Chapter 25.2 Stars and galaxies
Stable stars
Hot bodies radiate heat, creating a force known as radiation pressure.
The radiation pressure increases with the object's temperature. The hotter the object, the
greater the radiation pressure.
High temperatures in stars produce outward radiation pressure, which causes expansion. This
pressure counteracts the inward pull of gravity, which causes contraction.
When these two forces are balanced, the star remains stable and maintains its size. If a star's
core temperature rises, radiation pressure increases, and the star increases in size. Conversely,
a decrease in core temperature causes the star to shrink.
The life cycle of a less massive star like our Sun
Red giant
After billions of years, the hydrogen fuel used for nuclear reactions starts to run out.
Once this occurs, the fusion rate declines, leading to decreased radiation pressure
and the star contracting.
As it does, some gravitational potential energy converts into thermal energy,
increasing the temperatures of both the star's core and its outer hydrogen shell.
The core reaches temperatures sufficient for helium fusion, which requires higher
temperatures due to the greater electrostatic repulsion between helium nuclei—each
having a charge of +2, compared to hydrogen's +1.
This heating of the outer shell results in its expansion and then cooling, turning it red.
Therefore, the star evolves into a red giant, a larger star with a cooler surface.
As the fusion energy produced decreases, the gravitational pull becomes stronger
than the thermal pressure pushing outward. Eventually, the star transforms into a red
giant as the core reaches sufficient temperatures for helium to fuse into carbon. The
energy from renewed fusion reactions causes the star's outer layers to expand and
cool.
White dwarf / Black dwarf
Eventually, the core collapses into a white dwarf star. A white dwarf cannot exceed a
mass of roughly 1.4 solar masses and typically has a radius of about 1000 km.
Although it has a white-hot surface (resulting in its colour), it is not hot enough
internally to fuse heavier elements, leading it to cool down into a black dwarf over
time.
As the white dwarf continues to cool, the amount of energy it emits decreases
significantly.
Planetary nebula
Radiation pressure blows away the outer layers of a star, forming a planetary nebula.
When the helium in the star's core runs out, fusion reactions cannot continue.
The star becomes unstable, and the core collapses under its gravity, causing the
outer layers to be ejected into space and forming a beautiful planetary nebula.
Extra info
Our Sun is approximately 4.6 billion years old and halfway through its stable phase
as a main sequence star In around 5 billion years, it will transform into a red giant,
expanding past the orbit of Earth..
When the Sun becomes a white dwarf, its radius will shrink to about 1% of its
present radius, which is about the size of the Earth.
red giant → planetary nebula → white dwarf
Stable stars
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Stars' high temperatures produce outward radiation pressure, causing the star to
expand.
Radiation pressure counteracts gravitational pull, which causes contraction.
The balance between these forces keeps stars stable.
Increased core temperature leads to expansion; decreased temperature causes
contraction. (shrink)
Radiation pressure depends on the star's temperature; if a star's core temperature
rises, radiation pressure increases, enlarging the star.
Outward pressure prevents collapse from gravity.
Stable stars maintain their size when forces are balanced.
- Like all stars, it starts as a protostar before entering a stable phase.
- Nuclear reactions begin to slow down when the star starts running out of hydrogen.
- Decreased radiation pressure causes the star to contract.
- As the star contracts, some gravitational potential energy converts into thermal energy,
increasing core and outer shell temperatures.
- The core reaches temperatures sufficient for helium fusion due to greater electrostatic repulsion
(+2 for helium nuclei vs. +1 for hydrogen).
- Heating of the outer shell results in its expansion, cooling, and turning red, evolving the star into
a red giant with a larger size and cooler surface.
- After billions of years, hydrogen fuel for nuclear reactions starts to run out.
- Once the hydrogen runs out, the fusion rate declines, leading to core contraction and increased
temperature.
- As fusion energy decreases, gravitational pull becomes stronger than thermal pressure pushing
outward.
- Eventually, the star transforms into a red giant as the core achieves sufficient temperatures
for helium to fuse into carbon.
- Energy from renewed fusion reactions causes the star's outer layers to expand and cool.
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Hydrogen shortage slows nuclear reactions.
Reduced radiation pressure causes contraction.
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Gravitational potential energy converts into thermal energy, increasing core and outer
shell temperatures.
The core reaches temperatures sufficient for helium fusion due to greater electrostatic
repulsion (+2 for helium nuclei vs. +1 for hydrogen).
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Outer shell expands, cools, and turns red.
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Hydrogen shortage leads to declining fusion rates.
Core contraction heats the star to fuse helium into carbon.
Fusion energy expands and cools the outer layers.
Red giants are larger with cooler surfaces.
Marks an advanced stage in a star’s lifecycle.
White dwarf / Black dwarf
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Core collapse forms a dense white dwarf.
Mass limit: approximately 1.4 solar masses.
White dwarfs cool over time, becoming black dwarfs.
No fusion occurs in white or black dwarfs.
Cooling diminishes energy emissions.
- The core of a star eventually collapses into a white dwarf star.
A white dwarf cannot exceed approximately 1.4 solar masses.
- The typical radius of a white dwarf is about 1000 km.
- It has a white-hot surface, which gives it its color, but is not hot enough internally to fuse
heavier elements.
- Over time, the white dwarf cools down and eventually becomes a black dwarf.
- As the white dwarf cools, the amount of energy it emits decreases significantly.
 Radiation pressure expels the star’s outer layers.
 Core collapse triggers outer layer ejection.
 Forms a stunning planetary nebula.
 Helium depletion ends fusion reactions.
 Nebula marks the final stages of a medium star.
- Radiation pressure blows away the outer layers of a star, forming a planetary nebula.
- When the helium in the star's core runs out, fusion reactions cannot continue.
- The star becomes unstable as a result.
- The core collapses under its gravity.
- The outer layers are ejected into space, forming a beautiful planetary nebula.