Big Bang, Light-Element Formation & Stellar Nucleosynthesis

These question-and-answer flashcards review key concepts from the Physical Science module on the Big Bang, formation of light elements, and stellar nucleosynthesis. They cover evidence for cosmic expansion, early-universe nucleosynthesis, star life cycles, fusion processes, and important scientists.

What are the three main pieces of evidence that support the Big Bang Theory?

Redshift of galaxies, Cosmic Microwave Background Radiation (CMBR), and the observed abundance of light elements (H ≈ 73 %, He ≈ 25 %).

Who first measured the redshift of spiral nebulae in 1910, indicating universal expansion?

Vesto Slipher and Carl Wilhelm Wirtz.

Which astronomer used redshift to calculate galactic velocities and confirm expansion in 1929?

Edwin Hubble.

What does a redshift in a galaxy’s light indicate about its motion relative to Earth?

The galaxy is moving away from Earth, implying an expanding universe.

What does a blueshift in a star’s light tell us about its motion?

The star is moving toward Earth (wavelengths are compressed).

What was discovered by Arno Penzias and Robert Wilson in 1965 that provided evidence for the Big Bang?

Cosmic Microwave Background Radiation (a low, steady ‘hum’ left over from the Big Bang).

According to Big Bang Nucleosynthesis (BBN), which four light nuclei were produced in the first few minutes?

Deuterium, Helium-3, Helium-4, and trace Lithium-7.

Approximately how long after the Big Bang did most light-element nucleosynthesis finish?

Within the first three minutes.

Define binding energy in nuclear physics.

The energy required to break a nucleus into its individual protons and neutrons.

At what approximate temperature did deuterium become stable enough to fuse into heavier nuclei?

Below about 10¹⁰ K.

Write the fusion reaction that forms Helium-3 from two deuterium nuclei.

D + D → ³He + n (neutron released).

Which unstable isotope produced in BBN decays spontaneously to Lithium-7?

Beryllium-7 (⁷Be).

What term describes the era when electrons first combined with nuclei allowing photons to travel freely?

Recombination (also called the beginning of the ‘dark ages’).

List the correct chronological order of early-universe stages: Big Bang Singularity, Inflation, Nucleosynthesis, Recombination.

Big Bang Singularity → Inflation → Nucleosynthesis → Recombination.

Who originally proposed an expanding-universe solution in the 1920s, later forming the basis of the Big Bang model?

Aleksandr Friedman and Georges Lemaître.

Which physicist developed the modern Big Bang model in the 1940s?

George Gamow.

Define stellar nucleosynthesis.

The process by which elements are formed in the cores and shells of stars through nuclear fusion.

What two dominant hydrogen-burning processes fuel main-sequence stars?

The proton-proton (pp) chain and the carbon-nitrogen-oxygen (CNO) cycle.

What is a protostar?

A collapsing stellar core formed from fragments of a molecular cloud before nuclear fusion ignites.

When does a protostar become a main-sequence star?

When gravitational equilibrium is reached as hydrogen fusion stabilizes the core.

Why can massive stars fuse elements heavier than carbon, while low-mass stars cannot?

Massive stars have higher core temperatures and pressures, enabling successive fusion stages (C→O→Ne→Mg→Si→Fe). Low-mass stars never reach the conditions needed for carbon fusion.

What happens to a low-mass star (< 2 M☉) after helium is exhausted in the core?

It expels outer layers and ends as a white dwarf composed largely of carbon.

What stellar event produces elements heavier than iron?

A supernova explosion, where rapid neutron capture (r-process) occurs.

Define supernova.

The explosive death of a massive star that releases enormous energy and scatters heavy elements into space.

What observational evidence supports the star-formation theory from molecular clouds?

Infrared (IR) radiation measurements from protostars and surrounding dust showing energy, temperature, and pressure differences.

Explain astronomical extinction.

Absorption and scattering of electromagnetic radiation by gas and dust between an emitting object and an observer.

What percentage (by mass) of hydrogen and helium does Big Bang theory predict for the universe?

Approximately 73 % hydrogen and 25 % helium.

During which stellar phase does a star swell after core hydrogen is depleted and shell hydrogen burning begins?

The red giant phase.

Why does fusion of elements lighter than iron release energy, whereas fusing iron does not?

For elements lighter than iron, the resulting nucleus has lower mass than the reactants, releasing the mass difference as energy. Iron’s binding energy per nucleon is maximal, so fusing iron requires energy input.

What triggers a supernova in a massive star?

Core collapse when iron fusion ceases to produce energy to counteract gravity.

Name the process that produced most hydrogen and helium in the universe.

Big Bang nucleosynthesis.

Describe the proton-proton chain in simple terms.

A sequence of nuclear fusion reactions converting hydrogen into helium in the cores of low-mass stars.

What is meant by ‘multiple-shell red giant’?

A massive star where successive fusion shells (H, He, C, O, Ne, Mg, Si) surround an inert iron core.

How do IR observations distinguish a protostar from nearby dust clouds?

Protostars emit excess infrared radiation compared with regions of zero extinction, indicating internal heating from gravitational contraction and early fusion.

Which light element produced during BBN is considered unstable and quickly decays?

Beryllium-7 (⁷Be).

What cosmological model states that the universe began from an extremely hot, dense state ~13.8 billion years ago?

The Big Bang Theory.

What is the Doppler Effect in astronomy?

The change in wavelength (and frequency) of light due to the relative motion of the source and observer—shortened (blue) when approaching, lengthened (red) when receding.

What stellar remnant is composed mainly of carbon and is the final stage for low-mass stars?

White dwarf.