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AQA A-Level Astrophysics > Stellar Evolution > Flashcards

Stellar Evolution Flashcards

1
Q

What do stars begin as?

A

Clouds of dust and gas

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2
Q

Where do clouds of dust and gas come from?

A

Previous stars blow themselves apart in supernovae

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3
Q

How do protostars form?

A

Denser clumps of cloud contract under the force of gravity, then fragments into regions called protostars

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4
Q

How do stars form from protostars?

A

Protostars contract and heat up. At a few million degrees, hydrogen nuclei fuse into helium, releasing a lot of energy creating pressure (radiation pressure) that stops the gravitational collapse

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5
Q

How does a star move through the different categories of star?

A

(cloud of dust and gas → Protostars) → Main Sequence → Red Giants → White Dwarfs → Fade away

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6
Q

What are the four stages of stars burning?

Where does main sequence → red giant slot in

A 
Core Hydrogen → Helium burning
(Main sequence → red giant)
Shell Hydrogen → Helium burning
Core Helium → Carbon and Oxygen burning
Shell Helium → Carbon and Oxygen burning
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7
Q

(1) What is core hydrogen burning?

A

The pressure produced from hydrogen fusion in a Main Sequence star’s core balances the compressive force of gravity

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8
Q

(1) In what kind of star does core hydrogen burning occur?

A

Main Sequence

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9
Q

(1) How does a Main Sequence star become a Red Giant?

A

Nuclear fusion stops as all of the hydrogen is used up (fused into helium). The now-helium core contracts and heats up under the weight of the star. As a result, outer layers expand and cool into a Red Giant

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10
Q

(2) What is shell hydrogen burning?

A

The material surrounding the core still has lots of Hydrogen. Heat from the contracting helium core raises the temperature of this material enough for hydrogen to fuse

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11
Q

(2) After shell hydrogen burning, what happens to very low mass stars?

A

They stop.

They use up their fuel and slowly fade away

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12
Q

(3) What is core helium burning?

A

The helium core contracts until it is hot and dense enough for helium to fuse into carbon and oxygen. This releases a huge amount of energy, pushing the outer layers of the star outwards

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13
Q

(4) What is shell helium burning?

A

When the helium runs out, the carbon-oxygen core contracts and heats a shell around it so that helium can fuse in this region

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14
Q

At which stage of star burning do low-mass stars stop fusing?

A

Shell hydrogen burning

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15
Q

Why do low-mass stars stop fusing after shell hydrogen burning?

A

The carbon-oxygen core isn’t hot enough for any further fusion and so it contracts under its own weight

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16
Q

How does a low-mass star become a White Dwarf?

A

Once the core shrinks to Earth-size, electrons exert enough pressure to prevent collapsing.

The helium shell becomes more unstable as the core contracts.
The star pulsates and ejects its outer layer as a planetary nebula, leaving behind the dense core, becoming a white dwarf.

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17
Q

What happens to a White Dwarf?

A

It cools down and fades away

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18
Q

Describe the sun’s evolution through the H-R diagram

A

Starts at (G,5) -> Main sequence

Goes to (middle of K and M, -5) -> Red giant/supergiant

Goes to (B,10) -> White dwarf

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19
Q

Which mass of star stays as a Main Sequence star the longest?

A

Low-mass

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20
Q

What do red giants with high mass do with core to shell burning?

A

Fuse beyond helium - building layers in an onion like structure = red supergiant

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21
Q

What can really massive stars fuse up to?

A

iron

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22
Q

What happens after iron core is formed?

A

Fusing beyond iron isn’t energetically favourable - after iron core is formed the star quickly dies

23
Q

What happens when a star runs out of fuel?

A

It contracts

24
Q

Beyond what mass can electron degeneracy not stop the core contracting?

A

1.4x the mass of the sun

25
Q

What happens when the core of the star keeps contracting?

A

Outer layers fall in and rebound off the core (which is now a neutron star).

Makes huge shockwaves, increasing the absolute magnitude rapidly.

Shockwaves make stars explode cataclysmically in a supernova.

It leaves behind the core - this will be either a neutron star or (if the star was massive enough) a black hole.

26
Q

In short, what happens to a high-mass star after nuclear fusion of iron?

A

It explodes in a supernova

27
Q

What are the products of a supernova?

A

Neutron star or black hole, and gamma ray bursts

28
Q

How does absolute magnitude change due to a supernova?

A

Star has a rapid increase in absolute magnitude.

Light can outshine an entire universe for a few weeks/months and fades out.

29
Q

Energy output of type 1a is 10^x Joules.

What is x?

A

x = 44.

10⁴⁴J

30
Q

How are neutron stars formed (particles in the core of the star)?

A

As the core of a massive star contracts, the electrons in the core material get squashed into an atomic nuclei and combine with protons to form neutrons and neutrinos.

31
Q

How heavy does a stars core have to be to become a neutron star?

A

Between 1.4 and 3 solar masses

32
Q

What are neutron stars made of?

A

Neutrons

33
Q

How dense, large, and fast (rotating) are neutron stars

A

very dense = 4x10^17 kgm^-3

Very small = 20 km across

Very fast = 600 rotations per second

34
Q

How many radio wave beams do some neutron stars emit?

A

2

35
Q

What makes some neutron stars pulsars?

A

Neutron stars emit radio waves in two beams as they rotate.

They sometimes sweep past earth = observed as radio pulses like flashes.

These pulsing neutron stars are called pulsars

36
Q

What is the mass of the core of a star who’s neutrons can’t withstand the gravitational forces?

A

3 times the Sun’s mass

37
Q

What happens to a star who’s neutrons can’t withstand the gravitational forces?

A

It collapses to an infinitely dense point called a singularity

38
Q

What is the Schwarzchild radius?

A

The distance at which the escape velocity is the speed of light.

Inside the radius, nothing can escape but only travel further in the black hole.

39
Q

What is the boundary of the Schwarzchild radius called, where light has just enough kinetic energy to escape the gravitational pull of the black hole?

A

Event horizon

40
Q

What is escape velocity?

A

The velocity an object would need to travel to have enough kinetic energy to escape a gravitational field

41
Q

What is a black hole?

A

An infinitely dense point where the escape velocity is greater than the speed of light.
(But nothing is faster than c).

It is a region where nothing can escape, including light.

42
Q

What is the structure of a black hole?

A
43
Q

What do scientists believe about a supermassive black hole?

A

(10^6 times more massive than the sun.)

Centre of every galaxy - as they consume stars close to them, they produce intense radiation, making the centre of galaxies very bright.

44
Q

How do you derive the equation for the Schwarzschild radius from KE and GP?

A
45
Q

What is the curve of a Type 1a Supernovae plotted against?

A

Absolute magnitude, M, against time since supernova began

46
Q

What are the defining features of a Type 1 Supernovae curve?

A

A sharp initial peak

then a gradually decreasing curve

47
Q

What absolute magnitude does the graph of a Type 1a Supernova go from and up to?

A

(2,-18) and peaks at (0,-19.6)

48
Q

If a type I has hydrogen lines, what will it be classed as?

A

Type II

49
Q

Type 1a supernova all have the same mass when they explode this mean that on the graph …

A

The peak is the same.

The curve is the same.

50
Q

Why are Type 1a Supernovae important?

A

They always happen the same way, with a star of the same mass.

On the graph:
The peak is the same.
The curve is the same.

So they can be used as a standard candle.

51
Q

How is type 1a formed?

A

Subset of type I:

Formed when a white dwarf core absorbs matter from a nearby binary partner.

52
Q

What distances can be measured because of type 1a supernova

A

1000Mpc

53
Q

How much energy is released in a Type 1a supernova?

A

10⁴⁴J

54
Q

What time does the graph of a Type 1a Supernova go up to?

A

40 days

AQA A-Level Astrophysics (11 decks)

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