The Elusive B Cassiopeiae: in which we track and capture a supernova remnant.
Although not the first to spot the event now cataloged as Supernova (SN) 1572, Tycho was the most thorough of all observers to document its light curve. His precise data would be used not only in his time, but more than three and a half centuries later to determine the explosion's origin. Copernicus fired the first shot against the fortified concepts of Aristotelian and Ptolemaic dogma, but this new, inexplicable star that ultimately inspired Tycho's 1573 book, De nova Stella, convinced many of his contemporaries that a new cosmology was needed.
In the pre-telescopic era of the late 1500s, the light from this stellar explosion remained visible to the naked eye for 16 months, from November 1572 until March 1574. When the nova was at its estimated -4.0 maximum magnitude around the middle of November, it was visible even in daylight. Even as its light faded from view, the remnant of the supernova expanded. It continued to do so, unobserved by human eyes for the next 375 years.
The Arc of Observational History
The optical observation of Supernova Remnant (SNR) 1572, also cataloged as B Cassiopeiae, or B Cas, started in the 20th century. Walter Baade, the German astronomer who defined the category of bright stellar explosions as "supernovae" in 1934, reexamined Tycho's data to produce a light curve smoothed by quarter-magnitude steps. After comparing it with similar curves, such as that of Kepler's Supernova of 1604, he categorized SN 1572 as a Type I object that showed a linear decrease in brightness. In his 1945 Astrophysical Journal paper, Baade noted that no apparent remnant of SN 1572 was visible on existing photographic plates. However, he held out hope for future observations to be made with the Hale telescope, then under construction at Palomar Observatory. Dedicated in 1948, the 200-inch reflector possessed four times the collecting power of the largest instrument then extant, the Hooker 100-inch reflector at Mount Wilson.
Baade imaged the area around SN 1572 with this powerful new instrument on November 23, 1949, and on the red plates detected nebulosity on the eastern edge of the SNR. But he didn't report these findings, concluding instead that the filaments were too distant from the point of origin to be related to Tycho's exploded star. Thus, credit for the discovery of the SNR goes to Hanbury Brown and Hazard (Jodrell Bank), who detected it in radio wavelengths in 1952. Baldwin and Edge (Cambridge University) corrected position errors in 1957, and B Cassiopeiae was subsequently cataloged in 1959 as 3C 10 in the Third Cambridge Catalogue of Radio Sources (3C). As it turns out, B Cas is the second-brightest radio object in that constellation after Cas A, itself the most luminous radio source outside our solar system.
Prompted by these radio surveys, Baade repeated his plate observations with the 200-inch telescope in 1955 and 1957, ultimately deciding that the filamentary structure he'd detected earlier was indeed related to Tycho's supernova. He'd been thrown off by the remnant's very large proper motion. In 1970 Sidney van den Bergh (University of Toronto) used the same telescope to take a two-hour red exposure of the same field with a more sensitive emulsion. After studying the plates, he noted two new sections of nebulosity at the remnant's northeastern and northwestern edges. Based on these images, taken 21 years after Baade's initial attempt at detection, van den Bergh estimated the expansion rate of the nebula at ~0.19" to ~0.21" per year.
The Guilty Party
Competing theories exist regarding the creation of Type 1 supernovae like the one watched by Tycho and his contemporaries. In one scenario, two white dwarfs merge, either as the result of a single, dramatic collision or a slow drawing together in a gravitational spiral, to form a supermassive star that then explodes. In another scenario, a single white dwarf gravitationally accretes material from a nearby "normal" companion star until compression triggers a thermonuclear explosion. The white dwarf explodes, altering the chemical state of the companion star and affecting its radial velocity and proper motion. Astronomers think both causes are likely to occur, but the second is most frequently proffered to explain the creation of B Cas.
Theories also differ as to the identity of the companion star of the pre-supernova white dwarf. My original interest in observing B Cas was spurred by a 2005 report on the relationship of the SNR to the G-type star "Tycho G" (the second "G" here comes not from the star's spectral type but its place in the list of possible candidate companion stars, labeled "Tycho A" to "Tycho V") (S&T Feb. 2005, p. 22). While spectroscopic and metallicity analyses point to Tycho G as the putative companion star, recent studies suggest otherwise. Tycho G may fall outside the region most likely to include an ex-companion star, and discrepancies in Tycho G's velocity and motion may be the result of other stellar processes.
Regardless of the final ruling on Tycho G's role in the explosive event, looking for this dim star adds another element of difficulty to the already tough task of visually observing B Cas. It's clear from the efforts of professional astronomers that spotting the SNR in the optical range--with or without the possible companion--has always been a difficult game. But how obtainable is a view of it for backyard observers? As Howard Banich's account of viewing Cassiopeia A shows, some supposedly unobservable objects fall well within reach of amateur scopes (S&T: Dec. 2014, p. 60). His article encouraged me to reconsider my own observations of Cas A, which I viewed with my 25-inch scope in 2003, and later with instruments of 15 and 32 inches of aperture, as well as my notes on B Cas and Tycho G.
Hunting the Remnant
To my knowledge, there were no published visual sightings of B Cas by amateur astronomers prior to 2006. With an extremely low surface brightness, B Cas is a ghostly shell that spans about 20 light-years, or 8' visually. Techniques for observing SNRs are similar to those for ultra-dim galaxies. SNR possess emission lines, but most of the fainter ones don't respond to filters. In March 2015 Myung Gyoon Lee (Seoul National University) et al. published a study on SNRs in M81, demonstrating that nearly all of their spectra showed O III lines at 5007 angstroms, but the lines were significantly weaker than those found in planetary nebulae. The Ha emission lines were much stronger, but these are most useful for imaging. (One exception to filter enhancement would be the Veil Nebula in Cygnus, the view of which is markedly improved with an O III filter.) I couldn't count on filters enhancing the view of B Cas for me.
To aid my search, I made finder charts accompanied by images from the Second Palomar Sky Survey (POSSII) red plates as well as those from the results van den Bergh published in the Astrophysical Journal (1971). The eastern and northwestern areas of the SNR are faintly seen on the POSS-II red plate, serving as a guide to what might be visible in my large reflectors. I also printed a Hubble photo montage to take into the field. There's no substitute for having an image to compare with faint objects, but I suggest observing the field containing the target first, and using the image only for confirmation; otherwise, you can be misled into imagining you see something you really don't. In the case of SNRs, you should also keep in mind that they're expanding and may be in a different place than that shown in the image. For my finder charts, I used Megastar with USNO-A2.0 stars down to 21st magnitude to isolate the B Cas field 1.0[degrees] northwest of NGC 133.
Well-prepped and well-equipped, I transported my 15-inch f/5 reflector to a resort on Gull Lake in north-central Minnesota during a family vacation in early August 2006. I set up in a dark area known to have Sky Quality Meter (SQM) readings in the 21.0 range, indicating a very dark site. I rated the seeing as 7-8/10 and transparency 8-9/10. Cassiopeia was between 65[degrees] and 70[degrees] above the horizon between 2 and 3 a.m. as viewed from latitude 46.5[degrees] North.
I started my search with a 7-mm eyepiece, but when it wasn't adequate to spot the nebulosity, I bumped up the power. With a 3.5-mm Type 6 Nagler eyepiece at 544x, I spotted nebulosity on the northwest side of the SNR four times in 20 minutes, rating it extremely faint. The nebulosity appeared about 10" long and 2-3" in width, oriented northeast to southwest. It nearly touched a faint star to its northwest. A similar star rested 15" northwest of that star. I also managed to tag Tycho G at RA 00h 25m 19s, Dec +64[degrees] 08' 18" (J2000.0). Though sources give its magnitude as +19.1, I didn't have much difficulty observing it in such fine conditions.
Try as I might, I wasn't able to see the eastern portion of the SNR in the 15-inch scope, but I gave it another shot a few weeks later from my home in east-central Minnesota, where the SQM attains 20.7 but the sky generally isn't as transparent. On the other hand, I was working with a lot more aperture. I used my 32-inch f/4 reflector to view the remnant's eastern nebulosity with the same 3.5-mm eyepiece at 929x. Just southeast of a flattened triangle of 17- to 18th-magnitude stars, the fragment tipped north to south. It was several arcseconds wide and between 15-20" in length. This location is just east of the position shown on the POSS-II red plate, a change in position that correlates with the SNR's expansion rate of ~0.2" per year as calculated by van den Bergh. My friend Tim Parson confirmed the observation. On that night and again a few nights later with the same equipment, the SNR was seen quite well with averted vision and, at times of good seeing, was on the edge of direct vision. Tycho G was easily seen in the 32-inch.
Take a Shot At It
If you're interested in observing supernova remnants, you may find it helpful to start with brighter and closer objects, then work toward fainter members like SNR 1572. An excellent resource, updated regularly, is Dave Green's Catalogue of Galactic Supernova Remnants from Cambridge University (mrao.cam.ac.uk/surveys/snrs/). Images of SNRs in professional journal articles can be correlated with POSS-II data to determine visibility. Messier 1, the Crab Nebula in Taurus, is one of the best training targets; at magnitude +8.1, it's easily visible about 1[degrees] northwest of Zeta ([zeta]) Tauri with a small scope or binoculars. The challenge with M1 is to view the 16th-magnitude pulsar energizing the nebula. Similarly, IC 443, the Jellyfish Nebula in Gemini, is a large and beautifully detailed structure, offering hours of reward for careful observers. It contains a neutron star on its southern end that's plowing through nebular gas, leaving an observable wake.
After whetting your optical appetite on these targets, more difficult fare awaits. Objects for advanced observers include Simeis 147 in northern Taurus, near the Auriga border. This 200" remnant is diffuse, with only several of its brighter fragments visible. Cassiopeia A is surprisingly accessible, and it has been seen in a 9.25-inch scope by veteran observer Bill Gates. In its quadricentennial year, I observed several of the fragments of Kepler's SNR (1604) in Ophiuchus with my 25-inch scope at 454x without a filter. But I recommend observers of SNRs try different filters, particularly the O III. Having a filter slide that allows rapid switching between filters enhances the view and improves the chance of recovering these faint objects.
Lastly, don't give up if you don't capture your prey on the first attempt. For objects at the edge of visibility, sky conditions are critical. I keep a list of "challenge" targets for periods of excellent seeing and clarity, and in unusual cases have pursued objects for over a decade before they succumbed to diligence. Visually observing historical supernova remnants is rewarding on many levels. Their study can take us back in time to delve more deeply into the context of their occurrence, and they show how unexpected findings can push science and understanding forward. Technical observing obstacles hone skills that create a desire to seek new goals, extend visual limits, and encourage others to pursue challenging targets.
Nothing Ventured, Nothing Gained
Over my three decades in this hobby, my rate of visually recovering difficult objects has risen greatly. I've learned that with persistence and planning, many objects previously considered invisible through a telescope can be seen. I've experienced satisfaction in these moments when an observation offers a sense of being part of an event spanning centuries of time. In the case of SNR 1572 we have, through a shared love of the sky and careful documentation, a connection with one of the great observers of the past. Maybe someone four centuries from now will find fresh value in the remains of this elusive star.
David Tosteson has been an amateur observer for over 30 years. His main interest is observing rare and unusual deep-sky objects, including quasars, black-hole phenomena, and distant galaxy clusters. When he's not working as a family practice physician just north of St. Paul, Minnesota, traveling to regional star parties and solar eclipses with his wife keeps him busy.
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|Title Annotation:||Supernova Remnants|
|Publication:||Sky & Telescope|
|Date:||Jun 1, 2016|
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