How can the scientists say that there are other galaxies in the cosmos ?

Answer 1

Because they can be seen. And there are hundreds of thousands which have already been photographed. Some are close enough that we can see them in good binoculars or a modest telescope. Then we see the mosaic of the structure of our own galaxy from the inside and it is entirely consistent with what we see of other spirals. By measuring red shifts, we see that they are rotating as well, many just like ours.
A hundred years ago, astronomers thought these were nebulae in our own galaxy, but much more was learned when we applied red shift measurements to them, when individual stars could be resolved, and when Cepheid variable studies were done.

Answer 2

Galaxies are stars and panets, and a blackhole...we are in the milky way galaxy...the next galaxy closest to us is called Andromeda

Andromeda Galaxy
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See Andromeda (disambiguation) for other uses of "Andromeda".
Andromeda Galaxy
A visible light image of the Andromeda Galaxy.
Credit: John Lanoue.

Observation data: J2000 epoch
Constellation: Andromeda
Right ascension: 00h 42m 44.3s[1]
Declination: +41° 16′ 9″[1]
Redshift: −301 ± 1 km/s[3]
Distance: 2.54 ± 0.06 Mly
(778 ± 17 kpc)[2][3][4][5][6][a]
Type: SA(s)b[1]
Apparent dimensions (V): 190′ × 60′[1]
Apparent magnitude (V): 4.4[1]
Notable features:
Other designations
M31, NGC 224, UGC 454,
PGC 2557[1]
See also: Galaxy, List of galaxies
The Andromeda Galaxy (IPA: /ænˈdrɒmədə/, also known as Messier 31, M31, or NGC 224; older texts often called it the Great Andromeda Nebula) is a spiral galaxy approximately 2.5 million light-years away[4] in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way.

Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.[7] However, recent observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy.[8] 2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7.1×1011 solar masses.[3]

At an apparent magnitude of 4.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,[9] making it easily visible to the naked eye even when viewed from areas with moderate light pollution. It appears quite small without a telescope because only the central part is bright enough to be visible, but the full angular diameter of the galaxy is seven times that of the full moon.

Answer 3

Because we can see them.

The Great Debate (26 April 1920) was about whether some 'nebulae' were part of our Galaxy or if they were other galaxies outside our own.

At the time (due to poorer performance of instruments) the debate was won by those who thought that our Galaxy was the entire universe. The clincher was an observation that M31 (one such 'spiral nebula') was seen rotating. This would show that it is small and close.

Since then, we have been able to see that these 'special nebulae' are made up of stars. For example, Doppler shift observations show that M31 (now known as the Andromeda galaxy) cannot be seen as rotating, whatever instrument one takes. It too (like our Galaxy) takes hundreds of millions of years to turn around its centre, therefore it is impossible to see any rotation over a lifetime. Van Maanen's observations were optical illusions combined with a desire to see rotation even if it was not there.

Thanks to some (relatively common) events like supernovae and stars like Cepheids, we have been able to determine that the distance to these 'nebulae' is far greater than the size of our galaxy and that their sizes are properly galactic.

In other words, they too are galaxies as big and as rich in stars as our own.

Later, Hubble (the astronomer) was able to determine that the light we get from the fainter ones shows the spectral absoption lines of hydrogen, except that they are all shifted in frequency. From that, he was able to establish a relationship between a galaxy's distance from us and the rate at which space expands between that galaxy and us.

This too confirmed that these objects were clearly outside our galalxy, and very far away (meaning that there were things outside our galaxy and that these 'somethings' were very big).

Answer 4

The fact that we can actually SEE them is the most powerful reason.

One of the above answers points out that the Andromeda galaxy spans 190 minutes of arc across in the sky. The full moon is about 30 minutes wide. Just think about that for a while -- if M31 were as bright in the sky as it is in photographs it would stretch over six times as wide as the moon! It would be eye popping. And when you realize the thing is two million light years away the immensity of it is astounding.

Answer 5

How can anyone in their right mind imply that there are not? If I look at the constellation of Andromeda I can see one without any optical aid, and if I set up a camera and telescope I can get some fantastic pictures of it. And there are many others easily seen in telescopes. You've never actually looked at the night sky properly, have you?

Answer 6

Because they have seen these galaxies by their own eyes in the cosmos.

Answer 7

There are a good selection of issues to be chanced on, if via chanced on you advise proved. a great number of our cosmology is extremely theoretical, in keeping with mathematical modeling, and what we are able to work out, one way or yet another, from trillions of miles away. we are nevertheless being astounded as we reassess our understand-how of the image voltaic equipment, as probes and landers return extra no longer effortless data. you may use this actuality as a ruler to savour how little we could have particularly found out approximately platforms completely previous our testable attain. nevertheless, commentary maintains to yield extra insights, and smart speculations. information on the subject of the mechanics of cosmology that could at some point enable us extra applicable understand our place interior the universe, out origins, and the energies around us.

Answer 8

scientists have things at their disposal today that they didn't have yrars ago-like the hubble telescope which can see things in space that the strongest telescope on earth couldn't see. the universe is much like looking at smaller things such as through a microscope. aply that principle to the larger things and it makes sence.

Answer 9

with pictures... lights the fastest thing we know of and cameras can capture them , the galaxies they're mentioning may not exist anymore since its so far away and light does have travel time but some of them are still there and visible.

Answer 10

From pictures like the one on this link--------
The Hubble Deepfield
http://hubblesite.org/newscenter/archive/releases/1996/01