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1/ I've been asked a few times in recent years about current best estimates of the number of stars in our Milky Way (MW) galaxy. I'll show my estimate here but am curious if others can point me to better/published estimates.
2/ Recent estimates of the *stellar* mass of the MW in billions of solar masses BSM):
50+-10 BSM(Bland-Hawthorn & Gerhard 2016 https://ui.adsabs.harvard.edu/abs/2016ARA%26A..54..529B/)
54.3+-5.7 BSM(McMillan 2017 https://ui.adsabs.harvard.edu/abs/2017MNRAS.465...76M/)
50.4+4.3-5.2 BSM(Cautun+2020 https://ui.adsabs.harvard.edu/abs/2020MNRAS.494.4291C/).
Hasn't changed much.
50+-10 BSM(Bland-Hawthorn & Gerhard 2016 https://ui.adsabs.harvard.edu/abs/2016ARA%26A..54..529B/)
54.3+-5.7 BSM(McMillan 2017 https://ui.adsabs.harvard.edu/abs/2017MNRAS.465...76M/)
50.4+4.3-5.2 BSM(Cautun+2020 https://ui.adsabs.harvard.edu/abs/2020MNRAS.494.4291C/).
Hasn't changed much.
3/ What is the mean mass of a star in the Milky Way?
Using the nearest 100 star systems from RECONS (most complete sample; thanks Todd Henry!)
http://www.recons.org/TOP100.posted.htm,
for stars (>0.075 Msun, dumping lower mass brown dwarfs), one gets mean mass of 0.40 solar masses
(+- ~10%).
Using the nearest 100 star systems from RECONS (most complete sample; thanks Todd Henry!)
http://www.recons.org/TOP100.posted.htm,
for stars (>0.075 Msun, dumping lower mass brown dwarfs), one gets mean mass of 0.40 solar masses
(+- ~10%).
4/ If we assume the local sample is representative of the rest of the MW, then
N(stars)=M(stars,total)/M(stars,average)
Using the Cautun+2020 total mass and average star mass estimate from RECONS, I get total number of stars in Milky Way of about 125 billion (+-about 20 billion)
N(stars)=M(stars,total)/M(stars,average)
Using the Cautun+2020 total mass and average star mass estimate from RECONS, I get total number of stars in Milky Way of about 125 billion (+-about 20 billion)
5/ So I've pushed for the lower end (~100 billion) when asked for NASA pieces e.g.
https://exoplanets.nasa.gov/blog/1563/our-milky-way-galaxy-how-big-is-space/
But I still see stories quoting ~400 billion stars e.g.
https://astronomy.com/magazine/2019/08/the-milky-way-earths-home-galaxy
but I've never seen any analysis supporting a # that high.
https://exoplanets.nasa.gov/blog/1563/our-milky-way-galaxy-how-big-is-space/
But I still see stories quoting ~400 billion stars e.g.
https://astronomy.com/magazine/2019/08/the-milky-way-earths-home-galaxy
but I've never seen any analysis supporting a # that high.
6/ Regarding "average" star masses... I consistently get average masses of ~0.56 Msun for newly formed stars, but the high mass ones die. Average "in the field", along with white dwarfs, is ~0.4 Msun. But *median* mass is lower, probably about ~0.25 Msun, i.e. that of a M4 dwarf.
7/ So how many stars in the Milky Way by various types? Here's pie chart for local stars: https://figshare.com/articles/figure/Fraction_of_Stars_by_Spectral_Type_in_the_Solar_Vicinity/3206527
72% M (red) dwarfs =>~90 billion
13% K (orange) dwarfs =>~16 billion
6% G (yellow, like Sun) dwarfs =>~8 billion
6% white dwarfs, 3% F stars.
OBA stars => only 0.6%
72% M (red) dwarfs =>~90 billion
13% K (orange) dwarfs =>~16 billion
6% G (yellow, like Sun) dwarfs =>~8 billion
6% white dwarfs, 3% F stars.
OBA stars => only 0.6%
8/ What about brown dwarfs (~13-75 Jupiter masses= ~0.012-0.072 Solar masses)?
(ignoring even tinier 'rogue planets')
They are still being regularly discovered, even nearby.
Nearest brown dwarf system Luhman 16 (binary) was only discovered in 2013 at *2 parsecs*(6.5 light years)
(ignoring even tinier 'rogue planets')
They are still being regularly discovered, even nearby.
Nearest brown dwarf system Luhman 16 (binary) was only discovered in 2013 at *2 parsecs*(6.5 light years)
9/ The local space density of stars in the Sun's neighborhood is about ~0.10
star per cubic parsec ( http://www.pas.rochester.edu/~emamajek/memo_star_dens.html).
Looking at the latest estimates from Kirkpatrick+
https://ui.adsabs.harvard.edu/abs/2019ApJS..240...19K/
it looks like the local brown dwarf density is at least ~0.017 BDs/parsec^3.
star per cubic parsec ( http://www.pas.rochester.edu/~emamajek/memo_star_dens.html).
Looking at the latest estimates from Kirkpatrick+
https://ui.adsabs.harvard.edu/abs/2019ApJS..240...19K/
it looks like the local brown dwarf density is at least ~0.017 BDs/parsec^3.
10/ So roughly 6 stars for each brown dwarf.
This is similar to what has been detected in nearby, young star-forming regions (~5 stars per brown dwarf; Luhman 2012 https://arxiv.org/abs/1208.5800 ).
So this would only imply about ~20 billion brown dwarfs in the Milky Way.
This is similar to what has been detected in nearby, young star-forming regions (~5 stars per brown dwarf; Luhman 2012 https://arxiv.org/abs/1208.5800 ).
So this would only imply about ~20 billion brown dwarfs in the Milky Way.
11/ But again, this estimate of ~20 billion brown dwarfs in MW would be lower limit. I've seen claims that number of brown dwarfs (not including rogue planets!) might be similar to number of stars in MW e.g. https://www.space.com/37401-100-billion-brown-dwarfs-milky-way.html, but I'd be very surprised if this ends up true
12/ Recently it has become hard to find a *new* isolated star or BD w/i 5 parsecs. I'm unaware of any stars discovered within 5pc since 2004 (SCR 1845-6357A), no L or T dwarfs since 2013/14 (Luhman 16), and no Y dwarfs since 2014 (WISE 0855-0714). (please correct me!)
13/ There has been *intense* surveying of the solar neighborhood for BDs in recent years with WISE, Gaia, Backyard Worlds, etc. We're probably missing Y dwarfs (~<450K), but its getting harder and harder to hide
any L or T dwarfs within 5pc (perhaps hiding in Galactic plane?).
any L or T dwarfs within 5pc (perhaps hiding in Galactic plane?).
14/ If Number(stars) ~ N(brown dwarfs) locally, there should be *dozens* of missing L/T/Y dwarfs w/i 5 parsecs. No way. I think I'll now be pretty surprised if any new L/T dwarfs found w/i 5 pc, but won't be surprised to hear of new Y dwarf discoveries that close.
