Astrophysical Directions by Michael Erlewine

 

 

In Astrophysical Directions
Introduction Coordinate Systems The Solar System The Solar Neighborhood The Galaxy Galactic Objects The Fixed Stars Star Clusters & Nebulae Non-Visual Astronomy External Galaxies Finder-Lists, etc.
Galaxy

 

  The Galaxy

  Spiral Arms

  Differential Rotation

  Interstellar Dust

  Dark Nebulae

  Solar Motion (Apex of the Sun's Way)

  Star Streaming

  Galactic Center (Nucleus)

 

The Galaxy

Our Sun and the Local System of stars are part of a much larger disc-shaped collection of many billions of stars, gas, and dust that are bound together by mutual gravitational attraction to a vast flattened system that turns like a great pinwheel in space. A simplified sketch of our galaxy is presented above. There is a dense bright central region or nucleus and spiral arms extend from the nucleus outward into space to form a flat disk. These arms become increasingly thin until they are imperceptible. Our Earth and the solar system is embedded within the great disk that is our galaxy, and from our vantage point within this disk (and toward the edge of the great wheel), the plane of the galaxy appears as a great glowing arch in the night sky -- the Milky Way. All stars that we can see with the naked eye and almost all stellar material that is visible to our telescopes is concentrated within this galactic disk or plane, as it is called. The immense mass of stars and light has been known to man since time began through many names: River of Heaven, River of Light, Silver Street, Winter Street, Shining Wheel and The Ashen Path.

 

Our Local Galaxy

Galactic Coordinates

 

Figures A, B, and C below illustrate the general features of the Galaxy. Our Sun is located very much toward the edge or rim of the galactic disk, rather than toward the center. Keep in mind that we are embedded deep within the plane of the galaxy and that there are countless stars above and below us as well as toward the center and rim of the galactic disk. However, by far the greatest concentration of light and stellar matter appears to us in the direction of the Galactic Center and Anti-Center, as we look through or along the actual plane of the galaxy. If we look (from the Earth) in the direction of the North or South Pole of the Galaxy, we are not peering through the countless stars concentrated in the disk, but rather through a relatively thin sheet of stars between us and the intergalactic void beyond.

 

Galactic Equator or Rim

 

Because we are situated so far out on the galactic disc, there is also a great difference in what we see when we look toward the Center as opposed to the Anti-Center of our galaxy. There is much less material between us and the rim or edge of the galactic plane than there is toward the galactic center. When we gaze toward the galactic center (GC), we receive the combined light from all the stars between us and that center as well as the light stemming from the stars in the galactic disc beyond the center. In fact, as we look into the GC, we receive light (at once) that has been travelling to reach us for very different periods of time.

 

Galactic Rotation (clockwise)

 

Keep in mind that although it takes some 9 minutes for light to reach us from the Sun, it takes a period of around four years for us to receive light from even the nearest of stars. When we consider what we see as we gaze toward the GC, it becomes difficult to comprehend. We are looking at starlight that may have been travelling to reach us for 50-70,000 years! In other words we are looking at stars as they were a long, long time ago. We are looking into the past at the universe then. Who knows if these stars even exist now and, if so, what kind of light they give off today. We will not know for another 50,000 years of so. The stars in the anti-center direction are not so distant from us and we have a more up-to-date idea of how the galaxy is when we look in this direction.

 

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General Features of Galactic Spiral Arm Structure

Many distant galaxies appear to us through telescopes to have a lovely vortical or spiral structure. For over a century, astronomers have assumed that our Milky Way is also a spiral galaxy, although this has been difficult to prove. We are embedded within our galactic plane and surrounded in all directions by an apparently chaotic distribution of stars, clusters, nebulae, and dust clouds. It has been only since the 1950's that we have understood what the spiral arm structure of our galaxy looks like.

 

General Features of Galactic Spiral Arm Structure

 

At this point in time astronomers have distinguished three major spiral arms this side of the galactic center. The Sun seems to be about 1000 light years from the central part of a spiral arm that includes the Orion Nebula, the Coal Sack, and the North American nebula -- the Carina-Cygnus arm. An outer spiral arm (including the double star cluster in Perseus) passes about 6000 light years beyond us (the Perseus arm) and an inner arm (Sagittarius arm) has been discovered between the Galactic Center and us. Much of this research has been made possible through radio astronomy and in particular studies of the 21-cm line of emission in interstellar hydrogen. A rough idea as to the probable spiral arm structure as astronomers see it today is given in Figure A. Keep in mind that this structure is rotating toward the right (clockwise), so that the tilt of the arms to the radius vector from the center indicates that the spiral arms are trailing in the rotation as in a vortex. The spiral arms that exist today were probably formed not too long ago and in cosmic time may be rather short-lived.

The spiral arms are gaseous envelopes filled with stars and dust held together by gravitational and magnetic forces. These arms will cohere until the gas and dust has condensed into individual stars and these stars are dispersed throughout the galactic plane. In fact the whole sequence, if we could speed up the time process, would appear as a spinning pinwheel shedding sparks or stars as it whirls. The new-formed stars stay embedded within the spiral arm where they were born until galactic rotation forces them to migrate and be scattered through space, dissolving the spiral arms. New arms are continually being formed

 

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Differential Rotation

Our entire galaxy rotates about its center. The spiral arms revolve in a clockwise direction as seen from the north galactic pole and the velocity of revolution of stars about the center of the galaxy will decrease with increasing distance from the center. This is also true for the planetary orbits in the solar system; the far-out planets take longer to circle the Sun than do the inner planets. This is called differential rotation.

 

Differential Rotation

Differential Rotation

 

Figure B. will help to illustrate differential rotation. Stars or gas clouds that were lined up at one time (points 1) are spread out by the time they have gone 1/4 of the way (points 2) or about 3/4 of the way (points 3) around the galaxy. The small diagrams show the net velocity that stars or gas clouds at different distances from the GC would have with respect to our Sun. Objects within the Sun's orbit around the GC are orbiting faster than does the Sun and objects farther out than the Sun's orbit are orbiting more slowly than does the Sun. Our Sun complete one revolution about the GC in some 200 million years. The Sun was last on this side of the milky way center (with respect to the universe of external galaxies) at about the time that small dinosaurs were beginning to develop on the Earth's surface. It has moved through an angle of about 120° of its orbit since the last great dinosaurs vanished. Altogether, our Earth and Sun have completed only about 20 to 25 revolutions around the GC since their formation some 5 billion years ago.

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Interstellar Dust

Interstellar space is not empty, but filled with fine particles of dust (grains, smoke) and gas often mixed in clouds. These small grains of matter -- clouds of smoke or dust -- have absorbing efficiency and like cigarette smoke, diffuse or scatter starlight. Few single clouds absorb more than 3 magnitudes (see Stellar Magnitudes), but the accumulation in depth of many individual clouds in the vast cloud complexes of the "Great Rift" in the Milky Way can produce, in places, almost total obscuration. The Coal Sack dark nebula in the south Milky Way results from a dark cloud some 40 light years across absorbing somewhat more than one magnitude. It is located at a distance of some 500 light years from our Sun. Almost all of the gas and dust is concentrated in the equatorial plane of the galaxy and our observing situation in the Milky Way (in optical wavelengths) is similar to that of an edge-on external galaxy.

 

Distribution of Neutral Hydrogen in our Galaxy

 

The galactic center is totally obscured in the visual part of the spectrum by the dense dust clouds. Except for a few 'windows' (see below), this dust prevents us from seeing more than a few thousand parsecs in any direction in the galactic plane. The dust tends to clump in clouds associated with the spiral arms (see section on Spiral Arms). The most famous window through which we can look to greater distances than average is in the direction of the globular cluster NGC 6522. The most transparent or homogeneous window in the galactic equator is toward Puppis, between galactic longitudes 240°-250° (245° optimum view).

 

Visual windows in the plane of our Galaxy

Dust particles are not the only form of obscuring matter in interstellar space. Many kinds of gas pervade the space between the stars. The most abundant gas, Hydrogen, was discovered in the 1950s, when its emission at a radio wavelength of 21 centimeters was detected. Using radio astronomy, the first detailed map of the spiral-arm structure of our galaxy was produced. (see figure A) The gas is concentrated in the spiral-arms.

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Dark Nebulae

If there are no stars in or near the nebulosity, the nebula will obscure or block all light beyond or behind itself. The result are dark patches or 'holes' in the sky. The most celebrated dark nebua is the Coal Sack in the Southern Cross.

Dark Clouds and Interstellar Dust
Until the 20th century, astronomers assumed that the immense distance between stars was empty, in effect a perfect vacuum. Numerous dark patches were thought to be some sort of 'holes in space" where there were no stars. A few of these dark areas are visible to the naked eye, in particular the "Coal Sack" near the Southern Cross and the "Great Rift" in the Milky Way. The Great Rift splits the luminous background from Cygnus to Sagittarius through a succession of large overlapping dark clouds in the equatorial plane of the galaxy. It has been discovered that these "holes in the stars" are in fact obscuring clouds of small grains of matter, dust. Like cigarette smoke, this dust diffuses the light coming from behind them. There is no essential difference between a bright nebula and a dark one; it all depends on whether there are any suitable stars to provide illumination.

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Solar Motion (Apex of the Sun's Way)

The great disk of the galactic plane spins in space through time and carries with it all objects, including our Sun. Differential rotation causes objects that are located at different distances from the center of the galaxy to rotate at different speeds. In general, different groups of objects of a similar kind tend to move together through space. For instance, the Local System of stars that includes our Sun is moving in the general direction of the star Vega in the constellation Lyra. This apparent direction is termed the Solar Apex or Apex of the Sun's Way. The position given in astronomy books for the solar apex depends upon what group of stars we use to measure our Sun's motion. This can lead to a lot of confusion as to, which of several values is significant for our use.

 

Solar Motion

 

Solar motion is often explained as the deviation of the Sun's motion from a circular motion around the GC. This definition may help to clarify some of the confusion surrounding the use of the solar apex in astrological work. The standard solar motion (listed below) is the sun's drift with respect to the stars, which form the majority in the general catalogue of radial velocities and proper motions (A to G main-sequence stars, giants and super giants) and not the right-angle motion of the Sun and other galactic objects around the galactic center. The solar apex value depends upon what group of background stars we use to measure our Sun's motion and the more distant the objects (such as globular clusters), the more this apex approaches a simple right-angle to the GC. In fact, if we remove the effect of solar motion, the Sun and nearby stars are found to be moving at right angles to the GC.

Astronomers do this to arrive at a value called the local standard of rest. The local standard of rest is arrived at by removing what is termed the basic solar motion, and this motion is defined as the most frequently occurring velocities in the solar neighborhood, the "average" of local stars as measured from their geometric centers, rather than their centers of mass. Centers of mass for individual stars are not known. Therefore, the apex of the Sun's way (by definition) cannot be derived from the more distant stars, but should be determined using relatively near stars since it is a measure of the Sun's drift with respect to the centroid of motion of the local group of stars. It is similar to the slow drift to the side that often occurs to powerboats as they plow through the water. Astrologers will be interested both in the solar apex and in the right-angled motion of our Sun about the GC.

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Star Streaming

In the early 1900's, it was discovered, from proper motion studies of the, brighter stars, that the stars, in general, are moving in two preferred directions toward the apparent vertices. These points are situated in Lepus, at R.A 90° and declination, -15°, and in Pavo at R.A. 285°, declination -64°.

About 60% of the stars belong to Stream I, moving toward the Lepus vertex, and 40% belong to Stream II, moving toward the Pavo vertex at a velocoity about half that of Stream I. Not all stars share in the streaming, however; type A stars are very prone to do so, and type F and later classes in the spectral sequence show the same tendency, though less strongly. Most type stars are not members of either stream, but seem to be practically stationary. They are moving with the Sun. (see Local System).

If the apparent streaming is corrected for solar motion, the streams are found to be moving toward diametrically opposite points in the plane of the galaxy -- the true vertices at R.A. 95°, Declination +12° in Orion , and R.A. 275°, declination -12° in Scutum. Star streaming has been explained as the result of small deviations from circular orbits.

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Galactic Center (Nucleus)

Astrologers have become more aware of the existence and position of the Galactic Center (GC or nucleus in recent years thanks to the work of men like Theodor Landscheidt. Landscheidt points out in his seminal book Cosmic Cybernetics that there is an increasing tendency among astrophysicists toward consideration of our entire galaxy as a whole or living organism, capable of self-communication. The galactic nucleus may communicate "information" through electromagnetic gravitational radiation and other yet-undiscovered means.

It is estimated that the core of our galaxy has a million stars per cubic parsec, million times greater star density than in the solar neighborhood. If we lived on a planet circling a star near the galactic nucleus, we could see a million stars as bright as Sirius, and the sky each night would be as bright as 200 full Moons! However, our planet would be ripped out of its orbit every few hundred million years by close encounters with passing stars.

Vast clouds of obscuring dust prevent us from having a very spectacular view each night of the GC -- blaze with light. Most of our information concerning the GC has been obtained through the non-optical regions of the electromagnetic spectrum, such as the radio, infrared, and x-ray "windows" (which see). While light cannot penetrate the dust clouds, the radio and infrared waves, in effect, flow around the dust particles and on to reach us. The very energetic gamma and x-rays pass right through the intervening dust particles!

There appears to be an energetic flow of matter out from the core, and astronomers have located a ring of expanding particles at about 300 parsecs from the GC that is moving at 100 km/s. Two expanding arms of hydrogen (on either side of GC) at about 3000 parsecs have been found, one moving toward us at a velocity of 50km/s and the other away from us at about 135 km/s. There is some speculation that the nucleus of our galaxy may periodically explode (see Seyfert Galaxies) or that mass may spontaneously appear in the nucleus through some process that is beyond our comprehension at this point in time.

 

Radio Map of Galactic Center Region

 

Most radiation from the GC originated in an extended region about 20 across (Figure A). There are several "hot spot " or discrete emission sources located in the nuclear region. The GC appears somewhat different when viewed through the radio spectrum than it does through the x-ray or infrared windows. There is a powerful discrete x-ray source and at least three discrete infrared sources that each radiate a little less than a miIlion times more in the infrared than our Sun does at all wavelengths.

The GC has four ways of emission at radio frequencies:

  • Emission over a broad continuum of wavelengths by energetic electrons held in orbit by a magnetic field (synchroton radiation)
  • 21-cm hydrogen emission (hydrogen atoms whose electrons flip over from a higher to a lower energy state)
  • Similar molecular lines of emission.
  • Both line and continuum emission from H II regions.

 

Core of Sagittarius A (Radio Map)

 

The most powerful source of synchroton radiation and the traditional value given for the position of the GC is Sagittarius A which is a source about 12 parsecs in diameter of continuum emission generated by highly energetic electrons spiraling in a magnetic field. Embedded within Sag A and very near the actual center of-the galaxy are several small, bright knots of thermal radio emission about a parsec or less in size (see above). The general region of the galactic nucleus is located at about the 26th degree of Sagittarius in zodiac longitude and -5 degrees of odiac latitude. Every astrologer should be aware of this position.

 

 

© Copyright © 1997 Michael Erlewine

 

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