“Sorry, I Was Weighing a Milky Way”


(The Milky Way; NASA / JPL-Caltech)

I thought the title would be a fun play on the most current slogan for the Milky Way candy bar, “Sorry I was Eating a Milky Way”.  As difficult as the as people in the Milky Way commercials portray concentrating while trying to eat a Milky Way, it is sometimes very difficult to come up with titles for these blogs.  Although that may be true, it is even more difficult to determine the mass of a galaxy, especially one that you reside in.

In an article I found in Sky & Telescope entitled, New Mass Estimate for the Milky Way, author Monica Young talks about that difficulty.  In the article, readers come to realize the fact that residing in the Milky Way galaxy and not being able to see the totality of it due to dark matter are two of the main reasons why we have so many estimates on its mass.  She highlighted one of the methods astronomers use to help them determine the size of the galaxy, global cluster measurements.  But, even these measurements can only do so much in the effort of determining the mass of our galaxy.  Right now, the galaxy’s mass is estimated at 1 trillion (1,000,000,000,000) suns.

The author was correct.  Measuring the distance to global clusters within the galaxy is one way to help determine its size.


(The globular cluster Arp Madore 1, imaged here by the Hubble Space Telescope, is the farthest globular cluster in Eadie and colleague’s sample at 400,000 light-yeras from the galactic center.
Fabian RRRR / Wikimedia Commons / CC-BY-SA-3.0)

However, I didn’t see the author mention utilizing our knowledge of the gases present in the galaxy as a way to make sure we include all of the global clusters present within our galaxy.  Our galaxy produces some of the strongest sources of gamma rays, carbon dioxide, and atomic hydrogen as we learned from an dome slide demonstration in class.  Pictured below are wavelengths similar to what we saw on the overhead dome.


These wavelengths help astronomers to see the shape of our galaxy to further help them to know what to include as a part of it and what not to include when determining the mass.  The gas and dust within a galaxy are visible matter.  Because we don’t truly know what dark matter is, it is very hard to incorporate its mass into the total mass of the galaxy.  The Milky Way is a barred spiral galaxy.  Barred spiral galaxies produce more stars in the arms of the spirals.  So, hot, blue stars are present here.  Over time, we may need to incorporate a new measurement based on new star production within our galaxy’s arms.

I wonder if these measurements on mass may change drastically in the years to come as we hopefully begin to understand dark matter more.  I know we are looking at the galaxy in past tense, but who knows what the measurement would be if we were looking at it in real time.  Those measurements would be really interesting to compare.




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