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Southern Cross - February 1998

• Constellation of the Month - Dorado

Constellation of the Month - Dorado

Peter Ogilvie

Introduced by Bayer in 1604, the constellation of Dorado the swordfish is high in the southern sky throughout summer. It is notable because its southernmost portion includes most of the Large Magellanic Cloud (LMC) - the larger of two naked-eye satellite galaxies of the Milky Way (the other being the Small Magellanic Cloud in Tucana). Being so readily visible, the LMC helps in quickly locating Dorado, whose brightest star is magnitude 3.3.

There are lots of interesting objects to be found in Dorado, some of which are within the LMC itself. Although most of my observations were done with a 10 inch telescope at a suburban location, and some with the society's 14-inch at Mt Stromlo, most objects should be visible in smaller instruments.

Comet Hale-Bopp is visible in Dorado until March 1998, slowly moving NW. It is getting rather dim now at about magnitude 8 but still easily seen.

Double stars

Alpha Doradus (4h 34, -55deg 3m), the brightest star at magnitude 3.3 is a very close binary (13 year period). I mention it only as a matter of interest because it is certainly an extremely difficult one to split with even the largest amateur instruments. The mag 3.8 blue-white primary has a slightly fainter mag 4.3 companion at only 0.2 arcseconds separation.

Rmk 4 (4h 24, -57deg). An attractive pair of yellow stars, almost equal in brightness, mag 7.2 and 7.5 at 6 arcsec. Easy in any scope at 100X or more.

h 3683 (4h 40, -59deg). Another fine equal pair of yellow stars, mag 7.1 and 7.2 at 3.0 arcsec. A slow binary with a 240 year period. Nicely split at 156X.

Variable stars

Beta Doradus (5h 33.6, -62deg 29m). A Cepheid-type variable (spectrum F-G) with a magnitude range of 3.7 - 4.1 over a period of 9.84 days. Cepheid variables are used as distance indicators in other galaxies because their period is directly proportional to their intrinsic luminosity. Because their luminosity can be easily calculated, their distance can be determined from their apparent brightness.

R Doradus (4h 36.8, -62deg 5m). A red semi-regular variable (spectrum M), mag 4.8 - 6.6, period 338 days.

Galaxies

The Large Magellanic Cloud (LMC), one of the Milky Way's dwarf satellite galaxies, lies at a distance of 180,000 light years and is easily visible to the naked eye as a large hazy patch, several degrees in length, even in moonlight. Generally classed as an irregular galaxy, its appearance suggests a hint of barred spiral structure, though the arms have not properly formed - its mass is probably insufficient. It contains a number of interesting objects (see 'LMC objects' below).

NGC 1617 (4h 31.7, -54deg 36m). This barred spiral galaxy is easy to find being only 1/2 deg NW of Alpha Dor. Listed as mag 10.4, it appears as a small hazy oval with a tiny nucleus.

NGC 1549 and NGC 1553 (4h 16, -55 40). A bright, very easy pair, elliptical and lenticular (flattened elliptical) respectively, mag 9.9 and 9.4, among the better ones visible at this time of year (see article "The best galaxies for summer", Nov 96 journal ).

NGC 1566 (4h 20, -54 56). 2 deg W of Alpha Dor, a relatively bright mag 9.7 spiral, oval with an obvious nucleus.

NGC 1596 (4h 28, -55 01). About 1 deg W of Alpha Dor, a very small edge-on, highly flattened lenticular galaxy of mag 11, it appears like a miniature version of the famous 'Sombrero' (M104) in Virgo. Some big-scope owners might also spot the extremely faint 13th mag NGC 1602 very close to it. It is just detectable with the society's 14-inch.

NGC 1533 (4h 10, -56 07). A barred lenticular, mag 10.7 oval next to two stars.

NGC 1515 (4h 04, -54 06). An 11th mag spiral viewed nearly edge-on, it appears small and elongated with a small nucleus. Although this one isn't bright, edge-on spirals are visually the most interesting of galaxies.

NGC 1574 (4h 22, -57 00). Located on the border with Reticulum, only 1/3 deg W of the attractive binary star Rmk 4 mentioned above. It is a barred spiral, small and circular.

NGC 1672 (4h 46, -59 15). A fairly bright mag 9.6 barred spiral in a star-rich field, circular with several faint stars superimposed on its face. The small nucleus also looks star-like.

NGC 1947 (5h 27, -63 46). An unusual elliptical (possibly lenticular) galaxy with a thick dust band, not unlike the famous Centaurus-A. Listed at mag 10.6, I found it small and dim with no nucleus.

NGC 2150, 2187 (6h 04, -69 40). A very close pair of spirals, dim and very small but suprisingly easy to see in the society's 14 inch, just 0.9 deg SW of 5th mag Nu Doradus, or approx 2 deg E of 30 Dor. (Tarantula nebula).

LMC objects

Nebulae

NGC 2070 / 30 Dor / Tarantula nebula (5h 39, -69 06). One of the largest known emission nebulae. Even at the incredible distance of 180,000 ly it spans about 1/2 deg, which translates to an real diameter of about 1600 ly. If it was as close as the Orion nebula at a mere 1400 ly, it would span a spectacular 64 deg of sky! It is easily visible in binoculars, and a 6 inch scope or larger will show a great deal of structure. Larger instruments will show the spider-like loops and arcs of gas which give it its spider-like appearance. It responds well to nebula filters, particularly oxygen-III.

NGC 2074, 2085/86, 2078 (5h 40, -69 37). Three small but easy patches of nebula just S of 30 Dor (Tarantula), they also are improved with a filter.

NGC 1966 (5h 27, -68 48). Hazy patch, with a sprinke of stars.

NGC 1929 (5h 22, -67 58). Faint haze with some brighter patches, also with a number of stars.

NGC 1763 (4h 57, -66 22). This one covers a larger area. It has three main patches of nebulosity in a liberal sprinkling of stars.

Globular clusters

The LMC's globulars are all small and unresolved, but quite easy to see. The four brightest are: NGC 1786 (4h 59, -67 45), NGC 1835 (5h 05, -69 25), NGC 1978 (5h 29, -66 16), NGC 2210 (6h 12, -69 06). The easiest to find is 2210, just 1/3 deg SE of 5th mag Nu Doradus.

Supernova SN1987A

In January 1987 astronomers around the world dropped what they were doing in order to study the closest and so far only naked eye supernova since Kepler's supernova in 1604 (the same year Bayer introduced Dorado to his atlas). SN1987A was also the first naked-eye supernova since the invention of the telescope in 1609. I saw it by chance myself, nearly two days before any public announcement, when I glanced briefly at the LMC at about 10pm on 25th January '87 to see if I could see 30 Doradus (the Tarantula) with the naked eye. I would often do this as a guide to how transparent or dark the sky is.

On this occasion I had the impression 30 Dor was perhaps a little more obvious or sharp than usual. I didn't think any more of it until the supernova was announced on the 27th. Of course I immediately realised what I had seen, but because of cloud I had to wait another two nights to have a second look. When at last I did, I could see it was so close to 30 Dor that my initial mis-identification was understandable. By this time it was also brighter. Apparently I had first seen it about 16 hours after its discovery when it was around mag 5 but brightening rapidly.

From a pre-explosion magnitude of 12, it climbed to around mag 4 in three days. It continued to brighten more slowly over a month, pausing briefly before reaching a maximum of mag 2.8 in early March. It then took the rest of the year to fade slowly below naked eye visibility.

It took astronomers a little while to confirm exactly which star was the progenitor. There was initially some confusion because the apparent progenitor was a previously catalogued blue giant star, and according to theory main-sequence blue stars were not supposed to go supernova. Well, it was that star, however it seems it wasn't a main seqence star after all. It was a star of advanced age, having previously shed its outer layers, exposing a much hotter inner region. Despite this initial apparent conflict with theory, studies of the event over succeeding months did confirm many aspects of supernova theory, in particular the veritable 'flood' of neutrinos (they counted 19) which preceded the visible event.

It remains an interesting object with the expanding shock wave expected to hit the previously ejected material in the neae future, briefly lighting it up several magnitudes to perhaps be visible in amateur telescopes.