by MarkBour » Fri Mar 15, 2019 5:17 pm
Ann wrote: ↑Fri Mar 15, 2019 8:06 am
Wikipedia wrote:
M101 is a large galaxy, with a diameter of 170,000 light-years. By comparison, the Milky Way has a diameter of 258,000[11] light years. It has around a trillion stars, a twice the number in the Milky Way.[12] It has a disk mass on the order of 100 billion solar masses, along with a small central bulge of about 3 billion solar masses.
I doubt that the diameter of the Milky Way is 258,000 light-years. Maybe the dark matter halo surrounding our galaxy extends that far, but the obviously luminous disk of the Milky Way is hardly more than a bit over 100,000 light-years. The luminous disk of M101 is considered to be 170,000 light-years, and the Pinwheel Galaxy is believed to have twice as many stars as the Milky Way.
Ann
I see your point. That estimate of the Milky-way diameter in the Wikipedia article on M101 references a source article that used GAIA data to estimate the Milky Way's
mass. And the data source is specifically from globular clusters out in the halo. They don't comment on estimating our galaxy's
diameter in the abstract of that article (I did not read the full article). If they do work their way to a
diameter of the Milky Way, it is probably one that is quite different than the diameter of our luminous disk as it would be perceived by a distant outside observer.
So, it would seem that the sentence in the Wikipedia article is a poor one, since "by comparison" would be (to use an idiom) a comparison of apples to oranges. Or, possibly, the link is just wrong. Maybe you should post to the article's "Talk" page.
[quote=Ann post_id=290615 time=1552637172 user_id=129702]
[quote][url=https://en.wikipedia.org/wiki/Pinwheel_Galaxy#Structure_and_composition]Wikipedia[/url] wrote:
M101 is a large galaxy, with a diameter of 170,000 light-years. By comparison, the Milky Way has a diameter of 258,000[11] light years. It has around a trillion stars, a twice the number in the Milky Way.[12] It has a disk mass on the order of 100 billion solar masses, along with a small central bulge of about 3 billion solar masses.[/quote]
I doubt that the diameter of the Milky Way is 258,000 light-years. Maybe the dark matter halo surrounding our galaxy extends that far, but the obviously luminous disk of the Milky Way is hardly more than a bit over 100,000 light-years. The luminous disk of M101 is considered to be 170,000 light-years, and the Pinwheel Galaxy is believed to have twice as many stars as the Milky Way.
Ann
[/quote]
I see your point. That estimate of the Milky-way diameter in the Wikipedia article on M101 references a source article that used GAIA data to estimate the Milky Way's [i][b]mass[/b][/i]. And the data source is specifically from globular clusters out in the halo. They don't comment on estimating our galaxy's [b][i]diameter[/i][/b] in the abstract of that article (I did not read the full article). If they do work their way to a [b][i]diameter[/i][/b] of the Milky Way, it is probably one that is quite different than the diameter of our luminous disk as it would be perceived by a distant outside observer.
So, it would seem that the sentence in the Wikipedia article is a poor one, since "by comparison" would be (to use an idiom) a comparison of apples to oranges. Or, possibly, the link is just wrong. Maybe you should post to the article's "Talk" page.