Starship Asterisk*

Naked Eye Nova Sagittarii 2015 No. 2

Explanation: It quickly went from obscurity to one of the brighter stars in Sagittarius -- but it's fading. Named Nova Sagittarii 2015 No. 2, the stellar explosion is the brightest nova visible from Earth in over a year. The featured image was captured four days ago from Ranikhet in the Indian Himalayas. Several stars in western Sagittarius make an asterism known as the Teapot, and the nova, indicated by the arrow, now appears like a new emblem on the side of the pot. As of last night, Nova Sag has faded from brighter than visual magnitude 5 to the edge of unaided visibility. Even so, the nova should still be easily findable with binoculars in dark skies before sunrise over the next week.

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Nicely framed

I think you missed an obvious link to "Nova (Sag) has faded".
Edit – Though Jimmy " Never Failin' " saved the day.
As this is an "astronomy site" my comments should stick to business. but in the first sentence it is, sort of, true. "..went from obscurity to one of the brighter stars.." temporarily.

The way I understand it, the nova was yellow when it was discovered. It seems to me that the only possible reasons for it being yellow are either that Nova Sagittarii has already cooled, or that there is a lot of dust between us and the nova.

Bob King wrote:

As novae evolve, they’ll often turn from white or yellow to red. Emission of deep red light from hydrogen atoms – called hydrogen alpha – gives them their warm, red color.

It is easy to photograph Ha emission so that it looks red, but to the best of my knowledge it isn't possible for humans to actually see the red color of Ha emission with their own eyes. The human eye is quite insensitive to the wavelength of Ha, about 656 nm. When we see nebulas in the sky, such as the Orion Nebula, they never look red to us. That is because we detect other, shorter wavelengths - primarily OIII and Hβ - when we look at nebulas, and therefore they may look slightly greenish to our eyes. But never red. I can't believe that Nova Sagittarii will ever look yellowish to human eyes because of the contribution of red Ha light to the color of the nova.

Ann

Ann wrote:It is easy to photograph Ha emission so that it looks red, but to the best of my knowledge it isn't possible for humans to actually see the red color of Ha emission with their own eyes. The human eye is quite insensitive to the wavelength of Ha, about 656 nm.

Humans can readily see 656 nm light. I have a reference tube that produces this wavelength, and I can look through narrowband filters at the Sun and easily see it. Indeed, everybody who uses an Ha telescope to look at prominences is seeing this color directly. There are even credible reports of a few observers directly seeing red in bright Ha emission nebulas viewed telescopically.

Chris Peterson wrote:

Ann wrote:It is easy to photograph Ha emission so that it looks red, but to the best of my knowledge it isn't possible for humans to actually see the red color of Ha emission with their own eyes. The human eye is quite insensitive to the wavelength of Ha, about 656 nm.

Humans can readily see 656 nm light. I have a reference tube that produces this wavelength, and I can look through narrowband filters at the Sun and easily see it. Indeed, everybody who uses an Ha telescope to look at prominences is seeing this color directly. There are even credible reports of a few observers directly seeing red in bright Ha emission nebulas viewed telescopically.

I'm not saying we can't see 656 nm light. I'm saying our eyes aren't very sensitive to it. Certainly, when you look at something that contains Ha light through an Ha narrowband filter you will see this wavelength as long as you see anything at all, because other wavelengths that you would have seen so much more easily are blocked by the filter.

What I'm wondering about is the fact that the nova looks yellow so soon after its discovery. Surely the yellowness of the nova itself can have nothing to do with the Ha light of the nebula around it?

Are there any known examples of yellow-looking but intrinsically blue light sources that look yellow not because of dust, and not because their blue light has been scattered in a reflection nebula around them, but because of the bright red Ha light surrounding them?

Ann

Ann wrote:What I'm wondering about is the fact that the nova looks yellow so soon after its discovery. Surely the yellowness of the nova itself can have nothing to do with the Ha light of the nebula around it?

A nova initially shows a thermal emission spectrum, generally appearing white. This is quickly replaced by a collection of highly broadened emission lines, dominated by hydrogen and iron (both in the red). The hydrogen is typically an order of magnitude or more brighter than any other bands. That's why novas progress from yellow to redder colors as the continuum contribution declines and the Ha dominates.

The "brighter" in the explanation is a link to a report in the Sky & Telescope website that has a blinked image showing before and after the nova appeared. In the blinked image there is however another very obvious object that is not in the before but is in the after image. It is a little above and right of the centre bottom where in the before image there is what looks like a very thin arc feature (? image artefact) that is not there in the after image but there is now a new object. I wonder what it was/is

DavidLeodis wrote:The "brighter" in the explanation is a link to a report in the Sky & Telescope website that has a blinked image showing before and after the nova appeared. In the blinked image there is however another very obvious object that is not in the before but is in the after image. It is a little above and right of the centre bottom where in the before image there is what looks like a very thin arc feature (? image artefact) that is not there in the after image but there is now a new object. I wonder what it was/is :?:

Its PSF is quite different from that of the stars. I'd guess it's some sort of internal reflection of the nova.

The arc in the before frame looks like a calibration error (problem with a flat or dark frame). There are also some cosmic ray tracks in the after frame, suggesting it's been minimally processed.