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Milky Way galaxy: Facts about our galactic home

rod

Oct 22, 2019
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Some notes on this article. The article reports on the Milky Way is about 100,000 light years in diameter. Gaia results are referenced a number of times and how they are used to help understand distances and the structure of the MW. Question. How small are stellar parallaxes measured using Gaia? A stellar parallax of 2 mas (that is reliable) measures distances out to about 1631 light years. Other reports at space.com indicate stellar parallax good to about 100 light years distance or about 32 mas. Some celestial yardsticks are visible in the night sky, https://forums.space.com/threads/some-celestial-yardsticks-are-visible-in-the-night-sky.49582/, "Once we go beyond 100 light-years or so, the stars' parallaxes become too small to get meaningful measurements. How, then, is it possible to find the distances of galaxies?"

Another thought. Using a bright star with absolute magnitude of -6 and apparent magnitude of +20, the inverse square law of light allows a distance calculation out to about 5.17 million light years, without dust or other issues reducing the distance.

So, can I hold the 100,000 light year diameter for the MW with *heliocentric certainty*? :)
 
Aug 14, 2020
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Some notes on this article. The article reports on the Milky Way is about 100,000 light years in diameter. Gaia results are referenced a number of times and how they are used to help understand distances and the structure of the MW. Question. How small are stellar parallaxes measured using Gaia? A stellar parallax of 2 mas (that is reliable) measures distances out to about 1631 light years. Other reports at space.com indicate stellar parallax good to about 100 light years distance or about 32 mas. Some celestial yardsticks are visible in the night sky, https://forums.space.com/threads/some-celestial-yardsticks-are-visible-in-the-night-sky.49582/, "Once we go beyond 100 light-years or so, the stars' parallaxes become too small to get meaningful measurements. How, then, is it possible to find the distances of galaxies?"

Another thought. Using a bright star with absolute magnitude of -6 and apparent magnitude of +20, the inverse square law of light allows a distance calculation out to about 5.17 million light years, without dust or other issues reducing the distance.

So, can I hold the 100,000 light year diameter for the MW with *heliocentric certainty*? :)
And what is the diameter of Sagittarius A? I don't mean the apparent diameter from this distance and point out, but the diameter it might be as if you were closing upon its event horizon? Might it not be the very definition of expansionist universe, or at least, herein, expansionist galaxy, in the direction of inwardly rather than outwardly? Remember the article not long ago about measurements showing the possibility of the universe's expansionism pertaining to a greater dimensionality than just one. I would imagine, then, gravity's dimensionality pertaining to a greater dimensionality -- meaning pertaining to more dimensions -- than just one. The reduction-ism then being to galaxies actually, among other universes (my view), broader and deeper than showing and detected from any single point.
 
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rod

Oct 22, 2019
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Atlan0101 asked in post #3, "And what is the diameter of Sagittarius A? I don't mean the apparent diameter from this distance and point out, but the diameter it might be as if you were closing upon its event horizon?"

Using Einstein GR and the Schwarzschild radius for a 4 million solar mass BH, the radius in km is:
1.1813E+07​

or 11.813 million km. Using a circle with that radius, the diameter ~ 0.158 au. Using 8 kpc distance from Earth, angular size 1.974 x 10^-5 arcsecond, so an extremely tiny angular size to see :) Another observation about Gaia reported in the space.com article.

"The galaxy's evolution is, however, still shrouded in mystery. A discipline called galactic archaeology is slowly unravelling some of the puzzles of the Milky Way's life thanks to the Gaia mission, which released its first catalog of data in 2018. Gaia measures the exact positions and distances of 1 billion stars, as well as their light spectra, which enables scientists to understand the stars' composition and age. The position data allow astronomers to determine the speeds and directions in which the stars move in space. As things in space follow predictable trajectories, astronomers can reconstruct the paths of the stars billions of years into the past and future. Combining these reconstructed trajectories into one stellar movie captures the evolution of the galaxy over eons."...

Astronomers are extrapolating from Gaia position, magnitudes, and motion measurements for models like this. I still do not know how small Gaia stellar parallaxes are considered reliable, thus distance limits or constraints imposed. I use SIMBAD portal and see release 1 and release 2 data, some stars have negative parallaxes, so data is corrupt too. So here I summarize some notes after reading this interesting space.com article.

From the space.com article, the MW galaxy formed about 13.6 billion years ago using the BB model. The spiral arms are all young, perhaps no more than 100 million years old, thus the spiral arms must be regenerated, renewed again and again, otherwise astronomers today are living in a unique time period of the MW history to see such short lived spiral arms today. Gaia observes and measures only about 1% of the total number of stars in the MW. Astronomers extrapolate frequently from this astrometric collection. No doubt there is corruption in the database too along with limitations on good stellar parallaxes found thus distance constraints are in the database too.
 

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