"It is now thought that most galaxies possess a central super-massive black hole!"
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Galaxies like M 31 (the Andromeda Galaxy) are like "island universes" containing millions to trillions of stars! (1.6.x). (Note: you can vary CELESTIA's galaxy brightness with the parentheses, ( and ) , for better viewing.)
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DEEP-SKY OBJECTS (DSO's)
STAR CLUSTERS, NEBULAE & GALAXIES
Rather than referring to ordinary star systems (singles, binaries, multiples) or to their subordinate components (planets, moons, asteroids and such), the designation "deep-sky objects" (DSO's) is typically used to refer to appreciably larger celestial groupings, formations or "collective" structures. They include the various types of star clusters, nebulae and galaxies.
Today deep-sky objects rank among the most exciting and rewarding subjects in all of astronomy. In the first place, they have proved absolutely essential to our modern understanding of the true scale of the cosmos. Moreover, not only are they fascinating simply from the standpoint of physics, they are also among the most beautiful objects in nature!
The SEDS site's excellent Interactive Messier Objects page gives you quick acces to information about the most famous of the deeps-sky objects. In addition, the same site's Interactive NGC Catalog Online offers one of the best portals to information on thousands more deep-sky objects! Wikipedia's New General Catalogue and IC Objects pages are also good sources of related infomation.
Star clusters are formations whose stars a.) essentially all developed approximately concurrently and b.) are all gravitationally bound so that they move through space together. The two primary types of star clusters (open clusters and globular clusters) have markedly different properties.
Often irregular in shape, open clusters (OC's) generally contain no more than a few hundred stars in a volume no larger than 2-3 dozen light years across. These are usually young stars with ages on the order of tens of millions of years. An overwhelming percentage of open clusters are found in or near the galactic plane, most in the galaxy's spiral arms, i.e. in the regions of greatest continuing star-formation.
Among the most prominent and regularly viewed open clusters are the Pleiades (M 45) (labeled) in Taurus, the Southern Pleiades (IC 2602) in Carina, the Beehive Cluster (M 44) in Cancer, and the Ptolemy Cluster (M 7) in Scorpius.
As their name suggests, globular clusters (GC's) are roughly spherical in shape. And, although most are no larger than many open clusters, globular clusters can contain hundreds of thousands to millions of stars! (Can you imagine looking out at a star as near as Vega and seeing over a million stars filling your view?) In globular clusters an overwhelming majority of stars are red and yellow dwarfs that are quite old, over 90% of the age of the Universe! Their only younger stars are the rare "blue stragglers", thought to continually form from matter ejected when some old stars inevitably collide. The 150 or so globulars clusters that are part of the Milky Way are scattered throughout the approximate sphere of the galactic halo, so most lie away from the galactic plane. Example globular clusters: M 15 , Omega Centauri Cluster , M 80 .
INTERESTING STAR CLUSTER LINKS
Nebulae (or nebulas) are immense interstellar clouds of gas and dust. Some represent the regions of ongoing star production in the universe, while others are stellar "leftovers", discarded material which may one day help to spawn new stars. Some are remarkably stable, like diffuse nebulae, which if left undisturbed can remain essentially quiescent for hundreds of millions of years! Others, like protoplanetary nebulae, evolve relatively rapidly and represent a distinct phase in the waning lives of certain stars.
Nebulae are categorized in a number of different ways: a.) according to the nature of the light they give off or block, b.) according to the processes that produced or are producing them, and c.) according to their general shape and structure. It should be noted that a single cloud may be so large that various portions of it may be considered different types of nebulae. The famous Orion Nebula for example is customarily described as an emission nebula, though parts can certainly be regarded as reflective and dark nebulae.
EMISSION, REFLECTIVE & DARK NEBULAE
Emission nebulae, as their name suggests, emit visible light, primarily from their ionized gases (especially hydrogen) and free electrons energized by ultraviolet radiation from nearby stars. These nebula are typically reddish, the color of hydrogen's dominant H II emission line. Named specifically for this are H II regions, which are of a type of emission nebulae where considerable star development is taking place and where massive, young stars are ionizing its gases. Example emission nebulae: Tarantula Nebula , Trifid Nebula , Lagoon Nebula .
In reflection nebulae, the radiation from close-by stars does not possess enough energy to ionize their gases. Still, sufficient radiation is scattered to render them visible. Due to their inherently reflective nature, these nebulae typically share the spectrum of the stars that illuminate them. Example reflection nebulae: Witch Head , NGC 1999 , N30B "Double Bubble" .
In some cases, portions of an interstellar cloud block essentially all of the visible light coming from "behind" them. These formations are called dark nebulae, and they can be found "in front of" emission and reflection nebulae, and even "in front of" simple fields of stars. Example dark nebulae: Horsehead Nebula , Cone Nebula , Coalsack .
Most nebulae are diffuse nebulae. These are generally spread out and irregularly shaped, often to the extent that distinct boundaries are difficult to define. Diffuse nebulae can be emission, reflection or dark nebulae, or any combination of thereof. For instance, the Orion Nebula referenced above as predominantly an emission nebula is likewise a diffuse nebula.
Protoplanetary nebulae are among the "worst-named" objects in all of astronomy! Not to be confused with protoplanetary "discs", accretion discs that form around developing young stars, protoplanetary nebulae form during a relatively brief phase late in the lives of medium-mass stars. After such stars ultimately swell to become red giants, but before they produce conventionally more-spherical "planetary" nebulae, their lower-energy radiation can shape their ejected envelopes, which essentially become reflection nebulae. Eventually an aging star's directional stellar winds may "sculpt" its ejecta into lobes and, when the star radiates in the ultraviolet, its protoplanetary nebula becomes an emission nebula. Example protoplanetary nebulae: Boomerang Nebula , Red Rectangle Nebula , Rotten Egg Nebula .
Note: you will still find texts that treat protoplanetary nebulae and planetary nebulae as a single class, though they are now considered two distinct categories.
A planetary nebula is an expanding shell of gas that is expelled when a red giant finally dies. Surrounding the star's residual core, which continues to emit ultraviolet radiation, the nebulae is thus ionized and illuminated. So planetary nebulae are emission nebulae, and a good many of the most famous ones have a ring-like shape. Still, most are actually more complex in form. Example planetary nebulae: Ring Nebula , Bug Nebula , NGC 2440 , Cat's Eye Nebula .
From NASA's HubbleSite, here's an excellent Flash animation that highlights Planetary Nebula: A New Twist.
Supernova remnants are nebulae that are the "cosmic leftovers" of high-mass stars. A supernova explosion produces an expanding envelope around its residual core, which may be a neutron star (at times a pulsar), a quark star (theorized), a black hole or a white dwarf. Though similar to planetary nebulae in certain ways, supernova remnants are not nearly as numerous. This is because the stars that produce them are far more rare. Example supernova remnants: Crab Nebula , Cassiopeia A , LMC N 49 .
INTERESTING NEBULAE LINKS
From NASA's HubbleSite, here are Image Tours of several interesting nebulae: more Orion Nebula images, more Crab Nebula images, more Cassiopeia A images, more Cat's Eye Nebula images, more Tarantula Nebula images, more Bug Nebula images, Eagle Nebula, Carina Nebula, and Helix Nebula.
And here's the link to Wikipedia's Nebula page.
Galaxies are immense conglomerations of star systems, nebulae and interstellar matter, all bound together by gravitation. Generally classified according to shape as elliptical, spiral or irregular, most range in size from a few thousand to a half million light years across.
Here is an excellent interactive from ESA's Herschel Space Observatory mission that highlights the Major Classifications of Galaxies
From the SPITZER Space Telescope site, here is a high-resolution poster of the SPITZER Infrared Nearby Galaxy Survey (SINGS) Hubble Tuning Fork
From the famous Hubble Deep Field photograph, we now know that, in the observable universe, there are over 100 billion galaxies! To give us an appreciation of just how many galaxies that is, below is an excellent Sloan Digital Sky Survey video that accurately shows where 400,000 galaxies lie in relation to one another. However, this is only a tiny fraction of all galaxies! The Universe holds at least 250,000 times that number! And each galaxy is comprised of millions to trillions of stars!
From NASA's HubbleSite, here are two excellent Flash animations. The first examines When Galaxies Collide. The second, Cosmic Collisions, highlights the Milky Way's eventual collision with the Andromeda Galaxy!
Here's a NASA video that also shows Eventual Collision & Merging of the Milky Way with the Andromeda Galaxy.
Also from NASA's HubbleSite, here are Image Tours of several interesting galaxies: the beautiful Whirlpool Galaxy, the stunning barred spiral NGC 1300, the peculiarly intriguing Tadpole Galaxy, the dwarf galaxy NGC 4449, and the dark-rimmed Sombrero Galaxy.
OUR HOME GALAXY, THE MILKY WAY
Even as late as the 1920's, our own galaxy the Milky Way was believed to constitute the entire cosmos! But we now know that the universe is incredibly larger than was once believed! Current evidence indicates that the Milky Way is roughly 100,000 light-years across. Though this is but a tiny fraction of the entire universe, it is still immense by any earthly standards.
Structurally, the Milky Way is a barred spiral galaxy. Our Solar System lies in one of its spiral arms (the Orion Arm) about 28,000 light years from its core, as you can see in NASA's Annotated Roadmap to the Milky Way
The Photopic Sky Survey is a beautiful panorama that gives an illuminating perspective of the Milky Way as viewed from Earth! Because were are on the inside of our galaxy looking out, it stretches around our sky as an incredible band of stars and dust. (Note: there's a control (tiny " i " button) at bottom-center-left that turns on constellation asterisms, labels and the Ecliptic.)
From the SPITZER Space Telescope site, here is SPITZER's New 360° Infrared View of the Milky Way. You may also view this interactively and in impressively high resolution with the SPITZER site's Aladin Viewer or the WorldWide Telescope Viewer.
The image below shows what the Milky Way looks like "from the outside"!
In addition, NASA's Fermi Gamma-ray Space Telescope has recently revealed immense bubble-like gamma-ray-emitting structures above and below the plane of the Milky Way's disk! These seem to be beyond the visible spectrum and extremely tenuous. While the origins of these structures are unknown, it has been suggested that they are related to the super-massive black hole at the center of our galaxy.
Here's a link to a nice Flash demo from the Chandra site that begins to give you a good sense of the scale of our galaxy: Virtual Tour of the Milky Way.
OUR LOCAL GROUP OF GALAXIES
Our Local Group of Galaxies consists of more than 50 gravitationally bound galaxies nearest to our own, all within roughly 5 million light years. This includes the Large and Small Magellanic Clouds, the Andromeda (M31) and Triangulum (M33) galaxies, and many smaller (dwarf) galaxies. In the two maps below, notice how the smaller galaxies are not spread uniformly through space but rather they cluster around Andromeda and the Milky Way.
Here's a link to a nice 3-D interactive map from ESA: Our Local Group of Galaxies. (Requires: JAVA.)
BEYOND OUR LOCAL GROUP
Beyond our Local Group, galaxies have been found to be arranged in gigantic galaxy clusters of thousands or more members. Beyond this, the clusters stretch out to form long intertwining webs or filaments. This is the opposite of the prevailing theories of galaxy distribution just a half-century ago, when galaxies were believed to be dispersed rather uniformly throughout the universe! A lot has been learned since then. For example, quasars were once believed to represent an entirely different class of celestial objects than galaxies. Now they are thought to be highly active galactic nuclei!
INTERESTING GALAXY LINKS
And here's the link to Wikipedia's Galaxy page.
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FUN FACTS ABOUT
Though some people report seeing farther objects when viewing is exceptional, the Andromeda galaxy, M 31, is generally considered to be the farthest object normally visible to the naked eye! It's about 2½ million light-years away!
M 1, the Crab Nebula, is the remnant of a supernova explosion observed here on Earth in 1054!
The Omega Centauri cluster has a mass of over 4 million Suns!
The largest known galaxy is IC 1101. It has a diameter 50 to 60 times greater than the Milky Way's and a mass in excess of 100 trillion stars!
QUICK ACCESS LIST
Note: some links are echoed elsewhere on this page and may include descriptive text. Note too that there are a few links to interactives in the center column immediately next to this list, so they are not duplicated here.
SEDS site's Interactive Messier Objects page
NASA/JPL's The Two Micron All Sky Survey
Chandra's Virtual Tour of the Milky Way
Herschel's Major Types of Galaxies
ESA's Our Local Group of Galaxies. (Requires: JAVA.)
HubbleSite's Image Tours: