Stars | Physics for GCSE/IGCSE - Year 11 PDF Download

The Milky Way

• Galaxies: Galaxies, such as the Milky Way, are collections of billions of stars.
• Universe: The cosmos contains a multitude of diverse galaxies.
• Milky Way: The Milky Way galaxy houses our Sun and numerous other stars.
• Distance: Stars in the Milky Way are much more distant from Earth than our Sun.
• Planetary Systems: Some stars in the Milky Way possess planetary systems with orbiting planets.

Astronomical Distances

• When we talk about astronomical distances, we are discussing vast spans between stars and galaxies that are so enormous that physicists use a special unit to measure them known as the light-year.
• One light-year is the distance traveled by light through the vacuum of space in one year.
• The speed of light serves as the universal speed limit, with nothing capable of exceeding it.
• Despite this, light's journey over vast cosmic distances is notably unhurried.
• The Milky Way's diameter extends approximately 100,000 light-years, illustrating the considerable time light would need to cross it entirely.
• One light year is equal to 9.5 × 10¹² km, or 9.5 × 10¹⁵ m

Star Formation

Nebula

• All stars are formed from a massive interstellar cloud comprising hydrogen gas and dust, scientifically termed as a nebula.

Protostar

• Within a nebula, the force of gravity draws particles closer together, leading to the formation of a hot gas sphere known as a protostar.
• As the particles in the protostar are pulled closer, the density of the protostar increases, resulting in more frequent collisions among particles. This, in turn, causes a rise in temperature.

Main Sequence Star

• When the protostar reaches sufficient heat, nuclear fusion begins within its core.
  • Hydrogen nuclei combine to produce helium nuclei through fusion.
  • Each fusion reaction emits thermal (and luminous) energy, sustaining the core's high temperature.
• Once fusion commences, the star transitions into a main-sequence phase.
• Throughout the main sequence, the star maintains equilibrium and is considered stable.
  • The gravitational force inward is balanced by the outward pressure exerted by fusion reactions.

• The fate of a protostar is determined by its mass, leading to various life cycles as outlined below.
  

Life Cycle of Low Mass Stars

A low-mass star will go through the following stages:

Red Giant

• As billions of years pass, the hydrogen fueling the star's fusion reactions diminishes.
• Consequently, core fusion activity wanes, leading to core contraction and intensification of heat.
• The core contracts due to gravitational forces overpowering gas pressure as fusion declines.
• New fusion reactions, like helium nuclei fusing into beryllium, occur around the core, causing the star's outer layers to expand.
• A low-mass star, up to 8 times the Sun's mass or smaller, transitions into a red giant, characterized by its cooling outer surface.

Planetary Nebula

• Following the culmination of these subsequent fusion reactions, the star becomes unstable, shedding its outer layer of dust and gas.
• This ejected material forms what is known as a planetary nebula.

White Dwarf

• The remaining core undergoes complete collapse under gravitational forces, transforming the star into a white dwarf.
• As the white dwarf cools, its energy emission decreases.

Black Dwarf

• After shedding a significant amount of energy, the star evolves into a black dwarf.
• It continues to cool until eventually fading from view.

Life Cycle of High Mass Stars

A high-mass star will go through the following stages:

Red Supergiant

• After millions of years, the hydrogen fueling the star's fusion reactions starts depleting.
• A high-mass star (more than 8 times the Sun's mass) transforms into a red supergiant.
• Similar to low-mass stars, fusion reactions in the core diminish, leading to cycles of core contraction and heating.
• Consequently, the star's outer layers undergo expansion and contraction.
• Fusion reactions progress to produce elements up to iron, beyond which fusion ceases.

Supernova

• Once fusion reactions in the red supergiant's core halt, the core collapses suddenly, resulting in a massive explosion known as a supernova.
• Within this explosion, a dense body called a neutron star forms at the center.
• The outer debris of the star is expelled into space, forming new clouds of dust and gas known as nebulae.
• These nebulae from supernovae may give rise to new stars with orbiting planets.

Neutron Star (or Black Hole)

• In the case of the largest stars, the neutron star formed at the core continues to collapse under gravitational forces until it becomes a black hole.
• A black hole is an extremely dense point in space where not even light can escape.