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Learning objectives

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

  • Distinguish between population I and population II stars according to their locations, motions, heavy-element abundances, and ages
  • Explain why the oldest stars in the Galaxy are poor in elements heavier than hydrogen and helium, while stars like the Sun and even younger stars are typically richer in these heavy elements

In the first section of his chapter, we described the thin disk, thick disk, and stellar halo. Look back at [link] and note some of the patterns. Young stars lie in the thin disk, are rich in metals, and orbit the Galaxy’s center at high speed. The stars in the halo are old, have low abundances of elements heavier than hydrogen and helium, and have highly elliptical orbits randomly oriented in direction (see [link] ). Halo stars can plunge through the disk and central bulge, but they spend most of their time far above or below the plane of the Galaxy. The stars in the thick disk are intermediate between these two extremes. Let’s first see why age and heavier-element abundance are correlated and then see what these correlations tell us about the origin of our Galaxy.

How objects orbit the galaxy.

Orbital Motions in the Milky Way. In panel (a), at top and labeled “Thin Disk”, shows the orbits of stars as blue concentric ellipses centered on a + sign indicating the galactic center. The orbits are in the same plane, labeled “Galactic plane”. In panel (b), at bottom and labeled: “Halo”, shows the orbits of stars as blue ellipses of many different sizes and orientations extending above and below the galactic plane and centered on a + sign indicating the galactic center.
(a) In this image, you see stars in the thin disk of our Galaxy in nearly circular orbits. (b) In this image, you see the motion of stars in the Galaxy’s halo in randomly oriented and elliptical orbits.

Two kinds of stars

The discovery that there are two different kinds of stars was first made by Walter Baade during World War II. As a German national, Baade was not allowed to do war research as many other U.S.-based scientists were doing, so he was able to make regular use of the Mount Wilson telescopes in southern California. His observations were aided by the darker skies that resulted from the wartime blackout of Los Angeles.

Among the things a large telescope and dark skies enabled Baade to examine carefully were other galaxies—neighbors of our Milky Way Galaxy. We will discuss other galaxies in the next chapter ( Galaxies ), but for now we will just mention that the nearest Galaxy that resembles our own (with a similar disk and spiral structure) is often called the Andromeda galaxy , after the constellation in which we find it.

Baade was impressed by the similarity of the mainly reddish stars in the Andromeda galaxy’s nuclear bulge to those in our Galaxy’s globular clusters and the halo. He also noted the difference in color between all these and the bluer stars found in the spiral arms near the Sun ( [link] ). On this basis, he called the bright blue stars in the spiral arms population I and all the stars in the halo and globular clusters population II .

Andromeda galaxy (m31).

Visible Light Image of the Andromeda Galaxy (M31). The nearest large spiral galaxy to the Milky Way is slightly tilted from edge-on, allowing us to view the blue spiral arms as well as strong dust lanes that block some of the light from the central bulge.
This neighboring spiral looks similar to our own Galaxy in that it is a disk galaxy with a central bulge. Note the bulge of older, yellowish stars in the center, the bluer and younger stars in the outer regions, and the dust in the disk that blocks some of the light from the bulge. (credit: Adam Evans)

We now know that the populations differ not only in their locations in the Galaxy, but also in their chemical composition, age, and orbital motions around the center of the Galaxy. Population I stars are found only in the disk and follow nearly circular orbits around the galactic center. Examples are bright supergiant stars, main-sequence stars of high luminosity (spectral classes O and B), which are concentrated in the spiral arms, and members of young open star clusters. Interstellar matter and molecular clouds are found in the same places as population I stars.

Questions & Answers

Currently, I'm studying in 10th grade. What should I do after my 10th so that I can get an astronomy career?
Chaya Reply
try jee exam and achieve a good score in jee advance to join in IIST(Space research institute)
okay.. thanks
this famous person was the first to recognise earthshine on the moin
Rajan Reply
Leonardo Da Vinci.....
what is string theory?
sakshi Reply
A Cosmological Theory Based On The Exsistence Of Cosmic Vibrating Strings.....
Does MIT have good astrophysics courses ?
priyanshu Reply
Probably The Best In The United States.....
But Don't Get Caught Up On Who's The Best, You Can Be The Next Noble Winning Astrophysicist.....
hmmm..........that's nice one....Adam had said.....
How did ASTRONOMY helped you understand yourself ? 🤔
Hussain Reply
why do you want to study ASTRONOMY?
Hussain Reply
It is one of the natural sciences and therefore worth exploring. You are a part of the universe and it is a part of you. The sky is my classroom. (student of cosmology, Oxford Uni)
because everything in the space is just beautiful...interesting and worth studying and exploring...
I have been studying astrophysics an love it
I love to learn what makes up our lives an heavens an how it works there us so much more out there than books an our veiw of thw heavens can reveal
How did ASTRONOMY helped you understand yourself ?
Because it proves the impossibles and the limitless.
It helped me realize that there are like countless possibilities in life and get all through it.
It honestly showed me to view the world in a balanced way. Because space beautiful and calm yet violent, and so is the world, and we should still help, but there will always be violent people much like violent astrophysical jets.
Does anyone know where can I study astronomy in Spain? My budget is too low for traveling out there...
Valerian Reply
How long the duration was when the laws of physics became active after the big bang?And why the time is called plank's time?
Mostak Reply
how many galaxies in universe?
Shaikh Reply
10 to the power 11
if u can count all the sands on the beaches and on the deserts then u will know how much it will be.......some said more than 1,000,000,000 galaxies are there in our universe...
it's ok bro
I hope you will become a member of nasa
just new in this area, from art background not science
what's ur name wwe
The Observable Universe Contains Between 200 Billion To 2 Trillion Galaxies.....
are u sure?
It actually hasn't been proven, people have just made estimations.
the thing is u can't count,maybe the our number system will collapse there...who knows?
What's the exact time when the laws of physics became active after the big bang ?
Actually we still don't know the exact number of galaxies in the universe, since the universe is bigger than we can imagine and it is still expanding even bigger today than yesterday
we didn't actually find the exact number but may be it is around 100 billion
why this universe is expanding?
Science Does Not Concern It Self With Why Questions, Science Is Geared For How And What Questions.....
Why Questions Form And Endless Loop.....
According to the universal law of cause and effect .any phenomenon that occur in this universe should have any reason , nothing happen without any reason.
Can you tell me the expanding speed of this universe.
yes that would be a great question to have answered what is the expanding speed of the universe?
Remember that most of what you can see is light years away. We're looking back in time, we can never know the current nature of our observations, only glimpse the galaxies and stars as they once were. Strange but true, and a little sad.
Correct, That's Why Telescopes Are Like Time Machines, You See The Past.....
really telescope can see the past?....i meant everything is in its place, though it rotates or revolve or whatever it is.......but how can this telescope can see the past.....can we see how our Universe was created,how it formed out of it? thats my question to all of you guys can you plz tell me.
Because We Dont See Any Object In Space As It Is, We See It As It Was..... Hope This Clears That Up For You.....
And One Day We Might Have A Telescope That Will Show Us The Big Bang.....
ok....... that will be nice one
ok....... that will be nice one
Travel at light speed in a spaceship at 186,000 miles every SECOND. At this speed it will take you 100,000 years just to cross our Milky Way galaxy, which is just a dot in the sky. Next stop, the Andromeda galaxy, after 2.5 million years, still in the Local Group, Virgo Supercluster, still a dot.
Pretty Much.....
there are nearly 200 million I suppose.
hi i need a telescope would you help mee
Nauman Reply
i can only advice you to go to some museum
wher is it?
but before that.......... i wanna know where are you from and from which state...and if you are nearer to that of the museum or will be able to go then.,.. i donot have any more words.
What Do Plan To Observe?
sorry . i am from asia
Check on amazon.Celeston telescopes are good for beginners.
can yoy introduce your self
can you
Asia...? from which country you belong into ...,,is it India or where?
Do You Want A Telescope With ALot Of Maintenance Or Zero Maintenance?
with alot offf
Any second hand online stores in your area? great for unused Christmas gifts etc..
So You Want A Newtonian Reflector?
hey guys I wanna ask you all that will it be safe or helpful to communicate with an intelligent civilization like are we safe ?
Is the Orion Sky Quest XT6 a good Dobsonian?
Look for a Sky Watcher 130BKS, it has an upgraded dual speed 2" focuser. You can use 1.25" or 2" eyepieces with it and it is quite easy to pollinate using a simple visual collimating eyepiece. This scope is great and ready for prime focus astrophotography if you choose to go that route.
Sorry, spell check error. Pollinate should be collimate. lol
Dobsonians Require A Lot Of Maintenance, If You Don't Mind Cleaning And Collimating On A Regular Basis, Then Go Ahead A Dobsonian Puts Up Excellent Views Especially For Deep Space Objects..... Now If You Don't Want Constant Maintenance Then A Schmidt Cassegrain Might Be A Better Fit For You.....
So what i would put for that answer ?
Abdulla Reply
The largest observatory complex in the world is on Mauna Kea, the tallest mountain on Earth. Whatare some factors astronomers consider when selecting an observatory site? Don’t forget practical ones.Should astronomers, for example, consider building an observatory on Denali (Mount McKinley) or MountEverest?
Abdulla Reply
i need help on the the second part
Should astronomers for example considered building an observatory on Denali (Mount Mckinley) or MountEverest ?
I would take into consideration. climate weather pattern.
also, could an establishment be built and be able to sustain years of being beaten by the weather?
so far Mount Everest to my understanding is a difficult mountain
it is because of light pollution in the cities that the observatories are situated in extremely remote areas.
Everest ist the tallest mountain
how far us milky way galaxy?
ShowsSpy Reply
oh man
We are part of the Milky Way Galaxy. We are within it, so it's not really away from us. Our galaxy is 100,000 light years in diameter. The nearest big galaxy is Andromeda, around 2 million lightyears away.
its 200,000 light years
Where is Rice University located?
Amanda Reply
it is in your vagina
that couldn't be any more wrong 😂😂
what even is this conversation😂😂!?
Show Some Class.....
Houston, Texas.....
what is the first step to take as far as getting into the field of astronomy?
john Reply
probably by being type 1or2 civilization...launching more telescopes into space...and stepping on mars. probably.
John, the first step is to want to focus a large part of your life studying, researching and investigating the universe... Including the planet we know the most about. Do well in math, physics, and chemistry, and attend a college that either owns astronomical equipment or facilities and has courses.
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