The outer galaxy

July 31, 2017, Harvard-Smithsonian Center for Astrophysics
An artist's reconstruction of the Milky Way galaxy showing the locations of the various spiral arms. Astronomers have detected massive young stars forming in the outer part of the Scutum-Centarus Arm, the outermost portions of the galaxy. Credit: NASA

The sun is located inside one of the spiral arms of the Milky Way galaxy, roughly two-thirds of the way from the galactic center to the outer regions. Because we are inside the galaxy, obscuration by dust and the confusion of sources along our lines-of-sight make mapping the galaxy a difficult task. Astronomers think that the galaxy is a symmetric spiral, and about 10 years ago, CfA astronomers Tom Dame and Pat Thaddeus, using millimeter observations of the gas carbon monoxide, discovered symmetric components to the spiral arms deep in the inner galaxy that lent support to this model.

The galaxy is not perfectly flat. It has a slight warp that allows some distant structures, at least in the direction of the constellations of Scutum and Centaurus, to be seen more distinctly above much of the foreground confusion. In 2011 the same CfA astronomers were the first to discover a large-scale spiral feature within this distant warp which they called the "Outer Scutum–Centaurus Arm (OSC)." Subsequent studies placed the OSC at a distance from the galactic center of over forty thousand light-years; it appears to be a symmetric counterpart to a spiral arm on the opposite side, in the direction of Perseus.

CfA Tom Dame has joined with a set of collaborators to probe the extent of massive star formation in the OSC; sadly, his colleague Pat Thaddeus passed away earlier this year. Using radio measurements of ionized gas, which traces the hot ultraviolet from massive young stars, as well as bright emission from masers associated with massive star formation, the scientists observed 140 candidate locations and discovered evidence for massive young stars in about sixty percent of them. The study shows that the OSC is forming new stars, some with as much as forty solar masses each. These stars and their associated ionized environments, at least as far as we know now, mark the outer boundary for massive star formation in the Milky Way.

Explore further: A new, distant arm of the Milky Way galaxy

More information: W. P. Armentrout et al. High-mass Star Formation in the Outer Scutum–Centaurus Arm, The Astrophysical Journal (2017). DOI: 10.3847/1538-4357/aa71a1

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Mark Thomas
5 / 5 (2) Jul 31, 2017
Not sure where to place the "outer regions," but it seems to me like we are closer to half of the way out from the center to the edge, not two thirds.

"The diameter of the luminous Milky Way is between 100,000 and 120,000 light years across"


So for example, if the Milky Way is 100,000 ly across, that means a radius of 50,000 ly and at 25,000 ly out, we would be exactly half way between the center and edge. The range with the numbers above is 42% to 56%, all of which is closer to 50% than 67%.
Mark Thomas
not rated yet Jul 31, 2017
Somehow this was accidentally deleted from the comment above.

"The Galactic Center is . . . about 25,000 to 28,000 light years from Earth"

Captain Stumpy
not rated yet Jul 31, 2017
@Mark Thomas
but it seems to me like we are closer to half of the way out from the center to the edge, not two thirds.

"The Galactic Center is . . . about 25,000 to 28,000 light years from Earth"
offered IMHO- just because we're 25K-28K light years from it doesn't mean we're that distance from the center of it

you don't measure your distance from Germany to the US by the time it takes you to get to the dead center of the continent
... you measure to the distance when you cross the outer limit of US, which would usually be a land marker
(they don't even use the defining line of international vrs territorial waters to designate the distance between)

Perhaps Q-Star or another astrophysicists will post specific measurement standards for us to clarify the issue?
5 / 5 (3) Jul 31, 2017
I would think that the old data, that we are 2/3rds of the way from the Galactic Center, needs to be updated since we have found further evidence of dust, gas and stars at a farther distance than we had previously thought it went. So updating things to show that we are closer to half-way from the edge or center, either one, would be better.

That is one of the things about science, we learn new facts that overturns what is in the books, yet since the books do not always get changed out we get some people learning the wrong facts, and then they stick by them because "They Are In The Book".

From what I have seen, the 'closer to half-way' is the more correct answer.
Mark Thomas
5 / 5 (3) Jul 31, 2017
I believe Steelwolf got it exactly right. When I was considerably younger, the 2/3 estimate was probably correct given the best info we had at the time. Now that number needs to be revised to 1/2, but the writers are slow to catch on. I often find the failure to do elementary school math (division in this case) leads to inaccuracies in many articles like this.
Mark Thomas
5 / 5 (1) Jul 31, 2017
Captain, I agree with your point about measuring the distance between countries. However, and I may be wrong about this, I was simply using the Wikipedia definition which suggests the Galactic Center is a mathematical point.

"The Galactic Center is the rotational center of the Milky Way."

"The complex astronomical radio source Sagittarius A appears to be located almost exactly at the Galactic Center (approx. 18 hrs, −29 deg), and contains an intense compact radio source, Sagittarius A*, which coincides with a supermassive black hole at the center of the Milky Way."

"In 1958 the International Astronomical Union (IAU) decided to adopt the position of Sagittarius A as the true zero co-ordinate point for the system of galactic latitude and longitude."


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