The new Pixel 4 has been launched with some awesome photography chops. While Google will eventually bring a few of these older phones, that's not the case for the Android community. The most prominent Google Camera mod, GCam by cstark27, has been updated to bring several Pixel 4-exclusive features to older Pixels.
Astrophotography, the Pixel 4's most jaw-dropping photography skill, is the headliner of this mod update. It should work on both Pixel 1 and 2; Night Sight and keep your phone extremely steady then tap the shutter button when the "Astrophotography on" message shows up. I tested it on my Pixel 2 XL and was able to take many 60-second exposure photos. If I had less light pollution around me, it should have gone higher.
It's a very cloudy night, so you'll need to look closely to see some stars towards the upper part.
Another backported feature is Super Res Zoom (which came with the Pixel 3, not Zoom + which was launched with the 4). I tested it against the regular Google Pixel 2 XL.
left: 4x zoom with Google Camera. Right: 4x zoom with GCam mode 7.2. The bottomSame, cropped.
The mode also contains a lot of new settings and improvements, as well as focusing and zooming. against a bright background.
left: Night Sight focus distance and "Astrophotography on" toast. RightPhotobooth for selfies.
* Enable Astro mode for Pixel 1 & 2
* Enable Focus options in Night Sight (along with "infinity" – may be same as far)
* Modded lib for Learned Depth in Portrait Mode (Pixel 2)
* Enable Super Res Zoom (no zoom required) – this is like Pixel 3 stock
* Enable auto timer (photobooth), it's in the selfie camera in the Timer options
* Enable synthetic fill flash for Pixel 2 and enable SFF in Camera mode for 3a
cstark27 says he tested these on Pixel 2, as I did, but some of them should work on the first and third-gen Pixels as well. To try them out, you can grab GCam PXv3.0, which is based on Google Camera 7.2, from the official source or from APK Mirror.