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I recall that there are of order 100 billion stars in a galaxy and 100 billion galaxies in the (observable) universe, or something roughly like that.

Since we are IN a galaxy we have an opportunity to cataloging a lot of stars!

However, being big, the observable universe offers a view of a lot of galaxies in a volume way way larger than that where individual stars are still bright enough to detect.

So I'm curious about the following.

Question(s):

  1. Are more stars or more galaxies cataloged, currently?
  2. How has the ratio evolved over time? Roughly?

For example, in antiquity, pre-telescopes, stars are going to win. Early telescopes helped Messier count 110 non-stelar objects and only a fraction of those turned out to be galaxies.

But once big telescopes and photographic plates and now CCDs and infrared (red-shift) systems and AI and crowdsourcing (eg. zooniverse) are here, perhaps galaxies have made a comeback?

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  • $\begingroup$ Well as a galaxy contains stars, where do you want to go. $\endgroup$
    – Grimaldi
    Apr 8, 2023 at 19:20
  • $\begingroup$ @Grimaldi please read through the question more carefully $\endgroup$
    – uhoh
    Apr 8, 2023 at 21:18
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    $\begingroup$ Thanks for clarifying your question, see my answer. $\endgroup$
    – Grimaldi
    Apr 9, 2023 at 7:38
  • $\begingroup$ Zooniverse is about using crowdsourcing/"citizen science" to sub-classify objects previously identified as galaxies, so it doesn't add to basic numbers of "cataloged galaxies". $\endgroup$ Apr 9, 2023 at 11:20
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    $\begingroup$ @PeterErwin can we say that it adds to the catalog's accuracy? I thought one of the functions was to confirm if blobs were even galaxies or not to begin with? (stars, artifacts, other fuzzy things...) $\endgroup$
    – uhoh
    Apr 9, 2023 at 22:37

1 Answer 1

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There are a few easy points known to the graph of (#galaxies known) / (# stars known) over time:

  • let‘s say 2000 years ago: 5000 stars known, galaxies 1, as this is roughly the number of stars that can be seen with the naked eye in the whole sky (you can only see half of these at any given spot) and we count the milky way (note: +/- 1, see next item). But this number is not „catalogued“, the number catalogued is smaller, see next item.
  • 200 BC Hipparchus listed 850 stars in his catalogue
  • ca. 1845 Lord Rosse constructed a large telescope and was able to spot single stars and the spiral nature in the andromeda nebula lending credibility to the idea that those nebulosities consisted of stars themself. Let‘s count that as 2. Using that instrument, he was able to spot stars down to mag 17, according to the formula found in limiting magnitude, which is roughly 1.1B stars, according to the reference from first bullet.
  • By 1888 the NGC was compiled consisting of 7840 entries, out of which today (querying NGC2000 using VizieR) 6032 are still listed as galaxies. At that point in time Lord Rosse‘s telescope was still the largest telescope around.
  • The Bonner Durchmusterung conducted 1859-1863 listed 340.000 stars for the northern hemisphere, those that were visible with the Fraunhofer instrument they were using.
  • today: Gaia catalog DR3 contains roughly 1.5B confirmed stars and 4M galaxy candidates.

So, to answer your questions:

  1. there have (very probably) always been more stars known than galaxies, not taking into account that galaxies consist of stars.
  2. Roughly the gap has widened first, then closed up a bit.

This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France (DOI : 10.26093/cds/vizier). The original description of the VizieR service was published in 2000, A&AS 143, 23

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    $\begingroup$ The original Bonner Durchmusterung, published in 1863 and covering the Northern sky, had about 340,000 stars. $\endgroup$ Apr 9, 2023 at 9:23
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    $\begingroup$ Thanks @PeterErwin! IANA Historian of Astronomy. Let‘s make this answer better together! @uhoh here have been star charts and listings of stars even in ancient times, e.g. the original magnitude scale was developed by Ptolomy and Hipparchus. Hipparchus listed 850 stars 200 BC. $\endgroup$
    – Grimaldi
    Apr 9, 2023 at 9:50
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    $\begingroup$ Check: en.wikipedia.org/wiki/Chinese_star_maps $\endgroup$
    – Grimaldi
    Apr 9, 2023 at 10:06
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    $\begingroup$ The New General Catalog is mostly galaxies, but there are a hundred(?) or so globular clusters and assorted nebulae. $\endgroup$ Apr 9, 2023 at 11:23
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    $\begingroup$ This is the TAP VezieR query, I used to arrive at the 6000 Gx: -- output format : csv SELECT TOP 100 ngc.Type, count(*) FROM "VII/118/ngc2000" ngc where ngc.Name not like 'I%' group by ngc.Type $\endgroup$
    – Grimaldi
    Apr 9, 2023 at 15:18

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