When a high-mass main sequence star runs out of both hydrogen and helium in its core, the core begins to fuse

The core beings to fuse carbon into heavier elements.

Explanation:

As the star runs out of hydrogen and helium, the energy production in the core stops, and there isn't enough outward pressure to hold the star up. The start contracts, and this heats up the core (pressure increases, temperature increases)  creating temperatures high enough to fuse carbon into heavier elements like oxygen, silicon, neon, sulfur, magnesium, and iron.

But a star can sustain itself on Carbon fusion only for so long. In about 600 years (how long exactly depends on the star ) the star runs out of carbon and what happens next depends on the mass of the star; the star either collapses into a neutron star, or begins fusion heavier elements like neon.

d₁ = displacement of tern from arctic to antarctic = 11000 miles

d₂ = displacement of tern from antarctic to arctic =   - 11000 miles

d = total displacement for the journey

total displacement for the journey is given as

d = d₁ + d₂

d = 11000 + (-11000)

d = 0 miles

t₁ = time of travel from arctic to antarctic = 3 months = 3 x 30 days = 90 days

t₂ = time of travel from antarctic to arctic = 3 months = 3 x 30 days = 90 days

t = total time of travel

total time of travel is given as

t = t₁ + t₂

t = 90 + 90

t = 180 days

average velocity is given as

average velocity = total displacement/ total time

v = d/t

v = 0/180

v = 0 miles/day