According to Lehnert (of the Observatoire de Paris), it was forming the same number of stars per year as our galaxy, but they were much smaller and less massive, making it "intensely star forming". The light travel distance of the light that we observe from UDFy-38135539 (HUF.YD3) is more than 4 billion parsecs (13.1 billion light years), and it has a luminosity distance of 86.9 billion parsecs (about 283 billion light years). There are a number of different distance measures in cosmology, and both "light travel distance" and "luminosity distance" are different from the comoving distance or "proper distance" generally used in defining the size of the observable universe (comoving distance and proper distance are defined to be equal at the present cosmological time, so they can be used interchangeably when talking about the distance to an object at present, but proper distance increases with time due to the expansion of the universe, and is the distance used in Hubble's law; "see" Uses of the proper distance for more on the physical meaning of this notion of 'distance'). The luminosity distance "DL" is related to a factor called the "comoving transverse distance" "DM" by the equation , where "z" is the redshift, and the comoving transverse distance is itself equal to the radial comoving distance (i.e., comoving distance between an object and ourselves) in a spatially flat universe. So with and , the comoving distance would be about 9.1 billion parsecs (about 30 billion light years). The infrared light that we now observe from the galaxy was emitted as ultraviolet radiation toward the end of an era when the universe was filled with atomic hydrogen, which absorbed at ultraviolet wavelengths. Because the galaxy's own light alone would not have been intense enough to ionize a large region and render it transparent, scientists suspect that a population of smaller, undetected galaxies, contributed to the reionization making UDFy-38135539 visible. The period of universal star birth was the reionization epoch. The universe's first stars were massive, ionizing hydrogen in the surrounding environment ("Trenti"). UDFy-38135539 (HUDF.YD3) is thought to be one of the first galaxies observed in the reionization epoch. Caltech astronomer Brant Robertson, commenting on the study, stated that the "galaxy happens to reside at a very special time in cosmic history when the properties of gas in the universe were changing rapidly, and therefore this galaxy and others like it may teach us a lot about the early history of the universe".