by neufer » Tue Jun 21, 2016 1:21 pm
Dad is watching wrote:
In a spiral galaxy, to be actually considered an 'arm', how far around the galaxy does a trail of stars and matter have to extend?
That sort of depends if you, yourself, believe that you live in the Orion–Cygnus Arm or in the Local/Orion Spur:
https://en.wikipedia.org/wiki/Orion_Arm wrote:
<<
The Orion Arm is a minor spiral arm of the Milky Way some 3,500 light-years across and approximately 10,000 light-years in length. The Solar System, including the Earth, lies within the Orion Arm. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion Bridge, Local Spur and Orion Spur.
The Orion Arm is named for the Orion constellation, which is one of the most prominent constellations of Northern Hemisphere winter (Southern Hemisphere summer). Some of the brightest stars and most famous celestial objects of this constellation (Betelgeuse, Rigel, the stars of Orion's Belt, the Orion Nebula) are located within the Orion Arm, as shown on the interactive map below.
The Orion Arm is located between the Carina–Sagittarius Arm (toward the Galactic Center) and the Perseus Arm (toward the outside Universe), the latter one of the two major arms of the Milky Way. Long thought to be a minor structure, a "spur" between the two longer adjacent arms Perseus and Carina-Sagittarius, evidence was presented in mid 2013 that it might be a branch of the Perseus Arm, or possibly an independent arm segment itself.
Within the Orion Arm, the Solar System, including Earth, is located close to the inner rim in the Local Bubble, about halfway along the Orion Arm's length, approximately 8,000 parsecs (26,000 light-years) from the Galactic Center.>>
[quote="Dad is watching"]
In a spiral galaxy, to be actually considered an 'arm', how far around the galaxy does a trail of stars and matter have to extend?[/quote]
That sort of depends if you, yourself, believe that you live in the Orion–Cygnus Arm or in the Local/Orion Spur:
[quote=" https://en.wikipedia.org/wiki/Orion_Arm"]
[float=right][img3="[b][color=#0000FF]Observed (normal lines) and extrapolated (dotted lines) structure of the spiral arms. The gray lines radiating from the Sun's position (upper center) list the three-letter abbreviations of the corresponding constellations.[/color][/b]"]https://upload.wikimedia.org/wikipedia/commons/1/1d/Milky_Way_Arms.svg[/img3][/float]<<[b][color=#FF8000]The Orion Arm is a minor spiral arm of the Milky Way some 3,500 light-years across and approximately 10,000 light-years in length.[/color][/b] The Solar System, including the Earth, lies within the Orion Arm. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion Bridge, Local Spur and Orion Spur.
The Orion Arm is named for the Orion constellation, which is one of the most prominent constellations of Northern Hemisphere winter (Southern Hemisphere summer). Some of the brightest stars and most famous celestial objects of this constellation (Betelgeuse, Rigel, the stars of Orion's Belt, the Orion Nebula) are located within the Orion Arm, as shown on the interactive map below.
[b][color=#FF8000]The Orion Arm is located between[/color] [color=#FF2000]the Carina–Sagittarius Arm (toward the Galactic Center)[/color] and [color=#008080]the Perseus Arm (toward the outside Universe), the latter one of the two major arms of the Milky Way[/color]. [color=#FF8000]Long thought to be a minor structure, a "spur" between the two longer adjacent arms Perseus and Carina-Sagittarius, evidence was presented in mid 2013 that it might be a branch of the Perseus Arm, or possibly an independent arm segment itself.[/color][/b]
Within the Orion Arm, the Solar System, including Earth, is located close to the inner rim in the Local Bubble, about halfway along the Orion Arm's length, approximately 8,000 parsecs (26,000 light-years) from the Galactic Center.>>[/quote]