Wednesday, 27 June 2018

NGC5694: bye bye Milky Way

After last week's spectacle with M80, pointing your telescope at globular cluster NGC5694 can be a bit disappointing. It lies deep in the tail of Hydra, the water snake, and with small to medium-sized telescopes you'll either have to be very lucky or have the eyes of an eagle if you want to resolve any stars in it. Still, NGC5694 is not only bigger but also a lot more interesting than its brighter counterpart in Scorpius. 

The reason for its relative faintness is its distance - a whopping 113,000 light-years, making it one of the most remote known globulars of our Milky Way. Furthermore, its position at the far end of our galaxy implicates that a lot of its light's being absorbed by interstellar dust, making it appear even fainter than it already is. Its extremely low metals content (meaning any substance heavier than hydrogen) makes us conclude that it must be one of the oldest known globular clusters and hence one of the oldest known entities in our Universe. You may recall that globular clusters are usually older than the galaxy they accompany. In the case of NGC5694, its age has been estimated to be no less than 13.4 billion years! If you compare this to the estimated age of our Universe, 13.8 billion years, you'll realise that this cluster and the stars of which it is composed already came into existence shortly after the Big Bang. With "shortly" I mean in astronomical terms of course. 

Now we move on to interesting fact number two. NGC5694's approaching us at the unusually high speed of 144km/s, but relative to our Milky Way's centre it's zooming past at 273km/s. Warp factor 9, Mister Scott! At the cluster's distance from the galactic centre, this is 60% higher than the escape velocity, meaning that NGC5694's definitely leaving our galaxy and will never come back. 

A 1976 study suggests that a high-mass object may have flung NGC5694 into its fast, hyperbolic orbit and that the Magellanic Clouds may have been the culprits. The same study also theorises that this globular may even have belonged to one of these dwarf galaxies after which the tidal forces of our galaxy tore it away.

So, doesn't all of this make this faint little blob a little bit more interesting than you would have guessed at first sight?


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