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“In truth, all the pictures that depict our galaxy are either artist’s renditions or pictures of other spiral galaxies, and not the result of direct observation of the whole. Until recently, it was very difficult for scientists to gauge what the Milky Way really looks like, mainly because we’re inside it.”
“If you are just measuring the normal matter we can see (in visible, infrared, X-ray and ultraviolet light), then the Milky Way is at least 100,000 light years across. The diameter is a bit larger (120,000 light years) if you take into account tidal streams – basically other galaxies the Milky Way is eating, such as the Sagittarius Dwarf Elliptical Galaxy.
But normal matter isn’t all that makes up the Milky Way. Simulations of our galaxy show that it has a “halo” of dark matter, which makes up about 10 times the mass of the visible matter in the Milky Way.”
“It turns out we’re not located in one of the Milky Way’s two primary spiral arms. Instead, we’re located in a minor arm of the galaxy. Our local spiral arm is sometimes Orion Arm, or sometimes the Orion Spur. It’s between the Sagittarius and Perseus Arms of the Milky Way.”
“The Carina–Sagittarius Arm (also known as Sagittarius Arm or Sagittarius–Carina Arm, labeled -I) is generally thought to be a minor spiral arm of our home galaxy, the Milky Way.”
“The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called Scutum–Centaurus Arm. Perseus Arm begins from the distal end of the long Milky Way. Previously thought to be 13,000 light-years away, it is now thought to lie 6,400 light years from the Solar System.”
“The Orion Arm, or Orion Spur, has other names as well. It’s sometimes simply called the Local Arm, or the Orion-Cygnus Arm, or the Local Spur.”
“There are two methods traditionally used to map the spiral structure of our Galaxy. The first method is to study the density of the neutral hydrogen (HI) in the plane of the Galaxy which is enhanced in the spiral arms. This was first attempted by Jan Oort, Frank Kerr, and Gart Westerhout in 1958. They studied the galactic system as a spiral nebula by using radio-telescopes in the Netherlands and Australia. The early version of their map (incomplete on the left side) shows various sections of the spiral arms. The second method is to plot the giant HII regions (bright nebulae of ionised hydrogen) which are usually formed in the spiral arms. This was attempted by Yvonne and Yvon Georgelin in 1976. They studied the spiral structure of our Galaxy determined from H II regions. Their map allowed them to determine where the spiral arms are.”