3. Discovery of the Galaxy
Overview
Structure of galaxy
The Spiral Arms and Star Formation
Origin and History of the Milky Way Galaxy
The Nucleus of the Galaxy
7. The Great Star System
The ancient Greeks named that band
galaxies kuklos, the “milky circle.” The
Romans changed the name to via
lactia,meaning “milky road” or “milky
way.”
8. Galileo’s telescope revealed that the
glowing Milky Way is made up of
stars, and later astronomers realized
that the sun must be located in a great
wheel-shaped cloud of
stars, which they called the star
system.
9. In 1750 Thomas Wright
Wright used the term
universe because, so far
as was known at the time,
the Milky Way star
system was the entire
universe
10. In 18th Century
Astronomers Sir William Herschel and Caroline Herschel (Sir
William’s sister) set out to map the three-
dimensional distribution of stars in the
Milky Way.
11. ….The sun is located near the center of this
“grindstone” model universe
12. Variable Stars and
Distance Estimates
The variable stars Leavitt saw, Cepheids, are named after the first
such star discovered, d Cephei. Many Cepheid variables are known
today; they have pulsation periods from 1 to 60 days and lie in a region
of the H–R diagram known as the instability strip
13. Cepheid's are clearly
giant and supergiant
stars. A related kind of
variable, RR Lyrae stars,
are named after the
variable star RR in the
constellation Lyra. They
have a pulsation period of
about half a day and lie at
the low-luminosity end of
the instability strip
14. Size of the Galaxy
The Milky Way is
only 100,000 light years in
diameter.
17. Disk Component
It contains most of the galaxy’s
stars and nearly all of its gas and dust.
Because the disk is home to the
giant molecular clouds within
which stars form nearly all star
formation in our galaxy takes place in
the disk.
18. Halo
a spherical cloud of stars and star
clusters that contains almost no
gas and dust. Because the halo
contains no dense gas
clouds, it cannot make new stars
19. Central Bulge
a flattened cloud of billions of stars
about 6 kpc in diameter. Like the
halo, it contains little gas and dust.
Astronomers often refer to the halo
and the central bulge together as the
spherical component of
the galaxy.
21. Current observations indicate that the sun
orbits the center of our galaxy at about
240 km/s, moving in the direction of
Cygnus.
The evidence suggests the sun’s orbit is
nearly circular, so given the current best
estimate for the distance to the center of
our galaxy, 8.2 kpc, you can find the
circumference of the sun’s orbit
by multiplying by 2p.
23. Tracing the Spiral Arms
Those spiral segments have been
named for the prominent
constellations through which they
pass. Astronomers are now using
infrared and radio telescopes to
penetrate the interstellar dust, locate
more distant OB associations, and
trace the spiral arms even farther
Objects used to map spiral arms are called spiral tracers
24. Radio Maps of Spiral Arms
• Radio astronomers use the
strong spectral line emission
from carbon monoxide (CO) to
map the location of giant
molecular clouds in the plane
of the galaxy
25. The Spiral Density Wave Theory
In the spiral density wave theory, spiral
arms are dynamically stable regions of
compression that move slowly around
the galaxy, just as the truck moves
slowly down the highway. Gas clouds
moving at orbital velocity around the
galaxy overtake the slow-moving arms
from behind and slam into the gas
already in the arms.
27. Observations of the Galactic Nucleus
1.Observations at radio and infrared wavelengths reveal complex structures
near Sgr A* caused by magnetic fields and by rapid star formation. Supernova
remnants show that massive stars have formed there recently and exploded at
the ends of their lives.
2.The center is crowded. Tremendous numbers of stars heat the dust, which
emits strong infrared radiation
3. Finally, there is evidence that Sgr A* is a supermassive black hole into which
gas is flowing. Observations of the motions of stars orbiting the central object
indicate its mass is at least 4 million M}
30. The Element-Building Process
• The first stars that formed early in the universe’s history therefore
had to be nearly pure hydrogen and helium. All of the
other chemical elements have been produced by
Nucleosynthesis
-the process instars that fuses hydrogen and helium to make the
heavier atom
31. Stellar Populations
• They form and evolve in similar ways, but they differ, especially in
their abundances of metals.
>Population I stars are metal rich, containing 2 to 3 percent metals,
whereas…..
>Population II stars are metal poor, typically containing less than 0.1
percent metals.
32. The Age of Galaxy
• The oldest open clusters are 9 to 10 billion years old.
These ages come from the turnoff points in their H–R
diagrams, but finding the age of an old cluster is difficult
because old clusters change so slowly
Studies of the Studies of the oldest
globular clusters suggest that the halo of
our galaxy is at least 13 billion years
33. Formation of the Galaxy
• The monolithic collapse hypothesis (Top-down)says that
the galaxy formed from a single large cloud of turbulent gas over 13
billion years ago. That cloud contracted to form our galaxy. As
gravity pulled the gas inward, the cloud began to fragment into
smaller clouds, and because the gas was turbulent, the smaller
clouds had random velocities.
35. In a nutshell
• The sun, with Earth in its clutch, is ripping along at about 240
km/sec (that’s 540,000 mph) as it orbits the center of the Milky Way
Galaxy. We live on a wildly moving ball of rock in a large galaxy that
humanity calls home, but the Milky Way is more than just our home.
Perhaps “parent galaxy”would be a better name
• Except for hydrogen atoms, which have survived unchanged since
the universe began, you and Earth are made of metals—atoms
heavier than helium. There is no helium in your body, but there is
plenty of carbon, nitrogen, and oxygen. There is calcium in your
bones and iron in your blood. All of those atoms and more were
cooked up inside stars or in their supernova deaths
36. • Stars are born when clouds of gas orbiting the center of the galaxy
collide with the gas in spiral arms and are compressed. That
process has given birth to generations of stars, and each generation
has produced elements heavier than helium and spread them back
into the interstellar medium. The abundance of metals has grown
slowly in the galaxy
37. References
• Universe (Solar systems, Stars, and Galaxies) 7th Edition by Seeds and
Backman
• http://www.setterfield.org/Astronomy/Galaxies.html
• http://astronomy.nju.edu.cn/~lixd/GA/AT4/AT425/HTML/AT42504.HT
M
You will learn in this chapter how evidence reveals that you are inside a great wheel of stars, a galaxy. The Milky Way Galaxy is over 80,000 ly in diameter and contains more than 100 billion stars.
It is your galaxy because you live in it, but you are also a product of it, because the stars in the Milky Way Galaxy made many of the atoms in your body. You begin this chapter by pondering
the notion that the stars belong to you, but when you reach the
end of the chapter, you may realize that you also belong to the
stars
The large group of stars outside of our own milky way
Also may have gas and dust
It was not until Galileo used his telescope in 1610 that anyone realized the Milky Way is made of a huge number of
stars
at star system appears from our location inside it as the band of the Milky Way encircling the sky.
They assumed that they could see to the outer boundaries of the Milky Way in all directions and hypothesized that by
counting the number of stars that were visible in different directions they could find the relative distances to the edges
of the star distribution.
In 1785, William Herschel published this diagram showing the shape
of the star system as if it could be viewed edge-on from outside. The
sun is located near the center of this “grindstone” model universe.
>1785 - Herschel attempted to determine the shape and size of Galaxy
Assumptions:
All stars have same intrinsic brightness
Star are arranged uniformly throughout the MW
He could see to the edge of the MW
>It is a Common Misconceptionthat the stars are eternal
and unchanging, but astronomers have known for centuries that
some stars change in brightness.
?In 1912, Henrietta Leavitt was studying a star cloud in
the southern sky known as the Small Magellanic Cloud. On
her photographic plates, she found many variable stars, and
she noticed that the brightest had the longest periods.
Most of the stars in the disk are
middle- to lower-main-sequence
stars like the sun, a few are red
giants and white dwarfs, and
fewer still are brilliant blue O and B stars. Th ose hot, massive
stars are rare, but they are so luminous that they provide much
of the light from the disk
(intro) If you look “up” or “down,” out of the galaxy’s disk, you are
looking away from the dust and gas, so you can see into the galaxy’s halo
>Halo stars are old, cool, lower-main-sequence stars, red giants, and white dwarfs. It is difficult to judge the extent of the halo, but it could have as much as 10 times the diameter of the visible disk
side from OB associations, spiral tracers include young open
star clusters, clouds of hydrogen ionized by hot stars (emission
nebulae), and certain higher-mass variable stars. Notice that all
spiral tracers are young objects, formed recently, astronomically
speaking. O stars, for example, live for only a few million years.
. (The dust that blocks our
view at visual wavelengths is transparent to radio
waves because radio wavelengths are much larger
than the diameter of the dust particles.)
e sudden compression of the gas can trigger the collapse of the gas clouds and the formation of new Newly formed stars and the remaining gas eventually move on through the arm and emerge from the front
of the slow-moving arm to resume their travels around the galaxy
The most mysterious region of our galaxy is its very center,
the nucleus. At visual wavelengths, this region is totally hidden
by dust and gas that dim the light it emits by 30 magnitudes
A supermassive black hole is an exciting idea, but scientists
must always be aware of the difference between adequacy and
necessity. A supermassive black hole is adequate to explain the
observations
As you already know, medium-mass stars like the sun cannot
ignite carbon fusion, but during helium fusion the heat and
>Population I stars belong to the disk component of the galaxy and are sometimes called disk population stars. Th ey have nearly circular orbits in the plane of the galaxy and formed
within the last few billion years. The sun is a Population I star, as
are the type I Cepheid variables discussed in this chapter.
>Population II stars belong to the spherical component of the
galaxy and are sometimes called the halo population stars. These
stars have randomly tipped orbits ranging from circular to highly
elliptical. They are old stars that formed when the galaxy was
young. The metal-poor globular clusters are part of the halo
population, as are the RR Lyrae and type II Cepheids.
Because astronomers know how to find the age of star clusters,
they can estimate the age of the oldest stars in the galaxy, giving
a lower limit to the age of the entire galaxy. The process sounds
straightforward, but uncertainties make the easy answer hard to
interpret.
Also, the exact location of the turn off point
depends on chemical composition, which differs slightly among
clusters. Finally, open clusters are not strongly bound by gravity,
so older open clusters may have dissipated as their stars wandered
away. It is reasonable to suppose that the galactic disk is somewhat older than the oldest remaining open clusters, which suggests that the disk is at least 10 billion years old.
Th at caused the stars and star clusters that
formed from these fragments to have orbits with a wide range of
shapes; a few were circular, most were elliptical, and some were
extremely elliptical. Th e orbits were also inclined at different
angles, resulting in a spherical cloud of stars—the spherical component of the galaxy. Of course, these first stars were metal poor
because no stars had existed earlier to enrich the gas with metals
