In our Milky Way galaxy, the orbital speed of stars around the galactic center varies in ways that don't follow the predictions of classical Newtonian gravity based on visible matter alone. Here's an overview of how these speeds vary with distance from the center and why:
1. Central Regions (Near the Galactic Center):
• Stars close to the Milky Way's center, within a few hundred light-years, generally orbit rapidly around the galactic center. For instance, stars orbiting within the dense central bulge or near the supermassive black hole (Sagittarius A*) can reach speeds of several hundred kilometers per second.
• The supermassive black hole's gravitational influence is significant here, creating a steep gravitational potential well that accelerates nearby stars, resulting in high orbital velocities .
2. Disk Region (Outer Parts of the Bulge and Galactic Disk):
• Moving outward from the galactic center, into the disk region, orbital speeds surprisingly remain relatively high and stable. Stars and gas clouds orbit at roughly 200-250 kilometers per second even as they move farther from the center. Our Sun travels at 230 km/sec .
• According to classical expectations, orbital speeds should decrease with distance if only visible matter (stars, gas) is taken into account. This stability in orbital speed at greater distances implies the presence of additional mass, which astronomers attribute to dark matter. Dark matter's gravitational effects are thought to create a "halo" that extends well beyond the visible disk, stabilizing star speeds throughout the galaxy's disk .
3. Outer Halo:
• At extreme distances from the galactic center, in the halo where stars and globular clusters orbit sparsely, velocities generally begin to decline. However, stars in this region are still observed to move faster than would be predicted if only the visible galaxy's mass were considered.
• These observations further support the existence of dark matter, which seems to exert gravitational effects over a wide radius, influencing stellar motions even in these outer reaches .
The relatively flat rotation curve of the Milky Way—where orbital speed doesn't decline significantly with increasing distance from the center—is a major line of evidence for dark matter. Without it, the galaxy would lack the gravitational binding needed to maintain the observed high orbital velocities at such large radii, suggesting the need for mass beyond what is visible .
This pattern of orbital velocity—rising near the center, flattening in the disk, and decreasing slowly in the far halo—is observed in many spiral galaxies, underscoring the influence of dark matter on galaxy dynamics across the universe.
