> When I see sky I only see small stars twinkling at night.
>
> Now When I see sky with a large telescope I will again see many stars.
>
> How do I know whether a twinkling Object is a Star from our Galxy or
> it is some other Galaxy.
>
> How do scientist see the very far away Galaxies and see Billions of
> Stars in it.
>
> Can they really see each star in the Galaxy and how do they count that
> there are 100 Billion stars in that galaxy.
>
The Official Story;
Toward the end of the 18th century, Charles Messier compiled a catalog
containing the 109 brightest nebulae (celestial objects with a
nebulous appearance), later followed by a larger catalog of 5,000
nebulae assembled by William Herschel. In 1845, Lord Rosse constructed
a new telescope and was able to distinguish between elliptical and
spiral nebulae. He also managed to make out individual point sources
in some of these nebulae, lending credence to Kant's earlier
conjecture.
In 1917, Heber Curtis had observed the nova S Andromedae within the
"Great Andromeda Nebula" (Messier object M31). Searching the
photographic record, he found 11 more novae. Curtis noticed that these
novae were, on average, 10 magnitudes fainter than those that occurred
within our galaxy. As a result he was able to come up with a distance
estimate of 150,000 parsecs. He became a proponent of the so-called
"island universes" hypothesis, which holds that spiral nebulae are
actually independent galaxies.
In 1920 the so-called Great Debate took place between Harlow Shapley
and Heber Curtis, concerning the nature of the Milky Way, spiral
nebulae, and the dimensions of the universe. To support his claim that
the Great Andromeda Nebula was an external galaxy, Curtis noted the
appearance of dark lanes resembling the dust clouds in the Milky Way,
as well as the significant Doppler shift.
The matter was conclusively settled by Edwin Hubble in the early 1920s
using a new telescope. He was able to resolve the outer parts of some
spiral nebulae as collections of individual stars and identified some
Cepheid variables, thus allowing him to estimate the distance to the
nebulae: they were far too distant to be part of the Milky Way. In
1936 Hubble produced a classification system for galaxies that is used
to this day, the Hubble sequence.
http://en.wikipedia.org/wiki/Galaxy
A Cepheid variable or Cepheid is a member of a particular class of
variable stars, notable for a fairly tight correlation between their
period of variability and absolute luminosity. The namesake and
prototype of these variables is the star Delta Cephei, discovered to
be variable by John Goodricke in 1784.
http://en.wikipedia.org/wiki/Cepheid_variable
The Hubble sequence is a morphological classification scheme for
galaxies invented by Edwin Hubble in 1936. It is often known
colloquially as the Hubble tuning-fork because of the shape in which
it is traditionally represented.
Hubble’s scheme divides regular galaxies into 3 broad classes -
ellipticals, lenticulars and spirals - based on their visual
appearance (originally on photographic plates). A fourth class
contains galaxies with an irregular appearance. To this day, the
Hubble sequence is the most commonly used system for classifying
galaxies, both in professional astronomical research and in amateur
astronomy.
http://en.wikipedia.org/wiki/Hubble_sequence
