Milky way radius
Pro Research Analysisby 
Searched over 200M research papers
Understanding the Radius of the Milky Way Galaxy
Milky Way's Circular-Speed Curve and Mass Density
The circular-speed curve (v_c(R)) of the Milky Way provides critical insights into the galaxy's mass distribution. Near the Sun, the curve suggests a linear rise with radius (R). This observation implies that the Galactic mass density remains relatively constant beyond the solar radius ((R_0)). However, this apparent linear rise can be explained if the true circular-speed curve is constant or gently falling at (R_0), with mass concentrated in a ring at approximately 1.6 times (R_0)1.
Evolution of the Milky Way's Half-Mass Radius
The structural evolution of the Milky Way's progenitors over the past 10 billion years reveals that the half-mass radius ((r_{50})) has remained relatively constant at around 2-3 kpc. Despite a significant increase in stellar mass, there has been little-to-no inside-out growth. This stability in the half-mass radius challenges existing semiempirical models and numerical simulations, suggesting a unique evolutionary path for the Milky Way2.
Effective Radius vs. Physical Size
The effective or half-light radius is a common measure for galaxy size but does not always align with the physical boundary of a galaxy. For the Milky Way, using a size measure based on the gas density threshold required for star formation provides a more accurate representation. This method shows that ultra-diffuse galaxies (UDGs) are significantly smaller than Milky Way-like galaxies, highlighting the importance of choosing appropriate size estimators3.
Outer Edges of the Milky Way Halo
Recent measurements have identified the outermost edges of the Milky Way halo using galaxy kinematics. The inner depletion radius ((r_{id})) and the turnaround radius ((r_{ta})) are located well outside the virial radius, at approximately 559 kpc and 839 kpc, respectively. These measurements indicate that the Milky Way's halo is still growing and evolving, providing new constraints on its current dynamical state4.
Impact of Galaxy Mergers on Star Clusters
N-body simulations show that star clusters accreted by the Milky Way from dwarf galaxies undergo significant structural changes. The half-mass radius of these clusters adjusts quickly to the Milky Way's tidal forces, becoming indistinguishable from clusters that originated within the Milky Way. This suggests that the structural properties of accreted clusters are not unique, although other properties like metallicity may still indicate their extragalactic origins5.
Radial Orbit Migration in the Galactic Disk
Radial orbit migration plays a significant role in the Milky Way's disk structure. Stars in the Milky Way disk migrate significantly from their birth radii, with an average migration efficiency of about 3.6 kpc. This migration contributes to the structural regularity observed in disk galaxies and suggests that the Sun itself may have migrated from a birth radius of approximately 5.2 kpc6.
Conclusion
The radius of the Milky Way galaxy, whether considering the circular-speed curve, half-mass radius, or the outer halo edges, reveals a complex and dynamic structure. The stability of the half-mass radius over billions of years, the significant radial orbit migration, and the ongoing growth of the halo all contribute to our understanding of the Milky Way's unique evolutionary history. These insights challenge existing models and highlight the need for accurate size estimators and further research into the galaxy's structural properties.
Sources and full results
Most relevant research papers on this topic
THE OUTER ROTATION CURVE OF THE MILKY WAY
The apparent linear rise in v_c near the Sun is due to the true circular-speed curve being about constant or gently falling at R0, rather than the Galactic mass density being independent of radius at R > R0.
No Evolution in the Half-mass Radius of Milky Way–type Galaxies over the Last 10 Gyr
Milky Way-type galaxies have grown with a constant half-mass radius over the past 10 Gyr, indicating little-to-no inside-out growth and challenging previous models.
Are ultra-diffuse galaxies Milky Way-sized?
Ultra-diffuse galaxies (UDGs) are within the size range of dwarfs, not comparable to Milky Way-like objects, and using effective radius for galaxy size can lead to misleading results.
The Outermost Edges of the Milky Way Halo from Galaxy Kinematics
The Milky Way halo's depletion radius is 1.5 times its virial radius, suggesting a unique evolution history and providing constraints on its current dynamical state independent of internal dynamics.
The size of star clusters accreted by the Milky Way
Star clusters accreted by the Milky Way maintain their structural properties, making them indistinguishable from clusters born in the Milky Way.
Measuring Radial Orbit Migration in the Galactic Disk
Radial orbit migration in the Milky Way's main disk is rather strong, with stars migrating by about a half-mass radius over the age of the disk.
Highly CitedThe Inside-out Growth of the Galactic Disk
The Milky Way's stellar disk has grown by 43% over the last 7 Gyr due to radial orbit mixing, but the present-day age gradient remains weaker than 0.1 Gyr kpc1.
The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater
Crater 2 dwarf galaxy is the fourth largest in the Milky Way and is aligned with the Crater-Leo group, suggesting it may have been accreted as part of the Crater-Leo group.
Highly Cited