Astrophysics Flashcards
How is a star born?
- Nebula collapses and fragments. 2. Contraction due to gravity (GPE decrease, KE increases, T increases) (Molecular hydrogen H2). 3. Nebula density rises above critical value, therefore, thermal energy cannot easily escape (GPE decrease, KE increases, T increases) (Atomic hydrogen H) 4. Contraction continues until ‘T’ at center is bigger than or equal to 4*10^6 k. (Ionised hydrogen +,-) 5. Fusion starts - ‘hydrogen burning’ (Extremely high pressures and temperatures inside the core are needed in order to overcome the electrostatic repulsion between hydrogen nuclei). Star collapse halts, supported by i. gas pressure if M is smaller than 5 solar mass; ii. radiation if M is bigger than 5 solar mass. 6. A star is born.
What does it mean by main sequence?
A star is experiencing ‘hydrogen burning’ in the core.
What are planets?
A planet is an object in orbit around a star with three importnat characteristics:
- It has a mass large enough for its own gravity to give it a round shape.
- It has no fusion reactions.
- It has cleared its orbit of most other objects.
What are dwarf planets?
Dwarf planets have not cleared their orbit of other objects.
What are asteroids?
Asteroids are objects too small and uneven to be planets, usually in near-circular orbits round the Sun and without the ice present in comets.
What are planetary satellites?
A planetary satellite is a body in orbit around a planet. This includes moon and man-made satellites.
What are comets?
They are small irregular bodies made up of ice, dust, and small pieces of rock. All comets orbit the Sun, many in highly eccentric elliptical orbits.
What are galaxies?
A galaxy is a collection of stars, and interstellar dust and gas.
What are brown dwarfs?
Objects that are contracting due to gravity but never gets hot enough to trigegr fusion.
How do stars die?
- Stars between 0.5 solar mass and 10 solar mass will evolve into red giants. Reduction in energy released by fusion in the core —- Gravitational Force greater than Reduced Force from radiation and gas pressure —- Core collapse —- pressure increases to start fusion (H burning) in a shell around the core —- star expands (red colour) —- leaving planetary nebula and a white dwarf. 2. Stars with a mass greater than 10 solar mass —- the same —– core hot enough to trigger fusion of He nuclei (electrostatic repulsion is overcome) into heavier elements (Til Fe) —- expand (red supergiant) —- (type II) supernova —– neutron star (mass of the core is greater than Chandrasekhar limit, overcome neutron degeneracy pressure)/black hole (core has a mass greater than 3 solar mass)
Pauli Exclusion Principle
Two electrons cannot exist in the same energy state.
Electron Degeneracy Pressure
When the core of a star begins to collapse under the force of gravity, the electrons are squeezed together, and this creates a pressure that prevents the core from further gravitational collapse.
Chandrasekhar Limit
1.44 solar mass. The electron degeneracy pressure is only sufficient to prevent gravitational collapse if the core has a mass less than that.
Hertzsprung-Russell Diagram
A graph of stars in our galaxy showing the relationship between their luminosiry on the y-axis and their average surface temperature on the x-axis.
Black Body
An idealised object that absorbs all the electromagnetic radiation that shines onto it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature.
Wien’s Displacement Law
Maximum wavelength is inversly proportional to the absolute temperature T of a black body. (Wien’s constant: 2.90*10^-3 mK)
Stefan’s Law
The total power radiated per unit surface area of a black body is directly proportional to the fourth power of the absolute temperature of the black body. L = 4pir^2sigmaT^4
Astronomical Unit
The average distance between the Earth and the Sun. 150 million km.
Light-year
The distance travelled by light in a year. 9.46*10^15 m.
Parsec
The distance at which a radius of one AU subtends an angle of on second (1/3600 degree). [ d = 1/angle in arcseconds unit: parsec / d = 1 /angle in radians unit: AU ]
The galactic ladder
A set of techniques/methods for measuring disrance in space.
- direct —- stellar parallax
- standard candles (known luminosity) —- cepheid variables/Type IIa supernova
- Hubble’s Law (V = H0d)
Cosmological Principal
When viewed on a large enough scale, the Universe is homogeneous and isotropic.
What are the evidences for the Big Bang theory?
Hubble’s law and the microwave background radiation.
- If a galaxy at a distance d is moving away at a constant speed v, then a time d/v must have elapsed since it was next to our galaxy. This time is therefore roughly the age of the universe. d/v = 1/H0.
- Expansion of space over billions of years has reduced the original temperature of the universe to around 2.7 K. At this temperature the peak wavelength would correspond to about 1 mm, in the microwave region of the spectrum.
How was the universe born?
Big Bang —- time and space are created. The universe was infinitely dense and hot —- expanded rapidly (inflation). Full of eletromagnetic radiation in the form of high energy gamma photons. —- first fundamental particles (quarks, leptons etc.) gain mass through a mechanism which involves Higgs boson. —- quarks combine to form the first hadrons. Mass was created through the process of pair production. —- temperature dropped, creation of matter stopped. —- atoms were created.
Dark Energy
The expansion of the universe was accelerating. This acceleration needed a source of energy.
Dark Matter
The Doppler shift in light from galaxies found that the velocity of the stars in the galaxis did not behave as predicted. It was expected that their velocity would decrease as the distance from the center of the galaxy increases….The observations can be explained if the mass of the galaxy is not concentrated in the center…
Types of Galaxy
- spiral galaxy (disc, nuclear bulge, spiral arms, halo)
- lenticular (spiral without arms)
- elliptical (very little gas, elliptical shape)
- irregular
Possible shapes of spacetime
- flat geometry - spacetime looks like an infinite plane.
- spherical geometry - spacetime looks like a sphere.
- hyperbolic geometry - spacetime looks liek a saddle.
