Sunday, 23 December 2012

Greetings from Gamma Velorum


C P Gaposchkin, when asked to name the best moments in her astrophysics career didn't hesitate: "My first sight of the spectrum of Gamma Velorum".

   What makes it so special? This star ( we now know) has exhausted its hydrogen fuel and has a superabundance of helium: the byproduct of hydrogen fusion. Gamma Velorum now converts helium into carbon. The resulting spectrum is dominated by bright carbon bands (and lesser helium bands) - much like the spectrum of a WWII searchlight!
   The background continuum is that of a larger type O companion - but it's the Wolf-Rayet bands that are so distinctive. This is a 'sketch' compiled from spectroscope views, published plots and a generic white light spectrum. It's as close to the ep view as I can get. Photographs are rather different and dominated by a strong deep violet continuum - but my eye doesn't detect deep violet so well. The strongest part of the visible continuum is green, and the blue and yellow carbon bands make the Gamma Vel primary seem 'greenish' to my eye when viewed (undispersed) in the eyepiece.
   Big question: did CPG see it in colour, or only as a BW photo? We may never know; but I guess 'colour' for it to be her most favourite moment in a career of looking at spectra.
   Christmas greetings to all viewers. (by the way: the two smaller spectra are added for a Christmas effect)

Friday, 31 August 2012

AR11520: Ghost that Walks!

An impressive prominence at the sun's SE limb on 2012 August 27 suggested a large filament was about to appear. Next day it was very distinct on the disc at the foot of the prominence.  On the 30th it was wholly on the disc - an impressive sight - flanked by tiny spot groups11562 and 563. 'Helio' sited the filament, apparently, right on the old AR11520 site from early July (posts below). Big active region filaments like this can erupt spectacularly.  And it did so as I slept: on Aug 31 22:00 UT an array of bright flare ribbons were strewn across the site (a-a and b-b), as well as a few filaments that had somehow survived the ejection's turmoil. GOES logged a long-lived C8.4 flare at the site with a beautiful flare arcade in SDO AIA EUV images. The old AR11520 site is far from dead yet!

And readers, last night my friends at ASNSW saw fit to grant me the McNiven Award for astronomy : a very proud moment indeed. http://www.asnsw.com/node/753

Friday, 3 August 2012

Post flare loops in AR11520

So AR 11520 has not returned after its second transit of the solar far-side. This 'rough' sketch of coronal loops above the AR11520 site on July 18 (as it passes behind the limb) is posted to show some of the southern giant's more extreme behaviour. I will eventually redraw it for presentation. The physics of the loops formation is discussed in earlier posts - they are one of the most beautiful sights anywhere - and notoriously hard to photograph. Webcam users have the best chance as the bright loops saturate in most photos. The human eye almost never saturates, and can see the thin brilliant loops - and follow the tiny bright droplets of condensate (~3000km dia) as they slide down the loops to the flare ribbon sites. The ribbons were no longer visible to me, the M1.7 flare was eight hours earlier. These loops were growing faint - but superb nonetheless. The tall straight ones may be edge-on loops or surges caught up in the event. The grid lines are 20,000Km apart and the loops are a little above 100,000km (100Mm) high.

Wednesday, 18 July 2012

AR11520: southern giant!

The sketch shows AR11520 on July 13 - the biggest group of SC24 IN THE SOUTH. It had an area about 1400 units - still smaller than northern group AR11339 in 2011 that reached 1600 units. Still it was a giant, stretched across twenty degrees longitude, and easily seen with the PROTECTED but unaided eye!
     Here we see the two dominant red spots surrounded by lesser violet ones. The boundary between is termed the inversion line - and separates fields of opposite polarity. A dark active region filament there with a ring of bright plage 'cocooned' the main spots. This was the site of major flares - though cloud and time-zone meant I saw only some modest C-class events. The group was to have a long-lived M class at the west limb five days later, producing fine post flare loops. This group is the second transit of AR11504 (June) - will there be a third in two weeks time?

Monday, 11 June 2012

James Dunlop and early Australian Astronomy

"Jamie" Dunlop was the first to catalogue southern "deep sky" objects, ie stars clusters and nebulae - if we don't count Lacaille who mapped some with a tiny half inch telescope! Dunlop used a home-made 9 inch reflector. It's not hard to show he was a superb observer and draughtsman, and his catalogue of 600 southern objects mapped and described in seven months (in 1826) is an extraordinary feat! But why are half of these objects now "missing"?

ASNSW has honoured me with a invitation to deliver a talk: "Dunlop the Draughtsman",  in July, when this and other questions will be discussed. Tracking down "missing" Dunlop objects is an exciting project for amateur astronomers.

The image compares well-known southern galaxy N4945 with Dunlop's sketch of it from almost 200 years earlier - and 4945 is not a bright galaxy! Background image courtesy Johannes Schedler, Panther Observatory.

Friday, 1 June 2012

AR11476 Returns!

AR11476 Returns! A great sunspot group is the product of a much larger region of magnetic polarity - called by some :"activity nests". While sunspots arise only where emerging fields exceed ~1500 gauss and are fairly small, the whole region or "nest" can be huge, ten times larger than the spots. Here we see (SDO EUV 304A) the region that produced 11476 a month ago returning at the sun's NE limb. There are spots here, not seen as the nest saturates the SDO detector. White light will show them well. In the corona above the site there's lots to see: PFL are post flare loops above a sunspot and surges or filaments hover above. As well a large coronal hole has developed 30 deg west of the site (black arrows). Is it involved with the "nest" in any way? How long will the nest last - what will it produce during this transit?
"Activity Nest" is a useful term used by Schrijver and Zwaan in "Solar and Stellar Magnetic Activity" Cambridge Uni. 2008. P142~

Sunday, 20 May 2012

AR11476 - Surges at the Limb

AR11476 - when a great spot group rounds the east limb we see the chromosphere side-on, and a range of complex detail is revealed - detail often unseen when the group is on the disc and we view from above.
    Surges: here we see two large surges emerging (apparently)  from small spots half way between the preceding (p) and following (f) spots of the group. Views 1 and 2 show the outflow stage of the surge - and 3 and 4 the retraction phase. At full extent the surges reached ~120Mm, before retracting. View 4 shows some "wisps" left behind when the bulk of the material dived back into the sun.
     Surges take some explaining: they seem to be closed field structures emerging in areas of opposite magnetic polarity (see Schrijver and Zwaan "Solar and Stellar Magnetic Activity" P199). But why do they so often return along the same path? Often they do so many times over.
     Field configuration: fields emerging from spot umbrae are near-vertical, while those at the penumbra are near-horizontal - as shown in red and black (dotted) respectively.  Surges mostly emerge near the outer penumbrae and seem to follow the suggested trajectory dotted in black- the spots umbral fields  Velocity in this case was ~75km/sec - a typical surge value.

Sunday, 13 May 2012

AR11476:Gamma-Delta configuration

By May 9 this huge group has evolved strong R (red) polarity spots in the large violer (V)preceding spot - a mix of opposite sign spots in the one penumbra. It is now Hale Gamma-Delta class. A salvo of GOES Class M flares are the result - though none are logged here. There were, however, several small flares along the inversion line between opposite sign spots. The area of the group is now over 1000 units.

Thursday, 10 May 2012

Cycle 24: Another Northern Giant!

AR11476 swung into view on May 5, 22:00UT - helio freeware showed a longitude-length (LL)of 14 degrees - and a spectacular array of surges adorned it.(To be posted when Ive done the graphics!). Here's the group May 7 (local 8th) showing an area of almost 900 units. There is a hint of emerging Delta magnetic mixture in the huge preceding spot. (See above post)
       The small red dots were twice as bright as the chromosphere and are presumed to be Ellerman Bombs.

Sunday, 25 March 2012

AR11429 An Island Delta group

Island Delta is a term used by Zirin, Liggett and others to describe the most active class of sunspots - and AR11429 was the first example for SC24, I think. The post below shows the group when it contained mixed polarities within a single penumbra (Mar 5 and 6) - and those polarities are reversed.
    In a northern hemisphere sunspot like this one, violet polarity normally precedes in a spot group. In this group we see the large preceding spot is red rather than violet. And the following spot (at the other end of the group) is reversed too; its violet, when it should be red. This a fully reversed group! Such groups have the "greatest" flares. But AR11429 disappointed in this respect, its strongest flare was modest X5.4, the second strongest of SC24. The strongest of this cycle was a GOES X6.7 in 2011 August. This group is perhaps the most interesting from a polarity stand-point for SC24. Solar rotation is to the right in all my logs. This Fig shows the evolution of the group in detail after the single large penumbra split into many smaller ones. The reversed arrangement persisted. Zirin suggests the "true" preceding spot, the smaller spot with violet polarity just behind the large red one, should have forced its way into leading position - but this did not happen (weakness of the Hale-Nicholson force perhaps?. For an example where it did, take a look at AR 10930 in 2006 - try to find the Swedish telescope animation - its amazing)!
     In such a reversed group the members may attach to other distant polarities rather than its own group members. The SDO image (far right) shows AR11429 attaches to AR11431 way across the sun's equator in the south (white dotted line). This was the largest reversed group for SC24, and I think the largest reversed group I've seen in 13 years! It may return at the end of 2012 March - despite the "rule" that they are normally short-lived groups.

Monday, 19 March 2012

Island Delta spot group

The term "Island Delta" was coined by Zirin and Liggett in the mid-eighties to describe this rare class of spot group. Magnetic complexity and reversed polaritiy are the chief characteristics of "island delta" class.  When it was first seen at the sun's east limb AR11429 showed a mix of (p) violet and (f) red polarities, all in the one penumbra - by definition a Hale Class delta group. But the polarities were "reversed" - that is, normal northern groups have violet polarity preceding (p) but this group had RED preceding. As well, it had violet following - when it should have been red. It was, and remained, a fully reversed spot group. The invisible barrier between opposite polarities, the inversion line, is dotted in blue (speculative. Also rotation is to the right in Fig. The preceding spot has polarity R23 at longitude 303.)
          "Island deltas produce the strongest flares"(Zirin). But this one did not act as expected - it did have strong flares, up to GOES X5.4 - but the largest flare of SC24 remains an X6.9 in a normal Beta-Gamma-Delta group 2011 Aug 9.  AR11429 was the first "island delta" for SC24, and I dont recall any from SC23 - but they must have been there! I checked AR486 that had the largest recorded flare (http://users.telenet.be/j.janssens/Flares/Powerflare.html)- but it seemed a normal Beta-Gamma-Delta. Comments anyone? (Polarities (c)Regents of University of California, Mt Wilson).

Sunday, 5 February 2012

WR Stars in Eta Carina Nebula

Wolf-Rayet  stars are rare - but we have two quite close to the enigmatic hyper-star Eta Carina. Both belong to the WR subtype WN. WR stars have exhausted their hydrogen fuel, and are, it is said, fusing helium. What causes the nitrogen abundance in WN stars? Is it  the C-N-O cycle? These two stars are fairly faint and no real detail was seen in their spectra in the C8. Perhaps a larger 'scope will show emission bands?

The two WR stars are tagged - the dotted lines and star 'X' aid in locating them in this very starry region. The Fig is a 1990's VR1000 photo with the C8 - a fifty minute hand-guided exposure. It captures the complexity of the huge rich field - and I have tagged some major features. Eta Car is at the top (see spectrum below), with cluster Trumpler 14 rhs - where hyperstar HD93129A lurks. A dark trunk, I dub the "python" (arrowed), enters the field at left and seems to terminate at the famous "keyhole" dark nebula. or does it? An extension, bright in OIII, may continue across the central nebula. (Yes, I know its not a great photo - but it's still useful)

Sunday, 15 January 2012

Gamma Velorum - multiple star

Gamma Velorum is a superb multiple star - all four components visible in a 60mm refractor - though star D at 9.1mg needs good seeing. Colour contrasts are not large, but primary star A, also called Gamma Velorum 2 has a green or pale yellow tint.  Its spectral type is O7.5e+WR8. The spectrum has been viewed often and photographed (below). It's possible the blue and yellow emission bands combine with strong violet and green in the continuum  to give it a green or violet tint (?) Secondary B (type B) is very blue to my eyes, and C and D seem white (both type A)
Gamma Vel is the brightest of the rare type O stars - none are close to Earth (thank heavens!) and the second brightest is nearby Zeta Puppis. Both are sited within the Gum Nebula, a recent (~5000y BP) and huge supernova remnant. The whole region is rich in stellar exotica. 
(This is a sketch - photos of this object are usually much overexposed)

The Spectrum of Gamma Velorum

Gamma Velorum - the brightest Wolf-Rayet star in the sky is sited at RA 08h09m Dec -47:20, and shines brilliantly in constellation Vela, close to the second brightest type O star Zeta Puppis. This (1998 3M1000) photo is to show what the Baader Spectroscope records. The visual view is much nicer! The star is multiple, A is the WR type, but fainter B has recorded too - & their spectra overlap.The stars were drifted in RA to widen the prime spectrum - tricky with film. The brilliant emission bands are arrowed, wavelengths in Angstroms. The faint orange line 5876A, easy visually, has barely recorded - film's response was far from linear.The UV enhancement at rhs is partly from the WR star A and secondary star B's overlapping spectrum. The bright blue and lemon-yellow lines (all carbon) have recorded well. Problem: the Airy disc of the star broadens the lines - a slit-type spectroscope would solve this, but brings more problems of design and control. Ideas welcome!

Monday, 9 January 2012

Spectrum of Wolf-Rayet Star

The Baader Planetarium spectroscope would not normally show detail in the spectrum of any star as faint as ~7mgv - but there are exceptions. This is one!  Years back I had seen the faint blue carbon line in the spectrum of star WR79 in rich galactic cluster NGC6231, close to Zeta Sco. Recently, after removing the cylindrical lens that broadens the spectrum - producing a prime spectrum - I had sucess seeing emission lines in the spectrum of Eta Carina (see post below)

The same setup, when used on N6231 produced this view. It was spectacular! This star is a Wolf-Rayet type WC6, of 6.7mgv. The bands are almost identical to the brightest WR star Gamma Velorum. The blue line is a blend of two carbon lines, as is the lemon-yellow line. The fainter orange line is due to helium. Kaler J. tells us the WR stars are helium burners - having used up their hydrogen through fusion and mass-loss - exposing a helium shell.  Carbon is a product of helium fusion - hence these gaudy bands.  A nice example of stellar physics in a rare star type.  No cameras used for this sketch - but software helped to create the colours.

The other cluster stars are all type O and B - and showed no details in my setup. There are several WR stars in the southern summer sky. I hope to sketch them soon.