by NoelC » Wed Mar 07, 2007 7:48 pm
BMAONE23 wrote:Every time I have noted a localized star or cluster in most images, the individual stars will appear to have defraction spikes but galaxies do not. If this is the cast then there is only one individual star in this image and all remaining light is from galaxies.
You see the diffraction spikes because the exposures are made to such a depth as to be able to see tremendously dim objects, and the stars are being overexposed. The imagers and telescope are so good that you do not usually see the starlight very much spread out as a white blob as is the case with less sophisticated telescopes/imagers, such as amateur equipment. The only thing giving away the fact that the maxed-out pixels are so extremely overexposed is the diffraction effect of the telescope's secondary mirror support structure. Thus, the bright stars - in our own galaxy - are showing up with spikes.
However...
Do not infer that if there are no spikes the object must not be a local star. Indeed, it could well be a star in our galaxy that just isn't so bright/close as to overexpose to the level of exhbiting diffraction spikes.
Examining a Hubble image at full resolution, before processing into a presentation quality image, it is a bit easier - though notably not definitively so - to tell which point-like light sources are stars and which are galaxies or nebula components.
It should also be noted that energetic particles quite often leave marks in Hubble image data. Again, these are reasonably easy to differentiate when working on the images, but not absolutely so. If there is more than one exposure of the same object, then the knowledge that a light pixel or group of pixels shows up in one exposure and not the others can be used to help clean up these spots.
Looking closely at this image, I would say that there is definitely more than one star shown, but that the majority of light sources are extragalactic.
-Noel
[quote="BMAONE23"]Every time I have noted a localized star or cluster in most images, the individual stars will appear to have defraction spikes but galaxies do not. If this is the cast then there is only one individual star in this image and all remaining light is from galaxies.[/quote]
You see the diffraction spikes because the exposures are made to such a depth as to be able to see tremendously dim objects, and the stars are being overexposed. The imagers and telescope are so good that you do not usually see the starlight very much spread out as a white blob as is the case with less sophisticated telescopes/imagers, such as amateur equipment. The only thing giving away the fact that the maxed-out pixels are so extremely overexposed is the diffraction effect of the telescope's secondary mirror support structure. Thus, the bright stars - in our own galaxy - are showing up with spikes.
However...
Do not infer that if there are no spikes the object must not be a local star. Indeed, it could well be a star in our galaxy that just isn't so bright/close as to overexpose to the level of exhbiting diffraction spikes.
Examining a Hubble image at full resolution, before processing into a presentation quality image, it is a bit easier - though notably not definitively so - to tell which point-like light sources are stars and which are galaxies or nebula components.
It should also be noted that energetic particles quite often leave marks in Hubble image data. Again, these are reasonably easy to differentiate when working on the images, but not absolutely so. If there is more than one exposure of the same object, then the knowledge that a light pixel or group of pixels shows up in one exposure and not the others can be used to help clean up these spots.
Looking closely at this image, I would say that there is definitely more than one star shown, but that the majority of light sources are extragalactic.
-Noel