Chapter 5.5 - Astrophysics and Cosmology Flashcards
Nuclear Fusion
The process of two nuclei joining together and releasing energy from a change in binding energy
Planet
Large bodies that move in circular or elliptical orbits around a star
Planetary Satellite
A smaller body than a planet that orbits the planet (e.g the Moon)
Comets and where they come from
Rocky ice balls that travel in highly elliptical orbits around the Sun. Come from the Oort cloud
Solar System
A star (or a binary star pair) orbited by one or more planets
Galaxy
A cluster of many millions of stars
The Universe
The space in which everything exists
Gravitational Collapse
The inward movement of material in a star due to the gravitational force caused by its own mass
When gravitational collapse happens
- When a star is formed the cloud of gas undergoes gravitational collapse
- In a mature star when the internal gas and radiation pressure can no longer support the stars own mass
Radiation Pressure
An outwards pressure caused by the momentum of photons released in fusion reactions
Gas Pressure
An outwards pressure caused by the movement of the high energy gas particles inside a star
Main Sequence Star
A star in the main part of its life cycle, where it is fusing hydrogen to form helium in its core. The star is stable since the gas pressure and radiation pressure counteract the gravitational force
Red Giant
A star in the later stages of its life that has nearly exhausted its hydrogen supply and is now fusing helium. It is larger than a main sequence star and the outer layers are cooler, giving it its red colour
White Dwarf
The end product of the life cycle of a low mass star. It is very dense but does not undergo any fusion and will slowly cool down. It does emit light as photons from past fusion reactions leak away
Planetary Nebula
An expanding shell of ionised hydrogen and helium ejected from a red giant star at the end of its life
Electron Degeneracy Pressure
An outwards acting pressure that prevents stars of mass beneath the Chandrasekhar limit from collapsing further. Since two electrons cannot occupy the same states in an energy level of an atom, when electrons are being pulled into the star due to gravity they will reach a point where they cannot be added to the volume of the star. This has the effect of exerting an outwards force.
Chandrasekhar limit
1.4 times the mass of our Sun.
The mass at which a Star will collapse further than a white dwarf and become either a neutron star or a black hole
Red Super Giant
A Red Giant that has a mass much higher than that of our Sun
Supernova
An explosion produced when the core of a red super giant collapses
Neutron Star
The remnants of the core of a red super giant after it has undergone a supernova explosion. It is very dense and composed mainly of neutrons
Black Hole
The core of a massive star that has collapsed almost to a point. They are incredibly dense and their gravitational field is so strong that, past the event horizon, not even light can escape
Hertzsprung-Russel diagram
A luminosity-temperature graph
Luminosity
The total energy that a star emits per second
How stars form
Dust and gas come together through gravitational attraction. The work done on moving these particles increases their kinetic energy resulting in an increase in temperature. This large core is called a protostar. The gravitational field of the protostar will attract more matter until the temperature and the pressure in the core is enough for the hydrogen to fuse and create helium. The gravitational pressure will become balanced with the gas pressure and radiation pressure from the fusion. It is now a main sequence star.
Overall fusion reaction
4 proton goes to
- helium-4 (2 proton + 2 neutron)
- 2 positrons
- 2 neutrinos
- gamma rays
Lifecycle of a star of mass beneath the Chandrasekhar limit
Stellar Nebula -> Main Sequence
Hydrogen runs out and fusion halts, stopping the outwards pressure and causing the core to collapse and the outer layers to expand and cool as the star becomes a red giant. Core collapses further until helium fuses into carbon and oxygen preventing the core from collapsing further. Once all the helium is fused the core collapses further and ejects its outer layers which form a planetary nebula. The remaining core is a white dwarf which is stable as gravitational forces are counteracted by the electron degeneracy pressure
Lifecycle of a star of mass greater than the Chandrasekhar limit (up to supernova)
Stellar Nebula -> Main Sequence
Hydrogen runs out and fusion halts, stopping the outwards pressure and causing the core to collapse and the outer layers to expand as the star becomes a red super giant. As the core collapses, heavier elements are created by fusion. At each stable fusion stage, the further collapse of the core is prevented by the electron degeneracy pressure and the radiation pressure. Once an iron core has built up the fusion will stop and the core will undergo further gravitational collapse. The immense gravitational forces force protons and electrons to combine to form neutrons, which releases an incredible amount of energy causing a supernova as the outer shell is blown off. During a supernova heavier elements the iron can be formed when the nuclei fuse with neutrons.
What happens after a supernova
Depending on the mass remaining in the core a neutron star may be formed. These are very small and have very high density and are composed mostly of neutrons. The magnetic field of the neutron star can cause the star to emit vast amounts of high energy radiation from its poles. This is called a pulsar.
If the neutron star is massive enough the pressure can become so large that the neutron star would collapse to a point and become a black hole
Continuous spectrum
A spectrum that contains all wavelengths over a wide range
Energy Level
Discrete energies that electrons can have when occupying specific orbits.
Emission line spectrum
The spectrum of frequencies emitted due to electron transitions from a higher energy level to a lower one
Absorption line spectrum
The pattern of dark lines that would appear on a continuous spectrum when the spectrum is shone through a medium that would absorb some frequencies of light
Energy of energy levels
0 at infinity]
-13.6eV at the lowest energy level
varying between these but the energy level is always negative
Ground State
The lowest energy level
Emission spectrum source and how they appear
They come from hot gasses when an excited electron moves to a lower energy level and emits a photon.
