Stars & Cosmology Flashcards
(31 cards)
Definition of a planet:
An object in orbit about a star
KEY DEFINING TERMS FOR PLANET:
- Mass large enough for its own gravity to give it a round shape
- No fusion reactions
- Cleared its orbit of most other objects
Definition of Planetary Satellite:
A body in orbit around a planet
(Could be man- made, could be natural)
Give an example of a natural satellite and give an example of a man-made satellite:
Natural- Moon
Man-made - Artificial satellites e.g. Hubble Telescope
Definition of a comet:
Small object in orbit around a star
- Irregular bodies made of ice, dust and rock
- Offen have highly eccentric elliptical orbits
- Some develop tails near the Sun
Definition of Solar System
A star and the objects that orbit it
E.g. the sun and all the planets that orbit it
Definition of a galaxy:
A collection of stars and interstellar dust and gas
- On average contains 100 billion stars and associated solar systems held together by gravitational attraction
- Usually have a supermassive black hole at their centre
What’s our galaxy called and what’s the galaxy nearest to us called?
Our galaxy - Milky Way
Galaxy nearest us - Andromeda
Definition of the Universe:
- All matter, all energy, space-time
Definition of a nebula:
- Interstellar dust and gas is drawn together over millions of years due to gravitational attraction forming a nebula
How do nebulas form?
- They form because of irregularities. Some irregularities are slightly denser than others as they have a higher gravitational force of attraction and draws together other dust and gas to start forming a star.
- An area of slightly higher density
Protostar
- These regions become hotter and the pressure will increase as gravitational potential energy is transferred to thermal energy
- A protostar is a hot sense sphere of dust and gas
If temperatures and pressure are high enough then:
KE of hydrogen nuclei is large enough to overcome electrostatic repulsion
Nuclear fusion of hydrogen into helium starts
Main Sequence Star
Once formed, a main sequence star remains in stable equilibrium with almost constant size because of a balance between :
- Gravitational forces acting to compress the star inwards
- Radiation pressure - from photons emitted during fusion and gas pressure (from nuclei in the core) pushing outwards
Formation of a Star Sequence
Nebula->Protostar->Main Sequence Star
Low mass main sequence star.
What’s its mass?
0.5 - 10 solar masses
What’s the sequence of a star after low mass main star?
Low mass main sequence star-> Red giant -> White Dwarf
What happens to make a low main mass sequence star to form a red giant?
- After a long time runs low on hydrogen
- Reduction of energy released by fusion means gravitational force>force from radiation and gas pressure
- Core of star starts to collapse
- Pressure increases enough to start fusion in shell around the core
- Periphery of star expands and cools into red giant
- Eventually most layers drift off forming planetary nebula
At what point does a star become a white dwarf?
- When no more fusion happens
White dwarf - has a hot dense core
Electron Degeneracy Pressure:
- Pressure exerted outwards by electrons which stops a white dwarf collapsing further
Chandrasekhar Limit:
Maximum mass for which the electron degeneracy pressure can counteract the gravitational force
- 1.44 solar masses
What is the mass of a high mass main sequence star?
> 10 solar masses
High mass main sequence star:
- After shorter time runs low on hydrogen
- Core starts to collapse and outer star expands forming red supergiant
- She’ll fusion of hydrogen and core fusion of helium
- Formation of shell structure with heavier and heavier elements fusing in core until iron is formed
- Core collapses and if mass>Chandrasekhar limit, electron degeneracy can’t stop collapse
- Shockwaves cause star to explode in a supernova leaving neutron star or black hole
Supernova
- Brief rapid increase in brightness (could outside a galaxy)
- Creates all elements heavier than iron
- Distributes heavier elements throughout universe
When is a neutron star formed?
If core of red supergiant is 1.4 - 3 solar masses
Neutron Star
- Electrons combines with protons to form neutrons
- Density similar to that of a nucleus
- Small (20km) , rotate very fast (600Hz), some are pulsars
