Abell 2744 Parallel Deep Field from the Hubble Frontier Fields ProjectCredit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)
[Note: this blog post also appears on the Hubble’s Universe Unfiltered blog.]
One of the more philosophical concepts that astronomers have to deal with on an everyday basis is the commingling of space and time in astronomical images.
The underlying idea is straightforward. The speed of light is finite. Light from a star or nebula or galaxy takes a measurable amount of time to cross the space between it and us. Hence, the light we see now left that object at some previous time. We view astronomical objects as they were in the past. As I like to say, looking out in space is also looking back in time.
The implications of this maxim are considerable, especially in dealing with the deep field images from Hubble (see the accompanying image of the Abell 2744 Parallel deep field). Such images contain a wonderful assortment of galaxies, with a few stars here and there. Each object is at a different position in space, both in the two-dimensional sense of a different position within the image and in ...