<p>The sensationalist headlines mask a rather more prosaic result that has absolutely no effect on the number of stars we might estimate are present in the observable universe <em>today</em>.</p>

<p>The study in question has discovered (actually it was always supposed and suggested that this was the case), that the distribution of galaxy masses was much more skewed to lower mass galaxies in the past. That is, there were many more small galaxies in the past. But this happens at the expense of the large galaxies - there were fewer large galaxies in the past. To first order, the overall mass contained within galaxies stays the same and so long as the ratio of stellar mass to luminosity (since this is actually what is measured) remains roughly constant, than any estimate of stellar numbers stays the same.</p>

<p>Actually, a better estimate for the number of stars in the observable universe uses the present day galactic mass function and then extrapolates that to the size of the observable universe today. This is a far more honest estimate, since we have a much better understanding of how many stars there are in our local part of the universe and can then invoke the cosmological principle to say that other places (but not times) in the universe are similar. This cannot possibly be affected by the new study since that is dealing with the numbers of galaxies <em>in the past</em>.</p>

CC BY-SA 3.0
2017-03-05T02:17:17.483Z
Appreciate the excellent explanation. Thank you
Jayrok94
CC BY-SA 3.0
2017-03-04T11:31:44.613Z
2017-03-04T11:31:44.613Z
ProfRob
<p>The result from the Christopher Conselice of the University of Nottingham is based on the number of galaxies that could be visible to us. This is not the same as the number of galaxies that exist "today", since when we look into the deep space, we also look into the past. We can think of a "light cone" that stretches out in space-time. The number of galaxies means the number of galaxies on our light-cone. This makes sense because the value 200 billion, or 2 trillion, is the number of galaxies in the observable universe. (The entire Universe may be infinite)</p>

<p>The conclusion is that in the Early universe, galaxies were smaller, dimmer, and more numerous. Over time these small galaxies have merged to form the large galaxies that we see in our local neighbourhood. </p>

<p>Now it should be noted that this is a model based conclusion. Conselice has looked at the distribution of galaxies at different distances, and hence different times, and used a model of galaxy formation to infer that at great distances (and early times) there are a great many galaxies that can't be seen. These distant, early, small and dim galaxies greatly outnumber those that can be seen. The conclusion is that there are many more galaxies on our light-cone, but most are invisible.</p>

<p>Since these galaxies are small and dim they contain fewer stars. There is no change to the estimated number of stars or to the estimated mass of matter in the universe, only to the distribution of these stars in the early universe.</p>

<p><a href="https://www.nasa.gov/feature/goddard/2016/hubble-reveals-observable-universe-contains-10-times-more-galaxies-than-previously-thought" rel="nofollow noreferrer">https://www.nasa.gov/feature/goddard/2016/hubble-reveals-observable-universe-contains-10-times-more-galaxies-than-previously-thought</a></p>

CC BY-SA 3.0
2017-03-05T02:18:43.123Z
I appreciate this very much both your answers help really sort all of that out and help with what I was researching!
Jayrok94
CC BY-SA 3.0
2017-03-04T07:14:06.307Z
2017-03-04T07:14:06.307Z
James K
<p>I have found oddly different answers through my multiple google searches.</p>

<p>I found one article that mentioned this did not alter the estimated number of stars in the universe. Link: <a href="http://www.universetoday.com/30305/how-many-galaxies-in-the-universe/" rel="nofollow noreferrer">http://www.universetoday.com/30305/how-many-galaxies-in-the-universe/</a></p>

<p>Yet other article gave the estimate of stars an increase. Link: <a href="http://gizmodo.com/we-were-very-wrong-about-the-number-of-galaxies-in-the-1787750693" rel="nofollow noreferrer">http://gizmodo.com/we-were-very-wrong-about-the-number-of-galaxies-in-the-1787750693</a></p>

<p>There were other examples but they also had similar contradictory answers.
Thank you.</p>

CC BY-SA 3.0
2017-03-04T00:34:41.840Z
A Link to galaxies estimate increase article. https://www.nasa.gov/feature/goddard/2016/hubble-reveals-observable-universe-contains-10-times-more-galaxies-than-previously-thought
Jayrok94
CC BY-SA 3.0
2017-03-04T11:11:49.570Z
Closely linked http://astronomy.stackexchange.com/questions/18708/does-the-recent-news-of-ten-times-more-galaxies-imply-that-there-is-correspond?rq=1
ProfRob
CC BY-SA 4.0
2021-01-15T20:16:18.450Z
Recently: New Horizons data cut the galaxy estimate down to $2\times 10^{11}$ from $2\times 10^{12}$ https://www.nasa.gov/feature/new-horizons-spacecraft-answers-question-how-dark-is-space
alexchandel
<universe><observable-universe>
The # of Estimated Galaxies increased 10 fold. Where does that leave our # of stars estimate?
Jayrok94
# The # of Estimated Galaxies increased 10 fold. Where does that leave our # of stars estimate?

(Asked by: username_0 on 2017-03-04)

I have found oddly different answers through my multiple google searches.

I found one article that mentioned this did not alter the estimated number of stars in the universe. Link: <http://www.universetoday.com/30305/how-many-galaxies-in-the-universe/>

Yet other article gave the estimate of stars an increase. Link: <http://gizmodo.com/we-were-very-wrong-about-the-number-of-galaxies-in-the-1787750693>

There were other examples but they also had similar contradictory answers.
Thank you.

**Tags:** universe, observable-universe

> **Comments:**
> **username_0**: A Link to galaxies estimate increase article. https://www.nasa.gov/feature/goddard/2016/hubble-reveals-observable-universe-contains-10-times-more-galaxies-than-previously-thought  
> **username_1**: Closely linked http://astronomy.stackexchange.com/questions/18708/does-the-recent-news-of-ten-times-more-galaxies-imply-that-there-is-correspond?rq=1  
> **username_2**: Recently: New Horizons data cut the galaxy estimate down to $2\times 10^{11}$ from $2\times 10^{12}$ https://www.nasa.gov/feature/new-horizons-spacecraft-answers-question-how-dark-is-space  

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## 2 answers

### Answer by username_1 on 2017-03-04. Score: 4 | Accepted answer

The sensationalist headlines mask a rather more prosaic result that has absolutely no effect on the number of stars we might estimate are present in the observable universe *today*.

The study in question has discovered (actually it was always supposed and suggested that this was the case), that the distribution of galaxy masses was much more skewed to lower mass galaxies in the past. That is, there were many more small galaxies in the past. But this happens at the expense of the large galaxies - there were fewer large galaxies in the past. To first order, the overall mass contained within galaxies stays the same and so long as the ratio of stellar mass to luminosity (since this is actually what is measured) remains roughly constant, than any estimate of stellar numbers stays the same.

Actually, a better estimate for the number of stars in the observable universe uses the present day galactic mass function and then extrapolates that to the size of the observable universe today. This is a far more honest estimate, since we have a much better understanding of how many stars there are in our local part of the universe and can then invoke the cosmological principle to say that other places (but not times) in the universe are similar. This cannot possibly be affected by the new study since that is dealing with the numbers of galaxies *in the past*.

> **Comments:**
> **username_0**: Appreciate the excellent explanation. Thank you  

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### Answer by username_3 on 2017-03-04. Score: 3

The result from the Christopher Conselice of the University of Nottingham is based on the number of galaxies that could be visible to us. This is not the same as the number of galaxies that exist "today", since when we look into the deep space, we also look into the past. We can think of a "light cone" that stretches out in space-time. The number of galaxies means the number of galaxies on our light-cone. This makes sense because the value 200 billion, or 2 trillion, is the number of galaxies in the observable universe. (The entire Universe may be infinite)

The conclusion is that in the Early universe, galaxies were smaller, dimmer, and more numerous. Over time these small galaxies have merged to form the large galaxies that we see in our local neighbourhood.

Now it should be noted that this is a model based conclusion. Conselice has looked at the distribution of galaxies at different distances, and hence different times, and used a model of galaxy formation to infer that at great distances (and early times) there are a great many galaxies that can't be seen. These distant, early, small and dim galaxies greatly outnumber those that can be seen. The conclusion is that there are many more galaxies on our light-cone, but most are invisible.

Since these galaxies are small and dim they contain fewer stars. There is no change to the estimated number of stars or to the estimated mass of matter in the universe, only to the distribution of these stars in the early universe.

<https://www.nasa.gov/feature/goddard/2016/hubble-reveals-observable-universe-contains-10-times-more-galaxies-than-previously-thought>

> **Comments:**
> **username_0**: I appreciate this very much both your answers help really sort all of that out and help with what I was researching!  

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