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The Stars Are Yours was the title of a popular astronomy book written by James S. Pickering in 1948.
The point of the title is that the stars belong to everyone equally—and you can enjoy the wonders of the night sky as if you owned it.

You live inside one of the largest of the star systems that fill the universe.
Milky Way is over 80,000 ly in diameter and contains over 100 billion stars.

As you read this chapter, you will be learning about our home galaxy.
You will also be learning how the stars of the galaxy have cooked up the atoms heavier than helium.

You know already how the cores of massive stars make the heavier atoms up to atomic number 26, that is, iron.
You also know how supernovae blast those atoms back into space and add even heavier atoms made only during the supernova explosion.

You will see how the stars in Milky Way have, generation after generation, made the atoms in your body.

It seems odd to say astronomers discovered something that is all around.
However, until the early 20th century, no one knew what Milky Way was.

The Discovery of the Galaxy

It isn’t obvious that we live in a galaxy.
We are inside.
So, we see nearby
stars scattered all
over the sky—whereas
the more distant clouds
of stars in the galaxy
make a faint band of
light circling the sky.

The Discovery of the Galaxy

The ancient Greeks named the band galaxies kuklos—the “milky circle.”
The Romans changed the name to via lactea—“milky road” or “milky way.”

The Discovery of the Galaxy

bigger

It was not until early in the 20th century that astronomers understood that they lived inside a great wheel of stars and that the universe is filled with other such star systems.
Drawing on the Greek word for milk, they called them galaxies.

The Discovery of the Galaxy

Galileo’s telescope revealed that the glowing Milky Way was made up of stars.
Later astronomers realized that the great cloud of stars in which the sun is located, which they called the star system, must be wheel-shaped.

The Great Star System

If the star system were spherical, for example, you would see stars scattered more or less uniformly in all directions over the sky.
Only a wheel or disk shape would look, from the inside, like the Milky Way band encircling the sky.

The Great Star System

The English astronomer Sir William Herschel (1738–1822) and his sister Caroline Herschel (1750–1848) attempted to gauge the true shape of the star system.
They did this by counting stars in 683 different directions in the sky.
Where they saw more stars, they assumed the star system extended further into space.

The Great Star System

They concluded that the star system has a disk shape with some noticeable “holes” lacking stars around its edges.

The Great Star System

Modern astronomers know that those apparent holes are caused by dense clouds of gas and dust that block the view of more distant stars.
The Herschels counted similar numbers of stars in most directions around Milky Way and concluded that the sun and Earth are near the center of the star system.

The Great Star System

As the 20th century began, astronomers still believed that the sun was located near the center of a wheel-shaped star system that they estimated was about 15,000 ly in diameter.

The Great Star System

How humanity realized the truth about the size of the galaxy, Earth’s location in it,
and the galaxy’s location in a much larger universe of other galaxies is quite an adventure.
It begins with a woman studying stars that pulsate and leads to a man studying star clusters.

The Great Star System

The next two sections will follow that story.
It is an important historical moment in human history.
It illustrates how scientists build on the work of their predecessors and step-by-step refine their ideas about the natural world.

The Great Star System

It is a common misconception of poets that the stars are eternal and unchanging.
Astronomers have known for centuries that some stars change in brightness.
Of course, novae and supernovae burst into view, grow brighter, and then fade.
However, many other variable stars actually pulsate like beating hearts.

The Size of Milky Way

The period of pulsation is the time it takes for a star to complete a cycle from bright to faint to bright again.

The Size of Milky Way

Next, you will learn:
How the properties of some types of variable stars allowed astronomers to measure the size
of the galaxy

The Size of Milky Way

In 1912, Henrietta Leavitt (1868–1921) was studying a star cloud in the southern sky known as the Small Magellanic Cloud.
On her photographic plates, she found many variable stars.
She noticed that the brightest had the longest periods.

The Size of Milky Way

However, all the variables were in the same cloud at nearly the same distance.
She concluded that there was a relationship between the pulsation periods and luminosities (true total power output) of those variable stars.

The Size of Milky Way

The stars Leavitt saw—Cepheid variable stars—are named after the first such star discovered, δ Cephei.

The Size of Milky Way

They are giant and supergiant stars.
They have a pulsation period of 1 to 60 days and lie near the top (the high-luminosity end) of a region of the H-R diagram known as the instability strip.

The Size of Milky Way

As stars evolve, and the points that represent their temperatures and luminosities move in the H-R diagram, they can cross into the instability strip and start pulsating.
They stop pulsating when they evolve out of the strip.

The Size of Milky Way

Massive stars are larger and pulsate slower—just as large bells vibrate slower and have deeper tones.

The Size of Milky Way

Lower mass stars are less luminous and, being smaller, pulsate faster.
This explains why, as first noticed by Leavitt, the long-period Cepheids are more luminous than the short-period Cepheids.

The Size of Milky Way

That is known as the period-luminosity relation.
You may be interested
to learn that the North
Star, Polaris, is a
Cepheid variable with
a pulsation period of
4 days.

The Size of Milky Way

A young astronomer named Harlow Shapley (1885–1972) began the discovery of the true nature of Milky Way.
He noticed that different kinds of star clusters have different distributions in the sky.

Star Clusters and the Center of the Galaxy

You have learned about two types of star clusters.
Open clusters which are concentrated along Milky Way.
Globular clusters which are widely scattered.

Star Clusters and the Center of the Galaxy

Shapley noticed that the globular clusters were more common toward the constellations Sagittarius and Scorpius.

Star Clusters and the Center of the Galaxy

Shapley assumed that this great cloud of globular clusters was controlled by the combined gravitational field of the entire star system.

Star Clusters and the Center of the Galaxy

If so, he could study the size and extent of the galaxy by studying the globular clusters.
To do that, he needed to measure the distances to as many globular clusters as possible.

Star Clusters and the Center of the Galaxy

Globular clusters are much too far away to have measurable parallaxes.
They do, however, contain variable stars.

Star Clusters and the Center of the Galaxy

Shapley knew of Leavitt’s work on these stars.
However, it depended on their relative, rather than true, luminosities.
Cepheids are relatively rare.
Also, there are none near enough to have measurable parallaxes.
So, their true luminosities were not then known.

Star Clusters and the Center of the Galaxy

Shapley knew that he could calculate the distance to the globular clusters if he found the true luminosities of the Cepheid variable stars in the clusters.

Star Clusters and the Center of the Galaxy

Shapley used a statistical process involving measurements of position shifts due to their motions through space—called proper motions.
Thus, he was able to find the average distances of a few of the nearest Cepheids.
From that, he found their average luminosities.

Star Clusters and the Center of the Galaxy

That meant he could replace Leavitt’s apparent magnitudes with absolute magnitudes on the period–luminosity diagram.
Astronomers say that Shapley
calibrated the variable stars
for distance determination.

Star Clusters and the Center of the Galaxy

Finally, having calibrated the Cepheids, he could find the distance to the globular clusters.
He could identify the variable stars in the clusters, and could find their apparent magnitude from his photographs.
The comparison of apparent and absolute magnitude gave him the distance to the star cluster based on the inverse square law for light.

Star Clusters and the Center of the Galaxy

Shapley finally plotted the direction and distance to the globular clusters.
He found that they form a great swarm that is not centered on the sun.

Star Clusters and the Center of the Galaxy

Instead, the center of the swarm of clusters lies many thousands of light-years in the direction of Sagittarius.

Star Clusters and the Center of the Galaxy

Consider the analogy of a bunch of tall buildings that are all together in one direction away from you.
You might conclude that downtown is over there, and you are in the suburbs.
Evidently, the center was in Sagittarius—and it was
very far away.
The star system was much bigger than anyone had suspected.

Star Clusters and the Center of the Galaxy

You live not at the center of a small star system, but in the suburbs of a very big wheel of stars, a galaxy.

Star Clusters and the Center of the Galaxy

Why did astronomers before Shapley think they lived near the center of a small star system?
Space is filled with gas and dust that dim the view of distant stars.
When you look toward the band of Milky Way, you can see only the neighborhood near the sun.
Most of the star system is invisible and, like travelers in a fog, you seem to be at the center of a small region.

Star Clusters and the Center of the Galaxy

When you look toward the band of Milky Way, you can see only the neighborhood near the sun.
Most of the star system is invisible and, like travelers in a fog, you seem to be at the center of a small region.

Star Clusters and the Center of the Galaxy

Shapley was able to see the globular clusters at greater distances.
They lie outside the Milky Way plane and are
not dimmed very much by the interstellar dust.

Star Clusters and the Center of the Galaxy

Building on Shapley’s work, other astronomers began to suspect that some of the faint patches of light visible through telescopes were other galaxies like our own.

Star Clusters and the Center of the Galaxy

However, in 1923, Edwin Hubble photographed individual stars in the Andromeda Galaxy and, in 1924, he identified Cepheids there.
This allowed its distance to be estimated.
As a result, it became clear that our galaxy is just one in a universe filled with galaxies.

Star Clusters and the Center of the Galaxy

Our galaxy, like many others, contains two primary components—a disk and a sphere.

An Analysis of the Galaxy

The disk component consists of all matter confined to the plane of rotation—that is, everything in the disk itself.
This includes stars, open star clusters, and nearly all
the galaxy’s gas and dust.

An Analysis of the Galaxy

As the disk contains lots of gas and dust, it is the site of most of the star formation in the galaxy.
So, it is illuminated by recently formed brilliant, blue, massive stars and has an overall relatively blue color.

An Analysis of the Galaxy

The diameter of the disk and the position of the sun are also difficult to determine.
Gas and dust block the view in the plane of the galaxy—so, astronomers cannot see to the center or to the edge easily.
Also, the outer edge of the disk is not well defined.

An Analysis of the Galaxy

The best studies
suggest the sun is
about 8.5 kpc from the center.
1 kpc is a kiloparsec, or 1,000 pc.

An Analysis of the Galaxy

The sun and Earth seem to be about two-thirds of the way from the center to the edge.
So, the diameter of
our galaxy appears to be
about 25 kpc or about
80,000 ly.
However, that isn’t known
to better than 10 percent
precision.

An Analysis of the Galaxy

That is the diameter of the luminous part of the galaxy—the part you would see from a distance.
You will learn later that strong evidence shows that our galaxy is much larger than this—but that the outer parts are not luminous.

An Analysis of the Galaxy

Observations made at other wavelengths can also help astronomers see through the dust and gas.
Infrared photons have wavelengths long enough
to be unaffected by the dust.

An Analysis of the Galaxy

Thus, a map of the sky at long infrared wavelengths reveals the disk of the galaxy.

An Analysis of the Galaxy

The most striking features of the disk component are the spiral arms—long curves of bright stars, star clusters, gas, and dust.
Such spiral arms are easily
visible in other galaxies.
Also, you will see later how
astronomers found that our
own galaxy has a spiral
pattern.

An Analysis of the Galaxy

The second component of the galaxy is the spherical component.
This includes all matter
in the galaxy scattered in
a roughly spherical
distribution around
the center.
This includes a
large halo and
the nuclear bulge.

An Analysis of the Galaxy

The halo is a spherical cloud of thinly scattered stars and globular star clusters.
It contains only about 2 percent as many stars as the disk of the galaxy and has very little gas and dust.
Thus, with no raw
material available,
no new stars are
forming in the
halo.

An Analysis of the Galaxy

In fact, the halo stars are mostly old, cool giants or dim lower-main sequence stars.

An Analysis of the Galaxy

Recent careful studies reveal that they are old white dwarfs that are difficult to detect.
Nevertheless, astronomers can map the halo of the galaxy by studying the more easily detected giant stars.

An Analysis of the Galaxy

The central bulge is the dense cloud of stars that surrounds the center of the galaxy.
It has a radius of about
2 kpc and is slightly flattened.

An Analysis of the Galaxy

It is hard to observe because thick dust in the disk scatters and absorbs radiation of visible wavelengths.
However, observations at longer wavelengths can penetrate the dust.

An Analysis of the Galaxy

The bulge seems to contain little gas and dust.
Thus, there is little star
formation.
Most of the stars in the nuclear bulge are old, cool stars—like those in the halo.

An Analysis of the Galaxy

The vast numbers of stars in the disk, halo, and nuclear bulge lead to a basic question.
How massive is the galaxy?

The Mass of the Galaxy

When you needed to find the masses of stars, you studied the orbital motions of the pairs of stars in binary systems.
To find the mass of the galaxy, you must look at the orbital motions of the stars within the galaxy.

The Mass of the Galaxy

Every star in the galaxy follows an orbit around the center of mass of the galaxy.
In the disk, the stars follow parallel, circular orbits.
Astronomers say the disk of the galaxy rotates.

The Mass of the Galaxy

That rotation can allow you an estimate of the mass of the galaxy.
So, any discussion of the mass of the galaxy is also a discussion of the rotation of the galaxy and the orbits of the stars within the galaxy.

The Mass of the Galaxy

Astronomers can find the orbits of stars by finding how they move.
Of course, the Doppler effect reveals a star’s radial velocity.

The Mass of the Galaxy

In addition, if astronomers can measure the distance to a star and its proper motion, they can find the velocity of the star perpendicular to the radial direction.
Combining all this information, astronomers can find the shape of the star’s orbit.

The Mass of the Galaxy

The orbital motions of the stars in the halo are strikingly different from those in the disk.
In the halo, each star and globular cluster follows its own randomly tipped elliptical orbit.

The Mass of the Galaxy

These orbits carry the stars and clusters far out into the spherical halo—where they move slowly.
When they fall back into the inner part of the galaxy, though, their velocities increase.

The Mass of the Galaxy

Motions in the halo do not resemble a general rotation.
They are more like the random movements of a swarm of bees.

The Mass of the Galaxy

In contrast, the stars in the disk of the galaxy move in the same direction in nearly circular orbits that lie in the plane of the galaxy.
The sun is a disk star and follows a nearly circular orbit around the galaxy that always remains within the disk.

The Mass of the Galaxy

You can use the sun’s orbital motion to find the mass of the galaxy inside the sun’s orbit.
By observing the radial velocity of other galaxies around the sky, astronomers can tell that the sun moves about 220 km/s in the direction of Cygnus.
It carries Earth and the other planets of our solar system along with it.

The Mass of the Galaxy

As its orbit is a circle with a radius of 8 kpc, you can divide the circumference of the orbit by the velocity.

You will find that the sun completes a single orbit in about 220 million years.

The Mass of the Galaxy

If you think of the sun and the center of mass of our galaxy as two objects orbiting each other, you can find the mass of the galaxy.
Milky Way must have a mass of about 100 billion solar masses.

The Mass of the Galaxy

This estimate is uncertain for various reasons.
One, you don’t know the radius of the sun’s orbit with great certainty.
Astronomers estimate the radius as 8.0 kpc—but they could be wrong by 10 percent or more.
This radius gets cubed in the calculation—which has a large effect.

The Mass of the Galaxy

Also, the estimate includes only the mass inside the sun’s orbit.
Mass spread uniformly outside the sun’s orbit will not affect its orbital motion.
Thus, 100 billion solar masses is a lower limit for the mass of the galaxy.
No one knows exactly how much to increase the estimate in order to include the rest of the galaxy that lies outside the sun’s orbit.

The Mass of the Galaxy

The measured motion of the stars shows that the disk does not rotate as a solid body.
Each star follows its own orbit.
Stars in some regions have shorter or longer orbital periods than the sun.
This is called differential rotation.

The Mass of the Galaxy

A graph of the orbital velocity of stars at various orbital radii in the galaxy is called a rotation curve.
If all the mass in the galaxy were concentrated at
its center, then orbital velocity would be high near the center and
would decline
away from the
center.

The Mass of the Galaxy

This kind of motion has been called Keplerian motion.
It follows Kepler’s third law—as in the case of our solar system, where nearly all the mass is in the sun.

The Mass of the Galaxy

Of course, the galaxy’s mass is not all concentrated at its center.
Nevertheless, if most of the mass is inside the orbit of the sun, then orbital velocities should decline at greater distances.

The Mass of the Galaxy

Many observations confirm, however, that velocities do not decline and may actually increase at greater distance.
This observation shows that larger and larger orbits enclose more and more mass.

The Mass of the Galaxy

It is difficult to determine a precise edge to the visible galaxy.
However, it seems clear that large amounts of matter are located beyond what seems to be the limit of the galaxy’s luminous matter.

The Mass of the Galaxy

The evidence is clear that extra mass lies in an extended halo sometimes called a dark halo.
It may extend up to 10 times farther than the edge of the visible disk and could contain up to two trillion solar masses.
Some small fraction of this mass is made up of low-luminosity stars and white dwarfs.
Most of the matter, though, is not producing light.

The Mass of the Galaxy

Astronomers call it dark matter and conclude that it must be some as yet unknown form of matter.
It is one of the fundamental problems of
modern astronomy.

The Mass of the Galaxy

The most striking feature of galaxies like Milky Way is the system of spiral arms that wind outward through the disk.
These arms contain
swarms of hot, blue stars,
clouds of dust and gas,
and clusters of young stars.
These young objects hint
that the spiral arms involve
star formation.

Spiral Arms and Star Formation

As you try to understand the spiral arms, you face two problems.
How can you be sure the galaxy has spiral arms when you view is obscured by gas and dust?
Why doesn’t the differential rotation of the galaxy destroy the arms?
The answer to both questions involve star formation.

Spiral Arms and Star Formation

Studies of other galaxies show that spiral arms contain hot, blue stars.
Thus, one way to
study the spiral
arms of Milky Way
is to locate these
stars.

Tracing the Spiral Arms

Fortunately, this is not difficult, since O and B stars are often found in associations.
Being very luminous, they are easy to detect across great distances.

Tracing the Spiral Arms

Unfortunately, at these great distances, their parallax is too small to measure.
So, their distances must be found by other means—usually by spectroscopic parallax.

Tracing the Spiral Arms

The O and B associations in the sky are not located randomly but reveal parts of three spiral arms near the sun.
These have been
named for the
prominent
constellations
through which
they pass.

Tracing the Spiral Arms

Objects used to map spiral arms are called spiral tracers.
O and B associations are good spiral tracers because they are bright and easy to see at great distances.
Other tracers include young open clusters, clouds of hydrogen ionized by hot stars (emission nebulae), and certain higher-mass variable stars.

Tracing the Spiral Arms

All spiral tracers are young objects.
O stars, for example, live for only a few million years.
If their orbital velocity is about 250 km/s, they cannot have moved more than about 500 pc since they formed—less than the width of a spiral arm.
As they don’t live long enough to move away from the spiral arms, they must have formed there.

Tracing the Spiral Arms

The youth of spiral tracers
provides an important clue
about spiral arms.
Somehow, they are associated with
star formation.

Tracing the Spiral Arms

Radio telescopes can detect emission from clouds of cool neutral hydrogen gas.
Radio maps of the galaxy disk, combined with optical and infrared data, allow astronomers to deduce the spiral pattern of our galaxy.

Tracing the Spiral Arms

The segments near the sun are part of a spiral pattern that continues throughout the disk.
However, the maps show that the spiral arms are rather irregular and are interrupted by branches, spurs, and gaps.
The stars in Orion, for example, appear to be a detached segment of a spiral arm—a spur.

Tracing the Spiral Arms

There are significant sources of error in the mapping method.
However, many of the irregularities along the arms seem real.
Images of nearby spiral galaxies show similar features.

Tracing the Spiral Arms

Studies comparing all the available data on our galaxy’s spiral pattern with patterns seen in other galaxies do not necessarily agree.

Tracing the Spiral Arms

The newest models suggest that
the nuclear bulge in our galaxy is elongated into a bar.

Tracing the Spiral Arms

Spiral tracers show that the arms contain young objects.
That suggests active star formation.
Radio maps confirm this suspicion by showing that the material needed to make stars is abundant in spiral arms.

Tracing the Spiral Arms

Having mapped the spiral pattern, you can ask, “What are spiral arms?”
You can be sure that they are not physically connected structures.
Like a kite string caught on a spinning hubcap, such arms would be wound up and pulled apart by differential rotation within a few tens of millions of years.
Yet, spiral arms are common in disk-shaped galaxies and must be reasonably permanent features.

Star Formation in Spiral Arms

The most prominent theory about spiral arms is called the density wave theory.
It proposes that spiral arms are waves of compression that move around the galaxy, triggering star formation.

Star Formation in Spiral Arms

The density wave is a bit like a traffic jam behind a truck moving slowly along a highway.
Seen from an airplane overhead, the jam seems a permanent, though slow-moving, feature.
However, individual cars overtake the jam from behind, slow down, move up through the jam, wait their turn, pass the truck, and resume speed along the highway.

Star Formation in Spiral Arms

Similarly, clouds of gas overtake the spiral density wave and become compressed in the “traffic jam.”

Eventually, they move out in front of the arm, leaving the slower-moving density wave behind.

Star Formation in Spiral Arms

As you would expect, star formation will occur where the gas clouds are compressed.
Stars pass through the spiral arms unaffected—like bullets passing through a wisp of fog.
Large clouds of gas,
however, slam into the
spiral density wave
from behind and are
suddenly compressed.

Star Formation in Spiral Arms

You have learned that sudden compression could trigger the formation of stars in a gas cloud.
Thus, new star clusters should form along
the spiral arms.
The spiral arms are not wound up by differential rotation because they
are patterns, not
physically connected
structures.

Star Formation in Spiral Arms

The brightest stars—the O and B stars—live such short lives that they never travel far from their birthplace and are found only along the arms.

Star Formation in Spiral Arms

Their presence is what makes the spiral arms glow so brightly.
This is due to both their own light and the emission from clouds of gas excited by the stars’ UV radiation.

Star Formation in Spiral Arms

Lower-mass stars, like the sun, live longer and have time to move out of the arms and continue their journey around the galaxy.

Star Formation in Spiral Arms

The sun may have formed in a star cluster about 5 billion years ago when a gas cloud smashed into a spiral arm.
Since that time, the sun has escaped from its birth cluster and made about 20 trips around the galaxy, passing through spiral arms many times.

Star Formation in Spiral Arms

The density wave theory is very successful in explaining general properties of spiral galaxies.
However, it has two problems.

Star Formation in Spiral Arms

First, what stimulates the formation of the spiral pattern in the first place?
Theorists calculate that minor fluctuations in the galaxy’s disk shape or gravitational interactions with passing galaxies may be able to start a density wave.

Star Formation in Spiral Arms

Second, the density wave theory does not account for the branches and spurs observed in the spiral arms of our own and other galaxies.
The solution to this
second problem may
lie in a process that
sustains star formation
once it begins.

Star Formation in Spiral Arms

Star formation can also control the shape of spiral patterns—if the birth of stars in a cloud of gas can cause the birth of more new stars.

Star Formation in Spiral Arms

Massive stars evolve so quickly that their lifetimes are only an instant in the history of a galaxy.
Then, they explode as supernovae.

Star Formation in Spiral Arms

You learned earlier that bursts of luminosity and jets from newborn stars and the expanding gases of supernova explosions can compress neighboring clouds of gas.
These trigger more star formation.

Star Formation in Spiral Arms

This process is known as
self-sustaining star formation.
The Orion complex—consisting of the Great Nebula in Orion plus the protostars buried deep in the dark interstellar clouds behind the nebula—seems to be a region of self-sustaining star formation.

Star Formation in Spiral Arms

Astronomers have calculated models indicating that the differential rotation of the galaxy can drag the inner edge of a star-forming region ahead.
It lets the outer edge lag behind to produce a cloud of star formation shaped like a segment of a spiral arm.

Star Formation in Spiral Arms

Self-sustaining star formation plus differential rotation may produce the branches and spurs so prominent in some galaxies including our own.
However, only the
spiral density wave
can generate the
beautiful two-armed
grand spiral patterns.

Star Formation in Spiral Arms

Paleontologists reconstruct the history of life on Earth from the fossil record.
Similarly, astronomers try to reconstruct the galaxy’s past from the fossil it left behind as it formed and evolved.

The Origin and History of Milky Way

The fossil is the spherical component of the galaxy, and the stars in the halo formed when the galaxy was young.
The chemical composition and the distribution of these stars can provide clues to how our galaxy formed.

The Origin and History of Milky Way

To begin, you should ask yourself how old the galaxy is.
That question is easy to answer because you already know how to find the age of star clusters.
However, there are uncertainties that make that easy answer hard to interpret.

The Age of Milky Way

The oldest open clusters have ages of about 9 billion years.
These ages are determined
by analyzing the turnoff point
in the cluster H–R diagram
and are somewhat uncertain.
Nevertheless, from open
clusters, you can get a rough
age for the disk of our galaxy
of at least 9 billion years.

The Age of Milky Way

Globular clusters have faint turnoff points in their H-R diagrams and are clearly old.

The Age of Milky Way

Finding their ages is difficult, though.
Clusters differ slightly in chemical composition—which must be accounted for in calculating the stellar models from which ages are determined.
Also, to find the age of a cluster, astronomers must know the distance to the cluster.

The Age of Milky Way

Precise parallaxes from the Hipparcos satellite have allowed astronomers to increase the precision of the Cepheid variable stars’ calibration.
Careful studies with the newest large telescopes have refined the H-R diagrams.

The Age of Milky Way

Globular cluster ages seem to average about 11 billion years.
The oldest are a bit over 13 billion years old.
So, the halo of our galaxy must be at least
13 billion years old, older than the disk.

The Age of Milky Way

In the 1940s, astronomers realized that there were two types of stars in the galaxy.
The stars they were accustomed to studying
is located in the disk—such as the stars
near the sun.
These they called population I stars.

Stellar Populations

The second type—called population II stars—are usually found in the halo, in globular clusters, or in the nuclear bulge.

In other words, the two stellar populations are associated with the two components of the galaxy.

Stellar Populations

The stars of the two populations fuse nuclear fuels and evolve in nearly identical ways.
They differ only in the abundance of atoms heavier than helium—atoms that astronomers refer to collectively as metals.
Note that this is definitely not the way the word metal is commonly used by nonastronomers.

Stellar Populations

Population I stars are metal-rich—containing 2 to 3 percent metals.
Population II stars are metal-poor—containing only about 0.1 percent metals or less.
The metal content of the star defines its population.

Stellar Populations

Population I stars, sometimes called disk population stars, have circular orbits in the plane of the galaxy.
They are relatively young stars that formed within the last few billion years.
The sun is a population I star.

Stellar Populations

Population II stars belong to the spherical component and are sometimes called the halo population stars.
These stars have randomly tipped orbits with
a wide range of shapes.
A few follow circular orbits, but most follow
elliptical orbits.
These stars are all lower-mass main-sequence
stars or giants and are old stars.
The metal-poor globular clusters are part of
the halo population.

Stellar Populations

Since the discovery of stellar populations, astronomers have realized that there is a gradation between populations.

Stellar Populations

Why do the disk and halo stars have different metal abundances?
The two types of star must have formed at different stages in the life of the galaxy—at times when the chemical composition of the galaxy differed.

Stellar Populations

That is a clue to the history of the galaxy.
To use the clue, though, you must first discuss the cycle of element building.

Stellar Populations

The atoms of which you are made were created in a process that spanned a number of generations of stars.
The process that built the chemical elements over the history of our galaxy led to the possibility of Earth and life on Earth.

The Element-Building Cycle

You have learned how elements heavier than helium but lighter than iron are built up by nuclear reactions inside evolving stars.

The Element-Building Cycle

Atoms more massive than iron are made by the short-lived nuclear reactions that occur during a supernova explosion.
This explains why lower-mass atoms—such as carbon, nitrogen, and oxygen—are so common.
This also explains why atoms more massive than iron—such as gold, silver, platinum, and uranium—are so rare, and often valuable.

The Element-Building Cycle

The figure shows the abundance of the chemical elements.
The graph has an
exponential scale.
To get a feeling for the true
abundance of the elements,
you should draw this graph
using a linear scale.

The Element-Building Cycle

Using a linear scale, you can see how rare the elements heavier than helium really are.

The Element-Building Cycle

Most of the matter in stars is hydrogen and helium.
Other elements, including carbon, nitrogen, oxygen, and the rest of what astronomers call metals, were cooked up inside stars.

The Element-Building Cycle

When the galaxy first formed, there should have been no metals.
The stars had not yet manufactured any.

The Element-Building Cycle

Judging from the composition of stars in the oldest clusters, the gas from which the galaxy condensed must have contained about 90 percent hydrogen atoms and 10 percent helium atoms.
The hydrogen and helium came from the big bang that began the universe.

The Element-Building Cycle

The first stars to form from this gas were metal-poor.
Now, 13 billion years later, their spectra still show few metal lines.

The Element-Building Cycle

Of course, they may have manufactured many atoms heavier than helium.
As the stars’ interiors are not mixed, those heavy atoms stay trapped at the centers of the stars where they were produced and do not affect the spectra.
The population II stars in the halo are the survivors of an earlier generation of stars that formed in the galaxy.

The Element-Building Cycle

Most of the first stars evolved and died.
The various types of star death throes, including supernovae, enriched the interstellar gas with metals.
Succeeding generations of stars formed from gas clouds that were more enriched.
Each generation added to the enrichment with its death.

The Element-Building Cycle

By the time the sun formed, roughly 5 billion years ago, the element-building process had added about 1.6 percent metals.

The Element-Building Cycle

Since then, metal abundance has increased further.
Stars forming now incorporate 2 to 3 percent metals and become extreme population I stars.
Thus, metal abundance varies between populations—due to the production of heavy atoms in successive generations of stars as the galaxy aged.

The Element-Building Cycle

The oxygen atoms you are breathing, the carbon atoms in your flesh, the calcium atoms in your bones were all created in the interiors of red and yellow giant stars.
They were hot enough to sustain fusion reactions beyond helium.

The Element-Building Cycle

The gold and silver atoms in your jewelry and dental fillings, the iodine in medicine, the mercury (quicksilver) atoms in old-fashioned thermometers were all created during supernova explosions.
Those are the only places in the universe hot enough to make these types of atoms.

The Element-Building Cycle

This one idea is perhaps the most significant idea to take away from an introductory astronomy course and textbook.
You, and everything on Earth, are made
of stardust.

The Element-Building Cycle

The lack of metals in the spherical component of the galaxy tells you it is very old—a fossil left behind by the galaxy when it was young and drastically different from its present disk shape.
Thus, the study of element-building and stellar populations leads to the fundamental question—How did the galaxy form?