APOD: Gems of a Maldivean Night (2022 Mar 28)

[APOD Robot]

Gems of a Maldivean Night

Explanation: The southernmost part of the Milky Way contains not only the stars of the Southern Cross, but the closest star system to our Sun -- Alpha Centauri. The Southern Cross itself is topped by the bright, yellowish star Gamma Crucis. A line from Gamma Crucis through the blue star at the bottom of the cross, Acrux, points toward the south celestial pole, located just above the small island in the featured picture -- taken in early March. That island is Madivaru of the Maldives in the Indian Ocean. Against faint Milky Way starlight, the dark Coal Sack Nebula lies just left of the cross, while farther left along the Milky Way are the bright stars Alpha Centauri (left) and Beta Centauri (Hadar). Alpha Centauri A, a Sun-like star anchoring a three-star system with exoplanets, is a mere 4.3 light-years distant. Seen from Alpha Centauri, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia.

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[Cousin Ricky]

I can barely make out the stars of Chamaeleon, and they looked too close to the horizon for the south celestial pole to be visible. To rule out camera lens distortion, I looked up Madivaru’s latitude. It is 4.5° north. The south celestial pole cannot be above the island in this picture.

[Cousin Ricky]

Fun fact: Alpha Centauri A is the same spectral class as the Sun, and Alpha Centauri B is cooler. Therefore, if we could see it at a distance, the Sun would appear no more yellow than Alpha Centauri.

[Chris Peterson]

Fun fact: Alpha Centauri A is the same spectral class as the Sun, and Alpha Centauri B is cooler. Therefore, if we could see it at a distance, the Sun would appear no more yellow than Alpha Centauri.

Which is to be expected, as the Sun pretty much defines "white".

[Chris Peterson]

I can barely make out the stars of Chamaeleon, and they looked too close to the horizon for the south celestial pole to be visible. To rule out camera lens distortion, I looked up Madivaru’s latitude. It is 4.5° north. The south celestial pole cannot be above the island in this picture.

Another fun fact: the maximum atmospheric refraction value observed appears to have been about 4°. So under extraordinary conditions, it's remotely possible for the SCP to lie very close to the horizon from this location. (Of course, I agree with you that it's not present in this image.)

[Ann]

Fun fact: Alpha Centauri A is the same spectral class as the Sun, and Alpha Centauri B is cooler. Therefore, if we could see it at a distance, the Sun would appear no more yellow than Alpha Centauri.

Which is to be expected, as the Sun pretty much defines "white".

Exactly, Chris. Which is why I insist that the Sun is a white star.

The reason why the Sun is white - to us!! - is that human color vision has evolved so that it sees sunlight (make that daylight, particularly on an overcast day) as white, and then the other colors are either redder or bluer than the color of daylight.

Ann

[Chris Peterson]

Fun fact: Alpha Centauri A is the same spectral class as the Sun, and Alpha Centauri B is cooler. Therefore, if we could see it at a distance, the Sun would appear no more yellow than Alpha Centauri.

Which is to be expected, as the Sun pretty much defines "white".

Exactly, Chris. Which is why I insist that the Sun is a white star.

The reason why the Sun is white - to us!! - is that human color vision has evolved so that it sees sunlight (make that daylight, particularly on an overcast day) as white, and then the other colors are either redder or bluer than the color of daylight.

Ann

Of course, scientifically, a star's color is largely unrelated to its appearance. It's a classification based on the shape of its blackbody spectrum. It's temperature. So the Sun is, quite correctly, classified as yellow.

[Ann]

Which is to be expected, as the Sun pretty much defines "white".

Exactly, Chris. Which is why I insist that the Sun is a white star.

The reason why the Sun is white - to us!! - is that human color vision has evolved so that it sees sunlight (make that daylight, particularly on an overcast day) as white, and then the other colors are either redder or bluer than the color of daylight.

Ann

Of course, scientifically, a star's color is largely unrelated to its appearance. It's a classification based on the shape of its blackbody spectrum. It's temperature. So the Sun is, quite correctly, classified as yellow.

The scientific classification of the Sun as "yellow" is about as abstract as the color force in quantum mechanics.

(Tiny nitpick, Chris. I myself make more and more spelling and grammar mistakes nowadays, sadly enough. But now you made one, which is extremely rare. The corrects spelling is "its classification and temperature", not "it's classification and temperature".)

Ann

[Chris Peterson]

Exactly, Chris. Which is why I insist that the Sun is a white star.

The reason why the Sun is white - to us!! - is that human color vision has evolved so that it sees sunlight (make that daylight, particularly on an overcast day) as white, and then the other colors are either redder or bluer than the color of daylight.

Ann

Of course, scientifically, a star's color is largely unrelated to its appearance. It's a classification based on the shape of its blackbody spectrum. It's temperature. So the Sun is, quite correctly, classified as yellow.

The scientific classification of the Sun as "yellow" is about as abstract as the color force in quantum mechanics.

(Tiny nitpick, Chris. I myself make more and more spelling and grammar mistakes nowadays, sadly enough. But now you made one, which is extremely rare. The corrects spelling is "its classification and temperature", not "it's classification and temperature".)

Ann

Indeed. Those slip through once in a while.

But I do not think the Sun's classification as "yellow" is in the least abstract.

[MarkBour]

Of course, scientifically, a star's color is largely unrelated to its appearance. It's a classification based on the shape of its blackbody spectrum. It's temperature. So the Sun is, quite correctly, classified as yellow.

The scientific classification of the Sun as "yellow" is about as abstract as the color force in quantum mechanics.

(Tiny nitpick, Chris. I myself make more and more spelling and grammar mistakes nowadays, sadly enough. But now you made one, which is extremely rare. The corrects spelling is "its classification and temperature", not "it's classification and temperature".)

Ann

Well, 3 out of 4 correct on the punctuation of "its" is better than most articles these days.

I'm a bit concerned that I haven't seen any posts from neufer in a while.

[orin stepanek]

I love the photo1 To me though, the Southern Cross
looks more like a kite!

Kitty is amazed at what she is watching!

[Cousin Ricky]

I'm a bit concerned that I haven't seen any posts from neufer in a while.

Neufer shows up in the “yellowish star” link (one of the solar spectrum illustrations). But yeah, he hasn’t posted here since March 1.

[Ann]

I have lost my software, so I don't know if my annotations are correct, but here goes anyway...

Here are a few closeups of some of the stars, clusters and nebulas in Petr Horalek's image:

Alpha Centauri (left), Beta Centauri (right) and Proxima Centauri (inside the red circle). Image: Skatebiker at English Wikipedia.

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The Southern Cross, the Coalsack Nebula (left) and
the Running Chicken Nebula (lower right). Image: Naskies at English Wikipedia.

Beautiful Carina Nebula. Eta Carina is the bright white star near center. The Gabriela Mistral Nebula is at lower left. Image: Fedaro (Fernando da Rosa) y Santiago Roland.

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Stunningly beautiful picture of the Gabriela Mistral Nebula (left)
and star cluster NGC 3293 (right). Picture: Gábor Tóth.

Omega Centauri, the brightest globular cluster of the Milky Way. Photo: Jose Mtanous.

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NGC 3532, beautiful and rich open cluster in Carina. Photo: ESO/G. Beccari.

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IC 2602, the Southern Pleiades. Photo: Roberto Mura.

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The Dark Doodad and globular cluster NGC 4372. The blue star is Gamma Musca. Photo: Naskies at English Wikipedia.

---

Ann

[johnnydeep]

"Seen from Alpha Centauri, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia."

Is this really saying the Cassiopeia would look about the same to us if we were standing on a planet around Alpha Centauri? At first think, that seems like it would be quite a coincidence. But on second thought, all it really means is that all the stars in Cassiopeia are much farther away from both the Sun and Alpha Centauri than they are from each other. And it also means that ALL familiar constellations would look pretty much the same from either star!

[Cousin Ricky]

"Seen from Alpha Centauri, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia."

Is this really saying the Cassiopeia would look about the same to us if we were standing on a planet around Alpha Centauri? At first think, that seems like it would be quite a coincidence. But on second thought, all it really means is that all the stars in Cassiopeia are much farther away from both the Sun and Alpha Centauri than they are from each other. And it also means that ALL familiar constellations would look pretty much the same from either star!

Except Cassiopeia and Centaurus, of course. And Canis Major, too; Sirius is close enough to both stars to show in greatly different directions.

[Ann]

APOD Robot wrote:

Seen from Alpha Centauri, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia.

Our Sun would be a bright YELLOWISH star in constellation Cassiopeia?

Okay. So please spot the yellowish star in constellation Auriga below:

Constellation Auriga. Photographer: I don't know. I found the picture here.

Right. So can you spot the yellowish star, the star whose color and temperature is relatively similar to the color and temperature of the Sun, in constellation Auriga?

No? The answer is of course Capella, seen at far left in the picture. Mind you, Capella is both spectroscopically and photometrically cooler and yellower than the Sun. The spectral class of the brightest component of Capella (Capella Aa) is K0III, according to Wikipedia, the fainter Capella component (Capella Ab) is spectral class G1III, and the combined spectral class of Capella Aa and Ab is usually given as G3III. The spectral class of the Sun is G2V. The B-V index of Capella is +0.80, while the B-V index of the Sun is +0.656. So the Sun is indeed whiter than Capella.

Click to play embedded YouTube video.

The Sun is, frankly, a whiter shade of pale!

Ann

[johnnydeep]

"Seen from Alpha Centauri, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia."

Is this really saying the Cassiopeia would look about the same to us if we were standing on a planet around Alpha Centauri? At first think, that seems like it would be quite a coincidence. But on second thought, all it really means is that all the stars in Cassiopeia are much farther away from both the Sun and Alpha Centauri than they are from each other. And it also means that ALL familiar constellations would look pretty much the same from either star!

Except Cassiopeia and Centaurus, of course. And Canis Major, too; Sirius is close enough to both stars to show in greatly different directions.

What do you mean? Here's a list of stars in the constellation Cassiopeia - https://en.wikipedia.org/wiki/List_of_s ... Cassiopeia. And these are the 5 main stars that make up the "W":

Code: Select all

Name	B	F	Var	HD	HIP	RA		Dec		v.mag.	a.mag.	Dist.ly Sp.class	Notes
----------------------------------------------------------------------------------------------------------------------------------------------------------
α Cas	α	18		3712	3179	00h 40m 30.39s	+56° 32′ 14.7″	2.24	−1.99	228	K0II-IIIvar	Schedar
β Cas	β	11		432	746	00h 09m 10.09s	+59° 09′ 00.8″	2.28	1.17	54	F2III-IV	Caph; δ Sct variable, Vmax = 2.25m, Vmin = 2.29m, P = 0.10 d
γ Cas	γ	27		5394	4427	00h 56m 42.50s	+60° 43′ 00.3″	2.47	−4.22	613	B0IV:evar	Tsih, Navi; prototype γ Cas variable, Vmax = 1.6m, Vmin = 3.0m
δ Cas	δ	37		8538	6686	01h 25m 48.60s	+60° 14′ 07.5″	2.68	0.26	99	A5Vv SB		Ruchbah; Algol variable, Vmax = 2.68m, Vmin = 2.76m, P = 759 d
ε Cas	ε	45		11415	8886	01h 54m 23.68s	+63° 40′ 12.5″	3.35	−2.31	442	B2pvar		Segin; variable star, ΔV = 0.002m, P = 0.09 d

Is β Cas, at 54 ly, close enough to show a marked difference in position when viewed from Sol versus Alpha Centauri?

And all but one (Sirius) of the 10 brightest stars in Canis Major are closer than 60 ly to us. See https://en.wikipedia.org/wiki/List_of_s ... anis_Major. Would any of them except Sirius show a marked difference in position when viewd from Sol versus Alpha Centauri?

[Ann]

[float=]

"Seen from Alpha Centauri, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia."

Is this really saying the Cassiopeia would look about the same to us if we were standing on a planet around Alpha Centauri? At first think, that seems like it would be quite a coincidence. But on second thought, all it really means is that all the stars in Cassiopeia are much farther away from both the Sun and Alpha Centauri than they are from each other. And it also means that ALL familiar constellations would look pretty much the same from either star!

Except Cassiopeia and Centaurus, of course. And Canis Major, too; Sirius is close enough to both stars to show in greatly different directions.

What do you mean? Here's a list of stars in the constellation Cassiopeia - https://en.wikipedia.org/wiki/List_of_s ... Cassiopeia. And these are the 5 main stars that make up the "W":

Code: Select all

Name	B	F	Var	HD	HIP	RA		Dec		v.mag.	a.mag.	Dist.ly Sp.class	Notes
----------------------------------------------------------------------------------------------------------------------------------------------------------
α Cas	α	18		3712	3179	00h 40m 30.39s	+56° 32′ 14.7″	2.24	−1.99	228	K0II-IIIvar	Schedar
β Cas	β	11		432	746	00h 09m 10.09s	+59° 09′ 00.8″	2.28	1.17	54	F2III-IV	Caph; δ Sct variable, Vmax = 2.25m, Vmin = 2.29m, P = 0.10 d
γ Cas	γ	27		5394	4427	00h 56m 42.50s	+60° 43′ 00.3″	2.47	−4.22	613	B0IV:evar	Tsih, Navi; prototype γ Cas variable, Vmax = 1.6m, Vmin = 3.0m
δ Cas	δ	37		8538	6686	01h 25m 48.60s	+60° 14′ 07.5″	2.68	0.26	99	A5Vv SB		Ruchbah; Algol variable, Vmax = 2.68m, Vmin = 2.76m, P = 759 d
ε Cas	ε	45		11415	8886	01h 54m 23.68s	+63° 40′ 12.5″	3.35	−2.31	442	B2pvar		Segin; variable star, ΔV = 0.002m, P = 0.09 d

Is β Cas, at 54 ly, close enough to show a marked difference in position when viewed from Sol versus Alpha Centauri?

And all but one (Sirius) of the 10 brightest stars in Canis Major are closer than 60 ly to us. See https://en.wikipedia.org/wiki/List_of_s ... anis_Major. Would any of them except Sirius show a marked difference in position when viewd from Sol versus Alpha Centauri?

I don't think that the "W" of Cassiopeia would change much as seen from Alpha Centauri, except for the addition of our own Sol:

Cassiopeia as seen from Alpha Centauri. Photo: I don't know.

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Small and blurry picture of a larger piece of sky as seen from Alpha Centauri. Note Sirius in Orion right next to Betelgeuse. Image: I don't know.

---

Note that in the picture at left, the Sun is nearly as yellow as the yellowest star in Cassiopeia, Alpha Cas. That's not how it would be in reality. Alpha Cas is a star of spectral class K0III, with a B-V index of +1.12, whereas the Sun is spectral class G2V, with a B-V index of +0.656. That's a very obvious difference.

Note in the terribly tiny and blurry image at right of the skyscape as seen from Alpha Centauri that Sirius is right next to Betelgeuse in Orion. Note, too, that Sirius is markedly fainter as seen from Alpha Cas than when seen from the Earth. This may very well be true, because Sirius is one of the closest stars to the Earth, and it owes its high brightness as well as its position as seen from the Earth from its proximity to us. If Sirius is closer to us than to Alpha Centauri, the difference in its brightness as seen from Alpha Centauri would be noticeable right away.

But you have to admit that it would be fun to see still-bright blue-white Sirius right next to bright deeply golden-orange Betelgeuse. What a pair!

Ann

[johnnydeep]

"Seen from Alpha Centauri, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia."

Is this really saying the Cassiopeia would look about the same to us if we were standing on a planet around Alpha Centauri? At first think, that seems like it would be quite a coincidence. But on second thought, all it really means is that all the stars in Cassiopeia are much farther away from both the Sun and Alpha Centauri than they are from each other. And it also means that ALL familiar constellations would look pretty much the same from either star!

Except Cassiopeia and Centaurus, of course. And Canis Major, too; Sirius is close enough to both stars to show in greatly different directions.

What do you mean? Here's a list of stars in the constellation Cassiopeia - https://en.wikipedia.org/wiki/List_of_s ... Cassiopeia. And these are the 5 main stars that make up the "W":

Code: Select all

Name	B	F	Var	HD	HIP	RA		Dec		v.mag.	a.mag.	Dist.ly Sp.class	Notes
----------------------------------------------------------------------------------------------------------------------------------------------------------
α Cas	α	18		3712	3179	00h 40m 30.39s	+56° 32′ 14.7″	2.24	−1.99	228	K0II-IIIvar	Schedar
β Cas	β	11		432	746	00h 09m 10.09s	+59° 09′ 00.8″	2.28	1.17	54	F2III-IV	Caph; δ Sct variable, Vmax = 2.25m, Vmin = 2.29m, P = 0.10 d
γ Cas	γ	27		5394	4427	00h 56m 42.50s	+60° 43′ 00.3″	2.47	−4.22	613	B0IV:evar	Tsih, Navi; prototype γ Cas variable, Vmax = 1.6m, Vmin = 3.0m
δ Cas	δ	37		8538	6686	01h 25m 48.60s	+60° 14′ 07.5″	2.68	0.26	99	A5Vv SB		Ruchbah; Algol variable, Vmax = 2.68m, Vmin = 2.76m, P = 759 d
ε Cas	ε	45		11415	8886	01h 54m 23.68s	+63° 40′ 12.5″	3.35	−2.31	442	B2pvar		Segin; variable star, ΔV = 0.002m, P = 0.09 d

Is β Cas, at 54 ly, close enough to show a marked difference in position when viewed from Sol versus Alpha Centauri?

And all but one (Sirius) of the 10 brightest stars in Canis Major are closer than 60 ly to us. See https://en.wikipedia.org/wiki/List_of_s ... anis_Major. Would any of them except Sirius show a marked difference in position when viewed from Sol versus Alpha Centauri?

Hey, I was going to try to correct my original post to say "And all but one (Sirius) of the 10 brightest stars in Canis Major are farther closer than 60 ly from us.", but the interface doesn't give me the edit button! I wonder why.

EDIT: but it's letting me edit this post just fine. Strange.

[Chris Peterson]

Hey, I was going to try to correct my original post to say "And all but one (Sirius) of the 10 brightest stars in Canis Major are farther closer than 60 ly from us.", but the interface doesn't give me the edit button! I wonder why.

EDIT: but it's letting me edit this post just fine. Strange.

A post I made yesterday is editable. Earlier ones are not. I presume that when the board was updated, a change was introduced (accidentally or deliberately) that limits editing to a certain period of time after a post is made. Which isn't unreasonable.

[johnnydeep]

Hey, I was going to try to correct my original post to say "And all but one (Sirius) of the 10 brightest stars in Canis Major are farther closer than 60 ly from us.", but the interface doesn't give me the edit button! I wonder why.

EDIT: but it's letting me edit this post just fine. Strange.

A post I made yesterday is editable. Earlier ones are not. I presume that when the board was updated, a change was introduced (accidentally or deliberately) that limits editing to a certain period of time after a post is made. Which isn't unreasonable.

Yeah, that makes sense. It must have been changed or I'm sure I would have encountered this before. Based on the not yet 48 hour old post that I couldn't edit, the window must be less than 48 hours, and could be 24 hours I suppose.

[alter-ego]

Hey, I was going to try to correct my original post to say "And all but one (Sirius) of the 10 brightest stars in Canis Major are farther closer than 60 ly from us.", but the interface doesn't give me the edit button! I wonder why.

EDIT: but it's letting me edit this post just fine. Strange.

A post I made yesterday is editable. Earlier ones are not. I presume that when the board was updated, a change was introduced (accidentally or deliberately) that limits editing to a certain period of time after a post is made. Which isn't unreasonable.

Yeah, that makes sense. It must have been changed or I'm sure I would have encountered this before. Based on the not yet 48 hour old post that I couldn't edit, the window must be less than 48 hours, and could be 24 hours I suppose.

Over the years I've occasionally edited a post, one day has always been the limit. I believe 24 hours from the original posting time has been the limit.