Astrophysics Flashcards
Describe the evolution of the universe.
- The universe is infinitely dense and hot.
During the inflation phase there is no matter and only electromagnetic radiation ( gamma photons ). - The first fundamental particles (quarks leptons) gain mass.
- The quarks combine to form hadrons such as protons and neutrons.
-The creation of matter stops when the temperature drops to 10^9K
-Protons and neutrons fuse together to form deuterium and helium nuclei. At this point 25 percent of matter is helium nuclei.
How is a proto star formed and how does it become a main sequence star?
-An interstellar dust cloud collapses under the force of gravity.
-The gpe of gas is converted into kinetic energy as the particles are drawn towards the cloud’s centre of mass.
- The gain in kinetic energy increases the temperature causing the proto-star to glow.
No fusion takes place at this point.
- Once the temperature reaches 10^7 Kelvin fusion begins to take place and hydrogen nuclei fuse into helium, mass is converted into energy in this process.
High temperatures are necessary to overcome the electrostatic repulsion between the nuclei.
-Now the proto-star has become a main sequence star.
What is meant by the big bang and what are observations that support it?
Big bang theory is the creation and expansion of the universe.
Evidences are:
- the existence of primordial helium.
-microwave background radiation.
State the cosmological principal.
The universe is homogenous and isotropic.
ISOTROPIC- the universe looks the same from ever direction.
HOMOGENOUS- matter is distributed uniformly everywhere.
What is the evolution of a star similar to our sun?
-Hydrogen fuel runs out.
-Gravity overcomes the radiation pressure.
-More gravitational potential energy is converted into kinetic energy due to the collapse and of course temperature increases even more.
-The core becomes more dense and hot and helium fuses into heavier elements.
- The star’s outer layers begin to expand and the star turns into a red giant. The outer layers are cooler hence the red.
-The star remains in this state for a shorter amount of time and runs out of helium.
-Again gravity wins and the star forms a planetary nebula which then turns into a white dwarf.
-White dwarfs are extremely dense and have a small volume.
- What opposes the crushing force of gravity is electron degeneracy pressure.
- The electrons oppose the gravitational force. No two electrons can occupy the same quantum state and so they provide an opposing force.
- A star will end its life cycle as a white dwarf only if its below the Chandrasekah limit ( 1.4 solar masses.
Evolution of a massive star.
-Massive star turns into a super red giant.
-Fuses helium into heaver elements all the way up to iron.
The nuclear fuel runs out and they could progress into a neutron star or black hole.
-Neutron star has very high density, small volume and a strong gravitational field around them.
-Black hole ( all mass has collapsed into a singularity ). Infinitely dense.
Life cycle of low mass star.
Stars with low mass are cooler so hydrogen fuel isnt depleted as quick. They remain as main sequence stars for much longer. They eventully run low on hydrogen fuel and move on to the next stage in the cycle.
Red Giants
Between O.5 and 10 (solar masses) stars will evolve into red giants.
-At the start the reduction in energy created by fusion means grvaitational force overcomes radiation pressure so the core begins to collapse.
-This pressure increases neough to make fusion occur again in shell around the core.
-Red giants have inert cores no fusion takes place as the temperature is not enough to fuse helium nuclei.
- However fusion of hydrogen to helium occurs in the shell of the core.
-This causes the star to expand and cool giving off the red colour.
