Stars & Cosmology Flashcards
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
When is a black hole formed?
If core of red supergiant >3 solar masses
Black Hole
- Core collapses to a singularity (infinitely dense point)
- Gravitational field so strong that the escape velocity is greater than the speed of light
- Boundary of region is called event horizon (where escape velocity = speed of light)
Define Stellar Parallax
Apparent motion/ displacement of a star relative to the position of more distant stars. Caused by Earths orbit about the Sun.
How is parallax used to measure distances in space?
-Apparent motion/ displacement of a star relative to the position of more distant stars.
-Caused by Earths orbit about the Sun.
- An angle of parallax of 1 arc second when displacement of Earth is 1AU corresponds to distance 1pc
Describe the Doppler Effect
Observed frequency is different to source frequency when source moves relative to observer.
State Hubble’s Law
Recessional velocity of a galaxy is directly proportional to its distance from us (Earth)
State the cosmological principle:
The universe is homogeneous and isotropic (on a large scale) and the laws of physics are universal
