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A2 Physics > 3.9.2 - Classification of Stars > Flashcards

3.9.2 - Classification of Stars Flashcards

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1
Q

1 parsec?

A

The distance to a star with a parallax angle of 1 arcsecond.

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2
Q

1 arcsecond?

A

1/3600 of a degree.

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3
Q

How do you find the distance to a star in parsecs?

A

d = 1/p (p = parallax angle in arcseconds)

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4
Q

1 light year?

A

The distance light can travel in one year in a vacuum.

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5
Q

1 astronomical unit?

A

The mean distance from the centre of the earth to the centre of the sun.

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6
Q

Apparent magnitude?

A

The measure of how bright a star appears when viewed from earth.

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7
Q

What is the Hipparcus scale and how does it work?

A

A scale used to label stars. Dimmest visible star = 6 and brightest 1. A logarithmic scale.

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8
Q

What is the difference in intensity between stars with a difference of 1 on the Hipparcus scale?

A

Corresponds to a difference in intensity of 100^(1/5) = 2.51 times.

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9
Q

How and why has the Hipparcus scale been extended?

A

Since telescopes have become more advanced. 30 = dimmest, -30 = brightest.

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10
Q

What is the scale tied to?

A

The star Vega, which is defined as having an apparent magnitude of 0.

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11
Q

What is absolute magnitude?

A

What the apparent magnitude of the star would be if the star was 10 parsecs away. Does not depend on the distance of the star from earth.

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12
Q

M?

A

Absolute magnitude.

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13
Q

m?

A

Apparent magnitude.

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14
Q

What is a black body?

A

A body that absorbs all electromagnetic radiation of all wavelengths and can emit all wavelengths of electromagnetic radiation.

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15
Q

What are black body curves?

A

Graphs of radiation power output against wavelength for a black body at different temperatures.

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16
Q

What are black body curves used for?

A

Make estimations about an objects temperature.

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17
Q

What do all black body spectra have and what does this mean?

A

Peak intensity - this means that the wavelength this occurs at is the peak wavelength.

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18
Q

What does a higher surface temperature mean?

A

Shorter peak wavelength.

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19
Q

What will a hotter black body emit?

A

More radiation than a cooler one, and so its peak wavelength will be a shorter wavelength. It will also have a higher power output.

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20
Q

In Wien’s displacement law what is the constant measured in?

A

metres kelvin.

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21
Q

What can you use Wien’s law to calculate?

A

The surface temperature of the black body.

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22
Q

What is Stefan’s law?

A

The power output of a star is proportional to the fourth power of the star’s surface temperature and is directly proportional to the surface area.

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23
Q

To use the inverse square law what do you assume?

A

That the star is spherical and that it gives out an even amount of power in every direction.

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24
Q

What does E=hf mean?

A

Only certain frequencies and so certain wavelengths of light can be absorbed by electrons.

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25
Q

What happens when you split light from a star?

A

You get a spectrum. Stars are approximately black bodies so they emit a continuous spectrum of electromagnetic radiation.

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26
Q

When do you get absorption lines in the spectrum?

A

When radiation from the star passes through a cooler gas. Dark lines correspond to the absorbed wavelength.

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27
Q

What does the intensity of the absorption line mean?

A

How dark it is - the more intense the absorption line at a particular wavelength, the more radiation of that wavelength has been absorbed.

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28
Q

What causes the wavelengths corresponding to the visible part of the hydrogen’s line absorption spectrum, and what are they called?

A

Electrons in atomic hydrogen moving between the first excitation level (n=2) and higher energy levels. This leads to the Balmer series.

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29
Q

When are Balmer lines seen?

A

When the star has been absorbed through hydrogen atoms in the stellar atmosphere.

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30
Q

What needs to have happened for hydrogen absorption lines to occur in the visible spectrum? When does this happen?

A

Electrons in the hydrogen atoms already need to be in the n=2 state. This happens at high temperatures where collisions between the atoms give electrons extra energy.

31
Q

What does the intensity of the Balmer lines depend on?

A

The temperature of the star.

32
Q

What is the order of the spectral classes?

A

O B A F G K M

33
Q

O

A

Blue, 25000 - 50000K, He+, He, H

34
Q

B

A

Blue, 11000 - 25000K, He, H

35
Q

A

A

Blue-white, 7500 - 11000K, H (strongest), ionised metals.

36
Q

F

A

White, 6000 - 7500K, ionised metals.

37
Q

G

A

Yellow white, 5000 - 6000K, ionised and neutral metals.

38
Q

K

A

Orange, 3500 - 5000K, neutral metals

39
Q

M

A

Red, <3500K, neutral atoms, TiO

40
Q

Why can FGKM form metals?

A

Only cooler stars contain atoms with electrons in low enough energy levels to bond and form molecules and therefore show molecular bands in their absorption line spectra.

41
Q

What is the main sequence?

A

Where stars are in their long-lived stable phase, fusing hydrogen into helium.

42
Q

What are red giants?

A

Stars that have moved off the main sequence, fusion reactions other than hydrogen to helium are also happening in them.

43
Q

White dwarfs?

A

Low luminosity, but high temperatures and small. About the size of the earth and are at the end of their lives, where all of their fusion reactions have stopped and they are cooling down.

44
Q

What are the scales of the Hertzbrung-Russel diagram?

A

-10 to +15 on y axis (absolute magnitude) and 50 000K - 25 000K (OBAFGKM) on x axis.

45
Q

How are stars formed?

A

Denser clumps of clouds of dust and gas contract under the force of gravity. When they get dense enough the cloud fragments into protostars, that continue to contract and heat up. Eventually the temperature reaches a few million degrees and the hydrogen nuclei fuse together forming helium.

46
Q

What happens during hydrogen burning?

A

The pressure produced from hydrogen fusion in the core balances the force trying to compress them.

47
Q

What happens when the core runs out of hydrogen?

A

Nuclear fusion stops and with it the outward pressure stops. The helium core contracts and heats up under the weight of the star. The outer layer expands and cools and then the star becomes a red giant.

48
Q

What is and when does helium burning occur?

A

When the core gets hot enough and dense enough due to contractions, helium fuses into carbon and oxygen. This releases huge amounts of energy which pushes the outer layers of the star further outwards.

49
Q

What happens when the helium runs out?

A

The carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region - shell helium burning.

50
Q

What is electron degeneracy pressure?

A

When electrons in the core exert enough pressure to stop it collapsing any more.

51
Q

What happens to the helium shell as it contracts?

A

It becomes more unstable - the star pulsates and ejects its outer layers into space as a planetary nebula, leaving behind the dense core. The star is now a very hot, dense solid called a white dwarf.

52
Q

What can happen to the shell burning process of red giants?

A

It can continue past helium, building up layers on an onion-line structure. For really massive this can go up to iron.

53
Q

What happens once iron is formed?

A

Nuclear fusion is no longer energetically favourable so the star dies.

54
Q

What happens during the formation of a white dwarf?

A

The outer layers of the star fall in and rebound off the core, setting off huge shockwaves.

55
Q

What do the shockwaves cause?

A

Cause the star to explode in a supernova, leaving behind the core which will either be a neutron star or black hole (if big enough).

56
Q

What happens when a star explodes in a supernova?

A

It experiences a brief rapid increase in absolute magnitude. The light from a supernova can briefly outshine an entire galaxy before fading.

57
Q

What can also be released by a massive star during the explosion?

A

High energy gamma rays. These gamma bursts can go on for minutes or rarely hours.

58
Q

How can supernova be represented?

A

Using light curves.

59
Q

What is a light curve?

A

A graph of absolute magnitude, M, plotted against the time since the supernova reached peak magnitude.

60
Q

What is a type 1 supernova?

A

One which has no hydrogen lines in its spectrum.

61
Q

When are type 1a supernova formed?

A

When a white dwarf core absorbs matte from a nearby binary partner.

62
Q

Why do type 1a supernova have identical light curves?

A

They have the same critical mass when they explode - so they can be used as a standard candle.

63
Q

What happens when the core of a star contracts?

A

The core material gets squashed onto the atomic nuclei and combine with protons to form neutrons and neutrinos/

64
Q

When is a neutron star formed?

A

If the stars core is between 1.4 and 3 solar masses.

65
Q

What are characteristics of neutron stars?

A

Incredibly dense, very small (20km across) and rotate very fast.

66
Q

What do neutrons stars emit and why?

A

As they rotate they emit radio waves in two beams.

67
Q

What can be detected on earth from neutron stars?

A

The beams sometimes pass the earth and can be observed as radio pulses. These pulsing neutron stars are called pulsars.

68
Q

When are black holes formed?

A

When the core of the star is more than 3 times the Sun’s mass, the core contracts until neutrons are formed, but now the gravitational force on the core is greater so the neutrons cannot withstand this gravitational force.

69
Q

What is the escape velocity?

A

The velocity that an object would need to travel at to have enough KE to escape a gravitational field.

70
Q

What happens to the escape velocity of a black hole and why?

A

It becomes greater than the speed of light as the region has such a strong gravitational field due to the star collapsing into an indefinitely dense point.

71
Q

What is the event horizon?

A

The boundary of the region around the indefinitely dense point. The distance at which the escape velocity = c so light has just enough KE to escape the gravitational pull of the black hole.

72
Q

What is the radius of the event horizon called?

A

The Schwarzschild radius.

73
Q

Where do astronomers believe black holes are?

A

At the centre of every galaxy. As they consume stars close to them, they produce more intense radiation, making the centre of galaxies very bright.

A2 Physics (25 decks)

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