The sun and stars

Stars, including the Sun, produce energy through nuclear fusion, where hydrogen fuses into helium under extreme conditions. This process powers stars and determines their life cycle, from formation to their dramatic end as white dwarfs, neutron stars, or black holes. Understanding these reactions helps explain the energy that sustains our universe.

The sun

The Sun is a star of medium size compared to other stars in the universe. It is not the largest, nor is it the smallest; it falls into the main sequence category, which means it is in a stable phase of its lifecycle.

Composition:

• The Sun consists mostly of hydrogen (about 75%) and helium (about 24%). The remaining 1% is made up of heavier elements such as carbon, oxygen, and other trace elements.

Energy Radiation:

• The Sun radiates most of its energy in three regions of the electromagnetic spectrum:

  • Infrared Radiation (IR): This radiation is responsible for the heat we feel from sunlight.

  • Visible Light: This is the light that allows us to see. The Sun is the most significant natural source of visible light.

  • Ultraviolet Radiation (UV): The Sun also emits ultraviolet rays, which are responsible for causing tanning and, in excess, sunburn.

Nuclear reactions in stars

Nuclear Fusion:

• The Sun, like all stars, is powered by nuclear reactions. These reactions occur in the Sun’s core, where extremely high temperatures and pressures cause hydrogen atoms to collide and fuse together.

Fusion of Hydrogen into Helium:

• In a stable star like the Sun, the primary nuclear reaction is the fusion of hydrogen into helium. This process is called nuclear fusion and it releases a tremendous amount of energy.

• During fusion, two hydrogen nuclei combine to form one helium nucleus. During this process, some of the mass is converted into energy

• The energy produced by fusion in the Sun’s core travels through several layers before it is emitted from the surface. This energy provides the heat and light that we receive on Earth.

• A star is considered stable when the outward pressure from the energy produced by fusion balances the inward pull of gravity. In the Sun, these two forces are in equilibrium, which is why it has maintained a stable size for billions of years.

The lifecycle of a star

Stars go through a life cycle that can last millions to billions of years. Here is a description of the stages in the life cycle of a star:

• (a) Formation of a Star:

  • Stars form from interstellar clouds of gas and dust that primarily contain hydrogen. These clouds are called nebulae.

  • Gravity causes the gas and dust to clump together and collapse, eventually forming a protostar.

• (b) Protostar Stage:

  • A protostar is an early stage of a star where the cloud of gas and dust is collapsing under its own gravitational attraction.

  • As the cloud collapses, its temperature increases, and it starts glowing due to the energy produced.

• (c) Stable Star (Main Sequence):

  • When the inward force of gravity is balanced by the outward pressure from the high temperature caused by nuclear fusion, the protostar becomes a stable star.

  • The star enters the main sequence stage, where hydrogen in its core fuses into helium, releasing large amounts of energy. The Sun is currently in this stable phase.

The formation of a main sequence star from a nebula cloud.

• (d) Running Out of Fuel:

  • All stars eventually run out of hydrogen fuel for nuclear fusion. This marks the beginning of the end of their stable phase.

• (e) Red Giants and Red Supergiants:

  • When the hydrogen in the core is exhausted, the star expands:

    • Smaller stars (like our Sun) become red giants.

    • More massive stars become red supergiants.

  • The core temperature rises, and helium starts to fuse into heavier elements like carbon and oxygen.

• (f) Planetary Nebula and White Dwarfs:

  • For a less massive star (like the Sun), the outer layers are eventually blown away, forming a planetary nebula. What remains at the center is a white dwarf, which is a dense, hot core.

The transition of a smaller main sequence star into a planetary nebula and white dwarf.

• (g) Supernovae and Neutron Stars/Black Holes:

  • For massive stars, the collapse is more dramatic. When fusion stops, the core collapses and the outer layers are blasted away in a supernova explosion.

  • After a supernova, the remnants may form a neutron star or, if the star is very massive, a black hole.

• (h) Nebulae from Supernovae:

  • The explosion of a supernova leaves behind a nebula. The material from the nebula can form new stars, continuing the cycle of star formation.

The transition of a main sequence star into a neutron star or black hole.