Transcript Slide 1
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 21
st
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
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 non uniform 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:
F
G Mm r
2 •
This applies to
ALL
objects
G is the Universal constant of gravity G = 6.67 × 10 -11 N∙m 2 /kg 2 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.
v v
• Earth’s gravitational force constantly accelerates the moon towards Earth.
Moon • This acceleration is constantly changing the moon’s direction of motion, holding it on its almost circular orbit.
F
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
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 × = 3.0 × 10 m/s)(1.0 s) 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.
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 × 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.)
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 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 ► ~ 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:
G Mm r
2
m v
2
r
→ M ≈ 3 × 10 41 kg 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
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 20 th 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 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. 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.
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 2246 Nebula
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;
2) Reflection Nebulae
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