Chapter 19 Stars

Question 1

Definition of a planet

Answer 1

An object in orbit around a star

• has mass large enough for gravity to give it a round shape
• has no fusion reactions (else a star)
• has cleared its orbit of most other objects

A dwarf planet hasn’t cleared its object of most other orbits, thus not a planet

Question 2

Planetary satellites

Answer 2

Include both moons and man made satellites, anything that orbits a planet

Question 3

Comets

Answer 3

Small irregular bodies made of ice dust and small pieces of rocks

All comets orbit the sun in highly eccentric orbits

Question 4

Solar system

Answer 4

Contains the sun and all the objects that orbit ti
- planets, comets etc

Question 5

Galaxies

Answer 5

A galaxy is collection of stars , interstellar dust and gas

A lot of these stars will have their own solar system too

Question 6

Universe

Answer 6

Everything that existed
- em rsdistiln , energy, space time, stars matter everything

Question 7

What is nebullae?

Answer 7

A collection of dust and gas (mostly hydrogen)

They form over many years as tiny gravational attraction between particles pull them together , forming vast regions of cloud

Question 8

How does a nebullae become a protostar

Answer 8

As they get closer to Esch other gravational collapse accelerates

Eventually a denser region of Nebullae forms, and now it begins to heat up as gravational energy is transferred to kinetic and so the dense region begins tk heat up

This forms a protostar

Question 9

When does a protostar become a star

Answer 9

Only if the temperatures in the core and pressures are high enough for fusion of hydrogen tk occur , if this point never reaches a star is never born . This is because for fusion tk occur, electrostatic forces of repulsion between hydrogen nuclei must be overcome such that they can get close enough to experience the string nuclear force , and fuse

As soon as hydrogen fuse, it enters the main sequence of a star

Question 10

What happens when the star is in its main sequence

Answer 10

The gravational force of attraction acts tk compress the star

But this is countered by the RADIATION and GAS pressure due to fusion

As a result the star remains in a steady state as long as it is fusing

Question 11

Why do bigger stars have a much lower time as main sequence than smalller stars?

Answer 11

Bigger stars mean much hotter cores, due to more gravational potential energy being converted to kinetic in the core

As a result the rate of fusion of hydrogen is massively faster, as higher temps and pressures are achieved , so their sequence is less

But smaller mass has lowe temps and pressures, so rwte of hydrogen fusion is less and its sustained over lomger periods of time

Question 12

Again so why are higher mass stars more hotter cores?

Answer 12

Because more scope for more GPE to be converted to kE and thus temp in the core due to bigger mass

Question 13

Remember what is the outward pressure (2 things)

Answer 13

Radiation AND gas pressure due to fusion

Question 14

How do low mass stars become red giants

Answer 14

Once fusion of hydrogen in core is run out, there is no longer any outward pressure
- thus the gravational force is greater and acts to compress the star. The core begins to collapse and shrink
- as the core shrinks, this increases the pressure and temperature inside the core.
- as a result , conditions are right for fusion of hydrogen in the SHELL around the core.
- this creates some last outward pressure which causes the outward layers of star to expand outwards and move away from the core
- eventually they cool and form a red layer

The star is now a red giant

Question 15

What happens once the hydrogen in the shells of the core run out too?

What is left

Answer 15

• By now most of the layers have drifted away as planetary NEBULAE
• And so there is no more radiation pressure, and so the gravational force will act to compress

What’s left behind is a what dwarf, which is extremely hot and no more fusion

But doesnt collapse immedialty yet

Question 16

What’s stopping the white core from collapsing?

Answer 16

There is the pauli exclusion principle which stated two electrons can’t exist in the same energy state at the same time

• thus a certain pressure exists due to electrons squeezing closer to each other called ELECTRON DEGENERACY pressure .
• this counters the extreme collapse of the whit dwarf core due to gravity

However this only happens to a limit

Question 17

What’s the Chandrasekhar limit?

Answer 17

The limit it can withstand grav Collapse is if the core is less than 1.44M of the sun

More than that, and it camf withdntajd the collapse

This is known as the Chandrasekhar limit

Question 18

What are freaturee of the white dwarf?

Answer 18

• very dense
• mass similar to the sun (remember has tk be less than 1.44M if it was to even exist)
• no fusion left
• very hot surface temp due to photons from earlier life

Question 19

Finally a low mass star white dwarf ends as what?

Answer 19

Cools to form a black star and that’s it

Question 20

HIGH MASS STAR

What happens after main sequence for a high mass star?

Answer 20

1) again the outward radiation and gas pressure drops, and now the gravational force exceeds the outward pressure
2) this causes the star to collapse, along with the core
3) core gets hot enough now , but because much more mass, the core gets hot enough for fusion of HEAVIER ELEMENTS to fuse in the core (as we,k as hydrogen in shells)
4) this causes immense radiation pressure and caused the outer layers to expand and cool snd become red again

5) as the most inside part of core is the hottest, this is the one that does fusion the most. It fuses until it becomes iron, as after this fusion csn’t happen anymore as iron the most stable isotope . This forms a core of shells of heavier elements, with iron in the middle, and les heavy as you come out

6) however eventually the whole core finally fused to become an iron core.

Question 21

What happens when fusion stops and thr core is iron for a high mass star?

Answer 21

Now no fusion = no outward pressure
- the gravational force is much stronger and pulls in the layers
- these bounce of the layers as an implosion leading tk a shockwave that ejects all the layers into space

This is a SUPERNOVA

Question 22

After the supernova, what’s left of the core for a super red giant

What are rhe two possibilotedfirst

Answer 22

It can either become a neutron star, or a black hole

Question 23

How does the core after supernova of a super red giant become a neutron star

Answer 23

• if the mass of the core exceeds the chanreskehar limit, then thr grav force will counter the electron degeneracy pressure
• thus the collapse continues, and you form a neutron star

As it collapsed a lot, the neutron star is a very small volume but HIGH DENSITY object

Question 24

Features of a neutron star

Answer 24

Mad elf entirely neutrons

• very small volume
• very high density
  # therefore very high mass

Question 25

How does core after supernova of super red giant become a BLACK hole instead?

Answer 25

• if the mass exceeds 3M , then the gravational force will of course counter electron degeneracy and so collapse of core comt8nues
• however it’s collapsed so hard that you are left with something with craxy mass such that it’s gravational field is so stromg that you’ll need an escape velocity of the speed of light to escape it

As nothing ks faster, nothing escapes a black hole, not even em radisrion which travels at speed of light

So massive

Question 26

Features if a black hole

Answer 26

Crazy mass number

• such high grav force that escape velocity greater than spedd of light is needed to escape it
• therefor even em radiating Escape

Question 27

You said that fusion in the super red giant core stops at iron. Thus how is any heavier elements made.

Answer 27

This is all made in a supernova

Thus any heavier lament we see today came from a supernova in the past!

Question 28

Summary
1) How is a protostar formed.

2) how is a star born

Answer 28

Nebullae are collections of cast dust and gas which formed over millions of years due to small attrsvtions of grvatisonl force bringing the particles close together

As more and more mass is collected the grav force gets stronger. Eventually a denser area of the nebullae is formed, and here GPE due to attraction is converted to Ke. This makes it hotter snd higher pressure

This dense region of mass of high temp and pressure is known as a protostar

2) when the temp and pressure is high enough, fusion of hydrogen can take place in thr protostar. As soon as hydrogen fuses, the star is born

Question 29

Explain the main sequence which ALL STARS FACE

Answer 29

after first hydrogen fuses, it generated an outward pressure due to radiation pressure from thr PROTONS it emits and gas pressure due to Ke that’s produced

• this counters the gravational force of attraction that aims to compress thr star .
• as a result, for as long as hydrogen is available in the core of the star and is fusing, the star is in its MAIN SEQUENCE

Question 30

Why do heavier stars have a much lower time in main sequence and thus life cycle?

Answer 30

Higher mads Star = More gpe is converted into ke = more rwte of hydrogen fusion due to higher temps and pressure

As a result the hydrogen is used much quicker and thus higher mass stars have a lower life cycle

Question 31

How does a star and why become a red giant after main sequence

Answer 31

After main sequence, fusion in hydrogen stops and so the grav force now exceeds the radiation and gas pressure . This caused the star to collapse , including the core

• as the core collpases, the core becomes hotter and higher pressure .
• for lower mass stars, this still is NOT enough for fusion of heavier elemenrs to take place.
• however it is enough for fusion of hdydorgen in thr SHELLS to take place
• this creates some outward pressure, which caused the layers of the star to expand away from the star
• eventually this cools down, and becomes red in colour. The star is now a red giant

Question 32

What happens to red giants then

Answer 32

• eventually thr fusion of hydrogen in shells have ran out, and there is no more outward pressure again. By now most of layers have expanded off as planetary nebullae
• thus the gravational force aims to collapse the star snd the core again
• however due to the Pauli exclusion principle, electrons can’t exist in the same energy state. Thus as core collapses and electrons begin to saueexe together, a Pressure I generated and this prevents the collapse of the core anymore
• this is called electron degeneracy pressure. For stars of cores up to 1.44M, the electron degeneracy pressure counters the grav force, leaving the core as a WHITE DWARF. I’d higher than 1.44m likely to be a super red gisjt anyways

White dwarf has mass similar to sun , no fusion, but very high surface temp. This is due to phomts left behind from earlier.

Evermtuslly white dwarf cools to black dwarf

Question 33

Summar of high mass star

Answer 33

1) once main sequence up it colapses, however due to being heavier thr core is much hotter .
2) therefore fusion of HEAVIER ELEMENTS CAN HAPPEN, such as helium
3) this generates a greater outward force, and thus outer layers expand and cool down a lot becoming red. This forms a super red giant
4) fusion of heavier elements continue to happen until iron as this is the most dtsble isoptope. You get a core of different shells of heavy elements, eventually thr whole core has fused to become iron core.
5) at this point no more fusion can take place, and so outward pressure gone, the hrav force starts to collapse the star
- this causes an Implosion , where the outer layers are pulled and bounce off the iron core, cresting s shockwave thst ejects all the material into space with craxy energy, supernova

After supernovs ,

These stars will have higher mass core than Chandra had . If higher than 1.44M , collapse continues and doesn’t stop, and so the core is reduced to a very small volume neutron star, made of mostly neutrons, but extremely high density and mass

If it’s even higher than 3M, then it collapsed to such a high mass object it becomes a black hole. Here the grav force of the black hole so string thst need higher escape veocllty than speed of light to escape, therefore nothing can escape its pull, not even photons

Question 34

Summary

Answer 34

Protostar
Main sequence
Low mass = red giant
Collapse = white dwarf (less than 1.44M)
Black dwarf

Greater than 10M!
Main sequence
Collapse and heavier element + expand = super red giant
Once iron core supernovs

If core left nehidn 1.44 m to 4 = neutron star
If more than 3 = black hole