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Astronomy > Intro > Flashcards

Intro Flashcards

1
Q

What do we see in the sky?

A

constellations

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2
Q

What is a celestial sphere?

A

The name given to an imaginary sphere

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3
Q

•From northern hemisphere, stars, Sun, Moon and planets appear to move from

A

east to west in a circle around North Celestial Pole: Polaris, the North Star

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4
Q

In a day, celestial sphere appears to rotate

A

•In a day, celestial sphere appears to rotate once • rotates 360° in 24 hours, so each hour rotates 360° / 24 = 15° • stars appear to move 15° per hour east to west

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5
Q
A

6 hours

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6
Q

Zenith

A

is the point on the celestial sphere directly overhead.

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7
Q

Horizon

A

is a circle on the celestial sphere 90 degrees from zenith.

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8
Q

The meridian

A

is a line from horizon to horizon, passing through the zenith.

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9
Q

What you see at night depends on where you are standing

A
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10
Q

The altitude of polaris equals

A

Your latitude

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11
Q

Because Madison is 43 degrees north of the equator,

A

Polaris is 43 degrees above our horizon. From south of equator, Polaris is below the horizon, never visible.

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12
Q

What are longitude and latitude?

A
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13
Q

Celestial Coordinates

A
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14
Q

Earth’s Orbital Motion: What is a day?

A
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15
Q

Each time Earth rotates on its axis, it also moves a small distance along its orbit. This means Earth has to rotate through a bit more than 360 degrees in order for the Sun to return to the same apparent location in the sky.

How much more?

A

Earth takes 365 days to revolve around the Sun, so in 1 day it travels through 1/365 of its orbit or 360/365=0.986 degrees. This is the extra amount the Earth needs to rotate for the Sun to return to the same apparent position.

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16
Q

How long does this take?

A

The Earth rotates by 360 degrees in one day, so it rotates by 1 degree in 1/360 of a day.

This is 4 minutes: 1 day = 24 hours = 24x60 minutes = 1440 minutes. 1/360 (1440 minutes) = 4 minutes (Or: 24x60/360 = 24/6 = 4)

The time from sunrise to sunrise (1 day) is 4 minutes longer than the time from one rising of the star Betelgeuse (the brightest star in Orion) to the next.

Stars rise 4 minutes earlier (and set four minutes earlier) each day, returning to their original positions after 1 year. This is why we see different stars in the summer and winter.

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17
Q

Earth’s Orbital Motion: Seasonal Changes

A
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18
Q

The stars that the sun appears to move over are the

A
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19
Q

Explain the seasons and the tilt of the Earth?

A

The Earth’s axis of rotation is not perpendicular to the plane of the Earth’s orbit about the Sun:

•The Earth’s rotation axis is tilted by 23 1⁄2 degrees away from perpendicular to its orbit—

the plane of the equator is 23 1⁄2 degrees from the plane of the orbit.

On the summer solstice, when the axis is most directly tilted toward the Sun, the Sun is directly over a point 23 1⁄2 degrees north of the equator.

On the winter solstice, with the north part of the axis tilted away from the Sun, the Sun is over a point 23 1⁄2 degrees south of the equator.

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20
Q

The Sun is directly over the equator on the

A
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21
Q

The seasons are caused by the tilt of the Earth’s axis. The tilt has two effects:

A
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22
Q

If Earth were upright with no tilt, would the temperature in Madison in January be colder, warmer or the same as it is currently during the month of January?

• Colder • Warmer • The same

A

Warmer

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23
Q

True or False: Summer is warmer than winter because the Earth is closer to the Sun.

A

False

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24
Q

Why does the Moon shine?

A

Reflected sunlight:

– The side of the Moon facing the Sun is lighted

– The side of the Moon facing away from the Sun is dark

• Moon orbits Earth in 1 month (29 days)

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25
Q

The fraction of the Moon’s disk that is visible

A

=

the fraction of the night that the moon is up When the Moon is in the first part of its cycle (waxing) it is up for the first part of the night. When in the last part of its cycle (waning) it is up for the last part of the night.

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26
Q

Seen from the northern hemisphere, the star Polaris

A

Is always above the northern horizon

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27
Q

Relative to the stars, the sun appears to move

A

About one degree westward each day

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28
Q

Northern hemisphere winter are colder than Northern Hemisphere summers because

A

The light from the sun shines more directly on the Northern Hemisphere during the summer

The period of sunlight is longer in during the summer than in winter

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29
Q

The sun is on the celestial equator at the times of the

A

Autumnal equinox and vernal equinox

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30
Q

The ecliptic is

A

The centerline of the zodiac

The projection of Earth’s orbit on the sky

The apparent path of the sun around the sky

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31
Q

On the vernal equinox the sun is

A

On the celestial equator and moving north with respect to the equator

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32
Q

A solar or lunar eclipse will occur

A

When the sun is near the line of nodes of the moon and the moon is new or full

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33
Q

The ___________ moon is visible low in the sky near western horizon a few hours before sunrise:

A

Waxing gibbous

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34
Q

A waxing crescent moon is visible

A

Near the western horizon just after sunset

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35
Q

During a total lunar eclipse

A

The moon’s color will be affected by Earth’s atmosphere

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36
Q

The first quarter moon rises

A

At about noon

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37
Q

Total lunar eclipses always occur

A

At the time of full moon

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38
Q

Why do we send telescopes into space?

A
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39
Q

What do charged particles create?

A
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40
Q

How does light and electric force work?

A

To understand light and how it is produced, we first need to review some facts about the electric force Charges can be positive or negative Particles or larger objects with the same charges (two positively charged particles or two negatively charged particles) repel each other Particles with opposite charges (one positive and one negative) attract each other

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41
Q

What are electric forces and electric fields?

A
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42
Q

What does charge change effect light?

A
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43
Q

What is the speed of information?

A

This information moves at a speed of 300,000 km/s, the speed of light, or the maximum speed in the universe

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44
Q

What are the characteristics of Light?

A
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45
Q

What is all electromagnetic radiation (LIGHT) defined by?

A
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46
Q

What does the complete spectrum of light look like?

A
47
Q

What is the relationship between frequency and wavelength?

A
48
Q

What is the second crest that passes you?

A
49
Q

Relation between frequency and wavelength This is a very long radio wave with wavelength λ = 300,000 km, nearly the distance to the Moon. Now suppose that in 1 second, 100 crests pass you

A

This is a very long radio wave with wavelength λ = 300,000 km, nearly the distance to the Moon. Now suppose that in 1 second, 100 crests pass you (frequency f = 100 Hz). Then the time between crests is 1/100 s and the wavelength is λ = 300,000 km/s x 1/100 s = 3,000 km. A higher frequency (faster, more energetic electron) gives a shorter wavelength.

50
Q

In general, for a frequency of f crests per second, you can see that the time between crests is

A

T = 1/f . The distance between crests is then given by distance = speed x time:

i.e.

λ = c T or λ=c/f

We can also reverse the relation to solve for the frequency

f=c/λ

51
Q
A

B

52
Q

What are the colors of

X-ray, Optical, Infrared, Radio, and Ultraviolet.

A

Purple, Green, Yellow, Red, Blue

53
Q

Astronomy across the spectrum

A
54
Q

What does temperature measure?

A

Temperature

Temperature is a measure of how fast atoms or molecules are moving.

● Hot – atoms moving fast

● Cold – atoms moving slowly

● When atoms stop moving – lowest possible temperature.

This lowest temperature is called absolute zero,

which is -273°C (-459°F).

55
Q

What is thermal radiation?

A
56
Q

What is the peak of intensity of light emitted by a star?

A
57
Q
A

What do hotter stars have?

58
Q

What is black body radiation?

A
59
Q

How do you calculate wavelength with temperture?

A
60
Q
A

Colder

Longer peak wavelength = colder temperature

61
Q
A

10,000 K

62
Q

Light from the Sun looks like a continuous

spectrum with a set of thin dark lines.

What’s going on? What happened to the

continuous spectrum?

A

electrons orbiting the nucleus of an atom are restricted to a discrete set of orbits, at fixed distances from the nucleus. In particular, there is a closest allowed orbit.

When an electron absorbs light, it moves from a closer

to a more distant orbit.

When an electron moves from a more distant to a closer

orbit, it emits light.

63
Q

What wavelengths are emitted and absorbed?

A

Atoms emit and absorb only those wavelengths of light that can

correspond to the energy differences between orbits.

The light emitted or absorbed by isolated atoms is called a

discrete spectrum because only discrete wavelengths are

emitted and absorbed: one sees the spectrum as a set of

distinct lines. Each type of atom (each element) has a different

set of energy levels and therefore a different set of spectral

lines.

●Each element can be identified by its discrete spectrum.

●The spectral lines (wavelengths) that an atom emits are the

same as the spectral lines it absorbs

64
Q

What is absorption and emission?

A
65
Q

Elements in stars can be identified by recognizing the

patterns of their spectral lines.

A
66
Q

A neutral atom has the same number of protons and electrons, However…

A

A neutral atom has the same number of protons and

electrons.

But if you hit its electrons hard enough (e.g. in

collisions with other atoms in a hot gas) or if you hit its

electrons with energetic enough light (short wavelength

light), you can knock them entirely off the atom.

● Knocking electrons off an atom is called ionizing the atom

Ionization

This is very closely related to the “photoelectric effect” for which

67
Q

What is the doppler shift?

A

• When a source of light (or sound) is moving away

from you, its wavelength, seen by you, is

longer.

• When a source moves toward you, its wavelength, seen by you, is

shorter

68
Q

Light and Matter

A

•

All objects emit

continuous, thermal radiation

because

of their

temperature

–

Hot objects emit more radiation at all wavelengths,

and emit their peak radiation at shorter wavelengths

than cooler objects

•

Atoms create

emission or absorption lines

by absorbing

or emitting light

–

They emit light when an electron moves to a lower

energy level, and absorb light when an electron

moves to a higher energy level

•

The

Doppler effect

: light or sound moving toward you is

shortened in wavelength (blueshifted), and light or

sound moving a way is longer in wavelength (redshifted)

69
Q

What are optical telescopes?

A
70
Q

When does light travel fastest?

A
71
Q

What happens when focusing light on a lense?

A
72
Q

What are the two types of optical telescopes?

A
73
Q

Modern telescopes are all reflectors - Why?

A
  1. Light traveling through a lens is refracted differently depending

on wavelength (chromatic aberration). Mirrors don’t suffer from

this.

  1. Some light traveling through lens is absorbed (especially IR

and UV light). Mirrors can be made to reflect this IR and UV.

  1. Large lens can be very heavy, and can only be supported at
    edge. Mirrors are supported at the back.
  2. Lens needs two optically acceptable surfaces, mirror only

needs one, though mirror surfaces have to be more precise.

74
Q

What do the largest optical telescopes use?

A

The largest optical telescopes on earth use segmented

mirrors.

75
Q

How do we gather as much light as possible?

A
76
Q

What is angular resolution?

A
77
Q

The__________limits how clearly we can

see from Earth. Ways to solve this problem:

A

The

atmosphere

limits how clearly we can

see from Earth. Ways to solve this problem:

1.

Avoid it as best as

possible – put

telescopes on

mountains

2.

Get lucky

3.

Fix it

4.

Go to space

78
Q

What are the other ways to observe parts of the electromagnetic spectrum?

A
79
Q

Radio telescopes in a nutshell?

A
80
Q

What are the advantages of radio astronomy?

A

Can observe 24 hours a day

•

Clouds, rain, and snow don’t interfere (though this

depends somewhat on wavelength)

•

Observations at an

entirely different

frequency; get

totally different

information

81
Q

What are the wavelengths of light we would like to observe?

A
82
Q

What about shorter wavelengths?

A
83
Q

What are the general features of the SUN?

A

● Radius about 700,000 km, 100 times radius of Earth

● Composition: 3/4 hydrogen, about 1/4 helium by mass

(90% of the atoms are hydrogen)

● Density: Roughly the density of water

(1.4 times the density of water: 1.4 g/cm3 or 1400

kg/m3)

● Temperature very high at center (over 15 million K),

dropping to 6,000 K near surface

84
Q

What is the energy of the sun?

A

● The amount of energy per second hitting each

square meter of the Earth from the Sun is

1400 watts. This is called the solar constant.

If you covered your ceiling with hundred-watt

light bulbs, with 14 bulbs in in each square

meter of ceiling, the room would be as bright

as daylight.

● The luminosity of the Sun is 4x10^26

watts

● Total average power use by the entire world:

About 10 ^13 watts

A watt is a unit of power: energy per some unit of time.

Energy from the Sun

85
Q

What are the different parts of the sun?

A
86
Q

What is the interior of the sun?

A
87
Q

How does Energy get out of the Sun?

A

Radiation and Convection:

The radiation zone is relatively transparent; the

cooler convection zone is opaque.

88
Q

What is radiation?

A

All objects give off and absorb electromagnetic radiation.

But hotter objects gives off more than cooler objects.

89
Q

What is convection?

A

Convection transfers heat by moving stuff

around. Hot stuff rises, cool stuff sinks.

The sun transports energy by radiation in certain parts and

convection in others.

This is due to the different transparency of hydrogen and helium

as a function of temperature.

90
Q

The Solar Atmosphere

(photosphere, chromosphere, corona)

A

● The Sun is mostly hydrogen; about 10% of its atoms are

helium and there is a much smaller amount of heavier

elements.

● In nearly all of the Sun’s interior, the temperature is too hot

for electrons to stay attached to protons: The particles are

moving fast enough that any collision with a bound electron

will knock the electron off its atom. All the atoms are ionized:

The Sun is a collection of free protons and electrons.

● Near the surface of the Sun, the temperature is low enough

for some of the protons and electrons to form atoms.

91
Q

Electrons in atoms absorb only the wavelengths that correspond to the energy differences between their allowed orbits.

A
92
Q

What are sunspots?

A
93
Q

What are the features of the photosphere?

A

● Heat from the interior moves by

convection

(hot hydrogen

gas rising) to the photosphere

● The tops of convection cells are called

granules

●Sunspots are the result of strong magnetic fields going in or

out of the Sun’s surface

●The magnetic field drains energy from the surrounding

photosphere, cooling it; because the cooler gas is darker, it is

seen as a

dark spot

.

94
Q

Photosphere and Corona

A

The temperature of the photosphere is about 6,000 K.

But outside the photosphere, the temperature (surprisingly) increases, reaching 3 million K in the Corona.

95
Q

Solar Wind

A

Finally, particles that escape the Sun form what is

known as the solar wind.

Gas is so hot in the corona that particles move fast

enough to escape. The Sun is slowly losing mass (it is

evaporating). But don’t worry, over the last 4.6 billion

years only 0.1% of its mass has disappeared.

96
Q

What is a manisfestation of solar wind?

A

● A manifestation of the solar wind is the Aurora

Borealis or Northern Lights. It is a result of

the interaction between the solar wind and the

earth’s magnetic field.

● The solar wind blows Earth’s magnetic field

backward and drives high energy electrons to

the magnetic poles. These electrons cause

air molecules to glow.

97
Q
A

B) The equator

98
Q

What are features above the photosphere?

A

Associated with sunspots are

magnetic storms

that give rise to:

● Flares:

spectacular, hot explosions that

release UV and X-rays and eject electrons

and protons from the Sun’s surface.

● Prominences:

Trapped gas from the surface

of the sun. Trapped by magnetic fields

99
Q

Why does the Sun shine?

A

Only one known process can account for the

huge amount of energy generated by the Sun

Conversion of mass into energy

via nuclear fusion

E = mc^2

Energy = mass x (speed of light)^2

100
Q

Nuclear fusion vs nuclear fission

A

● Nuclear reactors on Earth use fission: heavy

elements are split into lighter ones

● Stars generate power through nuclear fusion:

light elements are fused into heavier ones

101
Q

What is the energy of starlight?

A

The Sun turns hydrogen into helium, and the

mass of a helium atom is slightly less than

the mass of 4 hydrogen atoms (by 0.7%=0.007)

Arthur Eddington (1920):

Hydrogen can turn into helium,

and when it does,

0.7% of its

mass changes to energy, and

that energy powers the Sun

102
Q

The whole is less than the sum of the parts

A
103
Q

How do protons manage to fuse together?

A
104
Q

Evidence of fusion in the Sun

A

Light

○Gamma rays produced in the center are absorbed and

re-emitted many many times before they reach the

surface of the Sun, more than 10,000 years later

○As they pass through cooler outer layers blackbody

spectrum shifts to lower temperatures

○We finally see visible radiation from the photosphere –

this is not direct evidence of fusion

●Neutrinos

105
Q
A

C. Nuclear Fusion

106
Q

What is a Stellar Parallax?

A

For the very closest stars we can measure

how much they shift against more distant

stars over the course of six months – the

time it takes for the earth to move from one

side of its orbit to another.

That tiny shift can tell us the distance to

that star. So tiny that no one saw it until

1838.

Shift is about 1/3600 of a degree or 1

arcsecond.

107
Q

What is the Parallax angle?

A
108
Q

Spectroscopic Parallax

A

If you know how bright something is, you can tell

how far away it is by looking at how bright it

seems.

109
Q

Spectroscopic Parallax

A

How bright something seems depends on its distance

apparent brightness = luminosity/(4π d^2)

How bright it seems (apparent brightness)

How bright it is (luminosity)

How far away it is (4π d^2)

Method 2: Spectroscopic Parallax

110
Q

The brightness of sunlight at the Earth is 1400

watts/meter

2

. What is the brightness of sunlight

at Saturn, 10 AU from the Sun?

A

Example: The brightness of sunlight at the

Earth is 1400 watts/meter2. What is the

brightness of sunlight at Saturn, 10 AU from

the Sun?

●

Saturn is 10 times farther away from the Sun

than the Earth, so sunlight is 1/10^2= 1/100 times brighter.

● The brightness of sunlight on Saturn is 1400/100 =

14 watts/meter^2. This is why the outer planets are cold!

111
Q

What is the brightness of the sun at 40

A.U. if it is 1400 watt/m^2 at 1 A.U?

2) How about at 100 A.U.?

A

= 1400 X 1^2/100^2 = 0.14 watts/m^2

112
Q

Magnitudes

A

Apparent brightness of stars is measured in units of

watts/meter

2

.

There is a much older scale, invented by the Greek

astronomer Hipparchus after whom the Hipparcos satellite

was named.

Hipparchus ranked the stars by apparent brightness, with the

brightest stars assigned magnitude 1, the dimmest magnitude

6.

Magnitude 1 stars are about 100 times brighter than

magnitude 6 stars (as seen from Earth).

113
Q

Stellar Spectra and Classification

A

The classification of a star is its spectral type.

Ordered from hottest to coolest, the spectral types are:

O, B, A, F, G, K, M (L, T)

(Use the mnemonic Oh, Be A Fine Guy/Girl Kiss Me,

or make up your own!)

● O stars are hottest with surface temperature > 25,000 K.

● G stars (like the Sun) have surface temperature of

approximately 6000 K.

● M stars are coolest (Betelgeuse for example) with surface

temperatures approximately 3000 K.

114
Q

Lifetime of star =

A

10^10 years M/L

Astronomy (4 decks)

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