D - Astrophysics

Question 1

When was the planets formed?

Answer 1

Formed from a gas disk around the sun ~4.6 billion years ago

Question 2

How are stars formed?

Answer 2

Formed from collapsing nebula gas. Forming protostar that collapse until fusion. Radiation pressure equal gravitational pressure - Hydrostatic equilibrium (on main sequence).

Question 3

What is a stellar cluster?

Answer 3

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.

Question 4

What are nebulae?

Answer 4

Regions of intergalactic dust or gas. Stars born of matter from supernovae.

Question 5

What are galactic clusters?

Answer 5

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.

Question 6

What types of galaxies are there?

Answer 6

Elliptical
Spiral
Irregular

Question 7

How far is a light year in meters?

Answer 7

9.46 * 10^15 meters

Question 8

What is the Astronomical unit?

Answer 8

Average distance between the sun and Earth.

1.5 * 10^11 m. Roughly 8 light minutes.

Question 9

What is a parsec?

Answer 9

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.

Question 10

What is Stellar parallax and to what extent is it useful?

Answer 10

It can determine distances up to 100 pc by the principle of nearby objects appearing to move relative to distant objects.

Question 11

What is black Body?

Answer 11

The normalized intensity (intensity of wavelengths to other wavelengths.
Luminosity is relate to surface area and temperature^4

Question 12

What is stellar spectra?

Answer 12

Absorption spectra from stars which gives the chemical composition of them, as well as their density, surface T and their rotational and translational movement!

Question 13

How are stars composed?

Answer 13

Hot dense region surrounded by cooler lower density gas.

A stars theoretically smooth BB-curve is modified by absorption dips.

Question 14

What is Wien’s displacement law?

Answer 14

A stars temperature times the wavelength at which its light peaks, equal a constant

Question 15

What are Cepheid variables?

Answer 15

Extremely luminous stars with periodic changes in their size and brightness. Finished Hydrogen fusion in their cores and not in main sequence.

Question 16

How are Red Giants formed?

Answer 16

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

Question 17

What happens to a star with 8 times or less the mass of the sun?

Answer 17

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.

Question 18

What is the fate of large stars?

Answer 18

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.

Question 19

What is the Oppenheiser-Volkoff limit?

Answer 19

Mass between 1.4 and 3.0 the mass of the sun. Mass that cause neutron degeneracy.

Question 20

What is Hubble’s law?

Answer 20

The velocity at which a distant object is moving away from us is proportional to the distance to it.

Question 21

What is the cosmological redshift?

Answer 21

The redshift of EM radiation at space-time stretches.

Question 22

What is the Cosmological Microwave Background (CMB)?

Answer 22

The “afterglow” of the Big Bang. Photons traveling in all directions after the universe turned transparent - redshifted to microwave length.

Question 23

What is the cosmic scale factor?

Answer 23

The factor of which distances are stretched.

Question 24

What is Jeans criterion?

Answer 24

If the mass of a gas cloud is greater than this number, it will collapse.

Question 25

What type of gas cloud is most likely to collapse?

Answer 25

A cool dense cloud is more likely than Hotmail low density cloud. Colliding clouds can destabilize the clouds and lead to collapse.

Question 26

How does Sun-like stars fuse H?

Answer 26

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).

Question 27

How does larger stars fuse H?

Answer 27

``` 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... ```

Question 28

What is nucleosynthesis and when does it happen?

Answer 28

Fusing heavier and heavier elements after finishing H fusion (after MS). He+He+He -> Be+He -> C

Question 29

How are elements heavier than are formed?

Answer 29

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).

Question 30

What is a type I supernova?

Answer 30

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.

Question 31

What is a type II supernova?

Answer 31

Result of young massive star collapsing and exploding.

Question 32

What were Einstein's assumptions?

Answer 32

The universe is homogenous and isotropic. (Same everywhere and in every direction)

Question 33

What three shapes can the universe have?

Answer 33

``` Spherical: Finite space, closed Negatively shaped(saddle): infinite size, never return to same place. Flat and infinite, never return to same position. ```

Question 34

What is the Critical density?

Answer 34

Density of matter to keep universe flat. ~10 particles/m3.

Question 35

What is the cosmic scale factor?

Answer 35

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.

Question 36

What is Dark Matter and why do we think it is there?

Answer 36

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.

Question 37

What is Dark Energy?

Answer 37

Potential explanation of why the expansion of the universe accelarate.

Question 38

Where and what is comets?

Answer 38

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.