Practice Questions

1.What is the name of the large cloud of gas and dust where stars form?
A nebula.

2.What type of nuclear reaction happens in the core of a main sequence star?
Hydrogen fusion into helium.

3.What two forces are in balance during the main sequence phase of a star’s life?
Gravitational force and outward pressure from fusion.

4.What is the final stage for a low-mass star like the Sun?
A white dwarf.

5.What stellar event can form either a neutron star or black hole?
A supernova.

6.Why does a main sequence star eventually become a red giant or red supergiant?
Because the hydrogen in the core runs out, fusion stops, the core contracts, and the outer layers expand and cool.

7.Why can only high-mass stars produce elements heavier than iron?
Because only in the extreme energy of a supernova can fusion of elements heavier than iron occur.

8.What causes a white dwarf to slowly become a black dwarf?
It gradually cools down and stops emitting visible light and heat.

9.Explain why massive stars have shorter lifespans even though they contain more fuel.
They burn through their fuel much faster due to higher core temperatures and pressures.

10.How does the death of a high-mass star contribute to the formation of new stars and planets?
The supernova ejects heavy elements into space, enriching nebulae that can form new stars and planetary systems.

11.Suggest what might happen to a star that is not massive enough to start hydrogen fusion?
It becomes a brown dwarf — not a true star, as fusion never begins. (This question is not on the course, but is extension material.)

12.Why doesn’t a white dwarf collapse further into a neutron star or black hole?
Its mass is too low; 

13.If a star produces mostly carbon and oxygen in its core, what does this suggest about its mass and stage?
It’s a low to medium-mass star in its red giant phase or forming a white dwarf.

14.Why is the presence of iron in a star’s core a ‘tipping point’ in stellar evolution?
Fusion of iron absorbs energy instead of releasing it, causing the star’s core to collapse.

15.How does the balance of gravity and radiation pressure influence the stability of a star?
If gravity outweighs radiation pressure, the star contracts; if radiation pressure is greater, it expands. Balance maintains a stable main sequence star.

16.Why is it accurate to say that humans are made from “star dust”?
Elements like carbon, nitrogen, oxygen, and iron that make up our bodies were formed in previous generations of stars.

17.Explain how a black hole forms, and why not all supernovae result in black holes.
If the stellar core remnant after a supernova is massive enough, gravity compresses it into a point of infinite density. Less massive remnants become neutron stars instead.

18.What would happen if fusion in a main sequence star suddenly stopped?
Without outward pressure from fusion, gravity would cause the star to collapse, potentially leading to a red giant or supernova, depending on the star’s mass.

19.How would the death of the Sun as a red giant affect the Earth?
The Sun would expand and possibly engulf the inner planets, including Earth, making the planet uninhabitable or destroying it.

20.Describe how the life cycle of stars is a key part of the recycling process of matter in the universe.
Stars form from nebulae, fuse elements, and then explode or shed their outer layers, returning enriched material to space for new stars and planets to form.