Section 1 Space Phenomenon

1.9 The Structure of The Milky Way.

Almost all events in Star trek take place within the Galaxy. Notable exceptions to this include "Where no Man has Gone Before" where the Enterprise was hijacked and attempted to leave the Galaxy and "Where No One Has Gone Before" where the Traveller first takes the Enterprise to M33 and then beyond our current understanding of space-time (see warp fields).

The Galaxy is divided into a number of regions, a central bulge, that thins into a disc which is divided into clumps of matter forming arms and regions of more tenuous dust and mass. Finally the disc is surrounded by a number of smaller halo galaxies. In 1750, the English theologian Thomas Wright correctly hypothesized, from the presence of the Milky Way across the sky, that the Galaxy must be a slab-like arrangement of stars. Other theoreticians, such as Immanuel Kant working at the same time were able to use Newton's laws of gravity to prove the theory. 

William Herschel, a German born man living in England used a self built telescope to more accurately map the Galaxy. He constructed a 1.2-m mirror (a size not surpassed until the 1840s).and mapped the skies under the assumption that the fainter the stars he could see in any direction, the farther away they were. He found far more faint stars in the Milky Way than in other directions.  He correctly mapped the Milky Way system as a disk of stars, with us inside it, but he was not able to determine the size of the disc. By 1918 Harvard astronomer Harlow Shapley showed that globular clusters are distributed in a spheroidal swarm extending above and below the disk. This swarm of clusters, which also includes some sparsely scattered individual stars and gas, is called the Galactic halo. Shapley also showed that the halo is centered not on the Sun but on a distant point in the disk in the direction of the constellation Sagittarius. He correctly hypothesized that this point is the center of the Galaxy. In 1951, a team from Harvard conducted observations based on the theories of H.C. van de Hulst to detect the interstellar 21 cm radiation line.  The 21 cm line is an emission from interstellar neutral hydrogen, the change of quantized spin of hydrogen causes an emission, for a given molecule this change may only occur once every 400 years, but the concentration in the ISM of hydrogen is so high that this means the 21cm line is the most pervasive and common emission line. The discovery of the line allowed probing of the disc that had until that time been obscured optically by high dust concentration. Not only does hydrogen emission allow probing of the Galaxy, the doppler shift of these lines tells us something about the speed of rotation of the disc. Unfortunately these measurements are only effective relative to the Sun. However given that the Sun is in its self an inertial frame of reference it was needed to constrain the movement of the Sun relative to the Galactic Centre. Distant Galaxies and globular clusters move in a generally random motion not related to the movement around the center of the Galaxy. Bertil Lindblad used the random motion to construct a speed for the Sun. He calculated that the Sun moves at 230 kms-1 about the Galactic center. 

In the Star Trek Universe, the Galaxy has been divided into four quadrants, Alpha, Beta, Gamma and Delta. (Okuda & Okuda 1997). These are superimposed across a plan view of the disc such that a and d are diagonally opposite. Within the disc one can determine various patterns, the most striking of which being the spiral arms. The relative brightness of the disc "arms" should not be confused with concentrations of stellar density. More so they are dust clouds caught up in a shock wave that propagates through the Galaxy and the comparative brightness of the young metalliforous stars. The density of the gases a ship in the Star trek Universe has to navigate through effects the actual velocity as a multiple of c the warp field creates. (Sternbach and Okuda 1991 pg. 55). The Galaxy is roughly 20 kiloparsecs in diameter but around 0.3 kiloparsecs in depth. This excludes the central bulge, a large oval mass 1 kiloparsec deep and 7 kiloparsecs across, about which the Galaxy rotates. The Star Trek Universe also divides the quadrants into sectors each of volume approximately around 6 parsec . Rather curiously these are labelled from Earth at sector 0 0 1 outwards though sometimes, especially pre Best of Both Worlds, in an inconsistent manner. (Piller 1990). The Galaxy contains around 100,000,000,000, one hundred billion, stars, each sector containing somewhere between 6 and 10 star systems if in the disc, and its age is comparable to that of the entire universe. The Galaxy formed along the cosmic wrinkles in space some 300,000 years after the Big Bang. (Kaufmann 1991). The Sun is located in the Orion Arm a short arm segment that includes the Orion Nebula (Kaufmann 1991) The Sun'a arm is bordered by the Sagittarius arm, this is towards the Galactic centre and the Perseus arm which is situated further out  from the central point. Work in 1990 by H. Zinnecker, M. Rosa and A. Monetti, using the New Technology Telescope at La Silla sate the distance to GZ-A Sagittarius to be 28,000 light years. (Moore 1995) If we know the Sun's speed and distance we should be able to determine some facts about the Galaxy. If we assume that the Sun travels in a circular orbit the distance travelled will be given by 2πr. Now given that we know the orbital speed we can say that:

T =  2πr /230 = 2π 28,000 (c*60*60*24*365.225636) /230

6.283185*[264905102984981760] / 230 = 1,664,447,850,872,132,610.2596 / 230

= 7,236,729,786,400,576.5664 seconds 

229,314,135.9725 (to 4 dp)

 

That means it takes some 230 million years to make one orbit of the Galaxy. From these basic facts we can use Kepler's Third law: The square of the orbital period is directly proportional t the cube of the separation of the two masses, to calculate the mass of the Galaxy. 

 

p2 4π2a3 / GM

We can simplify this to:

 

52370258001377334535441435166348.8 = (1.8584949051350829581563698227431*1061)* 39.4784176043574344753379639995046 / 6.668 *10 -11 M

3492048803531840666823.23489689208 = (1.8584949051350829581563698227431*1061)* 39.4784176043574344753379639995046 /  M

7.3370437980493459204616063347341061 / 3492048803531840666823.23489689208 

 

M = 2.1010713798239865884179062156259*1041

M = 2.1 *1041 Kg

M in solar masses = 2.1 *1041 Kg / 1.989 *1030 Kg

M in solar masses = 105,634,559,066.0627 (to 4 dp)

M in solar masses = 1.0563 *10+11 to four decimal places.

 

The internal part of the Galaxy contains some 105 billion times the mass of the Sun. There is of course a large amount of matter outside of the orbit of the Sun but this has no bearing on Kepler's law so is not shown in these calculations. It is by using radio analysis of the doppler shift that scientists have determined the rotation curve for the outer Galaxy.  Kepler's law shows that the orbital speeds beyond the centre should decrease, however the rotation curves are anomalous and show that the outer regions are being dragged seemingly by an invisible form of matter. This is part of the phenomenon named dark matter.

Figure 1 Sketch of the Milky Way Galaxy showing the Position of Sol in the Galactic Spiral Arms

 

As stated in the introduction some loose conglomerations of stars and satellite galaxies known as globular clusters surround the main disc of the Galaxy. The two most famous of these are the two Magellenic Clouds, which are found at a distance of around 1/10 that of Andromeda. These are both irregularly shaped satellites. Though globular clusters are a tremendous distance away, the Enterprise has visited them notably in Schisms (Matthias & Wilkerson 1993) where the crew explored the Amargosa Diaspora Globular Cluster, FGC13. If one assumes the Federation stretches 3 kiloparsecs in diameter across the disc then the vertical then even this would be insufficient to reach a true globular cluster and the act of crossing Federation space at maximum warp of 9.2, if it could be sustained, would take 6 years (Okuda & Okuda 1997). Galaxies are not distributed randomly but are grouped into clusters which are a consequence of the wrinkles from space time. These clusters are termed rich or poor depending on the concentration of galaxies within them. The Milky Way lies in the Local Group, which is a cluster containing the larger M31 Andromeda Galaxy and some twenty smaller galaxies. Most of these are dwarf ellipticals and the Local Group is considered to be a poor cluster. The Virgo Cluster is the nearest rich cluster, this is a conglomeration of over one thousand galaxies and is some 50 million light years away and has a radius of around 3.5 million light years. The center of Virgo is dominated by three giant ellipticals, these are each 2 million light years across and alone are around the size of the Local Group. Cluster shape can be described as either regular or irregular depending on the symmetry of the distribution of matter within them. Both Virgo and the Local Group are irregular due to their asymmetry. The Coma Cluster at 300 million light years distant is an example of a Rich - Regular galaxy. (Kaufmann 1991). 

 

Kaufmann 1991

 

Stars are assigned to two broad categories Population I and Population II depending on composition, speed and location. (see also stellar evolution) There is a tentative third population Population III outlined, but as this until recently contained a single star the classification is more theoretical than practical at present

1.9.1 Population III.

This is a hypothetical group of stars that formed previous or synchronous with the actual formation of the Galaxy. Their existence is theorised as an attempt to describe the metallicity present in population II stars where nucleosynthesis should not have been able to produce heavy elements. Population III would have almost all burned out by now and would have been bright and thus short lived. (Gribben 1996)

1.9.2 Extreme Population I.

Stars of this type are found mainly in the spiral arms of the Galaxy. Their orbits are generally circular about the central nucleus. Extreme population I stars are young with age ranges from 20*106 to 50*106 years. They have average metallicites of about 3% bearing in mind metal when referring to stars is used slightly differently to the traditional chemical sense and refers to elements heavier than helium. This convention is probably still in use due to initial spectroscopic analysis being dominated by iron and nickel. (Red Shift). Extreme population I are generally short lived. During this time they are very bright, of spectral class O, B1 and B2 as well as including T Tauri stars and certain supergiants they include high mass Cepheid variables those with periods in excess of 10 to 13 days.

(RoyandClarke(b)1988).

1.9.3 Intermediate Population I.

Stars of this stellar type move in circular orbits but are distributed throughout the disc not just in the arms. They are somewhat older than the extreme population I stars with ages in the range of 0.2*109 to 10*109 years. The metallicites form 1-2% of the overall chemical composition. Our Sun is one of these stars as are most of the visually observable stars in the disc

(RoyandClarke(b)1988).

1.9.4 Intermediate Population II.

These are old stars with ages between 2*109 to 10*1010 years. They have low metallicities typically 0.8%. Stars of this population are found in the inner parts of the halo and the nuclear bulge. The bulge does, however, also contain stars with high metallicities that are population I. Intermediate population II stars outside of the halo are not confined to circular orbits or to the disc instead moving in moderately elliptical orbits though still centred on the Galactic Nucleus.

(RoyandClarke(b)1988).

1.9.5 Extreme Population II.
These are the oldest stars with typical ages of 10*109 to 14*109 years. Their metallicity is very low usually less than 0.8% and they are found mainly in the halo, particularly in the ancient globular clusters. Many of these stars move around the Galactic centre in highly elliptical orbits that are often steeply inclined to the disc. Typical members of this population are low mass stars that have left the main sequence and RR Lyrae variables somewhat more massive stars that have evolved onto the horizontal branch of the HR diagram. Problems exist in some globular clusters in that the ages reported are in actuallity older than the theoretical maximum age of Population III stars and encroach on the age of the Universe.

(RoyandClarke(b)1988).

The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is beginning to change. The recent theoretical results provided by an improved physical understanding of the stars are reported by Filippina Caputo, with a particular emphasis upon those stellar evolutionary phases of fundamental importance for distance and age determinations. The data appears to reveal globular cluster ages younger than 10 Gyr-13 Gyr, weakening the conflict with current cosmological estimates. These recent results derived from the fitting of HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up-to-date theoretical models. Though the data seems to suggest that the problem of the globular cluster distance and age is still open due to remaining discrepancies in the absolute brightnesses reported by HIPPARCOS. (Filippina Caputo, Online publication: December 14, 1998)

It must be noted that since the disc lies within the halo, there is a degree of intermingling of different stellar populations. Therefore, though our part of the disc is generally Population I we do find occasional Population II stars. However as these stars generally follow highly elliptical orbits they often have very different velocities and as such are termed high velocity stars. (Roy and Clarke (b) 1988).

It is believed that initial stellar compositions are essentially the same, and that the difference between the stellar composition is a product of the differing time of evolution of a star. However, nebulas from super nova when the star collapses, will throw off gasses that are related to its composition. These gases will then condense into other stars that will have elements that are products of the parent star. (this process and the theories behind the collapse are discussed in more detail in the accompanying section: stellar evolution). The structure of the Milky Way as seen in the diagrams above has differing components at different points in its form.

 

Component Dimensions Stellar Populations
Disc 30kpc diameter Intermediate Population I
  1 kpc thick Extreme Population I in spiral arms
Halo 40 kpc diameter Extreme Population II
Nuclear Bulge 6kpc diameter Intermediate Population II, Extreme Population II in inner parts
    High metallicity stars close to centre

 

(FromS281book3page18)

Optical views of the central bulge are limited and most observations are conducted at radio wavelengths. However there is a narrow tunnel known as Baade’s window that gives relatively unobscured views, however even this is not perfect and the view is 4o away from Galactic centre. A prominent feature of the disc is the conglomeration of stars into open clusters. Such clusters are localised regions where the number density of stars is greater relative to that of the immediately surrounding area. The central bulge is believed to harbour a 2.6 million solar mass singularity. Recent measurements carried out with optical and radio telescopes have resolved the mass that has long been seen to exert a huge gravitational pull at the heart of our galaxy in the constellation Sagittarius. Andreas Eckart of the Max Planck Institute in Garching, Germany presented a film at the AAS meeting showing the proper motions (recorded over five years) of several stars within a few light days of the heavy object. The measured velocities of these stars, some as great as 1000 km/sec, lead to a mass estimate for the object of 2.6 (with an uncertainty of only 0.3) million solar masses. Considering that all of this mass must fit into a volume much less than the distance between us and the nearest star, Eckart asserted that the object could only be a black hole. (The American Institute of Physics Bulletin of Physics News Number 354 January 12, 1998 by Phillip F. Schewe and Ben Stein) The Nuclear Bulge is pervaded by a thermal plasma at at least 7000 K which is responsible for the diffuse free-free emission and magnetic fields in the Nuclear Bulge are strong as compared with the Galactic Disk with the field lines are oriented parallel to the galactic plane inside giant molecular clouds and perpendicular to the plane in the intercloud medium. Recent observations of the central bulge have shown that the surrounding the central mass is a cloud glowing in gamma rays produced by annihilating antimatter particles. Positron/electron annihilation energy is emitted as gamma rays with photon energies of 511,000 electron volts. Searching for these high energy photons, the OSSE instrument onboard NASA's orbiting Compton Gamma Ray Observatory has recently produced this map of the Galactic Center region. The Galactic Central object was first optically resolved in October 1990 by H. Zinnecker, M. Rosa and A. Monetti, using the New Technology Telescope at La Silla. They constrained the distance to GZ-A Sagittarius to be 28,000 ly. (Moore 1995). The more recent work on the central object has as anticipated showed annihilation gamma rays as a bright spot at the Galactic Centre with fainter horizontal emission from the galactic plane. The data sets also reveal a large and unexpected cloud of annihilation radiation, probably about 4,000 light years across, extending nearly 3,500 light years above the Galactic centre. Though this cloud has never been observed related to any other stellar phenomenon it seems to imply that a stream of antimatter positrons is being focused and emitted from the Galactic centre. Present guesses about the source of the positrons include the violent and exotic environments surrounding starbirth, neutron star collisions, and of course the singularity at the Galactic central point.

How Active Galaxies Form

Although astronomers are now modelling the basic principles of the operation of the engines that drive active galaxies, many details still remain unclear. There is a vigorous debate about the nature of the processes that ignite a starburst or form a central black hole. Much discussion is focused on what mechanism is used to feed fuel/material into the almost point-like nucleus.  Most likely, gravitational interactions with gas-rich galaxies redistribute gas in the host galaxy, perhaps by forming a stellar bar such as the one in NGC 1068. Computer simulations appear to indicate that the bar, once formed, may be quite stable

Joshua Barnes, et al. 1991

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