Like lighthouse beacons in a dark ocean, stars act as tiny islands in the vast universe. Producing light at the atomic level from the powerful release of energy through fusion, they are the engines that drive the formation of new elements. But in the darkness there are plenty of other hidden objects that are cold and give off little to no light. Yet many of them are easily seen. Here’s Why!
The first thing to think about is infrared light, the radiation given off by warm objects. Large planets and brown dwarf stars are very bright in infrared, much brighter than in visible light even. But cooler objects, essentially anything rocky, are visible because of their ability to reflect light. This is called Albedo, and it’s represented two ways in astronomy. The geometric albedo is the relative optical brightness of an object when the illuminating source is behind the observer, ie looking at a full Moon illuminated by the Sun. The Bond Albedo on the other hand is the proportion of the total reflected EM radiation. These two values can vary significantly depending on the object.
Most objects in our solar system have a low albedo, less than 0.1, meaning they are quite dark. The Moon is around 0.136, only slightly brighter than most solar system rocks, but the full Moon is very bright because it is a relatively large object in the sky and we are comparing it to a very dark background. If you’ve ever seen the Moon in the daytime it blends in with the blue sky a lot more.
You may think the Earth would be high, since it’s covered in reflective water, but it’s only about 0.37. This is quite high for a rocky planet, and definitely way higher than the majority of solar system objects.
One of the brighter objects in terms of albedo is the planet Venus, at 0.65. It’s actually the dense clouds in Venus’ atmosphere that are so strongly reflective, whereas the rocky surface wouldn’t reflect very much sunlight. It’s this reflective atmosphere, combined with it’s proximity to Earth that makes it the brightest object in the sky after the Sun and Moon.
The highest known albedo is Enceladus, the Moon of Saturn, at 0.99. With a surface of fresh ice, it reflects so much energy that it doesn’t absorb much sunlight, making it extremely cold among Saturn’s Moons. But this value is very telling. Combined with a relative lack of craters, the albedo of Enceladus implies that it’s surface is being renewed regularly every few hundred million years. Now we know there are geysers on Enceladus that tap into the subsurface ocean. Albedo reflects that….pun intended.