How many spiral arms does the Milky Way have?

After nearly 50 years of intensive study, astronomers still cannot clearly deduce the exact shape of our galaxy in terms of how many arms this pinwheel-shaped galaxy has.

We know that the Sun is on the edge of what is probably a 'spur' called the Orion Spur which seems to merge with the Perseus Spiral arm towards the constellation Cygnus. The Perseus Arm is the one just outward from the Sun's location in the Galaxy about 700 to 1000 parsecs from us. Beyond the Perseus Arm, there may be a more distant one, but the arms become less distinct in the outer galaxy zone.

Interior to the Sun's orbit, which is about 8,500 parsecs from the Galactic Center, and at a distance of about 2000 parsecs, we encounter the so-called Sagittarius-Scutum Arm.

Interior to the Sagittarius-Scutum Arm, there are indications of a Centaurus- Carina Arm at a distance of about 3000 parsecs from the center. Here we enter a very complex region where recent work seems to indicate that the Milky Way takes on a distinct 'bar-like' geometry with a central core and two arms curling upon themselves like the Greek letter theta.

Maps of the giant molecular clouds in the Milky Way seem to indicate an overall 2 or possibly 4-armed spiral bared-spiral shape, but there may also be many 'spurs' jutting out from these main arms just like we find in other 'grand design' spiral galaxies. Data from the NASA, COBE satellite have also suggested that the nucleus of the Milky Way has a barred-spiral shape.

We may never know completely, just what the Milky Way actually looks like. For now, we can trace a few of the major arm segments nearest the Sun with high fidelity, and in the distance see indications of how these segments join into a larger pattern. But exactly what the pattern looks like is not very well constrained by the data we now have.

Copyright 1997 Dr. Sten Odenwald

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