It's not as simple as "FF has lower noise". There's a reasonable argument that once you factor in everything, most sensor size differences come out in the wash.

It's not as simple as "FF has lower noise". There's a reasonable argument that once you factor in everything, most sensor size differences come out in the wash. In the case of low-light performance, you have situations like: APS-C + 35mm f/1.4 ~ FF + 50mm f/2. Yeah, you need a larger aperture in f-stop terms for equivalent optical properties, but the cost of that is probably offset by the cost of APS-C lenses being otherwise cheaper (having a narrower optical path). The two biggest actual advantages of FF, to my mind, are lower sensitivity to small inhomogeneities in lens glass (but this is not super important) and larger maximum FET capacitor size. The good astro shots you've seen are probably compositions of multiple photos. That is arguably the only way to get good low-noise shots with any consumer sensor technology, just due to the extremely high dynamic range of the sky and the physical realities of shot noise.