Why Can't We See Individual Stars from Other Galaxies at Night?

We can't see individual stars from other galaxies at night primarily due to two main reasons: distance and brightness. The vast distances involved mean that even the brightest stars in other galaxies appear extremely faint by the time their light reaches us. Additionally, the inherent brightness of stars diminishes according to the inverse square law, meaning that as the distance from the observer increases, the apparent brightness decreases rapidly. This is further compounded by the limitations of the human eye and the physical properties of visible light.

Distance: Vast Cosmic Distances

Galaxies are incredibly far away from us. For instance, the nearest galaxy, the Andromeda Galaxy, is about 2.537 million light-years away. This enormous distance makes individual stars within these galaxies nearly undetectable to the naked eye. Even the brightest stars in other galaxies appear extremely faint when their light eventually reaches us. It's important to note that even closer galaxies, while visible to some extent, appear as fuzzy patches of light due to the immense distances involved.

Brightness: The Inverse Square Law

Stars are inherently not bright enough to be seen across such vast distances. The light from stars diminishes according to the inverse square law, meaning that the apparent brightness decreases rapidly as the distance from the observer increases. For example, a very bright, relatively close star can appear dimmer than a distant star that is not very bright. This effect is even more pronounced for stars in other galaxies, where the light must travel millions or even billions of light-years to reach us, further reducing their apparent brightness.

Limited Human Perception

While telescopes can help in resolving very close light sources, such as binary stars, there are still limits to what can be seen. Light is specified by its wavelength, which for visible light is around 500 nm. The wavelength of the light sets a limit on the minimum angular distance that can be distinguished, regardless of the capabilities of an optical device. No matter how powerful a telescope is, certain physical properties of visible light will prevent us from resolving dots that are too close to each other. This is why, even in the most advanced telescopes, galaxies appear as single dots rather than individual stars.

The Core of Our Galaxy

Interestingly, even in the core of our own galaxy, the Milky Way, we can't see individual stars. The immense density of stars in the central regions makes it impossible for us to distinguish individual stars with the naked eye or even with the most powerful telescopes. The core of the Milky Way is a dense cluster of stars, and the starlight from these stars blends together, making it difficult to resolve individual stars.

Other Galaxies and Telescopes

While some bright galaxies, such as the Andromeda Galaxy, are visible and can even be studied in some detail, most other galaxies are just single dots in the night sky, even when observed through telescopes. The physical limitations of human eyes and the limitations of visible light prevent us from resolving individual stars in these distant galaxies. Even the most advanced telescopes struggle to distinguish individual stars due to the vast distances and the spread of light from numerous stars.