Stellar evolution

Question 1

What is a star?

Answer 1

• A massive, glowing sphere of plasma held together by gravity.
• Produces energy through nuclear fusion.
• Made mostly of hydrogen and helium.

Question 2

What is nuclear fusion?

Answer 2

• A process where hydrogen nuclei fuse to form helium.
• Releases enormous amounts of energy in the form of light and heat.
• Occurs in the core of stars under extreme temperature and pressure.
• Responsible for the star’s energy output and stability.

Question 3

What is absolute magnitude and how does it relate to the brightness of a star?

Answer 3

• Absolute magnitude measures a star’s brightness at a standard distance of 10 parsecs.
• Unlike apparent magnitude, it removes the effect of distance.
• Brighter stars have lower absolute magnitude values.
• Example: Rigel (-7.0) is brighter than the Sun (+4.8).
• Used to classify stars on the Hertzsprung-Russell diagram.

Question 4

What is the Hertzsprung-Russell (HR) diagram?

Answer 4

• A graph showing the relationship between a star’s absolute magnitude and surface temperature.
• X-axis: Surface temperature (decreases from left to right, measured in Kelvin).
• Y-axis: Absolute magnitude (higher up = more luminous).
• Helps classify stars and predict stellar evolution.

Question 5

What are the main components of the Hertzsprung-Russell diagram?

Answer 5

• Main Sequence: Diagonal band from hot, bright stars to cool, dim stars.
• Red Giants & Supergiants: Large, bright but cool stars in the upper right.
• White Dwarfs: Small, hot but dim stars in the lower left.
• X-axis: Surface temperature (hot blue stars on the left, cool red stars on the right).
• Y-axis: Absolute magnitude (brighter stars at the top).

Question 6

What happens to a star after the main sequence phase?

Answer 6

• Stars run out of hydrogen fuel in their cores.
• Low-mass stars (like the Sun) become red giants, then white dwarfs.
• Massive stars become red supergiants, then explode as supernovae.
• Supernova remnants form either a neutron star or black hole.
• Supernovae spread heavier elements into space, aiding future star formation.

Question 7

What are the stages in the life cycle of a star similar to the Sun?

Answer 7

• Nebula: Cloud of gas and dust pulled together by gravity.
• Protostar: Gravity causes compression; temperature rises; nuclear fusion starts.
• Main Sequence Star: Longest stage; fusion of hydrogen into helium.
• Red Giant: Hydrogen runs out; outer layers expand.
• Planetary Nebula: Outer layers ejected into space, leaving the core.
• White Dwarf: Small, dense, hot core remains; cools over time.

Question 8

What are the stages in the life cycle of a massive star?

Answer 8

• Nebula → Protostar → Main Sequence Star
• Red Supergiant: Hydrogen runs out; fusion of heavier elements.
• Supernova: Outer layers collapse and explode.
• Neutron Star: If core is 1.4–3 solar masses.
• Black Hole: If core is more than 3 solar masses, collapses into a black hole.

Question 9

What determines whether a star becomes a black hole or a neutron star?

Answer 9

• The mass of the core after a supernova.
• Neutron star: Core between 1.4–3 solar masses.
• Black hole: Core greater than 3 solar masses.

Question 10

What happens in a supernova?

Answer 10

• A massive star explodes at the end of its life cycle.
• The core collapses, and outer layers eject into space.
• Produces heavy elements like iron and gold.
• Releases immense energy, briefly outshining an entire galaxy.

Question 11

What is a black hole?

Answer 11

• A region of space with gravity so strong that not even light can escape.
• Formed when a massive star collapses beyond the neutron star stage.
• Detectable by observing gravitational effects on nearby objects.
• Can distort spacetime and pull in nearby stars.

Question 12

What is a neutron star?

Answer 12

• A dense core left after a supernova.
• Composed almost entirely of neutrons, with incredible density.
• Has a strong magnetic field and can emit pulses of radiation (pulsars).
• One of the densest objects in the universe.

Question 13

What is the significance of supernovae?

Answer 13

• Creates and disperses heavy elements like carbon, oxygen, and iron.
• Provides materials needed for planets and life.
• Can trigger new star formation by compressing nearby gas clouds.
• Plays a crucial role in the chemical evolution of galaxies.