It is possible to watch Houston Astros games live online or on TV without an expensive Cable TV package. Usually these games are on AT&T SportsNet Southwest, but that may change depending on schedule, type of game or national interest.
The Houston Astros play most of their games on AT&T SportsNet Southwest, a regional sports network. Regional Sports Networks (RSNs) are not common on streaming services, but when they do exist they are usually only available for an additional fee.
Here are the live streaming services that carry AT&T SportsNet Southwest: Hulu Live, YouTube TV and AT&T Now
The MLB also has MLB.tv, which live streams all out-of-market games for all teams. Unfortunately, Houston Astros games on here are subject to blackouts if you are within what is considered the team's "home region".
If you find yourself trying to watch a game on the MLB.TV that is blacked out, you will need a VPN to mask your location.