The Andromeda Galaxy is the furthest, most remote night sky object visible to the naked eye. It measures 220,000 light years in diameter and appears as a blue smudge in the constellation Andromeda between the constellations Cassiopeia and Pegasus. Its distance from the Earth is estimated at 2.4 million light years – i.e. it takes the combined light of Andromeda Galaxy’s 1 trillion stars approximately 2.4 million years to reach the Earth.
Light travels through space and time in the dual form of particles and waves. As light from the stars in the Andromeda Galaxy finally reaches the front part of your eye called the cornea, the direction of the wave forms is bent at the outward surface of the cornea, and then again at the inner surface of the cornea. The same type of directional bending of light waves occurs at the outer and inner surfaces of the lens that is inside your eye. Assuming you do not need any glasses, the light from the stars that has passed through your cornea and lens is then focused onto microscopic light receptors called “photoreceptors” arranged in an array within your retina, the light sensitive membrane lining the inside compartment of your eye.
Once activated by the light from the stars, the photoreceptors undergo a biochemical change that subsequently causes neighboring retinal nerve cells to change at the level of their electrical state. The electrical signal is then carried by ganglion cells of the retina through the optic nerve sticking out at the back of your eye. This signal eventually reaches the visual cortex at the back of your brain, where it is integrated into the blue smudge image of the Andromeda Galaxy that you perceive.
In summary, to see the Andromeda Galaxy, your eye must focus light from 2.4 million light years away onto the photoreceptors of the retina, which then convert light energy into electrical signals that your brain then integrates into the image of the galaxy. Both the galaxy itself and the process by which we see it are staggering.