5.5 Astrophysics and cosmology Flashcards
What is nuclear fusion?
Nuclear fusion is the process of two nuclei joining together and releasing energy from a change in binding energy.
Describe a galaxy.
A galaxy is a cluster of many millions of billions of stars. Starts in a galaxy rotate around the galaxy’s center of mass.
How many years does it take for the sun to undo go one complete revolution of the galaxy?
230 million years
How do stars form?
Stars are formed when huge amounts of matter, gas or interstellar dust, are pulled together under the force of gravity. If there is enough matter to create the right conditions of extremely high temperature and density, nuclear fusion begins, releasing enormous amounts of energy as electromagnetic radiation.
What gas mostly makes up stars, what does this fuse to?
The vast majority of the gas present is hydrogen, which fuses into helium.
What is the mass of the sun?
2x10^30kg
Define a planet.
Bodies that move in circular or elliptical orbits around a star to form a solar system.
How are planets formed?
Planets are formed when interstellar dust is attracted into larger clumps, which accrete to form larger bodies.
What planets are being formed, why doesn’t nuclear fusion occur?
During the formation of a planet, there is less matter involved compared to when a star is being formed. This means planets don’t give out their own light.
What are planetary satellites?
Smaller bodies than the planets themselves can orbit the planet. For example, the earth has one planetary orbit called the moon.
How long does it take for the moon to complete a full revolution of the earth?
Just over 27 days.
What are comets?
Comets are large, rocky ice balls that travel in highly elliptical orbits around the sun.
How long ago was the big bang?
13.8 billion years ago.
What is the diameter of the observable universe?
93 billion light years.
What is gravitational collapse?
Gravitational collapse is the inward movement of material in a star due to the gravitational force caused by its own mass. Star formation is due to the gradual gravitational collapse of a cloud of gas and dust. Gravitational collapse occurs in a mature star when the internal gas and radiation pressure can no longer support the star’s own mass.
What is radiation pressure?
Radiation pressure is due to the momentum of photons released in fusion reaction, and acts outwards (in the direction of the energy flow).
What is the main sequence in the life cycle of a star?
A main sequence star is a star in the main part of its life cycle, where it is fusing hydrogen to form helium in its core.
What is a main sequence start shown as?
The main sequence stars are shown as a curved band on a plot of a star’s luminosity against temperature.
What is a red giant?
A red giant is a star in the later stages of its life that has nearly exhausted the hydrogen in its core and is now fusing the helium nuclei. It is bigger than a normal star because the surface layers have cooled and expanded.
What is a white dwarf?
A white dwarf is the end product of a low-mass star, when the outer layers have dispersed into space. A white dwarf is very dense, with a high surface temperature and low luminosity.
What is a planetary nebula?
A planetary nebula is an expanding, glowing shell of ionised hydrogen and helium ejected from a red giant start at the end of it’s life.
What is electron degeneracy pressure?
Electron degeneracy pressure is the pressure that stops the gravitational collapse of a low-mass star. This is the pressure that stops a white dwarf star from collapsing.
What is the Chandrasekhar limit?
The Chandrasekhar limit is the maximum possible mass for a stable white dwarf star and it equal to 1.4 times the mass of our sun. White dwarfs with masses above this will collapse further to become neutron stars or black holes.
What is a red super giant?
A red super giant is a star that has exhausted all the hydrogen in its core and have a mass much higher than the sun.
What is a supernova?
A supernova is a huge explosion produced when the core of a red giant collapses.
What is a neutron star?
A neuron star is the remains of the core of a red super giant after it has undergone a supernova explosion. It is incredibly dense and composed of mainly neutrons.
What is a black hole?
A black hole is the core of a massive star that has collapsed almost to a point. Black holes are very dense and very small, with a gravitational field strength so strong the light cannot escape (the escape velocity is greater than the speed of light).
What is a Hertzsprung-Russell (HR) diagram?
A Hertzsprung-Russell (HR) diagram is a luminosity-temperature graph.
Define luminosity of a star.
The luminosity of a star is the total energy that the star emits per second.
What is a protostar?
As the gravitational force pulls more and more matter together, work is done on the particles of gas and dust, leading to an increase in kinetic energy. This results in an increase in temperature until some of the denser areas of gas become hot enough to glow. This large core of material is called a protostar.
How can protostars be detected?
Protostars can only be detected through telescopes designed to observe infrared radiation, as the clouds of gas and dust around them (nebulae) absorb and scatter most visible light.
What happens once the temperature and pressure of the protostar becomes much greater?
The star will reach millions of degrees kelvin and the increased kinetic energy increases the chance of fusion reactions will begin.
What happens during a fusion reaction in a protostar?
Thew overall effect of the fusion of hydrogen nuclei to helium nuclei is that 4 protons are 1 helium-4 nucleus with the production of 2 gamma ray photons, 2 neutrions and 2 prositrons. During this enourmous amount oif energy will be produced.
What does the momentum of photons released by fusion lead to?
Leads to an outwards force called radiation pressure.
In a star of stable size, what will the radiation pressure be equal to?
The radiation pressure and the gas pressure are in equillibrium.
What is the inwards gas pressure in a star caused by?
The kinetic energy of the gas atoms.
When a star has a radiatrion pressure and a gas pressure in equilibrium, what stage if it’s life is it at?
It is a main sequence star.
At what stage will a star remain for the majority of it’s life?
When it is a main sequence star.
What do main sequence stars fuse in their cores?
They fise hydrogen to make helium in their cores.
What is the mass of a main sequence star in regards to the sun?
The mass of a main sequence star can vary about 10% of the mass of the sun up to 200 times the mass of the sun.
How does a stars life time change depending on it’s mass?
The greater the mass of a star, the shorter its life time will be.
How much power output is the sun currently producing?
4 x 10^26W
What will happen to a star 1.4 times the mass of the sun at the end of its hydrogen fusion phase?
It will kove off the main sequence anf first become a red giant before becoming a white dwarf.
When will a low-mass star become a red giant?
When most of the hydrogen nuclei present in the core have been fused to helium. Nuclear fusion will stop.
This means the radiation pressure acting oiutwards will also stop and the star will experince an inwards force due to gravitational attraction.
Describe the formation of a rad giant after fusion in the core of a main sequence star has stoppped.
The ner inwards force, causes the core to contract, leading to an increase of temperature as it compresses. There is still a large quantity of hydrogen gas surrounding the core. The core releases thermal energy. These outer layers expand to cover a greater volume than the original sta, cooling and leading to the formation of a red giant.
After the production of a red giant, and the star continues to collapse, what happens?
The core become hot enough for fusion to begin again, leading to heavier elements, inculding carbon and oxygen.
What is produced during the production of carbon and oxygen in the core of a red giant?
Enormous amounts of energy is released, increasing the outwards radiation pressure.
What happens once the fusion the the core of a red giant stops?
The star becomes unstable and begins to collase again. At this stage, the outer layers may be ejected back into space forming a plantary nebula while the rest of the star continues to collapse under ity’s own mass until it reaches a point where it can collapse no further,
What is produced when an unstable red giant can no longer collapse any further?
What is left is very hit, dense core called a whitye dwarf. No further fusion takes place but the star continues to radiate energy as the photons produced from past fusion leaks away.
Eventually, the white dwarf will gradually coll down with a surface tem erature of just a few kelvin.
What will a high-mass star beome after its main sequence star?
A beutron star, or a black hole, or a supernova.
What is a contiumous spectrum?
A spectrum that appears to contain all wavelengths over a wide a comparatively wide range.
Define energy levels of an atom?
Energy levels insode an atom are the specific energies that electrons can have when occupying specific orbits. Electrons can only occupy these discreet energy levels and cannit exist at other values between them.
What is the emission line spectum?
The emission line spectrum of an element is the spectrum of frequencies of electromagnetic radiation emitted due to electron transitions from a higher energy level to a lower one within an atom of that element. Since there are many possible electron transitions for each atom, there are many different radiated wavelengths. A line spectum consists of a series of bright lines agains dark lines.
What is an absorbtion line spectrum?
An absorbtion line spectum is the pattern of dark lines in a continuous spectrum from a lihht source and is caused by light passing through an absorbing madium such as gas. The dark lines represent the wavelengths that are being absorbed.
What is transmission diffracting grating?
A transmission diffracting grating is a glass surface having a large number of fine parallel grooves or slits, and is used to produce optical spectra by diffraction of transmitting light.
What is a maxima?
Maxima are reigions of brightness whuch will be seen when the pth difference between overlapping waves is equal to a whole number of wavelengths.
What did Joseph von Fraunhofer discover on the suns spectum?
He discovered dark lines on the suns spectrum where frequencies were missing.
What properties did Niels Bohr propose atoms have that explained the existance of spectra lines?
- Electrons orbit the nucleus.
- The electrons can occupy certain orbits. These orbits are associated with definite energies and are also called energy levels.
- Electrons loose or gain energy by moving from one allowed energy level to another, absorbing or emitting elecromagnetic radiation with a frequency determined by the energy difference of the energy difference of the levels according to Plank relation for the photons energy.
How can an electron be described if it moved from a lower energy level to a higher one?
It’s said the electron is excited.
When will an electron move from a lower energy level to a higher one in an atom?
Tis happens is a gas absorbs energy from the suroundings, for example, if it’s heated.
What is the energy required to move the electron to the higher energy level equal to?
The energy required to move an electron to a higher energy level is equal to the energy difference between the 2 levels.
What is emitted when an electron falls to a lower energy level in an atom?
Elecromagnetic radiation of a fixed energy and wavelength.
When is an electrins energy 0 in regards to it’s distance from the nucleus?
When it ois a long way from the atoms nucleus.
(Similar to how we define gravitational potential as 0 at infinity)
What happens to the engery of an electron when it moves closer to the nucleus of an atom?
As an electron moves towards the nucleus from very far away, its energy decreases below 0 so energy levels inside an atom have negative values.
How can an electron in it’s lowest energy level be described?
When an elecrtron is at it’s lowest energy level, we say this is ground state.
What is ground state equal to in a hydrogen atom?
-13.6eV
A hydrogen atom has a ground state of -13.6eV, what does this mean for an electron?
If means an electron would need to be supplied with energy equal to 13.6eV to remove it completly from the atom.
What is removing an electrom from an atom called?
Ionisation
When is an emission line spectrum produced?
When an excited electron in an atom moves from a higher to a lower energy level and emits a photon with an energy correponding to the difference between these energy levels.
In regards to electron movind to different energy levels, what can hot gases produce?
Hot gases produce emission line spectra.
What is an absorbtion line spectrum?
When an atom in a gas absorbes a photon of the correct energy, an elecron moves from a lower energy level to a higher one. The particular wavelengths of light corresponding to these transitions are then missing from the continous spectrum.
What is an absorbtion line spectrum?
When an atom in a gas absorbes a photon of the correct energy, an elecron moves from a lower energy level to a higher one. The particular wavelengths of light corresponding to these transitions are then missing from the continous spectrum.
What does the position of the peak pof a star depend on?
Its temperature.
On one of these diagrams, at lower temperatures, how is the peak affected?
At lower temperatures, the peak is shifted to the right and corresponds to a higher value of the wavelength on the x-axis.
On one of these diagrams, as surface temperature increases, how is the position of the peak affected?
The peak moves to the left. As temperature increases, the area under the curve also increases.
As temperature increases, the area under a curve on this diagram will increase. Why is this true?
The area under the curve represents the total energy emitted by the body and this is strongly-tempertaure dependant.
What does the peak in this diagram represent?
It represents the wavelength that corresponds to the peak intensity of power of the stars emission. (λmax)
How can you calculate one parsec?
What is this graph?
Hubble’s original graph of recessional velocity against distance shows a linear relationship, but with many points large distance from the line of best fit. More recent data for many more gaxies has less scatter.
