G485 - Modelling the Universe Flashcards
The Solar System
- the sun, 99.8% of matter in the solar system
- eight planets, relatively cold objects moving in slightly elliptical orbits
- asteroid belt between Mars and Jupiter
- moons, the natural satellites of planets
Life Cycle of a Star Like the Sun
Stages
Stellar Nebula Protostar Main Sequence Star Red Giant Planetary Nebula White Dwarf Black Dwarf
Life Cycle of a Star
Stellar Nebula
Interstellar dust and gas cloud drawn together by gravity
Life Cycle of a Star
Protostar
Loss of gravitational potential energy of gas particles and increase in kinetic energy and temperature
Hot ball of gas
No fusion
Life Cycle of a Star
Main Sequence Star
Temperature and pressure in the core is great enough for fusion of hydrogen nuclei Hydrogen nuclei fused to from helium releasing energy
Outward gas pressure is greater than gravity, the star expands
A stable star is formed when outwards gas pressure is equal to gravitational pressure
Life Cycle of a Star
Red Giant
After hydrogen fusion stops in the core, the star collapses under gravity
The core becomes hotter and pressure increases
Helium fusion in the core starts
Outwards gas pressure is greater than gravitational pressure, the star expands
This continues until helium runs out in the core and the star collapses again
Red giants can fuse up to oxygen
Life Cycle of a Star
Planetary Nebula
Pressure is not great enough for fusion to continue in the core
Fusion stop, no energy released
Outer layers drift away
Life Cycle of a Star
White Dwarf
Only the core if the star remains
Very dense, very high temperature
No fusion
Life Cycle of a Star
Black Dwarf
White dwarf gradually cools over time to form a black dwarf
When is a star brightest?
- red giant
- largest surface area
Life Cycle of a Star More Massive Than the Sun
Stages
Stellar Nebula Protostar Main Sequence Star Red Supergiant Super Nova Black Hole / Neutron Star
Life Cycle of a Star
Red Supergiant
Can fuse elements up to iron
When outward gas pressure is equal to gravitational pressure, expansion stops
When fusion stops in the core, the star collapses under gravity
Life Cycle of a Star
Supernova
The core of the red supergiant collapses then explodes outwards as a supernova
Any element can be fused
Life Cycle of a Star
Black Hole
After bigger stars supernova the core collapses to an infinitely small, infinitely dense point
Light cannot escape it
Life Cycle of a Star
Neutron Star
After smaller stars supernova the core collapses to form a neutron star
No fusion
Parsec
Definition
Distance from a base length 1AU that subtends an angle if one arc second
Approximately 3x10^16m = 3.26ly
Light Year
Definition
The distance travelled by light in a vacuum over the period of one year
Approximately 9x10^15 m
Astronomical Unit
Definition
The mean distance between the centre of the earth and the sun
Olber’s Paradox
If space is full of an infinite number of stars in an infinitely large, static universe, every line of sight should end in a star
The night sky should be if uniform brightness but it is not
Olber’s Paradox
Solutions
The space is expanding
The universe is finite
Visible light has red shifted due to the expansion of space
Light has a finite speed light that is further from Earth in ly than the age of the universe won’t have reached us yet
The Cosmological Principle
- the universe is isotropic, the same in all directions
- the universe is homogeneous, of uniform density
Hubble’s Law
Recessional speed of a galaxy is directly proportional to its distance from Earth
Redshift
Definition
- when a star, galaxy or other object is moving away from the observer the light is stretched out
- the wavelength increases and the light is red shifted
- the change in wavelength depends on the speed of recession
Redshift
Equation
∆λ/λ = v/c
v = speed of recession c = speed of light
Hubble’s Constant
H = speed of recession / distance from Earth
1/H = age of the universe
Hubble’s Law
Conclusions
Everything in the universe expanded from a single point
Space is expanding, everything is moving away from everything else
Hubble’s Experiment
Calculated recessional speed by comparing the absorption spectra of stars with the standard emission spectra of specific elements
He calculated the speed using ∆λ/λ = v/c
He measured the distance by observing the brightness of a standard type of star, s cepheid variable
He plotted a graph of distance, x, against speed, y, and found they were directly proportional
Cosmic Microwave Background
- detected in all directions
- uniform in all directions
- 2.7 K
- released about 380 000 years after the Big Bang
- as the universe expanded it was red shifted from gamma to microwave
- very small fluctuations in its temperature
The Hot Big Bang Model
Definition
- the creation of the universe from which space / time evolved
- approximately 13 billion years ago
- all of the matter and energy in today’s universe was compressed to a singularity, infinitely dense and very hot
- over time it expanded leading to cooling
- implies that the universe is not static and that it has a finite age
The Hot Big Bang Model
Evolution of the Universe
- The universe began from a singularity, extremely hot and dense
- All forces were unified
- The universe expanded which led to cooling
- Quark and lepton soup
- More matter than antimatter
- Quarks combine under gravity to form hadrons
- Imbalance of protons and neutrons, helium nuclei formed
- Atoms formed
- Gravitational force attracted matter together leading to the formation of planets and galaxies
- Temperature decreases over time to 2.7K
Critical Density
Definition
- The density at which the universe will expand towards a finite limit as the rate of expansion tends to 0 resulting in a flat universe
i. e. the gravitational force of all of the matter in the universe is equal to the force causing it to expand - The ultimate fate of the universe depends on its density
Critical Density
Equation
ρ = 3H² / 8πG
Galaxy
Definition
A cluster of many billions of stars rotating slowly around its centre of gravity
100 000s of light years across
Open Universe
Definition
If the density of the universe is smaller than critical density:
Gravity is insufficiently strong to counteract expansion, the universe will continue to expand forever
Flat Universe
Definition
If the density of the universe is approximately equal to critical density:
The universe expands more and more slowly, tends to 0, but never goes into reverse
Closed Universe
Definition
If the density of the universe is greater than critical density:
Gravity will be strong enough to eventually slow and reverse the expansion of the universe
Everything falls back towards a Big Crunch
What is the density of the universe currently believed to be?
Close to, or possibly exactly equal to, the critics, density required for a ‘flat’ cosmology
