[7] Like most irregular galaxies, NGC 5204 is relatively rich in gas and dust, although it lacks any prominent nebulas or broad areas of vigorous star formation.

[1][2] The galaxy also appears to have a larger than normal dark matter component as the estimated mass of its visible portion does not adequately explain the observed rotation curve of the individual stars, even very close to its center.

[5] The galaxy's most noticeable feature is a very powerful ultraluminous X-ray source (ULX) discovered in the early 1980s by the Einstein Observatory and designated NGC 5204 X-1.

[1][6][8] An optical counterpart to NGC 5204 X-1 was discovered in 2001 using the X-ray data from Chandra and a series of observations in the visible spectrum from the Hubble Space Telescope.

A 2003 study that performed a detailed spectral analysis of the optical source determined that its surface temperature is most likely less than 25,000 K. If accurate, this would suggest that the counterpart is a type B0 supergiant with a mass of about 25 M☉ and a radius of about 30 R☉, similar to Deneb.

[1][8] Both the X-ray source and the optical counterpart are located near the center of a massive void in the surrounding interstellar medium more than 150 parsecs (490 light-years) wide.

Supergiant stars are also known to experience large variations in luminosity more or less at random, which would heat the corona to varying degrees and explain the observed changes in X-ray emission strength.