Introduction • Motion of the moon and the stars was observed by ancient civilizations. • These cycles became associated with certain yearly events.
(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. Ancient civilizations used celestial cycles of motion as clocks and calendars.
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.
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 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.
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 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.
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.
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.
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).
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.
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).
A parsec is defined as the distance at which the parallax angle is 1 arc second. A parsec is approximately 3.26 light-years.
What is the distance to a star with a parallax of exactly 1 arc second? See example 16.1 for the answer.
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.).
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.
Light Year • This is the distance that light travels in one year, which is 9.5 X 1012 km or 6 X 1012 mi.
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 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
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.
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 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.
Cepheid Variable • A bright variable star that is used to measure distances.
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.
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.
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.
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
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.
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.
A star becomes stable when the outward forces of expansion from the energy released in nuclear fusion reactions balance the inward forces of gravity.