Galactic Bulge Simulator | mysimulator.uk

Galactic Bulge

Black hole mass 4.3 × 10⁶ M☉

Sérsic index n 4.0

Bulge-to-disk ratio 0.6

Probe radius 1000 pc

Orbital speed at r

–

Enclosed mass

–

BH mass share at r

–

Half-light radius Rₑ

1000 pc

How it works

The bulge is the dense, roughly spherical concentration of old stars packed around a spiral galaxy's center — distinct from the thin, flat disk around it. Its stellar density falls off with radius following a Sérsic profile, whose shape parameter n controls how sharply light is concentrated: n≈1 gives a gently fading exponential disk-like bulge, n≈4 (de Vaucouleurs) gives the sharply peaked profile of a classical bulge.

ρ(r) ∝ exp[ −bₙ (r/Rₑ)^(1/n) ]
bₙ ≈ 2n − 1/3 + 4/(405n)
v(r) = √( G·[M_BH + M_bulge(<r)] / r )

Black hole mass — the mass of the supermassive black hole (like Sgr A*) sitting at the very center; near the middle it dominates the enclosed mass and orbital speeds spike steeply.
Sérsic index n — how concentrated the star cloud is; higher n packs far more stars into the innermost region, raising density and mid-range orbital speeds.
Bulge-to-disk ratio — the bulge's total stellar mass relative to the surrounding disk; a higher ratio means a heavier, more prominent central bulge (and fewer relative disk stars, shown thinned out).
Probe radius — drag the marker outward from the black hole to read the orbital speed and enclosed mass a star would feel at that distance, using the formula above.

Real-world relevance: this is exactly how astronomers weigh a galaxy's central black hole — by measuring how fast stars near the nucleus orbit and how quickly that speed changes with distance, then subtracting the bulge's own gravitational pull.

Central black hole Bulge stars Disk stars Probe marker

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