Astrophysics - use of the techniques and ideas of physics to
Astrophysics - use of the techniques and ideas of physics to study
the heavens. study the heavens. Astrophysics and Cosmology in the
21 st Century Cosmology - the study of the universe as a whole -
search for a Cosmology - the study of the universe as a whole -
search for a theoretical framework to understand the observed
theoretical framework to understand the observed universe, its
origin, and its future. universe, its origin, and its future.
foundation: Einsteins general theory of relativity and its theory
of gravitationfor 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.)
Slide 3
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 didnt 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 humanitys
understanding of what we are. Well start with a quick overview of
the universe and what the universe looks like seen from the surface
of our spinning planet.
Slide 4
Stars and Galaxies geocentric model geocentric model 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.
Galileos 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 Moons of Jupiter Rings of Saturn Rings
of Saturn Surface structures on the moon Surface structures on the
moon Sun spots Sun spots Phases of Venus Phases of Venus
Slide 5
Johannes Kepler (1571 1630) Used the precise observational
tables of Tycho Brahe (1546 1601) to study planetary motion
mathematically. Used the precise observational tables of Tycho
Brahe (1546 1601) to study planetary motion mathematically.
1.Circular motion and Planets move around the sun on elliptical
paths, with non- uniform velocities. Planets move around the sun on
elliptical paths, with non- uniform velocities. Found a consistent
description by abandoning both Found a consistent description by
abandoning both 2.Uniform motion.
Slide 6
Slide 7
Isaac Newton (1643 - 1727) Building on the results of Galileo
and KeplerBuilding on the results of Galileo and Kepler Major
achievements: 1.Invented Calculus as a necessary tool to solve
mathematical problems related to motion Adding physics
interpretations to the mathematical descriptions of astronomy by
Copernicus, Galileo and KeplerAdding physics interpretations to the
mathematical descriptions of astronomy by Copernicus, Galileo and
Kepler 2.Discovered the three laws of motion 3.Discovered the
universal law of mutual gravitation
Slide 8
Newtons 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
Slide 9
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: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:
G is the Universal constant of gravity G = 6.67 10 -11 Nm 2 /kg 2 G
= 6.67 10 -11 Nm 2 /kg 2 This applies to ALL objects This applies
to ALL objects The first unification of earthly & heavenly
physics The universal law of gravity allows us to understand
orbital motion of planets and moons.
Slide 10
Earth and moon attract each other through gravitation. Earth
and moon attract each other through gravitation. Example: Earth
Moon vv F Since Earth is much more massive than the moon, the moons
effect on Earth is small. Since Earth is much more massive than the
moon, the moons effect on Earth is small. Earths gravitational
force constantly accelerates the moon towards Earth. Earths
gravitational force constantly accelerates the moon towards Earth.
This acceleration is constantly changing the moons direction of
motion, holding it on its almost circular orbit. This acceleration
is constantly changing the moons direction of motion, holding it on
its almost circular orbit. The universal law of gravity allows us
to understand orbital motion of planets and moons.
Slide 11
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.Edmond Halley Newton's
Slide 12
So What? Newtons contributions unified physical lawsNewtons
contributions unified physical laws His contributions reduced all
celestial motion to three laws of motion + law of gravitationHis
contributions reduced all celestial motion to three laws of motion
+ law of gravitation He explained other phenomenaHe 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
Slide 13
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 10 8 m = 3.0 10
5 km = 3.0 10 8 m = 3.0 10 5 km 1 light-minute = 18 10 6 km
light-year (ly) 1 ly = 9.46 10 15 m l0 13 km. light-year (ly) 1 ly
= 9.46 10 15 m l0 13 km. The EarthMoon distance is 384,000 km =
1.28 light-seconds. The EarthSun 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 l0 9 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.)
Slide 14
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 10 11 stars = 100 billion stars. Counting - over 3000
years! The total mass of all stars 3 10 41 kg. our Sun ~ 28,000 ly
from the center. Period around the galactic center Period around
the galactic center ~ 200 million years, ~ 200 million years, its
speed 250 km/s its speed 250 km/s relative to the center of the
Galaxy. relative to the center of the Galaxy. Our Galaxy If we
could only see it like this! Milky Way as seen from West Texas
Slide 15
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: In terms of numbers of stars, if
they are like our Sun (m = 2.0 10 30 kg), there would be about 10
11 or about 100 billion stars M 3 10 41 kg
Slide 16
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. The Universe got bigger on
October 6, 1923 Idea was too revolutionary. Over the centuries
telescopes have improved a lot, so that at the beginning of the 20
th century astronomers were able to peek much farther than
Galileo.
Slide 17
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
Slide 18
Today, the largest telescopes can see about 10 11 (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. Andromeda Galaxy (NASA image)
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 Milky Way galaxy. However, new research seems to
indicate that this is not the case. irregulardwarfgalaxiesLocal
GrouporbitingMilky WaygalaxyirregulardwarfgalaxiesLocal
GrouporbitingMilky Waygalaxy
Slide 19
after these short intermezzo we go back to nebula to see new
beautiful pictures
Slide 20
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 NGC 2246
The Trifid Nebula
Slide 21
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 its not cloudy). 2) Reflection
Nebulae
Slide 22
Dense clouds of gas and dust absorb the light from the stars
behind; Bernard 86 Horsehead Nebula appear dark in front of the
brighter background; 3) Dark Nebulae a lot of beautiful
nebulas
Slide 23
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.
Slide 24
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! Astronomy/Astrology began as the same
thingAstronomy/Astrology began as the same thing People looking at
sky, telling storiesPeople looking at sky, telling stories
Astronomy/Astrology began as the same thingAstronomy/Astrology
began as the same thing People looking at sky, telling
storiesPeople looking at sky, telling stories
Slide 25
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
Slide 26
Precession Earths axis wobbles like a top as it spins, making a
circle in the skyEarths 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.Pole happens to point to
Polaris now, but in ~13,000 years, Vega will be the north
star.
Slide 27
Major Discoveries of Galileo Moons of JupiterMoons of Jupiter
(4 Galilean moons) (4 Galilean moons) Rings of Saturn Rings of
Saturn (What he really saw)
Slide 28
Major Discoveries of Galileo Surface structures on the moon;
first estimates of the height of mountains on the moon Surface
structures on the moon; first estimates of the height of mountains
on the moon
Slide 29
Major Discoveries of Galileo (3) Sun spots (proving that the
sun is not perfect!)
Slide 30
Major Discoveries of Galileo (4) Phases of Venus (including
full Venus), proving that Venus orbits the sun, not the Earth!
Slide 31
without forces anything can be imaginable anything celestial
sphere