Sigma Aquarii
A white subgiant star in Aquarius, possibly an astrometric binary.
Sigma Aquarii, also known by its Bayer designation σ Aquarii, lies about 1.3° south of the ecliptic in the equatorial constellation Aquarius. This white-hued star is just bright enough to be seen without a telescope, with an apparent visual magnitude of 4.81. Because it sits so close to the ecliptic, the Moon can occasionally pass in front of it, an event called an occultation. Based on parallax measurements, the star is roughly 206 light-years away, and it is moving farther from Earth at a radial velocity of +11 km/s.
Classified as A0 IVs, Sigma Aquarii is a subgiant star. The "s" in its classification indicates that its absorption lines are unusually narrow, a result of its relatively slow rotation. It is estimated to be about 30 million years old, with a projected rotational velocity of 21 km/s. The star has 2.87 times the mass and 2.87 times the radius of the Sun, and it shines with 105 times the Sun's luminosity from a photosphere heated to 10,115 K.
Sigma Aquarii was once categorized as a hot Am star—a chemically peculiar type—but this classification is now considered uncertain. Its spectrum shows at least double the normal amounts of magnesium, aluminum, and silicon, while helium and scandium are under-abundant. Calcium, which is typically deficient in Am stars, appears near-normal here. The Hipparcos catalogue listed Sigma Aquarii as a possible astrometric binary, with an orbital period of 654 days.
- Apparent visual magnitude
- 4.81
- Distance
- approximately 206 ly
- Radial velocity
- +11 km/s
- Stellar classification
- A0 IVs
- Age
- 30 million years
- Projected rotational velocity
- 21 km/s
- Mass
- 2.87 times the Sun
- Radius
- 2.87 times the Sun
- Luminosity
- 105 times the Sun
- Effective temperature
- 10,115 K
Lore & Background
Sigma Aquarii has a stellar classification of A0 IVs, indicating it is a subgiant star. The 's' qualifier means its absorption lines are sharp (narrow) compared to standard stars, caused by a relatively slow rotation. It is an estimated 30 million years old and spins with a projected rotational velocity of 21 km/s. The star has 2.87 times the mass and 2.87 times the radius of the Sun, radiating 105 times the Sun's luminosity from its photosphere at an effective temperature of 10,115 K.
Sigma Aquarii has been categorized as a hot Am star, meaning it is chemically peculiar, although this is now considered doubtful. Its spectrum displays at least double the normal abundances of elements like magnesium, aluminum and silicon, while helium and scandium are under-abundant. Calcium, normally deficient in Am stars, has near-normal abundance.
The Hipparcos catalogue identified Sigma Aquarii as a possible astrometric binary with an orbital period of 654 days.
Reader's Guide
Sigma Aquarii serves as a notable example of a chemically peculiar star whose classification as a hot Am star is now considered doubtful. Its spectrum shows enhanced abundances of magnesium, aluminum, and silicon, while helium and scandium are under-abundant, and calcium—typically deficient in Am stars—appears near-normal. This unusual abundance pattern challenges the standard Am classification and makes the star a subject of interest for studies of stellar chemical peculiarity. Additionally, its status as a possible astrometric binary, with an orbital period of 654 days identified by the Hipparcos catalogue, adds a layer of complexity to its nature. The star's proximity to the ecliptic makes it a target for lunar occultation events, which can provide precise positional and angular diameter measurements. With a white hue and apparent magnitude of 4.81, it is accessible to naked-eye observers and serves as a reference point in the constellation Aquarius. Its slow rotation, indicated by sharp absorption lines, and its subgiant evolutionary stage at an age of 30 million years contribute to understanding the early post-main-sequence evolution of A-type stars.
Did You Know?
- Sigma Aquarii is positioned about 1.3° south of the ecliptic, making it subject to lunar occultation.
- It has a projected rotational velocity of 21 km/s, causing its absorption lines to appear sharp.
- The star's spectrum shows at least double the normal abundance of magnesium, aluminum, and silicon.
Frequently Asked Questions
What is Sigma Aquarii?
Sigma Aquarii (σ Aquarii) is a white-hued subgiant star located in the constellation Aquarius, sitting roughly 1.3 degrees south of the ecliptic. It sits at an apparent visual magnitude of 4.81, making it just barely visible to the unaided eye under dark skies.
Is Sigma Aquarii a confirmed binary or multiple star system?
Sigma Aquarii is not a spectroscopically confirmed binary, but it is suspected to be an astrometric binary based on subtle positional wobble in its proper motion. This makes it a candidate worth watching for multi-star enthusiasts hoping for a definitive companion detection.
What is Sigma Aquarii's stellar classification and how old is it?
The star carries the spectral class A0 IVs, identifying it as a subgiant that has evolved off the main sequence. It is estimated to be around 30 million years old and rotates with a projected velocity of approximately 21 km/s.
How far is Sigma Aquarii from Earth and is it approaching or receding?
Parallax measurements place the star at roughly 206 light-years from our solar system. It is currently drifting away from us at a radial velocity of +11 km/s.
Can the Moon occult Sigma Aquarii, and is it visible without a telescope?
Because the star lies so close to the ecliptic path, the Moon can occasionally pass directly in front of it, producing a visible occultation event. At magnitude 4.81 it is faintly detectable by the naked eye, though it sits right at the threshold of easy detection in suburban skies.
More in Binary and Multiple Stars, Part 5 1-24
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