12/23/05: Astrophysics and Atlases at Montebello

by David Kingsley

The lock on the gate to the Montebello parking lot was soaking wet with dew. But conditions turned out to be surprisingly mild from about 7 to midnight in the parking lot. Temperatures in the 50s most of the night, very little wind. Relative humidity started in the 80s and ended in the 90s, but it wasn't wet enough to form any troublesome dew on eyepieces, finders, charts, etc. Bands of high level clouds passed through parts of the sky all night. However, there were always large clear dark regions somewhere in the sky. And the fog in the valley clearly reduced the typical light domes from both San Francisco and San Jose. Quantitative measurements with a a Sky Quality Meter showed background brightness about a half mag darker than a couple weeks earlier from the same location earlier in December ( 20.6 instead of 20.1 magnitudes per square arcsecond).

I spent most of the evening hunting well placed objects from Karkoschka's "Observer's Sky Atlas" with my 14.5 inch Starmaster. Whenever a large clear patch showed up in a certain region of the sky, I'd head off to that area to explore.

"Hunt and peck" astronomy produced an interesting mix of objects during the night.

Shallow sky

Mars and Saturn. Mars very obviously gibbous now, well past opposition.

Home galaxy

Island Universes

NGC253, NGC247, M31, M32, M110

I have seen all these objects before, many of them under much better conditions than we had last Friday at Montebello. However, it is always a pleasure to compare a mixture of beautiful and varied objects through the eyepiece.

As I go back through old favorites, I am now paying more attention to physical attributes and astrophysical data for each object than I did when first going through the Messier and Herschel lists. The "mind candy" often helps me see objects in a new light, and adds another dimension beyond the simple visual appearance in the eyepiece.

Karkoschka is a great starting point for basic physical data on objects. For those who like extremes, he has a useful table at the back of biggest, smallest, brightest, dimmest, furthest, closest, reddest, bluist, quickest, or most variable of all kinds of different objects. (Hind's Crimson star R Leporis viewed on Friday wins the reddest individual star prize). His introductory pages summarize absolute sizes, distances, intrinsic brightnesses of all the stars and deep sky objects in the Atlas (not just their apparent size and apparent magnitudes in the eyepiece). From Figure 3 in the introduction, it is easy to see that almost all open clusters are roughly 10 to 20 light years across. I thought about that uniformity in size last Friday night as I looked at the Hyades, the Pleiades, M35, or M35's dim companion NGC2138. The huge disparity in apparent dimensions in the sky comes mostly from the vastly different distances to each cluster. The Hyades are a sizable splash of naked eye stars in Taurus extending over 300 arcminutes (5 degrees) in apparent size and located about 140 light years away. The Pleaides are obvioius smaller and tighter, extending over about 100 arcminutes. But the threefold difference in apparent size comes largely from looking at a similar sized cluster that is 3 fold further away (400 light years instead of 140). M35 is a very rich cluster in a telescope, covering about 30 arcminutes with sparkingly stars. That's 1/10 the apparent size of the Hyades cluster, and sure enough M35 is located roughly 20 fold further away (3000 light years). Nearby Ngc2158 provides a striking contrast to M35: a sprinkle of resolved stars in a much smaller dimmer, more concentrated patch. NGc2158 has an apparent size that is about 1/6th that of M35 in the eyepiece (5 arcminutes instead of 30). Once again the six difference in apparent size is almost completely due to a difference in the distance at which we view otherwise similar sized clusters (NGC 2158 is located at 16 thousand light years instead of 3000).

Our visual experience of the naked eye Hyades is very different than our visual experience of NGC2158. But at each step from the Hyades to the Pleides to M35 to NGC2158, we are peering at open cluster that cover roughly similar absolute dimension. We are so close to the Hyades that it stretches across 5 degrees of the sky. We are so far from NGC2158 that we can only see it by peering with a telescope. But the differences are like watching a football game from different distances. A seat in the end-zone provides a great view of the action when the players are only 150 feet away (like looking at the Hyades). The same players are a bit harder to make out when first jogging onto the field at the other end of the stadium (the increased distance about like looking at the Pleiades instead of the Hyades). I've actually watched part off Stanford football games with my kids looking through a Televue Ranger from a second story window located less than a mile away from Stanford stadium. That provides a view of the action from several thousand feet instead of 150 feet away (about like looking at M35 instead of the Hyades). You could try the same "football at a distance" trick from FootHills Park above Palo Alto. Trying to watch a Stanford football game from several miles away would be comparable to hunting down NGC 2158 in a telescope. The actual football game and cluster of players is similar in each case. But the game looks a lot different from 16,000 feet away than it does from the seats 150 feet away in the endzone.

Another handy resource for basic astrophysical mind candy is Finlay's Concise Catalog of Deep-Sky Objects. This book summarizes astrophysical information for 520 objects on the Messier, Herschel 400, and Finest 110 NGC object lists. Finlay states in the introduction that the bookd is intended "to give a concise summary of the more interesting astropysical facts that are known about objects commonly observed by amateur astronomers. Pondering this information while viewing an object in the filed has added a new level to the author's enjoyment of deep-sky observing, and it is hoped this information will be similary enjoyed by other amateur astronomers.".

The paperback is about the same size as the Karkosachka atlas, and I now keep it in my observing jacket so I can read more about objects when I revisit them in the eyepiece.

Friday night I got interested in whether the physical size of bright starbirth HII regions are roughly similar in size to the open clusters they will later spawn. Karkoschka only lists apparent sizes and distances to objects, not physical size, but a useful approximation for conversion using basic geometry is:

(Apparent size in arcminutes) x (Physical distance in kilo light years) / 3 is approximately = True diameter in light years.

Plugging in numbers from Karkoschka for M42 gives (40 arcminutes) x 1.5 thousand light years distant / 3 = 20 light years across in physical size . So for the Orion nebula, the brightest parts we see does correspond roughly to the size of a future open cluster.

I then tried the same thing for NGC 1491, another HII region in Perseus. Karkoschka lists an apparent size of 3 arcminutes, and a distance of 2.5 thousand light years. That gives a physical size of roughly 2.5 light years , substantially smaller than a future open cluster.

Double checking in the Finlay book, I was surprised to see NGC 1491 listed as 6 x 9 arcminutes and 12 thousand light years away instead of 2.5 thousand. The wildly different estimates of both apparent size and physical distance compound each other to give rise to a new calculated size of about 30 light years in diameter, more than 10 fold larger than the calculation based on Karkoschka.

Puzzled, I turned to Uranometria for another estimate. The new edition of the Atlas shows NGC1491 a huge hand drawn nebula on the observing chart, covering almost half a degree! The companion volume lists the apparent size as 25 x 25 arcminutes, though also comments that the nebula is considerably smaller (about 4 arcmiinutes) on blue photographs than on red.

So from three different sources, we have apparent sizes for the same object listed as 3 arcminutes to 25 arcminutes, and physical distances from 2500 to 12000 light years!

So who is right? For apparent size, the estimates around 3 to 4 arcminutes clearly correspond best to what I saw through the eyepiece. I looked up the same object in the Millenium Star Atlas, and found it drew a small bright nebula symbol for NGC1491, instead of the big 25 arcminute outline shown in Uranometria. When I checked the Digitiized Sky Survey plates myself later at home, the brightest part of NGC 1491, and the ONLY part of the nebula that is visible at visual wavelengths even in a photograph, is indeed about 3 to 4 arcminutes wide. So why the giant oval shown in the newest, most up to date version of Uranometria? The introduction to Uranometria says that several hundred bright nebula were hand drawn on the observing charts using DSS plates for any nebula over 10 arcminutes in diameter. But I think it is perverse to make this decision based on wavelengths that are completely invisible to visual observers, even with nebula filters. I have lots of respect for the new edition of Uranometria, but in this case the depiction of NGC 1491 is very misleading for visual observers.

How about the distance discrepancy seen in Karkoschka vs. Finlay? I looked up more data when I got back home from Montebello. The NGC 1491 HII region is illuminated by a hot young O5 or O6 class star (BD +50 886). Given the absolute magnitude of such young stars (- 6 to -7), and the much dimmer apparent magnitude of this star in the eyepiece (mag 11), there must be either a large distance or lots of extinction between us and the illuminating star. The 2mass site at Caltech has an image of Ngc1491 and a useful summary of physical data and references (http://www.ipac.caltech.edu/2mass/gallery/ images hii.html). They list the distance as 10,760 light years (Snell et al. 1990, ApJ, 352, 139), with about 4 magnitudes of extinction Picmis &Mampaso (1991, MNRAS, 249, 385). That distance estimate is pretty close to the 12 thousand light year estimate given in Finlay. With an apparent size of 3.5 arcminutes, and a distance of 11 thousand light years, the true size of the brightest region of 1491 thus turns out to be about 13 light years in diameter, right back in the range of the bright region of M42, and the same order of size as a typical open cluster.

Forgive the blathering on about numbers and details. But in this case an enjoyable night under the stars with some favorite winter objects and observing guides left me with lots of questions. Karkoschka did a better job than Uranometria or Finaly on the apparent size of the nebula at visual wavelenghts, but was off in physical distance by almost five fold. No distance estimates are provided by Uranometria at all, even in the fat companion volume. The useful verbal description of the object by Finlay, and the accurate estimate of physical distance confirms my impression of the general usefulness of his Concise Catalog.

With the weather we have been having the last two weeks, the sight of stars alone last Friday was well worth the trip. I didn't look at a single object that I haven't seen before. But it was great to be out with a telescope, and to see some of old favorites in a new light.

Happy New Year and Clearer Skies to everyone in 2006.

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Posted on sf-bay-tac Dec 31, 2005 21:17:37 PT
Converted by report.pm 1.2 Mar 16, 2006 23:03:35 PT