One of the brightest Mira type variables, going up & down in brightness every 7.5 months.
Like other Mira type variables, R Trianguli is a red giant star whose brightness variations are due primarily to pulsations in its outer layers but also, to a lesser extent, to the formation and destruction of very simple molecules in its (relatively) cool outer layers.
Also, as for other Mira type variables, the brightness changes don’t repeat from one cycle to the next. The brightness at maximum changes from one maximum to the next and there is always an uncertainty of around +/- 2 weeks in any predicted dates of maxima.
There is a gap each year when R Trianguli is near conjunction and not visible in the night sky. This runs from mid April to mid June and was responsible for the 2015 maximum being missed in the above light curve.
| Extreme brightness range | 5.4 – 12.6 |
| More typical range | 6.0 – 11.7 |
| Period of variation | Approx 7.5 months |
| Frequency of observation | Worth checking a few times per month |
| Observe using | 40mm or 50mm binoculars when near maximum. 50-80mm binoculars when fainter. A telescope is required to follow it all the way down to minimum |
| Visibility | late June to early April |
| Dates of maxima | September 2025 |
The finder chart below shows the location of R Trianguli.
