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# Astronomy/Counting stars within given time & volume

Question
Dear Mr. Gort,

I would like to make two separate estimates of the total number of stars that have ever existed (now and in the past) given specific parameters of time and space.  I'd like the estimates to be as accurate as possible, but I understand that are estimates.

1st estimate:
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The total number of stars that have ever existed:
* since the origin of the universe
* within the observable universe

This should include all currently "burning" stars and any stars that have since faded-out or become black holes.  Wikipedia says that the observable universe contains 170B galaxies and 100B stars per average galaxy, which I think means 17*10^21 current stars.  I know that star formation is slowing, but I haven't found a count of stars formed per year during different time periods, so I don't know how to estimate past stars.

2nd estimate:
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The total number of stars that have ever existed:
* during the past 9.2 billion years
* whose light could have reached our point in space during that time.

Again this would include current and stars that have since disappeared.  In addition to not knowing how to calculate the number of former stars, I don't know how to factor in the expansion of the universe.  Leaving that aside, my best guess is to calculate that the volume of a sphere with radius 9.2Bly is 0.825% of the volume of the observable universe, and assuming an equal distribution of galaxies, I think that means 14*10^19 current stars.  As I understand, that number would be larger if we take the expansion of the universe over the past 9.2B years into account.

Thank you so much for your help!

Sincerely,

Mattthew Brauer

Hi Mattthew,

First of all, I like your two estimates. They're very well thought out. And they identify some of the open issues. But I'd like you to more clearly state the problem in order to ensure your estimate reflects the "number of stars" you're looking for.

It's not clear how you define a "star". Is it nuclear burning entity, or do we include brown dwarfs, black dwarfs, white dwarfs, neutron stars, and/or stellar black holes? All may be considered "stars" (with the possible exception of black dwarfs) because they emit more radiation than they receive (which differentiates them from planets and asteroids, which receive more radiation from an external source than they emit).

And are you only considering "our universe", and not the (possible) large number of multiverses? And are you considering our "entire" universe or only the observable universe?

And, finally, I assume you are adopting the "classic" view of Big Bang cosmology - which is still argumentative. It is 'generally' assumed that the Big Bang occurred some 13.7 billion years ago. Although this idea is believed to be valid by the majority of cosmologists, it is by no means universally accepted. The "Big Bang", based on Hubble's velocity-distance relation and the microwave background radiation, is very much still a theory, and there are alternative theories. Some observations do not support an expanding universe or the Big Bang. There are several references concerning this, but perhaps the most authoritative person is Halton Arp, a leading astronomer and researcher on galaxies, who wrote "Seeing Red". That book is highly recommended to get an alternative view.  Or read "A Different Approach to Cosmology" by Hoyle, Burbidge, and Narlikar.  That's another great book which gives a scientific view on how the universe has always been in a steady state (and stars will have been created and will have died for an indefinite period of time).

But I assume you mean a nuclear burning entity in our universe, with a space-time origin some 13.7 billion years ago. That puts some bounds on the problem.

That said, the rate of star formation in the early universe was very much greater than it is now. In fact, it's estimated that 95% of all stars that will ever exist have already been born (albeit some have died). If you're interested in the classic paper on the subject, it's at http://arxiv.org/pdf/1202.3436v2.pdf

The other pertinent fact is that many more massive stars (with short lives) were created in the early universe. The resulting supernovae spread heavier elements through the universe and resulted in second-generation (Population I) stars.

So the question becomes - what are supernova rates and how have they changed? Less massive stars (which will not supernova) are generally long - lived. Most will not have died off since the Big Bang or when they were formed. A good paper on the evolution of supernova rates is http://iopscience.iop.org/0004-637X/486/1/110/fulltext/35162.text.html.

So that's about the extent of my advice. Define the problem clearly, and then use the best research and follow it to its logical conclusion. You're on the right path. If you refine it just a bit, it could be very useful, publishable material. Good luck!

Prof. James Gort

Astronomy

Volunteer

#### James Gort

##### Expertise

Questions on observational astronomy, optics, and astrophysics. Specializing in the evolution of stars, variable stars, supernovae, neuton stars/pulsars, black holes, quasars, and cosmology.

##### Experience

I was a professional astronomer (University of Texas, McDonald Observatory), lecturer at the Adler Planetarium, professor of astrophysics, and amateur astronomer for 42 years. I have made numerous telescopes, and I am currently building one of the largest private observatories in Canada.

Publications
StarDate, University of Texas, numerous Journal Publications