by Chris Peterson » Fri Oct 16, 2020 8:13 pm
johnnydeep wrote: ↑Fri Oct 16, 2020 6:31 pm
Ann wrote: ↑Fri Oct 16, 2020 3:23 pm
shaileshs wrote: ↑Fri Oct 16, 2020 3:10 pm
I wonder - 1) All objects without spikes are galaxies then ? 2) What's the blue spike/colored star towards top right corner ?
The bright blue star is HD 205514. It is a star of spectral class A0, not unlike Vega. The star is located some ~ 1.000 light-years away, and at that distance, it is a faint 7th magnitude star as seen from the position of the Earth.
I'd say that most of the fainter objects in the APOD are stars even if they don't have very obvious spikes. As you can see if you look carefully, there are some obviously elongated, otherwise extended or even spiral-shaped objects in the image that are clearly galaxies. Most of the bright points in the APOD don't appear to be extended, however, and most of them are unlikely to be galaxies.
Abell 78 is located in Cygnus, a star-rich constellation with comparatively few visible background galaxies.
Ann
Isn't the rule with diffraction spikes that the higher the magnitude of the object (that is, the dimmer it is), the less pronounced the spikes? At least, that seems to be the case for this picture. So, the dimmest stars would have barely discernible spikes.
For any given optical system, a fixed percentage of the energy coming from the object (star or galaxy or anything else) is diverted into the diffraction spikes. So an object that is twice as bright will have diffraction spikes that are twice as bright.
In an
image, however, things can be confused by non-linear intensity mapping. The high dynamic range of the original data is almost always remapped to emphasize dim structure and minimize blowing out of bright structure. Usually, but not always, relative brightness is maintained (that is, if one star appears brighter than another, it really is) even if the absolute photometric relationship is lost.
[quote=johnnydeep post_id=307309 time=1602873088 user_id=132061]
[quote=Ann post_id=307298 time=1602861814 user_id=129702]
[quote=shaileshs post_id=307296 time=1602861032 user_id=143908]
I wonder - 1) All objects without spikes are galaxies then ? 2) What's the blue spike/colored star towards top right corner ?
[/quote]
The bright blue star is HD 205514. It is a star of spectral class A0, not unlike Vega. The star is located some ~ 1.000 light-years away, and at that distance, it is a faint 7th magnitude star as seen from the position of the Earth.
I'd say that most of the fainter objects in the APOD are stars even if they don't have very obvious spikes. As you can see if you look carefully, there are some obviously elongated, otherwise extended or even spiral-shaped objects in the image that are clearly galaxies. Most of the bright points in the APOD don't appear to be extended, however, and most of them are unlikely to be galaxies.
Abell 78 is located in Cygnus, a star-rich constellation with comparatively few visible background galaxies.
Ann
[/quote]
Isn't the rule with diffraction spikes that the higher the magnitude of the object (that is, the dimmer it is), the less pronounced the spikes? At least, that seems to be the case for this picture. So, the dimmest stars would have barely discernible spikes.
[/quote]
For any given optical system, a fixed percentage of the energy coming from the object (star or galaxy or anything else) is diverted into the diffraction spikes. So an object that is twice as bright will have diffraction spikes that are twice as bright.
In an [i]image[/i], however, things can be confused by non-linear intensity mapping. The high dynamic range of the original data is almost always remapped to emphasize dim structure and minimize blowing out of bright structure. Usually, but not always, relative brightness is maintained (that is, if one star appears brighter than another, it really is) even if the absolute photometric relationship is lost.