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Learning objectives

By the end of this section, you will be able to:

  • Describe the structure of the Milky Way Galaxy and how astronomers discovered it
  • Compare theoretical models for the formation of spiral arms in disk galaxies

Astronomers were able to make tremendous progress in mapping the spiral structure of the Milky Way after the discovery of the 21-cm line that comes from cool hydrogen (see Between the Stars: Gas and Dust in Space ). Remember that the obscuring effect of interstellar dust prevents us from seeing stars at large distances in the disk at visible wavelengths. However, radio waves of 21-cm wavelength pass right through the dust, enabling astronomers to detect hydrogen atoms throughout the Galaxy. More recent surveys of the infrared emission from stars in the disk have provided a similar dust-free perspective of our Galaxy’s stellar distribution. Despite all this progress over the past fifty years, we are still just beginning to pin down the precise structure of our Galaxy.

The arms of the milky way

Our radio observations of the disk’s gaseous component indicate that the Galaxy has two major spiral arms that emerge from the bar and several fainter arms and shorter spurs. You can see a recently assembled map of our Galaxy’s arm structure—derived from studies in the infrared—in [link] .

Milky way bar and arms.

Map of the The Milky Way Galaxy. Over-plotted on this data-based illustration of the Milky Way is a coordinate system centered on the Sun, which is located about half way from the center and the bottom of the image. It is a polar coordinate system, with zero degrees straight up from the Sun, 90O to the left, 180O straight down and 270O to the right. Distances are shown as circles of increasing radius centered on the Sun. Distances from 15,000 ly to 75,000 ly are indicated in increments of 5,000 ly. Moving outward from the Sun along the zero degree line are the “Near 3kpc Arm”, “Far 3 kpc Arm” and the “Sagittarius Arm”. Moving outward from the Sun along the 330O line (to the right of zero) are the “Norma Arm” and the “Scutum-Centaurus Arm”. Moving outward from the Sun along the 90O line are are the: “Orion Spur”, “Perseus Arm” and the “Outer Arm”.
Here, we see the Milky Way Galaxy as it would look from above. This image, assembled from data from NASA’s WISE mission, shows that the Milky Way Galaxy has a modest bar in its central regions. Two spiral arms, Scutum-Centaurus and Perseus, emerge from the ends of the bar and wrap around the bulge. The Sagittarius and Outer arms have fewer stars than the other two arms. (credit: modification of work by NASA/JPL-Caltech/R. Hurt (SSC/Caltech))

The Sun is near the inner edge of a short arm called the Orion Spur, which is about 10,000 light-years long and contains such conspicuous features as the Cygnus Rift (the great dark nebula in the summer Milky Way) and the bright Orion Nebula . [link] shows a few other objects that share this small section of the Galaxy with us and are easy to see. Remember, the farther away we try to look from our own arm, the more the dust in the Galaxy builds up and makes it hard to see with visible light.

Orion spur.

The Sun and the Orion Spur. Portions of three spiral arms of the Milky Way are shown in this illustration. The “Cygnus arm” at top, the “Perseus arm” at center and the “Sagittarius arm” at bottom. The “Orion spur” is a stream of stars and gas runs from the Cygnus arm diagonally downward to the right through the Perseus arm and on to the Sagittarius arm. The Sun is located in the portion of the spur between the Perseus and Sagittarius arms. Objects of interest are indicated with arrows from above and below the figure. At top, from left to right are: “Sn2 289”, “Perseus transit”, “Rosetta nebula” and the “Orion nebula”. At bottom, from left to right are: “Turner S”, “Vela molecular ridge”, “Gum nebula”, “Sun”, “Cygnus X-1” and “W51”.
The Sun is located in the Orion Spur, which is a minor spiral arm located between two other arms. In this diagram, the white lines point to some other noteworthy objects that share this feature of the Milky Way Galaxy with the Sun. (credit: modification of work by NASA/JPL-Caltech)

Formation of spiral structure

At the Sun’s distance from its center, the Galaxy does not rotate like a solid wheel or a CD inside your player. Instead, the way individual objects turn around the center of the Galaxy is more like the solar system. Stars, as well as the clouds of gas and dust, obey Kepler’s third law. Objects farther from the center take longer to complete an orbit around the Galaxy than do those closer to the center. In other words, stars (and interstellar matter) in larger orbits in the Galaxy trail behind those in smaller ones. This effect is called differential galactic rotation    .

Questions & Answers

what is siphon
pankaj Reply
what is mama radiation
Pragiti Reply
the time for planet formation must vary with the luminosity of the star? Disks must last much longer around a M-dwarft? please explain how planet formation varies as a function of star luminosity.
Jeffrey Reply
what do you mean by light year
Prabha
A lightyear is an astronomical distance of 5.9 trillion miles.
James
a light year is the speed of light times the total amount of seconds in a year. distance in physics is speed x time, so a light year refers to the total distance the light travels in a year
Levis
thanks for helping me
Prabha
Explain the formation of a black hole?
Prabha
what is atoms
Suman Reply
atoms are the smallest partiles that make the whole universe.For a measurement. if an atom were the size of a ball......The ball would be size of the Earth.
Sangeet
the smallest unit of an universe but it is not true the smallest thing in universe is quarks
Pragiti
The smallest thing in the universe is the arrogance of scientists who think they are never wrong.
Loren
You mean the greatest?
Cyclone
i believe so
daniel
do we are alone in universe
ninja Reply
is the astroid belt shaped like a torus? or like Saturn's disc?
Abhishek Reply
torus
ninja
torus
Robert
can i know deep about black hole
Monisha Reply
yupp ask questions about whatever you wanna know in black hole?
Paras
where does it lead
Vanessa
how black holes are detected in deep space as we are unable to see them.
jawad
if black holes are hypothetical then how do they detect them?
Alvis
who said blackholes are hypothetical?
Ehan
They are detected by their gravitational pull...
Ehan
also we can detect them by X-rays as they emit hawking radiation as they 'shrink'
Levis
hawking radiations are not detected yet...
Ehan
nopes that's not how you detect black holes. Gravity curves space, and light bends around heavy objects. Black holes have very very very high mass. So light bends so much that it cannot pass. When the light of the stars behind suddenly vanishes. => black hole is present in between us and that star.
Abhishek
to find astronomical units do you divide or multiply?
Bethany Reply
I think it depends what you're converting from. i.e. from km you divide
Fab
I tried that for question a in section 0.3 but the answer doesn't seem right
Bethany
make sure you're using consistent units m/m
Fab
i too had the same doubt
Monisha
hi dear
jawad
can you tell me what is ecliptic ?
jawad
the great circle formed by the intersection of the plane of the earth's orbit with the celestial sphere; 
Monisha
OK thanks
jawad
who is the father of Astronomy?
subhajit Reply
Me ;)
daniel
I think Galileo
Prabha
Copernicus, Nicolaus.....
Adam
which telescope is best for beginners ?
jawad
Do You Want Low Maintenance Or High Maintenance Optics? Do You Want To Look At Solar System Objects Or Deep Space Objects? Or May Be You Want To Split Doubles, Do You Live In A Big City Or Suburbs?
Adam
I want low maintenance. I want to see deep Space objects. I live in big light polluted city.
jawad
sir I have Nikon 16*50 binocular but now I want to buy a telescope.
jawad
i have got money that i earn with i am a doctor that working at st.louis jewish Centre.i am very helpful for people that they need help with.
Asali
What's Your Budget?
Adam
Deep Space Objects Are Gonna Hard To Come By In A Light Polluted City..... So Far I Would Recommend 8 To 10 Inch Schmidt Cassegrain Telescope, For Deep Space Objects You Need Light Gathering Power, Now A Decent Scope Like What I Just Described Is Gonna Run You About $800 To $1000 Dollars.....
Adam
And That's Just The OTA, Optical Tube Assembly..... Then We Gotta Talk Mount And Tripod, And For Good Stable And Steady Views We Need About Another $1000 Dollars For Mount And Tripod.....
Adam
my budget is almost 1000 dollars.
jawad
my budget is almost $1156.00 dollars each years
Asali
big diameter and lenth are good, i got my first telescope for $300 and it works real well for planets.
daniel
if you want to do amature astrophotography, what you can do is mount a webcam or your phone to your telescope and have a tracking mount attached so you can look for different deep space objects and take pictures. I have a couple I did using this method
Mark
A Big Dobsonian Might Be In Order, But That Scope Is Very High Maintenance..... But For $1000 Dollars, You Can Get A Real Nice Dobsonian And They Perform Excellent On Deep Space Objects, The Darker Your Observing Location Is The Better.....
Adam
guys this is my first day of i arrived in the u.s i need your help guys what does telescope means
Asali
so, a telescope is a thing you look through to see things far awY
Mark
A Tool Used To Observe The Sky.....
Adam
thank you guys for you help me i appreciate with that i will always thank you guys
Asali
what does physiognomys mean?
Asali
what happen if earth stop to rotate?
Sijan Reply
well first of all we wouldn't have day and night and it would get cold in the dark side humans and animals would have to get use to the darkness. plant would not he able to grow on the dark side either. Ad for the light side or "day" its the same thing but just...hot.
Ghost
and contestant datlight such could be harmful beacuase of the constant if rays
Ghost
Our gravitational field would collapse.
Hannah
Magnetic field sry. That means no shielding from cosmic rays.
Hannah
actually we would have day and night...but each day would last a year. We would also still have a magnetosphere unless the Earth's core also stopped rotating. Gravity would increase as we would lose any centripetal effect. We would only have one tide per 29 days as the moon orbits us.
Joe
we would still have gravity Hannah because we would still have mass!
Joe
pretty sure there would be no more rain or wind because any warm air and water vapour would just rise and fall above wherever it came from.
Joe
As we are rotating right now, if the earth suddenly stopped, objects that are not bound to the ground will get flung due east at very high speeds. It would be very catastrophic. That's the first thing that'll happen if such an event were to happen.
Cave
what is comets?
Ido Reply
the bottom of the page of chapter 13 thinking ahead describe it
Chelsea
I also don't know but I think it is star
Sijan
A comet is composed of small rocks and dust particles cemented together by frozen gas and ice.
shiela
and when the ice and the cold stuff on a comet heats up, by the sun as it closes by, some matter escape and we see a tail.
Abhishek
I once watched a show discussing finding dark matter ON planet earth, deep down inside caverns where sound and light are absent. I seem to remember them also saying that the slightest of sound would "charge" the matter up. If the solar system is primarily dark matter, could we use sound to travel?
Jamie Reply
I thought the same thing. If space is a vacuum and sound does not travel in a vacuum, it would be difficult?
James
What exactly is dark matter?
Sharadhi
from what i gathered, dark matter is incredibly motionless atoms. Almost dormant, and they are easily disturbed? i am by NO MEANS an expert on any of this. just a curious mind... if sound doesnt travel in a vacuum, maybe the light then? disturb the matter with light and maybe 'swim' it?
Jamie
yes, the sound can be converted as an electrical impulse, which can be converted to light. and if we can detect this light at the receiving end then we can convert it back to sound but only if a material medium is present for the sound to travel to our ears.
Abhishek
what are whitehole a wormholes
gajanan Reply
holes
xBeex
***en.m.wikipedia.org/wiki/White_hole ***en.m.wikipedia.org/wiki/Wormhole
xBeex
wormholes are those holes through which we can travel to past
ninja
is the reason why you can't see the carp nebula in some towns and city's due to other lights?
Ghost Reply
yes and the atmosphere fades the dimmer lights as well. and you can only see if the carp nebula emits visible light. otherwise you'll need appropriate detectors to detect the frequency.
Abhishek
in cities there are levels of light pollution which stop light from stars, nebulas, etc. also some nebulas are brighter than other's, but rural area are good places to see the sky with little light pollution.
daniel
Which of the following transformations is (are) fusion and which is (are) fission: helium to carbon, carbon to iron, uranium to lead, boron to carbon, oxygen to neon? (See Appendix K for a list of the elements.)
Nanea Reply
fission refers to conversion of helium into two hydrogen atoms fusion is combination of hydrogen atoms to helium
Aditya
but acc to the question helium to carbon is fission after hydrogen
Aditya
how is blackhole formed?
Prabha
When the center of a massive star collapses in on itself.
Radonwolf
collapse?
basim
is it exploding?
basim
what does ekg echocardiogram means?
Asali
The star does explode.
Radonwolf
what does gravity means?
Asali
it means to help people stay on ground but in space it makes things float around that would mean with out gravity we would be on cellings or even in space
Olivia
can you guys help me because i needs to know a lot about science i have a question that i would like to ask you. my question is what does conduction,convection,insulator,and thermal radiotion means?
Asali
conduction is when a source for some type of energy let's energy flow into things around it. convection is what happens with boiling water. currents of hot water form and heat the surrounding water. this is convection. an insulator is something that doesn't let energy flow into or out of it very wel
Mark
like rubber or glass. thermal radiation is heat energy being transferred to the surroundings
Mark
can you have heat feom convection?
Asali
convection is a way to transfer thermal energy and other types so yes.
Mark
and the definition of heat is the process of transferring thermal energy into or out of a closed system
Mark
id two object reffer to the other object and than what will happen?
Asali
what do you mean
Mark
what does newton third law of motion said?
Asali
i mean like when two object reffer to the other object what will happen to both of them?
Asali
it is understand by zeroth law of thermodynamics
Arun
what is the zeroth law anyway
Olivia
of two systems are in equilibrium with a third, then they're in equilibrium with each other
Mark
I think it was called the Zeroth law bc scientists discovered the first and the second law before it but they still thought they needed to state the obvious beforehand
Levis
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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