APOD: Rainbow Aurora over Icelandic Waterfall (2023 Dec 27)

[APOD Robot]

Rainbow Aurora over Icelandic Waterfall

Explanation: Yes, but can your aurora do this? First, yes, auroras can look like rainbows even though they are completely different phenomena. Auroras are caused by Sun-created particles being channeled into Earth's atmosphere by Earth's magnetic field, and create colors by exciting atoms at different heights. Conversely, rainbows are created by sunlight backscattering off falling raindrops, and different colors are refracted by slightly different angles. Unfortunately, auroras can't create waterfalls, but if you plan well and are lucky enough, you can photograph them together. The featured picture is composed of several images taken on the same night last month near the Skógafoss waterfall in Iceland. The planning centered on capturing the central band of our Milky Way galaxy over the picturesque cascade. By luck, a spectacular aurora soon appeared just below the curving arch of the Milky Way. Far in the background, the Pleiades star cluster and the Andromeda galaxy can be found.

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[VictorBorun]

here are my 5 layman's constellations

[johnnydeep]

here are my 5 layman's constellations
ArchFalls_Pellegrini_2000+.jpg

Nice! Thanks for providing some land (sky) marks.

[Ann]

here are my 5 layman's constellations

Click to view full size image

Thanks a lot, Victor! Ursa Minor, the Little Dipper, looks particularly impressive in the APOD, sitting square in the middle of the broad green band just below the arc of the Milky Way.

I have a number of questions about the colors of the aurora and some deep-sky objects. The entire landscape seems to be bathed in green, or rather teal-green, light. The color matches the broad green band in which Ursa Minor appears to be embedded, and it is somewhat reminiscent of OIII, doubly ionized oxygen, which produces the green light that we find in planetary nebulas. Admittedly OIII appears to be just a little bluer than the broad green band below the Milky Way.

IC 1295, a planetary nebula completely dominated by green OIII emission. Image credit: Adam Block.

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Below the broad green band in the APOD is a reddish-purple band, and below that is a brilliantly bright yellow-green band. This yellow-green light does not appear to light up the landscape below, because if it did, the landscape, including the waterfall, would be yellow-green too. It clearly isn't.


Note that, while the aurora is non-blue, the Milky Way looks decidedly pearly-pale bluish. So how blue is the Milky Way really? Let's compare two images:

The northern (top) and southern Milky Way. Credit: Maroun Habib.
Note the relatively blue color of the Milky Way.

The Milky Way in Sagittarius. Note the bright yellow light from the bulge of the Milky Way. It seems highly likely that this bright yellow light dominates the overall color of the Milky Way, and that the Milky Way is a relatively yellow galaxy. Credit: Terrence Dickinson.

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Today's APOD shows us parts of the Milky Way that are bluish. The Pleiades stand out because of their blue color. But the Alpha Persei Moving Cluster is also quite bluish, as is the Double Cluster.

Compare the color of these young Milky Way clusters with the beige color of Andromeda. Andromeda is really a quite yellow galaxy, strongly dominated by the light from its large bright yellow bulge.

Andromeda is dominated by its yellow light. Credit: David Baumgartner

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The color contrast between Andromeda the young blue clusters of the Milky Way, and between Andromeda and the young stellar populations of Cygnus, Cassiopeia and Perseus, is striking. But from a distance, as seen from outside, we are probably just a bit less yellow than Andromeda, and not overwhelmingly so.

Ann

[VictorBorun]

The Milky Way in Sagittarius. Note the bright yellow light from the bulge of the Milky Way. It seems highly likely that this bright yellow light dominates the overall color of the Milky Way, and that the Milky Way is a relatively yellow galaxy. Credit: Terrence Dickinson.

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Ann

But what if the Milky Way in Sagittarius is in fact the core looming from both sides of the disk? It would not be fair then to attribute all that yellow to the whole galaxy

[MarkBour]

Thanks for the notes, Victor. I can't answer any the questions about the colors, but I appreciate the answers you supplied yourself, Ann.

The caption for today kind of begs the question of whether one could get a photo composition including both an auroral "pseudo-rainbow" and a regular rainbow together. Since I suppose auroras that happen to have the fortuitous shape are pretty rare, it would be quite hard to capture. You'd maybe have to get a shot right near sunset when it was dark enough to see the aurora, but still having enough sunlight to make a rainbow.

Actually, including a waterfall would make this a little easier. You could get in a position where you knew the falling water would make a nice rainbow, even in clear weather, and then wait for a very lucky aurora to show up. To make it easier, I suppose you could position a floodlight to make a rainbow instead of needing the sun; then you could catch it if ever you got lucky at any time of night. Since this is Iceland, after all, perhaps even throw in some molten lava. I don't ask for much!

[Ann]

The Milky Way in Sagittarius. Note the bright yellow light from the bulge of the Milky Way. It seems highly likely that this bright yellow light dominates the overall color of the Milky Way, and that the Milky Way is a relatively yellow galaxy. Credit: Terrence Dickinson.

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Ann

But what if the Milky Way in Sagittarius is in fact the core looming from both sides of the disk? It would not be fair then to attribute all that yellow to the whole galaxy

In large galaxies, such as the Milky Way, we always expect the central bulge to be bright and yellow, and to provide most of the light of the galaxy.

There are exceptions. M101, the Pinwheel Galaxy, has a bright but small yellow bulge and very large, sprawling arms richly populated by young blue star clusters and pink emission nebulas. M101 is actually exceptionally "blue" as large galaxies go.

Messier 101. Credit: Hubble Legacy Archive, ESA, NASA; Processing and additional imaging - Robert Gendler

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But by far most spiral galaxies have less impressive sets of arms than M101, and they have fewer young blue stars and less star formation. Instead, they have bright yellow bulges, which have been built up over billions of years as the galaxies have undergone burst after burst of star formation and one galactic merger after another.

All these stellar streams are likely remnants of dwarf galaxies merging with the Milky Way over billions of years.

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Galaxies are blue or bluish because of recent high-mass star formation. That is the only mechanism by which they can be blue. But the bright blue stars quickly die, and unless they are replaced by new young clusters, the blue light of the galaxy will start to fade away.

The Pleiades, at left, is a young cluster of some 100 million years. All the bright stars of the Pleiades are blue B-type stars of at least 12,000 K. By contrast, the Hyades, at right, is some 600 million years old, and all the B-type stars have died. A few A-type stars remain. The bluest star of the Hyades is delta 3 Tau, a blue straggler of spectral type A2 and a likely temperature of, at most, 9,000 K.

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But when the blue stars die, the yellow and orange stars remain:

Trumpler 5. This cluster is possibly some 5 billion years old.
Credit: zirl/Photocommunity Deutschland.

In order to maintain its blue light, galaxies have to continually form new young clusters of massive blue stars. But each new star cluster also contains many low-mass, non-blue, typically yellow stars. These will live on when the blue stars die, adding to the yellow light of the galaxy. Every burst of star formation will eventually give the galaxy more yellow light.

This is the portrait of the Milky Way by ESA's Gaia telescope. You can see that most of the light from our galaxy comes from the yellow bulge.

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One more thing. We are used to seeing pictures of spiral galaxies where the (typically blue) arms have been made to look brighter than they are, compared with the galactic bulge. The galaxies simply look better that way, but they don't show us the "true" appearance of the galaxies. A good example is M81:

M81. The arms look splendidly long, bright and blue. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

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M81. The arms look nowhere near as impressive as in Adam Block's image. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

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Adam Block's portrait of M81 is gorgeous, but still, the spiral arms have been made to look brighter in his image than they "really" are, compared with the bulge of the galaxy. Also the bright light of the bulge has been somewhat muted, as we can see when we compare his image with the Hubble image. I love Adam Block's images and would never criticize them, but still, we might bear this detail in mind.

Ann

[VictorBorun]

This is the portrait of the Milky Way by ESA's Gaia telescope. You can see that most of the light from our galaxy comes from the yellow bulge.

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One more thing. We are used to seeing pictures of spiral galaxies where the (typically blue) arms have been made to look brighter than they are, compared with the galactic bulge. The galaxies simply look better that way, but they don't show us the "true" appearance of the galaxies. A good example is M81:

M81. The arms look splendidly long, bright and blue. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

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M81. The arms look nowhere near as impressive as in Adam Block's image. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

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Adam Block's portrait of M81 is gorgeous, but still, the spiral arms have been made to look brighter in his image than they "really" are, compared with the bulge of the galaxy. Also the bright light of the bulge has been somewhat muted, as we can see when we compare his image with the Hubble image. I love Adam Block's images and would never criticize them, but still, we might bear this detail in mind.

Ann

point taken. Still— what if we (Gaia included) do not do justice to the blue power of the Milky Way's arms because they are thinner than the core and more obscured by the dust of the disk. Maybe an IR panorama from JWST and Euclid would make a base for calculating a side view of our galaxy…

[VictorBorun]

All these stellar streams are likely remnants of dwarf galaxies merging with the Milky Way over billions of years.

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Ann

I was intrigued with colour-coding here and googled.
it's heliocentric distance: blue=closer, red=distant

Orphan stream (colour coded red to orange) is 20 kly long so I guess it is outside the disk.
I wonder if any blue-coded streams are inside the disk

[Ann]

This is the portrait of the Milky Way by ESA's Gaia telescope. You can see that most of the light from our galaxy comes from the yellow bulge.

---

One more thing. We are used to seeing pictures of spiral galaxies where the (typically blue) arms have been made to look brighter than they are, compared with the galactic bulge. The galaxies simply look better that way, but they don't show us the "true" appearance of the galaxies. A good example is M81:

M81. The arms look splendidly long, bright and blue. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

---

M81. The arms look nowhere near as impressive as in Adam Block's image. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

---

Adam Block's portrait of M81 is gorgeous, but still, the spiral arms have been made to look brighter in his image than they "really" are, compared with the bulge of the galaxy. Also the bright light of the bulge has been somewhat muted, as we can see when we compare his image with the Hubble image. I love Adam Block's images and would never criticize them, but still, we might bear this detail in mind.

Ann

point taken. Still— what if we (Gaia included) do not do justice to the blue power of the Milky Way's arms because they are thinner than the core and more obscured by the dust of the disk. Maybe an IR panorama from JWST and Euclid would make a base for calculating a side view of our galaxy…

As for why the yellow central parts of large galaxies are brighter than the blue arms of these galaxies, the answer is simple. It's because there are such mindbogglingly impossibly huge numbers of mostly small yellow and orange stars in the center of large galaxies, mixed with some relatively bright red giants, that the combined light from these astoundingly large collections of stars is so bright (and yellow). As for the arms, they typically contain some really bright blue stars, but they are few in number. They simply can't outshine the myriad of yellow stars in the center.

Consider the Andromeda galaxy, as portrayed by Hubble:

Hubble view of Andromeda. Credit: NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler.

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Now let's look at two closeups, one of the center of Andromeda, one of a region further out:

What we are seeing in the center of Andromeda is mostly light from red giant stars. Compared with the billions and billions of small main sequence M-, K- and G-type stars in the center of Andromeda, the red giants are few in number. Even so, they are numerous enough that the light from them is almost "blinding" in a picture like this.

As for the outer part of Andromeda, do note how very, very much darker it is than the center. The overall color is also much bluer than the central part, and we can indeed see individual blue stars or star clusters. The outer part of Andromeda looks very "grainy", because the individual stars stand out so much better, since they are so much fewer in number.

We always, always expect large galaxies to have bright yellow centers. And we expect most of the light from a large galaxy to come from the central part of the galaxy, because that is where most of the stars are.

Ann

[johnnydeep]

Ann said:

We always, always expect large galaxies to have bright yellow centers. And we expect most of the light from a large galaxy to come from the central part of the galaxy, because that is where most of the stars are.

Except perhaps for very early galaxies that still have only large bright blue young stars? Or does a galaxy only form after a period of time longer than the life span of large bright blue stars?

[Ann]

Ann said:

We always, always expect large galaxies to have bright yellow centers. And we expect most of the light from a large galaxy to come from the central part of the galaxy, because that is where most of the stars are.

Except perhaps for very early galaxies that still have only large bright blue young stars? Or does a galaxy only form after a period of time longer than the life span of large bright blue stars?

Large galaxies have either undergone multiple bursts of star formation or multiple mergers. Each burst of star formation initially forms blue stars, but eventually leaves only yellow stars behind. Therefore, each new burst of star formation ends up adding to the yellow population of the galaxy.

But galaxies in the very early Universe may indeed have been very blue. They had not had time to build up a bright yellow population of stars, but they could well be undergoing massive starbursts that produced huge numbers of brilliant blue stars.

Normale News wrote:

Among the more sensational discoveries of the James Webb Space Telescope, the largest space telescope ever built, in orbit since Christmas Day 2021, is undoubtedly the surprising abundance of giant galaxies, which already contain up to a billion stars like the Sun in the early Universe: a period that astrophysicists call the cosmic dawn, more than 13 billion light years away from us. Not only are these galaxies huge, but their light is very blue, which has earned them the nickname 'blue monsters'.

Here are some examples of at least reasonably large and very blue galaxies in the nearby universe:

NGC 4731. Credit: Warren A. Keller.

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NGC 4656. Credit: Mark Hanson.

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NGC 3310. Credit: Adam Block.

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M61. Credit: Robert Gendler, Roberto Colombari, Hubble Legacy Archive, European Southern Observatories

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Of these four galaxies, only M61 is likely to be bigger than the Milky Way. The other three galaxies are smaller. Large galaxies with predominantly blue colors are rare.

Ann

[johnnydeep]

Ann said:

We always, always expect large galaxies to have bright yellow centers. And we expect most of the light from a large galaxy to come from the central part of the galaxy, because that is where most of the stars are.

Except perhaps for very early galaxies that still have only large bright blue young stars? Or does a galaxy only form after a period of time longer than the life span of large bright blue stars?

Large galaxies have either undergone multiple bursts of star formation or multiple mergers. Each burst of star formation initially forms blue stars, but eventually leave only yellow stars behind. Therefore, each new burst of star formation ends up adding to the yellow population of the galaxy.

But galaxies in the very early Universe may indeed have been very blue. They had not had time to build up a bright yellow population of stars, but they could well be undergoing massive starbursts that produced huge numbers of brilliant blue stars.

Normale News wrote:

Among the more sensational discoveries of the James Webb Space Telescope, the largest space telescope ever built, in orbit since Christmas Day 2021, is undoubtedly the surprising abundance of giant galaxies, which already contain up to a billion stars like the Sun in the early Universe: a period that astrophysicists call the cosmic dawn, more than 13 billion light years away from us. Not only are these galaxies huge, but their light is very blue, which has earned them the nickname 'blue monsters'.

Here are some examples of at least reasonably large and very blue galaxies in the nearby universe:

NGC 4731. Credit: Warren A. Keller.

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NGC 4656. Credit: Mark Hanson.

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NGC 3310. Credit: Adam Block.

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M61. Credit: Robert Gendler, Roberto Colombari, Hubble Legacy Archive, European Southern Observatories

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Of these four galaxies, only M61 is likely to be bigger than the Milky Way. The other three galaxies are smaller. Large galaxies with predominantly blue colors are rare.

Ann

Ok, thanks.

[RJN]

The photographer has now said that this image was taken last November, in 2022. The text of the main NASA APOD has been corrected.
- RJN