Chapter 19 - Stars Flashcards
Define Nebulae
A gigantic cloud of dust and gas (mainly hydrogen)
Define Planets
An object that has a mass large enough for its own gravity to give it a circular shape. It has no fusion reactions and has cleared its orbit of most other objects.
Define Dwarf planets
They have not cleared their orbit of other objects.
Define Asteroids
Small, irregularly shaped bodies, composed of dust and metals. There have near circular orbit around stars and don’t contain any ice.
Define Planetary satellites
A body in orbit around a planet.
Define Comet
Small irregular bodies made up of ice, dust and small pieces of rock. They all orbit the Sun, in highly eccentric elliptical orbits.
Why do comets have ice?
They are so far away from the Sun that the ice doesn’t melt due to the lack of heat.
Define Solar system
The gravitational bound system of the Sun, and all the planets that orbit it.
Define Galaxies
A collection of stars, planets and interstellar dust and gas.
Define Protostar
A very hot, very dense sphere of dust and gas.
Star Birth
Nebulae form when tiny particles of dust and gas come together due to the force of gravitational attraction.
The denser regions of a nebula pull in more matter, becoming hotter as the gravitational potential energy is transferred to heat energy.
A protostar forms in the nebula.
Star from a protostar
Extremely high pressures and temperatures in the core will overcome the electrostatic forces of repulsion between hydrogen nuclei to fuse them together to form Helium nuclei.
Nuclear fusion of hydrogen nuclei begins and the protostar will become a main sequence star.
What does a fusion reaction produce
Energy in the form of kinetic energy
Star life
Once a star is born, it’ll stay the same size.
This is because the gravitational forces are equal to the radiation pressure and the gas pressure, meaning they’ll remain in equilibrium.
What is a star’s stable phase called
Main sequence.
What does a star with a low mass have as a consequence
A cooler core
What is a star with a low mass
0.5 Mo. to 10 Mo. They will turn into red giants.
Red giants
Energy is released by fusion will decrease, meaning the G.F increases. So the core will collapse, and the pressure increases. So, the fusion happens in a shell around the core. This means the periphery will expand, as the layers move away from the core.
What is a Mo.
The mass of the sun.
1.99 X 10^30 kg.
Why do red giants have an inert core?
So fusion doesn’t happen.
Because the temperature is too low.
What happens when the layers cool
It gives the red colours
White dwarf
It’s very dense.
There are no fusion reactions and it leaks photons created at an earlier evolution.
The Pauli Exclusion
2 electrons cannot exist in the same energy state.
Electron Degeneracy Pressure
When the core of a star collapses under the force of its own gravity, all the matter is forced together into a smaller volume. However, electrons aren’t allowed to occupy the same energy levels in an atom, so the atoms can’t move too close to each other. Therefore, a pressure s exerted by the electrons as the star collapses, which is called the electron degeneracy pressure.
The pressure outwards counters the gravitational attraction inwards, so that the core can’t continue collapsing.
Chandrasekhar Limit
The EDP will exist if the core has a mass less than 1.44 Mo. This limit is the max mass of a stable white dwarf star.
More massive stars
They have a mass greater than 10 Mo.
More massive stars and how they behave
When their hydrogen fuel runs out, the core collapses.
However, as the cores are much hotter, the helium can overcome their electrostatic forces of repulsion and fuse together. This means they can form even heavier elements.
Red Super-giants
Inside the core, the temperature and the pressures increase and are high enough to fuse massive nuclei together. They are sorted into many shells around the core.
Shells in a red super giant
Non-fusion hydrogen Hydrogen fusion Helium fusion Carbon fusion Oxygen fusion Neon fusion Magnesium fusion Silicon fusion Inert iron core
Supernovae- why does it happen in massive stars
The G.F crushed the nuclear fusion, and the star collapses in on itself, leading to a supernovae.
Supernovae- Neutron stars
Mass of the remaining core is greater than the Chandrasekhar limit. So, there is further gravitational collapse.
They are only 10km in diameter and entirely made up of neutrons and very dense.
Supernovae- Black holes
Mass is greater than 3M0.
The Gravitational collapse will continue to compress the core.
The G.F becomes immensely strong, and you would need something to travel faster than the speed of light, which is why photons of light can’t leave the holes.
The Schwartz child radius
Referred to as rs (small s)
It is the radius of an imaginary sphere so that if all the mass of an object is compressed into the sphere, the escape velocity would have to be greater than the speed of light.
Rs = 2GM/c^2.
What is the Hertzsprung- Russell diagram
It is a diagram of stars in our galaxy. It shows the relationship between their luminosity (y-axis) and their surface temperature (x-axis)
Define luminosity
Total radiant owner output of the star.
On a HR graph, how is everything plotted?
It is plotted relative to the Sun, where 1l (Sun) = 3.85 x 10^26
General pattern on the HR diagram
It allows them to classify different objects in the night sky
It can also be used to show evaluation
What is an energy level
A discrete set of energies
Why are energy levels negative
Because you have to add energy to remove the electron (due to the positive attraction)
How can you excite an electron
- using external energy
- supplying an electric field
- through heating
- when photons are absorbed of EXACT energy values (equivalent to the difference of energy levels)
- when colliding with an electron (they can transfer any amount of energy, doesn’t have to be exact)
Define emission line spectra
Each element produces a unique emission line spectrum because of its unique set of energy levels
Define a continuous spectra
All visible frequencies or wavelengths are present. The atoms of a heated solid metal will produce this type of spectrum.
Define absorption line spectra
This type of spectrum has series of dark spectral lines superimposed against a continuous spectrum. The dark lines have exactly the same wavelengths as the bright emission spectral lines for the same gas atoms.
How is an emission line spectrum formed?
If the atoms in a gas are excited and they then drop back into lower energy levels, they emit photons with a set of discrete frequencies specific to that element.
How is an absorption line spectra formed
It is formed when light from a source that produces a continuous spectrum passes through a cooler gas. As the photons pass through the gas, some are absorbed by the gas atoms, raising electrons to a higher energy level. Only photons with energy of exact energy levels are absorbed, so that only specific wavelengths are absorbed.
Why is it that some lines from the emission line spectrum may not be visible in the absorption line spectrum
Because in excited atoms electrons may return to their ground state in stages.
How can you detect elements within stars
When the light from a star is analysed, it is found to be an absorption spectrum, and some wavelengths are missing. If we know the line spectrum of a particular element, we can check if that element is present on that star.
Define a black body
It absorbs and emits all the electromagnetic radiation that shines onto it.
Wien’s displacement law
wavelength max is inversely proportional to Temperature (for any black body emitter)
λmax x T = 2.90 x 10-3.
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 = 4π r^2 σ T^4.
Stefan’s constant
5.67 X 10-8.
Define comets
Small, irregularly shapes bodies, composed of dust and ice and have highly eccentric orbits around stars.
Define universe
All the galaxies and all their mass and energy
Lifetime of a small star
Dust and gas -> protostar -> red giant -> white dwarf -> black dwarf.
Lifetime of a big star
Dust and gas -> protostar -> red super giant -> supernova -> neutron star -> black hole.
What happens during the main sequence phase?
This is when the radiation pressure outwards from the nuclear fusion is equal to the gravitational attraction inwards.
Why do more massive stars exhaust their hydrogen (or have a shorter lifetime) than smaller mass stars?
More mass means more gravitational potential energy. Therefore, there is more kinetic energy, as more of the GPE is converted to kinetic energy. Therefore, it has a hotter core and so a faster rate of fusion, which leads to a shorter life time.
Why do Super Giants look red?
Because the outer shells further expand and emit lower EM waves, which look red to us.
Supernovae of large mass stars
Finally, the star will develop an iron core. The iron nuclei cannot be fused together. So, the star becomes unstable and it implodes, leading to a supernova.
The outer shells shed and the denser core is all that remains, becoming either a neutron star or a black hole.
Energy levels- What is the energy number given on the y axis?
The energy at a given level, is how much energy must be transferred to the electron, for it to attain zero electrostatic potential energy and thus escape the atom.
Exciting an electron
When using a photon- the energy level of the photon must be EXACTLY the difference between 2 energy levels.
When using an accelerated electron- - the energy doesn’t have to be exact, just enough to move the original electron to a higher energy level.
How does a diffraction grating work with mononchromatic light?
When monochromatic light is incident on grating:
- It diffracts through each slit.
- Diffracted light waves interfere.
- Light waves from adjacent slits constructively interfere in certain directions only.
Therefore, light is transmitted in certain directions only.
Diffraction grating equation
dsin θ =nλ
d: grating slit spacing.
n : diffracting beam order.
θ : can only be observed on the screen for angles up to 90 degrees.
How does a diffraction grating work with non-monochromatic light?
- There are different wavelengths of light being diffracted through different angles.
The diffracted light waves interfere constructively at different positions..
Therefore, different wavelengths of light are seen separately forming a spectrum of the original incident light.
Why is the maxima produced from a diffraction grating more bright that those produced via the double slit experiments?
With many slits, more waves will interfere constructively, and give rise to a maxima, thus maxima are more intense.
