OC402: The Oort Cloud and Mass in the Galaxy
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.
This is Number 2 in a suite of web articles about the Oort Cloud, the volume of space immediately outside our Solar System.
The stars in the skies
Our knowledge of the Universe has until recently been entirely gained from studying the light we get from stars in the heavens. Even now, when we are getting information from radio, infrared, and other parts of the electromagnetic spectrum, the bulk of information comes from the visible-light spectrum.
Apart from the planets and other objects viewed by light reflected from the Sun, this means the stars, studied since ancient times. Stars vary considerably in colour, size, and mass. A useful graphical classification of stars was developed around 1910 by Ejnar Hertzsprung and Henry Norris Russell [reference B1], and represents a major step towards an understanding of stellar evolution.
Fig. OC402-F1. The Hertzsprung-Russell star diagram. From [B1].
The main axes on this beautiful diagram show luminosity (intrinsic brightness) and surface temperature. Most stars fall in what's called the "Main Sequence", that is, they lie on the band stretching from the top left corner down to the bottom right. These "normal" stars may be called "Fusion Stars" -- their light is ultimately generated from energy derived from the fusion of Hydrogen atoms into atoms of Helium and other heavier elements.
One of the first things revealed when this diagram was devised, is that the mass of a star determines its colour. Average-mass stars, such as the Sun, are yellow-white, while lower-mass stars are more red. And the smaller the mass, the redder the colour.
Main-sequence stars more massive than the Sun appear bluer, and with increasing mass their colour moves towards violet, the short-wave end of the visible spectrum. The mass of a star also determines its lifetime -- more massive stars burn out more quickly. On the diagram, masses are marked as multiples of MSun, the mass of our Sun. They range from 60 Suns with the violet stars on the top left, down to 0.1 Suns for the small faint red stars on the bottom right.
The sequence fades out with the small red stars because stars with mass less than about 7.5% of the Sun's mass are not heavy ("hot") enough to generate light from their internal processes. The upper limit occurs with the very bright violet stars because there seems to be a natural limit on how big stars can get. If bigger main-sequence stars did exist, they might radiate mostly in the ultraviolet, beyond the visible spectrum.
When we talk about the colour of stars, this refers to the peak wavelength at which they radiate. Their spectrum also includes radiation at either side of the peak -- we will mention more on this in OC403: Radiation in the Universe.
Sometimes the stars are grouped into bands (denoted by the letters at the base), so we might talk about "a G-class star like the Sun". The mnemonic is, Oh Be A Fine Girl, Kiss Me".
It might be wondered why White Dwarf and Red Giant stars don't fall on the Main Sequence line. This is because they are not "Fusion Stars", deriving energy from nuclear fusion, but get their energy elsewhere. White Dwarfs are part of the class of what can be called "Vortex Stars" -- very-rapidly rotating dense vortexes of matter/energy which emit energy in tight beams along their two axes. This class of Vortex Stars also includes Neutron Stars, Black Holes, AGNs (supermassive black holes at galaxy centres), Quasars, Pulsars, and Blazars. We will have more on this topic in OC403: Radiation in the Universe.
All Main Sequence stars evolve on a life cycle which includes a Blowup Phase, when they swell up and eject a big part of their mass in a roughly spherical mass/energy Explosion Front, leaving behind a Vortex Star. The Explosion Front has a very large surface area, which is the place of origin of the light it emits, and standard thermal-emission (black-body) laws mean that Red Giant and Supergiant "stars" are very luminous. Examples of these Transition Stars, which are essentially the Explosion Fronts of Fusion Stars undergoing change to Vortex Stars, are objects called Supernovas or Planetary Nebulas.
There will be more detailed examination of these matters later. The particular evolution path of a Fusion Star depends to a very large degree upon its mass. We now go on to look at the distribution of star masses.
How star masses are distributed
In 2005, Donald Figer published a study of star masses in the Arches cluster, the densest in our galaxy [B2]. He found that these stars appeared to have a mass limit of less than about 150 solar masses.
Fig. OCB-F2. Stellar masses and populations. From [B2].
Not unexpected was the fact that in the stars studied, the smaller their masses, the greater they were in number. This is a common occurrence where peak values of a physical quantity are attained -- below the peak, the number in a population at lesser quantity values increases continuously. There is a common example with the strength of earthquakes.
Fig. OC402-F3. Earthquake energies and numbers. From [B3].
From Figure F3 it can be seen that earthquakes of magnitude 9 or above are very rare, occurring only once on average per year over the whole Earth. Earthquakes of magnitude 7 (with one-thousandth of magnitude 9 energy) occur about 20 times more often, and numbers increase greatly with lesser energy, until down at magnitude 2, there are about 1 million earthquakes a year [B3].
And so with stars. Figure F2 on stellar masses only includes stars with mass of 10 solar masses and above -- the number of yellow G-class stars around 1 solar mass would be much greater. And the numbers of red-dwarf stars, with only about 7.5% solar mass, would be comparatively huge -- red dwarfs are easily the most common stars in the Universe.
We can get a better feel for the whole situation from the Universe Mass Wheel, which represents the masses of objects in the Universe, from the heaviest to the lightest, in terms of their Q-Scale Numbers. The Q-Scale number of an object is just the base-10 logarithm of its mass in grams.
Fig. OC402-F3. The Universe Mass Wheel. From [B4].
There is more about the derivation of the original Q-Scale in BS804: The Q-Scale method of representing masses [B5]. In that description, the Scale is flat. In the present case it has been curved round, head-to-tail, to form the Universe Mass Wheel.
This is a new way of representing masses, so a little explanation would be in order. The bold numbers round the rim of the wheel are the Q-Scale values, ranging from +40 down to -30. As the Q-Scale number of an object is the base-10 logarithm of its mass in grams, an object at Q = 40 has a mass of 1040 grams -- these are the most massive single objects currently known, the AGNs or supermassive black holes at the centre of galaxies.
Moving clockwise round the wheel brings you to less massive objects, such as those in everyday life. In the examples written in blue, the Q-Scale value for a man is put at Q = 2. This should actually have been at Q = 5, since a man might weigh 100 kg, or 105 grams.
Moving further clockwise round to the upper left, the wheel arrives at the lightest known objects in science, such as atoms and sub-atomic particles like the electron, which weighs about 10-28 grams, Q = -28. Obviously this Q-Scale representation is not minutely accurate, it's a way of representing visually a very large range of masses.
There is a feature of this wheel which may be helpful in our investigation of stars and other celestial objects. The assumption can be made, that when considering the total mass of a group of objects (for example stars), that total is proportional to the angle which they occupy on the Wheel. In this sense, the Wheel is similar to a Pie Chart, where a total population or group is split into divisions where the divisions are shown as pie slices according to their relevant sizes, or some other property.
As far as I am aware, the pie-chart-mass assumption above has not been proved by any mathematical or statistical analyses. It is only a working hypothesis which may be useful. But it should at least be approximately valid, because of the known fact that with objects like stars, their number increases regularly with lesser mass.
Objects smaller than stars, and their formation
It is generally acknowledged that stars, planets, and lesser objects such as asteroids are formed by the gravitational aggregation of matter in interstellar space. There is little room to argue against this. However, the actual mechanisms involved are still the subject of appraisal and not always widely accepted.
In 2014 I made a new analysis of the formation of solar systems (stars and their planets), in P1: The Cosmic Smog model for solar system formation [B4], providing evidence that the true picture differed considerably from earlier ideas. Back in the 1930s, one popular idea was that a passing star had drawn out material from the Sun, and this had condensed in blobs to form the planets. A later idea was that stars and their planets were formed from a rotating "Protoplanetary Disc" which was created spontaneously from matter in interstellar space.
The Cosmic Smog model article showed that all these ideas had significant defects. In the new-analysis picture, objects of every size are formed individually, by gravitational aggregation of matter, throughout interstellar space. There is some collision, fracture, and joining of these individual bodies, but generally every star, planet, and moon may have its own genesis.
This model has received major support from modern studies of exoplanets and star groupings which have been derived from increasingly improved astronomical instruments and techniques. None of the old ideas were capable of explaining new discoveries such as "Hot Jupiters" -- stars with huge planets orbiting around them very close in, sometimes in a few days. Also the discovery of many double and triple stars, some with planets, in the widest variety of arrangements.
Instead, it appears that the whole of space is occupied by objects which were formed individually, and rather randomly, in areas rich in matter material in the past. So stars are formed the same way as planets, the stars are just those aggregations which have happened to become massive enough to "achieve ignition", that is, to use hydrogen fusion to generate energy, which is then emitted in the form of light.
What we see today then, in any area of space, is what natural forces have produced in acting on a chaotic mix of bodies, formed in particular gas-rich areas of space at a particular time. All the bodies in our particular Solar System (including meteorites) appear to have aggregated about 4.7 billion years ago. Interestingly, there is some evidence that the Sun is slightly younger than some other Solar System bodies.
Much of the history of how our Solar System evolved is traced in P1: The Cosmic Smog model for solar system formation [B4]. This shows how an initially random assembly of what are now planets and moons was very gradually normalized so that they now orbit close to the Sun's equatorial plane, through a gravity mechanism called Equatorial Forcing, and how, after about a billion years, this compression of orbits into a single plane caused what is called the Late Heavy Bombardment event.
Fig. OCB-F4. The Solar System and the Oort Cloud. From [B4].
The effect of these forces has been to create a local bubble (which we call the Solar System) in the Oort Soup, with most of the mass ending up as planets and moons orbiting the Sun in its equatorial plane. Above and below this plane are hemispheres of fairly good vacuum, as most matter has been sucked down into the equatorial plane.
The gravity mechanism of Equatorial Forcing mentioned above is part of the relatively undeveloped field of Spin Gravity. Isaac Newton established the laws of what may be called Mass Gravity over 200 years ago, and the Newton laws are still the solid foundation of how we understand the interaction of bodies in space. But these take no account of the rotation of bodies, and an understanding of this Spin Gravity is needed to explain the finer points of the interaction of celestial bodies -- more on this later.
As the present work is devoted to the Oort Cloud, the history of the Solar System is mostly of interest as a model of what has apparently occurred in all areas of Oort Space. The important point is that most bodies in space have aggregated as a complete spectrum of sizes, from gas and dust particles up through asteroids, moons, and planets, and on through small, average, and large fusion stars.
The mystery of Dark Matter
Although apparently hypothesized earlier by Jan Oort, the term "Dark Matter" was brought into prominence in the 1930s by the brilliant Swiss-American astrophysicist Fritz Zwicky. Zwicky was a giant of his time, introducing major concepts such as Supernovas, Neutron Stars, and Gravitational Drag as the origin of cosmic red-shift, as well his work on Dark Matter. He also did major work for the US Government space program.
Fig. OCB-F5. Fritz Zwicky in 1970. From [B6].
Born in Varna, Bulgaria, to Swiss parents operating a business there, Zwicky studied in Zurich, Switzerland. But most of his professional life was spent in California, where he was able to use the huge new 200-inch Hale telescope, and also the 18-inch Schmidt telescope on Palomar Mountain, which he helped build [B8]. Zwicky was a practical working astronomer, and besides his hugely innovative ideas, he also discovered many new supernovas and asteroids, compiled stellar population charts, and analyzed the behaviour of galaxies and galactic clusters.
In analyzing the rotational behaviour of galaxies and galactic clusters, Zwicky showed that there was not enough mass evident in the stars contained in the galaxies to account for this behaviour. To comply with the laws of gravity, these galaxies would have to contain about nine times as much mass as the stars they contained, and he called this invisible material "Dark Matter".
This was in the 1930s, and since then, right up to the present day, scientists have argued as to the true nature of Dark Matter, postulating the existence of exotic particles which were subject to gravity but could not be identified by visible light. Equipment designed to detect the "new, strange" particles has been operated by many experimenters over decades, always without success. Here is what one report said [B7].
"Dark matter is not only the most abundant form of matter in the Universe, it's also the most mysterious. Whereas all the other particles we know of -- atoms, neutrinos, photons, antimatter and all the other particles in the Standard Model -- interact through at least one of the known quantum forces, dark matter appears to interact through gravity alone.
According to many, it would be better to have called it invisible matter, rather than dark matter. It not only doesn't emit or absorb light, but it doesn't interact with any of the known, directly detectable particles through the electromagnetic, strong, or weak nuclear forces. The most sought after dark matter candidate is the WIMP: the Weakly Interacting Massive Particle. The big hope was for a WIMP miracle, a great prediction of supersymmetry.
It's 2019, and that hope is now dashed. Direct detection experiments have thoroughly ruled out the WIMPs we were hoping for."
The Dark Matter mystery is now solved. Refer back to the The Universe Mass Wheel in Figure F3, and note the section within green dotted lines labelled "Green Zone". That is the section of the Universe's total mass represented by stars, and is about one-tenth of the total Wheel. So, if the assumption stated above is at all accurate, this means that stars represent only about one-tenth of the Universe's mass.
Dark Matter, then, is just all the all the smaller bodies in the Oort Soup -- bodies which never aggregated to sufficient mass to undergo ignition, become stars, and shine with their own light. Dark Matter is merely ordinary matter which happens to lie too far from a star to be detected by light reflected from that star, and is too cold to shine (within the visible spectrum) by its own light.
We'll go on, in Segment C, to show that this "Dark Matter" does, in fact, emit a great deal of electromagnetic radiation, in a manner which can enable us to find out a lot about everything which the Oort Cloud holds. Of course, this first exposure of the true nature of Dark Matter is only preliminary, much investigation is still needed. For example, Dark Matter would appear to include many of the stellar-mass black holes which we know exist, in our galaxy and others, but are still a long way from being properly numbered.
Appropriate comment on this segment comes from the strip cartoon Dilbert. When the subject of Dark Matter comes up, Dilbert's pointy-haired boss says it is just matter viewed "when the lights are off". Spot on!
Distribution of Oort Cloud objects
If, then, the Oort Cloud contains a very large number of solid objects (which we have now identified as the long-sought "Dark Matter"), can we work out anything about their distribution and masses? We can certainly speculate about possibilities and probabilities here, but these ideas are at the very beginning of their existence, and must certainly be modified with further data and analysis.
First, let's consider the total mass of objects in our own Oort Cloud, that which we have pictured as a sphere extending out 100,000 AU (a bit under 2 light-years) from the Sun. If, as indicated above, Oort Space contains nine-tenths of the mass of the Universe, then our local Oort sphere should contain, if typical of the whole, objects massing nine times that of the Sun (actually, the Solar System, but the Sun has almost all the mass of that).
We know that none of these objects has a mass as much as 7.5% of that of the Sun, or else they would have "ignited" and be shining by their own light, and so be visible to ordinary telescopes. Suppose we look at the next category down, Oort Objects massing around 5% of the Sun.
If our Oort Cloud contained only such objects, each with one-twentieth the Sun's mass, then there should be 20 times 9, or 180 of them. But assuming that Oort Objects have a mass distribution like those we have looked at, we can assume only half the Cloud's bodies will fall in this class, that is 90 objects which we would class as Brown Dwarfs or Super-Jupiters.
Half of the remaining mass may be expected to lie in the next category down, say with a mass similar to that of Jupiter, and because this mass is distributed among a larger number of bodies, there could be 200 Jupiters in our Oort Cloud. Repeating this reasoning for smaller and smaller bodies, we would get greater and greater numbers of these with decreasing mass.
At least, this would be the initial position at the time all the Oort Bodies were aggregating -- and it seems a reasonable assumption this aggregation would happen at the same time for all, as the density of interstellar material built up to a critical value, or an external force triggered the birth process. Since that time, perhaps 4.7 billion years ago, the actions of Mass Gravity and Spin Gravity may be expected to have altered the trajectories of these objects to form sub-solar systems and planet-moon aggregations -- the Oort Worlds.
We'll look further into this matter in < a href="http://www.aoi.com.au/OC/OC406/">OC405: "Chasing Planet Nine" '';;;;;;;;
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References and Links
[B1]. Hertzsprung–Russell diagram. https://en.wikipedia.org/wiki/Hertzsprung–Russell_diagram .
[B2]. NASA'S Hubble Weighs in on the Heaviest Stars in the Galaxy. http://www.spaceref.com/news/viewpr.html?pid=16334 .
[B3]. How Often Do Earthquakes Occur?. http://www.mgs.md.gov/seismic/education/no3.html .
[B4]. David Noel. P1: The Cosmic Smog model for solar system formation, and the nature of 'Dark Matter'. http://aoi.com.au/bcw1/Cosmic/index.htm .
[B5]. David Noel. BS804: The Q-Scale method of representing masses. http://www.aoi.com.au/BaseScience/BS804/index.htm .
[B6]. Fritz Zwicky at the International Astronomical Union meeting in Brighton, England, in 1970. https://www.researchgate.net/publication/310428923 .
[B7]. Ethan Siegel. The 'WIMP Miracle' Hope For Dark Matter Is Dead. https://www.forbes.com/sites/startswithabang/2019/02/22/the-wimp-miracle-is-dead-as-dark-matter-experiments-come-up-empty-again/#256b72096dbc .
[B8]. John Johnson. Zwicky: the Outcast Genius who Unmasked the Universe. Harvard University Press, 2019. ISBN 9780674979673.
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Version 1.0 published November 2019 as Segment B of the book "The Oort Cloud: Almost all the Universe". AOI Press, ISBN 9798614884314.
Version 2.0 placed on web at "AOI.com.au", 2022 Jun 22.