It is a variable star, and when it is near its maximum brightness, magnitude 6.1, it is faintly visible to the naked eye under excellent observing conditions.

A recent publication (2019) rules out this hypothesis, and the distance of HR 5171 has been revised to 5,200 ± 1,600 light years and its radius to 3-5 AU (i.e. 650 to 1080 times that of the Sun).

[11] The G8 spectral type was later adjusted to K0 0-Ia in the revised MK system, meeting the criteria of highly luminous supergiants.

[21] At the time it was considered a "cool S Doradus variable", a class including stars such as Rho Cassiopeiae that are now known as the yellow hypergiants.

These variables are usually classified as semi-regular (SRd) due to variations which are sometimes well-defined, at other times nearly constant, and may show unpredictable fading.

[22] In a 2014 paper, VLTI observations directly determined an unexpectedly large size for HR 5171 and revealed that it is a contact binary.

The stars and the nebulosity all show similar space motions that would place them in the Centaurus spiral arm about 4,000 parsecs (4 kpc) away from Earth.

[10] Early calculations based on the assumed luminosity of HR 5171B gave a distance of 3.2 kpc and 3.2 magnitudes of interstellar extinction.

[7] The open cluster NGC 5281 lies 19' from HR 5171, projected against the supernova remnant but only about 1,200 parsecs from Earth.

[23] The spectrum of HR 5171 is easily separated into a luminous yellow star and a hot blue supergiant.

It also shows a large infrared excess and exceptionally strong silicate absorption, both caused by a dust shell condensed out of material ejected from the star.

[12] An unusual blue excess near 383.8 nm may be due to polycyclic aromatic hydrocarbon (PAH) luminescence.

The shape of the eclipse light curve suggests that the orbit is almost edge on to Earth, and that the secondary is slightly hotter than the primary.

Colour changes at these minima suggest a transfer of luminosity from the visual to the infrared, either as a result of cooling or recycling by the surrounding envelope.

The best-fit model was a sharply-defined uniform disk with a small bright spot towards its edge, all surrounded by a fainter extended envelope.

The uniform disk, taken to be the photosphere of the larger star, was 3.39 mas across, corresponding to a radius of 1,315 ± 260 solar radii (915,000,000 ± 181,000,000 km; 6.12 ± 1.21 au).

[2] The luminosity has been calculated from spectral energy distribution (SED) fitting to be 630,000 L☉, assuming a distance of 3.7 kpc and 3.2 magnitudes of interstellar extinction.

Its exact properties can only be predicted from models since it is barely resolved from its larger companion and its spectrum cannot be distinguished.

[9] The evolutionary history of HR 5171A is complicated by its uncertain and unusual physical properties and binary companion.

The primary star is probably undergoing wind roche lobe overflow (WRLOF) with a portion of the material being transferred to the secondary.

The interaction between the pair should spin up the primary to synchronous rotation, which is a possible path to fast-spinning luminous blue variables or B[e] stars.