A Distant Outpost in the Galactic Halo
Posted by Brian Ventrudo
The tiny constellation Delphinus, the Dolphin holds a handful of extraordinary sights for small telescopes. Perhaps the most intriguing is the faint globular cluster NGC 7006, a distant ball of stars standing guard on the edge of intergalactic space.
NGC 7006 is a challenging object for a small telescope because it is quite faint (about magnitude 10.6) and tiny (just 3’ across). In a 3-4” scope, it will be nothing more than a dim smudge in a good star field. In a 6-8” scope, it will be a brighter smudge but still unresolved. Even a 12” scope cannot resolve the stars of this faint cluster. It took the extreme sensitivity of a Mallincam to resolve a few stars in the outer halo of this distant outpost (see image above, and click it to enlarge).
NGC 7006 appears so faint because of its extraordinary distance, some 140,000 light years. The orbit of this cluster about the center of the Milky Way is quite eccentric. At one end of its orbit it comes 60,000 light years from the galactic core, and at the other extreme it is some 330,000 light years away. That’s 1/6 the distance to the Andromeda Galaxy and further away than the Magellanic Clouds. At present, NGC 7006 is one of the most distant of all the Milky Way’s 150 known globulars. So while it’s not a striking object visually, NGC 7006 is worth observing for its extraordinary position in the Milky Way.
The cluster’s eccentric orbit suggests NGC 7006 was once part of another galaxy which was absorbed in whole or in part by the Milky Way.
The distance to globular clusters is determined by observing a select group of the cluster’s variable stars called RR Lyrae stars. The period of variability of these stars is proportional to their brightness. So when astronomers measure nearby RR Lyrae stars and calibrate the period-brightness relationship, they can use this relationship to indirectly measure the distance to objectsfor which there is no other means to do so.
This fortunate situation was used by the famous astronomer Harlow Shapley in the early 20th century to determine the distance to many globular clusters, and to determine such clusters are primarily gathered about the core of the Milky Way in a giant halo. That’s why we see more globular clusters in Sagittarius, near the galactic center, than we do in Ursa Major, for example, which is near the galactic pole.
NGC 7006 is located just 3º west of the star γ (gamma) Delphinus, itself an excellent double star for a small telescope (see map below).