APOD: Sigma Octantis and Friends (2018 Jun 28)

Comments and questions about the APOD on the main view screen.
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APOD: Sigma Octantis and Friends (2018 Jun 28)

Post by APOD Robot » Thu Jun 28, 2018 4:09 am

Image Sigma Octantis and Friends

Explanation: South pole star Sigma Octantis (of the constellation Octans) is on the left of this starry expanse spanning over 40 degrees across far southern skies. You'll have to look hard to find it, though. The southern hemisphere's faint counterpart to the north star Polaris, Sigma Octantis is a little over one degree from the South Celestial Pole. Also known as Polaris Australis, Sigma Octantis is below 5th magnitude, some 25 times fainter than Polaris and not easy to see with the unaided eye. In fact, it may be the faintest star depicted on a national flag. The remarkable deep and wide-field view also covers faint, dusty galactic cirrus clouds, bounded at the right by the star clusters and nebulae along the southern reaches of plane of our Milky Way galaxy. Near the upper right corner is yellowish Gamma Crucis, the top of the Southern Cross. Easy to pick out above and right of center is the long Dark Doodad nebula in the southern constellation Musca, the Fly.

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heehaw

Re: APOD: Sigma Octantis and Friends (2018 Jun 28)

Post by heehaw » Thu Jun 28, 2018 11:06 am

And of course Polaris was not always the north pole star - some thousands of years ago it was Vega!

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Re: APOD: Sigma Octantis and Friends (2018 Jun 28)

Post by starsurfer » Thu Jun 28, 2018 2:54 pm

Wasn't Canopus the south pole star once upon a time?

Also you can see the Chamaeleon I complex near the centre, Marco Lorenzi has an image here.

To the left of the Dark Doodad is the Chamaeleon II complex, which has actually been previously featured on APOD.

Also wasn't Thuban the north pole star a long time ago?

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Re: APOD: Sigma Octantis and Friends (2018 Jun 28)

Post by neufer » Thu Jun 28, 2018 4:49 pm

heehaw wrote:
Thu Jun 28, 2018 11:06 am
starsurfer wrote:
Thu Jun 28, 2018 2:54 pm

Wasn't Canopus the south pole star once upon a time?

Also wasn't Thuban the north pole star a long time ago?
And of course Polaris was not always the north pole star - some thousands of years ago it was Vega!
https://en.wikipedia.org/wiki/Pole_star wrote: <<Around 14,000 AD, when Vega is only 4° from the North Pole, Canopus will be only 8° from the South Pole and thus circumpolar on the latitude of Bali (8°S). Sirius will take its turn as the South Pole Star in the year 66,270 AD. In fact, Sirius will come to within 1.6 degree of the south celestial pole in 66,270 AD. Later, in the year 93,830 AD, Sirius will miss aligning with the south celestial pole by only 2.3 degree.

In 3000 BC, the faint star Thuban in the constellation Draco was the North Star. At magnitude 3.67 (fourth magnitude) it is only one-fifth as bright as Polaris, and today it is invisible in light-polluted urban skies. During the 1st millennium BC, Beta Ursae Minoris ("Kochab") was the bright star closest to the celestial pole, but it was never close enough to be taken as marking the pole, and the Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars. In the Roman era, the celestial pole was about equally distant between Polaris and Kochab.

The precession of the equinoxes takes about 25,770 years to complete a cycle. Polaris' mean position (taking account of precession and proper motion) will reach a maximum declination of +89°32'23", which translates to 1657" (or 0.4603°) from the celestial north pole, in February 2102. Its maximum apparent declination (taking account of nutation and aberration) will be +89°32'50.62", which is 1629" (or 0.4526°) from the celestial north pole, on 24 March 2100.

Precession will next point the north celestial pole at stars in the northern constellation Cepheus. The pole will drift to space equidistant between Polaris and Gamma Cephei ("Errai") by 3000 AD, with Errai reaching its closest alignment with the northern celestial pole around 4200 AD. Iota Cephei and Beta Cephei will stand on either side of the northern celestial pole some time around 5200 AD, before moving to closer alignment with the brighter star Alpha Cephei ("Alderamin") around 7500 AD.

Precession will then point the north celestial pole at stars in the northern constellation Cygnus. Bright first-magnitude Deneb will be within 7° of the North Pole in 10,000 AD, and third-magnitude Delta Cygni will be a pole star around 11,500 AD. Precession will then point the north celestial pole nearer the constellation Lyra, where the second brightest star in the northern celestial hemisphere, Vega, will be a pole star around 13,700 AD.

Precession will eventually point the north celestial pole nearer the stars in the constellation Hercules, pointing towards Tau Herculis around 18,400 AD. The celestial pole will then return to the stars in constellation Draco (Thuban, mentioned above) before returning to the current constellation, Ursa Minor. When Polaris becomes the North Star again around 27,800 AD, due to its proper motion it then will be farther away from the pole than it is now, while in 23,600 BC it was closer to the pole.

Currently, there is no South Star as useful as Polaris. Sigma Octantis is the closest naked-eye star to the south Celestial pole, but at apparent magnitude 5.45 it is barely visible on a clear night, making it unusable for navigational purposes.[16] It is a yellow giant 275 light years from Earth. Its angular separation from the pole is about 1° (as of 2000). The Southern Cross constellation functions as an approximate southern pole constellation, by pointing to where a southern pole star would be.

The Celestial south pole is moving toward the Southern Cross, which has pointed to the south pole for the last 2000 years or so. As a consequence, the constellation is no longer visible from subtropical northern latitudes, as it was in the time of the ancient Greeks. Around 200 BC, the star Beta Hydri was the nearest bright star to the Celestial south pole. Around 2800 BC, Achernar was only 8 degrees from the south pole.

In the next 7500 years, the south Celestial pole will pass close to the stars Gamma Chamaeleontis (4200 AD), I Carinae, Omega Carinae (5800 AD), Upsilon Carinae, Iota Carinae (Aspidiske, 8100 AD) and Delta Velorum (9200 AD). From the eightieth to the ninetieth centuries, the south Celestial pole will travel through the False Cross.>>
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Re: APOD: Sigma Octantis and Friends (2018 Jun 28)

Post by MarkBour » Fri Jun 29, 2018 3:03 am

neufer wrote:
Thu Jun 28, 2018 4:49 pm
Canopus
Thuban
Polaris
Vega
...
Iota Carinae (Aspidiske)
Delta Velorum
Quite a list! With Gaia's calculations of proper motions (although the Hipparcos survey probably would suffice for this issue), I suppose not only the motions of the Earth, but the motions of the stars themselves could be taken into account. Because when you're talking about 60,000 years ago or hence, this is not a question of simply the motion of our poles versus a fixed star map.
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Re: APOD: Sigma Octantis and Friends (2018 Jun 28)

Post by Ann » Fri Jun 29, 2018 9:17 pm

This is a beautiful and fascinating portrait of a part of our galaxy. I like the appearances of the Carina and Lambda Centauri clusters and nebulas. The blue starlight dominates over the red nebular light, which I believe is very realistic.

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