Weird Science

M13 - Great Globular Cluster in Hercules

Polish ver­sion is here

On a moon­less, clear night, with ideal atmo­sphe­ric con­di­tions, excel­lent eye­si­ght, and far from cities and other sour­ces of light pol­lu­tion, a nota­ble detail can be spot­ted with the naked eye in the con­stel­la­tion Her­cu­les. It appe­ars as a faint, dif­fuse point of light and lies almost exac­tly on the line con­nec­ting Eta (η Her) and Zeta (ζ Her) of this con­stel­la­tio­n—ap­pro­xi­ma­tely one-quar­ter of the way from Eta toward Zeta. Under nor­mal con­di­tions, howe­ver, this object is dif­fi­cult to see, and obse­rving its struc­ture cer­ta­inly requ­i­res the use of appro­priate opti­cal equ­ip­ment. Using a tele­scope with a mir­ror dia­me­ter of just 10 cm, one can obse­rve that the dif­fuse spot is actu­ally a sphe­ri­cal col­lec­tion of den­sely pac­ked stars.

This object is the Her­cu­les Clu­ster, disco­ve­red in 1714 by Edmond Hal­ley and later cata­lo­ged by Char­les Mes­sier as num­ber 13 (M13) in his famous cata­log. It is the lar­gest and bri­gh­test glo­bu­lar clu­ster in the nor­thern sky, loca­ted appro­xi­ma­tely 25,000 light-years from Earth, with a dia­me­ter of 145 light-years. Mea­su­re­ments and obse­rva­tions indi­cate that the clu­ster con­ta­ins at least seve­ral hun­dred tho­u­sand stars. The Her­cu­les Clu­ster has an appa­rent magni­tude of 5.8m, but as an exten­ded object span­ning about 20 arc­mi­nu­tes in dia­me­ter, it is much har­der to spot than a star of simi­lar bri­ght­ness.

Unlike stars that inde­pen­den­tly orbit the core of our galaxy, stars in glo­bu­lar clu­sters orbit a com­mon cen­ter of mass. Some of these stars fol­low cir­cu­lar orbits, while others have eccen­tric orbits exten­ding to the clu­ster's outer regions. Due to gra­vi­ta­tio­nal inte­rac­tions within the clu­ster, the orbi­tal speed of ligh­ter stars incre­a­ses, while that of hea­vier stars decre­a­ses. This results in a sort of stel­lar sor­ting: ligh­ter stars tend to gather in the clu­ster’s outer regions, while hea­vier stars con­cen­trate in orbits clo­ser to the core. Glo­bu­lar clu­sters like M13, along with the galac­tic bulge, are con­si­de­red the oldest com­po­nents of the Milky Way.

Obse­rva­tions

May 13, 2020 – Jaworzno (Poland), gar­den
urban con­di­tions, high level of light pol­lu­tion

The pho­to­graph was taken from my gar­den on a clear nigh­t—I mar­ked the clu­ster as well as two stars with magni­tu­des of about 7m. Howe­ver, it is worth noting that the bri­ght­ness of some of the visi­ble stars is as low as magni­tude 11 or even 12, making them impos­si­ble to obse­rve with the naked eye.

I must say that the Her­cu­les Clu­ster is a beau­ti­ful object, loo­king stun­ning thro­ugh a tele­scope eye­piece. The pho­to­graph below is the first I have taken of this object and, unfor­tu­na­tely, it is sli­gh­tly out of focu­s—so I now know what to improve during my next ses­sions.

Ilustracja: Photo 1 – without labels

Photo 1 – without labels

Ilustracja: Photo 1 – with labels

Photo 1 – with labels

The clu­ster was also visi­ble using 11x45 bino­cu­lars.

Photo 1 Para­me­ters:

  • Total expo­sure time: 10 minu­tes (stack of 10 RAW fra­mes at 60s each, using an appro­priate num­ber of dark, bias, and flat fra­mes)
  • ISO: 1600
  • Mak­su­tov-Cas­se­grain tele­scope (100/1400), prime focus expo­sure
  • A fil­ter was used to reduce the effects of arti­fi­cial light pol­lu­tion and atmo­sphe­ric glow
  • Mount: equ­a­to­rial mount with trac­king, ali­gned using the drift method and con­trol­led by a custom-built sys­tem.

Marek Ples

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