Chapter 19 - Stars Flashcards
1
Q
what are nebulae
A
giant clouds of gas and dust
2
Q
how does a protostar form
A
- nebulae form over millions of years
- the gas and dust are gravitationally attracted
- they become denser and gravitational energy is transferred to thermal energy
- in part of the nebula a protostar will form
3
Q
how does a protostar become an actual star
A
- it must have a large enough mass to get the centre hot and dense enough for nuclear fusion to occur
- this is the point where hydrogen particles have sufficient energy to overcome their electrostatic attraction
- for it to become a star fusion must occur
4
Q
what makes a star stable
A
- gravitational forces compress the star
- radiation pressure from photons and gas pressure from nuclei act outwards
- these forces cancel making the star stable and spherical
5
Q
what are planets/their key features
A
- they have sufficient mass to be spherical
- no fusion occurs
- it has cleared its orbit of other objects
6
Q
what are planetary satellites/their key features
A
- a body in orbit around a planet
- includes both man-made and natural satellites
7
Q
what are comets/their key features
A
- few hundred metres - few tens of kms in diameter
- small irregular bodies made from ice dust and small rocks
- all orbit the sun with very elliptical orbits
8
Q
what are solar systems/their key features
A
- A star and all objects that orbit them
9
Q
what are galaxies/their key features
A
- a collection of stars/solar systems and interstellar gas and dust
10
Q
what is a solar mass
A
solar mass
M(dot) is the mass of our sun = 2x10^30 kg
11
Q
what do smaller mass stars form after their main sequence and what are the masses that do this
A
any stars of 0.5-10 solar masses form red giants after their main sequence
12
Q
how do red giants form
A
- as the hydrogen runs out the energy released by the core decreases
- so gravity becomes the resultant force
- the core collapses gravitationally
- as the core shrinks, the temperature and pressure increase enough to start fusion again in a shell around the core
- the core is inert and contains very little hydrogen
- the star expands and the outer layers cool
13
Q
what do red giants tend to form after they have undergone all of their fusion
A
- white dwarfs
14
Q
how do white dwarfs form from red giants and what prevents them from collapsing further
A
- most of the outer layers of the red giants cool and drift into space forming a planetary nebula
- the hot core remains as a white dwarf
- this is very dense but no fusion occurs
- the pauli exclusion principle prevents electrons from existing in the same energy state
- this creates an electron degeneracy pressure which prevents further gravitational collapse
15
Q
what are the features of a white dwarf
A
- very dense but no fusion occurs
- surface temperature around 30,000K
- it doesn’t emit ‘new’ energy but ‘leaks’ photons from past fusion
16
Q
what is the Chandrasekhar limit
A
- 1.44 solar masses, this is the mass of a star’s core such that for any mass is greater than this, the electron degeneracy pressure is not sufficient to prevent further gravitational collapse
17
Q
what is the general life cycle of a lower mass star
A
nebula –> protostar —> main sequence —> red giant —> planetary nebula and white dwarf
18
Q
why do larger stars spend less time in their main sequence phase
A
- the fusion in larger stars occurs quicker
- because they have a greater mass
- so their cores are hotter so hydrogen atoms have more kinetic energy so can fuse more easily
- the cores are also at a higher pressure so the frequency of collisions is higher
- so they use up their hydrogen more quickly
19
Q
what classes as a higher mass star and what do they form after their main sequence
A
- anything with a mass greater than 10 solar masses
- they form red supergiants after their main sequence phase
20
Q
how do red supergiants form
A
- as the hydrogen in higher mass stars runs out, the core begins to collapse
- this makes the core hotter and denser
- so particles have a greater kinetic energy and more fusion can occur (helium atoms are then able to fuse)
- these changes cause the outer parts of the star to expand and cool
- inside the the pressures and temperatures continue to increase as the star collapses and heavier and heavier elements are made in a series of shells
21
Q
what occurs in supernovae
A
- when the red supergiant has an iron core, the star becomes unstable
- outer layers implode and bounce off the core and are ejected into space
- heavier elements can be made in this process
22
Q
what are the two options for what occurs to a core after a supernova
A
- if the mass of the core is more than the Chandrasekhar limit but less than 3 solar masses then they form a neutron star
- if the mass of the core is greater than 3 solar masses then gravitational collapse continues and they form a black hole
23
Q
what is a neutron star/its features
A
- stars formed almost entirely out of neutrons
- approx 10km in diameter
- mass around 10 solar masses
- density similar to an atomic nucleus
24
Q
what are black holes/their features
A
- occur where the gravitational collapse of a core after a supernova continues
- creates an object so dense that its escape velocity is greater than the speed of light
25
what is the general life cycle for higher mass stars
nebula ---> protostar ---> main sequence star ---> red supergiant ---> supernova ---> neutron star or black hole
26
what is the Schwartzchild radius
the Schwartzchild radius of an object is the radius such that if all the mass of that object was condensed into a sphere that size, the gravity of it would form a black hole
27
what does the Hertzsprung-Russel (HR) diagram show and what should we remember about it
- shows all the stars in our galaxy on a plot of luminosity (on Y-axis) against temperature (on X-axis)
- temperature increases right to left not left to right
- we often use log scales because values can vary massively
28
what is luminosity and what is a common measurement for it
- the luminosity of a star is its total radiant power output
- can be measured in Watts or solar luminosities
- 1 solar luminosity = 3.85x10^26 W
- related to both brightness and size
29
what do you find in the top left of an HR diagram
the hottest, most luminous stars
30
what do you find in the bottom right of an HR diagram
the coolest, least luminous stars
31
what do you tend to find on the line going from top left to bottom right on an HR diagram
- the main sequence stars, the red giants appear on a sort of branch off of this on the diagonal
32
what do you find along the top of an HR diagram
the most luminous stars
| the supergiants, (red supergiants in top right)
33
what do you find in the bottom left part of an HR diagram
hot, not very luminous stars, usually the white dwarfs
34
what are energy levels and what are the 4 things we need to remember about energy levels/electrons in energy levels
- energy levels are orbits around a nucleus in which an electron sits
- electrons cannot have a quantity of energy between two energy levels
- energy values for energy levels are negative because work must be done to remove an electron
- an electron with 0 energy is free from the atom, the most negative level is the ground state/level
35
what happens when an electron is excited and what can cause this
- when an electron gains energy, it becomes excited and moves up energy levels
- this can occur from a potential difference, thermal energy or absorption of a photon of the correct energy
36
what usually occurs following an electron becoming excited, how does this link to conservation of energy
- the atom becomes less energetically stable
- so the electron drops back down to a lower energy level
- a photon of energy equal to the change in energy between energy levels is emitted
- so conservation of energy holds
37
which two equations can we use to calculate the energy/frequency etc. of the electron
```
deltaE = hf
deltaE = hc/(lambda)
```
38
what are the three types of spectra
Emission Line Spectra
Continuous Spectra
Absorption Line Spectra
39
Define an Emission Line Spectrum
Emission line spectra show a spectrum of unique frequencies of light for each atom. These frequencies of light are produced when excited electron drop back to a lower energy level releasing a photon of a specific energy
40
define a continuous spectrum
A continuous spectrum is a spectrum that shows all the frequencies of visible light
41
how are continuous spectra usually produced
from heating a solid metal such as sodium
42
define an absorption line spectrum
An absorption line spectrum is a continuous spectrum containing dark lines where photons of those frequencies have been absorbed to excite an electron in an atom
43
explain what emission line spectra show and how they are formed
- Electrons are excited (usually through heating), they move to a higher energy level
- they then drop back to a lower energy level and emit a photon
- the photon's energy is equal to the change in energy between the energy levels the electron moved between
- this produces a set of discrete frequency emissions according to the element's energy levels
- these emissions form an emission line spectrum
44
Are emission/absorption line spectra specific to an element
yes
45
explain how absorption line spectra are formed and what they show
- light from a source which produces a continuous spectrum passes through a cooler gas
- photons at the correct energy (equal energy to the energy change between energy levels) excite an electron to a higher energy level and are absorbed
- this produces dark lines on the continuous spectrum
46
what should we note about what happens when an absorption line spectrum is formed
- the photons absorbed are re-emitted but in all directions rather than one, this makes it a lower intensity so a dark line still occurs
47
what is the link between emission spectra and absorption spectra
they are negatives of each other
48
how can we analyse star light using spectra
- light from stars produces specific absorption line spectra
- because some photons are absorbed by cooler gases in the outer layers of the star
- these missing wavelengths can be checked against known element's spectra to find the chemical makeup of the star
49
what is a diffraction grating
" a diffraction grating is an optical component with regularly spaced slits/lines that diffract and split light into beams of different colours, travelling in different directions"
50
what can be the issue with using a double slit, what do we use instead and why
- the fringes produced by passing light through a double slit are not very sharp and it can be difficult to determine the centre of each maximum
- to solve this we use a diffraction grating instead
- the greater number of slits means more light is diffracted and a clearer pattern is produced
51
what happens when white light passes through a diffraction grating
- the amount by which the light diffracts when passing through a diffraction grating depends on its wavelength
- therefore the different components of light diffract different amounts and the white light splits into its component colours
52
why do maxima occur for light that has passed through a diffraction grating
- as the light is diffracted, at certain points the waves meet in phase or with a path difference of n(lambda)
- this means when the waves superpose at these points, constructive interference can occur and maxima form
53
how can we derive the equation for diffraction gratings
- the angle theta is the angle between the normal line to the grating and the diffracted light ray/line to the maxima
- it is also the angle between the grating face and the shortest line from one slit to the other wave involved in superposition
- the opposite face for this mini triangle must be n(lambda)
- the hypotenuse is d (slit seperation)
- therefore n(lambda) = dsin(theta)
54
what is the diffraction grating equation and when can it be used
```
n(lambda) = d sin(theta)
- for monochromatic light
n = order number
lambda = wavelength
d = slit separation
theta = angle from normal to maxima
```
55
what is the practical we can do to find the wavelength of monochromatic light
1) shine monochromatic light (e.g. a laser) normal to a diffraction grating onto a white screen
2) calculate d, slit seperation
3) measure D and x (distance from grating to screen and distance from central order maximum to chosen maximum)
4) calculate theta using tan(theta) = x/D
5) repeat for each maxima
6) plot n against d sin(theta)
7) gradient = wavelength
56
what is black body
- idealised object
- absorbs all EM radiation that shines on it
- when at thermal equilibrium it emits a certain distribution of wavelengths and intensities
57
what is Wien's displacement law, state the proportionality
A law that relates the absolute temperature of a black body, T, to the wavelength of its radiation at which intensity is at a maximum, Lambda(max)
Lambda(max) = k/T
58
what is the constant of proportionality in Wien's Displacement law
Wien's constant = 2.9 x 10^-3 mk (metres-kelvin)
59
how does the distribution of an object's radiation change as the object's temperature changes
As temperature increases:
- peak moves left i.e. wavelength (max) decreases
- distribution becomes higher with a sharper peak
60
state Stefan's law
" Stefan's law states that the total power radiated per unit surface area of a black body is directly proportional to the fourth power of its absolute temperature"
61
state the equation for Stefan's law and what Stefan's constant is
L = 4 (pi) (r^2) (sigma) (T^4)
Stefan's constant is sigma = 5.67 x 10^-8 Wm^-2K^-4
62
what are the proportionalities that go with Stefan's law
Luminosity is directly proportional to:
- radius squared, r^2
- surface area, 4(pi)(r^2)
- (absolute temperature)^4, T^4
63
how will a more distant star's spectrum be different
- LOWER INTENSITY
| - more red-shifted because moving quicker
64
state how an emission line is formed
Electron makes a transition to a lower energy level and emits a photon
