S2 and the Silent Mass

Plate 35 speaks for itself: “We weighed it with a single star.”

The Physics

The star S2 orbits the Milky Way's centre every 16 years. Kepler's third law applied to its orbit weighs the dark object there, Sgr A*, at about four million Suns.

M = 4π² a³ / (G T²)

• M: central mass, kg
• a: semi-major axis, m
• G: gravitational constant, m³ kg⁻¹ s⁻²
• T: orbital period, s

4.15 million M☉ (GRAVITY, 2019). May 2018 pericentre: 120 AU, 7650 km/s.

Sources

• GRAVITY Collaboration, Astron. Astrophys. 615, L15 (2018).
• GRAVITY Collaboration, Astron. Astrophys. 625, L10 (2019).

On the poster

Plate 035, “S2 and the Silent Mass”: the pixel-art plate with its slogan “We weighed it with a single star.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.