Midterm 2 Flashcards
Preparation
How does Earth’s atmosphere affect observations?
- The atmosphere transmits light at radio and optical wavelengths, but absorbs most light at other wavelengths
- The atmosphere “blurs” our view of distant objects = decrease in angular resolution
- Solution: Adaptive Optics
Adaptive optics
Using another star or creating an “artificial star” using a laser, to distinguish stars between one another in the sky
Where is the best location for a telescope?
On Earth:
- High sites(above the atmosphere)
- Dry(good weather)
- Dark(minimal light pollution)
In space:
- Required for most wavelengths
- High angular resolution and optical wavelength
What prevents Sun and other stars from collapsing under the weight of their own gravity?
- Gas pressure opposes the weight of gravity
- Equilibrium between gravity and pressure keeps the Sun from collapsing
- Extremely high pressure in the center of the Sun results in extremely high density and temperature
Why does the Sun shine? How is energy produced in the Sun and other stars?
- Nuclear fusion - the centre of the Sun is so hot and Dense that H can fuse into He
- This provided the source of energy ( E = mc 2)
- Verified by measurements of neutrinos in the Sun
- Chemical and gravitational energy cannot explain the Sun’ luminosity = only enough for ~ 25 million years
How is energy transported outward from the center of the Sun?
- Radiative diffusion : photons “scatter”(randomly bounce) outward through the radiation zone
- Convection : rising hot plasma carries thermal energy through the convection zone, to the photosphere
What is the internal structure of the Sun?
- Core(where fusion happens)
- Radiation zone(where photons scatter)
- Convection zone(where hot gas rises and cool gas sinks)
- Photosphere(where light escapes and becomes visible)
How to measure properties of stars?
- Luminosity(from flux and distance)
- Temperature(from color and spectrum)
- Radius(surface brightness and luminosity)
- Mass(
How do star properties vary in different stars?
- Main sequence: most stars lie along the main sequence of Luminosity and Temperature. They all fuse H in their cores
-More massive stars are more luminous, hotter and larger - Red giants - low temperature, higher luminosity, larger radius
- White dwarfs - high temperature, lower luminosity, small radius
What causes the differences in stellar properties along the Main Sequence?
- Mass of the star = core pressure, temperature, fusion rate(=luminosity)
- More mass = hotter, larger, more luminous
- More mass = shorter lifetime
How do properties of stars change with time?
Life-track of a Sun-like star:
1. Main Sequence
2. Red Giant
3. Planetary Nebula
4. White Dwarf
Why are Red Giants giant?
-Hydrogen fusion is occurring in a ‘shell’ outside of the core, producing pressure which pushes up on the material above
- Larger energy output acting on less mass = outer layer expands to a huge radius
What conditions are required for elements heavier than Hydrogen to fuse?
Higher core temperatures
Why can high-mass stars use more varieties of fuel throughout their life-spans, than low-mass stars?
- More massive stars reach hotter core temperatures
- They have more gravitational potential energy to convert into thermal energy
What is a white dwarf?
The “naked” core of a star after it lost most of it’s layers in the “stellar wind” during the red giant phase
What is a planetary nebula?
The outer layers of a star which have been ejected by the “stellar wind”, forming a shell of gas around the white dwarf(The surrounding gas is illuminated by the white dwarf)
What are the life stages of a high-mass star?
- Main sequence
- Red giant
- Planetary nebula
- White dwarf
+fuse heavier elements after leaving the main sequence
How do high-mass stars die?
- Iron core collapses since it cannot release any more energy from fusion
- The core turns into a gigantic ball of neutrons
- Leads to a core collapse SUPERNOVA
What are the major ideas of General relativity?
Equivalence principle - effects of gravity = effects of acceleration
- Space + time = 4-dimensional spacetime
What is Einstein’s view of gravity?
- Gravity arises from “curvature” of spacetime
- Mass causes spacetime to curve, curvature determines the motion of objects
- Gravitational lensing - deflection of light when it travels near massive objects
Gravitational lensing
Deflection of light when it travels near massive objects
What is a black hole?
A region where spacetime curves so much that time appears to completely stop
- Nothing can escape, not even light
- Happens when large mass is packed into a small volume
How can a White Dwarf explode?
- Exceeds Chandrasekhar mass(whatever…)
- Can happen if they gain mass from a binary companion
What are the results of stellar evolution and how do the results depend on mass?
- Stars with mass > 8 MSun = > Burn all possible nuclear fuel => Core collapses into a NEUTRON star.
- Stars with mass < 8 MSun => Will not burn their fuel => Turn into a WHITE DWARF
- Stars with mass < 0.08 MSun => BROWN DWARFS
What evidence do we have that neutron stars exist?
- Pulsars: massive, rapidly spinning objects
- Must be very small because of their rapid rotation
What evidence do we have that black holes exist?
- We observe massive objects orbiting in binary systems with visible stars
- Masses are too large to be neutron stars
What does the Milky Way look like?
- Disk - stars, gas and dust. Young stars are found in SPIRAL ARMS
- Bulge - spherical. Stars are located in the center
- Halo - surrounds the other 2, has the remaining stars
How do stars orbit within our galaxy?
- Disk - orbit within the plane of the disk, moving in the same direction
- Bulge and halo - random directions
What lies in the centre of our galaxy?
A large amount of invisible mass - a SUPERMASSIVE black hole
What is the spectral sequence?
O - only
B - bad
A - astronomers
F - forget
G - gravity
K - kinetic energy
M - mass
L
T
Y
Neutrinos
Neutrinos are produced by fusion reactions in the Sun’s core, and are observable from Earth, allowing us to “see” into the center of the Sun.
End of life for low-mass stars
- Red Giant
- Stellar wind pushes masses from the outer shells into space
- The star can’t fuse heavier elements, because the core cools down
- Turns into a planetary nebula
- Turns into a white dwarf
- Cools down over time(Dark dwarf)
- IF mass surpasses the 1.4 because of a binary companion = explodes
- Thermonuclear(white dwarf) supernova
End of life for high-mass stars
- Red Giant - fuses heavier elements
- Core fuses into Iron, no longer can generate energy from nuclear fusion
- Core collapses into a ball of neutrons = Neutron star(OR a Black Hole)
- Creates a core-collapse supernova
