# Astronomy

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TEST 1 (Chapters 0,1,2,3)

### Geocentric model of Universe

-Earth at center
-Modeled after Aristotle's perfect form: circle
-Paths of celestial objects uniform and circular

### Kepler's 1st Law of Orbital Motion

Orbital paths of planets are elliptical, with the Sun at one focus

### Kepler's 2nd Law of Orbital Motion

Sweeps our equal areas of ellipse in equal intervals of time

### Kepler's 3rd Law of Orbital Motion

P² = a³
(P=orbital period) (a=semi-major axis)
-Planets further from Sun have longer orbital periods

### Why do we say that a baseball falls towards Earth, and not Earth toward the baseball?

-Earth's gravity modifies baseball's velocity, causing acceleration down toward Earth due to its larger mass exerting gravitational force on baseball

### Why would a baseball thrown upward from the surface of the Moon go higher than one thrown with the same velocity from the surface of the Earth?

Moon's gravity =1/6 of Earth's
Moon's gravitational force slows the baseball's velocity at a slower rate

### What would happen to Earth if the Sun's gravity were suddenly "turned off"?

The Earth would fly off its orbital path in a tangential line in accordance with Newton's First law

The apparent backward (Westward) motion of a planet as the Earth swings between it and the sun

### Heliocentric model of Universe

-Copernicus
-Sun at center
-Earth spins on its axis
-Earth and all other planets orbit Sun

### Kepler's Laws apply to

-All planets, known and unknown at the time

### Astronomical Unit

The average distance between Earth and the sun, about 150 million kilometers

### Period of a wave

-Time it takes for wave to repeat itself (cycle)
-Measured in seconds

### Amplitude of a wave

Highest displacement from an undisturbed state

### Frequency of a wave

-Inverse of a period. Instead of number of seconds per cycle, it is the number of cycles per second

### Spectroscopy

-The study of the properties of light that are wavelength dependent
-Analysis of the ways in which matter emits and and absorbs radiation
(To remember: Think of spectacles (eyeglasses)

### Why is Spectroscopy important to Astronomers?

Information about temperature, density, composition, and important physical properties of an astronomical object

### Continuous Spectrum

the emission of a continuous range of frequencies of electromagnetic radiation

### Describe how its blackbody curve changes as a red-hot glowing coal cools off.

As an object cools, then its peak will move to the right (in this pic), ie lower frequency/longer wavelength (Wien's Law).

### Absorption Spectrum

A continuous spectrum produced when white light is passed through a COOL GAS under LOW PRESSURE; The gas absorbs selected wavelengths of light, and the spectrum looks like it has dark lines superimposed
-Electrons are excited upward, and as they are excited upward they absorb a specific amount of energy in the same region (wavelength) that they will emit
-Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths

Red

-Temperature
-Motion
-Composition

### Advantages Hubble Space Telescope has over ground-based ones

-Hubble is sensitive to ALL
WAVELENGTHS from the ultraviolet to the near infrared.
-Many of these wavelengths are blocked by the atmosphere and inaccessible from Earth.
-Hubble images are STABLE
-Above atmosphere (clearer)
-RESOLUTION close to diffraction limit (20x sharper view)
(To remember: ALL WAVELENGTHS ABOVE THE ATMOSPHERE HAVE STABLE RESOLUTION)

### Disadvantages Hubble Space Telescope has

-Smaller field of view (Think of looking through a smaller telescope in a field)
-Cost

### What are the main advantages of studying objects at many different wavelengths of radiation?

By comparing the features visible in each, multi-wavelength observations can COMPLEMENT each other, greatly extending our perception of the universe around us.

### Astronomers use the term "seeing" to describe what?

The effects of atmospheric turbulence
(Use adaptive optics techniques to reduce effect)
(Think of looking out a plane window when it's going through turbulence)

### What is the main reason observatories are placed at high places?

To reduce atmospheric blurring
(high places-atmosphere)(observe-blurry image)

Infrared light

### Galileo's findings

(1600)
-Jupiter has moons (proves not all objects orbit earth)
-The Moon has craters (proves not perfect sphere)
-The Sun has spots (imperfections)
-Venus goes though phases (proves it orbits the Sun, not the Earth)
(To remember: JUPITER, MOON, SUN, VENUS)

### What does it mean when a celestial object has phases?

There is a change in relative position between two bodies

### Eccentricity

= distance between foci divided by length of major axis

### Tycho Brahe

Influenced by Copernicus; Built observatory and collected DATA on the locations of stars and planets for over 20 years; His limited knowledge of mathematics prevented him from making much sense out of the data.
(To remember name: Think of a tyco toy truck having braille words on it (make sure Brahe is spelled right)

186,000 mi/sec
300,000 km/sec

93,000,000 miles

8.33 min

### Actual size of object if measured in radians

=Angular size x Distance

### Actual size of object if measured in degrees

=Angular size x Distance x (2π/360)

A complete circle

### Parallax

-An apparent shift in the position of an object when viewed from different locations
-Can be used to measure distance

### What did Newton do?

(1642)
-Explained why there are elliptical paths
-Built first telescope
-Showed things don't move the way we think they do (inertia)
-Provided a consistent physical explanation which showed that the planets are kept in their orbits by the familiar force of gravity.

### Newton's 1st Law of Motion

An object at REST/MOTION will remain at REST/MOTION until acted upon by an outside FORCE

### Newton's 2nd Law of Motion

Force= mass x acceleration; the larger something is, the harder it is to move

### Acceleration

Any change in speed or direction

### Mass

-Measure of INERTIA
-Amount of material in an object
-Measured in kilograms

### Weight

-Force of gravity on an object
-Measured in Lbs or Newtons

### Newton's 3rd Law of Motion

-For every force there is an EQUAL and OPPOSITE force
-The effect of the forces depends on the mass of the two objects

### Newton's Law of Gravity

(Inverse Square Law)

### What is G?

Universal Gravitational Constant

### How far away is the Moon?

10x the radius of the Earth

### Center of mass

Point at the center of an object's mass distribution, where all its mass can be considered to be concentrated. For everyday conditions, it is the same as the center of gravity.

### Newton modified Kepler's 1st Law

Two objects will orbit one another in ellipses, with the center of mass of the two objects at one focal point

(Kepler said the Sun was at a focal point)

### Newton modified Kepler's 3rd Law

or m1+m2 (total mass) = A^3(distance of semimajor axis)/P^2 (period)

Frequencies

### Orbital

a region in an atom where there is a high probability of finding electrons

Wave

### When light interacts with matter it acts like a

Particle (particle of energy)

### Ground state

-Orbital closest to nucleus of atom
-Lowest possible energy
(Think of a kid being grounded)

### Photons of light can only be absorbed or emitted by an atom if energy in the photon

exactly matches the energy difference between the two shells.

### The temperature of an object determines

both the amount and type of radiation

### Temperature

Measure of the motion of molecules (Kelvin)

### Stefan-Boltzmann Law

A law of radiation which states that the amount of RADIANT ENERGY emitted from a unit SURFACE area of an object (ideally a blackbody) is PROPORTIONAL to THE 4TH POWER of the object's absolute TEMPERATURE.
(To remember: Think of a man stepping on the surface of a lightning bolt)

16x the energy

### In a Blackbody Scale, what happens as temperature goes down?

The wavelength of peak radiation goes down as well.

### Yang's theory

(1808)
Light is a wave!
For all waves: wavespeed = wavelength x frequency

RED

Blue

### Photoelectric effect

-Documented by Heinrich Hertz in 1887
-When a light source (or, more generally, electromagnetic radiation) is incident upon a metallic surface, the surface can emit electrons. Electrons emitted in this fashion are called photo-electrons (although they are still just electrons).

### Aphelion

the place in the orbit of a planet where the planet is farthest from the sun
(Think of the words APART(furthest) and HELIO (Sun)

### Celestial sphere

Imaginary sphere surrounding earth to which all objects in the sky were once considered to be attached

### Ecliptic

The apparent path of the Sun on the celestial sphere over the course of a year

(Instead of elliptical path its ecliptical)

### Lunar eclipse

Sun --> Earth --> Moon

### Perihelion

The point in the Earth's orbit where it comes closest to the sun.
(Think of word PERIMETER (closest) around the SUN)

### Phases of the moon

think of giving (gibbous) someone a wax crescent roll and then they start whining (waning)

-Takes a little over 29 days to complete

### Precession

The gradual conical path traced out by Earth's spinning axis; simply put, it is the "wobble" of the axis.

### Revolution

Orbital motion of one body about another, such as Earth about the Sun
(Think of Copernican REVOLUTION and the Earth revolving around Sun)

### Rotation

Spinning motion of a body about an axis

### Solar eclipse

Sun~>Moon(new moon)~>Earth

### Copernican revolution

(Late 16th century)
The dramatic change, initiated by Copernicus, that occurred when we learned that Earth is a planet orbiting the Sun rather than the center of the universe

### Parsec

The distance to a star that has a Parallax of 1 second of Arc.

### How did Kepler contribute to the Copernican revolution?

-He came up with the 3 Laws of Planetary Motion
-Kepler's Laws are emiprical laws providing a mathematical description of observed planetary motions.

### What some flaws Copernicus had with his model?

-Clung to the idea of CIRCLES to model the planets' motions
-Retained the idea of EPICYCLES

### Copernican Principle

Removal of Earth from any position of great cosmic SIGNIFICANCE

### How did Copernicus, Tycho, and Kepler challenge the Earth-centered idea?

-Copernicus created a SUN-CENTERED model
-Tycho Brahe provided the DATA needed to improve this model
-Kepler found a MODEL that FIT Tycho's data.

### How did Newton change our view of the universe?

-He discovered laws of MOTION and GRAVITATION.
-He realized these same laws of physics were identical in the universe(UNIVERSAL) and on Earth.
-Laws provide a means of explaining or predicting the motion of any object of given mass in response to a known force. Alternatively, the properties of the force can be inferred from the observed motions of the mass.

273

### How might the Doppler effect be used in determining the mass of a distant star?

We can measure the masses of binary stars, by detecting the VELOCITY changes as they ORBIT around their common center of gravity, and COMBINING these velocities with the PERIOD of the orbit.

### List electromagnetic wavelengths from low frequency long wavelengths to high frequency short wavelengths

-Infrared
-Visible
-Ultraviolet
-X rays
-Gamma rays

To remember: Rattlesnakes Inject Venom Unless eXtremely Gentle

### How is the area of light collected affected by doubling the diameter of the main lens or mirror of a telescope?

It is increased 4x the amount

### Resolution

-Ability to visually SEPERATE two objects that are close together
Angular resolution = .25x(wavelength/telescope diameter)

### What does 20/10, 20/20, 20/40 vision mean?

Ability to resolve a spatial pattern separated by a visual angle of
20/10 - 1/2 minute of arc
20/20 - 1 minute of arc
20/40 - 2 minute of arc

### Two categories of optical telescopes

Reflectors and refractors

### Refracting telescope

Uses a lens instead a mirror to focus the incoming light

### Refraction

The bending of a beam of light as it passes from one transparent medium into another.

### Diffraction

The bending of light when it goes through an opening (or past an obstacle)

### When is the amount of diffraction high?

When the wavelength is around the size of the opening or larger
(Think of it expanding out the opening)

### When is the amount of diffraction low?

When the wavelength is smaller than the opening (the wave can easily get through)

Reflectors

high

### What causes stars to twinkle?

Stars twinkle because of TURBULANCE in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions. This causes the star's image to change slightly in brightness and position, hence "twinkle.

temperature

16x the energy

### All magnets are created by ______.

electrical charges in a medium

### Interference

The ability of two or more waves to interact in such a way that they either reinforce or cancel each other.

### Hubble's law

The observation that the FARTHER away a galaxy is, the FASTER it is moving away.

### How to find the temperature, makeup, and velocity of an object?

Color ~> temperature
Spectral lines ~> makeup
Doppler shift ~> velocity

### In refraction, light is bent because

of speed difference.

### Largest optical/infrared telescope

Keck Telescope (Hawaii)

twice.

### Direction that light refracts when crossing a boundary

(FST) Fast to slow ~> bent towards normal
(SFA) Slow to fast ~> bent away from normal

### Hubble telescope uses a _______focus.

Cassegrain
(think of a casserole with grain in it)

### Magnification =

Primary focal length / eyepiece focal length

### What is the most important thing about a telescope?

The amount of light that can be gathered

force

### Radio telescopes need to be

large.
(Think of how a large antenna gives you better radio reception)

### Interferometry

A telescopic technique in which two or more telescopes are used in tandem to produce much better angular resolution than the telescopes could achieve individually.
(Think of how they don't interfere and distort each other)

### Spectroscope

Instrument used to view a light source so that it is split into its component colors
(Think of a spec of light)

### Emission Spectrum

-A series of bright lines of particular wavelengths produced by a HOT GAS under LOW PRESSURE
-What happens is an electron falls from an excited state to a lower or ground state, which emits a photon, which is what shows the emission spectra

### Focal length

Distance from a mirror or the center of a lens to the focus

### What causes seasons?

The TILT of Earth's axis, depending on where you are in the orbit around the sun, if you are tilted towards the sun, your hemisphere is in summer; away you are in winter

### Stellar parallax

the apparent shift in the position of a nearby star that occurs as we view the star from different positions in earth's orbit of the sun each year.

Radiation that is left over from the beginning moments of the universe and is coming from all directions in space

### Constellations

A group of stars that forms a shape or pattern

### Celestial coordinates

A more precise way of specifying a star's location on the celestial sphere

### Summer solstice

Sun is highest in the sky and the length of the day is greatest
(Think of long hot Summer days)

### Winter solstice

Sun is lowest in the sky and the day is shortest

### Which planet has the largest orbital eccentricity?

Mercury (.206)
(Pluto is .249, but it got demoted)

### Ptolemaic model

Ptolemy's model that stated that each planet moved on a small circle (epicycle) whose center moves around Earth on a larger circle (deferent)
(To remember: Pete told a maniac)

Galileo Galilei

### How is an astronomical unit determined?

By bouncing radar signals off Venus

small

X-rays

### What type of telescopes do astronomers prefer and why

Reflecting telescopes are preferred because large mirrors are lighter and easier to construct than large lenses, and suffer from fewer optical defects.

### Luminous solid or liquid, or sufficiently dense gas emits light

of all wavelengths (continuous spectrum of radiation)

### Low-density hot gas emits light

whose spectrum consists of a series of bright emission lines (these lines are characteristic of the chemical composition of the gas)

### Low-density cool gas does what?

Absorbs certain wavelengths from a continuous spectrum, leaving dark absorption lines in their place, superimposed on the continuous spectrum. (These lines are the characteristic of the composition of the intervening gas. They occur at precisely the same wavelengths as the emission lines produced by the gas at higher temperatures)

### Kirchhoff's laws

A set of laws that describe the ABSORPTION and EMISSION of light by MATTER.
(To remember: Think of Captain Kirk making David Hasselhoff admit he has an absorption problem)
(also remember it's kirc with a c, and 2h's and 2 ff's)

### How does the structure of an atom determine the atom's emission and absorption spectrum?

If an atom emits some energy in the form of radiation, that energy has to come from somewhere within the atom. Similarly, if energy is absorbed, it must cause some internal change. It is reasonable (and correct) to suppose that the energy emitted or absorbed by the atom is associated with changes in the motion of the orbiting electron.

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