Sanny wrote:
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.

Some of them are easy, the Andromeda galaxy, and the two Magellanic
Clouds, are both nearby independent galaxies, none of which we can see
from Earth's northern hemisphere, but you can see them from the south.
They look quite different from stars, they look like a spread out cloudy
smear to the naked eye. Andromeda looks bigger than the full moon,
though much fainter.

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.

They don't, they just see the smear, and they estimate the number of
stars in it. They may be able to distinguish specific individual stars
inside it, if one of those stars go supernova, or they are large
variable (blinking) stars.

The galaxies that are further away, wouldn't be visible to our naked
eyes anyways. But if they were naked-eye visible, they would be just
pinpricks to us.

The 100 billion star estimate might have to be revised to more like 500
billion per galaxy. As our telescopes get better we're discovering
within our own galaxy that the vast majority of stars are really red
dwarf stars, almost 80% of it in fact. We had previously thought that
stars around the size of our Sun were the majority, but we couldn't see
the faint little red dwarves until now. So the estimates are going up.

It may happen That is just a Star with a Billions of small planet
sized things.

They can tell it's not within our galaxy by the faintness of it. They
can tell it's not a star with many glowing planet-like things because
they wouldn't be nearly as bright as their own star.

And when we are living inside the Galaxy how do astronomers descide
that our galaxy is spiral.

We're not in the main disk of our galaxy, we're slightly higher off the
plane and looking down at it from an angle. Again, most of this is not
visible to us from the northern hemisphere, the people in the southern
hemisphere are much luckier than us in this respect.

As far as I know whichever direction I see I can see infinite number
of stars.

Using just your naked eye, all you are seeing are just stars that are
part of our own galaxy. I don't think there are any particularly
noticeable nearby galaxies visible from the northern hemisphere.

All galaxies we see from the north are only noticeable through telescopes.

So what are they basis of these assumption?

What I bilieve is stars are randomly distributed throughout the
Universe.

Sort of, stars are mostly distributed inside galaxies, and the galaxies
look quite randomly distributed. In actual fact, the galaxies are
distributed in a chaotic tangled spiderweb shape across the universe, so
it's not truly random just merely complex.

As a side note, where there isn't any galaxies, the universe is infused
with intergalactic gas. There's nearly ten times as much mass in the
intergalactic gas as there are in the galaxies. The gas could have
clumped together to form ten times as many galaxies, except that the
existing galaxies often capture some of this gas and incorporate them
into their own internal structures. Other times the jets, winds and
radiation emanating from the existing galaxies blow this gas away, so
they cannot form into new galaxies. There has been some instances of
stars observed that were not part of a galaxy, but floating around in
intergalactic space. Some of these stars may have been part of a galaxy
once, but during a galactic merger, they got tossed out. While others
seem like they may have been born right in the intergalactic gas; some
intergalactic stars were even large enough to go supernova, so they must
have gathered enough intergalactic gas to become truly colossal in size.

And Universe is Finite or Infinite is again a Question.

Say Universe is Finite then it must have some Shape.

There is no definitive answer for that, just theories. Even if the
Universe is finite, the part that we see right now is only a small part
of it. It is generally agreed that there might be some parts of the
universe that are expanding away from us *faster* than the speed of
light, due to the inflation of the universe. This doesn't violate the
laws of relativity because those laws only govern how fast matter and
energy inside the universe can travel through the universe, but not now
how fast space can travel in relation to itself. So space itself is
carrying away parts of the universe faster than light speed, and the
light from that part of the universe will never reach us. The Cosmic
Microwave Background is considered to be the sheath behind which the
rest of the Universe is.

It's all sort of like what happened when our universe was just the
Earth. And in fact, the Earth was merely everything we could see to its
horizons. Eventually we discovered that if we kept walking the horizon
always remained just as far away, except we were seeing new things. We
then eventually discovered that the Earth was not flat, but round. We
are in the same position now within the Universe, we are only seeing to
its horizons, but not beyond.

Lets assume it has a Shape of a Sphere.

Then That sphere must have some radius.

What happens about the place outside that sphere.

Say our Universe Radius is "r"

What are the things ar a distance 2r, 3r, 100r, 1000r. Why do we not
consider the things at a distance 1000r as part of our Universe?

Why we confine our Universe to be a Sphere of radius r ????


Anything outside our point of view cannot be observed, so there is no
way to make any measurements or take any readings. We can only consider
that which we can see. The stuff outside the horizon, whether it exists
or not, whether it is 2 times larger, or 10 times larger, 100 times
larger or 1000 times larger is irrelevant, we cannot even begin to guess
at its size.

Yousuf Khan