When one starts observing the heaven's, the stars initially appear at a constant brightness but closer inspections do reveal that some of the stars vary in brightness over time. The study of variable stars is subject that is well suited to the amateur astronomer and you can produce real scientific data for professional astronomers to use. Unlike professional astronomers, amateurs can observe as and when they want to and are only limited by the weather conditions. Professionals on the other hand have only limited access to telescopes at professional observatories and even then, they have to “book” telescope time in advance. Data collected by amateurs are normally pooled into data bases held by various bodies such as the Variable Star Section of the British Astronomical Association (BAAVSS) and the Association of American Variable Star Observers (AAVSO).

 The first periodic variable star discovered was Mira in the constellation of Cetus the whale in  1596 by David Fabricius, although its actual period was only determined in 1638 by Holwarda.  The number of variable star discoveries was slow in the early years and by the time William  Herschel discovered the planet Uranus in 1781, there where only 6 known variable stars. By the  year 1844, the total stood at 18 and gradually increased to 393 by 1896. Progress in variable  star detection was greatly advanced with photography and during the 20^th Century, the number  of variable grew much faster; 4000 by 1912, 22 650 by 1970 and 28 450 by 1983.

 There are many variable stars that can be observed with the naked eye, binoculars or telescope.  Obviously, the larger the instrument you use, the more variable stars there are. Also, the use of  CCD cameras will bring in a vast number of variables that only change with small amplitudes  that are undetectable to the human eye. The use of CCD cameras and determining the stars  brightness is a specialist operation and is for advance observers only. On this page, I will only  describe visual star observation.

How to make a variable star observation.

The following notes explain how to make an observation of how bright a particular variable star is, whether using your naked eye, binocular or telescope.

It is very important to ensure that your eyes have become dark adapted before you make the actual observation of the variable stars comparative brightness. To achieve this, you need to ensure that you are stay out of view of any lighting for a good half hour. If you need to use a star chart to identify the stars, then use a very dim flash light that is covered with a red filter. Once you have identified the variable star and the comparison stars, you are then in a position to estimate how bright the variable star is.

The graphic above left shows the location of the variable star R Corona Borealis which is abbreviated as R CrB. This star is normally just visible to the naked eye but is best seen in a small pair of binoculars such as 7x50 type. Every now and then, with no real set time scale, it will be seen to fade from about +6.0 to +15.0 and it will then require a large telescope to see. One theory as to why this star fades is a build of carbon dust particles around the star. The carbon density becomes so great as to block off most of the visible light from the star. Over time, the carbon is burnt off and the star regains its normal brightness.

Using the left graphic, you can use the constellation BoÖtes to find the smaller Corona Borealis. This is a near full ring of stars that make up the crown shape asterim. R CrB is marked with a red dot. The blue box on the graphic shows the field of stars that are drawn on the star chart right. This star chart is used to determine the magnitude (brightness) of R CrB and in called the “Sequence” chart. All observers will be using this same chart (if they are contributing observations to the BAAVSS) so as to give a common standard for all observations being made by different observers. This chart has a number on the top left corner “041.03” and this is the sequence number that must be quoted in you observation report. The stars that have fixed and known magnitudes are identified with Greek letters and a table at the bottom of the chart details what magnitude the specific stars are. These stars are the “comparison stars” that will be compared with the variable stars brightness. R CrB is identified with the letter “R” and symbol “?”. Other data on this charts shows the north/south direction and its position on the night sky with reference to Right Ascension and Declination. Details on where the chart was drawn from, who made the drawing and when drawn are also detailed. If this chart is ever changed, it will be re numbered as 041.04

To make the observation, look at R CrB and note its brightness. Then locate one of the comparison stars and determine if R CrB is brighter or fainter than the comparison star. You may find that R CrB is fainter than star marked “C” but brighter than star “D” marked on the sequence chart. With more concentration, you determine that it is halfway between the two comparison stars with regards its brightness. So with a bit of maths, you can work out how bright R CrB is at this particular observation. Using the table at the bottom of the chart, star C = 5.8 and star D = 6.2. So the magnitude of R CrB = 5.8+6.2/2 = 6.0

Make a formal note on this magnitude estimation by writing your observation as C(1)V(1)D. This shows that you have used stars C and D as comparison stars and the number in brackets represent the fractional difference the variable (V) is between these two stars. As the variable is half way between C and D, the numbers in the brackets are both 1. Your magnitude estimate written with the comparison star magnitudes is
5.8(1)V(1)6.2 and another way to calculate the magnitude of the variable is (1x5.8)+(1x6.2)/2 = 6.0 But always write the magnitude estimate using the letters as written on the sequence chart. This is to ensure at a later date what stars you actually used. Along with the magnitude estimate, remember to record the date and time. (normally to the nearest minute), the instrument you used and any local weather and other distractions that may affect your observation. (usually bright moon close by).

The observation may be recorded thus:-

Identification of the variable star. e.g. R CrB
Date and Time. e.g. 21.13h UT on 4^th April 2006
Instrument Used. e.g. 7x50B (B=Binoculars)
Magnitude Estimate. e.g. C(1)V(1)D
Deduced Magnitude. e.g. +6.0
Sequence Used. e.g. 041.03

The above observation has R CrB at magnitude +6.0 and as previously mentioned, this is the normal magnitude of this star but it is still a very important observation that is of value. You may observe this star on a daily basis (weather permitting) for months or years and it will appear to not change much in brightness, always staying at around +6.0. One day though, you will note a definite drop in its brightness and the determination of the stars new brightness may a bit more difficult to determine.

On a particular day, you note that R CrB is much fainter than normal and you have to use a star chart of the same sequence but with a smaller field of view that shows fainter comparison stars. The chart above shows stars between magnitude +7.1 and +9.5. As before, look at R CrB and compare its brightness to some of the comparison stars. You find that on this observation, R CrB is brighter than star “G” but slightly fainter than “HH”. The tricky part now is to estimate how much fainter R CrB is compared to “HH”. Looking alternatively between stars “G”, R CrB and “HH”, you may come to agree that R CrB is about ¼ fainter from “HH” than towards star”G”. Put another way, R CrB is ¾ brighter away from “G” towards star “HH”.
Record this magnitude estimation as HH(1)V(3)G. Putting the numbers in of the comparison stars gives 7.1(1)V(3)7.4 and this works out at
R CrB = (3x7.1)+(1x7.4)/4 = 7.175, which rounds to +7.2 (Record any magnitudes calculated to 0.1 accuracy only).

The above observation is recorded thus:-

Identification of the variable star. e.g. R CrB
Date and Time. e.g. 22.35h UT on 24^th July 2006
Instrument Used. e.g. 20x80B (B=Binoculars)
Magnitude Estimate. e.g. HH(1)V(3)G
Deduced Magnitude. e.g. +7.2
Sequence Used. e.g. 041.03

Keep a log book to record your observations. Preferably, use a bound notebook so as you can record in the front page details of who compiled the observations and other details of observatory location. A typical page from a log book is shown right.                                                                                               Click on the image for a larger view.

There are a few naked eye stars that can be easily watched over a period of time. As mentioned above, Mira does rise from an invisible magnitude +9 to magnitude +3 (or +2 in rare cases) in a period of about 330 days. Normally, it will be invisible to the naked eye for about 200 days and then comes into naked eye visibility for 130 days. The maximum brightness being attained half way through the 130 days. Each time the star reaches peak brightness, it doesn't attain the exact same brightness as the previous peak, but will be close to an average value of +3. It is also note worthy that during peak brightness, the star does take on a red hue as seen through a pair of binoculars. Note that this star is the prototype star that gives its name to many other stars showing similar characteristics. Such stars are know as Mira Type Variables.

 Another star that is suitable for observation without optical aid is the central star in the  constellation Cassiopeia. Gamma Cassiopeiae has a small fluctuation in magnitude that  amounts to 0.6 over an irregular period. Normally, this star shines at around +2.2 but can  flare up to +1.6 which occurred in 1937. By 1940, the star gradually faded until it dropped  below +3.0. It has since recovered to its present state of around +2.2. It is worth  watching this star every night as it is unknown when this star will flare in brightness.

 A star that is good to observe with the naked eye and through a pair of binoculars as it  fades to very faint magnitudes that require a telescope is Chi Cygni. This is a Mira Type  Variable and changes in magnitude from +3.3 to +14.2 in a period of 409 days. At  maximum, the star normally shines between +4 and +5 but can get as bright as +2.0 on  rare occasions. The star also takes on a reddish tinge when at maximum which becomes  readily visible in binoculars. Due to the large change in brightness, there are numerous  sequence charts required. One will be suited for naked eye observations when the star is  at maximum. As it fades, then binoculars will be needed and a sequence chart showing  the faintest stars that your binoculars can see will be needed. Finally, when it fades to  become invisible in binoculars, a sequence chart that shows stars down to magnitude  +15.0 will be needed, showing the very faint stars that a large telescope will reveal.

 Click on the images below for larger views of The American Association of Variable Star   Observer's sequence charts.

Naked eye sequence chart.        Binocular sequence chart.          Telescope sequence chart.          Telescope sequence chart.

Bibliography:

Norton's 2000.0 Star Atlas and Handbook. Edited by Ian Ridpath. Longman Scientific & Technical. 1989
The Variable Star Observer's Handbook. John Glasby. Sidgwick & Jackson. 1971
Guide to the Stars. Patrick Moore. Eyre & Spottiswoode. 1960
AAVSO Manual for Visual Observing of Variable Stars. AAVSO. January 2001.
The Observer's Year. Patrick Moore. Springer. 1998
Gamma Cassiopeiae. Variable Star of the Season. http://www.aavso.org/vstar/vsots/1001.shtml

BAA Variable Star Section.
The American Association of Variable Star Observers.
CVnet (Cataclysmic Variable Stars)

Thanks to Roger Pickard, President of the British Astronomical Association and Director of the BAA Variable Star Section (BAAVSS)
and Arne Henden, Director of The American Association of Variable Star Observers for permission to use their variable star sequence charts.

Guide 8 software used to produce graphics. Visit Project Pluto for more info on this great software.

Page Updated 9^th November 2008