THE BAA OBSERVERS’ WORKSHOPS

Cambridge Winchester York 2003 February 15 2003 April 26 2003 September 6 Karen Holland provides a summary of her talk at the first Observers’ Workshop in Cambridge.

Why observe variable Workshop No. 1: The Institute of Astronomy, stars? Cambridge 2003 February 15 by Karen Holland

second S And, and so main classes: pulsating variables, eclipsing Introduction on, all the way to Z variables and eruptive variables. Within And. However, it each main group, there are many sub- For many of us, there is much pleasure to was not long before more than nine variable types, and the sub-types are usually be gained from watching a star night after stars were discovered in a particular named after the first star of that sub-type night, seeing its brightness change, and constellation, so he continued, by doubling that was discovered. So, for example, Mira wondering about the reasons for that up the letters to produce a new sequence stars are so called after the first star, Mira, change. This activity becomes even more e.g. RR And, RS And…. down to ZZ. Then that was discovered to have what we now rewarding when professional astronomers when more letters were again required, call Mira-type behaviour (see later section request the data that we have collected, capital letters from the beginning of the on pulsating stars). take it away for analysis, and from the alphabet were used: AA to AZ, BB to BZ, Over the years, fashions change, and data deduce amazing snippets of informa- through to QZ. Then finally in desperation, whereas years ago, meticulously following tion regarding the system that we have when yet more stars needed a designation, the luminosity changes of long period monitored. Often, the brightness estima- he turned to numbers, starting with V335 variables was trendy, nowadays there is tions that we have made on the star, may (as 334 variable stars would already have great interest in cataclysmic variable stars. be the only observations that exist for that been numbered in the constellation using the To see how the number of observations of particular star at that specific time, making letters). This was a wise move, as there are different types of object has changed over them extremely valuable. now thousands of known variable stars in the years, see the Web page of the BAA Many stars are variable; even our own some constellations. Variable Star Section.1 Sun is very slightly variable, but happily it is not variable enough to affect us on Earth to any great extent. We are very fortunate that the Sun doesn’t go through some of Pulsating stars the massive changes experienced by some What are variable of the systems we will look at today! Variable star nomenclature can initially be stars? Why do stars pulsate, and what effect somewhat confusing. By the beginning of does this have on their light curve? the 17th century Bayer had designated the Variable stars are stars whose light output, Pulsating stars are stars that physically brightest stars in each constellation using for some reason, varies. We often plot light expand and contract; they pulsate. This Greek letters, e.g. γ Cas. At this time, lower curves showing the brightness (magnitude) pulsation causes the star to go through a case letters and the first part of the alphabet of the star versus time, to see how the light heating and cooling cycle, which changes had already been used to designate many output is changing. Where there is a regular its luminosity. That’s why we see the properties in astrophysics, but the end of periodicity to the variation, we can specify brightness of the star change; less notice- the alphabet still remained unused. When a period for the star, which is simply the able to us visually is the change in colour Argelander decided to instigate a naming length of time for the star to go through that takes place as the star heats up and convention for variable stars, he didn’t one cycle. The range of variation can be cools down. As the star cools down, the think that there would ever be any need to from barely detectable millimagnitudes to light becomes more red, and as it heats up name more than nine variables in each several whole magnitudes. the light shifts towards the bluer end of the constellation, so he started at the letter R, There are many different types of spectrum. Some stars have different calling the first variable star that was variable star, but for the purpose of this pulsation modes, so that rather than the discovered in a constellation (e.g. introductory talk, these many different whole star expanding and contracting, Andromeda) R Andromedae (R And), the types can be roughly grouped into three different parts of the star are affected, as if a J. Br. Astron. Assoc. 113, 5, 2003 273 Holland: Why observe variable stars? gravitational force of the star wins again, and the star starts to contract. Once again, as it passes the equilibrium position, it overshoots, the star becomes contracted, and the cycle starts all over again. Without the helium ionisation mechanism providing positive feedback, the initial disturbance would have been quickly damped out, but with it, the pulsation is sustained. The period of pulsation of a variable star can provide scientists with a wealth of information about its properties, arising from the fact that the period of pulsation is related to the density of the star. In fact the pulsation can be thought of as being similar to a sound wave travelling into the star and back out of it again. There are many different classes of pulsating star. R And is a typical Mira- type star. Figure 1 shows a light curve for R And constructed from 10056 visual estimates that were made between 1899 and 2000. This is typical of Mira-type stars, which have a light range of greater Figure 1. The light curve for R And, constructed from 10056 visual estimates made between than 2.5 magnitudes in the visual, and 1899 and 2000. BAAVSS archive periods of between 80−1000 days. Less regularly varying pulsating stars wave were running around its surface; these eyes (infrared light). The ionised helium may be classified as semi-regular variables, are called non-radial pulsators, as opposed therefore stops the heat and light from and this group is further divided according to the radial pulsators discussed above. inside the star from escaping as easily as it to how regular their light curves are, with What physically starts off this did previously, and this means that the SRa type stars being the most regular, pulsation is still not well understood, but temperature and pressure rise even more through to SRd type stars which are the scientists do have a fair understanding of rapidly inside the star. When the pressure most irregular. These stars generally have what happens during the pulsation cycle. becomes greater than the gravitational force smaller amplitudes than Miras, and there is Inside stars, the gravitational field due to holding it all together, then the star has to also a difference in the spectral classes of the mass of the star acts to hold the star expand. But when the star reaches what the various types of stars. material together, but the pressure that is was once the equilibrium position, it There are many other pulsating star exerted by this same hot gas, tries to overshoots, and keeps on going. As it does types too numerous to list here. L type make the star expand. In a star that is so, the pressure drops, the gas cools, and stars are slow, irregularly-varying stars, non-variable, these two quantities are the helium ions recombine to become and it may be that many stars have been perfectly balanced, so the star remains a whole atoms again; in this state, they placed in this category simply because constant size. But what if something become much more transparent to the heat they have not been well enough studied to happens to upset this balance a little? that is inside the star trying to escape, so place them more appropriately. Figure 2 Imagine that, for whatever reason, the star the star cools down even more rapidly. shows a light curve for BI Cyg, a slow is compressed a little and therefore This means that the pressure of the gas irregular variable supergiant of late spectral becomes slightly smaller; the pressure and drops sharply, and it isn’t long until the class (LC class variability). For an up-to- temperature inside the star would increase. In stars that do not have a predisposition to become variable, the star would quickly return to its equilib- rium position by expanding again. But in variable stars, there exists a layer of helium atoms at a crucial position in the star’s structure, that act as a positive feedback mechanism, and instead of any disturbance being quickly damped out, this mechanism keeps the pulsation going. So in such a variable star that has become a little contracted through some disturbance, as the temperature rises, the helium atoms in the star’s outer layers become ionised. Ionised helium is a bit like the sunblock that we put on to stop the sun from getting to our skin and burning it − it stops light and heat from passing through it; remember that heat is simply a Figure 2. The light curve for BI Cyg, a slow irregular variable supergiant of late spectral class colour of light that we cannot see with our (LC class variability). BAAVSS archive 274 J. Br. Astron. Assoc. 113, 5, 2003 Holland: Why observe variable stars? date reference section that lists all the many different types of pulsating variable stars, together with their respective subtypes, see Gerry Good’s book Observing Variable Stars.2

Eclipsing binary stars

Eclipsing binary stars are binary systems which are inclined at an angle to the Earth such that periodically one of the stars eclipses the other star, blocking out a fraction (or all) of the light that we would have received from it. Then, when the system has progressed through half an orbit the stars may again eclipse each other, with their roles reversed. This only occurs if the system is at the correct angle of inclination to us on Earth, but when it does take place it allows us to learn a great deal more about the system properties3 than would be the case for a single star. A visually-observed eclipse of RZ Cas is Figure 4. A light curve for SS Cyg. BAAVSS archive shown in Figure 3. There are several subgroups of eclipsing than one day, and it is thought that the two disc flips between a dimmer, cooler state, binaries and many stars of other types are stars in such systems are likely to be in to a hotter, brighter state, which we also eclipsing; for example, cataclysmic contact, with mass transfer taking place. observe as a sudden increase in the variables, which are classed as eruptive brightness of the system. After a period of variables, may also be eclipsing. Eclipsing time, the disc returns to its quiescent state. binary systems are classified largely The reason that there is such interest in according to the form of their light curves, Eruptive stars these objects at this time is that discs form which may differ depending on whether in many astrophysical situations: they the two stars in the system are detached exist around black holes, and in star from each other, are semi-detached, or are Eruptive stars are by far the most formation regions; planets form from in contact. EA (Algol) type eclipsing disparate group of objects. Within this protoplanetary discs. Dwarf novae, in binary systems are systems that are group are a host of categories of stars particular, are so interesting because the detached, producing a characteristic light that vary for very different reasons. timescales involved in the change in state curve, in which the start and end of Incorporated in this group are classes between the dim state to the hot state are eclipses are well-defined. EB (Beta Lyrae) such as Wolf−Rayet stars, R Corona relatively short, ranging from just a few type binaries exhibit light curves that vary Borealis stars, symbiotic stars, X-ray days, to a few years or more. This means more continuously throughout eclipses, Binaries, supernovae, T Tauri stars, flare that individual observers have an opportu- due to the ellipsoidal shape of one or both stars, gamma-ray bursters, pulsars, S nity to study these systems in both states, of the components. W UMa type systems Doradus type variable supergiants, and particularly during the transitions from have extremely short orbital periods of less Gamma Cas type stars, variable plan- one state to another. etary nebulae… the list is seemingly An example of such a dwarf nova is SS endless. Discussion of these many types Cygni. A light curve for SS Cyg is shown of eruptive star is beyond the scope of in Figure 4. In SS Cyg the two components this talk, and for a description of each of orbit around each other once every 6 hours these types, again, consult Gerry Good’s or so, and you can see that the disc goes book.2 There is however one group of into outburst every month or two. It is objects within this class, with its own also nice and bright at around magnitude set of subtypes, that deserves particular 12 in quiescence, and in outburst it almost mention, as it is one of the most widely becomes a binocular object. studied groups of stars in current times. Another interesting object of this type These are the cataclysmic variable stars, is RS Ophuichi, which is classed as a which include dwarf novae, novae and recurrent nova. A light curve for this object recurrent objects. is shown in Figure 5. Outbursts have been Cataclysmic variables are binary detected for this system in 1933, 1958, systems in which matter is transferred 1967 and 1985, so another outburst is Figure 3. The light curve of an eclipse of RZ from one star (the secondary star) to the expected any time now. Cas. This is based on observations of several other (the primary); the transferred matter Recurrent novae are an unusual class of eclipses observed and estimated by Des forms an accretion disc of material around eruptive star. They are a subclass of the Loughney in February to March 2003. the primary star. Periodically, the accretion cataclysmic variable stars: dwarf novae J. Br. Astron. Assoc. 113, 5, 2003 275 Holland: Why observe variable stars? How do we make visual observations of variable stars?

You have decided that you’d like to have a go at observing some variable stars. How do you get started?

Paper documentation available from the VSS

• The Guide to Making Visual Figure 5. The light curve for RS Oph. BAAVSS archive Observations; this is currently in the process of being updated and will be available shortly exhibit outbursts perhaps every few Variable stars need to be observed over • months to a few years, and their outbursts decades to determine their long-term Observation report forms are available, are of the order of 2−6 magnitudes; behaviour; some variables have periods of but the sending of observations by e- classical novae exhibit bigger outbursts (8− many years. Traditionally, professionals mail is strongly encouraged (see section 15 magnitudes) that have only been seen have not had the time, or the telescope later), to reduce the need for manual once; recurrent objects form a class access, to monitor stars over such long entry of paper reports between these two. Although this may periods. If a star does begin to exhibit • A Chart Catalogue is available which sound entirely reasonable, current theories unusual behaviour, we can alert profes- lists all the charts that are available from suggest that dwarf nova-type outbursts are sionals to changes, possibly resulting in the Section different in nature to nova-type outbursts, time allocation on world-class telescopes. • Charts and sample charts are available so to have systems (and there are very few Professionals often request that for binoculars, smaller telescopes and confirmed members of this class) that correlated data is obtained from ground- larger telescopes. These are available exhibit behaviour in between the two is based optical observations at the same from the Chart Secretary (Eclipsing exciting, and a great opportunity for time as their observations are made at binary charts from the Eclipsing Binary learning. Dwarf nova outbursts are thought other wavelengths. Secretary). Charts that have recently to be due to the accretion disc flipping into Determination of periods can lead to a been revised are also included in the a hotter state. Nova outbursts, on the better understanding of the pulsation Variable Star Circulars. Sample charts other hand are thought to occur when mechanism of stars. Albert Zijlstra and are provided if you are new to observing enough of the disc material has been Tim Bedding are doing a great deal of work variable stars and just want a taster accreted on to the white dwarf’s surface; in this area, and made a specific request pack! Charts are also freely available this leads to an increase in the gravitational recently to the Variable Star Section that from the VSS Web page. forces exerted, and the base of the material we monitor particular bright stars in an becomes compressed and hot, and triggers effort to determine their, as yet, unknown a thermonuclear reaction, resulting in a periods, so that they can add further points to their period−luminosity relation- Documentation available on the VSS shell of material being ejected from the 11 white dwarf’s surface. So what happens in ship graph, and further their research.6 Web page recurrent objects? This is why we need to O−C diagrams (Observed−Calculated There is a vast resource of additional observe these objects well. diagrams)7 can tell us about systems that information available on the Web page; For more on cataclysmic variables, see are changing. These diagrams chart the this is an invaluable source of up-to-date Coel Hellier’s book Cataclysmic Variable differences between the expected time of material, as its contents can be so Stars,4 and also Brian Warner’s definitive minimum of a system, and the actual time quickly updated. I summarise this professional-level volume of the same name.5 of minimum that is observed in a new observation. Often the period of a system information below for those who may changes, and much can be learnt about the not have easy internet access, as all of it system by monitoring this change. can be provided by the officers of the So, why observe Observation of previously unclassified Section on request. variable stars? systems can throw up surprises, and even • Details of the telescopic, binocular, result in new classes of variable star. For recurrent object, eclipsing dwarf nova, many years now, Mike Collins has been CCD and eclipsing binary programmes discovering new variable stars, about There are many reasons why the collection • A complete list of all objects for which which nothing is yet known.8,9 of data on variable stars is an important there is data in the database, with links Eclipse monitoring can teach us a great and useful activity. to files including details of period, type, Research on these stars provides a deal about binary systems.3 magnitude range, sequence, total number wealth of information about stellar Data is sometimes needed to test of observations made so far, links to properties, such as mass, radius, luminos- professionals’ theoretical models of articles on the stars, details of charts and ity, temperature, internal and external some systems. This is the case for the lightcurves to view structure, composition, and evolution. SS Cyg project that is part of the BAA This information can also be used to Variable Star Section’s (hereafter VSS) • PDF versions of past VSS Circulars understand other stars. CCD Target List.10 (including many finder charts) 276 J. Br. Astron. Assoc. 113, 5, 2003 Holland: Why observe variable stars? • Predictions of the time of minimum for eclipsing binary stars for the next three months • A list of Mike Collins’ discoveries (TAV and TASV stars) which need further observation • Details of special observing campaigns, in which professionals have asked us to monitor a particular object at a certain time • The most up-to-date charts are freely available on the Web page for download.

The Variable Star Section mentoring scheme The mentoring scheme was set up recently,12 in order to provide assistance and encouragement to those who are new to variable star observing. Under this scheme, beginners can be allocated a mentor, who will act as a friendly adviser throughout the early stages of learning. A number of experienced observers have offered to act as mentors; some will communicate by e-mail only, and some are willing to assist by sharing observing sessions with their observing partner, as well as being in contact by e-mail. In either case, an effort will be made to link students with a mentor who is as local to them as possible. It is intended that a mentor will provide support with all those little difficulties and Figure 6. The BAA ‘C’ chart for RX And. questions that arise during the early stages of a variable star observing career. Most of Choice of equipment Telescope these problems can be solved without A telescope offers the opportunity to assistance, of course, but they are often Naked eye observe a different set of objects, but also answered far more quickly by an experi- Whilst the VSS doesn’t have a formal makes life a little more complicated as you enced observer, who can pass on useful naked eye programme, there are a few need to start to consider what is the upper techniques and know-how. Even more stars in the database with observations magnitude limit to which you should importantly, a common fear amongst that are bright enough to be observed estimate. Each target star has its own ideal beginners is that they may not be estimat- by the naked eye, like Epsilon Aurigae. instrument combination (telescope and ing the star’s brightness correctly; this can Unfortunately, this varies by a fairly eyepiece). The general rule is only to use often stop an observer from contributing small amount (about magnitude 2.8 to enough power and magnification so that the results to the Section’s valuable 3.5), so it is not ideal for observers you can see the variable clearly, but not so archive, and in doing so, both robs a future new to this field. This is an eclipsing much that it is so bright that it is difficult to analyst of the data, and deprives the binary, that has a period of nearly estimate. In an ideal world, the target star observer from contributing to a potentially 9000 days! Gamma Cas is another should be a couple of magnitudes brighter important project. interesting star, but it too, only varies than the faintest star that you can see with One key way in which it is envisaged from 1.78 to 2.6; this is the prototype your telescope. Of course, in practice we do that a mentor might assist, is to observe a star of its class. try to estimate stars that are near the limit few of the same stars as the student, to of what we can see, out of necessity, but enable a comparison to be made between try to follow the above rule when possible. Binoculars the mentor’s estimation and the student’s You will find that there are some stars that estimation. In this way, the new observer Binoculars are good for observing stars in the 4 to 8 magnitude range if hand-held, at minimum you will barely see with a will start to gain confidence in the results, 300mm telescope, but at maximum you can and is more likely to submit them to the and down to magnitude 9 if mounted securely. With a pair of binoculars the observe with binoculars! Section’s archive; any problems encoun- Effective approximate magnitude limits tered in making the estimation will be whole binocular programme opens up to you. Many observers find that stars are for some common telescope sizes are picked up and corrected at an early stage. listed below: A map showing the locations of the easier to find with binoculars than with a mentors can be seen on the Web page,13 or telescope, and they are therefore a very Aperture (mm) 60 150 200 300 a copy obtained from one of the officers of good way to start practising variable star Brightest (mag) 6.4 8.5 9.0 10.0 the Section. observing. Faintest (mag) 11.0 13.0 13.5 14.5 J. Br. Astron. Assoc. 113, 5, 2003 277 Holland: Why observe variable stars? Use an eyepiece that gives the widest provided on the Web pages may prove Because there is a real art to good chart available field of view, so that you can see valuable in this respect, particularly the generation, it is vitally important that you as many of the comparison stars as archive statistics which demonstrate use BAA charts wherever they exist. possible in the same field of view as the which stars have been the most widely Comparison stars are chosen taking many target star. observed to date. factors into consideration. Clearly it is You will also need a notebook, a pen or • If you have a large telescope, then you important that as far as possible they are pencil, a loose-leaf binder for holding should concentrate on observing stars known to be non-variable, but good charts, a watch for noting the time, a dim that are relatively faint. comparison stars are also selected by their red light and warm clothes! colour, and a star that is a good comparison Once you have all this information, consult star for one variable, may not necessarily the list of stars for each of the VSS make a good choice as a comparison star for Compiling your personal observing programmes: telescopic, binocular, a different variable star. The VSS Chart list, and maximising the value of recurrent objects, eclipsing binary, CCD, Secretary, assisted by many other people, Mike Collins discoveries (TAV and your observations has built a wealth of experience in this field, TASV), and choose your targets! and is now working with international Once you have selected your equipment, variable star organisations to integrate the you are ready to choose some stars that combined knowledge and expertise of all you will begin to observe. In theory, you Charts concerned, and to standardise as many can choose to observe any stars you like; sequences internationally as possible. the VSS accepts observations of any star You will need to obtain a chart for every into its archive. However, because there star that you want to observe, although some charts cover more than one star. are only a limited number of observers, the Estimating magnitudes by the VSS selects a number of stars for its For some stars there may be an A, B and various programmes. These programmes a C chart, as is the case for RX And. The fractional method are a kind of preferred observing list, that A chart, in this case, covers an area of 9° if we all were to concentrate our efforts on, squared; the B chart covers 1° squared, Imagine that for the image shown in Figure would allow sufficient data to be obtained and the C chart (Figure 6) covers 20 7, with the stars labelled as shown, we are for a sensible analysis of the data to be arcminutes squared. I like to draw a given the following sequence: A=4.3, made. Each programme is updated circle on the chart that I will be using at B=5.2, C=6.3, D=8.2. regularly, at which time stars are added or the telescope, that represents my field of In order to estimate the brightness of removed from the list. In practice it makes view through the eyepiece so that I our variable star, marked V on the image, sense initially to choose stars that are on know exactly which stars should fit into then we need to compare the brightness of one of the VSS programmes; in addition to my view. the variable (V) with the brightness of the adding your data to a valuable database On the chart is also printed the sequence. comparison stars. If we do this with each along with that of other observers, you This is a list of stars in the field that are of the stars in turn, you can see from the will be able to compare the results of your given letters as identification, and their image that it is clear that star A is much estimations with those of others. magnitudes are listed. These stars, which brighter than the variable, V; star B is a bit As a first step to compiling your list are called comparison stars, have been brighter; star C is slightly fainter than V; establish the following: chosen to provide a graduated brightness and star D is much fainter. In this case, we sequence that you can use to help you to would choose to use stars B and C to • What is your possible magnitude range measure the brightness of the variable star compare the variable against, because one as dictated by your equipment? at the time that you are observing. You is a little brighter, and one is a little fainter • What type of stars are you interested in, might need to refer to a star atlas or chart than the variable. or do you want to sample many types? software to find the area covered by the You then need to look alternately at • How often do you want to observe, chart initially. On the charts of widest sky the stars, B, V and C, and try to deter- weather permitting? A short period coverage, there will usually be at least one mine the ratio of the brightness differ- eclipsing system will need intensive star identified that you can locate on your ences. I like to imagine the brightness monitoring throughout a single night; star chart or using star chart software. scale as a line drawn on paper, in this irregular stars and those that erupt unpredictably may need observing every clear night; long period stars will probably need observing less frequently. You should plan to observe a star no more frequently than is necessary to show its most rapid variations. • What area of the sky is available to you? Do you have restricted views in any direction? • What time do you prefer to observe? The most poorly-observed stars are those that are only visible in the later hours of the night, just before dawn, and observations of these stars are particularly valuable. Figure 8. Imagine the comparison stars’ • Try to select stars that are not already Figure 7. An image for the demonstration of brightnessess on a scale, and try to position grossly over-observed. The information the fractional method of estimation. the variable star some way between the two. 278 J. Br. Astron. Assoc. 113, 5, 2003 Holland: Why observe variable stars? case with star C’s brightness as the faint Negative observations be able to do anything about some of end of the line, and star B’s brightness as these, it is as well to be aware of them: the bright end of the line. Remember that if you cannot see the variable • The Purkinje effect is a tendency for Then you need to try to decide where star on a particular night, it is important to observers to estimate red stars at brighter on the line, the brightness of V should be make, and report, a negative observation. You magnitudes than bluer stars of the same positioned. I believe the trick here is to do this by recording the faintest comparison brightness; try using quick glances instead try not to think too hard about this, or star that you can see, and recording that the of long stares, so that the red light has less analyse what you’re doing too much; just variable star is fainter than that star: e.g. opportunity to build up on the retina; try make a decision based on your instinct. V < F, where the magnitude of the variable is defocusing the stars so that they are fuzzy Humans can do this surprisingly well. less than that of comparison star F. This balls of light instead of sharply focused The main thing is to find out how this provides an important upper bound on the points; try using averted vision, as the edge process best works for you, and then brightness of the star at the time of observa- of the retina is more sensitive to differences stick to it. Then you need to translate tion, so that if the star is seen at a much in light. Whatever you do, don’t try to what you have drawn into words that will brighter magnitude the following night, in correct your estimations, just make the record your observation. If you think that outburst, more precise limits on the time of best estimate that you can. the variable star brightness is one third onset of outburst can be calculated using • nearer to C than to B then write your negative observation, together with any Moonlight affects observations, as it is B(2)V(1)C; if you think that the vari- others that were made. scattered like sunlight to give the able’s brightness is exactly mid-placed impression of a bluer sky, making red stars between B and C write B(1)V(1)C; and if look brighter. If possible try to avoid you think that V is the same brightness as Minimising errors estimating red stars during moonlit nights, C then just write V=C. if not, just do your best! Now we need to convert this record There are a number of important factors that • Light pollution may affect estimations; into a magnitude. Take the estimate you should remember, in order to minimise do not use light pollution or other filters, B(2)V(1)C: V is one third nearer to C the number of errors that are made: as these filters change the relative than B, so we need to work out the • Allow 15–20 minutes for your eyes to brightnesses of the stars depending on their difference between B and C, and split become fully dark adapted before observing. colours. Just make a note on the this into thirds. The difference between This is important because the spectral observation report form, of the fact that B and C is 6.3 − 5.2 = 1.1. One third of response of your eye when fully lit, is very light pollution may have affected your this is 0.37. So V is estimated to be 6.3− different to that when dark, so that estimation. 0.37=5.93. Instead, we could have estimating a variable that has a different • Low altitude affects different colours of worked this out by adding two thirds of colour to the comparison star that you are light differently, and so may affect your the difference between B and C from the using whilst your eye is not dark-adapted, variable and comparison differently if they brightness of B [5.2 + (2 × 0.37) ] = may result in a different estimated value to are not exactly the same colour. Try to 5.94, and we would have got the same that which you would obtain once the eye observe stars at as high an altitude as answer. Magnitudes should be rounded is fully dark-adapted. possible to avoid low altitude effects, and off to the nearest whole fraction. • Take care to correctly identify the variable plan your observing session so that as and comparison stars; estimating the objects come to the zenith they are wrong star is one of the commonest errors observed. The Pogson method that is made in variable star observation. • High cirrus clouds can effect your • Take care to record observations correctly. estimation without you being aware that This is a different method of estimation in they are there; try to be careful and be • Try to avoid plotting light curves or which you compare the variable against only aware of such problems. one comparison star, and try to judge the learning of others’ estimations of the same brightness difference between the variable star on the night you go out to observe, as this may bias your results. and just one comparison star in steps of Recording observations tenths of a magnitude. It is an essential • Avoid observing when tired or ill, or after technique for occasions when the variable a few pints! It is important to make a note of your star has become fainter than the faintest star • Make multiple estimations using different observation whilst at the telescope, together that is on its sequence, for example. sets of comparison stars if you are keen to with all pertinent information. Figure 9 So if the variable were judged to be two be very precise, and take the mean of the shows a copy of an entry in my log book. It tenths of a magnitude fainter than a result. This is particularly useful when gives an idea of the bare minimum of comparison star A, for example, then we using the Pogson method. information that you should note down at − would write this down as A 2. If the • Avoid making estimations at low altitude, the telescope. Remember, also, to keep a variable was one tenth of a magnitude or through cloud or haze. note of which chart you are using for each brighter than B, we would write B+1. • star, and to note if your observing location Again, we can write =B, if we think the Bring each star, variable and comparison changes for any reason. You should add any variable has the same brightness as to the centre of the field of view to comments that you think might be relevant comparison star B. minimise errors that depend upon the such as the method that you have used to If you find it difficult to judge tenths of star’s position in the field of view. make the estimation, noting if you have a magnitude steps, try using a variation on used averted vision or defocused to make this method called the Herschel− the estimation, together with any notes Argelander method, which is very similar, Factors that affect your estimation relating to bad light pollution, haze, mist or but instead of using steps of 0.1 magni- a bright moon. You should also try to judge tude, you use the smallest step that you There are many factors that may affect what class you would give to the estimation can confidently detect instead. your estimation, and whilst you may not that you have made, labelling it as a class 1 J. Br. Astron. Assoc. 113, 5, 2003 279 Holland: Why observe variable stars? observation if it is of the watch stars that are best quality that you can millions of miles away achieve, class 2 for a changing, and to learn mediocre estimation, and about these systems from class 3 for a poorer data that is obtained using attempt. your own eyes. Your data, I would suggest, when combined with other therefore, that the observers’ data can be minimum to note in your valuable and unique, and log book would be the can make a real contribu- time and date of observa- tion to science. tion, star name, magnitude Finally, you can hope to estimate, the class of make many good friends all observation, the instru- over the world through ment used, and any variable star observing. Some comments relating to the years ago, I was fortunate to observation. Then visit a Japanese observatory additionally, you will want for a tour with a keen VS to keep track of the chart observer. After a short while used, and have somewhere of struggling, as I did not to note the reduced Figure 9. A copy of a page of the author’s log book. speak any Japanese, and my magnitude. host had very little English, I happened to mention a star Sources of more information that I was interested in by name − HT Cas. Submitting observations My host’s eye’s lit up, and he became very The VSS Circular is a quarterly publica- animated; he almost seemed to jump up and You then need to convert your log into a tion which is the newsletter of the down a little as he excitedly repeated ‘HT record to e-mail to the Computer Secretary Variable Star Section. It is now also Cas, yes, HT Cas!’ And with the aid of a (or send it on a disk), so that your available in electronic form, as a PDF pen and paper, we finally managed to start information can be added to the VSS file, saving postage costs for our many to communicate a little, comparing notes on archive. You can still send records on overseas members. PDF versions of the our favourite stars. paper if you do not have access to a Circulars are made available on the VSS computer, although there are still thou- Web pages approximately six months Address: 136 Northampton Lane North, sands of paper observations awaiting after they have been sent to subscribers. Moulton, Northampton NN3 7QW. typing into the archive, so your results Circulars include vital information on [[email protected]] will reach the archive more quickly if you Section activities such as news about can persuade a friend to help you out here! upcoming meetings (as well as summaries Before sending your first set of data, of meetings for those who were unable to you should contact the Computer Secre- attend), Pro-Am observing campaigns, References tary who will advise you of the format to and schemes such as the mentoring be used. He can supply an Excel scheme and CCD Photometry pro- spreadsheet which greatly speeds up data 1 http://www.britastro.org/vss/cmpyears. grammes amongst others. It is also a html entry if you have the necessary equip- useful medium in which members can ment, but there is also a standard format 2 Good G. A., Observing Variable Stars, share their experiences with others. There Springer−Verlag, 2003 for manual typing if required. have been articles recounting complete 3 Bohm−Vitense E., Stellar Astrophysics, beginners’ introductory experiences; Ch.9, Cambridge University Press, 1989 descriptions of experiments attempting to 4 Hellier C., Cataclysmic Variable Stars: How The data archive use standard digital cameras at the and Why they Vary, Springer−Verlag, 2001 eyepiece for photometry, through to 5 Warner B., Cataclysmic Variable Stars, The VSS archive contains over 1½ million articles about spectroscopy and precision Cambridge University Press, 1995 6 Zijlstra A. A. & Bedding T. R., ‘Periods for visual observations of over a thousand photometry. There is an occasional letters variable stars, with many more observa- semi-regular variables (a CCD target list page, and regular ‘Observer Profiles’ project)’, BAA Variable Star Section tions on paper still to be entered. There are articles recounting how members got over thirty stars that have more than ten Circular 115, p.5 (2003 March) started in their field, as well as regular 7 Cooper W. A. & Walker E. N., Getting the thousand observations each. All this data news on charts, recurrent objects, and Measure of the Stars, p.155 has been collected by over 940 observers topical thoughts From the Director. 8 Jones C. P., ‘Foster carers wanted for to date. You can view these statistics, and celestian orphans’, BAA VSS Circular 107, see how the type of object being observed p.9 (2001 March) has changed over the years by looking at 9 Hurst G., ‘Mike Collins’ discoveries’, BAA the VSS Web pages. VSS Circular 99, p.11 (1999 March) Both amateurs and professionals find In conclusion… Why 10 http://www.britastro.org/vss/variable_ this data extremely useful. In many cases, star_section_ccd_target.htm these observations are the only measure- 11 http://www.britastro.org/vss observe variable stars? 12 Holland K., ‘A new mentoring scheme for ments that exist of a given star at a the Variable Star Section’, J. Brit. Astron. particular time, and the data is frequently Assoc., 113(2), 115 (2003) used for analysis, resulting in publication There are so many reasons to observe 13 http://www.britastro.org/vss/mentormap. in professional journals or in the JBAA. variable stars. It is simply fascinating to htm 280 J. Br. Astron. Assoc. 113, 5, 2003