Sextans, the Sextant, is a faint constellation near the celestial equator, located just south of Leo. Unlike ancient constellations associated with mythological figures, Sextans represents an astronomical instrument used to measure the positions of stars.
Created by the Polish astronomer Johannes Hevelius in the late 17th century, Sextans covers 314 square degrees of sky, making it the 47th-largest constellation. Its brightest star, Alpha Sextantis, shines at only magnitude +4.5, so the constellation is difficult to recognize from light-polluted locations.
Despite its inconspicuous stars, Sextans contains several interesting galaxies, including the Spindle Galaxy (NGC 3115) and the interacting pair NGC 3166 and NGC 3169.
Where and When to Find Sextans
Sextans lies immediately south of Leo, with Hydra extending along its southern boundary and Crater lying farther east. The constellation is best observed during February, March, and April, when it is well placed in the evening sky. From mid-northern latitudes, Sextans rises reasonably high above the southern horizon. Dark skies are particularly important because even its brightest stars are near the naked-eye visibility limit in suburban conditions.
To locate Sextans, first find Regulus, the brightest star in Leo. Look southward into the relatively empty region of sky above Hydra. Sextans occupies this area, although its stars are too faint to form an obvious pattern.
History — Johannes Hevelius and His Sextant
Sextans was introduced by the Polish astronomer Johannes Hevelius, who included it in his celestial mapping work during the late 1600s. Hevelius used a large astronomical sextant to measure angular distances between stars. Unlike the smaller navigational sextants later used by sailors, his instrument was designed for precise positional astronomy.
In September 1679, a fire destroyed much of his observatory in Gdańsk, including valuable instruments and records. Hevelius subsequently commemorated his lost sextant by placing it among the stars. The constellation was originally called Sextans Uraniae, meaning Urania’s Sextant, after the muse of astronomy. Sextans remains one of seven constellations introduced by Hevelius that are recognized today.
Notable Stars in Sextans
- Alpha Sextantis is the constellation’s brightest star, a white giant approximately 280 light-years away. It shines at magnitude +4.5 and lies close to the celestial equator. Because of its faintness, it is best observed under dark skies.
- Beta Sextantis is a blue-white star roughly 350 light-years distant, with an apparent magnitude near +5.1. It is considerably hotter and more luminous than the Sun, although its distance makes it appear faint from Earth.
- Gamma Sextantis is a multiple-star system approximately 260 light-years away. Its two principal components form a close binary pair that requires a telescope to resolve.
Another interesting star is 24 Sextantis, an evolved star known to host giant exoplanets. Sextans also contains WASP-43, a star with a closely orbiting gas giant whose atmosphere has been extensively studied.
Notable Deep-Sky Objects
Sextans contains no Messier objects, but several galaxies make worthwhile targets for telescope observers.
Spindle Galaxy (NGC 3115)
The Spindle Galaxy, NGC 3115, is a lenticular galaxy approximately 30 million light-years away.
Seen nearly edge-on, it has a bright central bulge surrounded by a flattened disk, giving it the elongated appearance that inspired its name.
Unlike spiral galaxies such as the Milky Way, NGC 3115 contains relatively little cold gas and dust, and most of its stars are old. As a result, it has little ongoing star formation.
The galaxy also contains an exceptionally massive central black hole, estimated to have a mass of roughly one to two billion Suns.
At approximately magnitude +9.9, NGC 3115 is one of the more accessible deep-sky objects in Sextans and can be detected with moderate-sized telescopes under dark skies.
NGC 3166 and NGC 3169 — Interacting Galaxies
NGC 3166 and NGC 3169 are a pair of galaxies approximately 70 million light-years away.
Their mutual gravitational attraction is distorting their structures. NGC 3169 has warped spiral arms, while NGC 3166 shows disturbed dust lanes and an irregular appearance.
Astronomical photographs reveal how the interaction is reshaping both galaxies, providing a striking example of gravitational encounters on a galactic scale.
The pair can be observed as faint patches of light through moderate to large amateur telescopes under favorable conditions.
Sextans A and Sextans B — Nearby Dwarf Galaxies
Sextans A and Sextans B are dwarf irregular galaxies located several million light-years away, beyond the Milky Way. Both belong to the broader population of nearby galaxies studied by astronomers to understand star formation and galactic evolution.
Sextans A is particularly interesting because it contains regions of active star formation despite its relatively small size. These galaxies are extremely faint and are better suited to astrophotography than casual visual observing.
Observing Sextans
Sextans is not a constellation that immediately attracts attention. Its stars are faint, and it lacks the recognizable outline of nearby Leo. However, once you identify its position beneath Regulus, the constellation becomes an interesting region to explore with a telescope. The Spindle Galaxy is the best starting point for deep-sky observers, while NGC 3166 and NGC 3169 offer a more challenging view of interacting galaxies.
Sextans may be visually modest, but its history and distant galaxies make it a rewarding part of the spring sky.