Can you see the W????

Yes but nowhere near as obvious as it is to the the naked eye! Must be a difference between how our eyes work compared to photography.

Closer inspection reveals many faint subtleties. Interesting

What's the advantage of stacking images rather than one longer exposure? Are there any disadvantages?

Yes.... the sensor us far far far more sensitive than our eyes, but if we're are in a dark sky site you will see more with your etes.

In that shot there are 5 Nebulae and 2 star clusters!

Taking lots of images is used to collect data (basically your subject), and more importantly - increases the signal to noise ratio. When you stack the image, this reduces said noise. It also helps DSLR sensors not to heat up. The longer the exposure, it will enduce sensor burn if you don'tallow it to cool. As a rule os thumb with DSLR and Mirrorless, if you are shooting a 2 minute exposure, you'll set a gap that is half your exposure. This to allow the sensor to cool. Cooled cameras negate this, which means you can shoot half as long again. Also untracked long exposures will give you star trailing.

So.... say we're shooting Barnards Loop, in the Orion Constellation - untracked. You'll be using a high ISO, say 3200. You'll shoot say 256 2 or 4 second exposures. Rembering to move the camera every few minutes to recenter your subject. This will increae your signal to noise when you stack the image. So it helps to reduce high ISO noise as well as collecting photons!

I have tracking kit, so this means I can shoot up to 3 minute exposures. On my big kit with tracking, guiding, and a cooled camera (this also helps to reduce noise further). I regularly shoot 5 minute exposures, with the possibility of shooting 10 or 15 minutes exposures.

It's all about maximising data collection and increasing the signal to noise ratio. Bassically the more data you collect, the better!

Thanks for your explanation I'd not thought about the sensor heating up!

As a teen I used to lie on an old mattress in the back garden and count the Perseids! And, using a Praktica held by a brick on an angled plank I tried shooting toward Polaris. Boots got back with a sticker that said 'underexposed'! I was 13. I love this.