They change brightness and size over the course of a regular amount of time - generally between 100 and 1,000 earth days. It's believed this is caused by the entire star expanding and contracting almost rhythmically. Most Mira Variables are less than twice as massive as our Sun, but can be thousands of times more luminous than it. These transitions can cause a number of different effects:
- The size changes can happen in portions or stages - many Mira Variables are not spherically symmetric.
- Some Mira Variables cast out clouds of cosmic dust. These particulate shrouds partly obscure the stars, making their variations in brightness even more extreme.
- Material from deep in the core of the star can be ejected to the surface.
- They can suddenly heat up, and switch from fusing hydrogen-into-helium to fusing helium-into-carbon.
- They can emit microwave beams, becoming an astrophysical maser.
Game and Story Use
- Mira Variables can make an interesting navigational hazard or interstellar terrain feature. They change sizes, brightness and temperature, emit masers, and do other things that can ruin your day.
- A planet with a valuable resource might be close to a Mira Variable, and have extremely nasty weather patterns as a result. The atmosphere heats and cools rapidly as the Mira Variable reaches the end or midpoint of it's cycles.
- It may even be inhabitable for only short (but regular) periods. Say, 200 or 300 days out of every 1,000. Miners land, work for several months to a year, and then have to evacuate for a couple solar years before returning to do more mining. Such an unstable job would lend itself to tiny start-up operations, risky gambles, fat paychecks, quick turnover, etc. And there's always the danger of what happens if the evacuation ships arrive too late.
- If life evolves around such a star, it may have strange biological patterns based on the rhythms of the sun. Picture a world where every single animal hibernates for 400 out of every 600 days.
- For additional ideas, see Class M Star.