MIDTERM #2 Flashcards
(162 cards)
1
Q
Why does the sun shine?
A
The Sun’s mass contains more than enough energy to account for billions of years of sunshine. The Sun converts mass into energy through the process of nuclear fusion
2
Q
What is the Sun’s structure?
A
The Sun is essentially a giant ball of hot gas, or more technically, plasma (a gas in which atoms are ionized because of the high temperature)
3
Q
Gravitational Equilibrium
A
Energy supplied by fusion maintains the pressure that balances the inward crush of gravity
4
Q
Energy Balance
A
the rate at which fusion releases energy in the Sun’s core and the rate at which the Sun’s surface radiates this energy into space
5
Q
Energy Balance
A
the rate at which fusion releases energy in the Sun’s core and the rate at which the Sun’s surface radiates this energy into space
6
Q
Fission
A
Big nucleus splits into smaller pieces Example: nuclear power plants
7
Q
Fusion
A
Small nuclei stick together to make a bigger one Example: the Sun, stars
8
Q
How does the Sun release energy?
A
The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus
9
Q
What is the Sun’s radius?
A
700,000 kilometers - more than 100 times the radius of the Earth
10
Q
When is/was gravitational contraction an important energy generation?
A
During the contraction of the solar nebula
11
Q
Why does the sun remain roughly the same size?
A
It tries to expand because of the heat and light it generates, which balances the contraction it feels due to gravity
12
Q
Why do we know that gravitational contraction cannot be the source of energy which powers the Sun?
A
The amount of gravitational energy of the sun is too small to power the sun for billions of years
13
Q
What most accurately describes the overall nuclear process that powers our Sun?
A
Four protons combine to create an alpha particle, two electrons and neutrinos, and radiative energy
14
Q
Why won’t hydrogen fuse at low temperatures?
A
The protons need to undergo very energetic collisions to overcome electric repulsion and form helium. This happens much more often at high temperatures
15
Q
How does nuclear fusion occur in the Sun?
A
because the 15 million K plasma in the solar core is like a soup of hot gas fun of bare, positively charged atomic nuclei whizzing about at extremely high speeds
16
Q
Luminosity
A
Amount of power a star radiates (radiated energy per time; Unit: Joule/second = Watt)
17
Q
Apparent brightness
A
Amount of starlight that reaches Earth (energy per time and per area; Unit: Watt /square meter)
18
Q
Luminosity of a Star
A
the total amount of power that. star emits into space
19
Q
Parallax
A
is the apparent shift in position of a nearby object against a background of more distant objects
20
Q
Apparent Brightness of a Star
A
the amount of power (energy per second) reaching us per unit area
21
Q
Stellar Parallax
A
the small annual shifts in a star’s apparent position caused by Earth’s motion around the Sun
22
Q
Properties of Thermal Radiation
A
- Hotter objects emit more light per unit area at all frequencies
- Hotter objects emit photons with a higher average energy
23
Q
Remembering Spectral Types
A
(HOTTEST ) O B A F G K M (COOLEST)
Oh, Be A Fine Girl, Kiss Me
24
Q
Spectral Type
A
A way of classifying stars according to surface temperature - determined from spectral lines present in a star’s spectrum
25
Types of Binary Star Systems
Visual binary
Spectroscopic binary
Eclipsing binary
26
Visual Binary
We can directly observe the orbital motions of these stars
27
How do we measure stellar luminosities?
If we measure a star's apparent brightness and distance, we can compute its luminosity with the inverse square law for light
- Parallax tells us distances to the nearest stars
28
How do we measure stellar temperatures?
A star's colour and spectral type both reflect its temperature
29
How do we measure stellar masses?
Newton's version of Kepler's third law tells us the total mass of a binary system, if we can measure the orbital period (p) and average orbital separation of the system (a)
30
Spectroscopic Binary
identified through observations of Doppler gifts in its spectral lines
31
Eclipsing Binary
is a pair of stars that orbit in the plane of our line of sight
32
Hertzsprung-Russell diagram
Graphs of the type made by H-R - plots the luminosity and temperature of stars
33
Giant and Supergiant stars
Stars with lower temperature and higher luminosity - must have larger radii
34
White Dwarfs stars
Stars with higher temperature and lower luminosity than main-sequence stars - must have smaller radii
35
What does an H-R diagram depict
Temperature, Colour, Spectral type, Luminosity, Radius
36
What are the different Luminosity Classes?
```
Class 1 - Supergiants
Class 2 - Bright giants
Class 3 - Giants
Class 4 - Subgiants
Class 5 - Main-sequence stars
```
37
What dos the apparent magnitude of a start tell you about that star?
How bright it looks
38
What spectral type describes our Sun?
G
39
What are Main-Sequence stars doing?
fusing hydrogen into helium in their cores like the Sun
40
What colour are Luminous main-sequence stars?
Blue (hot)
41
What colour are less Luminous stars?
Yellow or red (cooler)
42
What main-sequence stars have a higher measurement?
hot blue stars are much more massive than the cool, red ones
43
What does higher core temperature of a star result too?
boosts fusion rate, leading to larger luminosity
44
How to measure Luminosity of a star
from brightness and distance
45
How to measure Temperature of a star
from colour and spectral type
46
How to measure Mass of a star
from period (p) and average separation (a) of binary star orbit
47
High Mass Star
- high luminosity
- short lived
- larger radius
- blue
48
Low Mass Star
- low luminosity
- long lived
- small radius
- red
49
Why does brightness in a star sometimes vary?
they cannot achieve proper balance between power welling up from the core and power radiated from the surface
50
What is the significant of the main sequence?
- Normal stars that fuse H to He in their cores fall on the main sequence of an H-R diagram
- A Star's mass determines its position along the main sequence (high-mass: and blue; low-mass: faint and red)
51
What are giants, supergiants, and white dwarfs?
- all stars become larger and redder after core hydrogen burning is exhausted: giants and supergiants
- most stars end up as tiny white dwarfs after fusion has ceased
52
Why do the properties of some stars vary?
Some stars fail to achieve balance between power generated in the core and power radiated from the surface
53
Interstellar Medium
the gas between stars
54
Molecular Clouds
- most of the matter in star-forming clouds is in the form of molecules
- These molecular clouds have a temperature of 10-30 K and a density of about 300 molecules per cubic centimeter
55
Interstellar Dust
material in a molecular cloud that is not gaseous. could contain carbon, silicon, oxygen, and iron
56
Mass of a star forming cloud
must contain at least a few hundred solar Masses for gravity to overcome pressure
57
Fragmentation of a Cloud
- gravity within a contracting has cloud becomes stronger as the gas becomes denser
- Gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments, each of which may go on to form a star
58
The First Stars
- elements like carbon and oxygen had not yet been made when the first stars formed
- Without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably warmer than today's molecular clouds
59
Where do stars form?
- stars form in dark, dusty clouds of molecular gas with temperatures of 10-30 K
60
Why do Stars form?
- stars form in clouds that are massive enough for gravity to overcome thermal pressure (and any other forms of resistance)
- Such a cloud contracts and breaks up into pieces that go onto form stars
61
Protostar
when thermal energy begins to build up inside a cloud increasing the internal pressure causing contraction to slow down and then the centre of the cloud fragment becomes a protostar - will eventually become a star
62
Main-Sequence star
contraction of a protostar continues until the core becomes hot enough for nuclear fusion - when the energy released by core fusion balance energy radiated from the surface contraction stops and the star is now a MAIN-SEQUENCE STAR
63
How long does it take for a protostar to form into a main sequence star?
30 million years
64
What is the smallest mass a newborn star can have?
Degeneracy pressure stops the contraction of objects <0.08Msun before fusion starts
65
What is the greatest mass a newborn star can have?
Stars greater than about 300Msun would be so luminous that radiation pressure would blow them apart
66
What are the typical masses of newborn stars?
star formation makes many more low-mass stars than high-mass stars
67
What is the interstellar medium composed of?
Quite sparse gas and dust particles
68
What type of cloud would you expect to find stars forming in?
a cold, dense molecular cloud
69
If you wanted to see a star behind an interstellar dust cloud, what "colour" of light should you look for?
infrared
70
What prevents the press from increasing as a cloud contracts due to its gravity?
Thermal energy is converted to radiative energy via molecular collisions and released as photons
71
Which process causes a single molecular cloud to produce multiple star systems?
Fragmentation
72
What stops the contraction of a collapsing star?
thermal and radiation pressure
73
what marks the birth of a main-sequence star?
the beginning of efficient, sustained hydrogen fusion
74
How does a star's mass affect nuclear fusion?
- a star's mass determines its core pressure and temperature and therefore determines its fusion rate
- higher mass stars have hotter cores, faster fusion rates, greater luminosities, and shorter lifetimes
75
A star remains on the main sequence as long as it can......
fuse hydrogen into helium in its core
76
Helium Flash
thermostat of a low-mass red giant is broken - core temperature rises rapidly when helium fusion begins - helium fusion rate skyrockets until thermal press takes over and expands the core again
77
What happens to a red giant after helium flash?
it should shrink and become less luminous after helium fusion begins in the core
78
What are the life stages of a low-mass star?
- hydrogen fusion in core (main sequence)
- hydrogen fusion in shell around contacting core (red giant)
- Helium fusion in core (horizontal branch), bellow hydrogen shell burning
- double shell burning (red giant)
79
How does a low-mass star die?
ejection of hydrogen and helium in a planetary nebula leaves behind an inert white dwarf
80
Double Shell Burning
after core helium fusion stops, helim fuses into carbon in a shell around the ecarbon core, and hydrogen fuses to helium in a shell around the helium layer
81
Planetary Nebulae
Double shell burning ends with a pulse that ejects the H and He into space as a planetary nebula. The core left behind becomes a white dwarf
82
Earth's Fate
The Sun's luminosity will rise to 1000 times its current level-too hot for life on Earth
83
What are the life stages of a high-mass star?
- Late life stages of high-mass stars are similar to thoxe of low-mass stars:
- Hydrogen core fusion (main sequence)
- Hydrogen shell burning (supergiant)
- Helium core fusion (supergiant)
84
CNO Cycle
High-mass main sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts - greater c ore temperature enables hydrogen nuclei to overcome greater repulsion
85
Helium capture reactions
reactions in which a helium nucleus fuses with some other nucleus
86
What elements do advance reactions in stars make?
Si, S, Ca, Fe
87
How does a high-mass star die?
Iron builds up in core until eletron degeneracy pressure can no longer resist gravity. The core then suddenly collapses, creating a supernova explosion
88
Supernova Expolsion
Core degeneracy pressure goes away because electrons combine with protons, making neutrons and nutrinos. Neutrons collapse to the center, forming a neutron star which is stabilized by neutron degeneracy pressure
89
How do high-mass stars make the elemensnts necessary for life?
Higher masses produce higher core temperatures that enable fusion of heavier elements
90
How does a high-mass star die?
Its iron core collapses, leaving to a supernova
91
Role of a Star's Mass
A star's mass determines its entire life story because it determines its core temperature
- high mass stars have short lives and end in supernova explosions
- low mass stars have long lives and end up as white dwarfs
92
How are the lives of stars with close copanions different?
stars with close companions can exchange mass, altering the usual life stories of stars
93
What does a low mass star (e.g. the Sun) become once it leaves the main sequence?
A red giant
94
What happends to a low mass star after the helium flash?
It contracts and warms up
95
What makes up a planetary nebula?
The outer layers of the dying star, illuminated by the remnants of the hot core
96
What is the CNO cycle?
A faster, more efficient way of converting protons into helium nuclei
97
Near the ends of its life, what lies at the core of a high mass star?
iron
98
Why is the density of a neutron star so much higher than that of ordinary amtter, like water or lead?
it is basically a giant nucleus, and has very little empty space
99
At what stage in a high mass star's life are very heavy elements like Gold or Uranium generated?
Supernova
100
What keeps a white dwarf from collapsing?
electron degeneracy pressure
101
What is a nova?
a white dwarf fusing hydrogen that has fallen on its surface
102
What happens to a white dwarf that grows to more than 1.4 solar masses?
Kaboom
103
What keep a neutron star from collapsing?
neutron degernacy pressure
104
What is a pulsar?
A rapidly rotating neutron star that beams radio waves towards Earth every few hudredths of a second
105
What is an event horizon?
The surface of the black hole from which nothing can escape
106
What are tidal forces with respect to black holes?
the extremely strong difference in the pull of gravity from one place to another near a black hole's singularity
107
Suppose you reside on a planet that is orbiting a star on the outer edge of the Milky Way galaxy's disk. Today is your friend's birthday, and she lives on the opposite edge of the galactic disk, so that the distance between you is the same as the diameter of the galaxy. You want to send her a message in the form of a radio broadcast. About how long after her birthday will she get the message?
a hundred millennia (1 millennium = 1,000 years)
108
You get invited on a galactic halo photo safari. What kind of objects do you expect to be photographing?
globular clusters
109
Why is it unlikely that you will photograph any rocky or metallic planets on your trip?
There weren't many heavy elements when objects in the halo formed
110
In astronomical terms, what is a bubble?
the expanding gas around a supernova or large star
111
What is a galactic fountain?
the idea that super bubbles blow hot gas out into the halo of the galaxy, which sinks back into the disk as it cools
112
When we map out our galaxy by looking at 21-cm wavelength radiation, what are we seeing?
atomic hydrogen
113
It takes one Earth year for our planet to travel around the Sun. About how Many Earth years does it take for our Sun to travel around the centre of the galaxy?
230,000,000
114
Where would you look to find very active regions of stellar formation?
in the disk of the galaxy in the spiral arms
115
Why do we think there's a supermassive black hole in the galactic centre?
we observe that motions of gas and stars in this region, and conclude that there must be a black hole there
116
What did Edwin Hubble find out about distant galaxies that changed our ideas of cosmology?
More distant galaxies are receding from us faster
117
What kind of galaxy is the Milky Way?
Spiral
118
If the Milk Way has a larger bulge, which of the following would likely be true?
It would have less interstellar dust and gas
119
Which of the following is true about elliptical galaxies?
They don't have as many star forming regions as spiral galaxies
120
If you wanted to determine the distance to a very distant galaxy (more than a billion light years away), which of the following techniques would you use?
white dwarf supernovae
121
Suppose you wanted to find the distance to a spiral galaxy, guy did not want to wait for a white dwarf supernova to occur. Which of the following would you want to measure? (and be able to measure)?
Rotation speed
122
What is Hubble's Constant?
A number that relates the speed of a galaxy away from us to its distance from us
123
How does Hubble's constant tell us the age of the Universe?
it tells us how fast the Universe is expanding, from which we can calculate its age
124
Which of the following is true about lookback time?
It is the amount of time travelled by light between emission and observation
125
Why do we not measure the expansion rate of the Universe from galaxies in our local group?
They are not moving away from us in the same way that distant galaxies are
126
What is a white dwarf?
White dwarfs are the remaining cores of dead stars
127
Size of a White Dwarf
White dwarfs with same mass as Sun are about same size as Earth
- Higher-mass white dwarfs are smaller
128
The White Dwarf Limit
Because nothing can move faster than light, a white dwarf cannot be more massive than 1.4MSun the white dwarf limit
129
Massive star (core collapse) supernova
Iron core of a massive star reachers white dwarf limit and collapses into a neutron star, causing total explosion (leaves behind nebula and neutron star)
130
White Dwarf Supernova
Carbon fusion suddenly begins as a white dwarf in close binary system reaches white dwarf limit, causing total explosion (leaves behind nebula)
131
What can happen to a white dwarf in a close binary system?
- Matter from its close binary companion can fall onto the white dwarf through an accretion disk
- Accretion of matter can lead to novae and white dwarf supernovae
132
What is a neutron star?
the ball of neutrons left behind by a massive-star supernova
133
What is a pulsar?
a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis
134
How were neutron stars discovered?
- Beams of radiation from a rotating neutron star sweep through space like lighthouse beams, making them appear to pulse
- Observations of these pulses were the first evidence for neutron stars
135
What is a black hole
an object whose gravity is so powerful that not even light can escape it
136
Singularity
When gravity crushes all the matter into a single point
137
Three populations of dead stars
1. white dwarfs (least massive and most abundant)
2. Neutron stars (more massive and less abundant that white dwarfs)
3. Black holes (most massive and least abundant of the three populations)
138
How do stars orbit in our galaxy?
- stars in the disk all orbit in the same direction with a little up-and-down motion
- orbits of stars in the bulge and halo have random orientation
139
Why do orbits of disk stars bop up and down?
Gravity of disk stars pulls them toward the disk
140
What does our galaxy look like?
Our galaxy consists of a disk of stars and gas, with a bulge of stars at the centre of the disk, surrounded by a large spherical halo
141
How is gas recycled in our galaxy?
star-gas-star cycle
| recycles gas from old stars into new star systems
142
Summary of Galactic Recycling
- stars make new elements by fusion
- dying stars expel gas and new elements, producing hot bubbles
- hot gas cools, allowing atomic hydrogen clouds to form
- further cooling permits molecules to form, making molecular clouds
- gravity forms new stars (and planets) in molecular clouds
143
Ionization nebulae
colourful, wispy blobs of glowing gas found near hot stars
144
Spiral arms are waves of star formation -
1. gas clouds get squeezed as they move into spiral arms
2. squeezing of clouds triggers star formation
3. young stars flow out of spiral arms
145
How is gas recycled in our galaxy?
- gas from dying stars mixes new elements into the interstellar medium, which slowly cools, making the molecular clouds where stars form
- those stars will eventually return much of their matter to interstellar space
146
Where do stars tend to form in our galaxy?
- active star-forming regions contain molecular clouds, hot stars, and ionization nebulae
- Much of the star formation in our galaxy happens in the spiral arms
147
How did our galaxy form?
our galaxy formed from a cloud of intergalactic gas
- halo stars formed first as gravity caused gas to contract
- remaining has settled into a spinning disk
- Stars continuously form in disk as galaxy grows older
148
What clues to our galaxy's history do halo stars hold?
halo stars are all old, with a smaller proportion of heavy elements that disk stars, indicating that the halo formed first
149
What lies in the centre of our galaxy?
orbits of stars near the centre of our galaxy indicate that it contains a black hole with 4 million times the mass of the Sun
150
What are the three major types of galaxies?
Hubble
Ultra Deep
Field
151
Barred spiral galaxy
has a bar of stars across the bulge
152
Lenticular galaxy
has a disk like a spiral galaxy but much less dusty. gas (intermediate between spiral and elliptical)
153
Elliptical galaxy
all spheroidal component, virtually no disk component
| Red-yellow colour indicates older star population
154
Are spiral galaxies found in groups or on their own?
Spiral galaxies are often found in groups o galaxies (up to a few dozen galaxies)
155
Are elliptical galaxies found in groups?
Elliptical galaxies are much more common in huge clusters of galaxies (hunters to thousands of galaxies)
156
How are the lives of galaxies connected with the history of the universe?
Galaxies generally formed when the universe was young and have aged along with the universe
157
What are the three major types of galaxies?
The major types are spiral, elliptical, and irregular galaxies
Spirals have both disk and spheroidal components; ellipticals have no disk
158
How do we measure the distances to galaxies?
- The distance measurement chain begins with parallax measurements that build on radar ranging in our solar system
- Using parallax and t he relationship between luminosity, distance, and brightness, we can calibrate a series of standard candles
- We can measure distances greater than 10 billion light-years using white dwarf supernovae as standard candles
159
How did Hubble prove that galaxies lie far beyond the Milky Way?
he measured the distance to the Andromeda Galaxy using Cepheid variable stars as standard candles
160
What is Hubble's law?
The faster a galaxy is moving away from us, the greater its distance:
Velocity = H0 x Distance
161
How do distance measurements tell us the age of the universe?
Measuring a galaxy's distance and speed allows us to figure out how long the galaxy took to reach it's current distance
Measuring Hubble's constant tells us that amount of time: about 14 billion years
162
How does the universe's expansion affect our distance measurements?
Three distances: distance at photon emission, light travel distance (loopback distance), distance today
Loopback distance is the default distance reported by astronomers
