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S0 galaxy

This is a disk galaxy; it would be classified as Hubble Type S0 by astronomers. It's diameter is 20 kpc (kiloparsecs), or about 65,000 light years. For comparison, the disk of our Milky Way galaxy is roughly 100,000 light years across. Six hundred stars, shown in blue, define the disk, while the central bulge comprises 400 orange stars. Each star masses one solar mass. Forty globular clusters, in red, move in a stellar halo enveloping the disk but much smaller than the DM halo. Each cluster masses 100,000 solar masses, and is represented by a single symbol instead of individual stars.

Kinematics: Stars in the disk move in circular orbits around the centre, while bulge stars and gobular clusters orbit in random directions in elongated orbits.

Simulated times: Each time step is 30 million years; the animation lasts 9 billion years - about twice as long as the age of the Sun.

NOTE: There are no simulated spiral galaxies on this web site. Spiral structure arises and is maintained by complex processes which can't be approximated on home-based computers. Interactions between galaxies, shown in INTERACTING GALAXIES, induce ephemeral arcs reminiscent of spiral arms, but stable spiral patterns do not emerge.

GRAVITATIONAL FORCES IN PLAY: Every star and cluster is acted upon by all the other stars and clusters in this simulation, but also by two invisible components: a spherical bulge and a spherical dark matter (DM) halo. Both components are centred on the visible galaxy. The bulge stars trace that component, whereas the DM halo is twice as large as the disk - diameter 40 kpc. Their masses, respectively, are 10¹⁰ (ten billion) and 5x10¹⁰ solar masses. The two components are believed to be present in most if not all real disk galaxies.

You can think of the bulge - here and in other disk galaxy simulations - as a mathematical representation of the combined gravitational force of ten billion stars - far too many to be accounted for individually in my efforts; the bulge stars you see contribute virtually nothing to the gravity of that component; they merely trace its presence. In reality DM halos make their presence known by their gravitational force on visible matter - stars in this case. It remains unknown what dark matter actually is.

The Sombrero Galaxy

The well-known galaxy NGC 4594, also known as M104, a favourite of amateur astronomers. The Hubble type is not certain - probably either Sa or E(pec), depending on the origin of the bisecting dust lane. That feature is prominent on James Webb Space Telescope images. The dust is mixed with atomic hydrogen and probably has a component of cool molecular gas and associated young stars. The Sombrero is as large and massive as our Milky Way, with a major diameter of 32 kiloparsecs (kpc) or about 100,000 light years. It's centre is occupied by a supermassive black hole of roughly a billion solar masses.

The simulation of the Sombrero features 1400 bulge stars in orange. The dust ring is delineated by 1800 blue stars. Forty red globular clusters orbit within and beyond the bulge - the actual galaxy probably has a thousand or more such clusters. A large stationary white symbol at the centre of everything represents the supermasive black hole.

GRAVITATIONAL FORCES: This is not a fully n-body simulation. The forces of gravity are provided by the 40 kpc diameter spherical bulge with mass 10¹⁰ suns and the concentric 40 kpc diameter DM halo five times as massive. The outer diameter of the blue "dust ring" stars is 24 kpc - well within the halo. The animation time step is 16 million years.

Notice the difference in smoothness between these two videos. Jerkiness is often the price paid for running true n-body simulations with limited computing power.