Astrophysics Flashcards
1
Q
Planets
A
A celestial body that orbits the sun and has sufficient mass for its self-gravity
2
Q
Constellations
A
A collection of stars that form a recognizable pattern as viewed from Earth
2
Q
Stellar clusters
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Stars that are gravitationally bound form an open arrangement and are close to each other in space
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Q
Galaxies
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Collection of a large number of stars mutually attracting each other through the gravitational force.
4
Q
nebula
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Giant cloud of dust and gas in space comprised mostly of hydrogen and helium.
5
Q
Light year
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Distance travelled by light in a vacuum in 1 year
6
Q
Astronomical unit (AU)
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Average distance between the center of the Sun and the center of the Earth
7
Q
Stellar parallax
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It occurs when an object seems to move when it is not the case. It is when the observer changes position. The angle P has to be taken 6 months apart and is measured in arc seconds
8
Q
Luminosity of star
A
A star is a source of light and has power output in watts. The total power output is known as luminosity. It is the total energy emitted per second, and as you go nearer, it appears brighter.
9
Q
Apparent brightness
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The apparent brightness of a star is the brightness measured from the Earth.
10
Q
Apparent magnitude
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Indicates the brightness of a star as seen from Earth
11
Q
Absolute magnitude
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It indicates the brightness of a star as seen from a distance of 10 pc, parsec
12
Q
Classification of spectral classes
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O- 30000-6000K
B-10000-30000K
A-7500-10000K
F-6000-75000K
G-5000-6000K
K-3500-5000K
M-2000-3500K
13
Q
Blackbody
A
A perfect absorber and emitter of radiation. The relative amounts of each type of radiation depends only on the surface temperature of the blackbody
14
Q
Blackbody spectrum
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Intensity against wavelength. As the surface temperature of the blackbody increases:
There is an increase in all types of radiation
There is a greater increase for shorter wavelengths
The peak of intensity shifts to shorter wavelengths
15
Q
Wien’s displacement law
A
States that the wavelength at peak intensity for a blackbody is inversely proportional to the surface temperature
15
Q
Visible light spectrum
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ROYGBIV
decrease in wavelength along the spectrum
16
Q
Stefan-Boltzmann’s law
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States that the luminosity of a star is directly proportional to its surface area and directly proportional to the temperature^4. This law is used to compare luminosities of known stars
17
Q
Main sequence stars
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Burn hydrogen by nuclear fusion to obtain helium. Heavier MS stars bur faster, look brighter and hotter, bluer
18
Q
Red Giants
A
They have used up their hydrogen. Their cores contract and heat up until helium burning starts, creating carbon. Their outer layer expands and cools, they become larger, more luminous and move higher on the HR diagram
19
Q
Supergiants
A
Have used up the helium in their cores, cores contract and carbon burning starts, causing their outer layer to expand, helium burning continues in a shell
20
Q
Binary stars
A
Two stars that appear close together in the sky and are maybe physically related. There are two types of binary stars, eclipsing and spectroscopic
21
Q
Eclipsing binary stars
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2 lose stars orbiting each other, where 1 passes in front of each other
22
Q
Spectroscopic binary stars
A
As the stars move away or towards the Earth, there is a Doppler shift in their spectra. Over time, the spectral lines regularly split into 2 lines and then recombine. As 1 star approaches the observer, the other recedes, leading to Doppler shifts in opposite directions.
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Composition of a star
When a pure gaseous element is heated, it radiates a very specific wavelength. A blackbody gives out a radiation of all wavelengths, when a continuous spectrum from a blackbody is shone through a pure gaseous element, the colours it would normally give are absorbed and dark lines appear.
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Chemical composition of a star
Specific wavelengths of light correspond to specific colours and each element absorbs its own colour. The dark lines in a star's spectrum allows the chemical composition of a star to be determined
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Physical composition of a star
from the colours of the stars and the intensity-wavelength distribution, the temperature of the stars can be obtained. We can also find if the star is moving towards or away from us.
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Formation of star
Stars are formed by interstellar dust/ nebula coming together through mutual gravitational attraction. The loss of potential energy is responsible for the initial high temperature necessary for fusion. The fusion releases so much energy that the pressure prevents the star from collapsing due to gravitational pressure. The main source of energy is nuclear fusion
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Condition for fusion in a star
As the particles such as hydrogen, helium and dust move together under gravitational attraction, they lose gravitational potential energy and gain kinetic energy.
The temperature of the system increases, ionization of molecules takes place and the system acquires its own luminosity
The mass of gas is called a protostar
As gravitational contraction continues, the temperature of the core increases until it is high enough for all the electrons to be removed from the atoms in the core
The core has now become plasma and nuclear fusion takes place where hydrogen burning occurs
The protostar has now become a main-sequence star
Nuclear fusion will eventually stop gravitational contraction and will reach hydrostatic equilibrium in which gravitational pressure is balanced by the pressure from nuclear fusion
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Placement of protostar
It is determined by the initial mass.
The greater the mass, the higher will be the surface temperature and the greater the luminosity.
The mass of a protostar is compared to the mass of the sun.
Protostars with greater mass will reach the temperature required for fusion faster
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lower and upper limit of mass for a protostar
Lower than 0.08solar masses- pressure and temperature required for fusion is not reached and the protostar will contract to a brown dwarf
Higher than 100 solar masses- internal pressure from contraction will overcome gravitational pressure, vast amounts of matter will be ejected and the evolution of the star is interrupted.
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Effect of a star's mass on the end-product of a nuclear fusion
At the end of its lifetime as a main sequence star, all the hydrogen in its core has been used up. The star will grow into a red giant.
The time and fate depends on the initial mass of the star
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Changes during nucleosynthesis when when a star leaves the main sequence and becomes a red giant
Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons, which are protons and neutrons
stellar nucleosynthesis is the process by which the natural abundance of the chemical elements within star vary due to nuclear fusion reactions in its core
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Red giant or red supergiant
MS star with mass less than 4 solar mass
MS star with mass between 4 and 8 solar masses
MS star with mass greater than 8 solar mass
MS with less than 4 solar mass- fuse elements up to carbon and oxygen in the core, the outer layer burns and the star becomes a red giant
MS with 4 and 8 solar mass- Fuse even more elements such as oxygen and Neon, the Outer layer burns, and star becomes a red giant
MS with greater than 8 solar mass- fuse elements such as carbon, oxygen, neon, and iron, Outer layer burns and star becomes a red supergiant
33
The Chandrasekhar limit
the core of a star like the Sun does not keep contracting under gravity. A point is reached where the electrons cannot be packed any closer. This is called electron degeneracy.
The core is now called a white dwarf
The Chandrasekhar limit is the greatest mass that a white dwarf can have before it goes supernova, a stellar explosion that briefly outshines an entire galaxy
The core makes up to 1/3 of the mass of a star, therefore stars with mass greater than 4 solar masses will not form a white dwarf.
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The Oppenheimer-Volkoff limit
When the core of a supergiant contract, the size of the core is not limited by electron degeneracy since the core mass is larger than the Chandrasekhar limit.
The core will continue to contract causing electrons to combine with protons to form neutrons
This continues until there is neutron degeneracy
The collapse between the core and the outer layers results in a rapid rise in temperature causing a huge explosion
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star after a supernova.
Neutron stars are the densest and smallest stars to exist in the universe
A rotating neutron star is known as a pulsar
The Oppenheimer-Volkoff limit is the upper limit of the mass of a neutron star beyond which it must collapse to become a black hole, 3 solar mass
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Black hole
It is a region of space where the pull of gravity is so strong nothing can escape from it, even light
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The Big Bang Model
Using the Doppler effect and cosmological redshift.
Outside of the local group, all other galaxies show a redshift, galaxies are moving away from us, and the universe is expanding
At some point in time, it occupied a much smaller volume and the universe started with an explosion
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Recessional velocity
The velocity at which galaxies are moving away from us
This can be obtained using the Doppler effect formula in the data booklet, v/c=change in wavelength/ wavelength emitted from galaxy= fractional increase(z)
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Hubble's law
Hubble's law states that the recessional velocity of a distant galaxy is directly proportional to the distance to the galaxy.
v=Hod
The Hubble's constant gives the current rate of Expansion of the universe
The effect of gravity might slow down this rate over time
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The cosmic scale factor
It gives a measure of how big the Universe is relative to today
z=Rtobs/RTem -1
z- fractional increase
Rtobs- wcale factor at the time of observing the light, now
Rtem-scale factor at the time the light was emitted
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Age of the Universe
At the time of the Big Bang, all parts of the Universe were a singularity
From speed=distance/time,
age of Universe= separation/recessional velocity
age of Universe=1/Hubble's constant
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Cosmic background radiation (CMB)
It is microwave radiation filling the Universe from all directions. The CMB has the same strength in all directions. By measuring the Intensity of the radiation, it was found to follow the Blackbody spectrum.
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CMB and Big Bang Model
The Big Bang Model predicts an expanding Universe that had a very high temperature at the beginning
During the expansion, the Universe cools down and the temperature of the radiation should fall to its present low value, which is what CMB measures
CMB is the leftover radiation that filled the Universe just before it became transparent
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The Jeans Criterion
Stars start their life inside giant molecular clouds which are held together by gravity. They are kept from collapsing by the pressure of the molecules moving in random motion.
However, the gas can be compressed by the shock wave of an exploding star or collision between 2 stars, causing gravity to exceed pressure.
Jeans criterion is
the specific mass required for a cloud of a given radius to collapse.
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Result of shock wave on molecular gas, Jeans criterion
When the nebula is by a shock wave, the cloud collapses and forms many stars of different sizes, which can be detected from the IR radiation they emit.
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Nuclear fusion of the main sequence
When nuclear fusion takes place in the core of a star, energy is generated and hydrogen burning occurs.
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Types of nuclear fusion of stars
Mass= mass of Sun
Mass > 4 Sun
Mass roughly equal to the sun- process of the proton-proton chain
Mass greater than 4 sun- series of reactions known as CNO cycle.
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Proton-proton chain
2 hydrogen nuclei, also known as protons fuse to form one hydrogen-2 nuclei, a positron, and a neutrino
A third proton fuses to form a helium-3 nucleus and a gamma ray photon
Two helium-3 nuclei fuse to produce a helium-4 and two hydrogen -1 nuclei
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CNO cycle
Starts with proton and carbon-12 to give unstable N-13 and gamma-ray photon, then undergoes positron decay to form carbon-13 and neutrino
Fuses with proton to give Nitrogen-14 and gamma ray
+proton to give unstable oxygen-15 and gamma ray
the undergoes positron decay to form Nitrogen-15
Finishes by fusing with proton to give carbon-12 and helium
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Recap and key points of CNO cycle of a star
carbon-12 is a fuel and a product
2 positrons, 2 neutrinos and 3 gamma ray photons are emitted
The CNO cycle is part of the main sequence and no heavy elements are synthesized in the process
Fusion of hydrogen into helium takes up the majority of a star's lifetime
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Nuclear fusion after the main sequence
Helium burning starts
Nucleosynthesis is the production of different nuclides by the fusion of nuclei
two helium nuclei fuse to produce an unstable Berrylium atom
+a helium nucleus to produce a carbon-12
The carbon fuses with a helium nucleus to produce oxygen-16
the process and fusion continue until elements such as silicon and iron are obtained
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Neutron capture
Neutrons are uncharged particles and do not experience electrostatic repulsion. When neutrons approach close to a nuclei, the strong nuclear force is able to capture the neutrons
No new element is produced but instead causes the nucleon number to increase
the newly created isotope will decay to a less energetic element by emitting a gamma ray photon
The neutron in the newly formed nucleus could decay into an electron and an antineutrino
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The 2 types of neutron capture
The slow neutron capture, the S-process
the rapid neutron capture, the R-process
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The slow neutron capture, S-process
There is time for nuclides to undergo beta decay before further nucleon capture.
Nucleon number increases, producing successively heavier isotopes of the original element
This occurs in massive stars
It ends in the production of Bismuth-209
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The rapid neutron capture, R-process
There is no time for beta decay to occur
Heavier isotopes are built up very quickly and neutrons are formed
Occurs in type 2 supernovae
Produces a large number of neutrons within short periods of time
Nuclides heavier than Bismuth can be obtained
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Supernovae
Supernovae are super rare events in a given galaxy
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Type 1 supernovae
Produced by old, low-mass stars, where they exceed the Chandrasekhar limit
If the white dwarf is part of a
binary system of stars, its large gravitational field can attract matter, causing the mass of the white dwarf to increase
Contains all elements fused in the core
No hydrogen since no hydrogen in Red giants
Massive release of energy
Peak luminosity - 10^10 L of sun
57
Type 2 supernovae
Produced by young massive stars
When there is a neutron degeneracy in the core of a supergiant, the outer layers (mainly hydrogen) collapse resulting in a rapid temperature rise, causing a huge explosion that blows everything except the core
Have hydrogen line in their absorption spectra
58
Differences in Type 1 and 2 supernovae
Different spectra
Type 1 is made from an exploding white dwarf and therefore contains all fused elements in the core
Type 2 are the outer layers which are mostly hydrogen
Different change in brightness over time
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Development of the universe
The universe is expanding but gravity is slowing down this expansion
The rate of slowing down of the expansion depends on the density of the universe
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The 3 models of the Universe
Closed universe
Flat universe
Open Universe
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Closed Universe
Has got enough mass and density to slow the expansion, and even revert the expansion
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Open Universe
Has not enough matter and the universe would expand at a steady rate. Our universe is an Open Universe
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Flat Universe
Has just the right amount of matter and the expansion will eventually stop
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Critical density
Consider a homogeneous sphere of gas, a galaxy of mass m at the surface of the sphere will move with a recessional speed away from the center of the sphere
From Hubble's law, total energy can be found.
Critical density is defined as the mass that will stop the expansion of the universe with its present volume
At critical density, total energy=0, and the equation is all constants which means that it only varies with the precision of the constants
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Density parameter of the universe
It is defined as the ratio of the actual density of matter in the universe to the critical density
There are 3 possibilities for the fate of the universe depending on the density parameter
Density parameter>1, closed universe
Density parameter<1, Open universe
Density parameter=1, flat universe
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Dark matter
dark matter is a type of matter thought to be responsible for much of the mass of the universe. It can be detected only from its gravitational effects on stars and gas since they do not emit light and we cannot see it
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Two possibilities of dark matter
WIMPs-Weakly Interacting Massive Particles
MACHOs-Massive and Compact Halo Objects
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WIMPs-Weakly Interacting Massive Particles
They hardly interact
WIMPs are sub-atomic particles that are not made up of ordinary matter(neutrinos).
To produce the amount of mass needed to make up the dark matter, there would need to be an unimaginable large quantity of WIMPs.
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MACHOs-Massive and Compact Halo Objects
They include black holes, neutron stars and small stars such as brown dwarfs.
These are all high-density (compact) stars at the end of their lives and might be hidden by being a long way from any luminous object.
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Evidence of dark matter
Consider a star of mass m rotating at an orbital speed v within a spiral galaxy at total mass M and radius r.
v= constant x r, data booklet
Now, if the star is towards the edge of the galaxy, the star would be free to move with nothing affecting its orbit.
But when the speed of rotating stars is measured from the red shifts, it shows that they are moving with the same speed when are in the outer arms of the galaxy as they are in the center of the galaxy.
One explanation for this effect is the presence of dark matter which forms a halo (surround) around the outer rim of the galaxy,
This is referred to as a rotation curve
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Dark energy
It is the increase of potential energy that reduces the kinetic energy of parts of the universe causing the expansion to slow down.
