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

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.

OPEN CLUSTERS

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.

An interesting interactive on  Star Cluster Evolution.

Wikipedia's  Open Cluster page.

GLOBULAR CLUSTERS

As their name suggests, globular clusters (GCs) or sim- ply "globulars" are dense concentrations of stars which gravity binds tightly into roughly spherical shapes.  Al- though most are no larger than many open clusters, a globular cluster can contain a truly staggering number of stars—hundreds of thousands to millions of stars!— in a region often less than 200 light years across!  Can you imagine looking out at a star as near as Vega and seeing over a million other stars filling your view?

Use the slider below to "zoom in" on a typical globular.  After zooming in and out, you may scroll to study the bottom-right quadrant of this densely packed region of space!  You'll soon gain insight into the different colors of stars that populate it!   Yes, that's a lot of stars!

In globular clusters the overwhelming majority of stars are red and yellow dwarfs that are very old, perhaps 90% of the age of the Universe!  The only younger stars in globular clusters are the rare "blue stragglers", thought to continually form from matter ejected when some old stars inevitably collide with one another.  Did you spot any blue stragglers when you zoomed in on this dense neighborhood of stars?  Look closely!  They're in there!

Almost all galaxies have been found to contain globular clusters, most lying well away from their inner regions.  In a spiral galaxy most globulars are found above and below its main disk, spread out in the roughly spherical volume of the surrounding galactic halo.

The approximately 150 globular clusters that are part of the Milky Way are no exception.  Scattered throughout the galactic halo, most are therefore found far above or below the immense sweeping arms that constitute the "great star factories" of the galaxy!

From  RASC  here is a good site that maps the  Loca- tions of the Milky Way's Globular Clusters.

From NASA here is an  Interactive Map of Several Milky Way Globulars.

Here is a truly awesome  3-D Model of a Globular Clus- ter.  But note: Its 15,000 plotted stars are only a small fraction of what might be found even in average globular clusters!  Still it does give an impressive sense of how densely stellar objects are packed into these relatively compact groupings.  A few example globulars:  M 15  ,  Omega Centauri Cluster  ,  M 79  ,  M 80 

Wikipedia's  Globular Cluster page  and  List of Globular Clusters page.

INTERESTING STAR CLUSTER LINKS

For more information and some beautiful images of star clusters, here are links to  NASA Hubble's Star Clusters,  ESO's Image Archive Star Clusters page.

And here's a link to Wikipedia's  Star Cluster page.

NEBULAE

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.

CATEGORIZING NEBULAE

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 

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   .

DIFFUSE NEBULAE

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

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.

PLANETARY NEBULAE

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   ,  Helix Nebula   .

SUPERNOVA REMNANTS

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 and GoogleArts, here is the  Orion Nebula Image Tour.

For more information and beautiful pictures of nebuale, here are links to  NASA Hubble's Nebulae,  ESO's Image Archive Nebulae page.

And here's the link to  Wikipedia's Nebula page.

GALAXIES

Galaxies  are immense conglomerations of star systems, nebulae, interstellar matter and energy, bound together by gravitation.  Huge in themselves, these vast amalga- mations, often aptly described as "island universes", are arranged throughout deep space in even larger  galaxy clusters    and  superclusters!  These are the "building blocks" of the unfathomably large  galaxy filaments  , which stretch throughout the universe in incredibly com- plex  3-dimensional webs  and networks!

Galaxies are generally classified according to shape as elliptical, spiral or irregular, most ranging in size from a few thousand to a half million light years across.  This  Galaxy Size Comparison Chart, showing the Milky Way at its center, begins to give us an idea of how varied the sizes of galaxies can be.

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 Ultra Deep Field     photo, 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 latest observations suggest that the Universe holds over a million times that number!  And each galaxy contains millions to trillions of stars!

Here is a NASA video that shows the  Eventual Collision and Merging of the Milky Way and Andromeda Galaxies.

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.

From the SPITZER Space Telescope site, it's New 360° Infrared View of the Milky Way may be viewed interac- tively and in high resolution with the  Aladin Viewer  or the  WorldWide Telescope Viewer  .

The animation 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.

And here are links to Wikipedia's  Milky Way  and  Orion Arm  pages.

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 are a few links to the Encyclopaedia Galactica's superb isometric  Map of Our Local Group of Galaxies  and  Map of Our "Extended" Local Group of Galaxies.

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

For more information and some beautiful pictures of star clusters, here are links to  NASA Hubble's Galaxies,  ESO's Image Archive Galaxy page.

And here's the link to  Wikipedia's Galaxy page.