I could have sworn this was Bradbury. Definitely published pre-1980. I feel like I should know this.
The female (?) protagonist is in an apartment suffering in the heat. Earth is moving toward the Sun / the Sun is going Nova. She shares one of her last cans of juice with an elderly female neighbor (perhaps named Ruth?)
The heat gets worse and worse. In the end, she falls asleep or passes out...and wakes.
She has been in a fever, the events of the story were a dream...but she is sick because of the cold, the bitter cold...because the Earth is moving away from the Sun / the Sun is dying.
I was trying to think of the kind of story where ending in a dream was not a cop out. I wanted this title to use as an example. Now I'm drawing a big blank. Can you help?