The red giant was a stage in the evolution of most stars.
While fighting the Greld in the 30th century, the Earth Empire used a quark bomb on a star in one of their star systems. The bomb changed the star from a white dwarf to a red giant. Afterward, the Empire built a prison on one of the surviving planets, Dis, which now orbited inside the photosphere of the red giant. (PROSE: Original Sin)
In the year 5000000000, Earth's sun "blossomed", as Cassandra O'Brien.Δ17 put it, into a red giant, finally resulting in the destruction of Earth. Gravity satellites had held back the sun until the National Trust ran out of money. (TV: The End of the World)
At the end of the universe, a red giant was the last star in existence. The War Doctor deployed the Tear of Isha into the star so the Time Lords could not use it to close the Tantalus Eye which would have disastrous consequences for the whole of the Tantalus Spiral during the Time War. Instead, the red giant collapsed without harming anyone. (PROSE: Engines of War)