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Monday, June 20, 2016


Thanks to the Hubble Telescope, we know definitively that galaxies come in many various shapes and sizes. Many of the objects we thought were planetary nebula, when seen from Earth based telescopes, have now been proven to be galaxies and interacting galaxies. In fact, it is almost impossible to view an image made with the Hubble space telescope without also seeing numerous galaxies in the background.
Most galaxies come in four basic types: spiral, barred-spiral, elliptical, and irregular. There are also globular clusters and lenticular types and the rarer ring type. Galaxies also come in three size classes: dwarf, galaxy, and giant. Virtually all galaxies fall into the Hubble Classification Scheme created by Edwin Hubble in first half of the 20th century. The original was dubbed the "tuning fork" diagram, but it has since been updated and revised. The below images show the original and updated versions.
Spiral and Barred-Spiral Galaxies
This is the common form of galaxy. It is also the shape we tend to think of when we think "galaxy." They have a round, spheroidal core surrounded by the classic "pinwheel" with at least two arms, but most often have more. The core can actually resemble an elliptical galaxy and contains the metal-poor stars found in elliptical galaxies. Thus, the core stars can have a distinct yellowish to whitish tint, where the arms will have a bluish-white tint due to the younger stars that can be found there due to the rich star-breeding grounds of large dust and gas clouds like the Orion Nebula. The arms can be loosely or tightly packed, intact or patchy, closely or loosely wound. Barred-spirals will tend to have arms that are more loosely wound than in a spiral galaxy. Although it may appear to be so, the space between galaxy arms is far from empty and can have healthy star-breeding grounds. In fact, our stellar system is situated on the inner side of the Orion Spur.
Ring Galaxies
Ring galaxies were perhaps spiral or barred-spiral before colliding with another galaxy. These are the rarest of all galaxies. So far, we have only found 43 ring galaxies out of the few billions of galaxies discovered. This works out to about 1 in 65,000,000 galaxies being a ring galaxy. This type of galaxy has a structure similar to the spiral and barred-spiral galaxy; however, instead of the classic "pinwheel" structure, the ring galaxy has an elliptical core surrounded by a ring of stars. Some traces of spiral structuring may be seen between the ring and the core. Although ring galaxies have a smaller habitability zone, they may still have stellar systems that can be habitable.
Irregular Galaxies
Irregular galaxies are just that, rough assemblages of stars with little or no regular structure. The Large and Small Magellanic Galaxies are irregular companions of our Milky Way Galaxy. In fact, they may be remnants of collisions or close encounters with other galaxies. Irregulars tend to be too small to generate star-breeding grounds, but on occasion, they can. The Tarantula Nebula in the Large Magellanic Galaxy is a star-breeding area. Because some irregular galaxies can have star-breeding areas, there is a chance for stellar systems possessing terrestrial planets.
Elliptical Galaxies
Also referred to as "dead" galaxies, this type of galaxy has very little gas and dust for star-breeding and are predominantly old, metallicity poor stars. Elliptical galaxies have the widest range of sizes ranging from a couple thousand light years to truly immense monsters such as M87 in the Virgo Cluster which is over 3000× the size of our Milky Way (>30,000,000 ly across). Monsters like M87 tend to sit in the centroid region of galactic superclusters. Some elliptical galaxies may possess a dusty disc which could indicate a near dying region of star-breeding. However, there will be very little new star-breeding.
Lenticular Galaxies
As with the elliptical galaxies, lenticulars are considered to be "dead" galaxies. They can have a spheroidal and/or barred core, but exhibit very little else in common with the spiral and barred-spiral, except for overall shape. These galaxies have used up all of their interstellar hydrogen and helium and, thus, they will have an orangish-yellow to orangish-red glow. Lenticulars also have very little in the way of star-breeding areas since they have very little gas and dust. Virtually all of the stars in a lenticular galaxy are very old, most being twice as old as our star or older.
Interacting Galaxies
This is at least two galaxies, or more, that are about to collide or have just collided. Interacting galaxies tend to be hot breeding grounds for new stars, revitalizing perhaps two dead galaxies. Some of these can produce spectacular layouts.
Globular Clusters
These are very similar to open clusters, except in size and age. Globular clusters tend to be fuzzy balls of stars with an orangish-yellow to orangish-red glow. Like a lenticular galaxy, the stars tend to be old stars, most being 2× or older than our sun. The stars tend to be more closely packed in the center than with open clusters. The core, which is usually one to two parsecs in size, can contain as many as one to three thousand stars. It has never been verified that any globular cluster has a black hole as most other galaxies have. Mostly there will only be Jovian planets and Pluto-like ice balls within a globular cluster.
Open Clusters
Although not true galaxies, some consider open clusters to be dwarf galaxies inside other galaxies. Open clusters are actually regions within galaxies where new star formation is currently occurring or has recently occurred. They tend to have only a few tens to a few hundred stars and are rarely larger than 15 to 20 parsecs (48.925 to 65.234 light years) and rarely have any definitive structure. Open clusters are dominated by the young Population I blue-white O and B class stars and are also associated with emission and reflection nebulae (q.v.). Open clusters make poor areas for habitable terrestrial planets due to the immense radiation pumped out by the O and B stars. However, asteroid, planetesimal, and planetoid mining might be profitable, albeit dangerous.
Globular Common Open
Group of tens of thousands to hundreds of thousands of stars Group of stars held together by mutual gravitational attraction Group of hundreds of star
Highly symmetrical ball of stars All of its stars are the same age, having formed from the same cloud of gas and dust Irregular shaped grouping of stars
Frequently contains bright red giant stars Stars in the cluster are basically at the same distance from Earth Contains bright blue stars
Located in the halo or bulge of a galaxy The star color indicates the age of the stars in the cluster Located in the arms of spiral and barred-spiral galaxies
Composed of old stars that formed when the universe was younger Orbits the center of a galaxy Composed of young stars that recently formed in the disks of galaxies
No longer forming in or near any galaxy Continues to form in the arms of spiral and barred-spiral galaxies

Image 1: Original Hubble Classification Scheme
Image 2: Revised Hubble Classification Scheme
Image 3: Milky Way Showing Our Sun's Approximate Position
Image 4: Example spiral galaxies.. M64 Hubble Images - The "BlackEye" Galaxy: a spiral choked with stellar dust.
Image 5: AM 0644-741, a ring galaxy in Volans
Image 6: NGC 3718 - "Bowtie" Galaxy: This one is the result of a collision/merge of at least two galaxies.
Image 7: M87: The largest know galaxy. Notice the jet from the black hole on the right side.
Image 8: Lenticular Galaxies - Centauri A
Image 9: Interacting Galaxies The "Rose" - Arp 273 is a group of interacting galaxies
Image 10: Open Clusters - NGC 290