The image above shows an annotated map of the Milky Way, based on recent distance measurements to distant stars located within dense and massive components of the galaxy.
Although it has long been known that the Milky Way galaxy has a distinct spiral structure, our position within the galaxy has thus far precluded astronomers from determining the diameter of the galaxy with any degree of accuracy. Various models of what the Milky Way might look like when seen from outside of the galaxy have all yielded size estimates of around 100,000 light years, but a new study has just doubled this value to about 200,000 light years.
Like other spiral galaxies, the Milky Way consists of a relatively flat disc that is composed of a spiral structure, with the whole being surrounded by a sparsely populated outer halo. Like other spiral galaxies, the bulk of the Milky Way’s stars are concentrated in the disc, which rapidly transitions into the halo, with the drop off point representing rough boundary of the disc.
Traditionally, the diameter of spiral galaxies is measured from the center of the disc to the point where the stellar population begins to thin out. However, in the case of our home galaxy, the exact distance from the center to the drop off point is not clearly visible, hence the need to use mathematical and theoretical modelling to determine the galaxy’s diameter.
Clearly, a more accurate way to measure the Milky Way’s diameter was required. Fortunately, since the bulk of the stars in the disc is much younger than the average age of halo stars, teams of researchers from the Instituto de Astrofísica de Canarias and at the National Astronomical Observatories of Beijing, hit on the idea of using spectroscopy to determine which distant stars belong to the disc, and which belong to the halo.
Working in concert, the two teams obtained spectra from a total of 4,600 stars, the idea being to use the stars’ individual chemical compositions to determine their age. As a rule, the younger a star is the more complex its chemical composition becomes, since each successive generation of stars contain progressively heavier elements. Thus, by separating the sample stars into discrete populations, the researchers found that young, chemically mature stars occur up to about 81,000 light years further from the galactic center that was though possible just a decade ago.
Martín López Corredoira, who is the lead author of the study that was published in the journal Astronomy and Astrophysics on May 7th this year, commented, “[certainly] one would expect the existence of stars at very far distances from the galactic center, as part of the halo. However, as far as we know, nobody could previously say that stars [farther than 81,000 light-years from the galactic center] are confirmed spectroscopically to belong to the disk.”
Nonetheless, and even though the disc tapers off into the halo at a greater distance from the galactic center than was thought possible, the additional 100,000 light years added to the galaxy’s girth adds almost no additional mass to the disc. When viewed objectively, the small number of stars that are now added to the galaxy’s disc represent less than the proverbial drop of water in a vast ocean, meaning that the mass of Milky Way’s disc remains unaffected in any meaningful way. In essence, no stars were added to the Milky Way; its existing stellar populations are now just rearranged somewhat differently.
Interestingly, even though the Milky Way’s diameter had grown by 100,000 light years in about a decade, it is still smaller than the Andromeda Galaxy, which measures 220,000 light years across from edge to edge.