Astrophysics Flashcards
What is a planet?
An object in orbit around a star with 3 important characteristics:
- Has a mass large enough for its own gravity to give it a round shape
- Has no fusion reactions
- Has cleared its orbit of most other objects
What is a planetary satellite?
A body in orbit around a planet
What is a comet?
A small irregular body made up of ice, dust and small pieces of rock. They all orbit the sun, mainly in high eccentric elliptical orbits
What is a solar system?
A star and the gravitationally bound objects that orbit it
What is a galaxy?
A cluster of billions of stars that are held together by gravity
What is the universe?
A large collection of billions of galaxies
What is the observable universe?
The portion of the universe from which EM radiation has had time to reach Earth since the formation of the universe
What stages are the same for all stars in their life cycles?
The first 4 stages
What are the 2 life-cycle branches?
- Low mass: mass between 0.5 and 10 times the mass of the sun
- High mass : mass over 10 times the mass of the sun
What are the first 4 stages in the life cycle of stars?
1) Nebula
2) Protostar
3) Nuclear Fusion
4) Main Sequence Star
Explain the first 4 stages in the life cycle of stars
1) NEBULA: all stars form from a giant cloud of hydrogen gas and dust called a nebula. Gravitational attraction between individual atoms forms denser clumps of matter. This inward movement of matter is called gravitational attraction
2) PROTOSTAR : the gravitational collapse causes the gas to heat up and glow, forming a protostar. Work done on particles of gas and dust by collisions between the particles causes an increase in their KE, resulting in an increase in temperature. Protostars can be detected by telescopes that can observe infrared radiation
3) NUCLEAR FUSION: eventually the temperature will reach millions of degrees kelvin and the fusion of hydrogen nuclei to helium nuclei begins. The protostars gravitational field continues to attract more gas and dust, increasing the temperature and pressure of the core. With more frequent collisions, the KE of the particles increases, increasing the probability that fusion will occur. 4 hydrogen nuclei are fused into 1 helium nucleus, producing 2 gamma ray photons, 2 neutrinos and 2 positrons. Massive amounts of energy are released. The momentum of gamma ray photons results in an outward pressure called radiation pressure
4) MAIN SEQUENCE STAR: the star reaches a stable state when the inward and outward forces are in equilibrium. As the temperature of the star increases and its volume decreases due to gravitational collapse, the gas pressure increases. The gas pressure and the radiation pressure act outwards to balance the gravitational force acting inwards
- If the temperature of the star increases, the outward pressure will also increase - this causes the star to expand
- If the temperature of the star decreases, the outward pressure will also decrease - this causes the star to contract
- If the 2 forces are balanced, the star will remain stable and a main sequence star
What is a low mass star?
A star that has a mass between 0.5 times and 10 times and mass of the sun
What is the evolution of a low mass star?
1) Nebula
2) Protostar
3) Main sequence star
4) Red giant
5) Planetary nebula
6) White dwarf
7) Black dwarf
Explain the evolution of a low mass star after the generic initial stages
1) RED GIANT: most of the hydrogen nuclei in the core of that star has been fused into helium and so nuclear fusion slows and the energy released from the fusion decreases. The radiation pressure, therefore, also decreases, so the gravitational forces become greater than the outward forces. The core collapses, causing a temperature increase as it compresses. Fusion in the core stops and the outer layers of the star expand and then cool forming a red giant.
There are still hydrogen nuclei in the areas outside of the core and the heat generated by the collapsing core provides temperatures high enough for this hydrogen to fuse in a process called shell hydrogen bonding. Contraction of the core continues, providing temperatures high enough to fuse helium into carbon and oxygen in a process called helium core burning
2) PLANETARY NEBULA: helium in the core releases massive amounts of energy in the fusion reactions. The outward radiation pressure increases, balancing the inward & outward forces. Once the helium in the core runs out, the core contracts, producing high enough temperatures for the helium outside the core to burn (helium shell burning). The carbon-oxygen core is not hot enough to fuse the heavy elements so the star becomes unstable and begins to collapse again. The outer layers of gas are ejected back into space, forming a planetary nebula
3) WHITE DWARF: the solid core collapses under its own mass, leaving a very hot, dense core called a white dwarf - no further fusion reactions take place. White dwarfs continue to radiate energy in the form of photons that were produced in previous fusion reactions
4) BLACK DWARF: eventually, the white dwarf will cool a few degrees kelvin and will so longer emit any significant light or heat
What are the characteristics of a white dwarf?
- Very hot
- Dense & solid
- Nuclear fusion
- Radiates photons from previous fusion
What is electron degeneracy pressure?
When the core of a star collapses, matter is compressed into a very small volume. This means that the electrons in atoms are no longer able to freely move between energy levels. Electrons are forced to fill the available energy levels.
- Electrons fill the lowest available energy levels
- Usually only excited electrons will fill the higher energy levels
- Compression of the matter in a collapsing core forces electrons into higher energy levels, not because they are in a higher energy state, but because there is nowhere else to go
- This rush of electrons to find an available space creates a pressure called electron degeneracy pressure, resulting in an outward acting force
What is the chandrasekhar limit?
The maximum mass of a stable white dwarf star - the mass of a core is up to 1.4 times the mass of the sun
What happens if a white dwarf exceeds the chandrasekhar limit?
- Electron degeneracy pressure can no longer prevent the collapse of the core
- Protons & electrons combine to become neutrons - forming a neutron star
What is a high mass star?
A star that has a mass of over 10 times that of the suns
What is the evolution of a high mass star?
1) Nebula
2) Protostar
3) Main sequence star
4) Red Supergiant
5) Supernova
6) Neutron star
7) Black hole
Explain the evolution of a high mass star after the generic initial stages
1) RED SUPERGIANT: the hydrogen nuclei in the core runs out and so the nuclear fusion slows down. The energy released decreases with thus and so does the radiation pressure - this means the the outward forces decreases and the forces are no longer in equilibrium. This causes the core to collapse. Fusion in the core stops and the outer layers of the star cool and expand to for a red supergiant.
The shell and core burning cycle in massive stars goes beyond that of low mass stars. Shell burning occurs due to high temperatures and nuclei are fused around the core. Contraction of the core generates high enough temperatures to fuse heavier elements in the core; this cycle continues fusing heavier and heavier elements until an iron core is formed
In each stable fusion phase, electron degeneracy pressure and radiation pressure balance the gravitational force and prevent the core from collapsing. If the mass of the core is less than 1.4 times the mass of the sun, the the electron degeneracy pressure is not enough to prevent the core from collapsing
2) SUPERNOVA: once the iron core forms, it becomes unstable and begins to collapse as no more fusion reactions can occur. The GPE transferred in the collapse produces intense heating. Gravitational pressure forces protons & electrons in the iron atoms to combing to form neutrons, releasing huge amounts of energy. The outer layers of the star fall inwards and rebound off the core causing shockwaves. The shockwaves cause the star to explode in a supernova. The supernova generates temperatures great enough to fuse heavy nuclei with neutrons to form all the known elements beyond iron
3) NEUTRON STAR: after the supernova explosion, the collapsed neutron core can remain intact - this is known as a neutron star
4) BLACK HOLES: if the neutron core mass is greater than 3 times the solar mass, the pressure on the core becomes so great that the core collapses even further. The gravitational forces are so strong that the escape velocity of the core is greater than the speed of light, hence the photons are unable to escape. This is a black hole
What are neutron stars?
Objects formed from stars which cores have masses greater than the Chandrasekhar limit. They are EXTREMELY dense and very small.
What are black holes?
What form when a core collapses into an infinitely dense point called a singularity
What is at A?
White dwarves
What is B?
Main sequence star
What is C?
Red supergiant
What is D?
Red giants
What is continuous line spectra?
This is where photons emitted from the core of a star contain all the wavelengths and frequencies of the EM spectrum. They are produced fro hot, dense sources, such as the core of stars
What is emission line spectra?
- When an electron transitions for a higher energy level to a lower energy level, this results in the emission of a photon
- Each transition corresponds to a different wavelength of light and this corresponds to a different line in the spectrum
- They are produced by hot, low pressure gases
What is absorption line spectra?
- When white light passes through a cool, low pressure gas it is found that light of certain wavelengths are missing; this type of spectra is absorption line
- It is a continuous spectrum with dark lines at certain wavelengths
- These dark lines correspond exactly to the difference in energy levels in an atom
- When these electrons return to lower levels, the photons are emitted in all directions rather than in the original direction of white light - therefore, some wavelengths appear to be missing
- The wavelength missing from an absorption spectrum are the same as their corresponding emission spectra from the same element
