Photons emitted by the Sun and other stars follow various paths through the universe. While some photons do interact with matter and get absorbed, many others continue to travel indefinitely, potentially for billions of years or even longer.
When photons are emitted from a star's surface, they initially possess a certain amount of energy corresponding to their frequency or wavelength. As they traverse space, photons can encounter various objects and substances, such as planets, asteroids, interstellar gas, and dust particles. When a photon interacts with matter, several outcomes are possible:
Absorption: The photon may be absorbed by an atom, molecule, or solid object. In this case, the absorbed energy is transferred to the material, increasing its internal energy.
Scattering: Instead of being absorbed, a photon can be scattered by the interaction with particles. Scattering can change the direction of the photon's path but does not typically result in the photon losing significant energy.
Transmission: Photons can pass through certain transparent materials without being absorbed or scattered, such as the Earth's atmosphere or interstellar space. This is why we can observe starlight reaching us from distant celestial bodies.
Regarding the fate of photons that do not encounter any matter and continue to travel through space, they do not naturally "lose" energy or stop completely. According to the theory of general relativity, photons traveling in a vacuum move at a constant speed, maintaining their energy and momentum.
However, it's worth noting that the universe is not entirely empty. Even in interstellar and intergalactic space, there is a low density of particles, such as hydrogen atoms and dust. Over extremely long distances, these rare encounters may cause photons to be absorbed or scattered. Additionally, the expansion of the universe can lead to a redshift of photons, causing their wavelength to stretch and their energy to decrease gradually over cosmological timescales.
In summary, while many photons emitted by stars do interact with matter and eventually get absorbed or scattered, a significant number of photons continue to travel indefinitely through space, maintaining their energy and momentum unless they encounter matter or undergo cosmological redshift.