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Physics AQA AS Level > Classification of Stars > Flashcards

Classification of Stars Flashcards

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1
Q

What is parallax?

A
  • Parallax is the apparent displacement of an nearby object e.g. a star because of a change in the observer’s point of view i.e. when looking at it from two different lines of sight.
  • The measurement of parallax is given as a semi-angle of inclination between those two lines.
  • The distance to the object is measured relative to stars that are so distant that the object does not appear to move - don’t quite understand.

*For stars nearby Earth, the two different lines of sight may be at two different points in the orbit of the Sun.

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2
Q

The greater the angle of parallax…

A

…the nearer the object is to the observer.

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3
Q

If the parallax angle, θ, for a star near Earth orbiting the Sun at radius r, how can we calculate the distance between the nearby star and Earth?

A

You use the equation:

tanθ = r/d
For small parallax angles, tan θ ≈ θ, so θ = r/d

θ = the parallax angle in radians 
r = the radius between sun and Earth, 
d = the distance between the nearby star and the sun.

d and r can have any units as long as they are the same.

*The distance between the nearby star and the Sun is basically the same as the distance from the nearby star and the Earth, so when asked out the distance of the star from Earth, we still use this equation (eventhough it tells us the distance to the Sun)

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4
Q

What is the distance to celestial objects usually measured in?

A

Parsecs (pc)
or
Lightyears (ly)

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5
Q

What is the parsec?

*Conversions between parsecs, lightyears, AU and metres will be given on data sheet.

A

This is the distance at which 1AU (astronomical unit) subtends an angle of 1 arcsecond.

i.e. an star is exactly one parsec away if the angle of parallax is 1 arcsecond.

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6
Q

What is 1 arcsecond in degrees?

You need to know this.

A

1 arcsecond = (1/3600) degrees

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7
Q

What is the lightyear?

CONVERSIONS BETWEEN LIGHTYEAR AND PARSECS AND METRES WILL BE GIVEN IN THE DATA SHEET

A

The is the distance the EM waves travel through a vacuum in one year.

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8
Q

What equation can you use to calculate the distance to an object?

A

θ = r/d

θ = half the angle subtended in radians (rad)
r = radius of the object
d = distance to the object

r and d can be any units as long as they are the same

  • This is similar to the parallax angle equation in that the parallax angle is ‘half the angle subtended’
  • Also in this case the angle is subtended at Earth by the radius of the distant object whereas for parallax the angle is subtended at the distat object by the radius of Earth’s orbit of the Sun.
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9
Q

What is the radius of the orbit of the Earth from the Sun?

A

1AU (Astronomical Unit)

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10
Q

What is ‘luminosity’ of stars?

A
  • Rate of light energy released/power output of a star

- (So the power emitted by a star)

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11
Q

What is the ‘intensity’ of a star?

A
  • The power received from a star (its luminosity) per unit area and has the unit, Wm⁻²
  • This is effectively the brightness of a star, as the brightness of the star depends the amount of energy detected pe unit time by the observer (i.e. power recieved per unit area). The power recieved by an area is dependant on the distance of that area from the star. So intensity depends on distance.
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12
Q

What is the relationship between intensity of a star and the distance from the star?

A

The intensity of a star follows an inverse square law. The intensity of a star is inversely proportional to the distance from it squared.

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13
Q

What is the apparent magnitude(m) of a star?

A
  • How bright the star appears from Earth

- It is the amount of light we can detect from Earth (it is based on intensity)

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14
Q

What does the apparent magnitude of a star depend on?

A
  • The luminosity of the star

- The distance of the observer from the star

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15
Q

What is the Hipparcos scale?

A

The Hipparcos scale was the original scale used to classify astronomical objects by their apparent magnitudes, with the brightest stars (greatest intensity) given an apparent magnitude of 1, and the faintest visible stars (lowest intensity)` being given an apparent magnitude of 6.

  • This scale has now been extended at both sides, so the apparent magnitude of stars can be below 1 and greater than 6.
  • This scale is continuos so you can have decimals, like 4.2
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16
Q

By what factor of intensity is an apparent magnitude of 1 greater than an apparent magnitude of 6?

A

The intensity of a magnitude 1 star is 100x greater than a magnitude 6 star.

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17
Q

The Hipparcos scale is logarithmic, as the magnitude changes by , the intensity changes by?

A

The intensity changes with by a factor 2.51.

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18
Q

How to calculate the brightness (or intensity) ratio between two stars using the equation…?

A

I₂/I₁ = 2.51ᵐ¹⁻ᵐ²

I₂ = Brightness/Intensity of star 2(Wm⁻²)
I₁ = Brightness/Intensity of star 1 (Wm⁻²)
ᵐ¹ = Apparent magnitude of star 1 (no units)
ᵐ² = Apparent magnitude of star 2 (no units)
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19
Q

What is the absolute magnitude of a star?

A
  • The apparent magnitude of a star if it were placed 10 parsecs away from Earth.
  • Like apparent magnitude, it also has a number scale: the lower the number, including negative numbers, the brighter the star.
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20
Q

What is absolute magnitude dependant on?

A

It is only dependant on the luminosity of the star (not distance, because it is always measured from a distance of 10pc)

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21
Q

What equation links apparent magnitude to absolute magnitude?

A

m - M = 5log(d/10)

m = Apparent magnitude of star (no units)
M = Absolute magnitude of star (no units)
d = Distance from Earth (in parsecs - pc)

*If you know the absolute magnitude of a star, you can use this equation to calculate the distance from Earth. This is because the distance to most stars is too big to measure using parallax - meaning of this?

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22
Q

How is the m-M = 5log(d/10) rearranged to calculate distance?

A

[ 10^(m-M/5) ] x 10 = d

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23
Q

You can also use the absolute magnitude formula to calculate the distance to distant object of standard candles. What are standard candles and give an example?

A
  • Standard candles are objects that you can calculate the absolute magnitude of directly e.g. type 1a supernovae
  • All type 1a supernovae have the same absolute magnitude.
  • If you then compare it with its apparent magnitude from Earth ,you can use the equation to calculate its distance.
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24
Q

Which objects emit EM waves?

A

Any objects which has a temperature above absolute zero emit EM waves.

  • At room temp, the object would emit waves of the infrared spectrum
  • At much higher temperatures, the object can emit waves of visible light.
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25
Q

SO…what does the wavelengths emitted by an object depend on?

A

The wavelength emitted by an object depends on its temperature.

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26
Q

What is a black body?

A

A ​black body is an object that is a ​perfect emitter and absorber of all possible wavelengths of electromagnetic radiation

  • Called a black body because they don’t reflect any light.
  • There is no such thing as a black body, but to a reasonably good approximation stars can behave as black bodies, so we can use balck body curves to make estimations about its properties. The sun can be assumed to be a black body.
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27
Q

What is black body radiation?

A

As black bodies emit all wavelengths of EM radiation, they produce a continuos spectrum of EM waves known as black body radiation.

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28
Q

What are black body curves?

A

A graph of radiation power output against wavelength of a black body. This varies with temperature, i.e. at certan temperatures, one type of wavelength may be emitted more per second, than another.

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29
Q

What is the general shape of a black body curve.

A

DRAW IT - pg223 of cgp.

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30
Q

What happens to the peak of the black body curve and the power output of a black body as the temperature of the black body increases?

A
  • The peak of the graph moves towards the shorter wavelengths as temperature increases.
  • The height of the peak (as well as the rest of the graph line) also increases i.e. the power output of the black body increases.
  • This means a lot more radiation is released, especially the wavelengths at and near the peak wavelength.
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31
Q

What is Wein’s displacement law?

*This equation is used for black bodies and objects, like stars that behave as black bodies.

A
  • All black body spectra have a peak intensity.
  • The wavelength at which the peak occurs at is known as the peak wavelength, λₘₐₓ.
  • The higher the surface temperature of the star, the short the peak wavelength, λₘₐₓ.

λₘₐₓ is related to the temperature by Wein’s displacement law:

λₘₐₓT = 2.9x10⁻³mK

T = Surface temperature (in Kelvin)
2.9x10⁻³ = Wein's constant (in metres Kelvin)
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32
Q

Why may a hotter star not appear as bright as a cooler one?

A

It may emit radiation that is mostly not part of the visible light spectrum, i.e. its peak wavelength may be located in the UV, X-ray or Gamma ray region whereas the cooler star may emit more radiation with wavelengths corresponding to visible light spectrum making it appear brighter.

LOOK AT PG224 ON CGP TO SEE THIS.

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33
Q

What is Stefan’s Law in words?

A

The power output of a star is directly proportional to its surface temperature to the power of 4 and the surface area of the star.

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34
Q

What is the equation which describes Stefan’s law?

*This equation is used for black bodies and objects, like stars that behave as black bodies.

A

P = σAT⁴

P = Power output of the star (W)
σ = Stefan's constant (5.67 x 10​⁻⁸​ Wm​⁻²​K​⁻⁴) 
A = Surface area of the star (m²)
T = Surface temperature of star (K)

*If a question says, star X has 10 times the power output of star Y, when equating this equation, this is how you would write it: X = 10Y

35
Q

What is the equation that links the intensity of a star and the distance from the star?

  • This equation is used for black bodies and objects, like stars that behave as black bodies.
  • This equation that assumes that the star is spherical and gives out an equal amount of power in every direction.
A

I = P/4πd²

I = Intensity of the star (Wm​⁻²​)
P = Power output of the star (W)
d = distance from the star (m²)
​
*As you can see, ​intensity is inversely proportional to the distance between the star and the observer.
36
Q

You can classify stars using the _____ ______ they produce, as well as their power output,.

A

line spectrum

37
Q

How is an absorption line spectra produced from a star?

A
  • If light from a star is split using a prism or a diffraction grating, you get a line spectrum.
  • Stars are approximately black bodies so they emit a continuous spectrum of EM radiation.
  • You get absorption lines in the spectrum when radiation passes through a cooler gas e.g. in a star’s atmosphere.
  • At certain temperatures, photons of particular wavelengths, released by the star, is absorbed by electrons of certain gas atoms in its atmosphere. This causes the electrons to move up to higher energy levels exciting the atom.
  • When the electrons de-excite, the same wavelength of light is emitted and radiated in all directions.
  • This means the intensity of radiation with this wavelength reaching earth is reduced. This is shown by dark lines in the otherwise continuous spectrum, corresponding to the absorbed wavelengths.
38
Q

The more intense the absorption line (the darker the line) at a particular wavelength….

A

…the more the radiation of that wavelength has been absorbed

39
Q

What is the Balmer series?

A
  • The wavelengths corresponding to the visible part of Hydrogen’s line absorption spectrum.
  • These absorption spectral lines are caused by the electrons in atomic hydrogen moving between the first excitation level (n = 2) and higher energy levels. This leads to a series of absorption lines called the Balmer series.
40
Q

When do hydrogen absorption lines occur in the visible part of a star’s spectrum?

A

When the electrons in the hydrogen atoms start in the n=2 state and are excited to higher energy levels.

41
Q

When are the electrons of hydrogen atoms (in the atmosphere/hydrogen clouds around the star) in the n=2 state?

A

When the temperature of the star is highness collisions of other gas atoms and the hydrogen give the electrons in the hydrogen atom more energy to move from ground state to n=2.
- It’s these hydrogens in n=2 that produce the Balmer series.

42
Q

What happens to hydrogen atoms when the temperature of a star is too high?
What can you conclude about the relationship between temperature and intensity of Balmer spectral lines?

A
  • When the temperature of a star is too high, The majority of the electrons in hydrogen hydrogen atoms will be in the n=3 state. This means there will not be many Balmer transitions (transitions of electrons from n=2 state).
  • So the intensity of the Balmer lines depends on the temperature of the star
43
Q

Because Balmer lines depend on the temperature of the star, you can use the intensity of Balmer lines to determine what?

A

The temperature of a star.

44
Q

What is the problem with using only Balmer lines to determine the temperature of a star?
How do astronomers overcome this?

A

For a particular intensity of Balmer lines, two temperatures are possible.
Astronomers overcome this by looking at the absorption lines of other atoms and molecules as well.

45
Q

How can classify stars?

What does it depend on?

A
  • We can classify stars into groups called spectral classes.
  • The spectral class of a star depends on their temperature (or relative strength of absorption lines of certain elements and molecules* like hydrogen, helium, neutral metals and TiO)

*For example if we look at CGP book pg.229 and concentrate on intensity of H Balmer spectral lines, the peak is greatest for A class relative to all other stars. Another thing is if we used intensity of Absorption spectral lines of only one element like Hydrogen, you can see that for the same intensity of H (Balmer), there are actually two temperatures, as discussed before.

46
Q

How many spectral classes are there and what are they called?
Order them in order of decreasing temperature.

A

OBAFGKM

“Oh Be A Fine Girl Kiss Me”

47
Q

Colour of stars in the OBAFGKM spectral classes?

A 
O = Blue
B = Blue
A = Blue-White
F = White
G = Yellow-White
K = Orange
M = Red
48
Q

Temperature of stars in the OBAFGKM spectral classes?

A 
O = 25,000 - 50,000
B = 11,000 - 25,000
A = 7500 - 11,000
F = 6,000 - 7,500
G = 5,000 - 6,000
K = 3,500 - 5,000
M = <3500
49
Q

Absorption of stars in the OBAFGKM spectral classes.

A 
O = Strong H+ ion, He atom absorption lines. Weak H atom (Balmer) absorption lines.
B = Strong He atom and H atom absorption lines.
A = Strongest H atom absorption lines. Some metal ion absorption lines too.
F = Strong metal ion absorption lines.
G = Metal ion and metal atom absorption lines.
K = Strong metal atom absorption lines.
M = Metal atoms absorption lines. Absorption lines from molecular bands like TiO also*.

*Cool enough for molecules to form.

50
Q

When Hertzsprung and Russel, independantly plotted __________ against ________, they produces the Hertzprung-Russel Diagram.
Draw the Hertzsprung-Rusell Diagram.

A

1) absolute magnitude - y-axis
2) temperature (or spectral classes) - x-axis

PG231 on CGP book

51
Q

What values of apparent magnitude goes on the y-axis of a Hertzsprung-Rusell diagram?
What values of temperature goes on the x-axis of a Hertzsprung-Rusell diagram?

A

1) 15 up to -15

2) 50,000K to 2500K

52
Q

The three main areas on the Hertzsprung-Rusell diagram correspond to? Name them.

A

It corresponds to the three main types of stars:
Main sequence stars
Red giants/supergiants (top right)
White dwarfs (bottom left)

53
Q

What type of stars are main sequence stars?

A

Stars which are in the stable, long-lived phase of their life. Hydrogen is being fused into helium at that point.

54
Q

What type of stars are red giants?

A

Stars with high luminosity and a low surface temperature. Using Stefan’s Law, this means the star must have a high surface area. Other fusion reactions on top of hydrogen fusing to form helium, is occuring.

55
Q

What type of stars are white dwarfs?

A

Stars with low luminosity but high surface temperatures. Using stefan’s law, this means the star must have a low surface area. These are stars in the end of their life, where all fusion reactions haev stopped and they are slowly cooling down.

56
Q

In which order do stars evolve?

A

Main sequence –> Red giants/supergiants –> White dwarfs.

57
Q

How are all mainsequence stars formed?

TO UNDERSTAND FROMATION OF STARS REALLY WELL LOOK AT THIS WEBSITE: http://www.alevelphysicsnotes.com/astrophysics/deadstars.php

A

Begins as a cloud of gas and dust (known as a nebula) left behind from blown up stars.
The denser parts of the cloud contract under the force of gravity until it is dense enough, such that the cloud, fragments into regions called protostars, that continue to contract and heat up.
When the protostar becomes hot enough to fuse hydrogen nuclei into helium nuclei, this reaction releases large amounts of energy and creates enough radiation to stop the gravitational collapse. At this point it has become a main sequence star.

58
Q

What is core hydrogen burning?

A
  • This occurs in main sequence stars. It is when pressure produced from hydrogen fusion in their core balances the gravitational force trying to compress them.
59
Q

What happens when all the hydrogen in the core have fused to form helium?

A

Nuclear fusions of hydrogen stops = outwards pressure stops = gravitational force causes the helium core to contract + heat up = this causes the outer layers to expand and cool = forms a red giant.

Why does it causes the outer layers to expand?

60
Q

What is shell hydrogen burning?

A

The material surrounding the core still as hydrogen. When the helium core contracts and heats up, this heat raises the temperature of this material high enough for the hydrogen to fuse together.

61
Q

What is helium core burning?

A

The helium core continues to contract until the core is hot enough for helium to fuse into carbon and oxgen. This releases huge amounts of energy that pushes the outer layers of the star further outwards.

*Hydrogen core burning causes the size of the star to remain the same as the outward pressure balances gravitational compression whereas helium core burning causes size of the star to increase.

62
Q

What is shell helium burning?

A

When helium runs out, the carbon-oxygen core contracts to heat the material around it which contains helium causing helium to fuse together.

63
Q

How is a white dwarf formed?

TO UNDERSTAND FROMATION OF STARS REALLY WELL LOOK AT THIS WEBSITE: http://www.alevelphysicsnotes.com/astrophysics/deadstars.php

A

For low mass stars, the carbon-oxygen core will not get hot enough for further fusion so still continues to contract under its own weight. When the core reaches the size similar to the Earth, electrons exert enough pressure outwards to stop any further collapse. When its contracting, it causes the helium shell to become more unstable until eventually the star pulsates and ejects its outer layers into space forming a nebula, leaving behind a white dwarf.
The star is very dense and very hot and will eventually cool down.

64
Q

What is the name of the pressure produced by the electrons?

A

Electron degeneracy pressure

65
Q

Which stars form supernovae type 2?

A

Stars of mass much greater than the sun become red giants, then red supergiants and then go supernova.

66
Q

How do type 2 supernovae form?

TO UNDERSTAND FROMATION OF STARS REALLY WELL LOOK AT THIS WEBSITE: http://www.alevelphysicsnotes.com/astrophysics/deadstars.php

A

For stars of much greater mass than the sun, when they become red giants and complete core helium burning, the core can continue to contract to increase the temperature high enough to allow for shell helium burning to occur but also core carbon-oxygen burning to occur, all the way up to core iron burning to occur (this forms a red supergiant). Once an iron forms, no more fusion can occur, so nothing is holding it up against gravity. Therefore the core continues to contract and the outer layers of the star fall in and rebound off the core, setting up huge shockwaves, that cause the star to cataclysmically explode in a supernovae type 2.

67
Q

When a star goes supernova, what is left behind?

A

A neutron star or if the star is large enough, a black hole.

68
Q

Define supernova.

A

A supernova is an astronomical object which exhibits a rapid and large increase in absolute magnitude because of an explosion that ejects most of its mass.

69
Q

What is a type 1 supernova?

A

A type 1 supernova is a supernova which does not have any hydrogen lines in its spectrum. If a supernova does have hydrogen lines in its spectrum then it is a type 2 supernova.

70
Q

What is a light curve?

A

A light curve is a graph of absolute magnitude against time since the supernova reached peak magnitude.
*(Different supernova form different light curves).

But if its reached peak magnitude why does the absolute magnitude on the graph increase again?

71
Q

What are the two defininf features of a supernovae light curve?

A
  • A sharp initial peak followed by a gradually decreasing curve.
    DRAW THIS!!
72
Q

You need to know about a specific subset of type 1 supernova known as type 1a supernova. What are type 1a supernova?

A

Type 1a supernova are supernova that are formed when a white dwarf core absorbs matter from a nearby binary partner.

73
Q

What do we know about the light curves of type 1a supernova?

A

Type 1a supernova all have the same mass when they explode so they have identical light curves, showing the same peak in absolute magnitudes.

74
Q

How do neutron stars form?

A

As the core of the giant star collapses after the initial supernova, the nuclei and electrons present get squashed against each other, until the electrons combine with the protons to create a dense ball of neutrons (as well as releasing neutrinos). The neutron degeneracy pressure stops further contraction.

75
Q

Structure of a neutron star??

A

Neutron stars are extremely dense + made up of mainly neutrons with a diameter of about 20km. It is hot and can sometimes spin rapidly (600 times per second), emitting radio waves as it spins. It evenutally loses energy and slows down.

76
Q

What are the names of neutron stars which spin rapidly?

A

Pulsars.

77
Q

When are black holes formed?

A

When the core of a star is more than 3x the mass of the sun, the gravitational force on the core will continue to occur even after neutrons are formed (proton-electron interaction). Here the neutron degeneracy pressure is not strong enough to withstand the gravitational force (due to the increased size of the core). Therefore the core of the star collapses to an infinitely dense point. This creates a region of space with an infinite gravitational pull known as a black hole. The laws of physics break down at this point.

78
Q

Define escape velocity?

A

Escape velocity is the minimum velocity an object must be given to escape from e.g. a planet when projected vertically.

To escape the gravitational field (gravitational field is only zero at infinity) of an object it must do work, the gain in gravitational potential energy. This is equalt to the loss of kinetic energy. Rearranging KE to give v, tells us the escape velocity.

79
Q

What is the escape velocity of a black hole and why?

A

The gravitational field strength of a black hole is so great that its escape velocity is greater than even the speed light, so not even light can escape it.

*(this is technically the universal speed limit so basically the fastest thing in the universe that we know about).

80
Q

What is the event horizon of a black hole?

A

The boundary of the region around the infinitely dense point at which the escape velocity is equal to the speed of light. Inside this boundary, the escape velocity is greater than the speed of light.

81
Q

What is the schwarzchild radius?

A

This is the radius of the event horizon of a blackhole.

82
Q

Equation to calculate the Schwarzchild radius?

A

Rs = 2GM/c²

Rs =  Shwarzchild radius (m)
G = Gravitational constant (Nm²kg⁻²)
M = mass (kg)
c = speed of light (m/s)
83
Q

Note: If asked to calculate the density of the black hole, when using mass/volume, the volume is calculated using 4/3πr³, because we assume the black hole is spherical.

A

N/A

Physics AQA AS Level (16 decks)

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