[Lvas] FW: Neglected Galaxy and Cepheid Variables

Wed Jun 17 07:50:30 PDT 2009

All,

For your consideration...

Since we have been discussing "E.E. Barnard" and objects in Sagittarius, please review M-24, also known as the Small Sagittarius Starcloud. 

Another object is immersed in this most interesting Messier. It is referred to as the dark nebula, Barnard 92 measuring 15 arc minutes across. It is located on the NE corner of the Starcloud. 

On nights of excellent viewing it can be easily seen as a dark hole or void in the field of shimmering stars. 

We may attempt to observe some dark nebulae discovered by Barnard in some future editions of the "Observers Challenge".   

Roger

All,

I mentioned in an earlier e-mail that NGC-6822 also known as "Barnard's Galaxy" was discovered by E.E. Barnard in the 1880's. He was working at Vanderbilt University in Nashville Tennessee and used a 6-inch refractor for the discovery. This galaxy has been named in his honor as "Barnard's Galaxy" by the astronomical community. Again Herschel missed this large, LSB galaxy, and of course is not listed in the H-400 or H-II list.   

In the 20's Edwin Hubble discovered 15 variable stars, 11 were cepheid variables in this galaxy. The Galaxy was calculated at a distance of between 1.6 or 1.8 million light years. 

This galaxy is relatively close, as M-31 is 2.4 million light years away...almost next door, so to speak. 

I have never discussed this galaxy or compared notes with an amateur that has personally  observed this galaxy. I have been unable to see this galaxy from my backyard due to my poor southerly sky, and have never attempted from a dark site. Weather permitting I may try this weekend with my 4-inch refractor. I have attempted many times from my backyard, but a "pesky" streetlight reduces the contrast too much. The LSB (low surface brightness) of this galaxy requires a dark sky and low magnification. 

Skiff and Luginbuhl "Observing Handbook and Catalogue of Deep-Sky Objects" observing with a 60 mm refractor from Arizona.  "A weak but definite glow in 6 cm, where it appears elongated N-S and shows a very slight central concentration". 

A Cepheid is a type of star whose brightness varies periodically at the beginning of a period, it's brightness increases very rapidly, for as long as several hours. That is followed by a gradual dimming which may continue for several days. The cycle is then repeated. 

Cepheid's variables are named for the prototype Delta Cephei (F-5 -G-2) with a period of 5.366341 days. If you have never attempted variable star observing, this is the perfect star as it is visible most of the year due to it's northerly position. Delta Cephei has a maximum magnitude of 3.48 and a minimum of 4.37.

It's variation was discovered by the gifted deaf astronomer, John Goodricke in 1784. 

Tom English allowed me to participate in many of his class activities when he was at GWU. I learned much about variables from Tom's many projects. As a learning experience, Tom, myself and his class plotted this star over a period of more than a month. Since this learning experience in variable star observing in December of 1998 I have followed and plotted quite a few others.    

These relatively rare variable stars are very important as the length of their pulsation or period is directly related to their absolute magnitude. The longer their period of variation the brighter the star. By observing the period of variation, astronomers can determine a Cepheid's absolute magnitude. By comparing the star's absolute magnitude to it's apparent magnitude, the distance can be easily calculated. 

It was in 1923 that Edwin Hubble found several cepheid variables in the Andromeda galaxy using the 100-inch at Mount Wilson. From these variables he was able to determine the true nature of the this object as an external galaxy and not a nebula within the Milky Way by calculating the distance. The period-luminosity relationship of Cepheid Variables was discovered by American Astronomer, Henrietta Leavitt in 1912.  

The best way to locate this galaxy is to find the relatively bright planetary nebula NGC-6818 as "Barnard's Galaxy" is only 45 arc minutes to the SSE. 

Sources: "Burnham's Celestial Handbooks" by Robert Burnham, Jr., "Observing Variable Star's" by David Levy, "Glossary of Astronomy and Astrophysics", and "Astronomy made Simple". 

Hope some of you are able to observe NGC-6822 this weekend. 

Roger

    The moon and stars to govern the night....   Psalm 136:9

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