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Small Wonders: Observing the Hubble Sequence in the Fall Northern Sky
Voice your opinion about this subject in our forums
Observing the Hubble Sequence in the Fall Northern Sky (Pt. 1)
Tom Trusock 9/09
A while back, I was thinking about finding a new observing project – something that could be done in just a few sessions, yet something – well – different. I've got several on tap for observing various extra-galactic DSO's (similar to observing M31's globulars), but I wanted something a little different. After reading The Day We Found the Universe (Marcia Bartusiak), I got to thinking about Hubble's galactic classification scheme.
How much, exactly, can be seen by a motivated modern amateur?
Granted, Hubble used photographic data from the largest telescopes of the time, but still – anyone who has seen M51 from a dark site would think it should be possible to see some of the detail necessary for Hubble Sequence classification. Not being one to completely reinvent the wheel, I spent some time researching the subject and it turns out that Brian Skiff already wrote an excellent article for Sky and Telescope about just this. But – somewhat predictably, as galaxy aficionado's are probably already thinking it's set for the spring sky.
I'm not the most patient man in many respects. I wanted something that I could do now – which for me is this fall. Ergo, this piece – observing the Fall Hubble Sequence. Predictably, it's biased towards northern observers. Unfortunately, trips to Australia or South America are most likely a ways off for me.
Lets back up a bit now and introduce the Hubble Sequence for those who may not be all that familiar with it.
Edwin Hubble (1889-1953) is unarguably one of the giants of American astronomy. Possibly his greatest contribution was the revelation of the modern universe by discovering the existence of other galaxies outside of the Milky Way. In some sense, modern cosmology started January 1, 1925. (Remind me to buy a birthday card.) Also vying for most significant contribution has to be the discovery by Humason and Hubble that a object's redshift is proportional to it's distance. Hubble's Law, as it became known, has been fundamental in forming our current cosmology of an expanding universe. (This was a time of giants for cosmology. Hubble used massive telescopes and Einstein the back of old envelopes – but that's another story.) And Hubble was one of the first to propose something that eventually became a foundation for a modern commonly accepted galaxy classification scheme that's still being used today.
Let's be clear, he wasn't the first. It can be argued that the William Parsons, third Earl of Rosse started the whole quest when he noted the sprial structure of M51 in 1845. One other system that saw use till the 1940's (several years after the introduction of Hubble's Sequence) . In 1908 the german Astronomer M. Wolf devised a system which rated targets from amorphous forms to well developed spirals. No distinction was made between planetary nebula and galaxies, but it still provides a more detailed description of the variations possible in the spiral patterns than does Hubbles scheme.
From Alan Sandage's - Classification and Stellar Content of Galaxies Obtained from Direct Photography
There were several others as well. Interested readers should refer to the link given at the end of this document to Alan Sandage's Classification and Stellar Content of Galaxies Obtained from Direct Photography.
However, visual astronomy being what it was, and is, for that matter (highly dependent on the observer, telescope, site and conditions), the real work on galaxy classification could only begin after photographic surveys became possible.
In 1926, Hubble first published (and later refined) his classification system based on data obtained with the 60 and 100 inch telescopes on Mt. Wilson.
From Alan Sandage's - Classification and Stellar Content of Galaxies Obtained from Direct Photography
Broadly speaking Hubble's classification scheme originally placed galaxies into three separate classes: Elliptical, Spiral and Irregular. The S0 class, a bridge between Elliptical and Spiral was added in a modification to his initial scheme.
We'll begin at the begining. Even though the Hubble Tuning Fork is not indicitive of galaxy evolution, it's standard phraseology to refer to galaxies at the left of the diagram as early and those towards the right as late.
So, in terms of the early galaxies, we find the Ellipticals. Elliptical (E) galaxies are characterized by the following:
Observationally (for our purposes) the brightness will fade quicker and more smoothly than a spiral
Spirals (S) and Barred Spiral (SB) are characterized by the following:
Observationally, (for our purposes) a weak disk means less fade (a uniform dim haze surrounding a brighter core) / Sc (and beyond) means a more starlike nucleus
Between the two, Hubble placed the S0 class. Commonly referred to as lenticulars, they can be very difficult to distinguish from the elliptical class, and as such were a class added by Hubble when he modified his system.
The S0 class is characterized by the following:
Finally, we have the irregulars.
It's important to note that this classification system as it's most commonly used today is in a form modified by de Vancouleurs to better account for the different shapes seen in spirals, and provides far more diversity than Hubble's original sequence. Further, that other systems exist and are in use today. In fact, classifying galactic morphology seems to be a never ending field. Ultimately, galaxies are like people. While they can be broadly classified, it's mainly on comparison to each other, and each individual is unique.
So, what can we see from our backyards?
Visual observers are limited by the light gathering ability of their eyes, and the eye's rather limited (comparatively speaking) angular resolution. However, it should be possible for observers with, good observing skills, sufficiently dark skies and moderately large apertures to discern between sufficiently distinct Hubble classes.
I wanted to select galaxies that were representative of their type (where possible), but I also wanted to point out some fall showpieces for readers who may not be familiar with them and – well, I had my own somewhat inscrutable reasons for a couple of choices. Ergo, the particular (and perhaps sometimes peculiar) choices made. If I've neglected your favorite fall showpiece, please let me know, and I'll add it to the list.
You can download an observing card list in PDF format here.
I've ordered the list by class (for obvious reasons) but most observers will probably want to reorganize it a bit by constellation so they can make quick comparisons – especially those without the aid of computerized pointing.
A few photographs for comparative purposes of the types:
NGC 7619 (E) - DSS
NGC 185 (E3) - DSS
NGC 5866 (S0) - DSS
M81(Sb) - HST
NGC 1055 (SBb) - DSS
M101 (SBc) - HST
M82 (Irr) - Chandra / HST / Spitzer
And now a clarification. As you probably noticed right off the bat, this particular edition of Small Wonders is a little different than my usual format. No Finder Charts – that's probably the biggest instantly notable difference. It also appears to be somewhat skewed to large scope users – and it probably is, but differentiating detail can be seen on many of these targets even in a small scope, and the galaxies chosen are for the most part some of the best fall showpieces, and beautiful even in a small telescope.
You'll also notice I'm not going into much, if any detail, on the objects except (of course) to provide you with a list. At this point, I'm specifically limiting the photos and descriptions in order to help observers from inadvertently using averted imagination (something I think all of us struggle with at times). Instead of the usual approach, we'll do the observation first, then in a later column (or two) I'll revisit these targets with more in-depth descriptions. To that end, I'd really like the next article in this series to be a community project. Please e-mail me with your photographs, sketches and descriptions on the objects for inclusion in the next phase. When observing, please pay particular attention to the Hubble Class, and what differentiates them and how it applies to what you're seeing in the eyepiece when you compare it to another target.
Additionally - I'd appreciate it if folks would please e-mail me with any necessary corrections or clarifications.
I look forward to hearing from you!
Additional Resources / References / Just Plain Cool Stuff
Sidney van den Bergh – Some Musings on Galaxy Classification
Alan Sandage – Classification and Stellar Content of Galaxies Obtained from Direct Photography
Brian Skiff – Exploring the Hubble Sequence by Eye
William C. Keel's – Galaxy Classification
Wikipedia - Galaxy morphological classification
Sings Hubble Tuning-Fork Poster
If you liked this article, you may want to check out the rest of the series.
I'd love to hear of your experiences under the night sky - please feel free to
e-mail me or send any observing reports to: firstname.lastname@example.org
(Please contact me via Forum PM if I don't respond.
I've been having some issues with spam lately, and probably
didn't receive your e-mail.)
Please indicate if I can cite your observations in future columns.
Photographic Images Courtesy DSS: copyright notice
Locational Images Courtesy Bill Tschumy, Where is M13?
Hubble Images Courtesy STSCI: copyright notice
Star Charts Courtesy Chris Marriott, SkyMap Pro 10 Used with Permission
Constellation Art from the Freeware Program Stellarium
Special Thanks to all those who take the time to read and contribute to this series.