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Cygnus X-1 · equivalent horizon (2021): size, dimension and comparison

The adopted reference is about 125.22 km.

Equivalent Schwarzschild diameter, non-rotating model

125.22 km ≈ 125,220 m. A kilometer contains 1,000 meters.

A dimension calculated from a measured stellar mass.

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Cygnus X-1 · equivalent horizon (2021), interpreted illustration
Interpreted illustration · dimension stated in the fact sheet.

What does this number measure?

Reference mass published in 2021: 21.2 ± 2.2 solar masses. We calculate D = 4 GM☉ × (M/M☉) / c², with nominal GM☉ = 1.3271244 × 10²⁰ m³/s² and c = 299 792 458 m/s. The resulting diameter belongs to a spherical, non-rotating model of the same mass. It is not a direct measurement of the actual horizon or the size of a photographed shadow. The artistic luminous ring is not included in the dimension.

Compare the stated dimensions

The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.

Compare with Gaia BH3 · equivalent horizon (2024) in the explorer →

Source and limitations

Reference mass dated 2021; its published spread is ±2.2 solar masses. The model's diameter is proportional to it. Rotation and optical appearance are excluded from this model, without certifying the real horizon.

A shared conceptual cross-section of the reference horizons, not a photograph. Only the diameter of the central dark disc follows the non-rotating model's dimension; the outer emitting ring is artistic and excluded from the measurement. This disc is not the shadow observed by the EHT.