Everything Science Forum

1).Why does Andromeda move towards Milky Way , since we know that galaxies at large move away from each other at a speed which is proportional to the distance between them ?
2).When two galaxies (spiral)  collide with each other : How many "suns" (the percentage) will be destroyed ? Can you describe the mechanism of collision ?  :wall

Andromeda is part of our local galaxy cluster.  Galaxy clusters interact withit's members due to gravitational links, hince, they can approach and merge.

When two galaxies collide, the percentage of accual collisions of stars is very minimal.  Though a galaxy looks thick, the distances between the stars is great, and as such they for the most part will pass through each other.  On the otherhand, we can expect a massive surge in star creation, as the gravitational wave front will disturb clouds of dust triggering star birth.

From observations of a collision we can see, the number of stars in the 2 galaxies that are colliding, the number of stars is dramaticly higher.  We can tell these are new stars due to their temperature and color.  And these are large blue giants which are short-lived stars (1 - 5 million years old).  We can not see how many stars the size of SOL due to the great distance.  They are too dim to see

In general all galaxies seem to moving away from some location (assuming the Big Bang Model, or expanding model of the universe).  However, on a local basis, galaxies can be moving laterally toward one another, because the angular momentum of the two produces a coincidence of their trajectories.  And as Dingo pointed out galaxies do exist in groups or clusters.

It is difficult to say exactly what fraction of stars interact - that would depend on how close the galactic core approach.

For a discussion of colliding galaxies, see -
http://csep10.phys.utk.edu/astr162/lect/galaxies/colliding.html


Here is a description of the Antennae galaxies with a picture from Hubble Space Telescope.

This Hubble Space Telescope image provides a detailed look at a brilliant "fireworks show" at the center of a collision between two galaxies. Hubble has uncovered over 1,000 bright, young star clusters bursting to life as a result of the head-on wreck.

[Left side of image]
A ground-based telescopic view of the Antennae galaxies (known formally as NGC 4038/4039) - so named because a pair of long tails of luminous matter, formed by the gravitational tidal forces of their encounter, resembles an insect's antennae. The galaxies are located 63 million light-years away in the southern constellation Corvus.

[Right side]
The respective cores of the twin galaxies are the orange blobs, left and right of image center, crisscrossed by filaments of dark dust. A wide band of chaotic dust, called the overlap region, stretches between the cores of the two galaxies. The sweeping spiral- like patterns, traced by bright blue star clusters, shows the result of a firestorm of star birth activity which was triggered by the collision.

This natural-color image is a composite of four separately filtered images taken with the Wide Field Planetary Camera 2 (WFPC2), on January 20, 1996. Resolution is 15 light-years per pixel (picture element).  STScI-PRC1997-34a

According to Spiral Galaxies (http://www.astro.umd.edu/education/astro/gal/spiral.html) - Spirals are not the most common type of galaxy. 

Most of the galaxies we know are elliptical in shape. The largest of these elliptical galaxies may contain as many as 10 trillion stars (10 trillion solar masses) and may be as large as 100,000 parsecs in diameter. (This is comparable to the size of our galaxy - including the entire disk of our galaxy - but with about 100 times more stars. No wonder they are so bright!) Such huge galaxies are called Giant Ellipticals (an example is shown above). They are rare but spectacular. Most of the ellipticals are Dwarf Ellipticals, which have approximately a few million solar masses and diameters of about 2000 parsecs. They are low surface brightness objects. Dwarf Ellipticals generally are found in galaxy clusters or near large galaxies.  from the Department of Astronomy, University of Maryland

Our neighboring Galaxy, M31 or Andromeda, is also a spiral galaxy.  Both are in the Local Cluster.

Milky Way Galaxy (http://en.wikipedia.org/wiki/Milky_Way_Galaxy) Wikipedia

Milky Way Spiral Structure (http://www.seds.org/messier/more/mw_arms.html)

The Milky Way Galaxy - A Grand Design (http://physics.uoregon.edu/~jimbrau/astr123/Notes/Chapter23.html#spiral) U. of Oregon

Spiral galaxy (http://www.absoluteastronomy.com/encyclopedia/S/Sp/Spiral_galaxy.htm)

Spiral galaxies (http://www.seds.org/messier/objects.html#galaxy)

Local Group of Galaxies (http://www.seds.org/messier/more/local.html)

Groups and Clusters of Galaxies with Messier objects (http://www.seds.org/messier/g-group.html)

Galaxy Cluster Mug Shots (http://www.astr.ua.edu/white/mug/cluster/)

also on Wikipedia -

Galaxy (http://en.wikipedia.org/wiki/Galaxy)
List of Galaxies (http://en.wikipedia.org/wiki/List_of_galaxies)
Groups or clusters of Galaxies (http://en.wikipedia.org/wiki/Groups_and_clusters_of_galaxies)

APOD, Oct 6 (ESO) - This gorgeous island universe lies about 85 million light-years distant in the southern constellation Fornax. Inhabited by young blue star clusters, the spiral arms of NGC 1350 seem to join in a circle around the galaxy's large, bright nucleus - giving the galaxy the appearance of a limpid cosmic eye. NGC 1350 is about 130,000 light-years across making it as large or slightly larger than our own Milky Way. For earth-based astronomers, NGC 1350 is seen on the outskirts of the Fornax cluster of galaxies, but its estimated distance suggests that it is not itself a cluster member.

STScI-PRC2001-10 Hubble Reveals the Heart of the Whirlpool Galaxy
http://hubblesite.org/newscenter/newsdesk/archive/releases/2001/10/

New images from NASA's Hubble Space Telescope are helping researchers view in unprecedented detail the spiral arms and dust clouds of a nearby galaxy, which are the birth sites of massive and luminous stars.

The Whirlpool galaxy, M51, has been one of the most photogenic galaxies in amateur and professional astronomy. Easily photographed and viewed by smaller telescopes, this celestial beauty is studied extensively in a range of wavelengths by large ground- and space-based observatories. This Hubble composite image shows visible starlight as well as light from the emission of glowing hydrogen, which is associated with the most luminous young stars in the spiral arms.

M51, also known as NGC 5194, is having a close encounter with a nearby companion galaxy, NGC 5195, just off the upper edge of this image. The companion's gravitational pull is triggering star formation in the main galaxy, as seen in brilliant detail by numerous, luminous clusters of young and energetic stars. The bright clusters are highlighted in red by their associated emission from glowing hydrogen gas.

This Wide Field Planetary Camera 2 image enables a research group, led by Nick Scoville (Caltech), to clearly define the structure of both the cold dust clouds and the hot hydrogen and link individual clusters to their parent dust clouds. Team members include M. Polletta (U. Geneva); S. Ewald and S. Stolovy (Caltech); R. Thompson and M. Rieke (U. of Arizona).

Intricate structure is also seen for the first time in the dust clouds. Along the spiral arms, dust "spurs" are seen branching out almost perpendicular to the main spiral arms. The regularity and large number of these features suggests to astronomers that previous models of "two-arm" spiral galaxies may need to be revisited. The new images also reveal a dust disk in the nucleus, which may provide fuel for a nuclear black hole.

The team is also studying this galaxy at near-infrared wavelengths with the NICMOS instrument onboard Hubble. At these wavelengths, the dusty clouds are more transparent and the true distribution of stars is more easily seen. In addition, regions of star formation that are obscured in the optical images are newly revealed in the near-infrared images.

This image was composed by the Hubble Heritage Team from Hubble archival data of M51 and is superimposed onto ground-based data taken by Travis Rector (NOAO) at the 0.9-meter telescope at the National Science Foundation's Kitt Peak National Observatory (NOAO/AURA) in Tucson, AZ.

 
 

Object Name:	Whirlpool Galaxy • M51 • NGC 5194
Object Description:	Face-on Spiral Galaxy with Companion
Position (J2000):	R.A. 13h 29m 52.37s
	Dec. +47° 11' 40.8"
Constellation:	Canes Venatici
Distance:	About 9.6 Mpc (31 million light-years)
Visual Magnitude:	8.4

The galaxy is roughly 9.3 kpc (30,000 light-years) in the vertical dimension in the image.

See also - http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/12/image/c - for an image in H-alpha light.

STScI-PRC2005-21 Hubble Pinpoints Doomed Star that Explodes as Supernova
http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/21/

Amidst the glitter of billions of stars in the majestic spiral galaxy called the Whirlpool (M51), a massive star abruptly ends its life in a brilliant flash of light. NASA's Hubble Space Telescope snapped images of the exploding star, called Supernova (SN) 2005cs, 12 days after its discovery. Astronomers then compared those photos with Hubble images of the same region before the supernova blast to pinpoint the progenitor star (the star that exploded).

The color image at left shows a section of M51 taken in January 2005 with the Advanced Camera for Surveys. The small green square marks the region where the progenitor star resides. The lower-right image shows a picture of SN 2005cs (the central bright object), taken July 11, 2005, by Hubble. By comparing the lower-right image with the color image at left, astronomers identified the supernova's progenitor star [marked by the arrow in the (pre-explosion) upper-right image]. The star was found to be a red supergiant whose mass is about seven to 10 times that of the Sun.

Every second, a star somewhere in the universe explodes as a supernova. Astronomers cannot see every supernova. Of the supernovas astronomers have seen, only six progenitor stars have been identified. Since Hubble can easily resolve stars in nearby galaxies, such as the Whirlpool, it allowed astronomers to track down the exploding star's identity in archival pictures.

SN 2005cs belongs to a class of exploding stars called "Type II-plateau." A supernova of this type results from the collapse and subsequent explosion of a massive star whose light remains at a constant brightness (a "plateau") for a period of time.

This finding is consistent with the idea that the progenitors of supernova explosions are red supergiant stars with masses eight to 15 times the Sun's mass. The progenitor star was found to be at the low end of the mass range for supernova explosions. Stars with masses lower than eight solar masses do not explode as supernovae at all, but rather contract to white dwarfs and blow off their outer atmospheres to become planetary nebulae.

Identification of the progenitor star was first reported in IAU Circulars 8556 and 8565 on July 3 and July 12, respectively, by Drs. Weidong Li and Alex Filippenko (University of California, Berkeley) and Schuyler Van Dyk (Spitzer Science Center, Caltech). The team submitted a paper describing their research to The Astrophysical Journal on July 18. A European team composed of Drs. Justyn Maund (Institute of Astronomy, University of Cambridge), Stephen Smartt (Queen's University, Belfast) and John Danziger (Trieste Observatory) reported similar results in a letter submitted on July 21 to the Monthly Notices of the Royal Astronomical Society.

For additional information, please contact: Weidong Li, Department of Astronomy, University of California Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411, (phone) 510-643-8973, (fax) 510-642-3411, (e-mail) wli@astron.berekely.edu or

Alex Filippenko, Department of Astronomy, University of California, Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411, (phone) 510-642-1813, (fax) 510-642-3411, (e-mail) alex@astron.berkeley.edu or

Justyn Maund, Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, England, UK, (phone) +44-1223-337544, (fax) +44-1233-337523, (e-mail) jrm@ast.cam.ac.uk

Object name: M 82, Messier 82, NGC 3034, Cigar Galaxy
Object type: Galaxy
Position (J2000): RA: 09h 55m 52.20s  Dec: 69d 40m 7.00s
Distance: 11.7 million light-years
Constellation: Ursa Major

NASA's Spitzer, Hubble, and Chandra space observatories teamed up to create this multi-wavelength, false-colored view of the M82 galaxy. The lively portrait celebrates Hubble's "sweet sixteen" birthday.

X-ray data recorded by Chandra appears in blue; infrared light recorded by Spitzer appears in red; Hubble's observations of hydrogen emission appear in orange, and the bluest visible light appears in yellow-green.

Instrument: IRAC
Release Date: 2006/04/24

Remember, this is false color.  It appears to me that there is a nebula in the forground, which partly covers M82.

Object name: NGC 1566
Object type: Spiral galaxy
Position (J2000): RA: 04h 20m 0.40s  Dec: -54d 56m 16.00s
Distance: 60 million light-years
Constellation: Dorado

This beautiful spiral galaxy NGC 1566, located approximately 60 million light-years away in the constellation Dorado was captured by the Spitzer Infrared Nearby Galaxies Survey (SINGS) Legacy Project using the telescope's Infrared Array Camera (IRAC).

The faint blue light is coming from mature stars, while the "glowing" red spiral arms indicate active star formation and dust emission. Much of the active star formation is seen in the two symmetric arms that are reminiscent of other grand design spirals such as the Whirlpool galaxy. The small and very luminous blue nucleus suggests that this is a Seyfert galaxy (a galaxy that is actively emitting radiation from a very small region in its core).

The SINGS image is a four-channel false-color composite, where blue indicates emission at 3.6 microns, green corresponds to 4.5 microns, and red to 5.8 and 8.0 microns. The contribution from starlight (measured at 3.6 microns) in this picture has been subtracted from the 5.8 and 8 micron images to enhance the visibility of the dust features.

Image Credit: NASA/JPL-Caltech/R. Kennicutt (University of Arizona) and the SINGS Team
Instrument: IRAC
Wavelength: 3.6 (blue), 4.5 (green), 5.8-8.0 (red) microns
Image scale: 12.2x12.2 arcmin
Orientation: North is 123 deg CW from up
Release Date: 2005/09/15

A GLIMPSE of the Milky Way
Object name: Milky Way, Galactic Plane
Object type: Spiral galaxy
Position (J2000): Galactic latitude: -1 to +1 deg  Galactic longitude: 327-335 deg

In visible light, the bulk of our Milky Way galaxy's stars are eclipsed behind thick clouds of galactic dust and gas. But to the infrared eyes of NASA's Spitzer Space Telescope, distant stars and dust clouds shine with unparalleled clarity and color.

In this panoramic image (center row) from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) project, a plethora of stellar activity in the Milky Way's galactic plane, reaching to the far side of our galaxy, is exposed. This image spans 9 degrees of sky (approximately the width of a fist held out at arm's length).

The red clouds indicate the presence of large organic molecules (mixed with the dust), which have been illuminated by nearby star formation. The patches of black are dense obscuring dust clouds impenetrable by even Spitzer's super-sensitive infrared eyes. Bright arcs of white throughout the image are massive stellar incubators.

With over 160 megapixels, the full detail in this panorama cannot be appreciated without zooming in to various areas of interest (top and bottom rows). Bubbles, or holes, in the red clouds are formed by the powerful outflows from massive groups of forming stars. Wisps of green indicate the presence of hot hydrogen gas. Star clusters can also be seen as the groupings of blue, yellow, and green specks inside some of the red nebulae, or star-forming clouds. In contrast to the plentiful examples of stellar youth in this montage, Spitzer also sees an object called a planetary nebula (top row, middle). Such nebulae are the final gasp of dying stars like our sun, whose outer layers are blown into space, leaving a burnt -out core of a star, called a white dwarf, behind.

Although this panoramic image captures a large range of the galaxy, it represents only 7.5 percent of the primary GLIMPSE survey, which will image most of the star formation regions in our galaxy.

The infrared images were captured with the Spitzer's Infrared Array Camera (IRAC). The pictures are 4-channel false-color composites, showing emission from wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange), and 8.0 microns (red).

Image Credit: NASA/JPL-Caltech/E. Churchwell (Univ. Wisconsin)
Instrument: IRAC
Wavelength: 3.6 (blue), 4.5 (green), 5.8 (orange), 8.0 (red) microns
Image scale: 9x2 degrees
Orientation: galactic north is up
Release Date: 2005/12/12

Image from - http://www.jpl.nasa.gov/multimedia/slideshows/spitzer-200605/
Specifically - http://sscws1.ipac.caltech.edu/Imagegallery/image.php?image_name=sig05-025

Object name: NGC2207, IC2163
Object type: galaxy, galaxies
Position (J2000): RA: 06h16m25.00s  Dec: -21d22m26.30s
Distance: 140 million light-years
Constellation: Canis Major

NGC 2207 and IC 2163 recently met and began a sort of gravitational tango about 40 million years ago. The two galaxies are tugging at each other, stimulating new stars to form. Eventually, this cosmic ball will come to an end, when the galaxies meld into one. The dancing duo is located in the Canis Major constellation.

The Spitzer image reveals that the galactic mask is adorned with strings of pearl-like beads. These dusty clusters of newborn stars, called "beads on a string" by astronomers, appear as white balls throughout the arms of both galaxies. They were formed when the galaxies first interacted, forcing dust and gas to clump together into colonies of stars.

This type of beading has been seen before in other galaxies, but it took Spitzer's infrared eyes to identify them in NGC 2207 and IC 2163. Spitzer was able to see the beads because the stars inside heat up surrounding dust, which then radiates with infrared light.

The biggest bead lighting up the left side of the mask is also the densest. In fact, some of its central stars might have merged to form a black hole.

This picture, taken by Spitzer's infrared array camera, is a four-channel composite. It shows light with wavelengths of 3.6 microns (blue); 4.5 microns (green); and 5.8 and 8.0 microns (red). The contribution from starlight (measured at 3.6 microns) has been subtracted from the 5.8- and 8-micron channels to enhance the visibility of the dust features.

Image Credit: NASA/JPL-Caltech/D. Elmegreen (Vassar)
Instrument: IRAC
Wavelength: 3.6 microns (blue); 4.5 microns (green); and 5.8 and 8.0 microns (red)
Exposure Date: 22 February 2005
Exposure Time: 30 seconds
Image scale: 5 x 5 arcmin
Orientation: North is 150 degrees CW from up
Release Date: 2006/04/26

Wonderful pictures, Astronuc! It's too bad that the community of astronomers may not have the  HST for much longer-hard to tell when the next generation instrument will become available.

Regarding galactic collisions, it should be remembered that large galaxies such as the Milky Way or M31 are theorized to have been built up by collisions between smaller galaxies-large mature galaxies are spoken of as "cannibalizing" their smaller neighbors. Galactic collisions were more common, apparently, in earlier times when the universe was smaller and all the distances were less but even today within local clusters of galaxies the probability of random motion of galaxies toward each other is still quite high-movement is not totally dominated by the expansion of the universe.

There are a number of interesting issues to ponder regarding galactic collisions-the behavior of the stars (spaced so far apart that actual collisions are highly improbable), the behavior of gas clouds (lots of turmoil there as interaction is inevitable) and the behavior of the black holes which are theorized to lie at the center of many if not most galaxies. A lot of thought has gone into detecting the gravity waves that should emanate from the collision of two massive galactic-center black holes.

How the Milky Way Formed; Mysteries of the Milky Way (http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ARTICLEID_CHAR=C94402DC-2B35-221B-6861A5FFD39952F5); Exclusive Online Issues; by Sidney van den Bergh and James E. Hesser; 5 Page(s)

Attempts to reconstruct how the Milky Way formed and began to evolve resemble an archaeological investigation of an ancient civilization buried below the bustling center of an ever changing modern city. From excavations of foundations, some pottery shards and a few bones, we must infer how our ancestors were born, how they grew old and died and how they may have helped create the living culture above. Like archaeologists, astronomers, too, look at small, disparate clues to determine how our galaxy and others like it were born about a billion years after the big bang and took on their current shapes. The clues consist of the ages of stars and stellar clusters, their distribution and their chemistry--all deduced by looking at such features as color and luminosity. The shapes and physical properties of other galaxies can also provide insight concerning the formation of our own.

The evidence suggests that our galaxy, the Milky Way, came into being as a consequence of the collapse of a vast gas cloud. Yet that cannot be the whole story. Recent observations have forced workers who support the hypothesis of a simple, rapid collapse to modify their idea in important ways. This new information has led other researchers to postulate that several gas cloud fragments merged to create the protogalactic Milky Way, which then collapsed. Other variations on these themes are vigorously maintained. Investigators of virtually all persuasions recognize that the births of stars and supernovae have helped shape the Milky Way. Indeed, the formation and explosion of stars are at this moment further altering the galaxy's structure and influencing its ultimate fate.

This seems like an interesting article.  I wonder how much data/information is consistent and what are the apparent discrepancies.

Newly Discovered Galaxy Is a Record-Breaker  (http://www.sciam.com/article.cfm?articleID=00016CE5-7A83-1032-BA8383414B7F0000&sc=I100322)
Well since the article is dated February 18, 2004, maybe it's not so new.

Astronomers have detected a galaxy located 13 billion light-years from Earth, making it the most distant such object on record. The find, described in a paper to be published in the Astrophysical Journal, should help scientists better understand the so-called cosmic Dark Ages, when the universe’s first galaxies and quasars transformed opaque hydrogen into the transparent cosmos that exists today.

The record-breaking galaxy is so far away and so dim that the astronomers needed considerable help to observe it. The galaxy cluster known as Abell 2218, which is very massive and located between the galaxy and Earth, bent and magnified its light. "As we were searching for distant galaxies magnified by Abell 2218, we detected a pair of strikingly similar images whose arrangement and color indicate a very distant object," explains lead author Jean-Paul Kneib of the California Institute of Technology. The data collected by NASA’s Hubble Space Telescope and the W. M. Keck Telescopes in Hawaii signal that the object’s redshift, which measures the shift of light to longer wavelengths and reveals its distance from Earth, lies between 6.6 and 7.1. "The galaxy we have discovered is extremely faint, and verifying its distance has been an extraordinarily challenging adventure," says Kneib. "Without the 25 times magnification afforded by the foreground cluster, this early object could simply not have been identified or studied in any detail at all with the present telescopes available."

See related thread in this forum Most distant object (http://www.everything-science.com/component/option,com_smf/Itemid,82/topic,641.0)

October 19, 2006—Astronomers, it turns out, like to rubberneck too.
http://news.nationalgeographic.com/news/2006/10/061019-galaxies.html

Scientists haven't been able to take their eyes off a nearby stellar collision. This image, snapped by the Hubble Space Telescope and released October 16, is the sharpest yet of the merging Antennae Galaxies.

The spiral galaxies, named for long antenna-like arms that extend far from the galaxies' centers, began colliding only a few hundred million years ago. The pair represents one of the closest galactic collisions to Earth, and one of the youngest known to science.

The reddish-orange blobs to the right and left of the picture's center are the two galactic cores—consisting mostly of old stars crisscrossed by filaments of dust (brown).

What has researchers really excited, though, are the blue star-forming regions, which are surrounded by hot glowing hydrogen gas (pink). Billions of new stars will be formed by the energetic collision, scientists say.

The brightest and most compact star-forming regions are called super star clusters.

http://news.nationalgeographic.com/news/2006/10/images/061019-galaxies_big.jpg


Tarantula Nebula Emerges in Detail
http://news.nationalgeographic.com/news/2006/04/0407_060407_tarantula.html

April 7, 2006—This leggy beauty harbors more than 200 massive stars in her brood. And there's more to come—new stars appear to be popping out all over this thousand-light-year-wide star nursery.

Released today by the European Southern Observatory, this image emphasizes the Tarantula Nebula's luminosity and spidery shape—both whipped into being by the intense radiation and winds of the superhot star cluster at the picture's center.

At 170,000 light-years away, the nebula hangs above one of the closest galaxies to our own. Though near enough, big enough, and bright enough to be seen with the naked eye, the Tarantula Nebula looks best through a large telescope—in this case, the Very Large Telescope, actually a cluster of four 8-meter-wide (26-foot-wide) telescopes in the Chile's Atacama Desert.

Planetary camera observations of the double nucleus of M31 (http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1993AJ....106.1436L)

Wow, I didn't know this.  ::)  I must have been off the planet when this was announced.

http://antwrp.gsfc.nasa.gov/apod/ap961011.html

http://antwrp.gsfc.nasa.gov/apod/ap061126.html

I would very much like a spacecraft that would travel round-trip to Andromeda in a short period of time.  But then I'd want to visit lots of places in the Milky Way galaxy - like Rigel, Betelgeuse, the Horsehead Nebula, . . . . .

I was prompted to look at two examples of spiral galaxies regarding a comment I heard about Hubble Ultra Deep Field survey.

Pin Wheel Galaxy (M101) - reportedly the best example of a spiral galaxy - that we can observe that is.
http://antwrp.gsfc.nasa.gov/apod/ap060302.html

http://antwrp.gsfc.nasa.gov/apod/ap030310.html

http://antwrp.gsfc.nasa.gov/apod/ap000610.html - this is interesting in the UV.  What strikes me is the asymmetry - and I wonder what that means in terms of mass distribution and energy generation.

http://antwrp.gsfc.nasa.gov/apod/ap970805.html


M83: The Southern Pinwheel Galaxy from VLT
http://antwrp.gsfc.nasa.gov/apod/ap051218.html

http://antwrp.gsfc.nasa.gov/apod/ap030511.html

http://antwrp.gsfc.nasa.gov/apod/ap991206.html

http://antwrp.gsfc.nasa.gov/apod/ap970419.html


NGC 3314: When Galaxies Overlap - two large spiral galaxies
http://antwrp.gsfc.nasa.gov/apod/ap000511.html

Mystery spiral arms explained? (http://www.esa.int/esaCP/SEMLVET4LZE_index_0.html)
10 April 2007

Using a trio of space observatories, astronomers may have cracked a 45-year old mystery surrounding two ghostly spiral arms in the galaxy M106 (NGC 4258).
 
The results, obtained by a team from the University of Maryland (USA), took advantage of the unique capabilities of the European Space Agency’s XMM-Newton X-ray observatory, NASA’s Chandra X-ray Observatory, and NASA’s Spitzer Space Telescope.

M106 (also known as NGC 4258) is a spiral galaxy 23.5 million light-years away, in the constellation Canes Venatici. In visible-light images, two prominent arms emanate from the bright nucleus and spiral outward. These arms are dominated by young, bright stars, which light up the gas within the arms. "But in radio and X-ray images, two additional spiral arms dominate the picture, appearing as ghostly apparitions between the main arms," says team member Andrew Wilson of the University of Maryland. These so-called "anomalous arms" consist mostly of gas.

. . . .

http://www.astronomy.ohio-state.edu/Gallery/galimgs.html

An interesting perspective on the Milky Way Galaxy and its neighbors - the Magellanic Clouds.
http://cseligman.com/text/galaxies/milkyway.htm