QUESTION: Good day
I recently read that there are more than 100 billion galaxies in the universe.
1. How and what method(s) do scientist use to calculate this figure?
2. How can a scientist state with confidence that there this amount when we cannot access other galaxies? Also how do scientist know where the universe ends and what if where we think it ends, it really does not end or what if the vast magnitude that we perceive is nothing but an illusion and it is not as large as we think it is
3. What really exists on these galaxies and how can we know?

Sunil Mahabir
Miramar / Pembroke Pines,Florida

ANSWER: Hi Sunil

There are a lot of questions here, but I will try to answer them as best I can.  

1.  The primary method astronomers use to calculate the number of galaxies in the universe is photography.  Using large telescopes and long time exposures, they can capture the light of very dim objects very far away.  And these objects look just like the galaxies that are closer to Earth, and can be seen more easily.  

2.  We can't travel to other galaxies, but we can certainly see them in the telescopes.  But you ask another interesting question--where does the Universe end?  And that's one that astronomers have not yet answered.  And so they actually calculate the number of galaxies in the "known' Universe.  Which is to say that they leave your question open---there could be more of the universe that we can't see, and they don't know what is in it.

3.  We can make a guess, based on what is on Earth and in our own Galaxy.  And while many of the galaxies are similar to our own Milky Way, others are quite a bit different.  And all we can do is estimate, based on our photographs and other observations, what conditions might be on those galaxies.   

hope that helps.

Paul Wagner

---------- FOLLOW-UP ----------

QUESTION: Thanks for the answer.
Even though you said astronomers use photography, I am still a bit confused since even if you had a photograph, it would take a tremendous amount of time to count 1 billion. How do they count these with photographs and how accurate are they. What kind of technology is used to count 1 billion + galaxies.

ANSWER: Hi Sunil

obviously, the 1 billion is an estimate.  But what they have done is count the number of galaxies in a sigle photograph of a small portion of the sky, and then multiply that to get the final number.  And So far, every time they take a photo of another section of the sky, they find MORE galaxies, not fewer!  So that number will probably go up over time.

Paul wagner

---------- FOLLOW-UP ----------

QUESTION: Thanks for the answer. It does give me perspective, but how can be sure of our perceptions that it is galaxies that we are seeing. What is there is some kind of phenonomenon like a mirage or peception error.

Hi Suhil

Well-it is not just visible light photographs.  These also involve infrared and ultraviolet parts of the spectrum, as well as raio waves, gravity waves...and even uses spectrometry analysis of the galaxies. All of those measurements indicate that these are galaxies--not just because they look like galaxies, but also because the act like galaxies as well.

Paul Wagner  


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Paul Wagner


Astronomy and telescope making. Have made at least seven telescopes, both refractors and reflectors, and have spent 30 years looking at the nighttime sky.

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