Up In Arms – Our Milky Way Galaxy Found to Have More 'Arms' Than Previously Thought

star-forming region in MIlky Way galaxy

A team of astronomers and astrophysicists has identified over 120 groups of “young, massive stars” located in well-known star-forming regions of our galaxy and found that they map faithfully onto a four arm model of our galaxy, strongly indicating that The Milky Way galaxy has four spiral arms, instead of two as previously thought.

Aided by ground and space-based telescopes, astronomers and astrophysicists are able to conduct stellar surveys of unprecedented scope. These surveys provide the raw data for a multitude of studies and calculations of the various features of our cosmos and, in particular, our galactic home: The Milky Way.

In this newest stellar investigation, reported yesterday in the Monthly Notices of the Royal Astronomical Society, researchers (Urquhart et al) began with a well-used sample of 1750 “embedded, young, massive stars” that were originally identified by the Red MSX Source (RMS) survey. The purpose here was to measure the distribution of our galaxy’s most recent, massive star formation.

These young, massive stars were selected because they are at an earlier stage in their stellar development and so would still reside (“embedded”) within the immense gas and dust clouds from which they formed.

In particular, the team look at approximately 800 of these stars (all being 8 times the size of our sun or larger, which is the size that leads to supernovae) that had not yet had their radial velocities (i.e., the speed of a star’s spin) measured. Upon calculating this factor, along with each star’s distance, position, (longitude and latitude) and luminosity, the team was able to identify about 120 small groups, or star-forming sources. Many of these sources were associated with previously identified formation complexes (see link to abstract, below).

Next, the team adopted (“assumed”) a model of our galaxy consisting of four gaseous (spiral) arms. They then compared the distribution of the stars’ positions with the expected locations of the spiral arms and found that the positions matched up well.

Quoting from the paper’s abstract:

The distribution of young massive stars in the Milky Way is spatially correlated with the [four] spiral arms, with strong peaks in the source position and luminosity distributions at the arms’ Galactocentric radii.” [source: see link below]

Additional comparisons of the stars’ “source and luminosity surface densities” also correlated well with the surface density of the molecular gas in these star-forming regions. According to the researchers, this suggests that the “massive star formation rate per unit molecular mass is approximately constant across the Galaxy.” Further comparison of the distribution of the gas and these young massive stars with other nearby spiral galaxies showed “similar radial dependences”. [source; see paper link, below]

Approximately 30% of the total luminosity of the embedded massive star population was found to be “associated with the ten most active star-forming complexes” known in our galaxy.

The fitting of these measurements with a four-arm spiral model of the galaxy stands in contrast to a 2008 survey of 110 million stars that found evidence supporting just two spiral arms (note: this survey included other categories of stars, including smaller and older stars). But lest one think that the scientists here have somehow fitted the data to the (pre-selected) model, it is interesting to note that these results concur with a much lower resolution survey conducted in the 1950’s, which indicated a four-arm spiral as the best fit for the data.

And while this survey adds to the total number of star-forming complexes, it does not (as of yet) mean that earlier estimations of our galaxy’s total star count are off, as these young stars represent only a small percentage of that total.

Top image: (artist rendition; star-forming regions are red dots in image; the black dot denotes the position of our solar system) Data: J. Urquhart et al., Monthly Notices of the Royal Astronomical Society; Background image: Robert Hurt/Spitzer Science Center

Source material (including quotes) for this post came from the paper ‘The RMS survey: galactic distribution of massive star formation’ (Urquhart, J. S. et al), paper abstract, published on-line in Monthly Notices of the Royal Astronomical Society (Oxford University), December, 17, 2013.
Some additional material came from the Science Shot article: ‘The Milky Way’s Missing Arms’ by Side Perkins




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