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A Level Physics > C19 (Stars) > Flashcards

C19 (Stars) Flashcards

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1
Q

Nebulae

A

-Gigantic clouds of dust and gas (mainly H)
-Often referred to as stellar nurseries, as they’re the birth place of all stars

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2
Q

How are Nebulae’s formed?

A

Tiny gravitational attraction between particles of dust and gas, pull particles together towards each other, forming the vast clouds.

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3
Q

How are stars formed

A

As dust and gas get closer, this gravitational collapse accelerates. Tiny variation in nebula causes denser regions to begin forming, also get hotter as grav potential energy is converted to thermal energy.

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4
Q

Protostar

A

Not yet a star, but a very hot, very dense sphere of dust and gas

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5
Q

What does a protostar need to undergo to become a star

A

Nuclear fusion in its core
Fusion reaction produces energy form KE
Extremely high pressures and temps inside core needed to overcome electrostatic force repulsion between H nuclei, in order to fuse together and form He nuclei.

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6
Q

What is the cosmological principle

A

The idea that the universe has the same large scale structure when observed from any point.

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7
Q

What are the 3 assumptions in cosmological principle

A

-Universe is homogenous (it’s density is the same everywhere)

-Universe is isotropic (it’s the same in all directions)

-The laws of physics are universal

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8
Q

What’s HR diagram

A

graph of stars showing relationship between their luminosity (y-axis) and average surface temp (x-axis, increasing right to left).

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9
Q

Luminosity of a star (definition):

A

the total radiant power output of the star, (luminosity related to brightness of star).

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10
Q

In HR diagram luminosity often plotted in…

A

units relative to our Sun, 1L = 3.85 x 10^26 W.

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11
Q

Why are both scales on HR normally logarithmic plots

A

due to the vast range of luminosity and surface temp of stars.

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12
Q

Main sequence star

A

Spends majority of it’s life span at this stage

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13
Q

Super Red Giants (structure)

A

Inside, core, onion-like layers, in which different elements are created by fusion, with heavier elements deeper in, up to the central core, made stable iron nuclei (cannot fuse any further).

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14
Q

Any star that’s x1.4-2.0 Ms becomes what kinda star?

A

Neutron star

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15
Q

Neutron star (structure):

A

Only the inner core of star remain, after supernova. Core, radius, 10km, extremely high density (close density of an atomic nucleus).

Outer shell, 1km solid crust of atomic nuclei
Inside, liquid interior, neutrons, increases in density towards centre.

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16
Q

Black Hole (formation):

A

When large star ends life (supernova), central core left, so massive that the neutrons inside are destroyed by grav forces.

It becomes smaller and denser than a neutron star, centre eventually collapses into a point of infinite density (called a singularity)

Grav force now so strong, nothing can escape it (including light), so appear black.

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17
Q

The Schwarzchild radius (Rs) formula

A

Rs = 2GM / c^2

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18
Q

How to derive the Schwarzschild radius

A

1/2 mv^2 > or = GMm / Rs

Sub v = c
And m’s cancel out

Rs = 2GM / c^2

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19
Q

Light can escape from the vicinity of a black hole if it’s outside a radius called…

A

The Schwarzchild Radius
(Inside this radius, nothing can escape, not even light).

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20
Q

What’s one Astronomical Unit (AU)

A

The average distance between the Sun and Earth

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21
Q

The Light-Year (ly)

A

The distance travelled by light in a vacuum in one year

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22
Q

The Parsec (pc)

A

Distance that gives a parallax angle of 1 second (most often used to measure the distances between stars and galaxies).

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23
Q

Formula for the Distance (pc)

A

Distance (pc) = 1 / parallax (arc second)

24
Q

Every black whole is surrounded by what?

A

An event horizon (nothing that occurs within can affect the universe outside it.

25
Q

To an outside observer what do they see, as an object is within the black wholes gravitational pull (and the objects point of view)

A

The object falling towards the black whole slows down as it approaches the event horizon, never quite crossing it.

From objects point of view, crosses the event horizon and falls towards the singularity.

26
Q

Chandrasekhar Limit

A

Concept of wether the star turns to a red giant or super red giant and therefore if it goes supernova

27
Q

What determines if a main sequence star turns into a red giant

A

Mass < 1.44 Ms (solar mass)

28
Q

What determines if a main sequence star turns into a super red giant

A

Mass > 1.44 Ms

29
Q

What determines if a super red giant turns into a neutron star

A

Mass 1.44-2.00 Ms

30
Q

What determines if a super red giant turns into a black hole

A

Mass > 2.00 Ms

31
Q

Stefan’s Law

A

luminosity star directly proportional to its surface area

32
Q

Stefan’s Law formula

A

(this formula assume black body radiation), and to the fourth power of its temp.
Theta is the Stefan-Boltzmann constant:

L= A x theta x T^4

33
Q

How can we measure brightness?

A

Light metres

34
Q

What can an electron not have (in terms of energy levels)?

A

An electron cannot have a quantity of energy between two levels.

When electrons bound to atoms, in a gas, can only exist in one of a discrete set of energies (energy levels).

35
Q

Why are all the energy levels negative values?

And therefore what does it mean if an electron has an a zero energy value?

A

-The energy levels are -ve, because external energy required, remove electron from the atom. The -ve values also indicate that the electrons are trapped within atom, or bound to +ve nuclei.

-An electron with zero energy is free from the atom.

36
Q

The energy level with most -ve value, known as the…

A

ground level / ground state.

37
Q

What happens when an electron becomes excited?

And what is required for this to be achieved?

A

-Electron moves, lower to higher energy level within an atom in a gas.

-This requires external energy (e.g from electric field , through heating, or when photons of specific energy (and therefore freq) are absorbed by atom).

38
Q

De-excitation:

A

electron moves higher energy level to lower one. Energy is conserved, so loss energy in form of a photon that is emitted.

39
Q

An electron in the -3.0eV energy levels requires at least how much energy to escape the atom?

A

3.0eV

40
Q

The energy of emitted photons (in electron transition, from higher to lower energy levels) is calculated using the equation:

A

Change Energy = Planks constant x freq

Change E = hf

Change E = hc / wavelength

Where f = freq of photon
Delta E = difference in energy between two energy levels.

41
Q

In terms of energy levels, what does each element have?

A

Each element has own unique set of energy levels.

42
Q

Diffraction

A

The bending of light around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle.

43
Q

Diffraction Grating:

A

-An optical component with regularly spaced slits or line that diffract and split light into beams of different colours travelling in different directions.

44
Q

What are the different beams of lights, created from a diffraction grating used for?

A

analysed determine wavelength of spectral lines in the lab or from starlight.

45
Q

How is it that a diffraction grating can form different beams of separate light (wavelengths)?

A

Different wavelengths of waves are diffracted different amount, so white light is split into it’s spectrum- red is diffracted the most, violet the least.

46
Q

The wavelength of red gets … the most, but … the least.

A

Red gets diffracted the most, but refracted to least.

47
Q

How is it, that when using a diffraction grating, we are able to see multiple spectrums?

A

-A diffraction grating has many slits, so we can see many spectrums.

-Can be useful as the spectrums overlap to produce maxima and minima from the interference pattern.

48
Q

The interference pattern depends on:
(Equation)

A

d x sin(theta) = n x wavelength

d- the distance between two slits (grating spacing)
Wavelength- the wavelength of light
Theta- angle from zero order maxima (when n=0)
n- the no. of the maximum used

49
Q

Why can the spectra from starlight can be used, identify elements within star?

A

Because different atoms have different spectral lines

50
Q

Detecting Elements within Stars:

A

-Light from star, found to be an absorption line spectrum.

-Some wavelengths light are missing (the photons have been absorbed by atoms of cooler gas in outer layer of star).

-If we know line spectrum of particular element, we can check if element is present in distant stars. If particular element present then its characteristic pattern of spectral line will appear as dark lines in absorption line spectrum.

51
Q

Emission line spectra

A

each element produces a unique emission line spectra, because of its unique set of energy levels.

52
Q

Continuous spectra

A

all visible freqs or wavelengths are present. Atoms of heated solid metal produce this type of spectrum.

53
Q

Absorption line spectra:

A

has series dark spectral lines against the background of a continuous spectrum. The dark lines have exactly same wavelengths as the bright emission spectral lines for the same gas atoms.

54
Q

What happens when an electron drops into a lower energy level?

A

When electron drops into lower energy levels, emit photons with set of discrete freqs specific to that element, producing a characteristic emission line spectrum.

55
Q

Each spectral line corresponds to…

A

Each spectral line corresponds to photons with specific wavelengths, these spectra, observed in laboratory from heated gases.

56
Q

How are Absorption Line Spectra formed?

A

-Formed when light (from source that produces a continuous spectrum) pass through a cooler gas. As photons pass through, some absorbed by gas atom, raising electrons into higher energy levels (exciting atoms).

-Only photons with energy equal to difference between different energy levels are absorbed, meaning only specific wavelengths absorbed, creating dark lines in spectrum. These lines show which photons have been absorbed by the gas atoms.

57
Q

Few lines from emission line spectrum may not be visible in absorption line spectrum, because…

A

In excited atoms, electrons may return ground state in stages, releasing photon each time, whereas absorption lines are mostly caused by electrons starting from ground state.

A Level Physics (28 decks)

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