What kind of star is Acrux?
Acrux, catalogued as Alpha Crucis (α Cru; name applies to component Aa), appears in the official constellation Crux. Its apparent visual magnitude is about 0.76 combined, while its estimated distance is approximately 320 light-years. Astronomers classify the object as B0.5 IV + B1 V. These labels describe temperature, luminosity class or system components; they should not be read as a claim that every naked-eye point is a single isolated star.
Alpha Crucis is built around the roughly four-arcsecond pair Alpha¹ and Alpha² Crucis. Alpha¹ is itself a short-period spectroscopic binary, and high-resolution work continues to refine additional components and their orbital relationships; component counts therefore depend on which wide companions are accepted as physically associated. The dominant stars are exceptionally hot early-B objects whose ultraviolet output exceeds what their visual brightness alone suggests. Space photometry has detected both pressure and gravity pulsation modes in the system, providing constraints on the interiors of massive stars. Acrux lies around 320–370 light-years away depending on the astrometric solution. The IAU proper name applies specifically to Alpha Crucis Aa, not automatically to every point in the wider hierarchy.
The coordinate pair RA 12h 26m 35.9s, Dec −63° 05′ 57″ fixes the object's direction on the celestial sphere for the J2000 reference epoch. Right ascension works like celestial longitude measured in hours, while declination works like latitude north or south of the celestial equator. Proper motion and improved measurements can refine catalogue positions, but these coordinates are sufficiently precise for a finder chart or planetarium search.
Acrux data at a glance
| Proper name | Acrux | Catalogue designation | Alpha Crucis (α Cru; name applies to component Aa) |
|---|---|---|---|
| Constellation | Crux | Apparent magnitude | about 0.76 combined |
| Spectral or system type | B0.5 IV + B1 V | Estimated distance | approximately 320 light-years |
| Right ascension (J2000) | 12h 26m 35.9s | Declination (J2000) | −63° 05′ 57″ |
Values are rounded for an observer-friendly overview. Consult the linked SIMBAD record for component-level identifiers, bibliography, measurement provenance and newer catalogue values.
The name Acrux
A modern contraction of “Alpha Crucis” that entered common use in the nineteenth and twentieth centuries.
Acrux is a comparatively modern contraction of Alpha and Crux, because the star's far-southern position limited its role in the Mediterranean naming tradition that supplied many bright-star names. European observers began recording its visual duplicity during the early telescopic era. Crux later became deeply important in Southern Hemisphere navigation and national symbolism, appearing on several flags; those modern uses coexist with much older Indigenous southern-sky traditions that do not reduce the region to a European cross.
Modern professional catalogues retain identifiers such as the Bayer designation alongside a proper name. The International Astronomical Union standardizes which name belongs to which component—an important distinction for systems whose combined light looks like one star from Earth.
How to find Acrux
Identify compact Crux beside Alpha and Beta Centauri. Acrux is the brilliant southern end of the Cross. Around 75–100× magnification can separate its close blue-white visual components under steady seeing.
Deep southern sky; circumpolar across much of the Southern Hemisphere and invisible from most of Europe. Visibility on a particular night still depends on latitude, season, time, horizon obstructions, weather and local light pollution. Use the interactive atlas below to check the surrounding field before observing.
Observation checklist
- View from March through June evenings in the Southern Hemisphere.
- Use 50–100× under steady seeing to separate the approximately four-arcsecond principal pair.
- Extend the long axis from Gacrux through Acrux to estimate the direction of the south celestial pole.
- Avoid judging the split when Acrux is close to the horizon, where atmospheric dispersion elongates its image.
Interesting facts about Acrux
- Acrux is the brightest system in Crux and the southernmost first-magnitude star.
- Crux and the Southern Pointers can be used to estimate the south celestial pole.
- Acrux appears single to the eye but separates into bright blue-white components in a telescope.
- Its wider visual components are separated by only about four arcseconds.
- It anchors the foot of the familiar four-star Southern Cross.
Continue from Acrux into the night sky
Use the Crux constellation guide to place this star within its larger pattern. When you are ready to observe, compare dark-sky places, find a nearby observatory, or follow our practical guide to finding a star in the night sky. Symbolically registered names are kept separately in the searchable Star-Register catalogue.
Sources and further reading
Measurements are rounded for readability. Multiple systems, variable stars and distant supergiants can have component-specific or model-dependent values. Survey panels show photographic sky fields, not the physical surface of the star.