Final Flashcards
What is the minimum temperature needed for nuclear fusion in the core?
10-15 million K
__% of the Sun’s mass is its inner half.
90%
What are neutrinos?
- Weakly interacting
- Low mass
- Fast
- Neutral particles
Needed to conserve angular momentum in beta decay (transforming into the next periodic element).
Neutrinos don’t usually interact with anything. When they do something, what do they do?
They can hit a neutron to produce a proton (beta decay).
Trillions of neutrinos pass through you every second. What was the rate of neutrino
detections for the Sudbury Neutrino Observatory?
About 3 neutrinos per hour.
What was the Solar Neutrino Problem? And the solution?
Early measurements only detected 1/3rd of the predicted amounts of neutrinos.
Turn’s out, there are three types of neutrinos, which oscillate between each other freely. We were only designing our detectors to detect one type.
Oscillations between neutrino types prove…
Neutrinos have mass!
Because of oscillation, the three types can’t have equal mass (or else there wouldn’t be different types).
What is helioseismology?
Like neutrinos, another way to study the interior of the Sun.
It’s the study of the sound waves produced due to the Sun’s pressure fluctuations.
Explain how we study helioseismology. (The Sun is an instrument?)
Sound waves bounce off the Sun to create vibrations with Doppler Shifts! The sound waves vary in pitch depending on the properties of the Sun (yup, it’s an instrument).
Explain the Sun’s Drunkard’s Walk.
Photons are produced from the core, and take a super long time to escape the Sun, since its path is completely random; scattering off & getting absorbed by other particles.
How long can it take for a photon to escape the Sun’s radiation zone?
Drunkard’s Walk = 10^5 - 10^6 years
Why does the Sun’s energy transport change from radiation (radiation zone) to convection (convection zone)?
Due to the decrease in temperature- different modes of heat.
Is energy transport the same in all star types?
Nope, it depends on mass/size –> layers.
Solar Chromosphere vs Corona
Chromosphere = outer activity layer (small red spots around solar eclipse)
Corona = outer atmosphere (white flashes around solar eclipse)
If you took a spectrum of the chromosphere during a solar eclipse, what would you see?
Emission lines (emitting photons).
What do Kirchhoff’s 3 laws imply? What are they?
They imply the way light and matter interact.
- Hot dense objects = continuous spectra
- Cool diffuse gas in front of a hot source = absorption spectra
- Diffuse gas against a dark background = emission spectra
What is a blackbody? What is a blackbody spectrum?
Blackbody: An ideal body that absorbs all incoming energy and emits a continuous spectrum of photons.
Blackbody Spectrum: The specific, continuous spectrum of a blackbody’s thermal radiation depending on its temperature.
What is the solar photosphere? Its significance?
The layer just below the active surface/chromosphere.
It’s the source of the Sun’s blackbody spectrum, and is broken into granules (top-of-convection bubbles) and sunspots.
Why are sunspots dark?
They’re cooler than the rest of the surface.
What are some applications/connections between the Sun’s magnetic field and different characteristics?
- Sunspots = pairs of spots from which magnetic field lines are concentrated and from to & from
- Flared & Prominences = twisted field lines
- Coronal Mass Ejections = giant bursts of releasing concentrated field lines (launch ionized material at Earth = solar winds = interaction with Eath’s magnetic field = auroras)
The number of sunspots indicates ____________________.
Solar activity.
Few = calm/weak
Many = angry/strong
What’s the significance of the number 11 concerning the Sun?
The Sun has an 11-year cycle in the number of sunspots, and its magnetic field reverses every 11 years.
What is the Interstellar Medium (ISM)? What is it best observed in?
The stuff (gas, dust & radiation) between stars.
Bets observed in infrared & radio wavelengths (real red/cool stuff).
A classmate suggests that the density of stars is almost constant everywhere in the disk of the Milky Way galaxy. Is this classmate correct?
Not at all
Roughly __% of the Milky Way’s stellar mass is in the ISM (gas and dust).
15% (though only 1% of this is dust mass, it’s mostly gas)
Main Components of the ISM
- Very cold MOLECULAR gas
- Cool neutral ATOMIC gas
- Hot IONIZED gas
Based on ISM temperature phases.
Dust absorbs roughly __% of all starlight.
30%
How Dust Affects Starlight
- Extinction: Light is absorbed or scattered so it doesn’t reach us (dims light).
- Reddening: Blue light is preferentially lost (makes things look redder).
- Polarization: Unpolarized light becomes polarized through dust.
If things look really red through dust, where is the line of sight? If things look really blue through dust, where is the line of sight?
Red through dust = Vantage aligned with star, dust cloud, and eye.
Blue through dust = Vantage is perpendicular to star and dust cloud.
You measure the magnitudes of a star through a thin dust cloud in the B and V filters. Is the extinction the same for these two filters (e.g., does AV = AB )?
No
How does polarization occur? Why is it useful?
Occurs when it hits a medium (like dust), it chooses the preferred orientation based on the alignment of that medium.
Ie through dust grains aligned by magnetic field, we can look at polarized light in a cloud to find the magnetic field (polarized light traces it).
The most abundant gas in the ISM is…
Neutral hydrogen (HI)
Can we use Lyman and Balmer lines (n=1 and n=2 energy level emission lines) for the ISM?
No, the ISM is too cold.
Instead, we use HI 21cm long wavelength emission measuring. It’s a very low-energy transition.
Different Ways Intersetllar Molecules Transition Between Energy States
- Electronic Transitions (interacting atoms/elements to release a photon) @>10,000 K
- Vibrational Transitions (fast vibration –> slow vibration + photon) @1,000-10,000 K
- Rotational Transitions (fast rotation –> slow rotation + photon) @10-100 K
A cloud has a temperature of 10 K. How could you best observe it?
A) neutral hydrogen (21 cm)
B) hydrogen Balmer series lines (e.g., Hα)
C) vibrational line of molecular hydrogen
D) rotational lines of CO gas
D) rotational lines of CO gas
How do stars form?
They form in the densest regions of molecular clouds, when these gas clumps collapse under gravity.
A molecular cloud collapses under gravity. What processes can help support the cloud against gravity?
- Thermal pressure
- Turbulence
- Rotation
- Magnetic field
We get collapse when gravity wins over these.
Which would collapse faster (based on the free-fall time), a large cloud or a small cloud of the same density?
Both collapse at the same rate (free fall time is merely density dependent).
For a 10 K gas cloud, the isothermal sound speed is v T = 0.2 km/s. Which gas particle
would move the fastest?
A) CO
B) NH 3
C) H 2 CO
D) H 2
E) All gas particles move at the isothermal sound speed
H2 –> it’s the lightest.
What is jeans instability?
The critical size where a cloud is supported against gravity, derived by Jeans.
Which is more likely to be unstable to collapse, a 1 M☉ cloud at 30 K or a 1 M☉ cloud at 10 K? Assume both clouds have the same density.
The 10 K cloud. Less thermal pressure to fight against gravity collapse.
Steps to Star Formation.
- Rotating material collapses inward
- Cloud shrinks, flattens & rotates faster due to conservation of angular momentum
- The center of the flat rotation material forms a protostar center bulge
- Continues to accrete material and form a “pre-main sequence” star (large, hot & inflated)
Pre-main sequence stars are slowly contracting. What should happen to their
luminosity?
Luminosity decreases
What is the Hayashi Track?
The vertical drop in luminosity on a HR diagram from red giant to main sequence as a young star contracts.
Describe the Stellar Evolution process along an HR Diagram.
Main Sequence: Hydrogen fuses into helium. Core pressure descreases, meaning:
1. Core contracts, and;
2. Core temperature increases
Then at the terminal age, hydrogen runs out and fusion stops. Gravity starts to win, core contracts more as it falls up off the MS branch. Once it’s hot enough, it starts fusing the H shell around the inert He core (red giant). Due to this, the star’s atmosphere expands, brightens, and cools.
The mass of inert He core of the red giant increases, making the core contract even more and get hotter again. When it gets hot enough, fusion of He begins (ex: explosive helium flash event). From red giant, it moves down and left on the HR diagram, to the horizontal branch stage.
It uses up all its He much quicker, and if it’s big enough it can contract to move onto to the next fusion stage… but for Sun-like stars, it’s done fusion for good and ascends the “Asymptotic Giant Branch”, where its atmosphere expands and cools, puffing out its outer atmosphere to create a planetary nebula, then descends to white dwarf.
When the core contracts and heats up, what happens to the rate of nuclear fusion?
It increases.
Stars in a cluster have similar…
- Age
- Composition
- Distance
How are Colour Magnitude Diagrams (CMDs) different from HR Diagrams? What can we learn from them?
They’re aimed at a single star population. The main sequence turnoff indicates the cluster’s age. The higher the turnoff, the younger they are.
Open vs Globular Clusters
Open Clusters: Less massive, less dense, and younger. Stars are “unbound”, so they tend to disperse (leave the cloud).
Globular Clusters: Massive, very dense, and much older. Very spherical cluster shape.
Would open or globular clusters be more metal poor?
Globular clusters, because they’re formed at the very beginning, before many supernovas and dying stars (where we got our metal from).
What type of system do you think the Sun formed in?
Open cluster (later dispersed).
Stars prone to variability (in brightness at regular intervals) are in the __________________.
Instability Strip
RR Lyrae Variables
Variable low-mass stars that are post-helium flash (between main sequence and main horizontal branch)
What is a standard candle? What is an example of one we’ve learned?
It’s a source with a known brightness, such as:
- RR Lyrae Variables
- Cepheid Variables
- Tip of the Red Giant Branch (brightness = standard helium flash brightness)
These can be used to find DISTANCE.
Cepheid Variables
Variable high-mass giant stars; yellow or super giant stars.
For stars with initial masses less than _________ the star ends in a planetary nebula and white dwarf star
less than 8 M⊙
What are white dwarf stars? Why don’t they collapse further under gravity?
They’re hot, small, dense stars. They are supported against gravity by electron degeneracy.
Pauli Exclusion Principle = electrons can’t have the same quantum numbers.
So, when protons and electrons are squished too close together, they’re forced to higher energy levels, creating the pressure required to rival gravity.
What is the Chandrasekhar limit?
The maximum mass a white dwarf can be to hold electron degeneracy: 1.44 M⊙
Explain the inverse mass-size relationship of white dwarfs.
Larger “mass” (more M⊙) means higher pressures and densities, meaning smaller “size”.
What happens if a white dwarf exceeds the Chandrasekhar limit (applicable for bigger stars)?
NEUTRONS can also exert degeneracy and can be packed even CLOSER together.
A white dwarf falls into a neutron star.
What are the “beams” observed from neutron stars?
The strong magnetic field funnels charged particles along the magnetic pole axis.
Neutron stars can rotate very quickly (a few hundred rotations per second). Why is that?
Because collapsing stars conserve angular momentum.
What are pulsars?
Young neutron stars. Neutron star spins slow down over time, so we only get strong pulses of beams traveling through our line of sight in these young stars.
What are two ways to form a supernova?
- Collapse of a massive star when gravity wins at the end of its fusion life (Type II Supernova).
- Add more mass to a white dwarf star to make it exceed the C limit (Type Ia Supernova)…
A. by mass accretion from being in a binary pair with a Red Giant (white dwarf star was much larger, and therefore reaches the end of its life much earlier)…
B. or merging with another white dwarf).
Is there any kind of degeneracy associated with black holes?
Nope. Gravity just wins entirely, collapsing into a singularity.
What is the Schwarzschild radius?
Also called the event horizon, the maximum radius beyond which nothing can escape a black hole.
What are some ways we indirectly detect black holes?
- X-ray binaries (black hole of one star eats another and produces X-rays)
- Stellar orbits (unseen mass affecting the orbit of a star)
- Gravitational waves of merging black holes
What is an example of a way we have DIRECTLY measured supermassive black holes?
The Event Horizon Telescope network & direct imaging.
Explain the Hubble Tuning Fork
Ellipticals
--> Unbarred spirals -- Lenticulars --> Barred spirals
and Irregulars at the end.
Components of the milky way barred spiral galaxy.
- Center bulge (“old” stars)
- Disk (youngest stars)
- Halo (oldest stars)
Where would you expect to find the most metal-poor stars in the milky way?
In the halo – oldest stars.
Why is it hard to see the milky way spiral structure from INSIDE the galaxy?
- Projection on the sky hides true structure
- Dust blocks light
- Not all stars are in the spiral arms
To get the distances to spiral arms you need…
1) Systems in the spiral arms
2) Their de-reddened brightness
3) Find their intrinsic luminosity (e.g.,
standard candle, spectral type)
How are spiral arms formed?
They’re spiral density waves, so clouds and stars move into the waves, compressing and rapidly rotating it- like a traffic jam.
How do we get the distance to galaxies?
We use a standard candle; Type Ia Supernovae (known magnitude).
Why are Type II supernovae not considered standard candles?
They have a wide variety of light curve shapes
What are the orbital motions in different parts of the milky way?
Disks: mostly circular and organized
Bulge: mostly random
Halo: random
What two properties are usefully linearly proportional for galaxies (scaling relation)?
Velocity of stars & mass in stars
