APOD: Red Aurora over Italy (2023 Nov 06)

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APOD: Red Aurora over Italy (2023 Nov 06)

Post by APOD Robot » Mon Nov 06, 2023 5:06 am

Image Red Aurora over Italy

Explanation: What was that red glow on the horizon last night? Aurora. Our unusually active Sun produced a surface explosion a few days ago that sent out a burst of electrons, protons, and more massive charged nuclei. This coronal mass ejection (CME) triggered auroras here on Earth that are being reported unusually far south in Earth's northern hemisphere. For example, this was the first time that the astrophotographer captured aurora from her home country of Italy. Additionally, many images from these auroras appear quite red in color. In the featured image, the town of Comelico Superiore in the Italian Alps is visible in the foreground, with the central band of our Milky Way galaxy seen rising from the lower left. What draws the eye the most, though, is the bright red aurora on the far right. The featured image is a composite with the foreground and background images taken consecutively with the same camera and from the same location.

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Ann
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Re: APOD: Red Aurora over Italy (2023 Nov 06)

Post by Ann » Mon Nov 06, 2023 7:20 am


The red stuff in today's APOD is easy to see, but can you find the one blue spot in this picture (apart from the lettering at lower right)?

Here it is:

APOD 6 November 2023 annotated.png

Blue Vega. Lovely Vega. If you see a picture of the Milky Way, and there is one blue star, bluer than anything else in the picture, close to but detached from the Milky Way, you can bet your boots it is Vega.

If you don't remember seeing a blue star in the sky, it's because Vega doesn't look blue to the naked eye. But look at it through a telescope, and its blue-white color is just jumping out at you. And in many RGB photographs of the northern Milky Way, the singularly blue hue of this beacon of the north is just ridiculously obvious.

And yet this blue star has been used for some 70 years as an astronomical standard of stellar whiteness! That's so horribly wrong!
Wikipedia wrote:

The UBV photometric system measures the magnitude of stars through ultraviolet, blue and yellow filters, producing U, B and V values, respectively. Vega is one of six A0V stars that were used to set the initial mean values for this photometric system when it was introduced in the 1950s. The mean magnitudes for these six stars were defined as: U − B = B − V = 0. In effect, the magnitude scale has been calibrated so that the magnitude of these stars is the same in the yellow, blue and ultraviolet parts of the electromagnetic spectrum. Thus, Vega has a relatively flat electromagnetic spectrum in the visual region—wavelength range 350–850 nanometers, most of which can be seen with the human eye—so the flux densities are roughly equal; 2,000–4,000 Jy. However, the flux density of Vega drops rapidly in the infrared, and is near 100 Jy at 5 micrometers.

So the magnitude scale has been calibrated so that the U-B and B-V indices of Vega are defined as zero. And Vega has a relatively flat spectrum in the visual region??? It emits the same amount of ultraviolet, blue and yellow-green light? Its light curve just suddenly drops off when we enter the infrared region?

NO!!!! It does not!

:facepalm:

And Vega's scientifically accredited (but totally inaccurate) stellar whiteness has led renowned astronomers to describe it as white even in captions of pictures plainly displaying its azure hue:


Okay! Vega does look white to the naked eye, I'll grant you that. But it sure looks blue in the picture, so why not mention that it looks blue through a telescope, and that a good camera will easily pick up its blue hue? It's like these people are denying what they are seeing.


Okay. Now that I've ranted :bang:, I'll leave it to the rest of you to discuss the red aurora! :D

Ann
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Ann
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Re: APOD: Red Aurora over Italy (2023 Nov 06)

Post by Ann » Mon Nov 06, 2023 7:17 pm

Okay, I can't help myself. How can we tell at a glance that the star pointed at by the arrow in the picture below isn't Vega?

Milky Way over a lake in southern Chile by Marcelo Maturana.png
The Milky Way of a lake in southern Chile.
Credit: Marcelo Maturana

Well, there are so many ways in which we can tell that the star by the arrow isn't Vega. First of all, the picture obviously isn't showing us the northern but the southern Milky Way. And second, there are just too many relatively bright stars near the arrowed one here for this particular bright star to be Vega.

But really, you can tell at a glance that this star isn't Vega because of its color! It is so very non-blue. Vega can never look yellow in a good RGB color picture, unless Vega is being temporarily just extremely reddened for some reason.

The non-Vega-colored yellow star in the apparent Vega position is Antares, the red supergiant! Let's use an angry-looking red emoji to represent Antares! 😡

Ann

P.S. And hey, there appears to be a red aurora in the picture by Marcelo Maturana, too. Anyone want to talk about that one?
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