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NEWEST, NEWEST NEWS
http://www.spacedaily.com/reports/Galaxy_May_Hold
_Hundreds_Of_Rogue_Black_Holes_999.html
http://www.nasa.gov/topics/universe/index.html
Astrophysics and Cosmology in the 21st Century
■ Astrophysics - use of the techniques and ideas of physics to
study the heavens.
■ Cosmology - the study of the universe as a whole - search for a
theoretical framework to understand the observed
universe, its origin, and its future.
foundation: Einstein’s general theory of relativity and its
theory of gravitation—for in the large-scale structure of
the universe, gravity is the dominant force.
The questions posed by cosmology are the biggest, complex and the
most difficult in all of the science; the possible answers are often
unimaginable. They are questions like “Has the universe always
existed, or did it have a beginning in time?” Either alternative is
difficult to imagine: time going back indefinitely into the past, or an
actual moment when the universe began (but, then, what was there
before?). And what about the size of the universe? Is it infinite in
size? (It is hard to imagine infinity.) Or is it finite in size? (This is also
hard to imagine, for if the universe is finite, it does not make sense to
ask what is beyond it, because the universe is all there is.)
Astronomy is about us. As we learn about astronomy, we
learn about ourselves. We search for an answer to the
question “What are we?” The quick answer is that we are
thinking creatures living on a planet that circles a star we
call the sun. But it didn’t always kook that way.
Appearances are deceiving. We will see how difficult it
has been for humanity to understand what we see in the
sky every day. In fact, the modern science was born when
people tried to understand the appearance of the sky. How
astronomers learned to understand what they saw in the
sky has changed humanity’s understanding of what we
are.
We’ll start with a quick overview of the universe and what
the universe looks like seen from the surface of our
spinning planet.
Stars and Galaxies
According to the ancients, the stars, except for the few that seemed to
move (the planets), were fixed on a sphere beyond the last planet.
geocentric
model
■ Galileo’s first telescopic observations of the heavens in 1610.
Invention of the modern view of science: Transition from
a faith-based “science” to an observation-based science.
Our view of the universe changed dramatically.
Major Discoveries of Galileo (1594 – 1642)
Moons of Jupiter
Rings of Saturn
Surface structures
on the moon
Sun spots
Phases of Venus
Johannes Kepler (1571 – 1630)
• Used the precise observational tables of Tycho Brahe (1546
– 1601) to study planetary motion mathematically.
• Found a consistent description by abandoning both
1. Circular motion and
2. Uniform motion.
• Planets move around the sun on elliptical paths, with nonuniform velocities.
Isaac Newton (1643 - 1727)
• Building on the results of Galileo and Kepler
• Adding physics interpretations to the mathematical
descriptions of astronomy by Copernicus, Galileo and Kepler
Major achievements:
1. Invented Calculus as a necessary tool to solve mathematical
problems related to motion
2. Discovered the three laws of motion
3. Discovered the universal law of mutual gravitation
Newton’s Laws of Motion
1. A body continues at rest or in uniform
motion in a straight line unless acted
upon by some net force.
An astronaut floating in space will
continue to float forever in a
straight line unless some external
force is accelerating him/her.
2. The acceleration a of a body is inversely
proportional to its mass m, directly
proportional to the net force F, and in the
same direction as the net force.
a = F/m  F = m a
3. Whenever one body exerts a force on a second
body, the second body exerts an equal and
opposite force on the first body.
and you swim
The Universal Law of Gravitation
• Any two bodies are attracting each other through gravitation,
with a force proportional to the product of their masses and
inversely proportional to the square of their distance:
Mm
F G 2
r
• This applies to ALL objects
G is the Universal constant of gravity
G = 6.67 × 10-11 N∙m2/kg2
The first unification of earthly & heavenly physics
The universal law of gravity allows us to understand orbital
motion of planets and moons.
The universal law of gravity allows us to understand orbital
motion of planets and moons.
Example:
• Earth and moon attract each other through gravitation.
• Since Earth is much more massive than
the moon, the moon’s effect on Earth is
small.
• Earth’s gravitational force constantly
accelerates the moon towards Earth.
v
v
Moon
F
• This acceleration is constantly changing
the moon’s direction of motion, holding it on
its almost circular orbit.
Earth
In 1705 Edmond Halley predicted, using
Newton's newly formulated laws of motion, that
the comet seen in 1531, 1607, and 1682 would
return in 1758 (which was, alas, after his death).
The comet did indeed return as predicted and
was later named in his honor.
So What?
• Newton’s contributions unified physical laws
• His contributions reduced all celestial motion to
three laws of motion + law of gravitation
• He explained other phenomena
– Tides, cannon ball motion, …
The laws of
physics are
universal!
A new era of science → mathematics as a tool for
understanding physics
The distances involved are so great that we specify them in
terms of the time it takes light to travel the given distance:
for example:
1 light-second = (3.0 × 10 m/s)(1.0 s)
= 3.0 × 108 m = 3.0 × 105 km
1 light-minute = 18 × 106 km
light-year (ly) 1 ly = 9.46 × 1015 m ≈ l013 km.
The Earth—Moon distance is 384,000 km = 1.28 light-seconds.
The Earth—Sun distance is 150,000,000 km = 8.3 light-minutes.
The most distant what used to be a planet in the solar system,
Pluto, is about 6 × l09 km from the Sun, or 6 × 10-4 ly.
The nearest star to us, other than the Sun, is Proxima Centauri,
about 4.3 ly away. (Note that the nearest star is 10,000 times
farther from us than the farthest planet.)
Our Galaxy
■ Galileo first observed, about 1610, that the Milky Way is
comprised of countless individual stars
■ diameter ~ 100,000 ly thickness ~ 2000 ly
■ it has a bulging central nucleus and spiral arms.
■ about 1011 stars = 100 billion stars. Counting - over 3000 years!
■ The total mass of all stars ≈ 3 ×1041 kg.
■ our Sun ~ 28,000 ly from the center.
► Period around the galactic center
~ 200 million years,
► its speed ≈ 250 km/s
relative to the center of the Galaxy.
Milky Way
as seen
from West
Texas
If we could only see it like this!
EXAMPLE
Estimate the total mass of our Galaxy using the orbital data of
the Sun (including our solar system) about the center of the
Galaxy. Assume that most of the mass of the Galaxy can be
approximated as a uniform sphere of mass
SOLUTION Our Sun and solar system orbit the center of the
Galaxy, according to the best measurements with a speed of
about v = 250 km/s at a distance from the Galaxy center of
about r = 28,000 ly. The gravitational force between the mass
of the galaxy M and the mass of our solar system m is
actually centripetal force:
2
Mm
v
G 2 m
r
r
→ M ≈ 3 × 1041 kg
In terms of numbers of stars, if they are like our Sun
(m = 2.0 × 1030 kg), there would be about 1011 or about
100 billion stars
The Universe got bigger on October 6, 1923
For some 300 years astronomers believed that our Galaxy,
Milky way is ALL there is in universe. That was universe.
Immanuel Kant (about 1755) seems to be the first to suggest
that some of the patches in the sky might be galaxies just
like our own, but are faint because they are so distant.
Idea was too revolutionary.
Over the centuries telescopes have improved a lot, so that
at the beginning of the 20th century astronomers were able
to “peek” much farther than Galileo.
Before October 6, 1923, astronomers thought the Andromeda
Nebula and similar objects were bright pockets of matter inside
the Milky Way. On that day astronomer Edwin Hubble noticed,
looking at the photograps, a particular type of star inside the
Andromeda Nebula. Hubble realized that the star (Cepheid
variable, a type of stars that astronomers use to measure
distances in the universe) must be far outside the Milky Way,
because of its great distance (which he was able to calculate).
By the way he developed the method for calculating star distance.
When Hubble reported his findings the following year,
astronomers realized that they had misnamed the Andromeda
Nebula. It's not a nebula at all. Instead, it's a galaxy -- the first
confirmed "city of stars" beyond the Milky Way.
Nebulae
latin: dust, cloud
Originally nebula was a general name for any extended
astronomical object – it was the name given to many faint
cloudy patches in the sky
Today – it is really a DUST
Today, the largest telescopes can see about 1011 (100 billion )
galaxies. They are sprinkled throughout the universe. Only three
galaxies outside the Milky Way are easily visible to the unaided
eye -- the great galaxy in Andromeda and the Large and Small
Magellanic Clouds. The distance to the Andromeda nebula (a
galaxy), for example, is over 3 million light-years, a distance 20
times greater than the diameter of our Galaxy. These are some of
our nearest galactic neighbors. The farthest galaxies ever
observed are more than 10 billion light-years away. These galaxies
formed soon after the universe itself was born.
The two Magellanic Clouds are irregular dwarf galaxies, which
are members of our Local Group of galaxies. Once they were
thought to be orbiting
our Galaxy
Milky Way
galaxy.
Andromeda
(NASA
image)However, new
research seems to indicate that this is not the case.
after these short intermezzo we go back
to nebula to see new beautiful pictures
Three Kinds of Nebulae
1) Emission Nebulae
Hot star illuminates a
gas cloud;
excites and/or
ionizes the gas
(electrons kicked into
higher energy
states);
electrons
recombining, falling
back to ground state
produce emission
lines.
The Fox Fur Nebula
The Trifid
NGC 2246Nebula
2) Reflection Nebulae
Star illuminates gas and dust
cloud;
star light is reflected by the
dust;
reflection nebula appear blue
because blue light is scattered
by larger angles than red light;
Same phenomenon makes
the day sky appear blue (if it’s
not cloudy).
3) Dark Nebulae
Dense clouds of gas and dust absorb the light from the stars behind;
appear dark in
front of the
brighter
background;
Bernard 86
Horsehead Nebula
a lot of beautiful nebulas
Besides the usual stars, clusters of stars, galaxies, and
clusters and super- clusters of galaxies, the universe contains
a number of other interesting objects. Among these are stars
known as red giants, white dwarfs, neutron stars, black holes
and exploding stars called novae and supernovae. In addition
there are quasars (quasistellar radio sources’’), which, if we
judge their distance correctly, are galaxies thousands of times
brighter than ordinary galaxies. Furthermore, there is radiation
that reaches the Earth but does not emanate from the bright
pointlike objects we call stars: it is a background radiation that
seems to arrive uniformly from all directions in the universe.
We discuss all these phenomena in due course.
• Astronomy/Astrology began as the same thing
• People looking at sky, telling stories
The view of the night sky changes as Earth moves in its orbit about
the Sun. As drawn here, the night side of Earth faces a different set
of constellations at different times of the year. The twelve
constellations that fall along the ecliptic are called the zodiac!
Here in northern hemisphere, sky seems to
rotate around North Celestial Pole.
As the Earth spins on its axis,
the sky seems to rotate around
us. This motion produces the
concentric arcs traced out by the
stars in this time exposure of the
night sky. In the middle of the
picture is the North Celestial
Pole (NCP), easily identified as
the point in the sky at the center
of all the star trail arcs. The very
short bright trail near the NCP
was made by the star Polaris,
commonly known as the North
Star.
Pole star a coincidence! No star
at southern celestial pole
Precession
• Earth’s axis wobbles like a top as it spins, making a circle in the sky
• Pole happens to point to Polaris now, but in ~13,000 years, Vega
will be the north star.
Major Discoveries of Galileo
• Moons of Jupiter
(4 Galilean moons)
(What he really saw)
• Rings of Saturn
(What he really saw)
Major Discoveries of Galileo
• Surface structures on the moon; first estimates
of the height of mountains on the moon
Major Discoveries of Galileo (3)
• Sun spots (proving that the
sun is not perfect!)
Major Discoveries of Galileo (4)
• Phases of Venus (including “full Venus”),
proving that Venus orbits the sun, not the Earth!
celestial sphere
without forces anything can
be imaginable