676 replies to this topic

[CAAD9]

I have looked at the various star groupings in Orion and thought "gee they look kind of like ill defined open clusters". I always put it down to just appearing that way from earth and the stars themselves not even being related.  But alas some of them are!

 

I must thank you both Allan and Glenn for introducing the OB associations to me.  I'm staggered that this is not better publicised in the popular astronomy literature.  Certainly deserves more attention. And more of our observing time when this can be viewed with modest equipment.

 

I will look at Orion all over again in a new light!  Then I'll follow Gould's Belt.  Fantastic pursuit for binocular viewing.

 

Thank you both again. Kindest regards,

 

Adam

Edited by CAAD9, 05 February 2017 - 03:59 AM.

[AllanDystrup]

     WOW Larry, your page on ->OB-associations<- is a GREAT resource for locating and identifying these large objects. I have searched for the S&T article on OB-associations from Jan.86, -- but wasn't able to find it.
So I for one will certainly return to your page time and again, to compare my own impressions with what you have recorded. Thanks for the link!

     As for "going over to the dark side" with EAA, I'm making that same transition in some ways, -- but I like to see it from another perspective. I see it as "crossing over to the light side" (a way of pushing the envelope), when my observing conditions for purely visual hits the wall   ... And even when observing using live video, I like to sit down and sketch my objects, because that, in my view, is the best way to REALLY see, experience, record - and enjoy - every detail.

 

Allan

 

Correction --
I found the S&T article on OB-associations mentioned above, in my latest search!

Here's the link:  IN PURSUIT OF OB ASSOCIATIONS (p 110-113)

Edited by AllanDystrup, 05 February 2017 - 08:24 AM.

[Larry Mc]

hi Allen,
Thanks! glad you enjoyed the page and was able to find that Sky& Tel article. I like your style of sketch recording keeping. Are you using a white conte pencil on black paper?
I use a #2 gray graphite pencil, and blending stump on a pre-printed home-made white sketch form, and later after scanning it in, will flip it to a negative so that the stars are white on a dark background. I would generally make a rough sketch out in the field and then the next day clean it up, make the stars a bit more rounder, and fix spelling typos.
I have a page on my website on sketching and here's a link to the form:  http://www.stellar-j...etchingform.htm
Back in the the mid-1980's thru 2000, I was quite a prolific sketcher. Then I kind of hit the wall where either I needed a much bigger aperture telescope to see the objects that I was interested in, or go the CCD imaging route.

Then I discovered video-astronomy, first using a home security style camera for lunar/planetary, and then working my way thru the line of deep-sky cameras offered by AVA-Astrovid. I still use their last camera, the StellaCam-3, peltier cooled with wireless remote with great results on my C8. I'm currently using it to work my way thru the Arp galaxies.
I agree with your comments regarding EAA. With how bad light pollution is getting, I think it's the future of amateur astronomy.
In my video-astronomy presentations that I do, I conclude with this line: "The future of video in amateur astronomy is quite bright."

--------------------------------

Also want to comment on Glenn's "Gould Belt" posts. Your two articles on the 'belt' was a great read! Loved the charts! (your chapter on the Milky-Way was the driving reason that I purchased "The Backyard Astronomer's Guide").

-------------------------------

Oh, and to Matt's question regarding my little 80mm, it's an old University Optics 'Super-finder', (actually a 320mm f4) that they sold back in the mid-80's.
It came with a amici diagonal for a non-inverted view and a cheap eyepiece that gave 11x. It was great for star-hopping using an atlas. The nice thing was it had a machined rack-pinion 1.25" focuser that let you switch to different eyepieces/diagonals, or even insert a camera. I found a picture of it mounted in Losmandy tube rings, which allowed me to move this about different telescopes as needed.
I bought it for my Coulter 13.1" Blue-Tube dob. After I sold the dob a few years later, (was killing my back), I kept the 80mm and used it on an Orion slow-motion alt-az tripod mount.

Larry

Attached Thumbnails

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[AllanDystrup]

Lambda Orionis Association
”Orion’s Head”

 

 

     The stellar association marking Orion’s Head consists of a group of stars around the type-O giant Lambda Orionis (Meissa). The association is not cataloged as part of the Orion OB-1 complex, but its estimated age of 11 MYR is in same timeframe as the creation of the earliest OB-1a association. Though the Lambda Orionis group of stars (also known as CR 69) is relatively young, it is never the less old enough, that one of its first stars went supernova ca. ~350 KYR ago.

 

     The supernova blew up a 7° large bubble of ionized gas (SH2-264) in the molecular cloud, which created the cluster; At the same time the explosion sent the remaining, spinning core of the supernova flying away NE in the direction of Gemini. The core of the supernova, Geminga, was discovered in 1972 close to the star 20 Gem (RA 6h33m, DEC 17°22m, almost 2° NW of Gamma Gem). It’s a neutron star, way too faint for visual observation (~25^m; gh'è minga in Italian dialect, meaning "it's not there"!…), but it is the closest neutron star to the sun, and the second brightest pulsar in gamma- and X-rays, that has been observed in the Milky Way galaxy.

 

 

Allan

 

[AllanDystrup]

λ-Orionis Cluster -- continued

 

 

     In the ~5° rich field view of my small 55mm refractor, the central λ-Orionis cluster (CR69) is dominated by the two giant stars: Lam Ori (3.4^m type O8) to the north, and Phi-01 Ori (4.4^m type B0) to the south; In between this pair of giants are seen a string of three ~7½^m type early B stars.

    

     Lambda Ori (Meissa) is a binary with a close B0-companion, plus three fainter and more distant components. I of course didn’t resolve these in my WF setup, but here’s the data:

 

Magnitudes   A:3.5  B:5.5   C: 10.7     D: 9.6     E: 9.2
Separation       AB:4.2″    AC: 28.7″   AD: 78.0″  AE: 150.4″
PA               AB:44°     AC: 185°    AD: 272°   AE: 279°
 

 

*click*

 

Allan

[AllanDystrup]

Hi Larry,

 

Your little UO-80mm setup is in some ways equivalent to the 80mm Vixen, I currently use for my DSO projects (described here: http://www.cloudynig...-ngc/?p=7440823): Amici diagonal + a turret for a set of eyepieces and my small camera. In the same thread you can also follow my journey from purely visual to also using EAA (http://www.cloudynig...sier/?p=7347470).

 

As for my sketching, I've described it briefly here: http://www.cloudynig...sier/?p=7229835. I use a refillable pencil with 0.5mm/2H leads for outlining, plus a fine (F) art design graphite pencil for shading. I use an observation template on standard A4 white paper. I make my drawing and notes at the telescope. Then I make a cleaned up version at the desk the next day, adding my notes and coordinates. For stellar objects I like to scan the drawing and make a "negative" (white on black) version. For fine detail DSO drawings, I lately mostly keep the black on white version, as I find I loose to much detail in the conversion.'

 

 

Allan

[GlennLeDrew]

Appreciating the larger scale structures such as star-forming regions and molecular cloud complexes is a worthwhile endeavor. It puts into perspective the interrelationships between what formerly were so often treated as individual objects in isolation. A larger instrument, with its restricted FoV, tends to foster the tunnel vision approach, rendering one's perception of the sky as principally a star-studded backdrop against which are scattered the clusters and nebulae to be sought out and contemplated mostly peicemeal.

 

Taking in a wider swath of sky permits to see the geography of the scene in a fuller context, perhaps even inspiring to consider the arrangement in depth. A dark sky that readily reveals the dark clouds really leads to a ready visualizing of the third dimension, allowing knowledge and imagination to bring this to the fore and push that into the background. This is the aspect from which I derive the most satisfaction; building up in my mind's eye a 3-D map of our galactic neighborhood. After more than two decades of observation and study directed to this goal, I've assembled a pretty conctrete map in my head of the Local, Perseus and Sagittarius spiral arm features within about 8,000 l-y.

[Organic Astrochemist]

I think one reason the piecemeal approach is adopted by amateurs and pros is that by excluding obvious field stars, one can often control both for distance and also for age.
Otherwise the the differences in stellar magnitudes we observe in an area of the sky is due not only to distance but also to stellar mass and age. Glenn's mental map is in part due to his deep understanding of the interplay of these factors.
An interesting aspect of Allan's project is that if he concentrates on young OB stars then he controls for both mass and age of the stars, and the differences in brightness are due to distance (and any extinction along the line of sight).
That seems like a great complementary approach.

[Far Star]

Thank you very much, Allan, for starting this thread and for your wonderful and inspiring work!

 

Ulrich

[AllanDystrup]

Ulrik, Organic Astrochemist -- Thank you for the kind feed back! Much appreciated   -- Here's an example of cosmography based on just OB-stars : http://sci.esa.int/h...rs-interactive/ (use the mouse for navigation!)

 

 

... This is the aspect from which I derive the most satisfaction; building up in my mind's eye a 3-D map of our galactic neighborhood. After more than two decades of observation and study directed to this goal, I've assembled a pretty conctrete map in my head of the Local, Perseus and Sagittarius spiral arm features within about 8,000 l-y.

Glenn -- this is indeed an important (long term) goal for my effort in starting the OB-1 survey. It's like when I'm observing the moon -- I want to, not just experience the beauty of the landscapes and to know their names, but also to understand the "grand picture" : by which processes did they evolve, and how was their structure determined. For our sattelite there's a book like "The modern Moon" by Charles A. Wood. I miss a corresponding introductory book for our solar neighbourhood (say out to ~600pc), with a focus on observational astronomy of large scale star clouds, associations, nebulae and clusters, -- from naked eye (possibly night vision) over binocular to rich field telescope observation  .

 

Allan

Edited by AllanDystrup, 08 February 2017 - 06:13 AM.

[AllanDystrup]

While I'm waiting for clear skies to proceed with my observations, here's a good illustration of the larger structures in our solar neighbourhood; It shows the Lindblad Ring of cold molecular gas, the Gould Belt of bright stars, including OB-associations, plus the smaller loops of expanding radiation shells and open star clusters :

 

©2014 Bruce MacEvoy. Published under Creative Commons 3.0.
May be freely reproduced without changes and with attribution.

 

Allan
 

[AllanDystrup]

A little more context :
 

     The first generation of recent star formation in the Solar neighbourhood was initiated ~60 Myr ago. What remains of this event is the Pleiades Supercluster (Ref.1) plus the extended, close by Cas-Tau OB association. Then, ~20 Myr ago, high-mass stars in Cas-Tau went supernovae and blew up a large bubble in the surrounding molecular clouds, resulting in the Gould Belt and the second generation of OB associations in Perseus, Orion and other constellations (Sco, Cen, Lup, Crux, Lac). The present star formation in Taurus-Auriga and Ophiochus is thus regarded the third generation.

 

     As can be seen from the maps below (Ref.2), the Alp Per Association (Per OB-3) can be considered a bound component in the otherwise unbound older Cas-Tau formation, which as mentioned is considered “ground zero”  for the Gould Belt. On this background my OB-association survey could naturally have started chronologically with Per OB-3, and then moved further outward to the Gould Belt OB-groups : Per OB-2, Orion OB-1, Lac, Sco, Vel, and beyond…

 

 

Ref.1: Gould’d Belt, Glenn leDrew
Ref.2: A Hipparcos Census of the Nearby OB Associations, A. Blaauw et al, Astronomical Journal, Jan.1999
 

 

Allan

[AllanDystrup]

Here's a plot I made showing the ~20° tilted Gould Belt with (some of the) major OB-associations, as it winds around the Milky way in the night sky. It's in equatorial coordinates (instead of galactic), just for a change...

 

*click* for larger image

 

 

Next up I plan to visit the Perseus OB-associations. The weather forecast however says overcast and snow for the next several days, so it may take a while... Mayby some of you have a better chance to go fishing in this pond the coming starry nights? I hope to see your results

 

Allan

Edited by AllanDystrup, 08 February 2017 - 10:41 AM.

[CAAD9]

Hi Allan,

 

Baby steps from me.  I managed to re-observe Orion. It's been cloudy since that night.  I got an appreciation of Orion's 1a,1b,1c and Lambda associations.  

 

To see is one thing. To see and be able to grasp some small piece of understanding is a totally higher plane of experience.

 

Thanks again for starting this thread and more so for continuing to fill it with so much useful information.  The maps are fascinating.

 

Cheers,

 

Adam

[AllanDystrup]

*click*

 

This is one plausible scenario for the creation of the Perseus OB associations :

 

     Around 35 Myr ago, a series of supernova explosions of massive OB stars in the Cas-Tau association created a large bubble, which by differential galactic rotation was extended to the expanding ring of gas we now know as the Gould Belt.

 

     Later, ∼10 Myr ago, the gas falling back at nearly right angles to the plane of the Gould Belt was swept up by the shell of an early type-B supernova explosion in Cas-Tau, distending the Perseus structure of high-latitude molecular clouds (the Lindblad Ring) and sparking the central Per-OB3 moving group. 

 

     Stellar winds from hot supergiants in the Per OB-3 finally created the HI “local bubble” void, and when the shell of this bubble rammed into  the Lindblad Ring around 6 Myr ago, it initiated  the creation of the more recent, distant OB associations like Per OB2 (Cep OB-6, Lac OB-1 etc.) :

 

• The Per OB-3 (Alpha Persei) Association, (Age: 35-10 Myr, Dist: 176 pc,  Diam:  13 pc), a bound moving stellar group close to the original center of the Cas-Tau halo (“Ground Zero” of the Gould Belt).
   
• The Per OB-2 (Zeta Persei) Association, (Age: 6 Myr, Dist: 305 pc, Diam: 50 pc), a moving group including 17 massive, high luminosity blue type OB stars, plus >1K lower mass stars. Recent supernovae in Per OB-2 have blow up a 20° expanding HI supershell in the Perseus Molecular Cloud, and at the border of this shell we find to the east the California Nebula (NGC 1499, Sh-2 220) illuminated by the run-away star ξ Persi, and to the west several  active star-forming clusters such as the ~3 Myr young IC 348 and the  NGC 1333 OC and Bernard-1 DN.
   
• The Per OB-1 (Double Cluster) Association, (Age:, 3(h)-5(chi) Myr , Dist: 2300 pc, Diam: each ~10 pc ), contains the double cluster NGC 869+NGC 884, two open galactic clusters in Perseus, each containing several thousand stars, many relatively young O-B type, but not considered an integral part of the Gould Belt OB associations.
   

*click*

 

Allan

[AllanDystrup]

Perseus OB-3 (Alpha Persei moving group)

 

 

Obs-1: Time: 2017-02-10, 18:00 UT,  Loc.:56N 12E Denmark, Alleroed,
       Setup: Vixen FL-55S/440mm, 1.5xGPC + Masuyama 2” 32mm
       Transp.: 3-2/7 (14dy/99% Moon, 5° Alt), Seeing: 4-5/10,
       Light wind; Bortle: Red, suburban (SQM 17.6 NELM 4.8m)
  

Obs-2: Time: 2017-02-11, 18:30 UT,  Loc.:56N 12E Denmark, Alleroed,
       Setup: Vixen FL-55S/440mm, 1.5xGPC + Masuyama 2” 32mm
       Transp.: 3-0/7 (15dy/100% Moon, 12° Alt), Seeing: 5/10,
       No wind; Bortle: Red, suburban (SQM 17.3 NELM 4.7m)

     

 

   It’s a couple of early evenings in mid February; It has been snowing the past days, but now a southerly wind is blowing the clouds away, -- almost. There’s a thin layer of high cirrus and some rags of lower altocumulus drifting up NW, and furthermore there’s a full moon (14-15 day, 99-100%)  at 5-10° altitude in LEO in the east.

 

     The overall transparency is correspondingly low, with a SQM  around 17.5 (NELM ~4.7^m), but I can trace the outline of Perseus almost overhead (~70° Alt), so I’m out under the night sky with my “little horse”, the Vixen FL-55-S/440mm refractor.  Why bother? you may ask…  I’m thinking of these lines by Robert Frost ^(*):

 

He gives his harness bells a shake / To ask if there is some mistake. / The only other sound’s the sweep / Of easy wind and downy flake.  
The woods are lovely, dark and deep,  / But I have promises to keep,  / And miles to go before I sleep,   / And miles to go before I sleep.

 

 

     So, tonight I’ll focus on the OB-associations in Perseus, starting with Per OB-3, the Alpha Persei Cluster, (=Melotte 20 =Collinder 39 =O’Meara Hidden Treasures 14). The most luminous stars in the association are the yellowish type-F supergiant Alpha Per (~2^m Mirfak) and ~4^m type-K Sigma, but the association also includes several blue type-B stars such as Delta, Psi, 29, 30, 31, 34 and 48 Persei. The cluster has a beautiful overall “S”-shape, somewhat like the Orion OB-1b association. O’Meara has described it as an outline of the serpentine body of the sea monster, that Perseus turned to stone (using Algol, the fierce eye in the snake-haired head of the Gorgon Medusa). I find this description very fitting (although O’Meara interpreted the outline a little different, than the way I see it).

 

 

*click*

 

 Allan

 

- - - - - - - - - -
(*) Footnote, on suburban observing:
     When I was doing construction, QA and teaching in the software business, we had this concept of “good enough quality”. It came from the realization, that any non-trivial software has such high complexity, that you can never prove or test it to be 100% correct (error free) viz. the specification. So instead you use smart heuristic techniques to develop and test it to a certified “good enough” level of error, that will allow you to ship a product on time and with a quality, that will keep the customer happy.

     Same thing with my observing conditions and equipment; There are maybe a handful of nights in a year here north of Copenhagen, that could be said to approach perfection, and there are other telescopes I could get, with better specs (Aperture, FOV, Strehl, etc. etc.). But I observe where I live for a reason, and I’ve acquired the scopes I own after doing my research. My conditions and equipment are past the burn-in test phase. They are of “good enough quality”, and now I’m luckily left with : “Many a wonderful sky to sweep, / And miles to go before I sleep”, to paraphrase Frost.[/end sidetrack].

 

[GlennLeDrew]

Sigma Per is not a member of Per OB3 because it's an older, lower mass giant, in the foreground, with quite discordant proper motion.

 

Alpha Per is transiting the Cepheid instability strip in the H-R diagram, thus nearing its demise, and probably as the last supernova of this group. All other members likely have insufficient mass to reach the iron fusion state at their centers.

 

Incidentally, the age cutoff for supernova candidates is about 50 Myr; a star which exceeds this likely is not massive enough. And so for associations much older than 50 Myr, all remaining stars are expected to expire less violently. In the case of Mirfak, the fact of a group age of about 50 Myr and the star's nearing its end make for a real possibility of a SN event.

 

Per OB1 is far removed from the Gould belt, lying out in the Perseus arm. Extinction by dust dims the double cluster by 1.5 magnitudes, or a factor of 4, and reddens the stars by E(B-V) 0.5 magnitude. This renders the bluish stars as nearer to 'white'. For instance, the bluest star possible is about B-V -0.34. A B-V color excess of 0.5 thus makes such a star exhibit an apparent B-V color of 0.16, like that of an A-type star.

[Sarkikos]

A garden variety ST80 with 2" Crayford replacement focuser would be good for these OB studies.

 

Mike

[Sarkikos]

It would be nice if SkySafari Pro had the OB's and such in their database.

 

Mike

Edited by Sarkikos, 14 February 2017 - 10:44 AM.

[AllanDystrup]

The Per OB-2 (Zeta Persei) Association

 

 

 

   The core of the Per OB-2 association is found at the right foot of Perseus, in the triangle between Xi, Zeta and Omicron PER. It consists of a handful of 5½^m-6^m type-B stars, plus many fainter members, the brightest of which, down to ~9^m-10^m, can be seen arranged in nice, winding patterns at 23x in my 3.2° FOV.

 

*click*

 

     The ~4^m giant, type-O7 Xi Per (to the NE on my drawing) is a run-away member of this OB-2 core. It is ionizing the emission nebula NGC 1499 (the California nebula) just 1° NE of the star– but I was not able to spot it with my RF setup.

 

     In the other direction, towards the SW on my drawing, is another 4^m giant, the type-B1 Omi PER, with the star forming region IC 348. It consists of a ~2 Myr young cluster (Col 41), still embedded in nebulosity (the reflection nebula VdB 19). I viewed the region at 135x in 20’ FOV, using live video with my small R2 security cam. (see lower right box on my drawing). There’s a nice multiple star system close to the center (STF 439) and a fine binary at the SW border of the drawing (STF 437). I was able to detect hints of nebulosity around the center of the cluster, -- but it could be just the glow of faint stars, and not actual nebula reflection.

 

     Both the NGC 1499 and the IC 349 nebulae lie at the border of the expanding HI supershell in the Perseus Molecular Cloud, that were blown up by recent supernovae in Per OB-2.  I looked for NGC 1333, another stellar group with reflection nebula in the Perseus MC – but I was not able to see it. However, on Larry’s page of Perseus objects, https://www.stellar-...org/perseus.htm, you can see a good image of NGC 1333, as well as of IC 348 – plus lots of other interesting DSOs in Perseus!

 

 

Allan

Edited by AllanDystrup, 15 February 2017 - 04:22 AM.

[GlennLeDrew]

Given the pretty intimate relationship between OB associations and runaway stars, here's a link to a club newsletter article of mine on runaways. The table of contents near the top of the page brings you down to the article itself...

 

Part 1.
http://ottawa-rasc.c...b/index.html#a4

 

Part 2.

http://ottawa-rasc.c..._mar/index.html
 

Edited by GlennLeDrew, 15 February 2017 - 11:27 AM.

[GlennLeDrew]

Re Per OB2... From a dark site NGC 1333 is rather easy in a 25X100 bino. IC 348 is not so obvious due to the brightness of omi Per. And the California is a ghostly but certain presence in a 10X50 (unfiltered). The molecular clouds, numerous of which have Barnard designations, are awfully subtle due to their angular distance from an already not bright milky way; the somewhat sparse background star field affords little contrast.

[Organic Astrochemist]

Here's an awesome image of the Taurus Molecular Cloud taken by a CN imager, RBA.

[AllanDystrup]

The Per OB-1 (h-Chi) Association
(Perseus ”Double Cluster”)

 

     NGC 869 (h Per) and NGC 884 (Chi Per) are a pair of young (~12.8 Myr) open clusters in the Perseus OB-1 association, only a few hundred Ly apart. Each cluster contains >300  hot and luminous early B supergiants, surrounded by large halos of stars.

 

     They are located in the next outer spiral arm of the Milky Way (the Perseus Arm), at a distance of ~7.600 Kly (2.3 Kpc). They are thus much closer to other galactic OB-associations such as the Cassiopeia OB’s, than to the previously described OB’s in Orion and Perseus (which are found within the 500 pc of the Gould Belt).

 

"Where is M13?" www.thinkastronomy.com (Bill Tschumy)

 

 

     I observed the Double Cluster on two nights back in mid-November of 2016, where I made these drawings:

• one using glass eyepieces (25mm ortho for 38x @ 1° FOV) : 
  http://www.cloudynig...-ngc/?p=7533618
   
• one using live video (R2 ccd/lcd  for  110x @ 2x 25’ FOV) :
  http://www.cloudynig...-ngc/?p=7533633

 

Allan

Edited by AllanDystrup, 17 February 2017 - 06:21 AM.

[GlennLeDrew]

To get a sense of the importance of supernovae in sculpting the ISM (interstellar medium), while gazing at the half dozen red supergiants scattered about the double cluster, consider that in the near future all will detonate. And perhaps a couple dozen of other currently bluish association and cluster members are likewise destined to follow suit. In a single RFT FoV lies a good concentration of upcoming fireworks. In cosmic terms those red SG stars will go off in very quick succession, with the remainder continuing in a tapering off sequence over the following few tens of Myr.

 

A single SN can blow out a fairly sizeable bubble of million-degree coronal gas. Dozens of such explosions will create a mighty chimney of such hot gas expanding more preferentially into the lower density halo on both sides of the denser, gas-rich thin disk. On each side of the galactic equator, such a chimney at that distance of about 7,000 l-y could extend to perhaps 10-15 degrees (20-30 degrees in total length.)

 

The shocks absorbed by the adjacent disk gas, more specifically the molecular clouds, would result in rashes of star formation propagating away from the site of the blasts. Something of a 'Gould belt' would likely manifest. Indeed, similar such 'Gould belts' must be not uncommon in gas-rich galaxies like ours, where star-forming activity is ongoing.