds (Earth, Venus)

• What’s the connection? Geological activity erases craters Larger planets must be more

geologically active.

Planets are Heated Internally &Smaller Planets Lose Their Heat Faster

Planetary Magnetic Fields

Measure Magnetic Fields with Spacecraft

Earth’s Magnetic Field Is Generated by the Motion of Electrically Conducting Liquid Iron in Its Core

(a.k.a., Dynamo Action)

Relic Magnetism on Mars:Magnetic fields are recorded in rocks that were once molten

Do All Planets Have Magnetic Fields?

• Not if it• Lacks a molten core (Mars), or

• Or doesn’t rotate much (e.g., Venus)

• Scientists were surprised to find a weak magnetic field on Mercury.• Some of the planet’s interior must still be molten

Magnetism of Small Bodies(iclickers Question)• In general small bodies in the solar system are less likely

than large bodies to possess a planet-wide magnetic field. Why should we expect size and magnetism to be correlated? •A) A small body cools more rapidly and is less likely to possess a molten liquid interior.•B) Small bodies are more likely to be heavily cratered and such impacts can destroy the mechanism that produces the magnetic field•C) Magnetic fields are produced by the entire volume of a body. Smaller bodies have smaller volumes and therefore smaller magnetic fields •D) Small bodies necessarily rotate more slowly and a rapid rotation rate is one requirement for a planet wide magnetic field

Summary• Properties of the Planets:

– Orbits in the same plane and direction

– Inner (terrestrial) planets are small and made of heavy elements

– Outer (Jovian) planets are big and made of light elements

• Other bodies in the Solar system– There are seven large satellites (like the moon)

– Asteroids in Asteroid Belt between Mars and Jupiter

– Outer solar system is populated by TNO and comets

• How do we learn about solar system bodies?– We send probes

– Spectroscopy reveals the composition of atmospheres

– Craters and magnetic fields reveal the presence of a liquid melted core

Homework

• HW Due Monday 10/27.• Do all review questions for chapter 7 on your own.

• Write up 7.23, 7.26, 7.27, 7.28 for the TAs.

• First Midterm Tuesday 10/28• Chapters 1-8

• Go over your answers to the review questions.

• You will be given an equation sheet.

• You may bring a calculator.

• Remember the course policies on academic honesty.