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Southern Cross - February 1999

• Constellation of the Month - Eridanus, Northern Half

Constellation of the Month - Eridanus, Northern Half

Albert Brakel

This is the second part of our tour through Eridanus, following on from Ross Gould's guide to the portion south of declination -30 degrees, in the December 1998 Southern Cross.

The River Eridanus is a very old constellation, probably dating from the ancient Sumerians and Babylonians, who doubtless had the River Euphrates in mind. In the form it has come down to us from the Greeks and Romans, it is unclear which river it represented to them. R.H. Allen gives no less than 14 possibilities! The most common interpretations though are that it represents either the Po River in northern Italy, or the Nile in Egypt. In these cases, the star Achernar at the end of the river could be thought of as representing Venice or Alexandria respectively (although Venice did not exist in Roman times). Hartung suggested that if the flow of the river is reversed, the disappearance of the river below the southern horizon as seen from Europe could symbolize the then unknown source of the Nile.

Despite beginning on the fringes of the Milky Way, Eridanus contains no open clusters, but it does hold a faint reflection nebula, a planetary nebula, and a large cohort of galaxies and double stars. Most of the galaxies in northern Eridanus belong to the Eridanus galaxy cluster, which is different from the Fornax galaxy cluster not far to the south. Using a C8 from suburban Downer, only three of the galaxies were visible (NGC 1332,1400 and 1407); interestingly, none of these were given in the observing guides I consulted, but they stand out on chart C-72 of the Herald-Bobroff Astroatlas. For the fainter objects, I used the 14" Bobroff reflector in the CAS Dome, located under the darker sky of Mt Stromlo. So let's take a sight-seeing trip down the Celestial River, starting from its source at Beta Eridani, near Rigel.

We don't have far to go for our first object, but seeing it is another matter! About 1 deg. south of Beta Eri lies the brightest part of IC 2118, the Witch Head Nebula, which is an extensive but very faint reflection nebula illuminated by nearby Rigel. I couldn't see it at all from the suburbs. It stretches for about 2.5 deg. in a southwesterly direction, and is brightest along the eastern edge. From Mt Stromlo I thought I could see a pervasive faint nebulosity in the region, especially with a broad-band Lumicon Deep Sky filter to increase contrast, but it was too faint to be absolutely certain. Larger apertures and darker skies should allow it to be seen better. It's harder than the Horsehead, so it's just as well it has never been a target for the John Morland Nebulous Award.

The first of several doubles eminently suitable for small 'scopes, 55 Eri (RA 04h 43.6m, Dec.-08 48') is a dainty, equal, deep yellow and pale yellow pair, that at a separation of 9.3" is dead easy to split, even with a power of 50x. The magnitudes are 6.7 and 6.8.

Have you ever seen a red dwarf and a white dwarf? White dwarfs especially are a rare species as far as amateur instruments are concerned, but in the Omicron-2 Eri system (04h 15.2m, -07 39') you can see both types of dwarfs side by side. The system is a triple one, dominated by a bright mag. 4.4 golden yellow star, around which the closer pair of dwarfs orbits at a very wide 83". The white dwarf is the brighter of the pair (mag. 9.6), and an easy 9" from the faint 11.2 mag. red dwarf. I saw all three components with 77x, and better with 117x, but with the C8 the red dwarf was unfortunately far too faint to detect colour.

Almost 3 deg. to the south, 39 Eri (04h 14.4, -10 15') forms a lovely unequal orange-yellow (mag. 5.0) and white (mag. 8.0) binary 6.4" apart, that was fairly easily resolved with 77x.

About the same distance south again, the bright planetary nebula NGC 1535 (04h 14.4m, -12 44') beckons the sky tourist. At low power it appears like a tiny (30") unresolved globular cluster, roughly aligned with two stars to the west. With the OIII filter it is brighter, and has a uniform disk that drops sharply in brightness near the edges. This is true of all magnifications tried except 380x, with which the outer fringe is wider in proportion. No central star was visible.

32 Eri (03h 54.3m, -02 57'), near the northern border of the constellation, is an attractive double comprising deep yellow 4.8 mag. and white 6.1 mag. stars that lie a comfortable 6.5" apart, again easily resolved with 77x.

Returning to the main course of the River, Epsilon Eridani (03h 32.9m, -09 27.5') is a Sun-like star 10.2 light years away, that was the second star targeted for a SETI search on the first day of Frank Drake's Project Ozma in 1960. We now know that Epsilon Eri is too young to harbour extraterrestrial intelligence. It is of spectral type K2, and looking at it you will see what the Sun was like some 4 billion years ago. A disk of dust has been found around it, with a solar-system-sized central cavity that is just what would be expected if planets had formed and swept up the available material close to the star.

1.7 deg. to the northwest, the galaxy NGC 1337 (03h 27.9m, -8 24') lies edge-on to our line of sight, making this mag. 12 object a bit easier to see than you might otherwise expect. Through the CAS 14" it was a spindle of very dim light.

Rho-2 Eri (03h 02.7m, -07 41'), near the western bend in the River, could only be split on a night of good seeing, as the separation is 1.6". The A star is a mag. 5.3 yellow-orange, and although the B star is a similar spectral type, at mag. 9.5 it was too dim to show colour.

NGC 1232 (3h 09.8m, -20 35'), 2.5 deg. northwest of Tau-4, is a face-on, multi-armed spiral. It was visible from Mt Stromlo only as a dim circular glow with a poorly condensed centre at all magnifications used (40x, 80x, 94x, 188x). The core was the most discrete with the highest power, but no arms were visible.

NGC 1300 (03h 19.8m, -19 24'), north of Tau-4 and forming an equilateral triangle with the star and NGC 1232, is a face-on, prominently barred spiral galaxy. With 40x it appeared as a dim elliptical haze without any condensation (elliptical because the bar is much brighter than the spiral arms), but doubling the power caused the elliptical smudge to start to develop a sharp core. Photographs show this object as a classic barred spiral, with two arms coming off the ends of the bar at right angles.

Tau-4 Eri (03h, 19.5m, -21 45') itself has bright orange and white components, but the magnitude difference of 3.7 and 9.2 is so great that despite a distance of 5.7" between them, the white star was lost in the glare of the primary until I bumped the power up to 117x.

About half-way between Tau-4 and Tau-5, NGC 1332 (03h 26.3m, -21 30'), a spiral with its brightness enhanced by its edge-on aspect, has a stellate nucleus enveloped in an elongate soft glow, that is quite distinct even at low powers.

While you're near Tau-5, see if you can spot close-by IC 1953 (03h 33.6m, -23 29'), another face-on barred spiral. The very faint, soft circular fuzzy spot showed no structure or nucleus with the 14" at 94x.

Over 3 deg. NE of Tau-5 lie two bright galaxies that could even be seen from suburbia: NCG 1400 and 1407 (03h 40.2m, -18 35'), lenticular and elliptical respectively. They can be seen in the same low-power field, with the brighter NGC 1407 to the east. With 62x, they were just roundish, slightly elliptical milky patches, rising in brightness towards their centres, but with an increase in power to 94x, sharp cores became visible.

Burnham 744 (04h 21.5m, -25 44') is a double star for those who think the preceding ones were too easy. The equal magnitude (6.6, 6.9) members are only 0.6" apart, right at the resolving limit of a C8 on an excellent night.

That brings us to where Ross Gould described the downstream sights of Eridanus last December. If you haven't already done so, do continue the journey with his notes as your guide.