chapter 13 universe 2 v2
TRANSCRIPT
THE UNIVERSE
• Introduction– Motion of the moon and the stars was observed by
ancient civilizations.– These cycles became associated with certain yearly
events.
Ancient civilizations used celestial cycles of motion as clocks and calendars.
(A) This photograph shows the path of stars around the North Star.
(B) A "snapshot" of the position of the Big Dipper over a period of 24 hours as it turns around the North Star one night. This shows how the Big Dipper can be used to help you keep track of time.
The stone pillars of Stonehenge were positioned so that the movement of the Sun and Moon could be followed
with the seasons of the year.
(A)The people of ancient civilizations observed that the apparent path of the sun throughout a year is within a band across the sky. For the north temperate latitudes, the southern limit of this band occurs on the day of winter solstice, when the sun rises south of east. The northern limit occurs on the day of summer solstice, when the sun rises north of east.
(B) Ancient civilizations also built monuments to mark movements of the Sun, and the shift defined a year.
• Geocentric model– The geocentric model was one in which the Earth was
seen as the center of the Universe and everything in the Universe moved around the Earth.
– The Egyptians believed that the Earth was surrounded by water and that the stars were lamps hung from a dome surrounding the Earth.• They also believed that the Sun was a disk of fire
carried by the sun god Ra.
A schematic representation of the geocentric Ptolemaic system.
The celestial sphere with the celestial
equator directly above
Earth's equator, and the celestial
poles directly above Earth's
poles.
• Babylonians– The Babylonians used the study of the stars to guide their
affairs.– This was developed as early as 540 BC
• Greeks.– Viewed the Earth as the center of the universe.– Failed to account for retrograde motion.
• Retrograde motion is the apparent shifting of the motion of the planets as they loop in their orbit.
The Sun, Moon, and planets move across the constellations of the zodiac, with the Sun moving around all twelve
constellations during a year. From Earth, the Sun will appear to be "in" Libra at sunrise in this sketch. As Earth revolves around the Sun, the Sun will seem to move from Libra into
Scorpio, then through each constellation in turn.
During the time of the Babylonians, the Sun rose with the constellation Taurus on the first day of spring. Today,
however, the Sun rises with the constellation Pisces on the first day of spring. Earth's precession will continue to change the position of the Sun during a particular month, and 25,780 years after the time of the Babylonians, the Sun will again rise
with the constellation Taurus on the first day of spring.
• The night sky
• Introduction– Stars twinkle due to differences in density of the Earth’s
atmosphere.– This difference in density causes the light to be refracted
one way and then another as the air moves.
• Celestial location– Celestial Equator
• This is the line where the Earth’s equator touches the celestial sphere
– North Celestial Pole• This is the line where the Earth’s North Pole touches the
celestial sphere– South Celestial Pole
• This is the line where the Earth’s South Pole touches the celestial sphere
– Celestial Meridian• The Celestial Meridian is located from where a person is on the
Earth.
The illustration shows an altitude of 30O and an azimuth of 45O east of due south
The North Star, or Polaris, is located by using the pointer
stars of the Big Dipper.
The altitude of Polaris above the horizon is approximately the
same as the observer's latitude in the
Northern Hemisphere.
• Celestial Distance– The Celestial Meridian is divided into 360 divisions
each known as 1O of arch.• Each of these degrees (O) is divided into smaller
divisions known as minutes (‘) and seconds (‘’).
The Moon and the Sun both have an
angular size of 0.5O, but the Sun is much farther away. The
observed angular size depends on distance and the true size of
an object. Thus during a solar eclipse the Moon appears to nearly cover the Sun (images are separated
here for clarity).
• Light Year– This is the distance that light travels in one year, which is
9.5 X 1012 km or 6 X 1012 mi.
The Starry Night
• Stars
• Origin of stars– Stars are born in nebulae, which is a swirling cloud of
hydrogen gas.– The random motion of stars can cause random shock
waves in the cloud, causing the molecules to collide and produce local compressions.
– The mutual gravitational attraction can then pull them together into a cluster.
– When enough atoms are pulled into this cluster it can produce a protostar, which is simply the accumulation of gases that will eventually become a star.
– As the gas molecules are pulled toward the center of the protostar, they gain kinetic energy.
– This increase in kinetic energy and the increase in mass at the center create a situation where nuclear fusion reactions can begin.
– The interior of a star has 3 shells.• The core.• A radiation zone.• The convection zone.
The structure of an average, mature star such as the Sun. Hydrogen fusion reactions occur in the core, releasing gamma and X-ray radiation.
This radiation moves through the radiation zone from
particle to particle, eventually heating gases at
the bottom of the convection zone. Convection cells carry energy to the surface, where
it is emitted to space as visible light, ultraviolet radiation, and infrared
radiation.
• Brightness of stars.– The classification is based on apparent magnitude scale– The apparent magnitude : how bright the star appears
from Earth– The absolute magnitude : how bright the star appears at
a standard distance of 32.6 light years, or 10 parsecs– Absolute magnitude is an expression of a stars luminosity
which is the amount of energy radiated into space per second
• Star temperature.– The color difference between stars is the relationship
between color and temperature of an incandescent object.– Cooler stars appear reddish and hotter stars appear bluish
white– Stars with temperatures in between, such as the Sun,
appear yellow.
Not all energy from a star goes into visible light. The graph shows the distribution of radiant energy emitted
from the Sun, which has an absolute magnitude of +4.8.
The distribution of radiant energy emitted is different for stars with different surface temperatures. Note that the
peak radiation of a cooler star is more toward the red part of the spectrum, and the peak radiation of a hotter star is
more toward the blue part of the spectrum.
Properties of Star
• Star types.– Stars are classified according to the Hertzsprung-Russel diagram
(HR diagram) which classifies stars based on temperature and luminosity
• The HR diagram plots temperature by spectral type sequenced O through M types, with the temperature decreasing from left to right.
• Each point on the graph represents the surface temperature and brightness of a star.
– Most stars are called main sequence stars as they fall in a narrow band that runs from the top left to the lower right of the HR diagram.
• These are mature stars which are using their nuclear fuel at a steady rate.
The Hertzsprung-Russell diagram.
The region of the cepheid variable, red giant, main sequence, and white dwarf stars and novas on the H-R
diagram.
– Red Giant Stars• Bright, low
temperature stars.• These stars are
enormously bright for their temperature due to their size.
– White Dwarf Stars• Faint, white
hot stars.• Faint due to its
small size.
The evolution of a star of solar mass as it depletes hydrogen in the core (1),
fuses hydrogen in the shell to become a red giant (2 to 3),
becomes hot enough to produce helium fusion in the core
(3 to 4),
then expands to a red giant again as helium and hydrogen fusion reactions move out into the shells (4 to 5). It
eventually becomes unstable and blows off the outer shells to become a white dwarf star.
The blown-off outer layers of stars form ringlike
structures called planetary nebulae.
Formation of stars
This flowchart shows some of the possible stages in the birth and aging of a star. The differences are determined
by the mass of the star.
Life cycle of a star
• Galaxies.
• The Milky Way Galaxy.– Like other galaxies, the Milky Way galaxy is a galactic
cluster of stars held together by gravitational attractions.– The Milky Way galaxy is made up of three distinct parts.
• A galactic nucleus which is a cluster of stars at the center• A rotating galactic disk, which contains most of the bright, blue
stars and the dust and gas.• A spherical galactic halo, which contains 150 globular clusters
that each contain millions of stars packed din tightly and are located outside of the galactic disk.
A wide-angle view toward the center of
the Milky Way galaxy. Parts of
the white, milky band are obscured from
sight by gas and dust clouds
in the galaxy
The structure of the Milky Way galaxy.
• Other Galaxies.– Sagittarius
• The nearest galaxy to the Milky Way• 80,000 light years from our solar system• A dwarf galaxy as it is only 1,000 light years across
– Andromeda• Nearest neighbor that is similar to the Milky Way.• About 2 million light years away.
The Andromeda
galaxy, which is believed to be similar in size, shape,
and structure to the Milky Way galaxy.
– Elliptical Galaxies.• Appear to be
spherical and flattened.
• Contain only old stars with little gas or dust.
– Spherical Galaxies• Small spherical
nucleus with two or more spiral arms.
• Young stars are found in the spirals
• Older stars and found in the globular cluster of the halo.
– Barred Galaxies• Shape of a bar• Spiral arms radiating
out from the ends of the bar
– Irregular Galaxies• Galaxies which lack
any symmetry at all.• Contain mostly
young stars and the greatest amount of dust and gases of any of the galaxies.
Different subgroups in the Hubble classification scheme: (A) elliptical galaxies, (B) spiral galaxies, and (C) barred
galaxies.
• The life of a galaxy.– The Big Bang
• The known Universe was created during the big bang, which was not an explosion, it was an event that created space.
• The space has been continually expanding ever since.
– The Big Crunch• Eventually all of the energy in the known Universe
may be locked up in White Dwarfs, and black holes.• This may create a reversal of the big bang where all
energy is locked up and attractive forces in the black holes will swallow up all of space until all of space has been condensed to a pinhead.
Will the universe continue expanding as the dust and gas in galaxies become locked up in white dwarf stars,
neutron stars, and black holes?
Big bang
The oscillating theory of the universe assumes that the space between the galaxies is expanding as does the big bang theory, but in the oscillating theory, the galaxies
gradually come back together to begin allover in another big bang.
Big Crunch