Australian Sky & Telescope - 04.2019

52 AUSTRALIAN SKY & TELESCOPE April 2019

HYDRA I CLUSTER by Steve Gottlieb

ON CLEAR APRIL EVENINGS, the splashy summer

constellations slide toward the western horizon, and deep sky

enthusiastsareitchingtodiveintothegalaxy-richfieldsof

Virgo,Leo,andComaBerenices.Theseconstellations,along

with Canes Venatici and Ursa Major, headline 32 of the 40

Messier galaxies. But farther south and often ignored is the

winding constellation of Hydra, the Water Snake, which

uncoils across 100° of the sky and contains its own bonanza

of galaxies.

THYDRAIFINDERCHARTFindtheclusternestledbetweenMu,

NuandXiHydraeandAlphaAntliae.Twooutliers,NGC3313andNGC

3393, lie slightly north and northeast, respectively, of the cluster core.

At a distance of 165 million light-years, the Hydra I

Cluster is one of the nearest rich galaxy clusters beyond the

VirgoCluster.Physically,thetwoclustersdisplayastriking

resemblance.HydraIisthreetimesasdistantastheVirgo

cluster, but its apparent size is^1 / 3 as large. So their linear

dimensions are nearly the same. And each cluster contains 50

galaxies within two magnitudes of their dominant members,

M87inVirgoandNGC3311inHydra,bothsupergiant

ellipticals with comparable absolute magnitudes.

Redshiftsurveyshavedemonstratedthattheclusteris

remarkably isolated in space, with no foreground galaxies

and a huge void in the background out to a distance of 400

million light-years. This allows for easy identification of

cluster members to fainter magnitudes based on redshift.

The NASA-IPAC Extragalactic Database lists upwards of 350

knownmembersina5°region.

John Herschel discovered nine of the central galaxies that

carry NGC designations in March 1835 and 1836 during his

surveyofthesouthernskyfromtheCapeofGoodHope.In

1835, he observed the cluster on three consecutive sweeps

(slowly swinging the scope up and down several degrees while

pointingduesouth)withhis18-inchspeculumreflectorin

an attempt to accurately determine positions. But timing the

closely spaced meridian transits while recording the altitude

wasdifficultastheflurryofgalaxiesdriftedby.Asasolution,

he made carefully drawn sketches in both years but realised

“in each diagram only 7 were seen and laid down, yet there

areinrealityatleast9inthewholegroup.”

Hydra I, also known as Abell 1060, just made the southern

cut-offlimit(–27°declination)ofGeorgeAbell’s1958

catalogue of 2,712 rich galaxy clusters.

A large aperture isn’t a requirement; several 11.5- to

12.5-magnitude members are within reach of a 15- to 20-cm

scope. But it’s always best to make your observations when

the cluster is highest near the meridian — about 11:00pm

_

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a

g

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p^1

h

+

11 h 00 m 10 h 30 m 10 h 00 m a^9 h^30 m

ANTLIA

CRATER

HYDRA

SEXTANS

44

Alphard

–10°

–20°

–30°

3393

3313

Hydra I

Star magnitudes

2 3 4 5 6 7

The

Asp

in

Autumn

Slide along the serpent’s

curves to explore the

Hydra I Galaxy Cluster.