Object of the Week 25th June 2023 - NGC 5897

NGC 5897

GCL 33, H VI-8

Globular Cluster

Libra

R.A. 15h17m24.0s

Dec. -21°00'36"

Magnitude: 8.40

Size: 11.0'

Class XI


Given there is no astronomical dark here at 52N now until sometime in August there
are few classes of objects worth going out to have a look at, however globular clusters
are one of them and my choice this week is the globular cluster NGC 5897 in Libra.
Discovered by William Herschel in 1785 NGC 5897 is a quite remote globular lying
about 12550 pc from us ad if at that distance is perhaps 170 light years in diameter.
It is perhaps 7400 pc from the galactic centre. Herschel classified it as part of his
group VI objects so it was a fairly faint compressed group to him. I am always
surprised given how many globular clusters he found that he did not have a separate
group for them as he did for planetary nebulae. It has a concentration class of XI, so
almost as diffuse as it gets. The cluster is classed as metal poor, hence old, although it
would appear to have some younger stars which suggest as second bout of star forming
later on. The two star formation events may have only been separated by a few hundred
million years. The metallicity would suggest an age of perhaps 13 billion years.

Attachment 5170

NGG 5897 from PanSTARRS

Interestingly this low metallicity renders the periods of the clusters RR Lyra variables
longer than normal. NGC 5897 is also home to a small number of blue straggler stars.
It is classified as a halo cluster. NGC 5897 is quite large and diffuse so only its core may
be seen from less than optimal locations. It can however been seen with relatively small
telescopes, although may not be resolved with those. The low central concentration and
large radius suggests that NGC 5897 may not have made too many passes through the
galactic plane as it has not suffered from significant tidal evaporation. Perhaps
unsurprisingly NGC 5897 despite its faintness makes many best of observing lists. As it
also lies close to the ecliptic it is occasionally photobombed by planets.

Attachment 5171

NGC 5897 from Hubble

Back in 2005, with a 20" at 272x: "Very much like an open cluster. One of the more larger and fainter ones. Looks like a Class X-XII. (really = Class XI)."

I logged this one with my 6" telescope back in August 2005 from Southern France. During astronomical twilight I managed to see one star on the western edge and either a star in the center or a very compact core. Given the compactness class of XI, I suppose now that it was a star.

Hi, here's my observation from 4/19/2023: Ngc 5897 globular cluster in Libra ; magnitude:8.6 ; size:12.6' ; fairly large but low surface brightness, a few stars do pop in and out of visibility. I used a 17mm Ethos at 133x with my 14.5 Starstructure f/4.3Attachment 5172

Surprisingly, in 1914 Joseph Hardcastle (a grandson of John Herschel) classified NGC 5897 as a known spiral nebula! Just a year later, this was put to rest when a photograph taken at Helwan Observatory in Egypt showed it was a loose globular cluster and "certainly not a spiral nebula" (Knox-Shaw). Here's a rather old observation I made with my former 18".

This low surface brightness globular was easily picked up 12.5x in the 80mm finder as a small, faint, hazy knot. Excellent view at 225x in the 18" with 60-70 stars resolved and a very weak concentration and no distinct core. At 325x, 90-100 stars were resolved over an 11' diameter though the halo doesn't have a well defined boundary so the count is somewhat arbitrary. The hazy background glow of unresolved stars is quite lively and irregular in surface brightness.

I will have to look at this again. My last observation was with an 8" in 2001:
med.size, faint, round, somewhat diffuse, slightly flatter 1 side, only few *'s vis, no strong core, slightly brighter in ctr.
The horizontal branch is mag.16.3 +/-, so the 8" fell shy of seeing stars across the face.
My 12.5" should reach the horizontal branch fairly easily.
The mag.22 isophote size is 4.5', while the mag.25 isophote is 11', so it would be a lot larger in dark skies.

Remember the good old Edmund Mag 5 Star Atlas? Flip to Map 5 and you'll see this globular plotted in Libra. In my beginner days, I generally trusted anything in Edmund Mag 5 as "visible easily in my 8 inch scope", but this one threw a curveball. I logged this object back in 2006 perhaps, as "Extremely faint even from the countryside. Maybe better in large binoculars."

This object is not easy. After a long gap, I took another look at it in March this year, with my 25x100 binoculars on a night in the Anza Borrego desert of southern California when it was too windy to set up my 18". The logs read:

"A substantially sized, elongated, fairly bright patch with a gradually brighter middle. A distinctive star pattern lies to the north east. Can be held continuously with direct vision although averted vision is best to detect the elongation."

I remember it being substantially brighter than NGC 5053 which I looked at earlier that night. Going from "extremely faint" to "fairly bright" in about 17 years, I guess, is a good sign :). But it must be noted that my descriptions aren't as calibrated as Steve Gottlieb's or some of the more experienced folks here.

Clear Skies
Akarsh