The changing brightness and colour of the kilonova seen in NGC 4993

This plot shows how the spectrum and brightness of the kilonova seen in the galaxy NGC 4993 changed over 12 days following the detection of gravitational waves on 17 August 2017. In blue light the object faded rapidly, but at longer wavelengths, in the near infrared part of the spectrum, it brightened a little and then faded much more slowly. As a result this object changed colour from very blue to very red during this period. Each line is labeled with the number of days since the explosion and the horizontal axis is the colour of the light, from ultraviolet to near-infrared. The colours of the lines indicate the overall colour of the object at the different times.