Since Mountain Lion, Apple hasn’t changed the system requirements for its desktop operating system. Since 2012, these Macs could run the current OS:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- Xserve (Early 2009)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)
With macOS Sierra, that list has changed:
- iMac (Late 2009 and later)
- MacBook (Late 2009 and later)
- MacBook Pro (2010 and later)
- MacBook Air (2010 and later)
- Mac Mini (2010 and later)
- Mac Pro (2010 and later)
It was initially though that Sierra requires Intel chips with the SSE4.1 instruction set, removing machines with silicon older than the 45nm Penryn Core 2 Duo family of processors.
However, if SSE4.1 was the hard cutoff, some older Mac Pros — that Apple has cut off — would be able to run macOS Sierra that Apple, so there may be other factors like GPU support in play as well.
Apple’s been really good about supporting old machines longer and longer, but some users are understandably upset that their machines are being left behind this time around. If you’re one of those users, I’ll just leave this link here, but something like this should is definitely not supported by Apple.