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Astronomy 101 > Chapter 5 > Flashcards

Chapter 5 Flashcards

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1
Q

How are Stars formed?

A

Cloud of dense, cold gas is compressed via gravity

It eventually becomes so dense that nuclear fusion starts to occur

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2
Q

How cold are the gas clouds?

A

30-100K

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3
Q

How hot is the Sun’s surface?

A

5800K

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4
Q

What colour is the Sun?

A

Green-yellow

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5
Q

Why do we see the Sun as white?

A

It is so bright that the green-yellow color is masked

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6
Q

How hot is the Sun’s outer atmosphere

A

1,000,000K

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7
Q

What’s keeping our atmosphere from sinking?

A

Hydrostatic equilibrium

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8
Q

What is Hydrostatic Equilibrium?

A

The weight of the uppermost layers crush the lower layers

Once the pressure gets to a high enough level, then the lower gases resist and push back

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9
Q

Where on stars do nuclear reactions occur?

A

At the core

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10
Q

What is the temperature of thee sun at its core?

A

1.5x10^7K

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11
Q

How old is the sun?

A

5bn y/o

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12
Q

What is the lifespan of a star like the Sun?

A

around 10bn years

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13
Q

What determines what happens to the star?

A

Its mass

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14
Q

What happens to less massive suns like the Sun?

A

They slowly die out as white dwarves

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15
Q

What happens to more massive stars than the Sun?

A

Some end up as neutron stars, others end up in massive cataclysmic explosions called supernovae

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16
Q

What is the raw material from which stars form?

A

Interstellar gas

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17
Q

What are the clouds of interstellar material called?

A

nebulae

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18
Q

Why are dust grains important in the formation of stars in the nebulae?

A

They radiate away energy, helping to cool the cloud to 30-100K

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19
Q

What is the relationship between temperature and the collapse of the gas clouds?

A

The colder the gas, the greater the chance the gas has of collapsing before the pressure to resist the collapse can build up again

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20
Q

How does the nuclear fusion within the protostar occur?

A

The gas heats up when it is compressed

Hydrogen is ionised so that it loses its single electron. The more electrons that are liberated, the more opaque the protostar becomes. This means that the photons trap energy within the star and the energy can cause nuclear fusion

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21
Q

What is the minimum mass for star formation?

A

0.08M.

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22
Q

What are red dwarfs?

A

Low-mass stars that are cool, glowing with only a faint red light

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23
Q

What are brown dwarfs?

A

Objects just below the mass for stellar recognition. They tend to be extremely dim, even to infrared light

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24
Q

What are stars primarily made out of?

A

Hydrogen gas

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25
Q

What was the original belief as to what powered the stars?

A

Gravity powered the stars

Compression of the gas would cause the gravitational energy to convert into heat and light

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26
Q

What discovery changed the tact from the belief that the Sun was powered by gravity?

A

The discovery of radioactivity

27
Q

What is the condition of the gas inside of the Sun?

A

They are the nuclei: their electrons have been stripped

28
Q

What is the relation between the size of a star and its central pressure?

A

The larger the star, the greater the central pressure

29
Q

What are the 2 factors that control the nature of a star?

A

Mass, composition

30
Q

Why do the protons in the core of a star stick together?

A

They are attracted to each other via nuclear force.

31
Q

What is the Coulomb barrier?

A

The gap created when protons repel each other

32
Q

What is key about the nuclear processes?

A

They provide the energy necessary to maintain the hydrostatic equilibrium

33
Q

What is the relationship between the luminosity of a star and its mass?

A

Luminosity = Mass cubed

34
Q

Who came up with the diagram relating luminosity and spectrum released by the star?

A

Hertzsprung Russell

35
Q

What is the main sequence?

A

The main line in the Hertzprung-Russel diagram

36
Q

For how long is the star is in the Main Sequence?

A

For as long as the fusion of hydrogen to helium dominates

37
Q

In the main sequence, what is the relationship between luminosity and mass?

A

Luminosity = Mass ^3.1

38
Q

What happens at the core when the hydrogen stops fusing with helium? (3)

A

The pressure drops, causing the force of gravity to make it contract.

As the core contracts, the temperature increases

When the temperature reaches a certain point, helium will begin to fuse into carbon in the deep interior whilst the hydrogen continues to burn normally in the shell surrounding the core

39
Q

How is a red giant formed?

A

When the core contracts after Hydrogen stops fusing with helium, the temperature goes up as the helium burns into a carbon core

This heat causes the outer regions to expand and thus, cool.

40
Q

What happens to large stars when the usable fuel has been converted into carbon?

A

The core contracts again. Under this pressure electron degeneracy occurs

41
Q

What is electron degeneracy pressure?

A

As the body that the electron resides in shrinks, the electron gains energy

42
Q

Who developed the idea of electron degeneracy pressure?

A

Chandresekhar

43
Q

As the core of the star compresses into a white dwarf, what happens to the rest of the star?

A

The star ejects its swollen outer layers and forms a planetary nebula

44
Q

What is the relationship between size and mass of White Dwarves

A

The more massive the WD gets, the smaller it gets

45
Q

What happens to the WD as it cools?

A

It crystallises

46
Q

What happens in a Type 1a Supernova?

A

A white dwarf “steals” matter from a companion star; the WD’s mass slowly increases until the WD’s mass exceeds 1.4 M¤ and it explodes.

47
Q

When can a White Dwarf be regenerated?

A

When it is in distance with a companion star to ‘steal’ matter from it. The mass slowly increases until it reaches around 1.4M and explodes

48
Q

What happens in the core of very heavy stars?

A

Increasingly heavy elements are subsequently burned at the core, fusing all the way to Iron

49
Q

What is the issue when Iron forms at the core?

A

Iron cannot provide the energy the star needs in order to support its weight.

50
Q

What is the Chandresekhar limit?

A

This is the limit of the mass that electron degeneracy pressure can support. When a white dwarf exceeds this, it collapses

51
Q

What is the maximum original mass that is required for a white dwarf to form from a star?

A

10M

52
Q

What happens if the electron degeneracy pressure falls short and can no longer support the star?

A

Massive explosion calles a Supernova

53
Q

Characteristics of a supernova

A
  • High energy (can have more than the other 100BN stars combined)
  • Creates heavier elements in the explosion
  • Ejects material at speeds of around 10,000km/sec
54
Q

What is a type 2 Supernova?

A

A massive star (> 10 M¤) explodes when its core starts to produce iron.

55
Q

How can a white dwarf explode?

A

When it accrues more than 1.4M worth of mass, it explodes as it has surpassed the Chandresekhar limit. The carbon in it ignites, which ignites all of the matter in the WD simultaneously and BOOM

56
Q

What is Neutron Degeneracy Pressure?

A

Neutrons, when compressed, can give significant amounts of energy (more than electrons)

57
Q

How are neutron stars formed?

A

As the star collapses into greater and greater compaction, the protons and electrons merge to form neutrons.

These neutrons exert neutron degeneracy pressure. this forms a massive neutron nucleus known as a neutron star

58
Q

What would the radius be of a neutron star with the density of the sun be?

A

10km

59
Q

What do Pulsars emit?

A

They emit highly regular bursts of electromagnetic radiation, generally as radio waves

60
Q

Why do neutron stars emit pulsars in the way that they do?

A

They are spinning very quickly due to the conservation of angular momentum

They rotate around 30x per second and are strong enough not to break up even when rotating at such great speeds

They have one specific beam of radiation. It appears to be flashing this radiation as we are detecting it when it spins towards us

61
Q

What is the lighthouse model

A

The theory that pulsars have one specific beam of radiation. It appears to be flashing this radiation as we are detecting it when it spins towards us

Radiation occurs at the north and south poles of the magnetic field

62
Q

Who is Annie Cannon?

A

She analysed and catalogued 350,000 spectra of Stars at Harvard and organised them in terms of how much hydrogen they had in them

63
Q

How did Cannon reorganise the orders of the stars?

A

OBAFGKM

64
Q

Why were the orders of the stars reorganised?

A

Because many of the stars had the same chemical make-up, but emitted different colours. Thus, they were reorganised according to temperature

Astronomy 101 (15 decks)

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