Spiral Galaxy

**Spiral Galaxies** are the most famous type of galaxy. They typically consist of a central bulge, a disk on the same plane as the bulge and a halo which surround the galaxy. Spiral galaxies are so named for the spiral structures which extend from the galactic bulge. Some spiral galaxies, such as the Milky Way, have a central bar which extends from the bulge. Roughly 2/3 of all spiral galaxies have a barred structure. An even larger halo of dark matter surrounds the galaxy, containing roughly 95% of its mass.

The bulge is typically a red-yellow color, as the stellar population is almost entirely old stars, whereas the spirals are typically a bright blue color of massive, young stars. The patches of pink that can be seen in spiral galaxies are in fact star-forming nebulae, and the brown patches are dust lanes. Roughly 60% of galaxies are spirals.

Very small, low-luminosity spiral galaxies are called dwarf spiral galaxies.

Examples of spiral galaxies include our own Milky Way galaxy, the nearby Andromeda Galaxy, the Pinwheel Galaxy and the Whirlpool Galaxy.

## Classification

   The Hubble Classification system separates spiral galaxies into two main groups: barred and unbarred galaxies. Barred galaxies are denoted by SB and unbarred galaxies by S. These are further separates into “a”, “b” and “c” type galaxies, based on how tightly wound the spiral arms are. Dwarf Spiral galaxies are denoted by dS (unbarred) or dSB (barred).

About 10% of spiral galaxies are so-called “grand design” spirals: spiral galaxies with only two, well-defined arms. Another 30% of spiral galaxies are “flocculent”, meaning that they have no well-defined spiral arms at all. The remaining 60% of galaxies are called “multiple-arm” galaxies, denoting the fact that they have more than two well-defined spiral arms.

## Physical Properties

The spiral arms are a great example of an interesting phenomena known as density waves. The spiral arms of a galaxy are not material, as was once thought, but are rather areas of higher density, analogous to a traffic jam: although individual cars move through the traffic jam, the jam itself does not move. As interstellar dust passes through a spiral arm, it is compressed, accelerating star formation (hence the population of young stars in the spiral arms).

A spiral galaxy’s classification sets the range for its pitch angle, rotational speed and the bulge-disk luminosity ratio. The relationship is tabulated below:

 Sb/SBb/dSb/dSBb
 Sc/SBc/dSc/dSBc

   Pitch angle defines how tightly wound the spiral arms are. Generally, a higher pitch angle is the result of a higher rotational velocity.

Spiral galaxies range in radius from about 1.3 kpc to 80 kpc, and range in mass from 109 to 1011 M☉. Dwarf spirals have smaller radii and masses.

Each spiral galaxy has a supermassive black hole (SMBH) at its centre. Its mass (M) is related to the stellar velocity dispersion (σ) of the galactic bulge (the standard deviation of stellar velocities from the mean stellar velocity). This relationship, known as the M-sigma relation, is a bit of overkill, but some worldbuilders may enjoy the extra depth and maths. The relationship is:

Where M is expressed in solar masses and σ in km/s.

If you wish, you can calculate just how many stars your galaxy has by calculating the volume of your galaxy and applying it to this formula:

Where ρs is stellar density. The stellar density of the Milky Way's disk is about 0.004 stars per light year cubed (0.139 stars per cubic parsec), whereas the bulge density is about 2.882 stars per light year cubed (99.993 stars per cubic parsec).

You can use this calculator to calculate the volume of the bulge and this one for the disk.

## Habitability

    The galactic bulge (and bar, for barred galaxies) are definitely not well suited for habitability. The high density of stars would prohibit the formation of planets and, based on data from our own galaxy, we could expect a stellar collision every million years. In addition to this, the SMBH would emit lethal amounts of radiation, killing all but the sturdiest life forms.

Spiral arms are also dangerous. Any star that spends too much of time in a spiral arm will have its Oort Cloud perturbed, sending comets and asteroids hurtling towards the inner planets. Worse still: the more time a star spends in the spiral arm, the higher the risk of exposing its planets to gamma-ray bursts, supernovae and the like, which would cause massive extinction events. Stars that orbit near a co-rotation radius of a galaxy (a radius at which objects orbit at the same speed as the density wave which carries the spiral arms) and have a very low orbital eccentricity (i.e. their orbits are very circular) would maximize the time spent outside the spiral arms.

In order to maximise habitability, a star should have a very low inclination with respect to the galactic plane. This means the star will be situated within the galactic disk, rather than the galactic halo. This is because stars that have high metallicity (metal-rich stars, known as Population I stars) are largely located within the galactic disk, whereas low metallicity stars (metal-poor stars, known as Population II stars) are largely located in the galactic halo. Planets require a high-metallicity environment to form, and thus will most likely be found orbiting Population I stars.

## Worldbuilding in Practice

> The Hathor Galaxy is a massive spiral galaxy located in the distant Thebes cluster. Roughly 151,000 parsecs across (about 493,000 light years), it is one of the largest spiral galaxies in the explored universe, and is certainly the largest in its cluster, with a population of roughly 23 trillion stars.
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> Hathor has three spiral arms - Hapi, Imsety and Ihy - and is therefore classed as a multiple-arm galaxy. It's galactic classification is Sa, for it has very tightly wound arms. Hapi, Imsety and Ihy dominate Hathor, and astronomers suspect Hathor's habitability to be greatly reduced as a result. For this reason, the Spacefarer's Union has devoted their efforts to explore Inti, an elliptical galaxy in the Thebes cluster, instead. This will give them the unique opportunity to explore ISG 23, a satellite dwarf spiral galaxy of Inti.
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> ISG 23 is a mere 4600 light years (1400 parsecs) across and, with only 146 million stars, it is a real cosmic lightweight. A barred, grand design dwarf spiral (classification dSBc), ISG 23 is characterized by its two long, sweeping arms which have been stretched by tidal forces from Inti. It is theorized that in about 1 billion years, ISG 23 will be consumed by Inti.
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> Even though Inti is much farther away than Hathor, exploration efforts are focused squarely on the former and its nearby satellite, thanks in no small part to ISG 23. A popular catchphrase from the proposal team: two galaxies for the price of one.
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## References

1. ↑ Wikipedia: Spiral Galaxy
2. ↑ Wikipedia: Dark Matter Halo
3. ↑ Wikipedia: Hubble Sequence
4. ↑ Chris Mihos: Spiral Structure
5. ↑ Wikipedia: Density Wave Theory
6. ↑ NED: Spiral Structure
7. ↑ Wikipedia: M-Sigma Relation
8. ↑ Wikipedia: Galactic Habitable Zone
9. ↑ Wikipedia: Stellar Population