Nova And Supernova Searches

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Stellar explosions are important laboratories for understanding the physics of stars. Before they can be studied, they must be found! Since they are relatively short-lived events, finding them requires continual searching; and amateur astronomers have demonstrated the necessary skill and persistence to succeed in this endeavor. By conducting a nova or supernova search in a consistent way you can contribute to the effort to understand these events, in three ways. By discovering a newly-exploded star you notify astronomers so that they can conduct detailed studies. By a record of your "unsuccessful" observations, you can help astronomers determine the statistics of nova and supernova occurrence. After discovery, you can contribute follow-up observations that determine the lightcurve of the event (using the techniques that were described in Chapter 3 for visual observers, or Chapter 4 for CCD imagers).

First things first: both novae and supernovae are rare events. When you're trying to detect a rare event, you're faced with two challenges: maximizing your "probability of detection'', and dealing with the inevitable "false-alarm rate''. It seems simplistic to point out that when an event is rare, you need to look long and hard before you're likely to see it. Nevertheless, it's a fundamental truth about these events. Also, when an event is rare, you're quite likely to see things that look like your target, but turn out not to be. Those things that look like a nova, might be a nova, get your heart rate up, and then turn out to be something much more pedestrian, are called "false alarms''. The trick is to distinguish them as false before you report them. There are far too many false alarms reported. Although such erroneous reports are certainly not the exclusive domain of amateurs, you should anticipate that the Central Bureau for Astronomical Telegrams will be much more leery of an amateur's discovery report than they would be of a comparable report from a well-known and well-regarded professional astronomer or observatory. They will expect you to take great care to confirm that your discovery is what you say it is before you make a report, and they are likely to query you about how you confirmed your discovery before they accept your report [13].

I'll describe specific advice regarding equipment and search methods in the following sections. Your most essential assets, however, are not equipment or procedures. Considering the challenges of low-probability and high false-alarm rate, amateur searches for novae and supernovae need two things above all. These are: (1) persistence, and (2) dedicated team members.

How rare are these events? Rare enough that you can't expect to make a discovery accidentally. Granted, it has happened [14], but it's sort of like winning the lottery—someone did win the big prize, but it probably wasn't you, and it probably wasn't anyone you know. If you want a secure financial future you need to learn a skill, report to your job, and work hard every day. Similarly, if you want to discover a nova or supernova, you need to learn the techniques that are appropriate for your equipment and your site; and you need to examine a great many star fields, consistently and frequently, for a long time. Bob Evans may be a familiar name to you. He discovered 23 supernovae during his visual search campaign. Over a period of eight years, he logged 74,648 observations of galaxies [15]. That averages out to one supernova for every 3,200 galaxy observations! The Puckett Observatory supernovae search team has discovered 100 supernovae (as of November, 2005), among 850,000 images of galaxies [16]. That's 1 supernova per 8,500 images. Looked at in a somewhat different way, a reasonable estimate is that a typical galaxy will host 1 to 3 supernovae per century [17]. Whichever way you look at it, you'll clearly need to examine a great many galaxies for quite a while before you find a supernova.

Novae are not quite as rare as supernovae, but the amateur nova hunter is restricted to searching a single galaxy—our own Milky Way. How often do novae occur in a single galaxy? Halton Arp once conducted a year-long campaign watching for novae in the Andromeda galaxy (M-31). He found 30 during that year [18]. (The brightest reached mag 15.7, so this is probably not a project that amateurs will want to replicate.) Taking that as a reasonable starting point, and considering that when we're looking at our own galaxy we're restricted to examining only the portion not hidden by the glare of daylight, and that a substantial portion of the galaxy is veiled from us by dense dark clouds of dust, we might expect to be able to detect somewhat fewer than 30 novae per year. Payne-Gaposchkin [19] estimated that the Milky Way should display an average of about 24 novae per year brighter than mag 9.

The fact is that we actually discover only 1 to 5 novae per year (down to magnitude 11). So, one suspects that there is fertile ground for the dedicated amateur astronomer to have a reasonable hope of discovering a new nova. Will the current "era of surveys'' make these amateur nova discoveries more difficult? Maybe, but probably not. The surveys are designed to detect faint objects. Even magnitude 11 is a glaringly bright object, too bright for most of them to deal with.

These statistics explain why an above-average level of patience and dedication is needed to be a supernova or nova discoverer. They also are part of the reason that it is useful to have a team, rather than trying to act as the "lone astronomer''. A huge amount of data is going to be gathered and examined. It's likely that some team members are more capable of doing one, or the other. It's also likely that the amateur astronomer has other obligations, in addition to the nova/supernova search. By being part of a team, the observational and data-examination duties can be parsed out so that they are fun, rather than a chore, for the team members. The other reason that it is important to be part of a team is that teamwork is the most effective way to deal with the inevitable high rate of "false alarms''. If you have other discovery-oriented friends, you can call on them for evaluation of any suspected discoveries. This evaluation is likely to require both independent observation of your target and critical examination of your data. These must be done promptly, since any delay risks losing your discovery to someone else. So, before embarking on either of these projects, do check your level of patience and dedication, and do discuss the project with astronomer friends and members of your astronomy club.

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