R Aquarii
Symbiotic Mira variable with jets and a dusty nebula.
R Aquarii, also designated R Aqr, is a variable star located in the constellation Aquarius. It is a symbiotic binary system composed of a red giant primary and a white dwarf secondary, with an orbital period of roughly 40 years. The primary is a Mira-type variable whose brightness fluctuates by a factor of several hundred over slightly more than a year; this variability was first noted by Karl Ludwig Harding in 1810. The star’s apparent magnitude ranges from 5.2 to 12.4, a variation of 750 times in brightness—meaning it can be visible to the naked eye at its brightest but fades beyond the reach of binoculars at its dimmest. Its pulsations occur approximately every 390 days but are not perfectly regular. R Aquarii is one of the closest symbiotic stars and is well known as a source of jets. The two stellar components have been resolved at a separation of 55 milliarcseconds.
The white dwarf’s gravity pulls material from the red giant, and occasionally some of this surplus matter is ejected in loops, forming the surrounding nebula. The entire system appears reddened because it lies in a heavily dusty region of space, which absorbs its blue light before it reaches Earth. The nebula around R Aquarii is also cataloged as Cederblad 211. It may be the remnant of a nova-like outburst possibly observed by Japanese astronomers in the year 930 AD. The nebula is reasonably bright but small, and its light is dominated by the central star, making visual observations difficult and rare. The central region of the jet contains an ejection that occurred about 190 years ago, along with much younger structures. Another jet launched in January 2020 was reported in a 2025 publication. The jet’s expansion rate places the system’s distance at 260 ± 27 parsecs (848 ± 88 light-years).
- Constellation
- Aquarius
- Type
- Symbiotic star (Mira variable primary + white dwarf secondary)
- Orbital period
- Approximately 40 years
- Brightness range
- 5.2–12.4 (factor of 750)
- Pulsation period
- 390 days (not entirely regular)
- Discovery of variability
- Karl Ludwig Harding, 1810
- Distance
- 260 ± 27 parsecs (848 ± 88 ly)
Lore & Background
R Aquarii is a symbiotic binary system where the white dwarf draws material from the red giant by its gravitational pull, occasionally ejecting surplus in loops to form a nebula. The whole system appears reddened because it lies in a very dusty region of space, its blue light absorbed before reaching Earth. The nebula around R Aquarii is also known as Cederblad 211; it is reasonably bright but small and dominated by its central star, making visual observations difficult and rare. The central region of the jet shows an ejection that took place around 190 years ago, as well as much younger structures. Another jet launched in January 2020 was announced by a 2025 publication. The expansion rate of the jet constrains the distance of the system to 260 ± 27 parsecs.
Reader's Guide
R Aquarii is significant as one of the nearest symbiotic stars and a well-known jet source, offering a close-up laboratory for studying binary interactions and jet formation. Its Mira variable primary, discovered by Karl Ludwig Harding in 1810, varies in brightness by a factor of several hundred with a period of slightly more than a year, though the pulsations are not entirely regular. The total brightness range of 5.2–12.4 corresponds to a variation of 750 times, from a naked-eye star to one beyond binocular range. The nebula Cederblad 211 may be the remnant of a nova-like outburst possibly observed by Japanese astronomers in the year 930 AD, though this is uncertain. The system's jets, including one launched in January 2020, provide temporal markers for studying ejection dynamics. The distance derived from jet expansion—260 ± 27 parsecs—anchors its astrophysical context. The system's reddening due to dusty surroundings further highlights the role of interstellar material in shaping observed properties.
Did You Know?
- R Aquarii's variability was discovered by Karl Ludwig Harding in 1810.
- A jet launched in January 2020 was announced in a 2025 publication.
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