The sun was formed about 4.6 billion years ago from a collapsing cloud of gas and dust in a spiral arm of the Milky Way galaxy. The sun is a star that emits light and heat by nuclear fusion of hydrogen into helium in its core. It is the main source of light for Earth and the solar system.
The light is not just something that comes from stars like the sun. Light is a form of electromagnetic radiation that can be produced by various physical processes in nature. And one of those processes is the Big Bang.
The Big Bang is the name given to the event that started the universe as we know it about 13.8 billion years ago. It was not an explosion in space, but rather an expansion of space itself from a tiny point of extremely high density and temperature. As space expanded rapidly, it cooled down and became filled with radiation and matter. This radiation was mostly in the form of photons (particles of light) that were very energetic and had very short wavelengths.
As time went on, space continued to expand and cool down further. The photons lost energy and stretched their wavelengths as they traveled through space. This process is called cosmological redshift. After about 380,000 years from the Big Bang, space became cool enough for atoms to form from electrons and protons (mostly hydrogen). This made space transparent to photons for the first time, allowing them to travel freely without being scattered by charged particles. This is when the first light in the universe was released.
This first light is what we call today the cosmic microwave background (CMB). It is a faint glow of radiation that fills the entire sky in all directions. It has a temperature of about 2.7 K (or -270 °C) and a wavelength of about 1 mm (in the microwave range). It is one of the most important pieces of evidence for the Big Bang theory.
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