Large gas bubbles of the Milky Way
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Updated November 11, 2014
The Gould Belt named in honor of Benjamin Gould (1824-1896), who identified it in 1879, is a young structure of our Milky Way, aged 100 million years. In reality it is a "small bubble" 3000 light years in diameter, composed of gas and stars in our galactic environment located very close, in which the solar system travel (red circle on the picture). Gould Belt is tilted about 18 degrees from the plane of the Milky Way.
We live in a spiral galaxy ≈100 000 light-years in diameter, but it is very difficult to map its structure because we are inside the structure, which explains the blur on the number of existing arms and even their appellation. However, what clearly appears from Earth, it is the white band continues the Milky Way, but this galaxy is actually composed of at least four major spiral arms (Sagittarius arm, Orion arm, Perseus arm, Norma arm). Its mass is ≈400 billion solar masses and our Sun rotates ≈217 km/s at a distance of ≈27 200 light years from the galactic center, in the Orion arm (orange arms on the image). At this speed it needs ≈240 million years, to go around.
Gould Belt contains a lot of young, hot stars, it could be a part of the embryo of the local arm of which the Sun belongs. This small local structure of the Galaxy is our "playground" is this ring of molecular clouds and bright young stars that we mask the background of the Milky Way. This area forms a bright bar to the Milky Way, visible especially from the southern hemisphere, this bar is inclined relative in front the Milky Way, which gives a curved shape to the Milky Way.
Indeed, when we look at the Milky Way, we see an especially, inclined band relative to the galactic plane because the stars near the Sun are very bright.
Gould Belt therefore draws a wide curve on the Milky Way, where many very active stars are forming.
|Structure of the Milky Way|
|Norma Arm or Cygnus Arm|| ||Discontinuous arm, which has an outer extension|
|Perseus Arm|| ||The Perseus Arm is one of two major spiral arms of the Milky Way|
|Orion Arm or Local Arm|| ||Minor spiral arm of the Milky Way galaxy in which our solar system is|
|Carina- Sagittarius Arm|| ||Sagittarius constellation as seen in the direction to the galactic center|
|Scutum–Centaurus Arm|| ||The Perseus Arm is one of two major spiral arms of the Milky Way|
Table: We live in a spiral galaxy ≈100 000 light-years in diameter, but it is very difficult to map its structure because we are inside the structure, which explains the blur on the number of existing arms and even their appellation.
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Image: Structure of the Milky Way, our galaxy. The Orion Arm or Local Arm contains the Sun, it is an embryo of arms, orange color on this image. The Sun (in the red circle) is located in the Orion arm and rotates at a speed of 217 km/s around the galactic center, completing one revolution every 240 million years. Gould Belt is a region of molecular gas and bright stars, located in the very close environment of the Sun (in the red circle). Each arm of the galaxy is a massive structure composed of billions of stars and thousands of gas clouds.
Origin of the Gould Belt
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Hipparcos satellite (HIgh Precision Parallax Collecting Satellite) of the European Space Agency, ran from 1989 to 1993. He has over four years, measured the position, the parallax and proper motion of stars in our Galaxy. Hipparcos data allowed the simulation (right image) showing the Gould Belt.
Gould belt appears clearly if we only photograph the stars of the Milky Way located within 1500 light-years from the Sun, while erasing small and old stars (bottom image).
This is what you see on the right image. At the top, the image shows an homogeneous distribution of all nearby stars within 500 parsecs (<1500 light-years) whose mass is roughly equal to that of our sun (if we photographed all the stars, near and far, we would see the Milky Way emerge above).
On the top image, we kept the stars of type A and F, whose mass is almost identical to our Sun, rather older and the bottom image, we kept the stars of type O and B, much more massive the Sun (7-20 solar masses) and of course, younger, less than 40 million years. The brightest stars of the Gould Belt are distributed over a curved band, compared to the Milky Way, is what Benjamin Gould was identified in the skies of the southern hemisphere in 1879.
Gould Belt is therefore a ring of gas, but what is the massive explosion that generated the gas ring inclined relative to the galactic plane?
Several scenarios are being considered, but it is probable that it is a gigantic hypernova equivalent to 10 supernovae, which engendered a shock wave that sweeps our immediate environment today. This circular shock wave 240 000 solar masses, is probably deformed in elliptical ring and inclined up the Milky Way because the gas density is less strong upward than on the galactic plane. Entire ring is leaned despite the gravitational force of the stars of the galactic plane which tends to return to the plan.
This shock wave ≈1000 light years, ballote the interstellar medium of our nearby immediate environment, from a few tens of million years. The Sun is going through this ring, where the periphery, exploding bubbles of stars forming in the Gould Belt.
The first generations of massive stars have already disappeared in supernovae and spilled their stellar matter in our environment. On the edges of the Gould Belt there are approximately 300 to 400 supernovae that will explode in a few million years. Statistically, one explosion occurs every 40,000 years, and we are well placed to attend this "stellar fireworks" because we are going through currently, a particularly rich supernovae region.
star||≥ 30000 K|
|W||Wolf-Rayet star||≥ 25000 K|
star||10000 - 30000 K|
|A||large star||7300 - 10000 K|
|F||solar type||6000 - 7300 K|
|G||solar type||5300 - 6000 K|
|K||solar type||3800 - 5300 K|
|M||sub solar||2500 - 3800 K|
|C||carbon star||2400 - 3200 K|
|S||sub carbon star||2400 - 3500 K|
|L||hot brown dwarf||1300 - 2400 K|
|T||cool brown dwarf||600 - 1300 K|
|Y||sub brown dwarf||< 600 K|
Table: Spectral types stars.
Image: Overview of the Gould Belt. This image homogeneous shows all the stars very close to the Sun, closer than 1,500 light-years distance (Hipparcos data from 1989 to 1993). At the top, stars of type A and F of almost identical mass to our Sun, rather older and to the bottom, stars of type O and B much more massive than the Sun (7-20 solar masses) and younger, less than 40 million years.
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