[Placidus (Mike and Trish)]

Originally Posted by strongmanmike

Hey look!...I found an old photo of you two...

Auntie Marie and uncle Pierre?

Originally Posted by PKay

..... I was wondering about the missing 03 in the image above.
Blue has turned up in the filaments using wide band pass.

Could this mean blue shift?
Is Gum15 moving toward us?

It is of course possible that the blue is 500 nM OIII which has been doppler shifted out of the 3nM bandpass, but is still visible in a broadband filter. But the shift would have to be of the rough order of 3/500 = 0.6% of the speed of light. That's about 40 times higher than the rate Andromeda is moving towards us.'


The likely explanation is that the blue is broadband starlight rendered blue by being scattered off dust, much like our sky is blue.

Edit: Just realized that the proposed doppler shift would have to be hugely more than that. The [OIII] is about 500 nM, or blue-green, but the image shows it to be a pure blue with very little green, so at the very most 480 nM, a shift of over 4%. That would put the H-alpha way out of its bandpass, which we did not see. So already not doppler shift. Perhaps more like 400 nM. Taking that into account, it would give a speed toward us of 25% of the speed of light. At a distance of 1500 light years, it will be upon us in only 6000 years. We'd better duck.