ASTR 100 Midterm 2

Light

made of photons

Photons

Particles, produced when charged particles accelerate (shake)

Light is also an

electromagnetic wave

light can travel through a

vacum

Interference

Light behaves like a wave, creating (blank) patterns where waves add up to make brighter spots or cancel out to make darker spots.

Photoelectric Effect

demonstrated that light sometimes behaves like a particle

Longer wavelength

lower frequency

Shorter wavelength

higher frequency

Wavelength

distance between adjacent peaks of the wave.

Frequency

The number of peaks going by you every second.

Speed

wavelength x frequency

• Color of the visible light is determined by

frequency

Blue

Shorter Wavelength; Higher Energy

Red

Longer Wavelength; Lower Energy

Visible light

only a small part of what we call the electromagnetic spectrum

The EM spectrum includes

radio waves, infrared, visible, ultraviolet, x-rays, and gamma rays

Sir Frederick William Herschel

discovered Infrared Light and Uranus

Completely transparent

light goes through without being changed.

Opaque

Does not transmit light (absorbs it).

The darker a surface

the more sunlight it absorbs

The lighter a surface

the less sunlight is absorbed

Albedo

Amount of reflection

Albedo = 0

means no reflection (all light absorbed)

Albedo = 1

means total reflection (no absorption)

Earth’s albedo

0.39

Moon’s albedo

0.07

Telescope

a tool used to gather light from objects in the universe

Refracting

telescope uses a glass lens to concentrate incoming light

reflecting

telescope uses mirrors to concentrate incoming light

Refractors

• The largest refracting telescope ever built is 40” (100 cm) in diameter.

• They suffer from “chromatic aberration” and are no longer used for astronomy.

Main functions of a telescope

Gather as much light as possible,
Resolve objects, Magnify images

A larger objective lens provides a

brighter (not bigger) image

Light gathering power

is proportional to area of mirror or lens, proportional to R2

Resolving power

is the ability to see detail, or to resolve two

objects near one another

The angular resolution

of a telescope is determined by the diameter of the

mirror or lens (smaller number for resolution=better resolution

Earth’s atmosphere limits resolving power to

1” (arcsecond)

Twinkle

Temperature and density differences in Earth's atmosphere bend starlight

Radio telescopes

“sees” radio waves, and can locate sources of radio waves from space.

Radio astronomy

Radio waves are not blocked by clouds of gas and dust and allow us to peer inside the center of the Milky Way.

Bigger wavelengths

need bigger telescopes

Luminosity

is the total energy (light) emitted by an object in each second.

Luminosity Equation

Luminosity = A x T 4

Big and Hot objects have

greater luminosity than small cool objects.

Blackbody Curve

a graph of an object’s energy output per wavelength. The peak of this curve tells us about the object’s temperature and color.

Color

when the lights are off

Wein’s law

Relates the temperature of an object to the wavelength of the peak in the black body curve. Hotter objects have shorter peak wavelength.

Protons, Electrons, and Neutrons

carry positive charge, carry negative charge, neutrons neutral

Charges create electromagnetic radiation

light

Elements

has a unique atom that is a combination of neutrons and protons in the nucleus, surrounded by a cloud of electrons.

Matter is made up of

atoms

Atomic Number

number of protons

Mass number

total number of nucleons

Hydrogen

one proton + one electron

Helium

two protons, two neutrons, two electrons

Carbon

six protons, six neutrons, six electrons.

Composition of the Universe

75% H + ~25% He (most of it produced in the Big Bang) + trace of heavier elements (most of it produced in stars).

Isotopes

Atoms of same element with extra neutrons.

• Adding a neutron to hydrogen atom gives

deuterium. Adding two gives tritium.

• Carbon has three natural isotopes, one of

which is unstable (and very important in determining age of fossils).

Molecules

When two or more atoms bond together.

• One atom of oxygen plus two atoms of

hydrogen give a molecule of water.

Organic Molecules

Contain C and H and are often complex chains of atoms

Gas

a collection of atoms and/or molecules in random motion, without a definite shape and volume

Plasma

ionized gas in which some or all of the electrons aren’t bound to atoms.

Emission of a photon

Photon is emitted from an atom when an electron moves from a higher energy level to a lower energy level.

Absorption of a photon

When a photons is absorbed by an atom, its electron moves from a lower energy level to a higher energy level.

Atomic energy levels

Structure of atomic levels determines which energies of photons are possible for the atom to absorb or emit.

Atomic “fingerprints”

Each chemical element produces its own unique set of spectral lines when it is excited!

Continuous Spectrum

Uninterrupted rainbow of wavelengths

Emission Spectrum

Specific wavelengths emitted

Absorption Spectrum

“Missing” wavelengths tell us about the gas cloud.

Doppler effect

the perceived frequency of a wave (like sound or light) changes depending on whether the source of the wave is moving towards or away from you

Redshifted

moving away

Blueshifted

moving toward us

The Sun

5800 K; Accounts for 99.86% of the total mass of the Solar System (1000 times more mass than all planets combined).

Terrestrial planets

Four planets closest to Sun (Mercury, Venus, Earth, Mars).

Jovian planets

Next four planets are “Gas Giants,” (Jupiter, Saturn, Uranus, Neptune).

Terrestrial vs. Jovian Planets

smaller size, higher density, solid surface, few moons

discovered uranus

William and Caroline Herschel

Discovery of Neptune

Le Verrier and Galle

Percival Lowell

constructed the Lowell Observatory to find Pluto (unnamed atp)

Who found Pluto?

Clyde Tombaugh

Sedna

86 AU (twice the distance to Pluto), the size of Charon.

Eris

More massive (but slightly smaller) than Pluto and has a moon Dysomnia.

Asteroid belt

Mars and Jupiter

Kuiper Belt

extends beyond the orbit of Neptune and includes a number of dwarf planets, including Pluto. About 10 times the mass of the Moon

Oort Cloud

a distant cloud of comets

Planetesimals

leftovers from planet formation

Asteroids

Rocky/Metallic planetesimals formed close to the Sun

Comets

Icy planetesimals formed outer planets

Definition of a planet

orbits the sun, round shape, cleared the neighborhood around its orbit

Solar Nebula Theory

Solar system formed from the collapse of an interstellar nebula (a cloud of gas). Proposed by Kant and Laplace (18th

century).

Condensation

is the formation of solids from the cooling gas of the solar nebula

Accretion

is the sticking together of solid particles to make bigger particles.

What Happens to the Solar Nebula as it

Collapses

Spinning: Solar nebula contracts and spins up –

conservation of angular momentum.

• Heating: temperature of solar nebula increases as it collapses – gravitational potential energy converted into thermal energy.

• Flattening: collisions result in orderly motion of its

components (while conserving momentum), the gas radiates away some energy, and the nebula flattens into a protoplanetary disk.

Frost Line

cold enough for icy planetesimals to form, and for planets to hold on to gases. Inside the frost line, only rocky and metallic

planetesimals formed.

Differentiation

the separation of material according to density

Nebular theory

the gas giants formed their own accretion disks, forming their moons

Nice Model

Neptune and Uranus formed closer in and migrated outward due to repeated interactions with the two gas giants

The Age of the Solar System

4.5 billion years old

Four Eons

Hadean, Archean, Proterozoic, and Phanerozoic.

Hadean

(hellish) Earth (till 3.85 billion years ago) during late heavy bombardment, continents just beginning to form