Bloom up your imagination. Consider a star which has nuclear fusion going on. Time to break things down. Now consider a small part of the star; assume the smallest part of the reaction which causes nuclear fusion. In this case, the hydrogen atoms. These atoms collide/ fuse with each other to produce heavier and heavier elements. Let us zoom in even deeper into the atomic world. When the fusion starts, the electrons get excited to different states. Whenever a beam of energy strikes that atom, the electrons jump to next excited levels, and during that process, a good load amount of energy is released. The energy released in the form of heat can be viewed in the form of light.
Now, we were assuming that happening in just part of the start, but stars are huge. Scaling up the things. The energy is not released individually but released in groups. These tiny packets of energy are known as quanta. A group of these quanta particles forms photons. Groups of these photons travel all across the cosmos in the form of light.
We were talking about this in a small quantity. Scale-up and you see the bigger picture, yes, emission of heat and light from stars. The fusion reaction is intense at the center, in the core of the stars. Here a tremendous battle is taking place between two massive forces. The fusion reaction which pushes the energy outwards- to the surface and the gravity-falling inwards. This battle goes on for years and years. For massive stars, stars much bigger than our sun, during the end of their life, the gravity wins over nuclear fusion and the whole star goes boom!! Into a supernova or a hypernova. One of the most energetic explosions in the universe. Our sun has been fighting this battle for 5 billion years. It has got 5 billion more years to turn into a white dwarf.
Battle apart, the light from the sunβs core takes nearly 40,000 years to reach its surface, and from there it covers a wobbling distance of 4.5 billion km in just under 8 minutes at the fastest speed- the speed of light-299,792,458 m/s. These light particles on their journey, bent by the gravity of other celestial objects and dark matter strikes up upon the atmosphere and travels all its way to the surface and then into our eye βs corona which enables us to see the world. What a beautiful journey!! Not just stars, but objects produce light when energy strikes the atoms. Iβm talking about this journey from the sun to the earth. The bigger scales.
The distance traveled by light in 1 minute is known as one light second and in 1 year it’s known as 1 light year (1 light year =9.46 trillion km). When we hear the news such as we found a planet/galaxy/ black hole or other objects at few light-years, it means that light took those many years to reach us. For example, the closest star system to the earth is the Alpha Centauri which is about 4.37 light-years away or 41.32 trillion km i.e even if we travel at the speed of light, it would take us 4.37years to reach that star system. It would take 100,000 years to reach the other end of our galaxy from one end at the speed of light. P.S: the size of the observable universe is 93 billion light-years ( 93,000,000,000 light-years).
When we observe deep into space, we are observing back into time. For example, light takes 8 minutes from the sun to reach the earth which means we are viewing the sun as it was 8 minutes before. Similarly, the event βthe kilonova- which happened in the year 2017. In an elliptical galaxy – NGC 4993- scientists observed the collision of two neutron stars. This collision produced a series of gravitational waves and this signal lasted for about 100 seconds. This event happened nearly 130 Million light-years away i.e we observed the event 130 million years after it happened.
When we say that the universe is 13.8 billion years old, we mean that the farthest light that we captured was from 13.8 billion years.
In this vastness of the cosmos, who are humans but a speck of life floating on a giant rock in the space over a suspended sunbeam. Tiny but powerful.
Let me end this by quoting a famous scientist
Cosmic Writer 
Great post π
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