The galactic center of our Milky Way holds secrets revealed through radio, X-ray, and near-infrared observations. These show Sagittarius A, a compact radio source believed to be a supermassive black hole, surrounded by a dense star cluster and hot gas.

This central black hole, weighing 4 million solar masses, shapes our galaxy's structure and evolution. By studying stellar orbits around it, scientists can calculate its mass and better understand its profound influence on the Milky Way's dynamics and evolution.

The Galactic Center: Key Observations and Implications

Radio and X-ray observations of galactic center

• Radio observations reveal Sgr A* (Sagittarius A*), a compact radio source at the center of the Milky Way galaxy that emits non-thermal radio waves suggesting the presence of a supermassive black hole
• Radio lobes and jets indicate an active galactic nucleus (AGN) powered by accretion of matter onto the central supermassive black hole
• X-ray observations show emission from the galactic center region, indicating the presence of hot gas and high-energy processes (supernovae, stellar winds)
• X-ray flares from Sgr A result from material falling onto the black hole and releasing energy, further evidence for a supermassive black hole

Near-infrared images of galactic center

• Near-infrared imaging allows observation of the galactic center through obscuring dust and gas, revealing a dense star cluster surrounding Sgr A
• Stellar orbits around Sgr A* show stars orbiting a central point coinciding with the position of Sgr A*, and these orbital motions are used to calculate the mass of the central object
• The high concentration of mass in a small volume suggests the presence of a supermassive black hole acting as the gravitational center of the Milky Way galaxy (4 million solar masses)
• The galactic bulge, a dense concentration of older stars, is visible in near-infrared images and surrounds the central black hole

Mass calculation of central gravitating object

• Kepler's third law relates orbital period $P$, semi-major axis $a$, and mass of the central object $M$ through the equation $P^2 = \frac{4\pi^2}{GM}a^3$, where $G$ is the gravitational constant
• Near-infrared imaging allows precise measurement of stellar positions over time to determine orbital parameters (period and semi-major axis)
• Using measured orbital parameters and Kepler's third law, the mass of the central object can be calculated
• The calculated mass combined with the small volume of the central region provides strong evidence for a supermassive black hole at the center of the Milky Way (Sgr A)
• Distances to stars near the galactic center are often measured in parsecs, a unit of astronomical distance equal to about 3.26 light-years

Implications and Significance of the Galactic Center

Understand the significance of the supermassive black hole at the center of the Milky Way

• The central object, known as Sagittarius A* (Sgr A*), has a mass of approximately 4 million solar masses and significantly influences the structure and evolution of the Milky Way galaxy
• The supermassive black hole dominates the gravitational dynamics of the central region, affecting the orbits of stars and gas in its vicinity (S-stars)
• When material falls onto the supermassive black hole, it can power an active galactic nucleus (AGN) that impacts the surrounding galaxy through feedback processes such as jets and outflows (radio lobes)
• The event horizon of the black hole marks the boundary beyond which nothing can escape its gravitational pull

Extended galactic structure

• The Milky Way is surrounded by a dark matter halo, which extends far beyond the visible disk and contributes significantly to the galaxy's total mass
• In some galaxies, the central black hole can become extremely luminous, forming a quasar that outshines the entire host galaxy