Week 7 Our Sun
1. Why does the sun shine? How do we know it's not 'on fire' or due to 'gravitational
contraction' ?
The shine of the sun is caused by nuclear fission happening at its core. We know that it is
not fire because the time it would take the fire to consume the mass of the sun is far less
than the actual lifespan which is also why it is not contracting.
2. What are the two types of equilibriums within the Sun (or any star for that matter)?
The Gravitational equilibrium where gravity pulls inward and the sun’s pressure forces
energy outward. Energy balance also occurs where thermal energy from fusion creates a
balance for the radiative energy lost by the sun.
3. What is the Sun's structure?
The outer most layer is the solar wind that radiates from the surface of the sun. Then, the
corona which is the outer most layer of the sun’s atmosphere. The chromosphere is the
middle layer of the atmosphere. The visible surface is the photosphere. Underneath is the
convection zone where energy is being moved around. Then it is the radiation zone where
energy is being pushed upwards. At the center is the core where nuclear fission is
happening.
4. How much larger is the Sun than Earth? How much more massive?
The sun has a radius of about 109 times the size of the earth and a mass equal to 300,000
earths.
12. Energy is created in the centre of the Sun, describe the pathway the photons take as
they leave the core and move through the radiation zone of the Sun. Describe the path the
photons take once they make it into the convection zone of the Sun.
Energy leaks out to the convection zone through randomly bouncing photons. Once they
reach the convection zone they rise to the surface.
13. Describe how we are able to know what is happening inside the Sun.
We are able to tell whats happening inside the sun by making mathematical models,
observing the solar vibrations and solar neutrinos. Observing the patterns of solar vibrations
give us an idea of what is happening inside. Observing solar neutrinos gives on insight on
what is going on at the core of the sun.
14. What are Sunspots, Solar Flares, and Solar Prominences?
Sunspots are the coolest part of the sun and have strong magnetic fields. Solar flares are
bursts of x-rays and charged particles that get shot out into space due to magentic activity.
Solar prominece are solar flares that occur high above the sun’s surface.
15. Compare the Sun's magnetic field to Earths.
The Sun’s magnetic field is on an 11 year cycles where is surges then dies down ultimately
fliping after 11 years. The Sun’s magnetic field is much stronger than Earth’s. Earth’s
magnetic field goes up and down while the sun’s twists and turns.
16. Why doesn't the Sun's magnetic field act like a bar magnet? What does it end up acting
like?
The Sun’s high temperatures cause the charged the particles that make up its plasma to
move around a lot. This causes the magnetic field to act out the way it does and not act like
a bar magnet.
17. What are coronal mass ejections?
Coronal mass ejections send bursts of energy throughout the solar system.
18. How does the solar wind create aurora on Earth?
When solar wind interacts with the the Earth’s magnetic field near the atmosphere the
particles collide with what’s in the atmosphere creating the aurora.
19. Why are sunspots dark?
Sunspots are dark because they are much cooler than the rest of the sun.
20. What is the solar cycle? What causes it? How do we know it's occurring?
The solar cycle is when the magnetic field gets so twisted that it flips. This occurs because
the plasma in the sun moves at different speeds causing the magnetic field to twist. We
know this is occurring because we can observe the sunspots that show up on the surface of
the sun.
21. Does the solar cycle have any measurable affect on Earth's climate? How much
does the Sun's luminosity vary over the solar cycle?
The solar cycle does not have any measurable affect on the Earth’s climate because there
has been no correlation between the rising temperatures and where the sun is in its cycle.
The Sun’s luminosity varies by 0.1%.
23. What are the top two elements found in the Sun by percentage of mass (and what are
those percentages)?
Hydrogen and Helium. About 73% and 25% respectively.
25. What does ionized and plasma mean?
Ionized is when an atom gains a positive or negative charge by losing electrons. Plasma is a
type of hot ionized gas.
Week 7 Other Stars
1. What is brightness? What is Luminosity? How do the two relate to each other?
Brightness is your perception of light. Luminosity is the amount of power that is being
radiated. These two relate because they both are a measure of how bright something is
although brightness can change depending on distance and luminosity is brightness void of
the distance.
2. What is the inverse-square law?
The inverse square law states that with every increase of square distance, intensity
decreases.
3. How would the apparent brightness of the star Sirius change if it were 5x further away?
The apparent brightness would seem about 5 times dimmer than before.
4. What is parallax? Why do stars exhibit parallax shift?
Parallax is the shift of an object against its background. Stars exhibit parallax shift because
as the Earth orbits around the sun, stars shift against their background.
5. How can we use parallax shift to measure the distance to a star?
We can measure by dividing 1 by the parallax angle.
6. What is the electromagnetic spectrum?
The electromagnetic spectrum is the range of electromagnetic radiation
7. How do we measure stellar temperatures? How is this related to thermal radiation?
We measure stellar temperatures by measuring the intensity and the wavelengths. This is
related to thermal radiation because we are using the light that every object emits to
determine the temperature.
8. What temperature is the hottest possible star? The coolest?
The hottest is 50,000k and coolest is 3000k
9. What is an absorption line? What is an emission line? How do these relate to the specific
atom doing the absorption/emitting?
Absorption line is material absorbing photons at a wavelength. Emission line is material
emitting photons at a wavelength.
10. How does temperature affect a spectra's emission or absorption lines?
The hotter the temperature the more light is emitted and the cooler the more is absorbed.
11. How would you define 'spectral type' when referring to stars? What are the 7
different spectral types? What is the order of these spectral types in descending
Temperature?
OBAFGKM
12. Why are the spectral types in such a weird order (ie, not alphabetical)?
The order relates to the amount of hydrogen observed in stars
13. How would you measure the mass of a star?
Newton’s version of Kepler’s 3rd 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)
14. What does centre of mass mean?
The centre of mass is the point in which an object orbits.
16. What's the most massive star that can exist? Why can't it get larger?
The most massive is 100Msun
17. What's the least massive star that can exist? Why can't it get smaller?
The least massive is 0.08Msun
18. What is the HR diagram? What is the main sequence?
The HR diagram plots the luminosities and temperatures of stars. The main sequence is the
category where most stars fit into on the HR diagram. These stars fuse hydrogen into
helium.
19. What role does mass play in the luminosity and temperature of a star? What about the
how long the star lives? How are these represented on the HR diagram?
The larger the mass the higher the luminosity and temperature. More mass causes the
lifespan of a star to decrease as it is burning out faster.
21. What are giants/super giants? What are white dwarfs? Are they on the main sequence?
Why or Why not?
Giants and super giants are stars whose core hydrogen have been exhausted. White dwarfs
are stars whose fusion has ceased. These are off the main sequence because giants and
supergiants are much larger and white dwarfs are much dimmer.
22. Compare open clusters vs globular clusters
Open clusters are a few thousand closely packed stars while globular clusters are made up
of millions packed densely.
Week 8 Star Stuff
1. Where do stars form? What do you call the space in-between stars?
Stars form in clouds of gas and dust in interstellar space. The space between them is called
the interstellar medium.
2. Discuss the interplay between gravity and pressure with respect to star formation
When gravity overcomes the thermal pressure in a cloud it can form a star. As the cloud gets
denser gravity grows stronger. Stars need a few hundred solar masses for gravity to be able
to overcome pressure.
3. What is the name of the type of cloud stars form in? Normally, how much mass can be
found in one of these?
They are called molecular clouds and millions of solar masses can be found in them.
6. What would happen to a contracting cloud fragment if it were not able to radiate away its
thermal energy?
The internal pressure of the cloud would increase.
7. Explain the generally agreed upon process that creates stars
Gravity cause clouds to contract and as they contract temperature increases and thermal
pressure is radiated away. As it gets smaller it starts to spin faster and faster. The spinning
causes the gas to form into a disk. As the pressure increases jets of gas burst from the
center of the disk. The center of the disk becomes very dense and extremely hot to the point
atoms start to fuse together causing the birth of a star.
8. What is conservation of energy? How is it related to star formation?
The conservation of energy states that the energy in a closed system must stay constant.
This relates to the formation of a star because clouds contract due to the conservation of
energy.
9. What is conservation of angular momentum? How is it related to star formation?
The conservation of angular momentum states that angular momentum cannot be changed
unless another force twists it. This relates to the formation of a star because the disk starts
to spin faster and faster as gravity cause the cloud to contract.
10. How do you go from contracting gas cloud to protostar to main sequence star?
Contracting gas cloud gets denser and hotter until a protostar forms at the center and this
turns into a main sequence star after its temperature can create hydrogen fusion and once
this energy is released and balanced with the energy radiating from the star’s surface then
itll become a main sequence star.
12. When a large cloud of gas forms stars, compare the number of high mass to low mass
stars that form there
Very few high mass stars are formed from stellar masses mostly low mass stars are formed.
14. What is the upper limit to a star's mass? Why?
300 Msun is the limit as more mass will cause it to blow apart.
13. What is the lower limit to a star's mass? Why?
0.08 Msun is the limit because fusion will not start lower than this.
15. What is degeneracy pressure?
Two electron cannot occupy the same state in the same place
16. What is thermal pressure?
The main form of pressure in stars
17. What are Brown Dwarfs?
Brown dwarfs are star like objects not massive enough to start fusion.
18. How long does it take for a protostar to become a main sequence star?
Anywhere from less than a million to over a hundred million depending on the mass of the
star.
21. What are the life stages of a low-mass star?
It starts off in the main sequence and once the hydrogen core is exhausted it turns into a red
giant where the outer shell of hydrogen starts fusing. Then it starts to burn hydrogen faster
and thermal pressure increase as well as the size. Then when hot enought it starts to burn
the helium core cause it to create steady energy making it hotter and smaller. The star then
collapses into a white dwarf.
22. What is a helium flash?
Helium flash is when the ignition of the helium core happens very quickly. This causes the
temperature to rapidly rise.
23. What does a low-mass star eventually turn into?
A low mass star will eventually turn into a planetary nebula with a white dwarf at the center.
24. How is a white dwarf supported if it is not fusing?
Degeneracy pressure supports the white dwarf.
26. What are the life stages of a high-mass star?
Similar to low mass stars: hydrogen core fusion, Hydrogen shell fusion and helium core
fusion.
27. What is the CNO cycle? Why does the CNO cycle NOT exist in low-mass stars?
Carbon-Nitrogen-Oxygen cycle. One of two known fusion sets where hydrogen turns into
helium. Found is most starts with large masses. High mass stars use carbon and oxygen as
catalysts to fuse hydrogen into helium at a higher rate. This does not exist in low mass stars
because they cant get hot enough for it to occur.
28. You may have heard that we are all 'made of star stuff' before, why do we say this?
Stars help create most of the elements necessary for life. CNO cycle produces carbon
nitrogen and oxygen. We’re made up of carbon so this why we hear that we are made up of
star stuff.
29. Why is iron the last element that can be made in the high-mass star life cycle?
Iron is the last element that can be made in the high-mass cycle because iron does not
release energy unabling further reactions.
30. What is a supernova? How does it occur?
A supernova is a massive stellar explosion. Iron builds up in the core until degeneracy
pressure can no longer resist gravity and then the star collapses and a supernova forms.
31. What does a high-mass star eventually turn into?
It eventually turns into a neutron star.
Week 8 Stellar Graveyard
2. How large are white dwarfs? Why does a white dwarf that has MORE mass also have a
SMALLER size?
White dwarfs tend to be the size of the earth. The more mass white dwarfs have the more
packed together their electrons are causing them to be small.
3. What is the upper limit to a white dwarf mass? What's it called? Why does it exist?
The upper limit to a white dwarf is 1.4 Msun. This is called the Chandrasekhar White Dwarf
Limit. This exists because they more mass a white dwarf has the faster the electrons go and
since nothing can move faster than light the limit is 1.4 Msun.
4. Describe can happen to a white dwarf in a binary system
The star with more mass evolves and transfers mass to the lesser star. The star that gave
mass becomes a white dwarf. The star gets mass transferred back until it explodes.
5. What is an accretion disk?
Mass falls towards a white dwarf and his matter form a disc around the dwarf due to the
angular momentum.
6. What is a nova? How does it compare to a supernova?
A nova is when the accreted material around a white dwarf heats up to the point of nuclear
fusion and then explodes. A supernova is much more massive caused by the collaspe of a
star. A nova still leave the star intact while a supernova leaves nothing behind.
7. Compare the two different types of supernovas. What causes them? How do they appear
different on our sky?
Massive star supernova is when the iron core reaches its limit and explodes into a neutron
star. A white dwarf is when carbon fusion reaches its limit causing a total explosion of the
star.
8. What is a neutron star? How does it support itself against gravity?
A neutron star is a ball of neutrons left behind by a supernova. The degenerecy pressure
supports a neutron star against gravity.
9. How does a neutron star compare in size and mass to a white dwarf or the Earth?
A neutron star is very dense around 10km in radius and could easily fit into the gta. They
have the mass of about 1.4 Msun around the same mass as a white dwarf.
10. If a supernova goes off, what may it leave behind as a remnant?
A neutron star or a black hole.
11. What is a pulsar? How are they related to neutron stars?
A pulsar is a neutron star that beams radiation not along its rotation axis.
13. Is it possible to see a neutron star on the sky, but that same neutron star could be a
pulsar from the perspective of some alien civilization? Why is that the case?
Yes because it beams it a certain direction and you only see it when it beams in your
direction like the light from a lighthouse.
14. Why do pulsars spin so fast?
Because of the conservation of angular momentum. A star has to speed up when collapsing
in order to keep angular momentum.
15. Is there a limit to neutron star mass?
The limit is 3 Msun. At his mass the neutron star can no longer be supported degenrecy
pressure.
16. What is space-time? How does mass affect space-time? Compare a small mass object
(like the Sun) with a large mass like a neutron star.
Space-time is the dimensions of space and time fused together. Mass bends the space-time
around it. The larger the mass the more spacetime being bent. This takes place as an orbit.
Something like the sun would bend spacetime little but a neutron star heavily distorts the
spacetime around it.
17. What is a black hole?
A black hole is an object with such a strong gravitational pull not even light can escape.
Massive star supernovas can create black holes with enough mass falling into the center.
18. What is the event horizon? Schwarzschild radius?
The spherical surface of a black hole where the escape velocity is light speed is known as
the event horizon. The schwarzchild radius is the radius of the event horizon.
19. What is gravitational redshift? Time dilation?
Gravitational redshift is when light takes longer to get out of a deep hole. Time dilation is
when time passes more slowly near the event horizon.
20. What is tidal stretching? Why doesn't the Earth stretch us?
Tidal stretching is when an object is stretched by gravtional pull. The Earth has a much
larger radius than a black hole and less mass.
21. What are the two different types of black holes
Stellar black holes and super massive black holes.
22. How do we know black holes exist?
We know black holes exist because we have observed patterns of objects with masses
exceding 3 Msun and are not stars.
23. What causes Gamma Ray Bursts?
Gamma Ray burst are massive explosions of energy coming from distant galaxies that could
possibly be the formation of black holes. They can be caused by supernova explosions or
the collsion between neutron stars.
24. What are Gravitational waves? How do we measure them?
Gravitational waves are ripples in spacetime. Massive accelerating objects disrupt spacetime
causing gravitational waves. Laser Interferometer Gravitational wave Observatory (LIGO), is
used to measure them. Ligo does this by using two identical and widely separated
interferometers.
Week 9 Our Galaxy
1. How does the Milky Way appear in our night sky? Describe how it appears.
Our galaxy appears like a faint band of light in our sky. Dust gas clouds obscure our view of
the galaxy as they absorb light. The galaxy is built like a disc.
3. What are the primary structural features of the Milky Way?
Its primary features are disc, bulge, halo, globalur clusters.
4. Describe the motions of stars in the Milky Way
Stars in the disc all orbit the same directions with a little up and down motion. Stars in the
bulge have random orientations.
5. Why do orbits of stars in the disk 'bob up and down?'
Gravity of stars pull them toward the disc.
6. How can we use the motion of the Sun through the Milky Way to calculate the mass of the
Milky Way?
The sun’s orbital motion tells us the mass of the sun’s orbit which we can use to find the
mass of the milky way. The orbital speed (v) and radius (r) of an object on a circular orbit
around the galaxy tells us the mass within that orbit.
7. Describe how the Milky Way recycles gas, ie, the Star-Gas-Star cycle
Gas from old stars is recycled to make new stars. The stellar winds from high mass stars
blow out gas. Lower mass stars also return gas to interstellar space. Gas from supernova
remnants contain heavy elements. Gas blows out the disk into the halo and rains back down
when cooled down. Atomic Hydrogen gas cools forming molecular clouds. Gravity forms
stars from this gas.
8. Describe the observational evidence to back up the idea of the star-gas-star cycle.
We observe this process using wavelengths of light. Much of our view is blocked by dust
clouds in visible view. As we get to smaller wavelengths we see more of the activity of the
gasses involved in the process.
9. Where will the gas of the Milky Way be in 1 trillion years?
Locked into white dwarfs and low mass stars.
10. What is an ionization nebula? What is a reflection nebula?\
Ionization nebulae are found near high mass star indicting active star formation. Reflection
nebulae scatter the light from stars.
11. Where does star formation occurs in a spiral galaxy like the Milky Way? Why?
Star formation happens in the spiral arms. Gas gets squeezed when they move into the
spiral arms which triggers star formation.
12. Describe the differences between stars in the disk and stars in the halo
The stars in the disk are made up of young and old stars with compositions similar to the
sun. While stars in the halo are all old low mass stars that move around randomly.
13. Provide a basic description of how the Milky Way formed.
The milky was form in a huge gas cloud. Halo stars were first made and after the gas leaked
out into a spinning disc. This is where disc stars were formed later.
14. What is the evidence for a black hole at our galaxy's centre? Provide an answer that
looks at our galaxy's core across the electromagnetic spectrum
When we look into the centre our view is obscured in visible light. But as we look deeper at
in other forms of light on the electromagnetic spectrum we see the stars are orbiting
something invisible with a mass 4 million times greater than that of the sun. We can also
observed the magnetic fields that indicate something with great mass. All this evidence
suggests a black hole at the centre.
Week 10 A Universe Full of Galaxies
3. What are the main types of galaxies?
spiral, elliptical, irregular
Spirals have both disk and spheroidal components; ellipticals have no disk
Spirals tend to have more gas, thus more young, blue stars than ellipticals
Spiral galaxies tend to collect into groups of up to a few dozen and elliptical are more
common in large clusters
5. What types of colour and luminosity patterns exist for galaxies?
Elliptical galaxies have an older star population leading to red-yellow stars. Irregular tend to
have blue-white colours indicting new star formation.
6. What is a galaxy group? What type of galaxies will you often find there?
Galaxy group is when a few dozen galaxy are huddle together. Spiral galaxies are often
found in galaxy groups.
7. What is a galaxy cluster? What type of galaxies will you often find there?
Galaxy clusters are when thousands of galaxies are huddle together. You tend to find
elliptical galaxies here.
11. What is Sagittarius dwarf?
A milky way satellite in the process of being torn apart by gravity.
12. What are the Large and Small Magellanic Clouds?
They are two irregular galaxies that are satelites of the milky way galaxy.
Hubble’s law: galaxies move at speeds proportional to their distance from the earth. The
farther they are the faster they move.
Hubble’s constant times distances equals velocity
Hubble’s constant is the speed at which the universe is expanding
The hxdf is a collection of photos taken from the Hubble ultra deep field and put together to
form one picture. This lets astronomers infer the size of things and distances between
galaxies.