Seven Stars in One System express
In the southern sky, the star Acrux shines brightly — the lowest point of the Southern Cross constellation. It is not a single star but a whole family. New observations revealed that it contains not five, but seven stars bound by gravity, like children holding hands in a circle dance.
Earlier, astronomers knew that at the center of the system there are two bright stars — Alpha-1 and Alpha-2—with three fainter ones orbiting them. Now, by combining light from several telescopes (like assembling a puzzle from pieces of one image), scientists have discerned that Alpha-1 and Alpha-2 are themselves binary. That makes seven stars: four in tight pairs and three on wide orbits.
Such tight groups of stars are born together from a single gas cloud. Their orbits are inclined to each other at a large angle, like carousels spinning in different planes. This suggests the system had a turbulent youth: the stars jostled each other until they settled into their current orbits.
If this system were farther away, we would not be able to see so many stars: they would merge into a single point. This means the Universe may harbor far more multiple stars than we think. We just cannot distinguish them yet.
Original abstract
as written by the authors arXiv:2603.11194
Alpha Crucis is the closest very high multiplicity massive star to the Sun. At its heart is the $$4" \leftrightarrow 430 \text{ au}$$ binary $$\alpha^1$$ (A) + $$\alpha^2$$ (B) Cru, which combined make up the 13th visually brightest star in the night sky. Here we make use of archival VLTI data of $$\alpha$$ Cru A+B in order to study its multiplicity and orbital architecture. The data spatially resolved the close (6 mas) companion in $$\alpha$$ Cru A (a known spectroscopic binary) and revealed that $$\alpha$$ Cru B is also a close (17 mas) binary, which upgrades $$\alpha$$ Cru to a seven star system. By combining the interferometric data with radial velocities, we solve for the full orbit of Aa+Ab and find dynamical masses $$M_{Aa}=17.2\pm1.2 M_{\odot}$$ and $$M_{Ab}=6.8\pm0.3 M_{\odot}$$. While the data on Alpha Cru B are not yet sufficient to tightly constrain all orbital parameters, we find that the orbital period is most likely 405 days (with 203 days also a possibility). The orientation of the orbital planes are sufficiently constrained to yield a mutual inclination between Aa+Ab and Ba+Bb of either $$50 \pm 5^{\circ}$$ or $$137\pm5^{\circ}$$, pointing to a dynamical formation scenario for the system. The photometric masses $$M_{Ba}=12.4 M_{\odot}$$ and $$M_{Bb}=9.8 M_{\odot}$$ together with the less massive wide component $$\alpha$$ Cru Ca+Cb+D yield a total mass $$M\simeq52 M_{\odot}$$. At larger distances, the seven-star nature of Alpha Crucis would be arguably very challenging to unveil, suggesting that the companion frequency in massive star surveys may be underestimated.
Alpha Crucis is the 13th brightest star in the night sky, but in reality it is seven stars shining together as one.