Hail Mary by Andy Weir

A man wakes up, disoriented, from a medically induced coma.  He is attended by an medical robot in the ceiling of his room.  When he is able to rise from his bed he discovers he has two companions in the room who are desiccated.  He begins to remember that he is on a spaceship.  He is on a mission to save Earth from a disaster.  The sun is getting dimmer.  So are several other stars surrounding Tau Ceti.  But Tau Ceti is not getting dimmer.  He is there to find out why in hopes of applying that same difference to Sol.  He also discovers that he is not alone.  There is another spaceship that has parked itself nearby.  And it has a survivor too.  They’re from one of the other affected stars.  Both civilizations figured out the cause of the dimming and in the process found they could use that to achieve near light speed travel.  Now they have to cooperate to save both civilizations.