The sensor format matching a 35 mm film frame. For low light and the night sky it is the practical minimum: bigger pixels keep the signal above the noise. Notice that the finest low-light bodies carry only 20 to 25 megapixels rather than 60 or more. Fewer, larger pixels are the point, not a compromise.
A sensor format smaller than full frame but larger than Micro Four Thirds, typical of Canon and Nikon starter systems. A capable general-purpose amateur format, but rarely the one professionals choose for the highest quality work, and rarely a system you can grow upward inside.
Sensor formats larger than full frame, favored where image quality outweighs everything else. The large sensor means large pixels, wide tonal range, and prints that hold together at any size.
Photography after sundown, which is less about darkness than about balance. The best night shots are usually made before total darkness, while the sky still carries light to weigh against the lamps and windows. Once the sky goes fully black, lights burn out against nothing and the picture loses its shape.
The measure that decides how clean an image looks, and the reason all noise lives in the shadows. A brightly lit pixel might put out five volts; a shadowed one, a hundredth of a volt. The same tiny error that disappears against five volts is ten times the entire signal of the dark pixel. Bigger pixels gather more light and keep the signal far above the error, which is why low-light cameras favor fewer, larger pixels.