D - Astrophysics Flashcards
When was the planets formed?
Formed from a gas disk around the sun ~4.6 billion years ago
How are stars formed?
Formed from collapsing nebula gas. Forming protostar that collapse until fusion. Radiation pressure equal gravitational pressure - Hydrostatic equilibrium (on main sequence).
What is a stellar cluster?
Group of star gravitationally bound to each other. Most likely formed at the same time. Ranging from dozens to billions of stars.
- Open cluster: several to hundreds af stars younger than 10 bil. Years
- Globular cluster: Many more stars, or older than 11 bil. year stars. Contain little gas and dust. ~150 globular clusters in Milky Way halo.
What are nebulae?
Regions of intergalactic dust or gas. Stars born of matter from supernovae.
What are galactic clusters?
Groups of galaxies containing dozens to 1000 of galaxies and dwarf galaxies. Local Groups has ~30.
Form superclusters which form networks of sheets and filaments with 90% of all galaxies.
What types of galaxies are there?
Elliptical
Spiral
Irregular
How far is a light year in meters?
9.46 * 10^15 meters
What is the Astronomical unit?
Average distance between the sun and Earth.
1.5 * 10^11 m. Roughly 8 light minutes.
What is a parsec?
The distance at which 1 AU substandard an angle of 1 arc second (1/3600 a degree).
3.26 ly / 3.09*10^16 m.
What is Stellar parallax and to what extent is it useful?
It can determine distances up to 100 pc by the principle of nearby objects appearing to move relative to distant objects.
What is black Body?
The normalized intensity (intensity of wavelengths to other wavelengths.
Luminosity is relate to surface area and temperature^4
What is stellar spectra?
Absorption spectra from stars which gives the chemical composition of them, as well as their density, surface T and their rotational and translational movement!
How are stars composed?
Hot dense region surrounded by cooler lower density gas.
A stars theoretically smooth BB-curve is modified by absorption dips.
What is Wien’s displacement law?
A stars temperature times the wavelength at which its light peaks, equal a constant
What are Cepheid variables?
Extremely luminous stars with periodic changes in their size and brightness. Finished Hydrogen fusion in their cores and not in main sequence.
How are Red Giants formed?
Star moving off main sequence stars contracting due to less radiation pressure from missing H fusion in core. Outer lays get hot and fuse H, outer layers expand
What happens to a star with 8 times or less the mass of the sun?
Don’t fuse C.
Core shrinks after He fusion while emitting radiation -> outer layers blown away.
Core stops shrinking due to Pauli’s exclusion principle (limit to how close identical particles can be to each other). -> Planet size White Dwarf emit radiation slowly.
What is the fate of large stars?
Continue fusion up to Fe, lose energy. Contract - dense core cause high T causing photo disintegration (high E protons split nuclei) - free e- combine with p to form n -> Neutron star (density of 10^17kg/m!)
Contracting gas hits neutron star and bounce off (Supernova).
Option 2: Star above Oppenheimer-Volkoss limit and collapses into Black hole.
What is the Oppenheiser-Volkoff limit?
Mass between 1.4 and 3.0 the mass of the sun. Mass that cause neutron degeneracy.
What is Hubble’s law?
The velocity at which a distant object is moving away from us is proportional to the distance to it.
What is the cosmological redshift?
The redshift of EM radiation at space-time stretches.
What is the Cosmological Microwave Background (CMB)?
The “afterglow” of the Big Bang. Photons traveling in all directions after the universe turned transparent - redshifted to microwave length.
What is the cosmic scale factor?
The factor of which distances are stretched.
What is Jeans criterion?
If the mass of a gas cloud is greater than this number, it will collapse.
What type of gas cloud is most likely to collapse?
A cool dense cloud is more likely than Hotmail low density cloud. Colliding clouds can destabilize the clouds and lead to collapse.
How does Sun-like stars fuse H?
Proton-Proton Chain (PP chain) is dominant up to 1.3*Msun.
- 1,1H tunnel and form 2,1H (takes billions of years)-> 1,1H and 2,1H form 3,2He(takes 1 sec) -> 3,2He + 3,2He form 4,2He +2 1,1H (takes millions of years).
How does larger stars fuse H?
CNO cycle (carbon, Nitrogen, Oxygen). More efficient than PP chain but doesn't exclude PP chain. C+H -> N -> C+e- -> C+H -> N -> N+H -> O -> N+e- -> N+H -> C+He. Repeat...
What is nucleosynthesis and when does it happen?
Fusing heavier and heavier elements after finishing H fusion (after MS).
He+He+He -> Be+He -> C
How are elements heavier than are formed?
Neutron get close enough to nucleus that Strong force pulls them in. (Neutron capture). Neutron might decay into proton, raising the atomic number.
Slow process: In massive stars nuclides up to bismuth-209 can be formed by small neutron flux as a byproduct of C,O and Si fusion.
Rapid process: Bismuth-209 doesn’t have time to decay and even heavier elements are formed (Type II supernovae).
What is a type I supernova?
Old (relatively) low mass stars (no H-lines in absorption spectrum).
- Ia is the result of accretion of matter in binary system, often white dwarf pulling mass from companion.
- High brightness and luminosity fall off over ~6 months.
What is a type II supernova?
Result of young massive star collapsing and exploding.
What were Einstein’s assumptions?
The universe is homogenous and isotropic. (Same everywhere and in every direction)
What three shapes can the universe have?
Spherical: Finite space, closed Negatively shaped(saddle): infinite size, never return to same place. Flat and infinite, never return to same position.
What is the Critical density?
Density of matter to keep universe flat. ~10 particles/m3.
What is the cosmic scale factor?
The actual density of matter divided by the critical density.
If it equals 1: flat universe
Less than 1: Open universe that continue to expand forever.
More than 1: closed universe, eventually stop expanding - collapse in Big Crunch.
What is Dark Matter and why do we think it is there?
WIMPs (Matter that does not interact with normal matter) or MACHO’s (Massive compact halo objects eg black holes, neutron stars, brown dwarfs)
Stars in spiral galaxy should move slower further from center, they do not, so more mass must be there than we can see.
What is Dark Energy?
Potential explanation of why the expansion of the universe accelarate.
Where and what is comets?
Irregular icy/rocky/dust objects in the Keiper belt. A few kilometers across. Orbit of a few to thousands of earth years.
Tail of ice vapor pointing away from the sun.
