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Differential rotation would appear to explain why so much of the material in the disk of the Milky Way is concentrated into elongated features that resemble spiral arms . No matter what the original distribution of the material might be, the differential rotation of the Galaxy can stretch it out into spiral features. [link] shows the development of spiral arms from two irregular blobs of interstellar matter. Notice that as the portions of the blobs closest to the galactic center move faster, those farther out trail behind.

Simplified model for the formation of spiral arms.

Simplified Model for the Formation of Spiral Arms. At left, the illustration begins with two irregular blue blobs, one above the other, with a short curved arrow at top pointing to the right indicating the direction of rotation. The next frame, with a longer curved arrow, shows how parts of the initial blobs have moved toward each other, but the parts further away have moved less, giving the appearance of two small comets. In the next frame, the curved arrow covers about 180O, and the blobs are now even more curved and elongated. In the final frame at right, the curved arrow covers 270O, and the classic spiral shape has emerged.
This sketch shows how spiral arms might form from irregular clouds of interstellar material stretched out by the different rotation rates throughout the Galaxy. The regions farthest from the galactic center take longer to complete their orbits and thus lag behind the inner regions. If this were the only mechanism for creating spiral arms, then over time the spiral arms would completely wind up and disappear. Since many galaxies have spiral arms, they must be long-lived, and there must be other processes at work to maintain them.

But this picture of spiral arms presents astronomers with an immediate problem. If that’s all there were to the story, differential rotation—over the roughly 13-billion-year history of the Galaxy—would have wound the Galaxy’s arms tighter and tighter until all semblance of spiral structure had disappeared. But did the Milky Way actually have spiral arms when it formed 13 billion years ago? And do spiral arms, once formed, last for that long a time?

With the advent of the Hubble Space Telescope, it has become possible to observe the structure of very distant galaxies and to see what they were like shortly after they began to form more than 13 billion years ago. What the observations show is that galaxies in their infancy had bright, clumpy star-forming regions, but no regular spiral structure.

Over the next few billion years, the galaxies began to “settle down.” The galaxies that were to become spirals lost their massive clumps and developed a central bulge. The turbulence in these galaxies decreased, rotation began to dominate the motions of the stars and gas, and stars began to form in a much quieter disk. Smaller star-forming clumps began to form fuzzy, not-very-distinct spiral arms. Bright, well-defined spiral arms began to appear only when the galaxies were about 3.6 billion years old. Initially, there were two well-defined arms. Multi-armed structures in galaxies like we see in the Milky Way appeared only when the universe was about 8 billion years old.

We will discuss the history of galaxies in more detail in The Evolution and Distribution of Galaxies . But, even from our brief discussion, you can get the sense that the spiral structures we now observe in mature galaxies have come along later in the full story of how things develop in the universe.

Scientists have used supercomputer calculations to model the formation and evolution of the arms. These calculations follow the motions of up to 100 million “star particles” to see whether gravitational forces can cause them to form spiral structure. What these calculations show is that giant molecular clouds (which we discussed in Between the Stars: Gas and Dust in Space ) have enough gravitational influence over their surroundings to initiate the formation of structures that look like spiral arms. These arms then become self-perpetuating and can survive for at least several billion years. The arms may change their brightness over time as star formation comes and goes, but they are not temporary features. The concentration of matter in the arms exerts sufficient gravitational force to keep the arms together over long periods of time.

Key concepts and summary

The gaseous distribution in the Galaxy’s disk has two main spiral arms that emerge from the ends of the central bar, along with several fainter arms and short spurs; the Sun is located in one of those spurs. Measurements show that the Galaxy does not rotate as a solid body, but instead its stars and gas follow differential rotation, such that the material closer to the galactic center completes its orbit more quickly. Observations show that galaxies like the Milky Way take several billion years after they began to form to develop spiral structure.

Questions & Answers

Is the Earth really circle?
Chu Reply
Earth is not a circle It is ellipsoid
Tanmay
What is Astronomy
eaten_and_swallowed Reply
Astronomy can be defined as the study of objects beyond earth, and the process by which these objects interact
Simnein
Jupiter has 79 known moons as of July 18, 2018. Saturn has 82 moons as of 2019.
Michael Reply
How nearby planets seem fixed relative to stars?
Megha Reply
what is astronomy?
md Reply
Astronomy is the study of physical universe beyond the earth's atmosphere, including the process of mapping locations and properties of the matter and radiation in the universe.
Tracy
Astronomy is the study about the physical and chemical properties of all the objects, and heavenly bodies in the outer space.
Sophia
distance from earth surface to its orbit
Ram Reply
who's orbit?
Tanmay
thanks
Ram
I am completely confused about the question and your thanks to my question
Tanmay
orbit meaning?
Monika
😂😂
anita
Can anyone explain me chanderasekhar limit
Asim
orbit is an imaginary path that is followed by an object because of gravitational pull of another object
Tanmay
Chandrasekhar limit is the amount of maximum mass a white dwarf can have before becoming a neutron star or a black hole value is 14 solar masses
Tanmay
I guess he's asking for the distance between the earth's surface and the orbit of earth itself which seems illogical to me
anita
yeah I also thought that but then thought this might be type error
Tanmay
what does chandrashekhar unit has to do with the actual question
anita
Someone asked it in the middle
Tanmay
do you mean at what distance from the surface are you considered to be in orbit?
dennis
it's approximately 160 miles
dennis
wouldn't the velocity (acceleration) of the object determine the distance it orbits at?
paul
what's did they use to detect dark matter again?
Creative
As we look at the rotation of many galaxies, ours included, they are spinning too fast to stay together if they only rely on the gravitational attraction of what we can see. So there must be more stuff we can't see, what has come to be called dark matter. It still isnt known exactly what it is.
paul
Is Hawking radiation a theory or we have already proved it?
Tanmay Reply
I believe the jury is still out. the physics seems ok but the evaporation of a black hole takes longer than the age of the the universe so can't be said to have been observed. Like all theories, they are always being tested so maybe by now it may have been shown to be correct
paul
So we haven't proved it completely but we can say that it can happen and might be true?
Tanmay
hello hooman
Alexis
Hello mortal earthling
Tanmay
can you please show me how to research the universe?
Alexis
By joining space research clubs
Asim
You will know about everything
Asim
If you are in a college there should be an astronomy club join it an share knowledge
Tanmay
Tanmay hii
Asim
hi
Tanmay
Your country?
Asim
india
Tanmay
yours?
Tanmay
Pakistan
Asim
But currently living in Australia
Asim
enjoying vacations at this time?
Tanmay
You are looking very interested in astronomy
Asim
yup
Tanmay
Your class or grade
Asim
what is astronomy
md
Who all beleive in multiverse theory?
Tanmay Reply
I don't believe in it
Asim
So you think that there is only one universe?
Tanmay
Its just an assumption
Asim
neither do I believe in it
anita
True we can't even think how we can prove
Tanmay
Tanmay do you know laplace theory?
Asim
His threoy can't be proved if we prove that theory we will exactly know the past and future of our universe and how its seems like in future
Asim
Just like that concept of multiple universe
Asim
oh yes
Tanmay
او ئاژەڵەی چاوی لەمێشکی گەورەترە
You forgot this ∞
Tanmay
Tanmay
You can't type infinity
Tanmay
In this
Tanmay
Can we see the rotation or magnetic attraction between two stars from naked eyes?
Asim
Or stars moving with speed?
Asim
You can't see most stars moving, with the exception of our sun. you can measure their movement by measuring the the Doppler shift of the light coming from the star, even measuring the rotation by comparing the Doppler shift of the Lightroom each side of the star.
paul
stars don't attract each other in any great way magnetically. Rather they attract each other gravitationally
paul
This is because the ions in the star are at chaos so some charges cancel out some other charges electromagnetic field so gravity becomes the winner
Tanmay
I am studying in 11th std (science stream) and want to know that how can I take astronomy field? as my physics and maths are better.
Centauri Reply
choose it😂
Astroker
hi
Saswat
i am also very much interested but i am in class 9th
Saswat
but i can help you somehow
Saswat
First of all you have to choose either observational astronomy or experimental astronomy
Satyam
Then get good grades and enter into a college and take physics major astronomy minor or astronomy major anything minor
Tanmay
Make or join astronomy club and have fun with ideas and telescopes. You might discover something new
Tanmay
how to become a astrophysicist
Saswat Reply
Study, Get a degree at a college; it's what I'm doing.
Simnein
distance between sun and earth
Nibha Reply
moon phases changes how
Nibha
Around 93m miles
Nick
average distance between earth and sun is 150 million km
Tanmay
Phases of moon is nothing but amount of light from sun being reflected by moon's surface. Sun is the light source and earth is the object that blocks light so we see the moon's phases according to the area not receiving light.
Tanmay
The Distance Betwern The Sun And Earth Is 149.7Million Km In Summer. While In Winter. its 151 Million Km Its Because Planets Have No Perfect Circular Orbit From Its Parent Star.
Anti-Capitalism
thanks
Nibha
what is anthropic principle
Nibha
Antropic principles is that the unvierse we see it isn't just like this. It may less or more we see
Asim
how universe is form
Nibha Reply
how can I get deeply knowledge about univers
Nibha
how universe was formed
Nibha
The big bang
Nick
hii
Saswat
hi
Nick
13.8 billion years ago the universe was just a tiny dot in space then something triggered it an it expanded at a tremendous rate which we call the Big Bang.
Tanmay
it's just a theory u have no proofs to say precisely it was imagined by scientists like that in simple terms it's hypothesis
Sreeman
Actually Cosmic Microwave Background Radiation is one of the best examples to say that Big Bang theory might be correct
Tanmay
We actually don't know what happened at t=0sec but we do know that inflation ended at t=10^-35 and then quarks are formed to produce matter
Tanmay
Astronomers have technology that examines the composition of objects in space; that gives evidence to a theory of the big bang
Simnein
Amongst the science community, a theory, law, or principle is accepted as truth or tought in schools unless solid proof is present. So the big bang, can be accepted as a truth.
Simnein
*Isn't accepted [I meant]
Simnein
I did not get what you said "Isn't accepted (I mean)" You are saying that science community has some solid proof that's why it is accepted but
Tanmay
then you say isn't accepted
Tanmay
I made a typo, just wanted to correct it for Sreeman
Simnein
Nibha still has a question Q- How can I get deeply knowledge about universe?
Tanmay
I did not understand the question can someone answer it or simplify it?
Tanmay
Most astronomers think that the Universe wasformed during an event called the Big Bang - a giant explosion which occurred between 10 and 20 billion years ago. During the Big Bang, all of the space, time, matter, and energy in the Universewas created.
Pixies
It was not an explosion in was expansion of the universe at a tremendous rate
Tanmay
There was no boom sound or shock waves to b propogate
Tanmay
well
Pixies
but for the very first question Nibha asked I recommend an awesome book "Astronomy:a beginner's guide to the universe" a perfect book to have deep knowledge about the cosmos
anita
but the reader must read it seriously as it is not a fun fact book made for kids, it's actual astronomy
anita
Can a class XII student understand it with the physics taught in school?
Tanmay
Is there a digital copy available on internet?
Tanmay
it easy for a 12 class student and I think some websites may provide you with its digital copy. just Google the title with the edition
anita
I guess edition 8 is the latest
anita
Thanks
Tanmay
***researchgate.net/publication/252761966_ASTRONOMY_A_BEGINNER'S_GUIDE_TO_THE_UNIVERSE I just found that page which might help you
anita
what is the difference between astrophysics& astronomy
etsegenet Reply
Astrophysics deals with the physics and interactions of celestial bodies and what their made up of while astronomy deals with the celestial body's beyond our atmosphere like their path around a star.
Andrew
Astronomy deals with what can be seen or measured. Astronophysics uses science to explain our observations
Nick
In what way asteroid is different from meteorite? Do they differ from each other? Anyone, please help me.
letmake
Asteroid: few kilometres wide, can cause massive destruction if entered the earth, majorly found in asteroid belt Meteorites: few meters wide, burn out before reaching Earth's surface, can easily be found in our universe
Tanmay
That is why we have regular annual meteorite showers.
Nick
***earthsky.org/astronomy-essentials/everything-you-need-to-know-eta-aquarid-meteor-shower
Nick
Asteroids are bigger and mostly in orbit between Mars and Jupiter. Meteorites are smaller and we pass through them as we orbit the sun
Nick
which is better for you astronomy or astrophysics?
etsegenet
I like both. To see but also to understand
Nick
why and how does diffraction limited resolution decrease on increasing the diameter of the telescope
anita Reply
somebody plzz answer
anita
I think nobody knows much about telescopes
Tanmay
if we consider that the light entering a scope is a series of waves, that are focused at a point, the eyepiece, then as the light moves away from 'straight in front of the scope, different parts of the wave reach the scope at different times.
paul
some peaks are cancelled out by some troughs. this means less light arrives at the eyepiece in a focused way. The diffraction angle is equal to the wavelenght of the light divided by the diameter of the scope, approximately anyway.
paul
I don't get it
anita
I mean ,what does it do with diffraction limited resolution
anita
.
paul
try this vid. easier to understand with pictures. ***m.youtube.com/watch?v=ZkUn9YSXDVg
paul
ok
anita
thanks
anita
worth looking at his other online courses on edx if this interests you.
paul
Practice Key Terms 2

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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