The primary source of light in the Andromeda Galaxy, also known as M31, is the billions of stars that make up the galaxy. These stars emit electromagnetic radiation, including visible light, which collectively illuminates the galaxy.
The Andromeda Galaxy is approximately 2.537 million light-years away from Earth, and its total luminosity is estimated to be around 2.5 trillion times that of the Sun. However, it's important to note that the brightness of the galaxy can vary depending on the distance and position of individual stars within it, as well as factors like dust and gas that can absorb or scatter light.
The apparent brightness or magnitude of an object in the night sky is measured on a logarithmic scale. For reference, the apparent magnitude of the Andromeda Galaxy is around 3.4, making it visible to the naked eye under dark sky conditions. This puts it among the brightest deep-sky objects in the night sky.
If we could somehow transport ourselves to a closer distance to the Andromeda Galaxy, its brightness would appear much more significant. However, since it is currently impossible for humans to reach such distances, we can only appreciate the beauty and brightness of the Andromeda Galaxy from afar.