Dark galaxy CDG-2 near Perseus Cluster (annotated compass image)

This image of dark galaxy CDG-2 was captured by the Hubble Space Telescope’s ACS (Advanced Camera for Surveys) with additional data from the European Space Agency’s Euclid space mission.

The image shows a scale bar, compass arrows, and colour key for reference.

The scale bar is labeled in light-years along the top, which is the distance that light travels in one Earth-year. (It takes about 36,000 years for light to travel a distance equal to the length of the scale bar.) One light-year is equal to about 9.46 trillion kilometers.

The scale bar is also labeled in arcseconds, which is a measure of angular distance on the sky. One arcsecond is equal to an angular measurement of 1/3600 of one degree. There are 60 arcminutes in a degree and 60 arcseconds in an arcminute. (The full Moon has an angular diameter of about 30 arcminutes.) The actual size of an object that covers one arcsecond on the sky depends on its distance from the telescope.

The north and east compass arrows show the orientation of the image on the sky. Note that the relationship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above).

The colour key shows which ACS filters were used when collecting the light. The colour of each filter name is the visible-light colour used to represent the light that passes through that filter.

[Image description: Image labeled “CDG 2 near Perseus Cluster, HST ACS WFC”. At left, a field of space with a dozen white foreground stars and a number of small, yellow background galaxies. An unremarkable area at center is outlined with a dashed red circle surrounded by a white box. Lines extend from the box to a pullout at right containing faint, grainy white light surrounded by a red circle labeled “Candidate dark galaxy – diffuse emission.” Four white dots are circled in blue and labeled globular clusters. Compass arrows at lower left show east pointing to 1 o’clock and north pointing to 4 o’clock. A scale bar is labeled 36,000 light-years and 30 arcseconds. It extends about one-eighth of the way across the image. A colour key shows F475W in blue, F814W in orange, and Euclid in gray.]