Astronomy Chapter Notes by xiagong0815


									del Campo Astronomy 2011

Comprehending Distances in the Universe.

In reality, with current methods of propulsion, it would take a Mars-bound crew
more than a year to reach the Red Planet safely. A trip to Jupiter would last several
more years, and a trip to Pluto might be a lifelong quest.

The Moon is 400,000km away, and this is the most distant destination that humans
have ever traveled. This was back in 1969 and during the 1970’s.

The astronomical unit (AU) is based on the average distance between the Earth and
Sun. 93,000,000 miles away.

The way that astronomers measure the distance to a star is similar to the methods
that surveyors use. It involves basic triangulation.

A light-year sounds as though it is a unit of time; because a year is a unit of time, but
really it is the distance that light travels in a year.

Measurement in Astronomy: Parallax and the Parsec.

Parallax is the apparent change in the position of an object due to a change in the
observer’s position.

The parsec is 3.26 light years.

The Nebular Theory.

The Sun contains over 99% of all of the mass of the solar system. All this mass came
form a nebula. The idea that the solar system evolved from such a swirling cloud of
dust is called the nebular theory.

In the nebula that gave birth to our solar system, gravity caused the gases and dust
to be drawn together into a denser cloud. At the same time, the rate of rotation
(swirling) of the entire nebula gradually increased. The effect is the same as when a
rotating ice skater draws his or her arms in, causing the rate of rotation to speed up.

Fusion reactions inside the Sun create very high pressure, and like a bomb, threaten
to blow the Sun apart. The Sun doesn’t fly apart under all this outward pressure,
however. The Sun is in a state of equilibrium. The gravity of the Sun is pulling on
each part of it and keeps the Sun together as it radiates energy out in all directions,
providing solar energy to the Earth community.
The Birth of our Planets and Solar System

The rest of our solar system formed in the swirling disk of material surrounding the
newborn Sun. Nine (9) planets, 67 satellites, and a large number of comets and
asteroids formed.

Mercury, Venus, Earth, and Mars—are called our terrestrial (“Earth-Like”) planets.
They formed in the inner part of our solar system, where temperatures in the
original nebula were high. They are relatively small, rocky, bodies.

Jupiter, Saturn, Uranus, and Neptune consist mostly of dense fluids like liquid
hydrogen. These gas giants formed in the colder, outer parts of the early solar
nebula. They have solid rocky cores about the size of Earth, covered with layers of
hydrogen in both gas and liquid form

Pluto is the most distant. Some astronomers do not consider Pluto a planet. Some
scientists think that it may not been part of the original solar system but instead was
captured later by the Sun’s gravity.

There are trillions of comets and asteroids scattered throughout the solar system.
Earth and other solar-system bodies are scarred by impact craters formed when
comets and asteroids collided with them. On Earth, erosion has removed obvious
signs of many of these craters; also do to Earth being geologically active.

Astronomers see these comets and asteroids as the leftovers from the formation of
the solar system.

Asteroids are dark, rocky bodies that orbit the Sun at different distances. Many are
found in the asteroid belt between Mars and Jupiter.

Comets are mixtures of ice and dust grains. They exist mainly in the outer solar
system, but when their looping orbits bring them close to the Sun, their ices begin to
melt. That is when you see tails streaming out from them in the direction away from
the Sun.

Unfortunately, those who like to view the night sky; light pollution in densely
populated area makes it impossible to see the Milky Way even when the nights the
atmosphere is clear and cloudless.

Galaxies are classified according to their shape: elliptical, spiral, or irregular. Our
home galaxy is a flat spiral, a pinwheel-shaped collection of stars held together by
their mutual gravitational attraction.

Our solar system is located in one of the spiral arms about 2/3 (two-thirds) of the
way out from the center of the galaxy.
Our Milky Way galaxy formed about 10 billion years ago.

The Universe itself formed somewhere between 12 to 14 billion years ago in an
even called the Big Bang.

This beginning, at what a scientist would call “time zero,” the universe consisted
almost entirely of energy, concentrated into a volume smaller that a grain of sand.

Most cosmologists think that most of the matter in the universe was formed within
minutes of time zero!

The 5 stars nearest our Sun:
   1) Proxima Centauri (4.24 light years)
   2) Alpha Centauri A (4.34 light years)
   3) Alpha Centauri B (4.34 light years)
   4) Barnard’s Star (5.67 light years)
   5) Wolf 359 (7.80 light years)

Lunar Effects on Earth

The force of gravity lessens with distance; we see its effect time and time again as
we study the interaction of comets with planets, stares with other stars, and even
the action of gravity across the gulfs that separate galaxies.

The Earth has a large iron core whereas the Moon does not.

The Moon formed in orbit around the Earth, accreting from small planetesimals just
as the Earth did. However, the lack of the iron core rules out that idea.

Another possibility is that the Earth captured the Moon.

Tidal Forces

The tidal forces exerted on the Earth’s oceans by the Sun and the Moon has played a
large part in the evolution of the length of the Earth’s day.

Soon after the Moon formed, it was probably no more than 24,000km away from the
Earth. Earth was spinning rapidly with a day that was probably no more than six
hours long.

Ocean tides could have been hundreds of meters high, and the pull of this nearby
Moon would have flexed the Earth’s crust, causing heating and melting.

As the motions of the Earth-Moon system evolved, tidal forces gradually caused the
Moon to move farther away from the Earth. As it moved away, it took longer to orbit
the Earth. The tidal bulges in the Earth’s surface led as the Moon orbited the Earth.
Although the dynamics are not easy to understand, this produced a torque that
slowed down Earth’s spin rate as the distance between Earth and Moon increased.

This slowdown continues today as the Moon continues to recede from the Earth at
the rate of about 3cm per year.

400 million years ago, the Earth had a 400-day-long year.

The Moon and the Origins of Life on Earth

Life on Earth would not have arisen without the presence of the Moon.

The formation and ongoing survival of life on this planet needed a stable
atmosphere, and the possibly the evolution of longer days and shorter years.

The Moon’s Effects

The two most obvious effects of the Moon are the phases of the Moon and the Tides.

For part of the month the Moon appears in the nighttime or early-morning skies.
The rest of the month it can be spotted in the daytime sky.

Over a period of 28 days, the Moon’s shape changes from a thin crescent to a full
Moon and back to a crescent.

Variations in the tides which are caused by the Earth’s rotation, the Moon’s orbital
motion around the Earth, and the Earth’s orbital motion around the Sun.

The Evolution of the Earth-Moon System

Small fragments of rocky material called planetesimals stuck together in a process
called accretion.

Most scientists believe that the Moon was formed when a Mars size asteroid
impacted the Earth during its early molten stages. Show Video.

After the Earth-Moon system became stabilized, incoming planetesimals continued
to bombard the two bodies, causing impact craters. The Earth’s surface has evolved
since then. Because the Earth is geologically an active place, very few craters remain.
The Moon however, is geologically inactive and this results in the Moon’s
pockmarked face that has preserved its early history of collisions.

As the Earth rotates through a 24 hour day, shorelines experience two high tides—
one when the tidal bulge that points toward the Moon passes by, and once when the
tidal bulge that points away from the Moon passes by.
The Moon rises and sets about 50min. later each day, and this is why high and low
tides are about 50min. later each day. Because there are two high tides each day,
each high tide is about 25min. later than the previous one.

The gravitational pull of the Sun also affects tides.

The changing relative positions of the Sun, the Moon, and the Earth cause variation
in high and low tides.

At a new Moon, the Moon is in the same direction as the Sun and the Sun and Moon
rise together in the sky.

When the Sun and Moon are adding together, high tides are even higher than usual,
and low tides are even lower than usual. These tides are called Spring Tides. Spring
Tides are when the Sun and Moon are in general alignment and raising larger tides.

At full Moon, the Moon and Sun are on opposite sides of the Earth. When the sun is
setting, the full Moon is rising. When the Sun is rising, the full Moon are on opposite
sides of the Earth. Therefore, spring tides occur twice a month at both the new-
Moon and full-Moon phases.

When the Earth and Sun make a right angle with the Earth, there tidal effects tend to
counteract one another. At those times, high tides are lower than usual and low
tides are higher than usual. These tides are called Neap Tides. They occur during
first quarter and third quarter Moons. As with spring tides, Neap Tides occur twice a

Another way to look at tides is that the tidal bulges are always located on the sides
of the Earth that point toward and away from the Moon, while the Earth with its
landmass is rotating below the bulges.

Over long periods of time, the tidal bulge has the effect of slowing down the rotation
of the Earth, and actually causing the Moon to move away from the Earth.

As the Earth systems evolves, cycles change as well.

Orbits and Effects

Objects in the solar system move around the Sun in easily predictable paths called

Nicolaus Copernicus was one of the pioneers of orbits.

He placed the Sun at the center of a set of circular planetary orbits.

Planetary Orbits are not circles, but ellipses.
Copernicus was the first to observe that the planets travel in an ellipse and it was
Johannes Kepler that devised three Laws:
      First Law: Each Planet orbits the Sun in an elliptical path with the Sun
      occupying one focus of the ellipse. The point nearest the Sun is called
      “perihelion” and the point in the ellipse farthest away is called “aphelion”.

       Second Law: As a planet moves in orbit around the Sun, it sweeps out equal
       areas in equal times. When Earth is at perihelion with respect to the Sun, it
       moves faster than it does when it is at aphelion.

       Third Law: The square of the length of time a planet takes to make one
       completer orbit around the Sun is equal to the cube of its average distance
       from the Sun.

Most bodies in the solar system follow an elliptical orbit of some kind.

Many comet and asteroids have orbits that cross Earth’s orbit.

Stability of Orbits

Imagine a car parked on a street. It doesn’t just start rolling down the street by itself.
It will stay stopped until it is acted upon by something else—perhaps another car
will smash into it and cause it to move.

It would take the collision of another body to push the Moon out of that orbit and
out into space or careening into the surface of the Earth.

Now in fact, cometary orbits can be changed by gravitational tugs from nearby

Have students think about what conditions would be like on Earth if our planet were
to be tipped over on its side by an outside force and made to roll around in its orbit
as Uranus does.

Eccentricity, Axial Tilt, Precession, and Inclination

The more flattened the ellipse is, the greater its eccentricity. Values of eccentricity
range from zero for a circle, to one, for a straight line.

Axial Tilt

The Earth’s axis of rotation is now tilted at an angle of 23.5 degrees to the plane of
the Earth’s orbit around the Sun.

The greater the angle of tilt, the greater the difference in solar energy, and therefore
temperature, between summer and winter.
The Earth has a slight wobble, the same as the slow wobble of a spinning top, called

The orbits of all the planets except Pluto stay within a narrow range called the
orbital plane of the solar system.

Asteroids and Comets and Meteoroids (Meteors & Meteorites)

Asteroids occupy very little space, with the asteroid belt; it is a misconception that
asteroids are the remains of a planet that exploded.

Comets are “loners” that periodically visit the inner solar system. They usually
originate in the outer solar system.

A rocky body is far more likely to survive the ride through our planetary
atmosphere to reach the ground and do damage. A comet, on the other hand, may
lose much or all of its mass on the way to Earth’s surface, depending on how much
ice it had before it began its trip.

Impacts in the early solar system figured significantly in a process called accretion.
This is how planets and moons are formed when smaller particles of material come
together to form larger ones.

The rate at which comets and asteroids strike the Earth in our time is fairly low.

Comets following high-inclination, eccentric orbits simply aren’t seen until the Sun’s
heat begins to melt their ices, forming a cloudy coma around the nucleus and
causing them to grow a pair of tails that stretch out in the solar wind.

No one knows when the next major impact will happen.

A 20 meter wide piece of space debris created Meteor Crater, some 50,000 years
ago. When we start to discuss objects 1 km in diameter, we are getting into the
realm of global catastrophic effects.

Astronomers think that asteroids at least 1km in diameter hit Earth every few
hundred million years.

Scientists are asking world governments to spend time and money building early
detection systems that could give people a chance to prepare themselves for the
effects of the impacts.

If you look at it in comparison to more commonly occurring events like earthquakes,
volcanic eruptions, or severe weather, then the risk of being affected by an impact is
very small indeed.
Asteroids are rocky bodies smaller than planets. They are leftovers from the
formation of the solar system.

Asteroids orbit the Sun in very elliptical orbits with inclinations up to 30 degrees.

Our planet has undergone at least a dozen mass-extinction events during its history.

NASA is currently forming plans to discover and monitor asteroids that are at least
1km in size and with orbits that cross the Earth’s orbit.

Comets are masses of frozen gases (ices) and rocky dust particles. Like asteroids
they are leftovers from the formation of the solar system.

The icy head of a comet (the nucleus) usually a few kilometers in diameter, but it
appears much larger as it gets closer to the Sun. That is because the Sun’s heat
vaporizes the ice, forming a cloud called a coma.

Along with Solar wind and the heat from the Sun this produces a tail that points
away from the Sun even as the comet moves around the Sun.

Meteoroids are tiny particles in space, like leftover dust from a comet’s tail or
fragments of asteroids.

Meteoroids are called meteors when they enter Earth’s atmosphere, and meteorites
when they reach the Earth’s surface.

About 1000 tones of material is added to the Earth each year by meteorites.

Chondrites may represent material that was never part of a larger body like a moon,
a planet, or an asteroid, but instead are probably original solar-system materials.

The Sun and its effects on Our Community

Everyday we are exposed to radiation from a nuclear power plant, that “nuclear
power plant” is the Sun!

The nuclear power plant we call the Sun is distant enough to spare us from instant
radiation-induced death but close enough to warm the Earth and promote the
continuation of life.

Dark spots exhibiting extreme magnetic fields rampage across its boiling face in
tandem with changes in its overall magnetic activity. Flares and huge eruptions
called coronal mass ejections send streams of ionized particles blasting past the
Earth, affecting everything from communication systems to weather satellites and
Earth-orbiting astronauts.
The Sun sustains a 16-billion-degree furnace.

The Sun has a core that is 13 times denser than solid lead.

The Sun undergoes nuclear fusion—this is the source of all the Sun’s power.

Life on Earth exists, in part because of the warming from the Sun.

Our upper atmosphere forms ozone to shield us from the most dangerous solar
radiation from our Sun.

The Equator rotates much faster than the polar regions do and this may be one of
the causes of Sunspots.

Sunspots are highly magnetic.

Collecting starspot data and evidence of flares and coronal mass ejections from
other stars will go a long way toward helping astronomers understand all the
complex mechanisms that govern star life.

The core is the source of all the energy the Sun emits. That energy travels out from
the core, through a radiative layer and a convection zone above that. Finally, it
reaches the outer layers: the photosphere, which is the Sun’s visible surface, the
chromosphere, which produces much of the Sun’s ultraviolet radiation, and the
superheated uppermost layer of the Sun’s atmosphere, called the corona.

The is the Earth’s main external energy source. Of all the incoming energy from the
Sun, about half is absorbed by the Earth’s surface, the rest is absorbed by the
atmosphere or reflected (30%) or scattered back into space by the Earth or clouds.

The flow of charged particles (also called plasma) from the Sun is called the solar

Molecules of dust and gas in the atmosphere with some of the incoming solar
radiation by changing its direction. This is called scattering, and it explains the blue
color of the sky.

The Sun heats the atmosphere not directly, but rather by warming the Earth’s
surface. The Earth’s surface in turn warms the air near the ground.

The Earth gains energy from the Sun and loses energy to space, but the amount of
energy entering the Earth system is equal to the amount of energy flow out, on a
long-term average.

Solar energy creates weather, drives the movement of the oceans, and powers the
water cycle. All of Earth’s system depends on the input of energy from the Sun. The
Sun also supplies most of the energy for human civilization, either directly, as with
solar power and wind power, or indirectly, in the form of fossil fuels.

The ill effects of sunlight are caused by ultraviolet (UV) radiation, which causes skin

Electromagnetic Spectrum

Astronomers routinely study the universe in x-ray and gamma-ray wavelength, as
well as the radio ultraviolet, and infrared ranges.

Electromagnetic radiation surrounds us in all its forms.

The Earth, and the universe in which it resides can be told only using the
electromagnetic spectrum as a tool of exploration.

Satellites that see the universe through x-ray and ultraviolet eyes.

Theoretically, there is no end to the electromagnetic spectrum: as long as something
has energy, it emits radiation.

Laboratory spectroscopists study the spectrum of each element as it burns under
tightly controlled lab conditions. This procedure allows them to construct a
chemical fingerprint for the elements.

When astronomers compared the spectrum of Chi Lupi to laboratory-standard
spectra for elements that emit light in that narrow wavelength range, they knew
immediately what they had.

Scientists use the tools of spectroscopy to study every kind of object in the universe,
including other plants, the Sun, stars, and galaxies.

Infrared studies of distant dust clouds can reveal the presence of newly forming
stars hidden within protected starbirth nurseries.

The range of dangers we face, harmful radiation from a distant star is not as big a
threat as the dangers posed by solar flares at the height of a sunspot cycle.

Make sure students use a spectroscope to observe the visible part of the
electromagnetic spectrum under natural sunlight, fluorescent light, and
incandescent light.

Spectroscopy- the science of studying the properties of light.

Each color has a characteristic wavelength. This range of colors, from red to violet, is
called the visible spectrum.
The visible spectrum is a small part of the entire spectrum of electromagnetic
radiation given off by the Sun, other stars, and galaxies.

The colors of the visible spectrum are best described as waves, but the same energy
that produces an electric current in a solar cell is best described as a particle.

Long radio waves have wavelengths from several centimeters to thousands of
kilometer, whereas gamma rays are shorter than the width of an atom.

Humans can see only wavelengths between 0.4 and 0.7 um, which is where the
visible spectrum falls. A micrometer (um) is a millionth of a meter.

Ultraviolet radiation gives you sunburn.

Infrared radiation you detect as heat.

Doctors use x-rays to help diagnose broken bones or other physical problems. Law-
enforcement officers use radar to measure the speed of a motor vehicle, and at
home you may use microwaves to cook food.

The wavelength of light with the most energy produced by any objects, including the
Sun, is called its peak wavelength.

Reddish stars are a “cool” 3000 to 4000 K.

Bluish stars are hot over 20000 K.

Kelvins are Celsius degrees above absolute zero, which is at minus 273 degrees C.

One of the most important tools in astronomy is the spectrum—a chart of the entire
range of wavelengths of light from an object.

Think of these spectra as “fingerprints” that reveal many kinds of things about an
object: its chemical composition, its temperature and pressure, and its motion
toward or away from us.

Each chemical element in the universe has its own unique spectrum. If you know
what the spectrum of hydrogen is, you can look for its fingerprint in a star.

A way to sort things by color, size, brightness, and shape.

The invention of multi-wavelength detectors extended our view and allowed us to
explore stares and planets and galaxies more fully.

Astronomers classify stars in many ways. Some of the most common sorting
routines place stars in categories according to their luminosities, masses, and radii.
Astronomers use the Sun as a baseline unit.

Luminosity can be defined in two ways—intrinsic brightness is a measure of the
total radiative output of an object. It is independent of distance from the observer. A
star’s apparent luminosity, on the other hand, describes how bright it appears to us
here on Earth. How bright it appears depends on a combination of its distance from
Earth and its radiative output.

Stars are also classified by their temperatures, which are released to their
brightness. Hot stars are bright, whereas cooler stars are dim.

Spectral classes or types may be the most important classification of all. They help
astronomers determine a star’s are, function, rates of mass loss, and history.

The hotter a star is, the greater its intrinsic luminosity.

G stars (which include our Sun).

From most supernova explosions, we would merely experience a rash of radiation
that our atmosphere should deflect.

In the far distant future, it is clear that our galaxy and the Andromeda Galaxy will
collide with each other.

Imminent danger from the stars is highly unlikely.

Students observe how the brightness of three light bulbs of different wattages varies
with distance.

A Star usually appears to twinkle and that effect has to do with slight distortions of
the path of the starlight as it passes through the Earth’s atmosphere.

Astronomers use a magnitude scale to describe the brightness of objects they see in
the sky. A star’s brightness decreases with the square of the distance.

Astronomers study stars with spectrographs to determine their temperature and
chemical makeup.

Luminosity of a star was related to its surface temperature.

The life cycle of a star begins with its formation in a cloud of gas and dust called a
molecular cloud.

The material in the cloud begins to clump together, mixing and swirling. Eventually
the core begins to heat as more material is drawn in by gravitional attraction. When
the temperature in the center of the cloud reaches 15 million kelvins, the steller
fusion reaction starts up and a star is born. Such stars are called main-sequences
stars. Many stars spend 90% of their lifetimes on the main sequence.

How long a star lives depends on its mass.

Stars like out Sun will live about 10 billion years.

Throughout its life a star loses mass in the form of a stellar wind.

In a bout five billion years the Sun will start to resemble.

The end of a supergiant’s life is a cataclysmic explosion called a supernova.

In an instant of time, most of the star’s mass is hurled out into space, leaving behind
a tiny remnant called a neutron star.

If the star is massive enough, into a stellar black hole—a place where the gravity is
so strong that not even light can escape.

Humans evolved on a planet that was born from a recycled cloud of stellar mass,
they are very much star “stuff”—part of a long cycle of life, death, and rebirth.

In some cases, starbirth is triggered when one galaxy collides with (actually passes
through) another. The clouds of gas and dust get the push they need to start the

Not only are nebulae interesting, but also they show scientists what the fate of our
solar system will be billions of years from now.

A Supernova some five billion years ago may have provided the gravitational kick
that started our own proto-solar nebula on the road to stardom and planetary
Key Vocabulary: AU, Light Year, Parsec, Nebula, Constellation, Tides, Spring Tides,
Neap Tide, Eccentricity, Comet, Asteroid, Meteor, Meteoroid, Meteorite, Solar Wind,
Photosphere, Chromosphere, Corona, Albedo, Sunspots, Solar Flares, Plasma,
Aurora, Spectroscopy, Electromagnetic Spectrum, Radio, Microwave, Infrared,
Visible, Ultraviolet, X-Ray, Gamma, Absorption, Emission, Luminosity, Supernova,
Black Hole, Neutron Star

Phases of the Moon

Earth and Moon

To top