16 03 2009

On a typical night, looking into the sky, you can see somewhere between 500 and 6000 stars, depending on how dark your night sky is. If you live in a reasonably dark area, you may be lucky enough to see a faint, diffuse band stretching across the sky, from horizon to horizon. This band, known as the Milky Way, is the plane of our home galaxy.

It was Galileo who discovered that this band consisted of an almost uncountable number of stars, after he turned his telescope to the sky. Prior to this, it was believed that the Milky Way was a ‘cloud’ that spanned the entire sky.

360-degree photographic panorama of the Milky Way. (Digital Sky LLC)

360-degree photographic panorama of the Milky Way. (Digital Sky LLC)

The centre of the galaxy is in the direction of Sagittarius, and the Milky Way then passes (going westward) through Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion & Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus & Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius.

It wasn’t until the 1920’s that certain ‘spiral nebulae’ were discovered to be too far away to be part of the Milky Way; up to that point, it was pretty much believed that the Milky Way was the entire observable universe.

Island Universes
In 1750, however, Thomas Wright, in his “An original theory or new hypothesis of the universe”, speculated (correctly) that Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. In 1755 Immanuel Kant introduced the term “island universe” for these distant nebulae.

In 1920 the so-called Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.

The matter was conclusively settled in the early 1920s. In 1922, astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object. Using the new 100 inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.

Galaxies come in all shapes and sizes, from the small (containing a mere 10 million or so stars) to giants, with an estimated 10 trillion stars. The basic shapes they come in are elliptical, spiral and barred spiral. Hubble sorted galaxies into a scheme we now call the Hubble Classification Scheme as demonstrated below.

Types of galaxies according to the Hubble classification scheme.

Types of galaxies according to the Hubble classification scheme.

Our Milky Way galaxy is a type SBbc barred spiral and, although we have never actually seen it from ‘outside’, we can infer its size and shape from the spread, density and layout of the stars within it. Our solar system lives in the Orion Arm, out in the ‘suburbs’ of the galaxy. The galactic disc, which bulges outward at the galactic centre, has a diameter of between 70,000 and 100,000 light-years.

The distance from the Sun to the galactic centre is now estimated at 26,000 ± 1400 light-years.

Messier Object 109 (2MASS),

Messier Object 109 (2MASS),

The galactic centre is home to a compact object of very large mass as determined by the motion of material around the centre. The intense radio source named Sagittarius A*, thought to mark the centre of the Milky Way, is newly confirmed to be a supermassive black hole.

The Galaxy’s bar is thought to be about 27,000 light-years long, running through its centre at a 44 ± 10 degree angle to the line between the Sun and the centre of the Galaxy. It is composed primarily of red stars, believed to be ancient red dwarfves and red giants. The bar is surrounded by a ring called the “5-kpc ring” that contains a large fraction of the molecular hydrogen present in the Galaxy, as well as most of the Milky Way’s star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy.
Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. There are believed to be four major spiral arms which all start near the Galaxy’s centre. These are named as follows: 3-kpc and Perseus Arm, Norma and Cygnus Arm, Scutum-Crux Arm, Carina and Sagittarius Arm and the Orion Arm (which contains our own Solar System and Sun)

Outside of the major spiral arms is the Outer Ring or Monoceros Ring, a ring of stars around the Milky Way proposed by astronomers Brian Yanny and Heidi Jo Newberg, which consists of gas and stars torn from other galaxies billions of years ago.

Peculiar Types
Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. An example of this is the ring galaxy, which possesses a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy. Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.

A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars. (Barred lenticular galaxies receive Hubble classification SB0.)

In addition to the classifications mentioned above, there are a number of galaxies that can not be readily classified into an elliptical or spiral morphology. These are categorized as irregular galaxies. An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme. Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted. Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.

NGC 5866 (Hubble)

NGC 5866 (Hubble)

Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. These tiny galaxies are about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.

Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.

The average separation between galaxies within a cluster is a little over an order of magnitude larger than their diameter. Hence interactions between these galaxies are relatively frequent, and play an important role in their evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.
Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars within these interacting galaxies will typically pass straight through without colliding. However, the gas and dust within the two forms will interact. This can trigger bursts of star formation as the interstellar medium becomes disrupted and compressed. A collision can severely distort the shape of one or both galaxies, forming bars, rings or tail-like structures.

Hoag's Object (NASA)

Hoag's Object (NASA)

At the extreme of interactions are galactic mergers. In this case the relative momentum of the two galaxies is insufficient to allow the galaxies to pass through each other. Instead, they gradually merge together to form a single, larger galaxy. Mergers can result in significant changes to morphology, as compared to the original galaxies. In the case where one of the galaxies is much more massive, however, the result is known as cannibalism. In this case the larger galaxy will remain relatively undisturbed by the merger, while the smaller galaxy is torn apart. The Milky Way galaxy is currently in the process of cannibalizing the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy.

Active Galaxies
A portion of the galaxies we can observe are classified as active. That is, a significant portion of the total energy output from the galaxy is emitted by a source other than the stars, dust and interstellar medium.

The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole at the core region. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc. In about 10% of these objects, a diametrically opposed pair of energetic jets ejects particles from the core at velocities close to the speed of light. The mechanism for producing these jets is still not well understood.

A Jet from M87 (Hubble)

A Jet from M87 (Hubble)

Active galaxies that emit high-energy radiation in the form of x-rays are classified as Seyfert galaxies or quasars, depending on the luminosity. Blazars are believed to be an active galaxy with a relativistic jet that is pointed in the direction of the Earth. A radio galaxy emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the viewing angle of the observer.

Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weakly ionized elements. Approximately one-third of nearby galaxies are classified as containing LINER nuclei.




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