Chapter 19

Question 1

What are nebulae?

Answer 1

Gigantic clouds of dust and hydrogen gas. It eventually gives to the formation of stars and planets.

Question 2

What are planets?

Answer 2

Spherical bodies that orbit a star

Question 3

What are dwarf planets?

Answer 3

A planet that has not cleared its orbit of other objects

Question 4

What are moons?

Answer 4

Spherical bodies that orbit planets

Question 5

What are asteroids?

Answer 5

Small, irregularly shaped bodies composed of dust and metal. Usually in near circular orbits of the sun.

Question 6

What are comets?

Answer 6

Small, irregularly shaped bodies composed of dust and ice. Usually in eccentric orbits around stars.

Question 7

What is a solar system?

Answer 7

A system of planets orbiting a central star.

Question 8

What is a galaxy?

Answer 8

A collection of solar systems

Question 9

What is a universe?

Answer 9

All the galaxies and all their mass and energy

Question 10

Formation of a star

Answer 10

1. Nebulae form over millions of years as tiny particles and dust come together under the force of gravitational attraction
2. Denser regions of a nebular pull in more matter, becoming hotter as grav potential energy is transferred to heat.
3. A protostar forms in the nebula which is a very hot and very dense cloud of dust and gas.
4. If the protostar becomes massive, hot and dense enough, the grav attraction between particles is able to overcome the electrostatic attraction between the hydrogen nuclei
5. Nuclear fusion of hydrogen nuclei begins and the protostar becomes a main sequence star

Question 11

Life cycle of small star

Answer 11

1. Radiation pressure outwards from nuclear fusion balances the gravitation attraction inwards. This causes it to be stable
2. Hydrogen nuclei eventually run out in the core. Star cools down and radiation pressure drops as the rate of fusion decreases
3. The gravitational force collapses the star, causing it to heat up again as GPE is transferred to KE
4. Fusion of hydrogen nuclei in the shells of the core begin, shells start to expand and emit lower red EM waves. This stage is known as the red giant
5. Rate of fusion decreases again and the star cools. Mass of shells are lost to space and the radiation pressure drops
6. The hot core remains and is now a white dwarf
7. White dwarf cools into black dwarf

Question 12

What is electron degeneracy pressure?

Answer 12

Pressure exerted by electrons as the star collapses as a result of electrons not being able to occupy the same energy levels within atoms. This pressure counteracts the gravitational attraction meaning no further collapse is possible.

Question 13

What is the Chandrasekhar limit?

Answer 13

The limit to the mass of core which prevents total collapse of the star due to electron degeneracy pressure. The core/white dwarf can only be 1.44 times the mass of the sun. If the mass is greater, gravitational collapse is not prevented by electron degeneracy pressure.

Question 14

Life cycle of high mass stars

Answer 14

1. Hydrogen nuclei in core run out
2. Rate of fusion decreases and star cools causing the radiation pressure to drop
3. Gravitational force collapses star, causing it to heat up again
4. Fusion of helium nuclei begins in the core, star heats up and expands. Fusion of hydrogen in the out shells begin, causing it to further expand and cool. Starts to emit lower red EM waves. Red super giant
5. Sun develops an iron core. Star becomes unstable and implodes, forming a supernova. Outer shells are shed
6. The dense core reminds and either a neutron star or black hole forms

Question 15

What is a neutron star?

Answer 15

It is composed of neutrons. It’s very dense but quite small.
It is formed when the mass of the remaining core is greater than the Chandrasekhar limit.

Question 16

What are black holes?

Answer 16

If the mass of the remaining core is greater than 3 solar masses, further gravitational collapse of core causes a very dense body to form. The gravitational pull is so strong that even light can’t escape it.

Question 17

What is a solar mass?

Answer 17

Mass of the sun

Question 18

What does the energy at a given level mean?

Answer 18

How much energy is required for the electron to escape the atom and achieve zero elastic potential energy.

Question 19

What condition must be met for electrons to be excited via photons?

Answer 19

The energy of the photon must match the difference between levels

Question 20

What’s different between the excitation of electrons with accelerated electrons?

Answer 20

The accelerated electron’s energy does not have to match the difference in energy levels

Question 21

What happens to an electron which excites to a higher energy level?

Answer 21

They must de-excite either directly to the ground level or in steps. Each de-excitation emits a a photon equivalent to the difference between the two levels

Question 22

What is an emission line spectra and what can it be used for?

Answer 22

Series of spectral lines do discrete frequencies which correspond to differences between energy levels. Can be used to identify the element.

Question 23

What is a continuous spectrum?

Answer 23

A spectrum produced by a body which includes all visible frequencies of light.

Question 24

What is an absorption spectra?

Answer 24

Spectrum considering a series of dark spectral lines superimposed against a continuous spectrum. The dark lines correspond to missing wavelengths of radiation and the emission spectra of elements.

Question 25

What is a black body?

Answer 25

Object which absorbs all electromagnetic radiation that shines onto it.

Question 26

What is Wien’s displacement law?

Answer 26

Maximum wavelength is inversely proportional to temperature

Question 27

What is Stefan’s law?

Answer 27

L=4 π R^2 σ T^4
L=luminosity
R=radius
T=surface absolute temperature