Galaxies And The Universe Midterm Review

Front 1

F= delta T^4

Back 1

flux = delta temperature ^4

(for blackbodies)

Front 2

L= FA

Back 2

luminosity = flux * area

Front 3

L = 4piR^2

Back 3

Luminosity = 4pi radius ^2

(for any sphere)

Front 4

Newton's First Law of Motion

Back 4

o A body continues in state of rest or uniform motion in a straight line unless made to change by forces acting on it

Front 5

Newton's Second Law of Motion

Back 5

o F=MA
o The amount of acceleration A that a force F can produce depends directly on the mass M of the object being accelerated
o Apply the same force to two objects, smaller mass gets the bigger acceleration- think about the basketball versus the tennis ball demonstration


o Force 2= Mass 2*Acceleration 2 (N2)
o Force 1 = mass 1 * acceleration 1 (N2)

Front 6

Newton's Third Law of Motion

Back 6

o When two bodies interact, they create equal and opposite forces on each other

Front 7

Newton’s Laws of Gravity

Back 7

• Every mass exerts a force of attraction on every other mass
• Force = [(gravitational constant)(mass1)(mass2)]/distance between them)
o F= (Gm1m2)/r^2

Front 8

Kepler’s Third Law of Orbital Motion

Back 8

• P^2 = [4Π^2/G(m1+m2)] a^3
• Newton’s Form of Kepler’s 3rd law
• This describes orbits of 2 bodies around their center of mass
o P= orbital period (time)
o A= semi-major axis of elliptical orbit (the radius if the orbit is circular) * note, this A is not acceleration
o M1, M2 = masses of the bodies

Front 9

What is light?

Back 9

• A form of energy that can travel through space without any medium to carry it
- a particle and a wave

Front 10

Blackbody radiation

Back 10

an idealized opaque body at a uniform temperature which reflects no light but emits light with a spectral distribution of wavelengths which are specified by Planck’s Law
o Not necessarily dense
o Not necessarily solid
o Blackbody is not a good name because it doesn’t have to look black. Same as thermal radiation. Stars are not perfect blackbodies but they resemble perfect blackbodies.

Front 11

Temperature

Back 11

 Related to the speed of Kinetic Energy of particles (atom, molecules, other) that make up a substance.
 Proportional to the average speed of the molecules squared.
 Absolute zero
• Particles have no motion
 Kelvin temperature scale- based on absolute zero
• = 0°Kelvin. = -273°C = -460°F
• TK = TC + 273°
• Water freezes at 32°F = 0°C = 273°K

Front 12

What does

λ Max = (.0029 Kelvin * Meter)/ temperature

mean?

Back 12

The wavelength (λ) at which the intensity of a blackbody spectrum peaks given by Wien’s Law

Front 13

flux

Back 13

energy per time per area

Front 14

Emission line spectrum

Back 14

Blackbody through a prism produces a continuous spectrum
• Cold background (colder than the transparent gas ) through transparent gas through a prism and you get bright lines, but no rainbow. Intensity versus wavelengths would have only sharp peaks at certain discrete wavelengths.

Front 15

Absorption line spectrum

Back 15

Hot blackbody (hotter than the gas) --> transparent gas--> through a prism -> a rainbow minus some lines. Gaps. Rainbow with dark lines. Will look like a continuous spectrum but it will have gaps at landa 1 and landa t.

Front 16

spectral lines

Back 16

if the lines of one element are much stronger/brighter/deeper than the others, there is more of that element

Front 17

4 fundamental forces of nature

Back 17

• gravity
o very weak but important on large scales
• electromagnetism
• strong nuclear force- binds nucleons (p, n)
• weak nuclear force- radioactive decay of nuclei.

Front 18

What is a star?

Back 18

• Big, hot, self-luminous ball of gas (plasma) held together by its own gravity
• Powered by nuclear fusion (at some point in its lifetime)
• Requires mass > 1/10 Mass of the sun

Front 19

What is gas pressure?

Back 19

o Occurs from collisions of gas particles. Those collisions produce a pressure which can resist other forces
o So what is actually happening? Electrostatic repulsion!! They don’t actually touch each other, they just kind of bouce away. Picture like magnets.
o As gas is squeezed, particles are pushed toward one another and they collide more rapidly with each other and rebound due to electrostatic repulsion thus resist compression

Front 20

Ideal Gas Law

Back 20

o P= (some constant) * density * temperature
o P= knT
• N= number of particles/volume

Front 21

The core of the Sun is undergoing the same process of nuclear fusion that occurs when a hydrogen bomb explodes. Why does the Sun not explode?

Back 21

• ANSWER: the huge energy release is contained by the pressure of the overlaying material.
o Imagine hydrogen bombs under a huge steal slab- it stays elevated because of all the bombing.

Front 22

Proton-proton chain (3 step process) this produces most of the energy in the sun

Back 22

• H1 + H1 → H2 +( e+, positron) + neutrino
• H1 + H2 → 3He + (gamma ray photon)
• He3 + He3 → 4 He + H1 + H1

Front 23

Gravity as energy

Back 23

• Gravity as a source (waterfalls on earth, black holes, quasars which feed black holes) Varies enormously from 0-30%!!!!!!
o On earth this is close to chemical, but the most extreme places in the universe like a black hole. This is the most important source of energy in the universe

Front 24

Lifetime of a star

Back 24

• Total lifetime = total energy supply
Rate at which energy is consumed

(t is proportional to Mass ^-2.5)

bigger the star with hotter temperature, the faster it uses up its supply

Front 25

Evolution of Stars

Back 25

o Pre main sequence stars are above main sequence but bellow giants. Not hot enough to fuse hydrogen in its core, but finally when it can fuse hydrogen it moves to main sequence. When it uses all the hydrogen, it becomes a Giant or Supergiant (depending on mass) and evolves off the main sequence. Then when the outer layer of the star is blow off after “things happen” it shifts again and becomes a white dwarf.

Front 26

Fusion in the evolution of a star

Back 26

1. Main sequence- fusing hydrogen in the core

2. Red giant I- fusion of hydrogen in the shell; no fusion in the core.

3. Red Giant II- you still have hydrogen fusion in a shell, but there is Helium fusion in the core.

4. Planetary Nebula/White Dwarf Phase: eventually, the envelope or outer layers of the star are ejected into space to form planetary nebula an the core of the star becomes a white dwarf.

Front 27

Fusion of high mass stars

Back 27

• Once the core turns to iron, there is no more heat/energy generated to support the core of the star against the weight of the overlying material. → Thus, the core of the star collapses and the star explodes in a Type II Supernova. The outer layers of the star shoot off really fast, violent ejection (not a gentle release like planetary nebulae) into interstellar space, carrying with it the stuff the star made- elements made during evolution and during explosion

Front 28

Parallax example

Back 28

• We look at the positions of the earth and Jupiter, and their moons, at distances of 6 months, noticed that there was a time delay. There is a difference however, because the distance between earth and Jupiter varies.
o First line (Jupiter to Earth) y= Distance between Jupiter and 2(distance between earth and sun)
o Time delay of 16.6 minutes in the orbital transits of Jovian moons at roughly 6 month intervals. Delay is due to extra light travel time across inner solar system.
o Difference of distances y-x = 2xdistance between earth and sun = 16.6 light minutes
o Des = 8.3 light minutes
o Distance = CT = (300,000km//sec)(8.3min) (60 sec/min)
• Des = 150,000,000 Km = 1AU

Front 29

Distance formula

Back 29

distance = 1/p
• if distance is in parsecs
 parsecs- invented to make this equation simple= 206,265 AU, or 3.25 light years.
if angle is in arcseconds
arcsecond: 1” = (1/3600) of 1 degree (a dime 2 miles away)

Front 30

Brightness

Back 30

• Brightness – luminosity/Area
• Brightness – luminosity over/ 4Πd^3

Brightness is how luminous a star APPEARS TO BE WHEN LOOKING FROM EARTH, not how luminous it ACTUALLY is

Front 31

Difference between flux, luminosity and brightness

Back 31

• Brightness: energy per time per area (Jsec^.2, m-2
• Luminosity- energy per time
• Flux and brightness similar qualities, but the difference is that flux is measured a the surface of emitting object. Brightness is measured at some distance away from object.

Front 32

Why did it take us so long to realize we were part of the milky way?

Back 32

o The galaxy is really big
o We’re inside of it (like being in a forest and trying to see how big the forest is)
o Dust obscures visible light from stars in the disk of the galaxy, especially distant ones

Front 33

What is dust & why does it matter?

Back 33

• Small specks of solid matter (biggest ones up to millions and billions of molecules)
Absorb and scatter light,

Front 34

Schematic of the galaxy

Back 34

• Schematic of the galaxy (edge on view) of the milky way Disk, thickness = 5,000ly. 35,000ly wide. No harp cutoff. Half the stars would be within this range.. Disk has most of the dust. The bulge is right in the middle Nucleus w/black hole. There are also stay clusters, or globular clusters, in the halo or region about the dist. We are within the thick part, about 2/3rds of the way out.
o Why can we see things in the halo without dust? Not too much dust in the halo.

Front 35

Star cluster

Back 35

• little piece of a galaxy at helps us figure out how the galaxy formed
• gravitationally; group of stars which all formed at the same time, so they have the same age. We can use the HR diagram or this.
• globular clusters. In a globular cluster, there is a well defined cutoff.

Front 36

What stellar properties can you estimate simply by looking at a star on a clear night?

Back 36

brightness and surface temperature

Front 37

The distance to the nearest star (other than the Sun) is

Back 37

250,000 times greater than the distance to the Sun

4ly

Front 38

A star with 10 times the mass of the Sun has a main sequence lifetime how many times shorter/longer than the sun?

Back 38

About 300x shorter than the sun

Front 39

What 2 forces are in balance in main sequence stars?

Back 39

gravity and gas pressure

Front 40

The energy source for stars is...

Back 40

Nuclear fusion

Front 41

What property of a star determines its evolution?

Back 41

Mass

Front 42

The 2nd most abundant element in the universe is

Back 42

Helium

Front 43

Much of the visible light in the Milky Way Galaxy is

Back 43

obscured by dust

Front 44

When we observe stars near the center of the Milky Way Galaxy, we detect light that was emitted from those stars about

Back 44

25,000 years ago

Front 45

The star Rigel in the constellation of Orion has a surface temperature which is about 3 times that of the Sun, and a diameter which is about 100 times greater than that of the Sun. Therefore its luminosity is

Back 45

800,000 times greater than that of the sun

Front 46

At 100 mph it takes about 10 days to
drive around the earth. How long
would it take to drive to the Sun?

Back 46

100 years

Front 47

If you started right now to count to
1 billion you would be finished by

Back 47

your 90th birthday

Front 48

Which of the following is a
non-scientific idea?
.

1. The earth orbits the sun
2. The sun orbits the earth
3. The shape of earth’s orbit
is an ellipse
4. The shape of earth’s orbit
is a rhombus
5. The universe is large
6. The universe is expanding

Back 48

5. The universe is large

Front 49

If you’re standing in a train moving
on level ground at a constant velocity
of 200mph and leap straight up where will you land?

Back 49

You will land in the same place

Front 50

An earth-sized extrasolar
planet
orbits 1 AU from a star of 4 solar
masses. What is the approx
period of the star’s wobble?

Back 50

0.25 yr

Front 51

If light is a wave, what is waving?

Back 51

Electromagnetic feild

Front 52

Lightning generates EM radiation at many
wavelengths. How much longer does a pulse of
radio energy take to travel to you than a pulse of
UV energy?

Back 52

the same amount of time

Front 53

A particle of interplanetary material is heated by friction
from 400 K to 4000 K as it falls into Earth’s atmosphere,
producing a meteor or a shooting star. If it behaves like a
perfect blackbody, how does its emitted radiation change
as it is heated?

Back 53

Its flux is 10,000x greater, while its peak
wavelength is 10x shorter (IR->VIS)

Front 54

Suppose Barack and Rush look at the
spectrum of the same gas cloud in a
laboratory. Barack sees emission lines and
Rush sees absorption lines. How can this be?

Back 54

Rush sees the gas against a
hot background

Front 55

Spectral lines provide
information on:

Back 55

1. Which elements are
present
2. The relative amounts of
different elements

Front 56

What property makes the Sun a
star, and the Earth not a star?

Back 56

Mass

Front 57

The core of the Sun is undergoing the same process of nuclear fusion that occurs when a hydrogen bomb explodes. Why does the Sun not explode?

Back 57

The huge energy release is
contained by the pressure of
the overlying material

Front 58

The Photosphere (visible surface)
of the Sun is like:
1. The surface of the earth
2. The surface of the ocean
3. An apparent surface
4. The surface of a trampoline

Back 58

An apparent surface you would notice very little change as you go through it, as when you fly through a cloud

Front 59

When 2 Hydrogen nuclei, each of
mass m, fuse to form Deuterium:

Back 59

A small part of their mass
is converted to energy

Front 60

Our Sun has a main sequence lifetime of 10^10 years. According to the mass-lifetime relation, a star with a mass of 10 Msun has a lifetime of:

Back 60

3x10^7 years

Front 61

Stars in the “red giant region”
of the HR diagram must be larger than the sun because they are:

Back 61

more luminous than the sun and
have a cooler temperature

Front 62

The word “sequence” implies that stars on the main sequence of the HR diagram can be put in some sort of order. Which of the following is not ordered as one goes along the main sequence?

1. Temperature
2. Luminosity
3. Mass
4. Age

Back 62

Age

Front 63

The story of any star's evolution is
the story of its constant battle to __

1. balance the effects of
gravity
2. fuse its entire supply of
hydrogen
3. maintain its core
temperature
4. maintain its high mass
5. lose 5 pounds

Back 63

Balance the effects of gravity

Front 64

Why does a Red Giant star
become so large?

Back 64

Fusion occurs outside core so
not as much overlying weight
to keep star compressed

Front 65

Why does fusion of heavier elements
occur in more massive stars?

Back 65

Cores are hotter so they have
faster particles

Front 66

Why can’t we use parallax to
measure very large distances?

Back 66

The parallax angle becomes
too small to measure.

Front 67

Why can’t we use Cepheid variable
stars to measure the distances to
the most distant galaxies?

Back 67

Their brightness is too small
to be detected

Front 68

Why can’t we use supernovae to
measure the distances to stars in
our Galaxy?

Back 68

There haven’t been any in
our Galaxy recently.

Front 69

If the distance to the center of the Milky Way were scaled to the size of this room, then on the same scale the solar system would be about the size of

Back 69

A molecule of DNA

Front 70

The distance to the Milky Way center is 10 pc, and the size of the solar system is 10-4 pc. If the distance to the Milky Way center were scaled to the size of this room, then on the same scale the solar system would be the size of

Back 70

A molecule of DNA (10^-7m)

Front 71

Why do the best IR images come
from telescopes in space?

Back 71

Earth’s atmosphere is only partly
transparent to IR photons

Front 72

Why do the best visible light images
come from telescopes in space?

Back 72

Earth’s atmosphere
causes images to be
blurred

Front 73

Star clusters are useful to
astronomers because the clusters...

Back 73

contain stars of the same age

Front 74

AM/FM

Back 74

AM = 10^3 hz
FM = 10^ 6 hz