5 ms·

They would have to be within ~100 light years of us which honestly is not a lot of places…

by flatiron 4y ago

umeshunni 4y ago
There are 59,722 stars within 100 light years of us.
- Teever 4y ago
  Oh man, I had no idea. Do you know where I could find a map of that?
  - raattgift 4y ago
    You probably can't, because your parent comment's figure is too large and too precise. You can get some maps at e.g. <https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brow...> and more if you explore through the cited papers. There are 10 known stars or star-systems within approximately 10 light-years of our local star (Proxima Centauri, the Alpha Centauri triple, Barnard's star, Luhman 16 in Vela, W0855 in Hydra, Wolf 359 in Leo, Gilese 411 in Ursa Major, the Sirius binary, and the UL Ceti / BV Ceti binary and right on the edge of our 10 light-year bubble V1216 Sagittari). There are 93 within 20 light-years. These counts include multi-star systems. I write "known" because there may be dim stellar objects we haven't found yet. The double in Vela are both dim brown dwarfs, much smaller than the sun, and were only discovered in the last decade or so, despite being less than seven light-years away. Brown dwarfs are hard to spot. Red dwarfs become similarly hard to spot at a distance of only 20 light-years. Without descending into power-law discussions (although see <https://news.ycombinator.com/item?id=32613762 https://news.ycombinator.com/item?id=32613762> if you are curious and want to think about the density of dim dwarfs vs bright naked-eye stars), we can just accept that the density of stars within 100 light-years is likely very similar to that within 10 light-years and within 20 light-years. The volume of a sphere goes 4/3 * pi * r^3. A 20 light-year (ly) sphere has 8 times the volume of a 10 ly sphere. We have roughly 10 stars vs roughly 100 stars, so that tracks roughly. Extrapolating and taking only orders-of-magnitude estimates of star counts, a 100 ly sphere has 125 times the volume of a 20 ly sphere, so we would expect the star-count to rise from roughly 100 to roughly 10000 stars. That's much less than half of the almost sixty-thousand number that your comment's parent suggests. To multiply the rough estimate based on the 10-ly and 20-ly count by between about four and six to match the "59,722" figure we would have to introduce one or more screening mechanisms or properties to hide so many more stars. Properties include intrinsic dimness and low mass. If we require that stars are self-luminous that means they must be massive enough for nuclear fusion in their cores, so at the lowest end that means brown dwarfs like the ones mentioned a couple paragraphs above, somewhat heavier and brighter red dwarfs, and so forth. Screening properties would involve arranging these unseen stars in such a way that they eclipse one another, are hidden behind brighter dust, and have low proper motion about the "local standard of rest" average motion stars moving in our part of the Milky Way. Not only do we have a sudden jump in the volume-to-mass relationship compared to 10 and 20 light-years, we would also have to cut off the volume-to-mass relationship not too far outside the 100 light year bubble, or dramatically change the shape of our own galaxy compared to that of spiral galaxies, and our understanding of the proper motion of the Andromeda galaxy towards us. We also have to deal with the results from gravitational micro-lensing surveys (e.g. from MACHO hunting), and the study of the fast orbits of stars in the Milky Way's central parsec, neither of which supports large numbers of dim-but-considerably-bigger-than-Jupiter masses lurking in our galaxy. If we go the other direction and start with the evidence that we are in a spiral arm of a galaxy like Andromeda, and that the "thin disc" stellar density is about 0.004 per cubic light year (<https://en.wikipedia.org/wiki/Stellar_density https://en.wikipedia.org/wiki/Stellar_density> and the recent <https://arxiv.org/abs/2207.03492 https://arxiv.org/abs/2207.03492> which maps comparable-scale variations in the stellar density in the Milky Way's thin disc), then in a 100-light-year sphere having a volume of about 4.2 million cubic light years we end up expecting a little fewer than seventeen thousand stars. Consequently, unless there's something almost uniquely special happening around 30-100 light-years from here, a much better estimate for the star-count within 100 light years is twelve thousand plus or minus five thousand. Note that this is a much less precise figure than the one given upthread.
    - freeqaz 4y ago
      This is probably the best comment I've ever read on HN. Thank you. This is why I love this community!
  - MrGLaDOS 4y ago
    The GAIA telescope is at your service :) “thanks to Gaia EDR3, the solar neighbourhood has been mapped with great precision out to 100 pc (326 light years).” https://www.cosmos.esa.int/web/gaia/edr3-gcns https://www.cosmos.esa.int/web/gaia/edr3-gcns The page links to the paper, the catalogue, notable figures and a “fly-through”.
giantrobot 4y ago
Wha? Atmospheric CO2 has been rising since the start of the Industrial Revolution in the early 1800s. Besides CO2 industry also spits out a lot of other air pollutants that take a while to break down in the atmosphere. I'd posit that we started polluting faster than natural breakdown rates in the mid-1800s. So the radius that would be able to detect industrial civilization on Earth would be about 170ly, not 100ly. There's also a few hundred[0] G-type stars within 100ly of Earth, if we include F and K types we've got thousands of stars within 100ly. We're just on the cusp of observatories that will be able to get spectra from terrestrial planets in close (<5 AU) orbits around their host stars. So a civilization within 200ly of us just slightly more advanced would be able to not just detect Earth, life signs on it, but also have at least a guess there's a technological civilization here.
analog31 4y ago
Sure, I'm assuming the usual disclaimers about the speed of light.

They would have to be within ~100 light years of us which honestly is not a lot of places…

umeshunni 4y ago

There are 59,722 stars within 100 light years of us.

Teever 4y ago
Oh man, I had no idea. Do you know where I could find a map of that?
- raattgift 4y ago
  You probably can't, because your parent comment's figure is too large and too precise. You can get some maps at e.g. <https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brow...> and more if you explore through the cited papers. There are 10 known stars or star-systems within approximately 10 light-years of our local star (Proxima Centauri, the Alpha Centauri triple, Barnard's star, Luhman 16 in Vela, W0855 in Hydra, Wolf 359 in Leo, Gilese 411 in Ursa Major, the Sirius binary, and the UL Ceti / BV Ceti binary and right on the edge of our 10 light-year bubble V1216 Sagittari). There are 93 within 20 light-years. These counts include multi-star systems. I write "known" because there may be dim stellar objects we haven't found yet. The double in Vela are both dim brown dwarfs, much smaller than the sun, and were only discovered in the last decade or so, despite being less than seven light-years away. Brown dwarfs are hard to spot. Red dwarfs become similarly hard to spot at a distance of only 20 light-years. Without descending into power-law discussions (although see <https://news.ycombinator.com/item?id=32613762 https://news.ycombinator.com/item?id=32613762> if you are curious and want to think about the density of dim dwarfs vs bright naked-eye stars), we can just accept that the density of stars within 100 light-years is likely very similar to that within 10 light-years and within 20 light-years. The volume of a sphere goes 4/3 * pi * r^3. A 20 light-year (ly) sphere has 8 times the volume of a 10 ly sphere. We have roughly 10 stars vs roughly 100 stars, so that tracks roughly. Extrapolating and taking only orders-of-magnitude estimates of star counts, a 100 ly sphere has 125 times the volume of a 20 ly sphere, so we would expect the star-count to rise from roughly 100 to roughly 10000 stars. That's much less than half of the almost sixty-thousand number that your comment's parent suggests. To multiply the rough estimate based on the 10-ly and 20-ly count by between about four and six to match the "59,722" figure we would have to introduce one or more screening mechanisms or properties to hide so many more stars. Properties include intrinsic dimness and low mass. If we require that stars are self-luminous that means they must be massive enough for nuclear fusion in their cores, so at the lowest end that means brown dwarfs like the ones mentioned a couple paragraphs above, somewhat heavier and brighter red dwarfs, and so forth. Screening properties would involve arranging these unseen stars in such a way that they eclipse one another, are hidden behind brighter dust, and have low proper motion about the "local standard of rest" average motion stars moving in our part of the Milky Way. Not only do we have a sudden jump in the volume-to-mass relationship compared to 10 and 20 light-years, we would also have to cut off the volume-to-mass relationship not too far outside the 100 light year bubble, or dramatically change the shape of our own galaxy compared to that of spiral galaxies, and our understanding of the proper motion of the Andromeda galaxy towards us. We also have to deal with the results from gravitational micro-lensing surveys (e.g. from MACHO hunting), and the study of the fast orbits of stars in the Milky Way's central parsec, neither of which supports large numbers of dim-but-considerably-bigger-than-Jupiter masses lurking in our galaxy. If we go the other direction and start with the evidence that we are in a spiral arm of a galaxy like Andromeda, and that the "thin disc" stellar density is about 0.004 per cubic light year (<https://en.wikipedia.org/wiki/Stellar_density https://en.wikipedia.org/wiki/Stellar_density> and the recent <https://arxiv.org/abs/2207.03492 https://arxiv.org/abs/2207.03492> which maps comparable-scale variations in the stellar density in the Milky Way's thin disc), then in a 100-light-year sphere having a volume of about 4.2 million cubic light years we end up expecting a little fewer than seventeen thousand stars. Consequently, unless there's something almost uniquely special happening around 30-100 light-years from here, a much better estimate for the star-count within 100 light years is twelve thousand plus or minus five thousand. Note that this is a much less precise figure than the one given upthread.
  - freeqaz 4y ago
    This is probably the best comment I've ever read on HN. Thank you. This is why I love this community!
- MrGLaDOS 4y ago
  The GAIA telescope is at your service :) “thanks to Gaia EDR3, the solar neighbourhood has been mapped with great precision out to 100 pc (326 light years).” https://www.cosmos.esa.int/web/gaia/edr3-gcns https://www.cosmos.esa.int/web/gaia/edr3-gcns The page links to the paper, the catalogue, notable figures and a “fly-through”.

giantrobot 4y ago

Wha? Atmospheric CO2 has been rising since the start of the Industrial Revolution in the early 1800s. Besides CO2 industry also spits out a lot of other air pollutants that take a while to break down in the atmosphere. I'd posit that we started polluting faster than natural breakdown rates in the mid-1800s. So the radius that would be able to detect industrial civilization on Earth would be about 170ly, not 100ly. There's also a few hundred[0] G-type stars within 100ly of Earth, if we include F and K types we've got thousands of stars within 100ly. We're just on the cusp of observatories that will be able to get spectra from terrestrial planets in close (<5 AU) orbits around their host stars. So a civilization within 200ly of us just slightly more advanced would be able to not just detect Earth, life signs on it, but also have at least a guess there's a technological civilization here.

analog31 4y ago

Sure, I'm assuming the usual disclaimers about the speed of light.