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Author Topic: Why is our galaxy a spiral ?  (Read 9478 times)

Offline neilep

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Why is our galaxy a spiral ?
« on: 17/03/2006 23:35:45 »
Dear Chums,

Why is our galaxy a spiral ?..what makes it a spiral ?...what about other galaxies...they're not all spirals are they ?

If there's no definitive answer then your comments would be most welcome too.

Many Thanks

neil
xx

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another_someone

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Re: Why is our galaxy a spiral ?
« Reply #1 on: 18/03/2006 14:36:58 »
http://www.damtp.cam.ac.uk/user/gr/public/gal_class.html
quote:

Spiral and Elliptical Galaxies


The classification of galaxies due to Hubble (1925) remains the most popular in use today. There are two main categories, the spiral and elliptical galaxies, but also lenticulars and irregulars.

Spirals


like our own galaxy, fall into several classes depending on their shape and the relative size of the bulge: ordinary spirals are labelled either Sa-d,m while those which have developed a bar in the interior region of the spiral arms are SBa-d,m. Spiral galaxies are characterized by the presence of gas in the disk which means star formation remains active at the present time, hence the younger population of stars. Spirals are usually found in the low density galactic field where their delicate shape can avoid disruption by tidal forces from neighbouring galaxies.

M100 and NGC1365 (AAO)

Ellipticals


are placed in the categories E0-7 depending on their degree of ellipticity. They have a uniform luminosity and are similar to the bulge in a spiral galaxy, but with no disk. The stars are old and there is no gas present. Ellipticals are usually found in the high density field, at the centre of clusters.

The giant elliptical galaxy M87 (AAO)

Lenticulars


are labelled S0 and, although they possess both a bulge and a disk, they have no spiral arms. There is little or no gas and so all the stars are old. They appear to be an intermediate.

Sombrero galaxy NGC4549 (AAO)

Irregulars


are small galaxies, labelled Irr, with no bulge and an ill-defined shape. The Magellenic clouds are examples.

 The Large Magellenic Cloud (AAO)





George
« Last Edit: 18/03/2006 14:39:41 by another_someone »
 

another_someone

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Re: Why is our galaxy a spiral ?
« Reply #2 on: 18/03/2006 14:59:38 »
http://en.wikipedia.org/wiki/Spiral_galaxy
quote:

A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties:

  • A considerable total angular momentum

  • Composed of a central bulge surrounded by a disk


    • The bulge resembles an elliptical galaxy, containing many old, so-called "Population II" stars, and usually a supermassive black hole at its center.

    • The disk is a flat, rotating assembly consisting of interstellar matter, young "Population I" stars and open star clusters.



Spiral galaxies are so named due to the bright arms of star formation within the disk that extend—roughly logarithmically—from the bulge. Though sometimes difficult to discern, such as in flocculent spirals, these arms distinguish spiral galaxies from their lenticular counterparts, which exhibit a disk structure but no evident spiral.
The disks of spiral galaxies tend to be surrounded by large spheroid halos of Population II stars, many of which are concentrated in globular clusters that orbit the galactic center.
Our galaxy, the Milky Way, has long been thought to be a spiral, with a Hubble sequence classification of Sbc (possibly SBb); recent research, however, suggests that it may in fact be a barred spiral.

Origin of the spiral structure


The early pioneer of studies on the formation of the spiral arms was Bertil Lindblad. He realised that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the speed of rotation of the galactic disk varies with distance from the centre of the galaxy, a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is not what is observed.
[img=right]http://upload.wikimedia.org/wikipedia/commons/thumb/5/5c/Spiral_galaxy_arms_diagram.png/180px-Spiral_galaxy_arms_diagram.png[/img=right]
The first acceptable theory was devised by C. C. Lin and Frank Shu in 1964. They suggested that the spiral arms were manifestations of spiral density waves. They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic centre. This is illustrated in the diagram. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.
Alternative hypotheses that have been proposed involve waves of star formation moving about the galaxy; the bright stars produced by the star formation die out quickly, leaving darker regions behind the waves, and hence making the waves visible.





George
« Last Edit: 18/03/2006 15:05:56 by another_someone »
 

Offline ukmicky

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Re: Why is our galaxy a spiral ?
« Reply #3 on: 18/03/2006 15:03:54 »
That's got to be the most gargantuan thing ever posted on TNS

Michael
 

Offline neilep

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Re: Why is our galaxy a spiral ?
« Reply #4 on: 18/03/2006 15:06:59 »
WOW !!..Wish I hadn't given  my 50" screen monitor to the kids.

THANK YOU George !!
 

another_someone

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Re: Why is our galaxy a spiral ?
« Reply #5 on: 18/03/2006 15:10:07 »
quote:
Originally posted by ukmicky

That's got to be the most gargantuan thing ever posted on TNS

Michael



Which is why I found a shrunk down version of it (it meant having to go through the HTML source code for the wikipedia page rather than just picking the image properties).



George
 

Offline neilep

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Re: Why is our galaxy a spiral ?
« Reply #6 on: 18/03/2006 15:11:21 »
Hey !!...WE were ALL attending to this thread at the same time !!..how's that for synergy ?...Thanks George...Hi  Michael !
 

another_someone

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Re: Why is our galaxy a spiral ?
« Reply #7 on: 18/03/2006 15:20:39 »
quote:
Originally posted by neilep

Hey !!...WE were ALL attending to this thread at the same time !!..how's that for synergy ?



Indeed – and now all the other reader's will wonder what all the fuss was about size (maybe I should repost the big image, just to let them know):D

It is not easy when people start commenting one one's responses just as one is still making corrections to them :D – I know, I should always get it right first time – but I never do – cela vie.



George
 

Offline neilep

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Re: Why is our galaxy a spiral ?
« Reply #8 on: 18/03/2006 15:29:26 »
quote:
Originally posted by another_someone

quote:
Originally posted by neilep

Hey !!...WE were ALL attending to this thread at the same time !!..how's that for synergy ?



Indeed – and now all the other reader's will wonder what all the fuss was about size (maybe I should repost the big image, just to let them know):D

It is not easy when people start commenting one one's responses just as one is still making corrections to them :D – I know, I should always get it right first time – but I never do – cela vie.



George




George..it just proves you're human like the rest of us !

..and you can't blame us for commenting so quickly George..after all..we  all think you're great :)

Men are the same as women, just inside out !
 

Offline ukmicky

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Re: Why is our galaxy a spiral ?
« Reply #9 on: 18/03/2006 16:31:28 »
quote:
George..it just proves you're human like the rest of us !

lol i wouldnt goo quite as far as that.:)








Neil when did you join the human race  :)
Michael
« Last Edit: 18/03/2006 16:36:30 by ukmicky »
 

Offline Soul Surfer

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Re: Why is our galaxy a spiral ?
« Reply #10 on: 19/03/2006 09:37:28 »
Good posting there.  It currently seems to be most probable that the spiral arm are shock waves propagaring slowly through the galaxy.  It is a recursive process.

Surpernova explosions from big stars create vast shock waves that can travel great distances.  These shock waves tend to compress the interstellar medium and trigger star formation this gets more stars forming and the biggest ones explode as supernovae quite quickly (a few million years in this scale) this tends to reinforce the original shock wave.  The main visibility of the spiral arms is caused by the bright stars the will explode quite quickly as supernovae.

The speed and shear of orbits in the galaxy tend to help to shape the shock waves to cause the structure.  The availability of material for new stars and the activity of the gaxlaxy nucleus is also a feature in defining the many different shapes that galaxies can form.

The trouble is that these features develop over periods of millions of years (the sun takes about 250 million years to go round our galaxy) so all we can see are a set of snapshots and no information about how things evlove

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Offline Bishadi

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Re: Why is our galaxy a spiral ?
« Reply #11 on: 21/04/2006 16:10:18 »
neilip .... although the posts share different views I still did not see anyone really answer the question.

So gravity causes a base spiral or attraction.  What has been missing in today's descriptions is the understanding of energy also having a mass property.

It is where entanglement steps in.  Presently our current models do not actively take this into account but for you I make the suggestion.

A simple test; take 2 base elements weigh them within a vacuum separately, then combine them within a vaccum fire them up with emr and then reweigh the results, if the weight exceeds the sum of the 2 separately, (as it will) .... this model is what is coming.

Mass and energy are entangled and will assist in describing the missing mass or dark energy currently sought.

Hope this help my friend..  great question and be sure to look into entanglement, casimir and feshbach management and casaulty.  
 

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Re: Why is our galaxy a spiral ?
« Reply #11 on: 21/04/2006 16:10:18 »

 

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