These pages chronicle the work and ruminations of Carolyn Collins Petersen, also known as TheSpacewriter.

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I am CEO of Loch Ness Productions. I am also a producer for Astrocast.TV, an online magazine about astronomy and space science.

For the past few years, I've also been a voice actor, appearing in a variety of productions. You can see and hear samples of my work by clicking on the "Voice-Overs, Videos and 'Casts tab.

My blog, TheSpacewriter's Ramblings, is about astronomy, space science, and other sciences.

Ideas and opinions expressed here do not represent those of my employer or of any other organization to which I am affiliated. They're mine.

Visit my main site at: TheSpacewriter.com.

**Comments are welcome; I do moderate them to weed out spam.

Contact me for writing and voice-over projects at: cc(dot)petersen(at)gmail(dot)com

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Does This Spiral Arm Make Me Look Fat?

May 29, 2008 at 13:48 pm | Leave a Comment

The Milky Way Loses Mass the Easy Way

So, let’s say you’re an average spiral galaxy doing your thing in the Local Group. You have a hundred billion stars (give or take), some nebulae, a big core (with a black hole or two hidden in the center), and some planets (but you’re not sure how many). But, according to astronomers who have measured you, it seems that you’re much more massive than you thought. Could it be all that dark matter you’ve been carrying around with you since you were a young galaxy? What’s the deal here?

So, you do like everybody else who’s overweight does — you think about going on a diet. Galactically speaking, the only way you’re going to do that is lose mass, which is tough to do. In fact, short of having a near-miss with another galaxy (which would tear away some of your stars and mass), it’s not easy for a middle-aged galaxy to lose weight.

But wait! There’s another way. Maybe, you think, they didn’t weight you correctly. Yeah, that’s the ticket. The scale is off a little…

As a matter of fact, that’s exactly the issue that scientists set out to solve, using the Sloan Digital Sky Survey to measure stars in the galaxy and recalculate its mass based. Basically, they did a more efficient survey which allowed them to come up with a better estimate of the true mass of our galaxy. The difference between the older, fatter Milky Way and the new, improved, skinnier one is a pile of mass equivalent to a trillion Suns.

“The galaxy is slimmer than we thought,” said Xiangxiang Xue of the National Astronomical Observatories of China, who led an international team of researchers on the project. “That means it has less dark matter than previously believed, but also that it was more efficient in converting its original supply of hydrogen and helium into stars.”

Our sun lies about 25,000 light years from the center of the galaxy, roughly halfway out in the galactic disk. The new mass determination is based on the measured motions of 2,400 “blue horizontal branch” stars in the extended stellar halo that surrounds the disk. These measurements reach distances of nearly 200,000 light years from the galactic center, roughly the edge of the region illustrated above.

The visible, stellar part of our Milky Way in the middle is embedded into its much more massive and more extended dark matter halo, indicated in dim red. The ‘blue horizontal branch stars’ that were found and measured in the SDSS-II study, are orbiting our Milky Way at large distances.

From their speeds the researchers were able to estimate much better the mass of the Milky Way’s dark matter halo, and found it to be much “slimmer” than thought before.

How They Did It

The discovery that the Milky Way is slimmer than we thought is based on data from SEGUE, an enormous survey of stars in the Milky Way — one of the three programs that comprise SDSS-II. Using SEGUE measurements of stellar velocities in the outer Milky Way, a region known as the stellar halo, the researchers determined the mass of the galaxy by inferring the amount of gravity required to keep the stars in orbit. Some of that gravity comes from the Milky Way stars themselves, but most of it comes from an extended distribution of invisible dark matter, whose nature is still not fully understood.

To trace the mass distribution of the Galaxy, the SEGUE team used a carefully constructed sample of 2,400 “blue horizontal branch” stars whose distances can be determined from their measured brightness. Blue horizontal branch stars can be seen to large distances, enabling the team to measure velocities of stars all the way out to distances of 180,000 light years from the sun.

The most recent previous studies of the mass of the Milky Way used mixed samples of 50 to 500 objects. They gave implied masses up to two trillion times the mass of the Sun for the total mass of the Galaxy. By contrast, when the SDSS-II measurement within 180,000 light years is corrected to a total mass measurement, it yields a value slightly under one trilliion times the mass of the Sun.

And that’s how the Milky Way got a little slimmer, without the galactic equivalent of a diet. Wish it were so easy for humans!!

This blog a wholly pwnd subsidiary of Carolyn Collins Petersen, a.k.a. TheSpacewriter.
Copyright 2008, Carolyn Collins Petersen
Inama Nushif!
Image of Horsehead Nebula: T.A.Rector (NOAO/AURA/NSF) and Hubble Heritage Team (STScI/AURA/NASA)

“It is by Coffee alone I set my day in motion. It is by the juice of bean that coffee acquires depth, the tongue acquires taste, the taste awakens the body. It is by Coffee alone I set my day in motion.”

TheSpacewriter