by Ann » Fri Dec 06, 2019 7:14 am
Chris Peterson wrote: ↑Thu Dec 05, 2019 10:36 pm
shaileshs wrote: ↑Thu Dec 05, 2019 7:31 am
A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.
We're not very well situated to see the contents of our own galaxy. So a large uncertainty in star count is not unreasonable. The planet estimate is based on the statistics of what we've observed, along with theory suggesting that most of the stars in the galaxy probably don't have planets because they're in regions of high density, where gravitational perturbations make star systems unstable. Of course, those systems probably formed planets, then lost them. But estimates for planet counts usually are about those in orbit around stars, not the much larger number of possible rogue planets flying around the galaxy unattached to their parent stars.
I'm going to guess that
Gaia will give us a better understanding of the size of our galaxy.
But I think that the term itself, "size", is problematic here. What do we mean by a galaxy's size? I think the word might refer to two things, the size of the optically bright disk, and the size of the low-luminosity galactic halo. (Not the the halo itself is all homogeneous.) When the caption says that the diameter of NGC 6744 is 175,000 light-years, it probably refers to the size of the visible disk of NGC 6744. I think most estimates of the size of the Milky Way agree that its visible disk is not as large as 175,000 light-years across.
Click to play embedded YouTube video.
The starry halo of Andromeda.
The gaseous halo of Andromeda. Higher resolution here. The light from
background quasars are used to probe the size of the Andromeda halo.
As Chris said, we can get a much, much better look at Andromeda's halo than at the Milky Way's halo. Andromeda has two halos, one relatively small and starry (but still faint) and one huge and gaseous. The Youtube video allows you to see the nature of the halo at different distances from Andromeda's luminous disk. The closer you are to the disk,
the "starrier" is the halo. The last cutout seen in the video, farthest from the disk of Andromeda, appears to be strongly dominated by background galaxies. But the gaseous halo of Andromeda stretches far, far beyond the point where the starry halo appears to end. The light from background quasars are used to determine the size of the gaseous halo of Andromeda.
Hopefully Gaia will give us a better estimate of the size of the visible disk of the Milky Way. Just maybe Gaia can also give us an idea of the size of our galaxy's "starry halo".
Ann
[quote="Chris Peterson" post_id=297708 time=1575585408 user_id=117706]
[quote=shaileshs post_id=297690 time=1575531097 user_id=143908]
A bit tangential question/topic - Do we really know how big is our own milky-way galaxy and how many stars and planets there are ? There is so much of ambiguity or variance, it feels it's such a wide guess (at times giving impression of such *wild* guess).. Some reliable sites say diameter is 100,000 ly, some say 150,000 ly, some say 200,000 ly.. Some sites say stars are 250 billion with +/- 150 billions.. Whaaaat ? If the variance is 5-10% then we can say we know reasonably well.. but with 50-100% variance, it feels it's wild. And, planets - they say around 100 billion+. Seriously ? How do we know ? Do we even know ? Are we saying on an average each star doesn't have even 1 planet (average, approximately) ? Finding planets is SOOOOOOOOOOO difficult, and here, someone claims there are 150 billions.. Difficult to believe these #s, right ? What do you guys think ? It's so unfortunate that it'll take maybe 1000 more years for humans (or whatever species it's called then) to send the spaceship/telescope outside of this galaxy and actually see/measure and validate what we currently are assuming or guessing.. No ? Thanks in advance for everyone's help/comments.
[/quote]
We're not very well situated to see the contents of our own galaxy. So a large uncertainty in star count is not unreasonable. The planet estimate is based on the statistics of what we've observed, along with theory suggesting that most of the stars in the galaxy probably don't have planets because they're in regions of high density, where gravitational perturbations make star systems unstable. Of course, those systems probably formed planets, then lost them. But estimates for planet counts usually are about those in orbit around stars, not the much larger number of possible rogue planets flying around the galaxy unattached to their parent stars.
[/quote]
I'm going to guess that [url=https://en.wikipedia.org/wiki/Gaia_(spacecraft)]Gaia[/url] will give us a better understanding of the size of our galaxy.
But I think that the term itself, "size", is problematic here. What do we mean by a galaxy's size? I think the word might refer to two things, the size of the optically bright disk, and the size of the low-luminosity galactic halo. (Not the the halo itself is all homogeneous.) When the caption says that the diameter of NGC 6744 is 175,000 light-years, it probably refers to the size of the visible disk of NGC 6744. I think most estimates of the size of the Milky Way agree that its visible disk is not as large as 175,000 light-years across.
[float=right][youtube]https://www.youtube.com/watch?v=a_2lw8wuWw8[/youtube][c][size=85]The starry halo of Andromeda.[/size][/c][/float][float=left][img2]https://hubblesite.org/uploads/image_file/image_attachment/27222/web.jpg[/img2][c][size=85]The gaseous halo of Andromeda. Higher resolution [url=https://hubblesite.org/uploads/image_file/image_attachment/27223/web_print.jpg]here[/url]. The light from
background quasars are used to probe the size of the Andromeda halo.[/size][/c][/float]
As Chris said, we can get a much, much better look at Andromeda's halo than at the Milky Way's halo. Andromeda has two halos, one relatively small and starry (but still faint) and one huge and gaseous. The Youtube video allows you to see the nature of the halo at different distances from Andromeda's luminous disk. The closer you are to the disk, [url=https://apod.nasa.gov/apod/ap030519.html]the "starrier" is the halo[/url]. The last cutout seen in the video, farthest from the disk of Andromeda, appears to be strongly dominated by background galaxies. But the gaseous halo of Andromeda stretches far, far beyond the point where the starry halo appears to end. The light from background quasars are used to determine the size of the gaseous halo of Andromeda.
Hopefully Gaia will give us a better estimate of the size of the visible disk of the Milky Way. Just maybe Gaia can also give us an idea of the size of our galaxy's "starry halo".
Ann