Intro Flashcards
What do we see in the sky?
constellations
What is a celestial sphere?
The name given to an imaginary sphere
•From northern hemisphere, stars, Sun, Moon and planets appear to move from
east to west in a circle around North Celestial Pole: Polaris, the North Star
In a day, celestial sphere appears to rotate
•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
6 hours
Zenith
is the point on the celestial sphere directly overhead.
Horizon
is a circle on the celestial sphere 90 degrees from zenith.
The meridian
is a line from horizon to horizon, passing through the zenith.
What you see at night depends on where you are standing
The altitude of polaris equals
Your latitude
Because Madison is 43 degrees north of the equator,
Polaris is 43 degrees above our horizon. From south of equator, Polaris is below the horizon, never visible.
What are longitude and latitude?
Celestial Coordinates
Earth’s Orbital Motion: What is a day?
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?
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.
How long does this take?
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.
Earth’s Orbital Motion: Seasonal Changes
The stars that the sun appears to move over are the
Explain the seasons and the tilt of the Earth?
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.
The Sun is directly over the equator on the
The seasons are caused by the tilt of the Earth’s axis. The tilt has two effects:
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
Warmer
True or False: Summer is warmer than winter because the Earth is closer to the Sun.
False
Why does the Moon shine?
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)
The fraction of the Moon’s disk that is visible
=
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.
Seen from the northern hemisphere, the star Polaris
Is always above the northern horizon
Relative to the stars, the sun appears to move
About one degree westward each day
Northern hemisphere winter are colder than Northern Hemisphere summers because
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
The sun is on the celestial equator at the times of the
Autumnal equinox and vernal equinox
The ecliptic is
The centerline of the zodiac
The projection of Earth’s orbit on the sky
The apparent path of the sun around the sky
On the vernal equinox the sun is
On the celestial equator and moving north with respect to the equator
A solar or lunar eclipse will occur
When the sun is near the line of nodes of the moon and the moon is new or full
The ___________ moon is visible low in the sky near western horizon a few hours before sunrise:
Waxing gibbous
A waxing crescent moon is visible
Near the western horizon just after sunset
During a total lunar eclipse
The moon’s color will be affected by Earth’s atmosphere
The first quarter moon rises
At about noon
Total lunar eclipses always occur
At the time of full moon
Why do we send telescopes into space?
What do charged particles create?
How does light and electric force work?
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
What are electric forces and electric fields?
What does charge change effect light?
What is the speed of information?
This information moves at a speed of 300,000 km/s, the speed of light, or the maximum speed in the universe
What are the characteristics of Light?
What is all electromagnetic radiation (LIGHT) defined by?
What does the complete spectrum of light look like?
What is the relationship between frequency and wavelength?
What is the second crest that passes you?
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
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.
In general, for a frequency of f crests per second, you can see that the time between crests is
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
B
What are the colors of
X-ray, Optical, Infrared, Radio, and Ultraviolet.
Purple, Green, Yellow, Red, Blue
Astronomy across the spectrum
What does temperature measure?
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).
What is thermal radiation?
What is the peak of intensity of light emitted by a star?
What do hotter stars have?
What is black body radiation?
How do you calculate wavelength with temperture?
Colder
Longer peak wavelength = colder temperature
10,000 K
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?
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.
What wavelengths are emitted and absorbed?
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
What is absorption and emission?
Elements in stars can be identified by recognizing the
patterns of their spectral lines.
A neutral atom has the same number of protons and electrons, However…
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
What is the doppler shift?
• 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
Light and Matter
•
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)
What are optical telescopes?
When does light travel fastest?
What happens when focusing light on a lense?
What are the two types of optical telescopes?
Modern telescopes are all reflectors - Why?
- Light traveling through a lens is refracted differently depending
on wavelength (chromatic aberration). Mirrors don’t suffer from
this.
- Some light traveling through lens is absorbed (especially IR
and UV light). Mirrors can be made to reflect this IR and UV.
- Large lens can be very heavy, and can only be supported at
edge. Mirrors are supported at the back. - Lens needs two optically acceptable surfaces, mirror only
needs one, though mirror surfaces have to be more precise.
What do the largest optical telescopes use?
The largest optical telescopes on earth use segmented
mirrors.
How do we gather as much light as possible?
What is angular resolution?
The__________limits how clearly we can
see from Earth. Ways to solve this problem:
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
What are the other ways to observe parts of the electromagnetic spectrum?
Radio telescopes in a nutshell?
What are the advantages of radio astronomy?
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
What are the wavelengths of light we would like to observe?
What about shorter wavelengths?
What are the general features of the SUN?
● 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
What is the energy of the sun?
● 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
What are the different parts of the sun?
What is the interior of the sun?
How does Energy get out of the Sun?
Radiation and Convection:
The radiation zone is relatively transparent; the
cooler convection zone is opaque.
What is radiation?
All objects give off and absorb electromagnetic radiation.
But hotter objects gives off more than cooler objects.
What is convection?
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.
The Solar Atmosphere
(photosphere, chromosphere, corona)
● 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.
Electrons in atoms absorb only the wavelengths that correspond to the energy differences between their allowed orbits.
What are sunspots?
What are the features of the photosphere?
● 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
.
Photosphere and Corona
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.
Solar Wind
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.
What is a manisfestation of solar wind?
● 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.
B) The equator
What are features above the photosphere?
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
Why does the Sun shine?
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
Nuclear fusion vs nuclear fission
● 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
What is the energy of starlight?
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
The whole is less than the sum of the parts
How do protons manage to fuse together?
Evidence of fusion in the Sun
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
C. Nuclear Fusion
What is a Stellar Parallax?
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.
What is the Parallax angle?
Spectroscopic Parallax
If you know how bright something is, you can tell
how far away it is by looking at how bright it
seems.
Spectroscopic Parallax
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
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?
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!
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.?
= 1400 X 1^2/100^2 = 0.14 watts/m^2
Magnitudes
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).
Stellar Spectra and Classification
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.
Lifetime of star =
10^10 years M/L
