A SEARCH FOR NOVAE AND SUPERNOVAE
by P. Clay Sherrod
Arkansas Sky Observatory
(modified as first published in: A Complete Manual of Amateur Astronomy, P. Clay Sherrod, 1981, Prentice-Hall Press)
The thrill of discovery in astronomy is second to no other experience, and it is why the comet searcher religiously scans the skies in the early mornings of the year, spending hundreds or thousands of hours peering at the same regions of sky. All the time is well spent, but not forgotten, at the moment a new object is found. To help in understanding the physics involved in the evolution of stars, no event is as important as the explosion of a star - the nova or supernova. These phenomena can occur instantaneously, and valuable records can be lost forever if the beginning of the nova is not monitored by the professional astronomer. Consequently, many amateur astronomers dedicate themselves to searching methodically through the sky for these new stars, either photographically or visually. If they are motivated by serious purpose, eventually they will find a previously undiscovered nova or supernova giving the professional a jump on a newly erupted star.
Lately there has been a renewed interest in the search for both novae within our own Milky Way galaxy - the "Galactic Novae" - and those that occur seemingly more frequently in other galaxies, or the "Extragalactic Supernovae." This overview is NOT intended to be all-inclusive, but to invite telescope users to examine the possibilities and the remote chance of the actual discovery of a "new star" in the heavens.
Novae and Supernovae searches can be conducted:
1) visually, using good star charts and the naked eye, binoculars or a telescope;
2) photographically, patrolling the same selected area(s) of sky at every opportunity and comparing images over time;
3) electronically, with CCD imagers which can provide not only rapid discovery information, but also serve as a photometer to accurately measure the brightness and color (hence an early indication of spectral type) of the new star.
Being Ready for the Nova Event -
It is likely that no amateur will be fortunate enough to be viewing, at just the right time, a starfield out of which one star will rapidly increase in brilliance by a magnitude of thousands. The rise to maximum light of the nova is quite rapid, requiring only hours to increase perhaps as much as 15 to 20 magnitudes. For discovery work, you should be concerned only about detecting a new nova as soon after the event takes place as possible. Others may jointly discover and report the new star, but it takes no worth away from your discovery.
This photograph of the constellation Cygnus (the outline of the 'Northern Cross" is indicated) shows the bright stars Deneb (top)
and Albireo (bottom). Note that the nova indicated by the arrow can easily be compared in magnitude to the nearby brighter stars
of the constellation. This was a 15-minute photo on Tri-X film taken through a Nikon 50mm f/1.2 lens guided atop the Observatory's 6" Refractor.
This very situation happened to me in the summer of 1975 when Nova Cygni....just a short distance from the "top" of the northern cross in bright Cygnus....popped into view. While I hesitated, opening up the observatory's roof, thinking that I might have been mistaken about that "new" third magnitude star being something I had not seen before.....others were reporting the event. At the time I was doing novae and comet confirmation for the Smithsonian Astrophysical Observatory and needed to call Dr. Brian Marsden that night anyway about Comet Bradfield 1975.....in our conversation, Dr. Marsden asked, ".....by the way have you seen the nova tonight?" Nova? What nova? Then it hit me and I asked him: "By any chance, would this nova be in Cygnus?"
The rest is history. Sherrod's Nova 1975 was never history.
Here are two Astronomical Telegrams from the International Bureau of Astronomical Telegrams from 1975.
The one on the right announces the discovery of Nova Cygni 1975 (shown in photo above)
and the one on right is the confirmation of the nova by the Smithsonian confirmation observers at that time.
Note that the Author is one of the confirming stations for the event.....but he was not credited with its discovery!
It is quite important for amateur astronomers to continue to monitor any nova - whether YOU find it or not - as long as it is visible since maximum light of a nova remains for only a short time. If it is a very bright outburst, this maximum light is the signal that draws your attention to the area in which the new star is located. However, the outburst soon subsides, and a bright nova can be quickly lost in a field of third and fourth magnitude stars if you are not familiar with the star fields and patterns of the constellations. Such a bright nova can result in the simultaneous "discovery" of the star by perhaps hundreds of independent observers. The competition - and chance of being first to report such a discovery - is slim and it is not this type of discovery that we should concentrate on....Rather, we should think about the planned, scheduled and methodical searching of specific areas over intervals of time for novae that are at or near the naked-eye limit. Of the many novae that occur each year in our galaxy, only a small fraction are ever detected. The brightest ones are discovered usually by happenstance while those fainter than the naked-eye limit go largely unnoticed. Galactic novae, or those occurring in our galaxy, occur more frequently in the dense clouds of the Milky Way, such as seen in the early morning skies of spring or the evening skies of summer. However, the detection of the novae in these regions is VERY difficult because of the enormous numbers of stars in your field of view, no matter what instrument or equipment you might be using. Ordinary novae that occur in other galaxies are well beyond the range of amateur equipment. However, the most violent of all stellar events - the supernova - can be, and often is, detected in the smallest of telescopes even though the event might occur in a galaxy millions of light years distant.
A Visual Search for Galactic Novae - "New Stars" within our own Milky Way galaxy
If you have only a small telescope, or even only binoculars, you can still search for novae within our Milky Way galaxy with some degree of success. If you have a telescope, select and area of the sky and examine it visually at low power and with as wide a field of view as possible for many nights in a row. Although some type of star chart or "planetarium PC program" is necessary to identify both the stars that are "supposed to be there" and any suspected interloper, you might wish to concentrate on three or four general areas and dedicate your time to them. After several nights of NOT finding your nova, you will be very tempted to move on to another part of the sky....don't do it: the grass is NOT always greener over there!
Sketch the field of view as it appears in the telescope. This method has the distinct advantage that all stars seen in the telescope will be on the chart you have drawn. If later, you see any that do not appear on the chart then they are candidates for further scrutiny. Concentrate on the densely packed Milky Way regions of our galaxy; most discovered novae have been found more frequently there than in the sparsely populated areas of sky, such as the star-poor spring skies. If you examine four areas at least every other night, about 15 minutes for each area will be required once you become familiar with the star fields. Thus, plan to spend about one hour total in your search nightly.
It is remarkable how familiar you become with a star field once you have searched it a few times. If you have binoculars or use the naked eye, you can look for the brighter novae if you examine a dedicated swatch of sky each night as soon as twilight ends. For such naked eye searches, concentrate on regions of the Milky Way and learn the constellations and the stars within them to the naked-eye limit. (see my constellation GO TO star charts as listed under the Guides of the ASO web page) A great help is a good star atlas such as Hans Vehrenberg's Handbook of the Constellations (no longer in print but can be found on the used book market via the Internet), or Atlas 2000 by Wil Tirion, both of which has each constellation indexed for quick reference and shows stars down to the naked eye limit.
NOTE: with all of the excellent computer "planetarium-type" programs with star chart capabilities, I highly recommend their use as it allows quick and easy determination of an EXACT Right Ascension (R.A.) and Declination (DEC) by merely placing the cursor at the exact point of a suspected new object. Also, if the object is bright enough, you can obtain an INSTANT magnitude from any nearby star by merely clicking on that star or moving your screen pointer to it.
A Photographic Program For Galactic Novae -
A more consistent and more rewarding search for galactic novae can be initiated through photographic (or digital camera/CCD) searching. The considerable benefits of such a program include the following:
- No memorization of star fields is necessary.
- The photograph will provide you with a permanent record and verification of any discovery, from which a magnitude and position may be derived from your star charts/PC planetarium program. Furthermore, examination of the field can be done at leisure, allowing you to take the image(s), store it or process it from film and look for suspect objects at a later time.
- More stars can be recorded on the photograph than can be discerned by the human eye and the limiting magnitude threshold will be greater than a visual examination of the field along.
- Hence, the chance of discovery increases because the number of stars on the photograph is greater than can be seen by the eye. Attempting to photograph a star field through an average telescope in search for novae is a perplexing and tedious task. A better method is to photograph as wide a field as possible on a single photograph and reach perhaps magnitude 10 or 11 with merely a good camera and standard lens attached (50mm up to 200mm f/2.8 to 3.5 if possible).
- If using a CCD system with a three-color capability (i.e., filter wheel) you can actually obtain spectral (color) data on the star, thereby giving a very early and valuable indication of the star's spectral type at or near brightest light.
- Also, with CCD, the telescope you are using can reach far fainter magnitudes that either visually or photographically, allowing even an 8" telescope to reach even magnitude 16 in very dark skies.....therefore the number of potential novae visible to you will dramatically increase.
The Equipment Necessary -
To systematically search for novae using the camera or CCD, only two things are required - a camera/CCD and some provision for tracking on the stars. A simple clock-driven equatorial mounting provides accurate tracking for standard 50mm camera lenses for up to 10 minutes if the polar axis has been accurately aligned to north and set at the proper latitude of your observing location.
However, telephoto lenses are advantageous and highly recommended for this study. Even so, you still must have some way to correct the tracking of the instrument. The piggyback method and a good camera lens is recommended with lenses of 100mm to 200mm focal length for the search, and exposure times on fast monochrome or color CCDs should be around 1 minute. If your observing station is located in a bright suburban area, exposure times should not be greater than about 20 to 30 sec, although the limiting magnitude achieved will be considerably less than if the photograph was taken in dark skies and exposed for a longer period of time.
If using the CCD, it is to your advantage to do PATROL searches in black and white and - if something is found - then do three-color imaging to provide accurate color characteristics of the early nova. With CCD equipment, even the most modest, the imaging can be done effectively in suburban skies which offers a tremendous advantage over traditional photographic means. Exposures can be kept quite short and many areas can be imaged with INSTANT results via the computer link in an extremely short period of time.
The Method of Search -
In order to convincingly search the sky for nova there must be two successive photographs, taking them no more than about 10 days apart. Any new object in the 10 day (or preferably shorter) interval will appear on one photograph but not on the other,and thus give you initial findings that something new has appeared on the last photograph (or actually in some cases, it could appear on the first photo only to have faded significantly on the last one!) If any object IS suspected, further confirmation using a telescope visually must be done without hesitation.
This photograph shows a closeup of the region of Nova Vulpeculae 1976 taken with the Arkansas Sky Observatory
6" f/5 Sky Patrol Refractor. This photograph was taken on the same night as the discovery after a phone call from the Smithsonian Observatory
requested early verification through the Observatory
You can see how important making sure that the star you have identified is the CORRECT star.....almost
every star in this small asterism in Vulpecula just happens to be about the same magnitude as the nova!
This all sounds rather simple until you try to compare two photographs, each containing thousands of stars. Keep in mind that the faint stars are the ones under surveillance! No one can possible inspect each and every star, one by one. The professional astronomer uses either a computer which can digitally search a field for "visitors" OR a large desk-mounted instrument known as a blink comparator, a device in which two negatives are centered on the same starfield and can be alternately compared through a binocular viewing eyepiece. If the two negatives are precisely lined up in the comparator, the effect is that the field is merely "flashing" on and off rapidly as the comparator shows first one negative and then the other. However, if a new object is apparent on one film and not the other, that new object will alternately flash on and off as the comparator shows first the negative with the object and then the negative without it.
This method is fast and reliable, but the instrument is quite expensive and beyond the budget of most amateur astronomers. Therefore, as in other cases of amateur research, some makeshift method must be devised by which YOU can accomplish similar results. Several small portable blink comparators aimed at the amateur astronomer have been developed by non-professional astronomers in past years. Most of these are adequate, although somewhat difficult to use. A simpler method is to devise a light box using a translucent material on top of which you can position the two negatives (or color transparencies). Then precisely aim two viewing eye-pieces, one at one slide or negative and the other at the second film. Install within the box, under the translucent material, a timer (either mechanical or electronic) that switches the current back and forth between a light under both slides (or negatives). HOWEVER, today there are excellent computer programs (i.e., CCDsoft from Bisque) which allow two or more individual digital photos to be aligned and blinked in an instant!
Another method that has had a great deal of success will allow you two use two SLIDE PROJECTORS, each of which is aimed so that it centers the projected image atop the other as they are projected onto the same screen. The method has the nice distinction of allowing you to sit back and enjoy the view as the slides flash onto and off of the simple projection screen As in the blink comparator, this method makes an object visible on at one moment, but not the other so that only one image appears to be blinking on and off. But in these more modern days, by all means, find a good computer program to do the job for you!
NOTE: Any method of comparing one image to the other requires exacting care to align the camera on the star field at the time the photograph is taken. The orientation and the center of each photograph should be the same, and of course,you must use the same optical system through-out. The formats of .fit, .tiff or in some cases raw or even .jpg is preferred for computer programs providing blinking programs in that the field will be dark when viewed by whatever comparator is used. Always blink white stars on black, rather than "negative, because a stark white background will quickly tires the eyes. I highly recommend Bisque's CCDsoft program for this:
Studying External Galaxies For Supernovae -
Although only one supernova per century on an average is thought to occur in our Milky Way galaxy, there is a way in which a hundred times that many super-novae explosions might be found in the same timeframe: Look for them in a hundred galaxies! There are hundreds of galaxies within reach of modest amateur equipment that are large and close enough to show any supernova that might occur within them. When most supernovae occur in these external galaxies, they appear as VERY faint stars embedded in the seemingly nebulous outer portions of that galaxy. The brightest are on the order of magnitude 12 or 13, so telescopes of 8 inches (20 cm) or larger are required to see them visually. However, the "typical" supernova in an external galaxy might be more expected on the order of 15th or 16th magnitude, still within the photographic or CCD-grasp of modest amateur equipment.
To search for supernovae in external galaxies requires that you know the appearance of the galaxies as they should appear in your telescope. Photographs of galaxies are often published that have had the central regions burned in in an effort to expose the outer regions of the galaxies.
Any bright knots or star-like images that might normally be seen visually are thus obliterated. Consequently, it is necessary that you set up a schedule for recording the fields of the galaxies that you plan to observe before you start your actual observations.
Recording the Galaxy Field -
You should choose a number of galaxies large enough to allow you to examine 15 galaxies for supernovae each night. With the great number of galaxies visible in our spring skies, there is a predominance of excellent subjects during that season. At the advent of the project, examine each galaxy on your list, preferably in sequence by right ascension. Using a circle about 4 inches(10 cm) in diameter, sketch the field as you see it through the telescope eyepiece, including every star within the field of view. Because most galaxies appear small to us, draw the galaxy field with moderate magnification, say about 100x to 150x, and only on the very best night. It will take you much longer to make the initial field sketch than it will to search or the galaxies on every subsequent night. Draw the fields with the telescope that you will use for the searches. Once you have drawn the fields, it is a simple matter to examine as many galaxies in sequence as possible or practical for the night. Eventually, you will become so familiar with the fields that referring to the charts you have drawn will be necessary only when you find an unfamiliar object that might indeed be a "new star" within that galaxy.
Galaxies Recommended -
For supernovae surveillance the galaxies listed in the following table are recommended as part of your regular patrol for the possible supernova event. Be aware that many of them might have star-like points near the galaxy, most if not all of these distant stars located in our own Milky Way between the observer and the distant galaxy. By all means - don't take a chance! - record all star-like objects on your initial drawing. Monitor each galaxy on every available night because the detection of a supernova in its early stages is of the utmost importance to the professional astronomer. Notice that only spiral galaxies are listed in the table, since the occurrence of the supernova is considerably more common in the spiral than in the elliptical galaxies.
REPORTING and CONFIRMING ANY DISCOVERY
So let's be optimistic and assume that you are going to find that "new star" you are searching for....or in the process you accidentally just happen to find a new COMET or asteroid. What do you do?
1. DON'T immediately assume that you have discovered something....this is the first mistake. Unfortunately there are TWO possible scenarios that are much more possible that you actually having discovered a nova or supernova: a) someone else has actually discovered it before you; b) it is an object that is "supposed to be there", but is not showing up for whatever reason on your star atlas, computer sky program or other source.
2. DO go back and check the same location after looking at your charts for verification or processing your photo/CCD image; if it is still there, then image it again or estimate its magnitude again visually.
3. WRITE DOWN EVERYTHING: including the time, date, telescope/equipment used, sky conditions and EXACT coordinates (R.A. and dEC) of the object.
4. If you are convinced that the object is "not supposed to be there" then you should proceed to notify someone. If there is a local public observatory handy, then check with them; if not, then check immediately via the Internet at: http://www.cbat.eps.harvard.edu/RecentCBETs.html which is the net's posting of ALL International Astronomical Union (IAU) telegrams announcing new discoveries, activities or events in the skies. Even if the "circular" is not active yet, it WILL be posted here and you can immediately see if there is a prior announcement of "Discovery of Nova in Cygnus" for example. That would be your first red flag that someone has beat you to the punch.
5. If you have been assured by your local observatory and/or the IAU telegrams posted that nothing exists where you "think" you have found your object, then by all means - with ALL data at hand - go ahead and report it as a "possible" discovery. Do NOT over-embellish what you have found....be very specific and realistic as to magnitude, color, position, etc. To report any astronomical discovery, go to the IAU page:
6. They will alert observatories around the world to confirm or deny the existence of what you might have found. Within a short period of time, usually less than one day, you will be notified if this checks out.
7. If it does, it's party time.
This is merely a very concise capsulated introduction into this exciting endeavor; cater YOUR observing plans to meet your equipment and needs; certainly CCD cameras can patrol in a mere snapshot many faint galaxies that cannot be accessed visually, and will record the faintest of stars possible in your equipment. But one does NOT necessarily need that sophistication....indeed, many bright novae have been discovered by a casual stargazer who happens out on a warm summer night and just happens to look skyward at a familiar constellation.....and there may be a bright star that causes him or her to freeze....is it or is it NOT an interloper in the heavens?
Only YOUR research might reveal the true answer!
Arkansas Sky Observatory