On 23 veebr, 18:25, Thomas Womack <twom...@[EMAIL PROTECTED]
>
wrote:
> In article
<26d06864-fa4c-44d1-8238-43c276203...@[EMAIL PROTECTED]
>,
> Crown-Horned Snorkack <chornedsnork...@[EMAIL PROTECTED]
> wrote:
>
> >How many stars can be seen by telescope?
>
> >Eye has standard angular resolution of about 1 minute (or larger) and
> >can see magnitude 6.
>
> >There are estimated to be something of 5000 to 6000 stars up to
> >magnitude 6 - over the whole heavenly sphere.
>
> >The whole sphere should be something like 40 000 square degrees, or
> >150 millions of square minutes. So, out of the 150 million pixels of
> >whole sky, 6000 are point sources of light, and the rest, 25 000 times
> >more, is empty dark sky.
>
> >What is the now resolution of telescopes? 0,1 seconds? 0,01 seconds?
>
> Ground-based telescopes, in a good site, without adaptive optics or
> lucky-imaging, get about 0.8 seconds. With lucky imaging you get 0.2
> seconds over a field of view of maybe a few square arc-minutes; with
> adaptive optics you get 0.05 seconds over a field of view of maybe a
> few square seconds.
>
> Hubble gets about 0.05 seconds over a field of view three arc-minutes
> on a side.
>
> >And what is the magnitude telescopes can see? +21? +25? +30?
>
> Hubble very long exposures can get to +30. People with very large
> amateur telescopes, good drives and CCDs can got to +21 in a
> three-hour exposure; below about +25 the fluctuations in the air-glow
> start drowning out the stars when working from Earth.
>
> >How many stars (point sources of light which can be seen by telescope,
> >resolved from others and believed to be a star or a close multiple)
> >can be observed in sky?
>
> I suspect every unobscured star in the Galaxy is in principle
> resolvable - lucky-imaging from Palomar gets well below the confusion
> limit even in the core of globular clusters. The question is how many
> resolvable extra-galactic stars there are; Hubble's resolution is 1
> light-year at M31, which is less than the average inter-stellar
> distance in the solar neighbourhood, but you'll only be able to get
> down to the absolute magnitude of the Sun (24 magnitudes of
> distance-modulus between 10pc and M31), so cutting off the whole
> bottom of the main sequence will reduce the number of stars
> enormously.
>
The far edge of Milky Way is something like 80 000 lightyears away: 30
000 lightyears to centre and perhaps 50 000 lightyears to the edge.
This is about 17 magnitudes of distance modulus. So, a Sunlike star
there is about +22 magnitude. Hubble very long exposures at +30 would
be about 1000 times dimmer.
Proxima Centauri is 18 000 times dimmer than Sun. And Proxima is not
the faintest red dwarf.
So, seeing red dwarfs in far edge of galaxy would be difficult.
> http://seds.org/messier/more/m031_g1hst.htmlis
a Hubble image of a
> globular cluster in M31, and I'd be pleased to get something that
> sharp of M13 in our own Galaxy from a dark-sky site with the $1000
> telescope I've borrowed.
>
> Tom
In an old globular cluster, stars should be easy to define. Old brown
dwarfs are cold and dim. Low metallicity red subdwarfs should be
hotter and smaller than high metallicity red dwarfs. Then you have the
old "white dwarfs", which are small, and not red any longer.
How completely visible are the closest Milky Way globular clusters -
that is, with respect to dim red dwarfs, brown dwarfs and "white
dwarfs"?
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