Our Own Galaxy Galactic Structures and Types

Our knowledge of galaxies and their structures has grown by leaps and bounds through the twentieth century including knowledge of our own galaxy. Our knowledge of the Milky Way continues to grow and evolve in fits and starts because in visible wavelengths we just cannot see a great deal of it. The core of the galaxy is completely hidden from our view in visible light because of thick interstellar dust clouds. We know that our galaxy is spiral in shape with several arcing structures or arms that wrap around the core. The Milky Way is some 100,000 light years in diameter and contains a mass equivalent to one trillion Suns. The best current estimate of the total galactic population is around four hundred billion stars. The Milky Way is now known to be spiral in layout. This was discovered by the American astronomer Harlow Shapley, who made radio observations of hydrogen clouds in the galaxy and from that was able to postulate the spiral structure. Shapley also correctly pinned down the approximate location of the galactic center in Sagittarius, near the borders of Scorpius and Ophiuchus. Subsequent studies, crowned by astrometric observations using the Hipparcos spacecraft have confirmed that the solar system lies approximately 28,000 light years from the core and 22,000 light years from the edge and is almost perfectly centered within the galaxy, lying only 20 light years above the plane of the galaxy's disk, the galactic equator if you will. We know now that the Milky Way is structured like most spirals with two distinctly different components. First are the spiral arms, which wrap tightly around the galaxy surrounding a bulging core, from which the arms emanate. The nature and content of the two structures are very different. The spiral arms contain most of the galaxy's dust and gas and therefore most of the star-forming regions. The stars in the arms tend to be young and hot. We call these Population I stars. The core contains no or very little in the way of star-forming materials and most of the stars are very old and have evolved off the main sequence. These stars are called Population II type stars. The environment of the galactic core very much resembles an enormous globular cluster with one important additional object. This object is a supermassive black hole around which the entire galaxy revolves. The region of the black hole is also one of the most powerful known radio sources in the sky, a point that coincides with the galactic center called Sagittarius A. Other such so-called supermassive dark objects have been located in the core areas of M31, M32 and six other Messier galaxies. Some galactic nuclei display tremendous levels of activity and radiate enormous amounts of energy across the entire spectrum from x-rays to radio. Remember that many galaxies contain supermassive central objects at the center of their cores. The electromagnetic activity is caused by interstellar matter falling into the central object at extremely high speeds. These galaxies are called Seyfert-type galaxies after their discoverer, Karl Seyfert. The brightest Seyfert galaxy in our sky is M77. Galaxies with even more exotic nuclei become so compact and bright that they outshine their entire host galaxy pouring out astonishing amounts of energy. Because these nuclei appear almost stellar within their galaxies, they have been given the name "quasi-stellar objects" or quasars for short. Quasars however are extremely remote, the closest being more than two billion light years away. No quasars appear in the Messier catalogue, the New General Catalogue or the Index (IC) Catalogue.

The Milky Way has five distinct spiral arms that have been clearly identified. Our solar systems orbits within the Orion Arm of the galaxy. This arm is also designated as "0" or the Local Arm. Arms that are outward from our position from the galactic center are numbered with a positive number while those arms inward of our position are noted with a negative number. The Orion Arm is actually a spur arm, a bridge connecting the Sagittarius Arm (-I) towards the center with the Perseus Arm (+I), which is outside our position. The innermost arm is called the Scutum-Crux Arm (-II), while the outermost arm is called simply the Outer Arm (+II). The arms are named for the constellations in the sky where the light of the arms is most heavily concentrated. There may be as many as three more arms that have not yet been positively identified. There are many other such spiral galaxies in the universe but not all of these spirals are the same in construction. Later on we'll look more closely at spiral galaxy make-up.

One thing that should have become very evident in our outward journey through the universe by now is that the universe for all its chaos is a very highly organized and well-ordered place. Organization does not stop with galactic structure. The intergalactic realm is also very highly organized. The Milky Way is not floating alone in space but belongs to an association of approximately 46 other galaxies gravitationally bound together39. Andromeda is the largest of these galaxies however the Milky Way may actually be more massive. Each of the two major galaxies has accumulated a large number of satellite galaxies. The third major member

39 We are not certain that every single one of these satellite galaxies are actually bound to their assumed primaries so the use of the word "approximately" is appropriate until further study of proper motion of these galaxies can be verified.

of the Local Group is the Triangulum Spiral M33, though some believe it may be a large satellite of Andromeda. Of the Milky Way's satellites, the most prominent are the Large and Small Magellanic Clouds, though they are only visible from the southern hemisphere. Others include the recently discovered Canis Major Dwarf, a galaxy that is being disrupted and integrated by the Milky Way, dwarf galaxies in Ursa Minor, Carina, Sextans, Sculptor, Fornax, Draco, Sagittarius and two in Leo. The Andromeda Galaxy has two companions that appear on the Messier list (M32 and M110) both of which are conspicuous in amateur telescopes. Andromeda has as many as 10 additional satellites. M33 has at least one satellite and possibly three others, though their orbital nature is highly in doubt. Several other small galaxies travel as part of the local group though without being gravitationally bound to one of the three large galaxies. The Local Group also is involved in gravitational interaction and member exchange with other groups. The giant elliptical galaxy Maffei 1 has split off from the Local group and has since become a group on its own with its satellite galaxies. The Sculptor or South Polar Group is dominated the galaxy NGC 253 and contains a handful of smaller galaxies and there are two other groups dominated by the galaxies M81 and M83. Maffei 1 and M83 seem to be heading away from the local group while the Local Group itself seems to be moving in the general direction of the Sculptor group. As for future interactions, the Milky Way and Andromeda galaxies are moving slowly towards each other and will likely merge into a massive elliptical galaxy, while the entire Local Group will eventually merge into the giant Virgo supercluster.

The Virgo supercluster is the closest major association of galaxies to our Local Group. Messier noticed an unusual accumulation of nebulae in the northern wing of Virgo. By the time the list was completed, Messier had listed sixteen nebulae in

Figure 14.2. The central portion of the Virgo supercluster. The image is centered on M87. NASA image, equipment unspecified.

Figure 14.2. The central portion of the Virgo supercluster. The image is centered on M87. NASA image, equipment unspecified.

this one area of Virgo. The cluster is so massive that many member galaxies that are on the far side of the center of mass and are being accelerated towards us in their orbits show the highest blueshifts of any galaxies in the heavens. Conversely the galaxies that are moving away from us exhibit some of the largest known red-shifts in the universe. The galaxy IC3258 is approaching Earth at 517km/sec and the lines in its spectrum are shifted towards the blue end of the spectrum. Since the entire cluster is moving away from us at 1,100km/sec, IC3258 is traveling around the cluster's center at 1,600km/sec. The galaxy NGC 4388 is receding from Earth at over 2,535 km/sec, giving it an orbital velocity of about 1,400 km/sec. These superclusters represent the largest structures in the universe. The Virgo super-cluster is now known to have more than 2,000 members. Near the center of the cluster is the giant elliptical galaxy M87. This massive galaxy appears to be the most dominant gravitational influence in the cluster.

Galaxies come in many shapes and sizes. The most visually compelling systems are the spirals, though not all spirals are created alike. Just like with everything else in the universe there is a system by which spiral galaxies are categorized. Edwin Hubble devised the scale. Elliptical galaxies are denoted with "E" then assigned a number 0 through 7 depending on the degree of elongations with 0 being assigned to galaxies that are closest to spherical and 7 to the most "football" shaped. Irregulars are given the letter "I". With spirals, the initial designation is with an "S". Galaxies are then sub-characterized by lower case letters "a" through "d." A spiral galaxy that is graded "Sa" is one that has a large central nucleus as its most dominant feature. If the galaxy also sports conspicuous arms with a dominant core, it is called "Sb." A spiral galaxy that has an identifiable core and dominant arms is graded "Sc." If the galaxy has visible spiral arms and no visible core at all, then it is graded "Sd." A spiral may also have an intermediate grading consisting of two lower case letters in case it does not exactly fit one sub-class or another. In addition, many spirals exhibit a structure in their cores that extends the classic elliptical structure into a bar shape. In such cases, the class of the galaxy will be noted as "SB" followed by the lower case letter denoting arm-core dominance. In addition, some galaxies may show a spiral shaped disk but with no definable features. Such a galaxy is called a lenticular galaxy. The loss of spiral structure may be attributable to extreme age or a lack of gravitational interaction over a long period of time. Lenticular galaxies are graded as "S0" galaxies. The Shapely grade for the Milky Way is SBbc. This indicates a barred spiral with roughly equally dominant core and arms. The discovery that the Milky Way is a barred spiral was one of the more important discoveries in galactic astronomy in the last twenty years and in fact some astronomers now believe that the bar is more dominant in the core than had been originally believed. The bar is now believed to be as long as 27,000 light years in length or more than one-quarter of the galaxy's diameter. Let's now see how the Milky Way compares to other relatively easy to view spirals.

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