(8.9-8.12P) Evolution of stars + HR diagrams

What is a solar mass star?

A star which has a mass equal to the mass of the Sun.

What are the stages in the life cycle of a solar mass star?

nebula → protostar → main sequence star → red giant → white dwarf

What is a nebula?

A giant cloud of hydrogen gas and dust where stars form.

What is a protostar?

A hot ball of gas formed when gravity in a nebula pulls the particles together.

As the density increases,

so do the collisions,

causing the temperature to increase.

What is a main sequence star?

Once the protostar is hot enough,

nuclear fusion reactions occur in the core,

forming helium and releasing heat energy.

It is in equilibrium and stable:

inward force from gravity = outward pressure force from expanding gases within.

What is a red giant?

The hydrogen eventually starts running out,

causing the fusion reactions to slow and

the core to shrink and heat up because

it is no longer in equilibrium.

Now helium fuses to form beryllium.

The outer part of the star expands and cools as

more reactions occur and it becomes a red giant.

What is a white dwarf?

The star becomes unstable and ejects the outer layer of gas and dust.

The remaining core will collapse completely due to gravity and the star will become a white dwarf.

The energy it emits will decrease as it cools.

What is the difference in lifetimes of a solar mass star and a larger star?

Stars larger than the Sun have shorter lifespans.

What are the stages in the life cycle of a star larger than the sun?

Nebula → Protostar → Main Sequence Star → Red Supergiant → Supernova → Neutron Star (Black Hole)

What is a red supergiant?

When the main sequence star starts running out of hydrogen,

the core shrinks and heats up because

it is no longer in equilibrium.

Helium fuses into beryllium and these reactions

cause the outer part of the star to expand,

forming a red supergiant.

What is a supernova?

When the reactions in the red supergiant stop,

the core will collapse, causing a

gigantic explosion called a supernova.

At the center, a dense neutron star forms.

The outer remnants of the star will be ejected during the supernova.

What is a neutron star?

In the case of the largest stars,

the neutron star formed

at the center of the supernova will continue collapsing

under the force of gravity until

it forms a black hole-

an extremely dense point in space that

not even light can escape from.

What is absolute magnitude?

The measure of stars at a standard distance

Why is absolute magnitude measured at a standard distance?

Distance affects how bright a star appears so a standard distance removes the effects of distance.

How does the brightness affect the magnitude in the scale?

brighter star - smaller magnitude

dimmer star - larger magnitude

What factors affect the brightness of stars?

• How much light is emitted

• How far away the star is

What is the value of absolute magnitude?

10 parsecs (3.04 × 10¹⁴ km) from Earth

What is a Hertzsprung-Russel Diagram?

A model showing the classification of stars, plotting luminosity against temperature.

Left to right, describe what the x-axis shows.

Decreasing temperature

From bottom to top, describe what the y-axis shows.

Increased brightness

Where is a star’s main sequence on a HR diagram?

A diagonal from top left to bottom right.

Where are white dwarfs on a HR diagram?

Below main sequence and to the left

Where are red giants on a HR diagram?

Above and to the right of main sequence stars

Where are red supergiants on a HR diagram?

Above red giants