2. Hydrogen in Galaxies
Universes contain a significant amount of pure Hydrogen;
- some of it is in the normal ‘H2‘ molecules, but even more of it in the atomic particle form; ‘H’.
- all of it experiences gravitational forces, just as do all the particles that are created in the
fusion in stars; helium, lithium, ...... uranium .......
It is always a temptation to ignore the fact that hydrogen experiences gravitational attraction, because a common experience of it is that it appears to be attracted upwards; as in hot air balloons etc. Such movements arise because hydrogen is the lightest gas.
Hydrogen is just as much a part of the matter in a galaxy as the stars, planets, moons etc..
etc. However, it is spread out more widely relative to other material in a galaxy; often over three or four times the radius of a normal visible galaxy. The reason for this, is that it plays a different role.
All the other particles are created as the result of the fusion of hydrogen. Periodically, the density and volume of the hydrogen becomes large enough for it to trigger a fusion reaction creating helium from hydrogen. In time, further fusion produces more complex atoms, such as lithium ...... uranium, and these combine to create a 'Solar System' centred on a sun.
Because these fusion transitions also radiate energy (photons), all the particles except the hydrogen atoms are effectively 'illuminated'. Hydrrogen atoms and molecules are invisible to our eyes.
Many other bodies are also created and these merge to form ‘planets ’ and ‘moons’. These different bodies adopt some parabolic orbit around their appropriate 'star'. Hence, almost all of the matter in a solar system, move in orbits that are centred on a solar system.
Most non-gasses reflect light and this is what we see in the sky.
Gasses can radiate light, but at specific frequencies depending on their make up. The frequency of the emitted light depends on the velocity of the gas relative to the observer. This is very useful, particularly on trying to identify the source of this light.
Sir Isaac Newton recorded that, within our solar system, most of the planets are also 'spherical' and he proved that, for bodies that are Spherically Symmetric, the force (F) that they experience from the spherical body is given by:
F = m x M x G / (r^2)
where m is the mass of the rotating body,
M is the massof the body that it is circling around,
G is the Gravitational Constant,
r is the radius of the orbit.
However, when writing Book 3 of the 'The Principia', concerning specifically the movement of the 'earth', he found that it was not spherical, not rigid, because of its tidal water and hence exceptionally difficult to predict.
This lead to him to write a 'Scholium' (Note) to the end of Secion 12 of Book 1.
'Section 12' was specifically dedicated to 'The attractive forces of spherical bodies'.
The final scholium in that section pointed out that, if the bodies are non-spherical, the techniques of this section MUST NOT be used.
It is these non-spherical properties, that cause a lot of erroneous conclusions.
'Galaxies are very different from solar systems.'
Galaxies form, when a number of solar system stars, start to move through the orbits of other solar systems. Usually, their orbits begin to be pulled into planes that are more or less parallel to one another and this causes their orbits to collapse into one single plane (disc). Typically, the thickness of the disc is about 5 or 10 percent of the radius, although some of them have a less dense spherical bulge near the centre.
This automatically confirms that the forces experienced within galaxies, clusters and possibly universes, DO NOT follow the the basic formula given above; the masses are NOT spherically symmetric and the Scholium from Section 12, requires an enhanced formula.
Basically, the large mass 'M', which in this case is NOT sperically symetric and it is sufficiently large that the value of 'r' in the term 1/r^2 is significantly different for different part of that mass M. This requires that the formulla has to replace above term with an integral over the shape of M. This in turn provides larger than expected forces.
A few of the orbits, which are closer to the centre, do exceptionally remain at their original angles to one another and form a central spherical 'bulge'.
Near the centre of the galaxy, the centrally attractive force can exceed the ability of stars and their emitted light to spread back to the outer part of the galaxy. They remain trapped in the centre of the galaxy. This is known as a 'black hole'.
(Fortunately, black holes and central bulges are not considered in this paper, because they occur at the centre of the galaxy and the hydrogen that we are discussing lies predominently outside the visible boundaries of the galaxy).
However, it should be noted that the bodies are constantly moving towards the centre of the galaxy:
- Black holes absorb particles from near the centre and somehow dispose of their energy.
- Bulges house a disproportionate number of stars that are 'dying'.
- Spirals show stars that are slowly being 'sucked' into the centre.
- Stars, are built around the fusion of Hydrogen to Helium,
Hence they extend out to the currently recognised limit of a 'illuminated' disc of a galaxy.
- Hydrogen clouds, generally lie outside of the 'illuminated' disc.
But when their density reaches some limit, they can trigger a fusion reation, and this, by definition, shows that they must have moved inside the illuminated galaxy or the galaxy has just expanded.
The resulting paths within a galaxy disc are more or less circular or spiral 3D shapes. However, unlike elements in a 'Solar System', they are NOT centred on any specific body. They are effectively rotating around the centre of gravity of all the stars in the galaxy (or possibly a Black Hole).
We are considering millions/billions of stars at a time.
The Outer Boundary of a Galaxy
What defines the outer boundary of a galaxy?
Is it at the outer limit of the visible stars?
Is it at the outer limit of any hydrogen that is rotating with the visible stars?
Is it at infinity?
It is almost universally accepted that the outer boundary of a galaxy is the outer boundary of the visible stars.
However, a lot of invisible hydrogen clouds exist outside of this. Certainly, near the visible boundary, they will just about as close to one another as the visible ones are.
It must not be forgotten that hydrogen atoms also respond to gravity. So how far out do they extend?
Rubins makes some very good observations:
"Roberts and Witehurst published their survey of the southern end of 'M31 galaxy' (Andromeda) . They traced the extent and velocity of neutral hydrogen gas using the infra-red radiation from the Hydrogen. To the limits of their detection,....the velocities remain 'flat' ('constant')."
She continued with some simple questions:
"What is spinning the stars and gas around so fast beyond the optical galaxy?"
It can only be the same rotational inertia.
Hydrogen responds to the rules of angular momentum just as well as any other atoms.
"What is keeping them from flying out into space?"
Gravity pulls them in, in exactly the same way as it attracts all visible atoms.
This has generally been ignored, because this radiation lies in the infra red.
It is these facts also cause the misconception that the boundary of the visible light identifies the boundary of the galaxy. It ignores the hydrogen atoms that are also orbiting the centre of the star, but happen to be at a distance that is mostly outside the visible light.
It is easy to think of hydrogen as something that just floats up into the sky. But it still suffers gravity, the same as all other atoms. It is only when in other more dense media, such as air or water, does hydrogen 'float off' into the night sky.
She doesn't see the barely visible hydrogen atoms. Instead she adopts the answer of 'Dark Matter'.
When these Hydrogen atoms are also taken into account, the boundary of the galaxy is commonly over three times the radius of the fusion material (helium etc). Hence the true outer limit of a galaxy is the outer limit of any hydrogen that is rotating with the disc?
The hydrogen atom do not radiate energy continuously of their own accord. However, if they are in orbit, the cetrigugal force (gravity), which holds them in orbit, radiates a small amount of low energy light in the red end of the spectrum. This light has been studied by Roberts, M.S. and Whitehurst .
Consequently, it really doesn't make much difference whether fusion has started or not.
Complete suns, based on Helium, and lying in the illuminated portion of a galaxy, emit a wide range of visible light waves that are easily seen.
In comparision, the individual hydrogen particles emit weak, Dopler shifted, light, which is far less visable.
A galaxy's effective gravitational boundry must lie outside of most of the hydrogen that is rotating around the galaxy.
It can be anticipated that the density of the hydrogen atoms will reduce with the distance they are from the outer boundary of the visible galaxy. This also makes this boundary even less visible.
In 1975, Roberts, M.S. and Whitehurst, R.N.  measured the red shift of the faint Hydrogen particles and showed that this decrease in density is slow; at least until the observer is several (more than 3) times the visible radius of the galaxy, away from the centre of the galaxy. In other words, galaxies can have significant volumes of hydrogen rotating around them for at least 3 times the radius of the visible stars.
When the hydrogen is significantly more than 3 times the radius of the galaxy, away from the centre of the galaxy, the number of stars with this velocity falls away very rapidly.
In the 1960's, this supposition was interpreted as being due to "Gravity, from matter that has not light"; (Rubin,V., 2006). They are identified by their speed, when compared with other random atoms and very little radiated light. The velocities of the atoms are more or less as predicted by Kepler, probably because they are not in the galaxy.
References and Notes
 Rubin,V., Physics Today, Dec 2006, p8.
 Roberts, M.S. and Whitehurst, R. N., Astro-Physics ,J. 201,327 (1975)
 Formation and shape of galaxies.
 Dust and Gas: https://www.space.com/19321-sun-formation.html
 UWA: New analysis by researchers at the University of Western Australia and the International Center for Radio Astronomy Research confirms even galaxies featuring intense rates of star formation host large amounts of atomic hydrogen.