Indigo_Sunrise wrote:This is an amazing image - or should I say:
these are amazing images!
I especially like the fact
that as the image changes, from the infrared to the optical, some of the stars that are noticeably brighter in one image, are less so in the other image.
Excellent APOD!
This infrared image, while remarkable and hugely interesting in many ways, doesn't do a good job of showing the bolometric (total) energy output of the stars. Of course, that isn't what it is meant to do, either. For all of that, it might be interesting to take a closer look at how the image represents the brightness of different types of stars.
Look at the visible-light image of the North America Nebula and note a reasonably bright blue star at top center or center-left. This is HD 199579, an intrinsically very, very bright and massive O-type star. In fact, its spectral type is the same as Theta-1 C Orionis, the star that is ionizing the Orion Nebula. We can be sure that the bolometric energy output of HD 199579 is at least 100,000 times that of the Sun. But because the star produces most of its energy in the ultraviolet, and because most of its visual light is blue and purple, the star is barely visible at all in infrared light. If you want to detect it at all in the infrared image, you'd better look at the visual-light image and put your finger on the screen where the star is to "mark the spot", as it were. In the infrared image this scorchingly hot mighty star all but disappears.
But take a look at the infrared image and note the apparently brightest of all blue-looking stars - that is, look at the star that is the apparently brightest of all stars in the near infrared, which is mapped as blue in this image. What star is this? Why, it's HD 199799, an M-type star, whose visible-light color is similar to that of reddish Betelgeuse. But HD 199799 is no M-type supergiant: its (uncertain) luminosity in visual light is about 200 times that of the Sun. However, because it is an M-type star, it emits most of its energy in the (near) infrared, which is why it looks so bright in the infrared image of the North America Nebula. There is no doubt that HD 199799 emits a lot more than 200 times the energy of the Sun if we take the infrared light into account. It may very well emit several thousand times the energy of our own G-type star! But even so, the bolometric, total, energy output of this M-type star is probably less than a tenth of the energy output of the hot blue O-type star. So you shouldn't use this infrared image to judge the bolometric energy output of the stars here!
Ann