Cetus

by http://a%20href=#molongui-disabled-linkShaphan%20Shank/a | Dec 1, 2022 | 0 comments

NGC 253
NGC 253. Photo © CC by 3.0.

The southern sky is nearly devoid of bright stars on evenings in late autumn and early winter. Cetus, the Whale, is a relatively large constellation that lies in the middle of this star-poor region, southeast of Andromeda and Pegasus. Fortunately, Cetus contains one relatively bright star, called Diphda, which makes the constellation easier to find. Diphda is one of the only bright stars that is currently high in the southern sky in the early evening at mid-northern latitudes. The nearest brighter star is Fomalhaut, which lies over 25° southwest of Diphda.

The most impressive deep sky object near Cetus is NGC 253, an edge-on spiral galaxy. This galaxy officially lies within the bounds of the neighboring constellation Sculptor, but it is easiest to find by starting from Diphda. To find NGC 253, start by finding the two little (~1° across) triangles of dim stars that lie 4–6° south-southeast of Diphda. (You may need a binocular to see the triangles if your sky is light-polluted.) NGC 253 lies just over 1° southwest of the bottom (closest to horizon) triangle.

NGC 253 is large and bright for a galaxy. You should be able to easily locate the galaxy and see its oblong shape with any quality binocular. A telescope won’t show the spiral arms of NGC 253 because it is nearly edge-on to us, but you may be able to see some mottling or bright knots in the galaxy with a mid-sized telescope.

The brightest galaxy within the official boundaries of Cetus is a barred spiral cataloged as M77. This galaxy is easy to find because it lies just under 1° east of the star Delta Ceti. M77 does not appear particularly impressive on its own, but it is separated only 0.5° from a slightly fainter edge-on spiral galaxy, NGC 1055. Both galaxies are visible in the same field of view at low to moderate magnifications. Mid-sized telescopes will probably not show any structure within these galaxies, but careful observers using larger telescopes may be able to detect some spiral structure in M77, as well as a dark dust lane in NGC 1055. A dark sky with minimal light pollution will make both galaxies much easier to observe.

Cetus contains one notable planetary nebula, NGC 246, or the Skull Nebula. To find this planetary, first locate Phi1 and Phi2 Ceti, an east-west pair of stars just over 7° north of Diphda. Phi1 and Phi2 are both relatively dim (magnitude 5), but they are the brightest in their immediate vicinity. NGC 246 lies south of this pair, forming the third corner of an equilateral triangle with the two stars.

The Skull Nebula is rather large for a planetary, with an apparent diameter of nearly 4 arcminutes. (“Large for a planetary” is still fairly small.) It is not particularly bright, but it is readily visible in a dark sky with a mid-sized (8”–10”) telescope. NGC 246 is nearly circular, and most of the nebula’s rim is a little brighter than its center. Several stars are visible, superimposed on the planetary’s disk, one of which is its central star. Try using moderate to high magnification to study NGC 246 once you’ve found it. An OIII or UHC filter will significantly increase the contrast between the nebula and the background sky.

Cetus is also home to Mira, one of the most famous variable stars in the sky. Mira varies in brightness over a period of about 332 days. Mira’s exact brightness range varies a little, but at its dimmest, the star often reaches a magnitude of about 9.5, which is too dim to be visible without optical aid. Mira often peaks at magnitude 3–3.5, which is not particularly bright, but is easily visible with the unaided eye. Mira reached its most recent peak in mid-July, so the star currently is at a dimmer time.

Star map of Cetus

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