Download Chapter 30 Notes

Chapter 30 Notes
The Big Bang Theory
Objectives
Explain how Hubble’s discoveries lead to an understanding that the universe is expanding.
Summarize the big bang theory
List evidence for the big bang theory
Cosmology- the study of the origin, properties, processes, and evolution of the universe
Hubble found that the spectra of galaxies, except for the few closest to Earth, were shifted toward
the red end of the spectrum. This means that these galaxies are moving away from ours and the
amount of red shift allows astronomers to measure the speed at which these galaxies are moving
away. The most distant galaxies are moving away the fastest. Recall the balloon activity where the
far away dot moved away at a rate much faster than the two that were close together, when you blew
up the balloon. All of this leads scientists to believe the universe is expanding
Big bang theory- the theory that all matter and energy in the universe was compressed into an
extremely small volume that 3 to 15 billion years ago exploded and began expanding in all
directions.
Cosmic background radiation- radiation uniformly detected from every direction in space;
considered a remnant (left over) of the big bang. Astronomers believe that this radiation formed
shortly (nanoseconds) after the big bang. This radiation was extremely hot, but has cooled over time
to about -270º C. Maps of this radiation show ripples over the entire sky. These ripples are caused
by small fluctuations in the distribution of matter in the early universe. These ripples may indicate
the first stages in the formation of the universe’s first galaxies. Below is a timeline from the big
bang to present.
Dark matter
The matter that makes up humans, the planets, the stars and the matter between the stars makes up
only about 4% of the universe.
About 23% of the universe is made up of a type of matter that does not give off light but has gravity.
This matter is called dark matter.
Dark energy
Most of the universe is made up of an unknown material called dark energy. Scientists think this
energy acts as a force that opposes gravity and is what is responsible for the expansion of the
universe.
Characteristics of Stars
Objectives
Describe how astronomers determine the composition and temperature of stars
Explain why stars appear to move in the sky
Describe one way astronomers measure the distances to stars
Explain the difference between absolute magnitude and apparent magnitude
Star- a large celestial body that is composed of gas and that emits light.
Nuclear fusion is the combination of light atomic nuclei (such as hydrogen) to from heavier atomic
nuclei (such as helium or carbon).
Astronomers use light from stars to learn about their composition and temperature.
Light passed through a spectrograph produces a display of colors called a spectrum.
All stars have dark line spectra.
Dark line spectra- bands of color crossed by dark lines where the color is diminished or reduced
The dark line spectrum tells us the star’s composition and temperature.
Different elements have different spectrums. So depending on a star’s composition its spectrum will
have bands of color that are the same color as the element it is made of.
The most common element in stars is hydrogen, followed by helium. There are small quantities of
carbon, oxygen, found in stars as well.
The temperatures of most stars range from 2,800º C to 24,000º C. There are some stars that have
higher temperatures. Blue stars have an average surface temperature of 35,000ºC. Yellow stars,
such as our sun, have surface temperatures of about 5,000º C. Red stars have the lowest average
surface temperatures, about 3,000º C.
Stars vary in size and mass.
Stars like are sun are considered medium-sized stars, with a diameter of 1,390,000 km. Most stars
visible from earth are medium sized stars. Many stars also have similar mass to our sun, although
there are a few that are more or less massive.
Apparent Motion- the motion visible to the unaided eye. Apparent motion is caused by the
movement of the Earth.
The daily rotation of the Earth makes it appear that (in the Northern Hemisphere) the stars are
moving in a counter clockwise motion around the North Star (Polaris). Earth’s revolution around the
sun makes it seem like the stars shift to the west slightly every night.
Circumpolar stars- stars that never pass below the horizon. In the Northern Hemisphere these stars
are the ones that appear to circle the North Star (Polaris).
Actual Motion
Stars actually move in a few ways. They rotate on an axis, like Earth. They may revolve around
another star. They either move toward or away from Earth.
Doppler Effect- an observable change in the frequency of a wave with the source or the observer is
moving, or both. This effect is what causes red and blue shift in the color of the light from a star.
Below is a chart of light affected by the Doppler Effect.
Light-year- the distance that light travels in one year. This is not considered the same as the speed
of light. The time it takes you to travel to Salt Lake is not the speed at which you traveled. That is
why the speed of light and a light year is not the same measurement. A light year is the time it takes
light to travel a particular distance and is just a measurement scientists use to account for distance in
space.
Parallax- the apparent shift in the position of an object when viewed from different locations. For
close stars scientists use parallax to measure the distance to those stars.
Apparent Magnitude- the brightness of a star as seen from the Earth. This depends on how much
light the star emits and how far the star is from Earth.
Absolute Magnitude- the brightness that a star would have at a distance of 32.6 light-years from
Earth. The brighter a star is, the lower its number of absolute magnitude. The diagram on the next
page shows an absolute magnitude scale.
The lower the number of the star on the scale shown on the diagram below, the brighter the star
appears to observers.
Stellar Evolution
Objectives
Describe how a protostar becomes a star.
Explain how a main-sequence star generates energy.
Describe the evolution of a star after its main-sequence stage.
One way scientists classify stars is by plotting the surface temperature against the star’s luminosity
(the amount of light it gives off). The graph that represents this plotting is called the H-R diagram.
Scientists use the H-R diagram to describe the life cycles of stars. Most stars fall within a band that
runs diagonally through the middle of the diagram. These stars are called main sequence stars.
Main Sequence-the location on the H-R diagram where most stars lie; it has a diagonal pattern from
the lower right to the upper left.
The diagram below is the H-R diagram
Star Formation
Nebula- a large cloud of gas and dust in interstellar space; a region in space where stars are born. A
star begins in a nebula. When the nebula is compressed, some of the particles move close to each
other and are pulled together by gravity. As they move closer together the gravitational pull
increases. As more particles come together, regions of dense matter begin to build up within the
nebula.
Protostars- As the gravity makes denser regions, these regions spin and shrink and begin to form
flattened disks. The central region of matter is called a protostar. This protostar contracts and
increases in temperature for several million years. Eventually it is so hot that electrons get stripped
from their parent atoms. These nuclei and free electrons mixed in a separate state of matter is called
plasma.
Birth of a star
A protostar’s temperature increases to 10,000,000º C. At this temperature nuclear fusion begins.
This process releases an enormous amount of energy. The start of nuclear fusion marks the birth of
a star.
As gravity increases the pressure on the matter with in the star; the rate of fusion increases. The
energy radiated from fusion reactions heats the gas inside the star. The outward pressures of the
radiation and the hot gas resist the inward pull of gravity. This balance makes the star stable in size.
This can continue for billions of years.
The second and longest stage of a star is called the main-sequence stage. During this stage energy is
generated from hydrogen being fused into helium. A star about the same mass as our sun stays in
the main-sequence stage for about 10 billion years. Scientists estimate that over about 5 billion
years our sun has converted only about 5% of its original hydrogen nuclei into helium nuclei.
Leaving the main-sequence
Giant stars
Giant- a very large and bright star whose hot core has used most of its hydrogen. This stage starts
when a star has fused most of its original hydrogen nuclei into helium nuclei. As this happens the
gravity at the core is reduced and the star starts to expand. With this expansion the gases in the outer
shell begin to cool. As they cool they begin to glow a reddish color. These stars are known as
giants.
Supergiants- Stars more massive (not bigger, but have more mass) than our sun, will become larger
giants in their third stage. These highly luminous stars are called supergiants. These stars appear
along the top of the H-R diagram above.
Planetary Nebulas
As the star’s outer gases drift away; the remaining core heats these expanding gases.
The gases appear as a planetary nebula; a cloud of gas that forms around a sun like star that is dying.
White Dwarfs
As the planetary nebula disperses; gravity causes the remaining matter in the star to collapse inward
until it cannot be pressed further together. A hot, extremely dense core of matter is left behind. This
mass is called a white dwarf and can shine for billions of years before it cools completely. Stars
more massive than our sun do not become white dwarfs.
Novas and Supernovas
Nova- a star that suddenly becomes brighter.
Some white dwarfs revolve around red giants. When this happens the white dwarf may capture
some gases from the red giant. As these gases accumulate pressure begins to build up. This pressure
can cause large explosions called novas. A supernova is when the explosion is so large that it
completely destroys the white dwarf and possibly much of the red giant. Massive stars (stars that
have a large mass, not necessarily size) become supernovas as part of their life cycle.
Neutron stars- a star that has collapsed under gravity to the point that the electrons and protons have
smashed together to form neutrons. After a star explodes as a supernova, the core may contract into
a neutron star.
The chart below is a chart of the possible life cycles of stars.
Pulsar- a rapidly moving neutron star that emits pulses of radio and optical energy. For each pulse
detected here on Earth, we know the pulsar has rotated once within the period between pulses.
Black hole- an object so massive (not always big but having a lot of mass) and dense that even light
cannot escape its gravity. Some massive stars produce leftovers too massive to become stable
neutron stars. These leftovers contract and the force of the contraction leaves a black hole.
Star Groups
Objectives
Describe the three main types of galaxies
Explain how a quasar differs from a typical galaxy
Constellation- a group of stars organized in a recognizable pattern. In 1930, astronomers around the
world agreed upon a standard set of 88 constellations.
Multiple star systems
Over half of all observed stars form multiple-star systems.
Binary stars are pairs of stars that revolve around each other and held together by gravity. The
center of mass or barycenter is somewhere between the two stars.
Star clusters
Sometimes, nebulas collapse to form groups of hundreds or thousands of stars called clusters.
Globular clusters are round and can contain around 100,000 stars
An open cluster is loosely shaped and rarely has more than a few hundred stars.
Galaxy- a collection of stars, dust, and gas bound together by gravity. These are the major building
blocks of the universe. Astronomers estimate that there are hundreds of billions of galaxies. A
typical galaxy has a diameter of about 100,000 light-years and may contain more than 200 billion
stars.
Types of Galaxies
There are three main types
Spiral Galaxy- has a nucleus of bright stars and flattened arms that spiral around the nucleus.
Elliptical Galaxies- have various shapes and are extremely bright in the center and do not have spiral
arms.
Irregular Galaxies- has no particular shape, and is fairly rich in dust and gas.
The Milky Way
It is a spiral galaxy in which the sun is one of hundreds of billions of stars. Two irregular galaxies
called the Large Magellanic Cloud and Small Magellanic Cloud are our closest neighbors. These
three galaxies are called the Local Group.
Quasars- quasi-stellar radio source; a very luminous object that produces energy at a high rate.
These appear as points of light similar to stars. They are located in the centers of galaxies that are
distant from Earth. They are among the most distant objects that have been observed from Earth.
End Chapter 30 Notes.