How can each individual photon of light radiating from a star millions/billions of light years away maintain its energy level long enough to reach our eyes?
First keep in mind that photons are not like ordinary matter because they are massless. They are such unusual particles that whenever we talk about physics and/or motion at (or near) the speed of light, we have to use an entirely new set of rules called relativity. Many aspects of relativity can be counter-intuitave because we usually don't observe relativistic physics in our everyday lives and thus this "new set of rules" I mentioned seem strange.
Now, if an ordinary object travels a long distance here on Earth (for example, imagine a ball rolling down a long hallway), friction will eventually slow the object down to a stop. A photon won't fall victim to this same friction because the photon has no mass (unlike the ball).
There are a few other ways that a photon can lose energy during its trip to us from a distant star. If the photon encounters an atom along its path, that atom could absorb it and then re-emit the photon at a lower energy. A photon could also encounter a particle and be scattered and in the process lose some of its energy to the particle. However, since the universe is mostly vast, open space, these events usually don't stop a star's light from reaching us.
Finally, there IS a mechanism at work that decreases the energy of almost all the photons we receive from distant objects (like other galaxies) - the expansion of the universe. You may have heard the term "redshift" which describes the shift from shorter to longer wavelengths of light leaving objects that are moving away from an observer. Since the universe is expanding, from our point of view other galaxies appear to be moving away from us. The light we then detect from these galaxies can still reach us but is a redshifted version of the light that actually left the galaxies Longer wavelengths mean lower energy photons. How much a photon's energy is decreased depends on how fast the far-off galaxy is moving away from us, the observers.
This page was last updated July 18, 2015.