Posted on 12/08/2005 3:16:16 PM PST by blam
Spiral arm of Milky Way looms closer than thought
19:00 08 December 2005
NewScientist.com news service
Maggie McKee
The Milky Way is made of four main arms curving around its centre – astronomers measured the distance from Earth to a star-forming region called W3OH inside the Perseus arm (Image: Y. Xu et al/Science)
One of the Milky Way's star-studded spiral arms lies twice as close to Earth as some previous estimates suggested. New research has produced the most accurate distance measurement ever made of the arm, which could help astronomers understand how our galaxy's spiral structure formed.
The Milky Way appears to be made up of four main arms that curve around its centre like a pinwheel. "However, our view from the interior makes it difficult to determine its spiral structure," writes a team led by Ye Xu of the Shanghai Astronomical Observatory in China, in Science.
Measuring the distance to the spiral arms can be particularly tricky. This is because astronomers can only measure the speed of an astronomical object in terms of how fast it is moving towards or away from the Earth. Comparing this speed to theoretical models, which assume the objects travel on circular paths around the centre of the galaxy, allows astronomers to deduce the object's distance from Earth.
Astronomers using this technique had previously estimated the distance to Perseus, the arm immediately beyond the Sun, at more than 13,000 light years. But other researchers arrived at half that distance using a method that compares the apparent brightness of massive, young stars with estimates of their intrinsic brightness.
Now Xu's team has used a third technique - 100 times more accurate than the other two - to conclude the Perseus arm is indeed relatively close, at just 6400 light years from Earth.
Hawaii to the Caribbean
They used a system of 10 radio dishes that boasts the sharpest vision of any telescope in existence. Called the Very Long Baseline Array (VLBA), the dishes - each spanning 25 metres - are scattered from Hawaii to the Caribbean Sea.
They focused on a star-forming region called W3OH inside the Perseus arm. Bright, young stars in the region heat methanol vapour in gas clouds around them, which in turn emits radio waves in what are called "masers".
The team tracked the masers at five intervals over the course of a year, determining their distance by "triangulating" their observed positions from different points along Earth's orbit.
"We used our changing vantage point to form one leg of a triangle," says team member Mark Reid, an astronomer at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, US. "Then, measuring the change in angle of the source as the Earth orbits the Sun, we can calculate the source's distance by simple trigonometry."
They found that W3OH is not moving in a perfectly circular orbit but instead follows an elliptical path, as if drawn along the Perseus spiral arm. "It seems to be indicating that the spiral arms may have a higher density than previously guessed," Reid told New Scientist.
The team will now use the VLBA to measure the distances to a dozen star-forming regions spread across several of the Milky Way's spiral arms. "We hope to use such data to better understand how spiral arms form," says Reid.
Journal reference: Science (DOI: 10.1126/science.1120914)
Check the links in post #3 and #6. Your answer may be there.
I assume that this really means "half as distant."
Yeah, that's about right we live in the suburbs.
... at many a trendy nightclub.
Someone help me here. I am familiar with Saggitarius, Perseus, Carina, Crux (the southern cross), but Scutum and Norma? Never heard of any constellation, or subset of one, by those names.
The Universe and the Curtis - Shapley Debate: Lecture
by Robert J. Nemiroff and Jerry T. Bonnell
http://staff.imsa.edu/science/astro/astrometry/historical/debate_1920a.html
"...Shapley thought that the Galaxy was so large it was essentially the whole universe. Shapley not only based his opinion on the distribution of globular clusters and the distances to globular clusters, but on the apparently measured rotation velocities of spiral nebulae... Shapley also did not believe that spiral nebulae were galaxies in their own right. At the time of the Great Debate, he thought that spiral nebula were gaseous clouds repelled by the light pressure of our Galaxy and that our Galaxy was moving through these clouds."
They still make Necco Wafers? Gawd, I loved those as a kid!
CA....
...heck.... I'll take it!!
I was actually offering a readable explanation for folks who may not understand the math or the theory.
I think most people on this forum are familiar with your wonderful efforts to provide knowledge to those of us less informed on space and the science involved. Unfortunately, some would reply to your generous offer with sarcasm. I love sarcasm but there is a time and a place. A reply to you wasn't it. Keep up the good work. Most of us enjoy your posts.
There's worse places!
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