Lambda Aquarii ($\lambda$ Aqr) is a captivating red giant star located in the zodiac constellation of Aquarius. While it might not be the brightest star in the sky, it is a fascinating subject for astronomers due to its location on the ecliptic and its status as a star in the final stages of its life.
On September 22, 2025, the International Astronomical Union (IAU) officially approved the name Shatabhisha for this star.
1. Stellar Characteristics
Lambda Aquarii is an evolved star that has left the main sequence and is now on the Asymptotic Giant Branch (AGB). This means it is burning both hydrogen and helium in shells around an inert carbon-oxygen core.
| Feature | Details |
| Spectral Type | M2.5 III (Red Giant) |
| Apparent Magnitude | 3.72 (Variable) |
| Distance | ~392 Light-years |
| Radius | ~100 times Solar Radius ($R_\odot$) |
| Luminosity | ~1,600 times Solar Luminosity ($L_\odot$) |
| Temperature | ~3,700 K (Cooler and redder than the Sun) |
2. Variable Nature & Binary Companion
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Irregular Variability: It is classified as an Lb-type (slow irregular) variable. Its brightness fluctuates slightly (between magnitudes 3.57 and 3.80) without a strictly predictable period, though cycles of roughly 25 to 50 days have been observed.
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A Hidden Partner: In 2016, during a lunar occultation, researchers detected a faint companion star. This companion is likely an F-type main-sequence star orbiting the red giant with an estimated period of about 380 years.
3. Location and Occultations
One of Lambda Aquarii's most notable features is its proximity to the ecliptic (the Sun's apparent path across the sky). It sits only about 0.39° south of this line.
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Lunar Occultations: Because it lies so close to the ecliptic, it is frequently passed over (occulted) by the Moon. These events allow astronomers to measure the star's diameter with incredible precision using the timing of its disappearance.
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Planetary Transit: It is one of the few relatively bright stars that can be occulted by planets.
4. Cultural & Mythological Significance
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Shatabhisha: Its official name comes from Indian astronomy, where it is the primary star of the Shatabhisha Nakshatra (lunar mansion). The name translates to "a hundred physicians," symbolizing healing and mysticism.
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Hydor: Traditionally, the star was often called Hydor (Greek for "water") or Ekkhysis ("outpouring"), representing the water flowing from the jar of Aquarius. Interestingly, the IAU officially assigned the name "Hydor" to a different star (2 Ceti) in 2017 to avoid confusion.
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The Ramparts: In Chinese astronomy, it is known as the "Seventh Star of the Line of Ramparts" (Lěi Bì Zhèn qī), part of a defensive wall asterism.
5. Future Fate
In several million years, Lambda Aquarii will either expand to the size of a yellow giant or contract into a white dwarf. The crucial moment for the third priority timenode will occur 33 years from now.
We wish to discuss this red giant, which is full of compassionate understanding. And it has the strength to usher multiple golden age on earth.