In about a billion years, the Sun is forecasted to brighten up and turn the Earth into a desert like Mars. Based on the star formation history of the Milky-Way galaxy, about half of the Sun-like stars formed more than a billion years before the Sun. If some of these suns hosted a habitable Earth-like planet, and life on that planet led to the emergence of an intelligent species more than a billion years ago, then this species must have suffered from a devastating astrophysical catastrophe by now. In that case, it would be appropriate for us, as cosmic citizens of the Milky-Way galaxy, to hold a memorial service. once a year commemorating the civilizations that may have tragically died this way within our galaxy. Their number is ten billion if about a fifth of the Earth-analogs had a similar biological history to that of Earth.

This math implies roughly one dead Galactic civilization per living human currently on Earth. If we only knew the identities of these lost civilizations, then each of us could have lit one candle in memory of one of them.