Ancient civilizations used the motions of celestial bodies like the moon and stars to develop calendars and tell time. They observed that the sun's path through the sky shifted over the course of a year, marking seasons. The geocentric model viewed Earth as the center of the universe, with objects like the sun and stars orbiting around it. Stars twinkle due to differences in the density of Earth's atmosphere refracting light. Stars are born from collapsing clouds of gas and undergo various stages of life depending on their mass, eventually ending as white dwarfs, neutron stars, or black holes.

1. •THE UNIVERSE

2. • Ancient ideas

3. • Introduction
– Motion of the moon and the stars was observed by
ancient civilizations.
– These cycles became associated with certain yearly
events.

4. 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.

5. 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.

6. (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.

7. • 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.

8. A schematic representation of the geocentric Ptolemaic
system.

9. The celestial
sphere with
the celestial
equator
directly above
Earth's
equator, and
the celestial
poles directly
above Earth's
poles.

10. • Babylonians
– The Babylonians used the study of the stars to guide their
affairs.
– This was developed as early as 540 BCE
• 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.

11. 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.

12. 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.

13. The apparent position of Mars against the background of
stars as it goes through retrograde motion. Each position is
observed approximately two weeks after the previous
position.

14. • The night sky

15. • 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.

16. • 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.

17. • Once you have established the celestial equator, the
celestial poles, and the celestial meridian, you can
use a two-coordinate horizon system to locate
positions in the sky.
– One popular method of using this system
identifies the altitude angle (in degrees) from the
horizon up to an object on the celestial sphere and
the azimuth angle (again in degrees) the object on
the celestial sphere is east or west of due south,
where the celestial meridian meets the horizon.

18. The illustration shows an altitude of 30O
and an azimuth of
45O
east of due south

19. The North Star, or Polaris, is located by using the pointer
stars of the Big Dipper.

20. The altitude of Polaris
above the horizon is
approximately the
same as the observer's
latitude in the
Northern Hemisphere.

21. • 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 (‘’).

22. 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).

23. • Parallax
– Distances can be measured using the apparent
shift of position that an object goes through when
viewed relative to a background and from
different positions.

24. The angle through which a star seems to move against the
background of stars between two observations that are 1
A.U. apart defines the parallax angle. The distance to
relatively nearby stars can be determined from the
parallax angle (see example 16.1).

25. A parsec is defined as the distance at which the parallax
angle is 1 arc second. A parsec is approximately 3.26
light-years.

26. What is the distance to a star with a parallax of exactly 1
arc second? See example 16.1 for the answer.

27. • Astronomical Unit.
– An astronomical unit is defined as the radius of
the Earth’s orbit.
– One AU is approximately 1.5 X 108
km (9.3
X 107
mi.).

28. • Parsec
– A parsec is the distance at which the angle made from 1
AU baseline is 1 arc second.
– Distance to stars in parsecs is the reciprocal of the
parallax angle in seconds of arc.

29. • Light Year
– This is the distance that light travels in one year, which is
9.5 X 1012
km or 6 X 1012
mi.

30. • Stars

31. • 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.

32. – 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.

33. 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.

34. • Brightness of stars.
– The classification is based on apparent magnitude scale
– The first magnitude is defined as 100 times brighter
than a sixth magnitude star.
– There are then 5 equal divisions between these two.
– Each magnitude is approximately 2.51 times fainter than
the next higher magnitude number.
– The absolute magnitude of a stars brightness is the
brightness it would have if it were measured at 10
parsecs (32.6 light-years)
– Absolute magnitude is an expression of a stars luminosity
which is the amount of energy radiated into space per
second

35. • 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.

36. 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.

37. 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.

38. • 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.

39. The Hertzsprung-Russell diagram.

40. The region of the cepheid variable, red giant, main
sequence, and white dwarf stars and novas on the H-R
diagram.

41. – Red Giant Stars
• Bright, low temperature stars.
• These stars are enormously bright for their
temperature due to their size.

42. – White Dwarf Stars
• Faint, white hot stars.
• Faint due to its small size.

43. – Cepheid Variable
• A bright variable star that is used to measure
distances.

44. • The life of a star.
– Life for a star begins as a giant cloud of gas and dust
settles and then begins to shine due to fusion of hydrogen
nuclei in its core.
• Can expand to a red giant, then blow off the outer
shell to become a white dwarf star.
• May also collapse on itself to become a neutron star
• A massive star may collapse to become a black hole.

45. – The first stage in the life of a star is the formation of a
protostar.
• As gravity continues to pull the gas of a protostar
together, density, pressure, and temperature increase
from the surface to the center of the protostar
• When the temperature reaches 10 million Kelvin,
nuclear fusion reactions begin in the core.

46. – The second stage begins when the hydrogen core becomes
fused to produce helium.
• As there are now less hydrogen fusion reactions, less
energy is produced, which means less outward pressure, so
the star begins to collapse due to gravitational pull.
• This collapse begins to heat the helium core of the star and
the hydrogen begins to expand.
• It now becomes a red giant and will remain so for about
500 million years.
• The red giant has helium fusion reactions occurring in the
core and hydrogen fusion reactions occurring in the shell.
• The radius and luminosity decrease and the star moves
backward to a main sequence star.

47. – After millions of years of helium fusion reactions the
core gradually is converted to a carbon core, with two
shells, one of helium fusion reactions and one of
hydrogen fusion reactions
• This results in a great release of energy and the star reverts
back to a red giant one more time.
• As the outer shells expand, however, they give off energy and
again contract.
• The shells sort of pulsate back and forth.
• Eventually the outer layers will be blown off, leaving just the
carbon core and helium fusing shell and becomes a white
dwarf.
• The blown off outer shell becomes a planetary nebula

48. – In a small star this will end as the star is converted into a
lump of stellar carbon.
– In a massive star, this process will continue until the
fusion ends with iron.
• Since iron cannot undergo fusion
• The star thus loses its outer pressure and explodes as a
supernova
– If the remaining core has a mass of 1.4 solar masses or
more, the remaining core that is left after the supernova is
pulled together by gravitational forces.
• This force collapses the nuclei forcing electrons and
protons together into neutrons and forms a neutron
star.

49. – If the neutron star becomes strongly magnetized and may
emit electromagnetic pulses and it is a pulsar.
– If the mass of the remaining core after the supernova has
a mass of 3 solar masses of more the star may collapse to
the point where all forces are overcome
• This may create a mass of matter so dense that even
light cannot escape.
• This is called a black hole.

50. A star becomes stable when the outward forces of
expansion from the energy released in nuclear fusion
reactions balance the inward forces of gravity.

51. The evolution of a star of solar mass as it depletes
hydrogen in the core (1),

52. fuses hydrogen in the shell to become a red giant (2 to 3),

53. becomes hot enough to produce helium fusion in the core
(3 to 4),

54. 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.

55. The blown-off outer layers of stars form ringlike
structures called planetary nebulae.

56. 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.

57. • Galaxies.

58. • 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.

59. 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

60. The structure of the Milky Way galaxy.

61. • 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

62. – Andromeda
• Nearest neighbor that is similar to the Milky Way.
• About 2 million light years away.

63. The
Andromeda
galaxy, which
is believed to
be similar in
size, shape,
and structure
to the Milky
Way galaxy.

64. – Elliptical Galaxies.
• Appear to be spherical and flattened.
• Contain only old stars with little gas or dust.

65. – 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.

66. – Barred Galaxies
• Shape of a bar
• Spiral arms radiating out from the ends of the bar

67. – 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.

68. Different subgroups in the Hubble classification scheme:
(A) elliptical galaxies, (B) spiral galaxies, and (C) barred
galaxies.

69. • 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.

70. – 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.

71. Will the universe continue expanding as the dust and gas
in galaxies become locked up in white dwarf stars,
neutron stars, and black holes?

72. 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.