Stars and The Universe — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

Section 1 of 23The Sun as a star

The Sun as a star

Type

• Type the Sun is a medium-sized star at the centre of the Solar System.

Composition

• Composition mostly hydrogen and helium.

Radiation

• Radiation most of its energy is radiated in the infrared, visible and ultraviolet regions of the electromagnetic spectrum.

Energy source (Extended)

• Energy source (Extended) in the core hydrogen nuclei fuse to form helium, releasing a huge amount of energy;
  • all stable stars are powered by fusion.

One possible reaction is hydrogen-2 + hydrogen-3 → helium-4 + a neutron, and its details are not required.

Section 2 of 23Structure of the Universe

Structure of the Universe

Universe

• Universe a large collection of billions of galaxies; it is also the name for all of space.

Galaxy

• Galaxy a large collection of billions of stars; the Milky Way is one of many billions of galaxies.

Our place

• Our place the Sun is one star in the Milky Way, at the centre of the Solar System.

Other stars

• Other stars are much further away than the Sun, and some have planets that orbit them.

Section 3 of 23Astronomical distances

1Light-year

• Light-year the distance travelled by light in one year.

2Size

• Size the Milky Way is about 100 000 light-years across, so light takes 100 000 years to cross it.

3Conversion (Extended)

• 1 light-year = 9.5 × 10¹² km = 9.5 × 10¹⁵ m.

Section 4 of 23Star formation (Extended)

1

A nebula, a giant cloud of gas and dust, is pulled together by gravitational attraction.

2

The particles collide more often as the density rises, so the temperature increases and a hot ball of gas called a protostar forms.

3

When the core is hot enough, hydrogen fuses to helium and the star becomes stable.

4

A stable star is in equilibrium: gravity acts inwards and thermal pressure from the hot expanding gas acts outwards, and they balance.

Section 5 of 23Life cycle of a low-mass star (Extended)

1Low-mass star

• Low-mass star up to about eight times the mass of the Sun.

2Red giant

• Red giant when hydrogen runs out the fusion rate drops, the core shrinks and heats, and helium fuses into carbon;
  • the outer layers expand and cool.

3Planetary nebula

• Planetary nebula when the helium runs out the core collapses and the outer layers are ejected into space.

4White dwarf

• White dwarf the collapsed core, which cools and emits less energy over time.

5Sequence

• Sequence nebula → protostar → stable star → red giant → planetary nebula → white dwarf.

Section 6 of 23Why the stages change

1Fuel running out

• Fuel running out when hydrogen in the core is used up, fusion slows, gravity wins and the core shrinks and heats.

2Helium fusion

• Helium fusion the hotter core fuses helium into carbon, so the outer layers expand and cool, giving a red giant.

3Ending

• Ending a low-mass star cannot fuse heavier elements, so it sheds its outer layers and the core cools as a white dwarf.

Section 7 of 23More from the specification

1

Stellar luminosity determination

• Stellar luminosity determination combining radiant apparent brightness with measured astronomical distance provides a reliable means to calculate solar luminosity.

2

Galactic Redshift and Receding Motion

• Light observed from distant galactic sources shows spectral lines shifted toward longer wavelengths, indicating that distant galaxies are moving away from Earth.

3

Hubble's Law and Expansion Rate

• Hubble discovered that a galaxy's recession speed v is directly proportional to its distance d from Earth (v = H₀ * d),
• where H₀ is the Hubble constant.

4

Cosmic Expansion of Space

• The observation that redshift increases with distance indicates that the fabric of space itself is continuously expanding in all directions.

5

Stellar Life Cycles

• Stars progress through an evolutionary cycle from interstellar gas clouds to stable hydrogen-burning stars, eventually ending as white dwarfs, neutron stars,
• or black holes.

6

Galactic redshift

• Shift of galaxy spectral lines to longer wavelength due to recession.

7

Background radiation

• Radiation that is always present in the environment.

8

Redshift

• Increase in the wavelength of light from a source moving away from the observer.

9

Wavelength

• Distance between a point on one wave and the same point on the next wave

10

Constant

• Not changing.

11

Recession

• Movement away from the observer.

12

v = H0d

What the symbols mean: v = recession speed of the galaxy (km s⁻¹) · H₀ = Hubble constant (km s⁻¹ Mpc⁻¹) · d = distance (Mpc)

Section 8 of 23Life cycle of a high-mass star (Extended)

1High-mass star

• High-mass star more than about eight times the mass of the Sun.

2Red supergiant

• Red supergiant after a few million years the core hydrogen runs low, the core shrinks and heats, helium fuses to carbon and heavier elements such as oxygen form,
• while the outer layers expand and cool.

3Supernova

• Supernova once iron forms, fusion cannot continue;
  • the core collapses rapidly and a gigantic explosion follows,
• producing the elements heavier than iron and ejecting dust and gas that may form new stars and planets.

4Neutron star

• Neutron star the dense body left at the centre of the explosion.

5Black hole

• Black hole in the most massive stars the core keeps collapsing to a point so dense that not even light escapes.

Section 9 of 23Doppler effect and redshift

Doppler effect

• Doppler effect the wavelength of waves changes when the source moves relative to the observer.

Blueshift

• Blueshift light from a source moving towards the observer has a shorter wavelength, shifted towards the blue end.

Redshift

• Redshift light from a source moving away has a longer wavelength, shifted towards the red end.

Observation

• Observation glowing hydrogen in distant galaxies gives light that is redshifted compared with hydrogen on Earth, so the galaxies are moving away.

Greater redshift

• Greater redshift means a more distant galaxy that is receding faster.

Spectrum fact

• Spectrum fact red light has the longest wavelength and lowest frequency of visible light.

Section 10 of 23The Big Bang theory

1The theory

• The theory about 13.8 billion years ago the Universe began from a single, extremely hot and dense point and has been expanding and cooling since.

2Evidence 1

• Evidence 1 distant galaxies are all moving away from us, so the Universe is expanding.

3Evidence 2

• Evidence 2 the further away a galaxy is, the faster it recedes and the greater its redshift.

4Why the wavelength stretches

• Why the wavelength stretches space itself is expanding;
  • a balloon with dots on it is a good model, as the dots all move apart when it is inflated.

Section 11 of 23Cosmic microwave background radiation (Extended)

CMBR

• CMBR electromagnetic radiation emitted shortly after the start of the Universe, detected from all directions.

Prediction

• Prediction the early Universe was very hot, so the radiation began as short-wavelength gamma rays.

Stretching

• Stretching as the Universe expanded, the wavelength increased until the radiation was redshifted into the microwave region.

Uniformity

• Uniformity the CMBR is very uniform, which shows the Universe was once much smaller and very energetic.

Section 12 of 23Comparing the life cycles of stars (Extended)

Stage	Low-mass star	High-mass star
After stable star	Red giant	Red supergiant
Explosion or ejection	Planetary nebula	Supernova
Remnant	White dwarf	Neutron star or black hole

Section 13 of 23More from the specification

1

Stellar life cycle and nuclear fusion

• Stars form from collapsing gas nebulae;
  • gravitational pressure ignites hydrogen fusion in the core, maintaining hydrostatic equilibrium against gravitational collapse.

2

Hertzsprung-Russell diagram

• The H-R diagram plots stellar luminosity against surface temperature, classifying main sequence stars, red giants, supergiants,
• and white dwarfs.

3

Redshift and cosmological expansion

• Spectroscopic redshift of distant galaxies demonstrates that space is expanding, supporting Hubble's law (v = H₀ * d) and the Big Bang theory.

4

Background radiation

• Radiation that is always present in the environment.

5

Reaction

• The partner force acting on the other object.

6

Constant

• Not changing.

7

Light-year

• The distance light travels in one year, 9.5 × 10¹⁵ m.

8

Recession

• Movement away from the observer.

9

Hubble constant

• H₀ = v / d, about 2.2 × 10^{-18} per second.

10

Nuclear fusion

• Joining of light nuclei to form a heavier nucleus, releasing energy.

11

v = H0d

What the symbols mean: v = recession speed of the galaxy (km s⁻¹) · H₀ = Hubble constant (km s⁻¹ Mpc⁻¹) · d = distance (Mpc)

Section 14 of 23Hubble’s law (Extended)

Observation

• Edwin Hubble found that all galaxies show redshift and that galaxies further away show a larger redshift.

Statement

• Statement the speed of recession of a galaxy is proportional to its distance from Earth.

Equation

• H₀ = v / d, so v = H₀ d.

H₀ is the Hubble constant, v the recession speed and d the distance to the galaxy.

Value

• H₀ ≈ 2.2 × 10^-18 per second.

Section 15 of 23Measuring recession speed and distance

Speed

• Speed found from the redshift: the change in wavelength of the galaxy’s starlight.

Distance

• Distance found from the brightness of supernovae in the galaxy.

Standard candles

• Standard candles some supernovae reach the same peak brightness, so how dim they look shows how far away they are, even for very distant galaxies.

Section 16 of 23Estimating the age of the Universe

Rearranged law

• 1 / H₀ = d / v, and since time = distance / speed, this is an estimate of the age of the Universe.

Reasoning

• Reasoning if the expansion is run backwards, galaxies were closer together in the past and all began at a single point, the singularity of the Big Bang.

Graph

• Graph a graph of recession speed against distance is a straight line through the origin; its gradient is H₀.

At distance zero all matter is at one point.

Result

• Result this gives an age of about 13.8 billion years.

Section 17 of 23Hubble’s law and the Big Bang

Evidence

• Evidence the law shows the Universe has been expanding since the beginning.

Pattern

• Pattern the furthest galaxies move away the fastest.

Conclusion

• Conclusion the Universe began from a single point and has expanded ever since.

Section 18 of 23Interpreting the Hubble graph

1Axes

• Axes recession speed on the vertical axis and distance on the horizontal axis.

2Straight line through the origin

• Straight line through the origin speed is directly proportional to distance.

3Gradient

• Gradient equals H₀; a steeper line means a larger H₀ and a younger Universe.

4Age

• Age the age is 1 divided by the gradient, since time = distance / speed.

Section 19 of 23Checking the age calculation

Units

• 1 / (2.2 × 10^-18 per second) = 4.5 × 10¹⁷ s.

Convert

• Convert dividing by 3.15 × 10⁷ seconds per year gives about 1.4 × 10¹⁰ years, close to 13.8 billion years.

Section 20 of 23Limitations

Measurement

• Measurement uncertainties in distance and redshift mean the age is only an estimate.

Assumption

• Assumption a constant expansion rate gives the simple estimate 1 / H₀;
  • the true age depends on how the expansion changed.

Section 21 of 23More from the specification

1

Thermonuclear Fusion in Stellar Cores

• In the core centre of main-sequence stars, extreme temperature and gravitational pressure sustain nuclear fusion reactions that fuse hydrogen into helium, releasing radiant energy.

2

Hydrostatic Equilibrium and Radiation Pressure

• A stable star balances the inward pull of gravitational collapse against the outward thermal gas and radiation pressure generated by core fusion reactions.

3

Cosmological Redshift and Expanding Space

• Spectroscopic analysis shows that absorption lines of glowing hydrogen in distant galaxies are shifted toward longer wavelengths, indicating that galaxies are moving away from Earth and that the metric of space is expanding.

4

Cosmic Microwave Background Radiation (CMBR)

• The cosmic microwave background radiation is uniform electromagnetic radiation permeating all space, representing remnant thermal radiation from the Big Bang stretched by cosmic expansion over the age of the Universe.

5

Cosmic microwave background radiation

• Cosmic microwave background radiation uniform microwave radiation permeating space, originating from the hot early universe.

6

Nuclear fusion in stars

• Nuclear fusion in stars core hydrogen fusion reactions generate outward radiation pressure that balances inward gravitational collapse.

7

Stellar life cycle progression

• Stellar life cycle progression stars transition from main sequence to giants and supernovas when core hydrogen fuel becomes depleted.

8

Light-year

• The distance light travels in one year, 9.5 × 10¹⁵ m.

9

Spectrum

• Range of colours in white light.

10

Stable

• Returns to its original position when tilted slightly.

11

Galactic redshift

• Shift of galaxy spectral lines to longer wavelength due to recession.

12

White dwarf

• Small, hot, dense core left by a low-mass star.

13

Background radiation

• Radiation that is always present in the environment.

14

Planetary nebula

• The shell of gas ejected by a red giant.

15

Fusion

• Joining of light nuclei into a heavier one.

16

Radiation

• Transfer by electromagnetic waves, requiring no medium

17

Reaction

• The partner force acting on the other object.

18

Nebula

• Cloud of gas and dust in which stars form.

19

Helium

• Product of the fusion of hydrogen in stars.

20

v = H0d

What the symbols mean: v = recession speed of the galaxy (km s⁻¹) · H₀ = Hubble constant (km s⁻¹ Mpc⁻¹) · d = distance (Mpc)

Section 22 of 23Key terms

Term	Meaning
Galaxy	A large collection of billions of stars.
Universe	All of space, containing billions of galaxies.
Light-year	The distance light travels in one year.
Protostar	A hot ball of gas formed as a nebula collapses, before fusion starts.
Red giant	A large cool star formed when hydrogen in the core runs out.
White dwarf	The small dense core left by a low-mass star.
Nebula	A cloud of gas and dust in space.

Term	Meaning
Redshift	An increase in observed wavelength from a receding source.
Doppler effect	A change in observed wavelength due to the motion of the source.
Big Bang	The theory that the Universe began from a single hot dense point about 13.8 billion years ago.
CMBR	Cosmic microwave background radiation left over from the early Universe.
Supernova	The explosion of a massive star at the end of its life.
Black hole	An object so dense that not even light can escape.
Neutron star	A very dense body left after a supernova.

Term	Meaning
Hubble constant	The ratio of the recession speed of a galaxy to its distance, about 2.2 × 10^{-18} per second.
Hubble’s law	The speed of recession of a galaxy is proportional to its distance.
Recession speed	The speed at which a galaxy is moving away.
Standard candle	An object, such as a certain supernova, with a known peak brightness used to find distance.
Singularity	The single point from which the Universe began.
Age of the Universe	About 13.8 billion years, estimated by 1/H₀.

Section 23 of 23Exam technique

1

Mixing up galaxy and Universe — the Universe contains billions of galaxies.

2

Treating a light-year as a time — it is a distance.

3

Saying fusion in stars is fission — fusion joins nuclei and fission splits them.

4

Thinking the Sun will become a black hole — a low-mass star becomes a white dwarf.

1

Saying redshift means the object is red — it means the observed wavelength has increased.

2

Thinking galaxies move through space — space itself expands and carries them apart.

3

Saying the Sun will end as a black hole — as a low-mass star it ends as a white dwarf.

4

Thinking the Big Bang was an explosion into empty space — it was the expansion of space itself.

1

Mixing up the gradient and the age — the gradient is H₀ and the age is 1/H₀.

2

Forgetting to convert light-years to metres before using H₀ in per second.

3

Saying closer galaxies are redshifted more — more distant galaxies show the larger redshift.

4

Thinking recession speeds are measured directly — they come from redshift.