The main component of this system was once recognized as the best candidate for the largest known star when it was a red supergiant,[7] until it gradually became a yellow hypergiant with half of its original size and 34% of its luminosity.

[3] WOH G64 is surrounded by an optically thick dust envelope of roughly a light year in diameter, containing 3 to 9 times the Sun's mass of expelled material that was created by the strong stellar wind.

The star's evolved state means that it can no longer hold on to its atmosphere due to low density, high radiation pressure, and the relatively opaque products of thermonuclear fusion.

[16][17] Based on spectroscopic measurements assuming spherical shells, the star was originally calculated to have luminosity around between 490,000 and 600,000 L☉, suggesting initial masses at least 40 M☉ and consequently larger values for the radius between 2,575 and 3,000 R☉.

[12][14][18] One such of these measurements from 2018 gives a luminosity of 432,000 L☉ and a higher effective temperature of 3,500 K, based on optical and infrared photometry and assuming spherically-symmetric radiation from the surrounding dust.

[7] It shows an unusual spectrum of nebular emission; the hot gas is rich in nitrogen and has a radial velocity considerably more positive than that of the star.

[7] The stellar atmosphere is producing a strong silicate absorption band in mid-infrared wavelengths, accompanied a line emission due to highly excited carbon monoxide.

[3][13] The interacting binary system HR 5171 is considered an analog to WOH G64, as it also contains a yellow hypergiant with a B-type star companion.