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Teaching astronomy for Sheridan College gave me the opportunity to prepare a laboratory activity that determines key characteristics of the open star cluster NGC 957 in the constellation Perseus. I wrote a detailed instruction manual for my students which you can view here: Star Cluster Photometry Instructions. You can also view plots of the color indexes, magnitudes, and temperatures at this site: Star Cluster Photometry Plots
The H-R diagram I created for the star cluster NGC 957 shows possible interpretations for the evolutionary stages of stars in the cluster. (Click on the article title to view the image and read the full text.)
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- Written by: Ray Oltion
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Riding out a 50 mph windstorm in my observatory tent convinced me that for desert camping, a hard-sided structure would be worth the effort. Inspired by the HexaYurt structures popular at Burning Man, my goal was to create a structure that would be easy to take apart and reassemble when moving from one campsite to the next. Rather than adhesive tape, machine screws and metal flashing would hold the panels together. The whole thing would break down into 4 foot panels that could be loaded into the bed of my pickup truck, and leave enough room for my telescope equipment and solar panels.
Click on the article title to read more and view additional images --> Hexagonal Observatory
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- Written by: Ray Oltion
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- Written by: Ray Oltion
- Category: RoGarOn Observatory
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The Arabs did it. The Polynesians did it. Almost anyone who traveled in the wilderness probably did it at some time. How else would you find your way in a featureless landscape, especially at night when visibility is limited to maybe 20 feet at best? Well, now I can say that I have entered that club, because the stars are what got me back to camp early this morning.
It started at 4:45 AM when I had to, ahem, take care of some urgent business, and walked some distance from camp with only my red headlamp for a light. The light wasn't much good except for dodging creosote bushes and avoiding rabbit holes. I wandered off in a general direction I usually take for such business, and wound up in an area indistinguishable from myriad other areas near my camp. It was dark and the stars were bright, the Moon having set hours ago.
After taking care of said business, I started back for camp. I walked for what I thought was the right distance and guess what? No camp. Hmm. That was embarrassing. How could I possibly get lost in the desert not more than 200 feet from camp? Well, it turns out to be surprisingly easy when there are creosote bushes every 10 feet and my light is only good for about that distance. The landscape was relatively flat, so there were no clues from slopes or drainages.
Standing in the dark in the early morning, not knowing where I was or which way to go, was an unnerving experience. It would be hours yet before sunrise. I didn't want to wait that long to find camp. I couldn't just keep walking, though. I could wind up miles from camp. That much was clear.
Then I looked up and saw all the constellations. Scorpius had been high in the sky outside my tent window. What direction was that, southeast? Oh, it was the opposite direction from which I started walking away from camp not ten minutes ago. Okay, there was the Dipper and the North Star. So East was to my right. Let me think. My camp was south of Highway 9, on a gravel road that ran pretty much north-south. The dirt two-track to my camp branched off the gravel road towards the west. If I could intersect this two-track that would lead me either to the gravel road, or to my campsite which was only 20 feet or so off the track.
I had walked northwest for my business appointment, so I needed to walk southeast to return to camp. If I overshot, I would hit either the two-track or the gravel road. Sure bet. Start walking toward Scorpius. Okay, there's the two-track. Turn left and follow it. Okay, there's the gravel road. Ah, too far. Turn around and follow the two-track back the way I came. Top the slight rise and go down into the bowl ahead. Yay! There is the reflection from my car's taillight. Home in sight!
The moral of the story? Don't think you can't get lost just because you are close to your camp. You could miss it by 40 feet in the dark and brush. Stop when you realize you are lost. Use the most powerful tool that you have: your mind. Reason out your directions and make a plan. Then act on the plan. If your thinking was sound, you will be back in camp cooking breakfast before long.
One of the exercises in a CHOICE course on how to use VStar, a neat variable star analysis tool provided by the AAVSO, was to use the Leavitt's Law plugin to calculate the distance to Delta Cephei. The following was my response, and it led to an interesting study of how we use Cepheid variables to compute distances to star clusters and galaxies. We might expect this to be cut-and-dried in our era of space-based astronomy, but it turns out to be "not so".
I downloaded data on Delta Cep from the AAVSO data download portal, not from the VStar menu that loads data from the AID, mainly because I wanted to store the raw data file locally. That means there was no associated period information from the VSX. With my new skills at determining periods with DCDFT, it was easy to narrow down on the period, first between 2 and 6 with resolution 0.01, which reported a top hit of 5.37 days, and then with a narrower search from 5 to 6 with resolution 0.001 days, which reported a period of 5.366 days. Using that with Leavitt's Law yielded a distance of 273.16 parsecs.
To find the distance of Delta Cephei, my Patrick Moore's Data Book of Astronomy was my first choice, but that didn't list a distance, only magnitudes and period. My next choice was Cartes du Ciel, the star mapping and planetarium software that is my favorite. It didn't list the distance either, but provided a link to Simbad. The Simbad page loaded lots of information on the star, but not the distance, until checking the distance box in the Measurements area yielded the following: 0.244 kpc, which is 244 parsecs. This compares fairly well with my calculation using Leavitt's Law, to within about a 12% error. That seems somewhat excessive, especially since Del Cep is the prototype star for this class of variable.
The problem seems to be calibrating Leavitt's Law to absolute distances, not the relationship itself. That is assuming you are using the right type of Cepheid, as there is a difference between two "overtone modes". This website explains how the Hubble Space Telescope used highly precise parallax measurements to determine distances to 10 nearby Cepheids to better calibrate Leavitt's Law. This sounds paltry, but Cepheids are relatively rare and there just aren't many that are close enough to measure by parallax. Hubble measurements claim an accuracy of better than 10%, though. Hubble also measured 10 Cepheid variables in the Large Magellanic Cloud and found the slope of the linear relationship between logarithmic period and luminosity to be very close, assuming that all of those variables are at about the same distance from Earth. The calculated distance to the LMC is about 49.4 kpc, which puts it outside the diameter of the disk of the Milky Way Galaxy, which is about 100,000 light years, or 30,674 parsecs, calculated by dividing 100000 ly by 3.26 parsec per ly.
Since Leavitt's Law calibrates to absolute distances via an assumed distance to the LMC, this can lead to a rather knotty problem of chicken-and-egg origins. Current estimates of distance to the LMC from various sources, described in this excellent lecture notes webpage, put the distance from about 44 to 51 kpc, a variation of about 15% from the average of the two estimates. So now we see where the uncertainty comes from. We just can't do any better for now.