What are these outward streaks in this globular cluster?

greatergood · Post by **greatergood** » Thu Dec 22, 2005 5:22 pm

In the corners of this image there are dozens of streaks going radially outward from the center of this globular cluster:

http://antwrp.gsfc.nasa.gov/apod/ap050905.html

Is this normal? I can think of three possible causes for the streaks:

1) At the center of the cluster there are explosions which eject material outward. In the vacuum of space however wouldn't the ejected matter stay as a cohesive unit, not as streaks - ie. there is no drag on moving matter in a vacuum though I suppose gravity of surrounding stars could provide the drag to rip apart the ejcted matter. If this is the case, then these explosions have to be incredibly huge to eject so much matter so far out from something with so much mass. Maybe these are the result of supernovas within the cluster.

2) The gravitational force of the cluster is pulling stars inward (ripping them apart actually) into the center of the cluster. This scenario suffers from the same problems noted in #1 above with regard to streaking in a vacuum. Also, why then aren't all the stars sucked back into the center of the cluster? Perhaps the strekaing ones are the stars that existed prior to the formation of the cluster, and all other stars are being uniformly ejected from the cluster center.

3) Gravitational lensing of stars in the background behind the cluster. But shouldn't such lensing result in streaks that are perpendicular to these streaks?

What other possibilities are there?

Globular clusters have always been a mystery - why don't they blow themselves apart, and what's keeping them from collapsing under their own mass? How do they evolve - are the stars born there in the middle or on the fringes? How will they die - through supernovas in the middle, or by burning out in the fringes? Maybe these streaks can provide some clues.

harry · Post by **harry** » Fri Dec 23, 2005 5:41 am

The Cluster is very common.

I would assume that a black hole is playing part.

As for the streaks I would say maybe lensing or possible objects burning towards the centre.

There is no reason for the stars to collapse, eventually they will be sucked into the black hole

Empeda2 · Post by **Empeda2** » Fri Dec 23, 2005 12:25 pm

Due to their position and shape I would say it's just a photographic exposure effect. It's still uncertain as to whether or not the globular clusters are housing intermediate mass black holes - the velocities of the central star does suggest that a lot of them do - though their motions can be explained without the need for a black hole so it is still uncertain.

greatergood · Post by **greatergood** » Fri Dec 23, 2005 10:42 pm

Empeda2 wrote:Due to their position and shape I would say it's just a photographic exposure effect.

Not that I doubt you, but why aren't the large stars in the immediate vicinity of the streaks also streaked? Also, why are the streaks so aperiodic, the upper left corner doesn't even show any streaks. Lastly, if it's exposure effects why are these streaks are not at isometric angles?
Bottom-left:

Bottom-right:

Top-right:

greatergood · Post by **greatergood** » Fri Dec 23, 2005 10:47 pm

harry wrote: As for the streaks I would say maybe lensing or possible objects burning towards the centre.

I don't think lensing, after some research I confirmed my suspicions: lensing effects are perpendicular to these streaks. For more info:
http://hubblesite.org/newscenter/newsde ... 32/image/a

harry · Post by **harry** » Fri Dec 23, 2005 10:47 pm

Are those streaks at the same distance.

Or are they objects closer to the lense.

I'm having second and third thoughts.

Smile, now I have to know.

Mitrovarr · Post by **Mitrovarr** » Sat Dec 24, 2005 12:36 am

My guess is that they are internal reflections from elements of the telescope or CCD. They're aperiodic because they're the reflection of stars outside of the camera field; only stars at a very specific distance outside of the field reflect right to appear as streaks. I'd guess they streak out radially because they're reflecting off a circular element.

greatergood · Post by **greatergood** » Sat Dec 24, 2005 2:49 am

Okay, so I chased down another image of the 47 tuc from the Hubble. It is a little overexposed compared to this one from SALT, and it was flipped vertically, was 44% smaller, and tilted by 6 degrees - still I managed to match them up to help clarify this issue:

It appears that these might be reflections of stars just outside the FOV. I wouldn't call it 100% conclusive, but highly likely.

...something to keep in mind for future reference.

harry · Post by **harry** » Sat Dec 24, 2005 5:02 am

I would tend to agree with you

Empeda2 · Post by **Empeda2** » Sat Dec 31, 2005 1:31 pm

That's some decent researching greatergood!