N. SCI. 1
SOME FACTS ON STARS AND GALAXIES
1. Study of stars dates back to 4000 BC, to the ancient Egyptians, Babylonians and Greeks whose
systematic observations yielded ideas which became the stepping stones to present day
knowledge.
2. We can see half the sky sphere at any one time. The position of constellations in the sky
changes every hour because they move westward. New constellations appear in the east and
constellations near the western horizon disappears.
3. Measurement of distances of stars by the parallax method and by the use of light period or
luminosity of cepheids, reveals that stars, even within a constellation are of different distances
from the earth.
4. Stars differ in apparent magnitude. To compare their actual brightness, astronomers compute
how bright they would be if they were 10 parsecs away. This is their absolute magnitude.
5. Stars differ in color because they differ in temperature. Red stars are coolest while blue stars are
hottest.
6. The spectral lines of stars reveal that stars are made up of 73% hydrogen (H), 25% helium (He),
and 2% all the other elements together.
7. Henry Norris Russel and Einjar Hertzsprung in plotting the absolute magnitude of stars against
their colors, discovered that most of the stars fell along a diagonal band which they called the
main sequence. Outside the main sequence are the very big but cool, red giants and the hot
tiny white dwarfs.
8. Large packets of gases sufficiently near one another are pulled together by gravity until they
become hot enough to glow as a protostar.
9. A protostar squeezed further by gravity becomes hot enough for hydrogen fusion to take place
and it begins life as a star.
10. A star remains stable and stays in the main sequence as long as the balance between the
gravitational force and the nuclear force is maintained. When nuclear reaction stops, the star
collapses. When nuclear force becomes greater, the star explodes.
11. A star is about to die when its hydrogen core is used up. Then it may explode as a planetary
nebula or as a supernova and it ends up as a white dwarf, a neutron star, or a black hole,
depending on its original mass.
12. Our sun belongs to a group of 100 billion stars held together by gravity, the Milky Way galaxy.
The sun, which is 30,000 light years away from the center of the galaxy, revolves around it once
every 250 million years.
13. Galaxies come in varied shapes and sizes, but the most common are the spiral galaxies to which
the Milky Way belongs..
14. Within a galaxy, stars die and new stars are formed from the hydrogen in space and the debris of
dead stars.
15. Doppler shifts have shown that the galaxies are moving away from each other at a speed of 55
km/sec/million parsecs.
16. The two most popular theories on the origin of the universe are the Big Bang and the Steady
State. The former claims that the universe will end with the last star while the latter claims that
the universe is endless, for hydrogen is continuously being formed.
PROPERTIES OF STARS
1. Magnitude. This refers to the brightness of the star.
2. Color and Temperature. Color is a result of the temperature.
3. Composition. A spectroscope is used to study the composition of stars. As stars become older,
their composition, surface temperature, and luminosity changes.
4. Proper motion. There is very little movement among stars. Some stars show proper motions of 1
sec or more of arc per year, others may show seconds of arc per 100 years.
5. Radial velocity. This type of motion shows whether a star is moving in a line away or toward the
earth.
6. Size. The size of a star may be determined by interferometry. The technique is called speckle
photography which obtains an image on the surface of a big star. The image is fed into a
computer to reassemble several short exposures into a single composite. If a star is small, its
luminosity and temperature may be used to determine its size. The diameter can be comuted by
using the Stefan Boltzmann Law which states that: “the hotter a star, the more it radiates per unit
area.”
7. Stellar distance.
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STAR GROUPINGS
1. Open Star Clusters. Are moderately close-knit, irregularly shaped groupings of stars. They
usually contain 100 – 1000 members and are about 4 to 20 parsec in diameter.
2. Associations. Are cousins of open clusters. They often have fewer stars but are larger in size
and have a looser structure. Some large associations include an open star cluster within them.
They may have 10 to a few hundred members and diameters of about 10 – 100 parsec. They are
rich in very young stars.
3. Globular Star Clusters. Are quite different from the two other types. They are more massive,
tightly packed, symmetrical and very old. Typically contain 20,000 to several million stars.
A STAR’S LIFE CYCLE
A. Star Formation
Stars are forming in the nebula of the Pleiades and in the Great Orion nebula.
a) The force of gravity causes the gas in the nebula to contract forming the protostar ( a very
young star that is not yet hot enough to shine by nuclear fusion).
b) Protostar’s matter is packed more tightly, its temperature rises.
B. Main Sequence Stage
a) Protostar’s temperature rises and nuclear fusion begins in its core. H21 + H31 = He + n + E
b) The protostar becomes a true star and becomes part of the main sequence.
c) If most hydrogen (H) is converted to helium (He), great changes occur.
i. The star expands and becomes a giant or super giant.
ii. The star’s core becomes extremely hot and may reach a temperature of 100 M°C.
iii. Nuclear fusion begins again. Helium fuses to become carbon (c).
C. Final Stage
a) White Dwarf.
When helium (He) is used up the star enters the final stages of its life cycle. The star’s final
stage depends on how much it contains. Small or medium sized stars may shrink and
become dimmer and end up becoming a white dwarf.
b) Supernova.
A massive star may have continuous increase in its temperature and end up becoming a
supernova.
c) Neutron star.
The remains of supernova that contracts; an extremely dense star made entirely of tightly
packed neutrons.
i. May have a diameter of 20 km yet contain twice or thrice the mass of the sun such that a
teaspoon of neutron star will be greater than all the people on Earth.
ii. May rotate rapidly in space. Hence, its beams sweeps through space.
Pulsar – a neutron star for which radiation is viewed in regular, rapid pulses.
d) Black Hole.
This is the fate of most massive star. The gravity compresses it to a single point that take up
no space at all. The star’s disappear leaving its gravitational force behind. A black hole is a
collapse object from which no light or matter can escape. It can be detected through effects
on radiation and effects on the motion of other stars.
EVOLUTION OF STARS
The evolution of the stars can be summarized into the following phases:
1. Condensation Phase
A star is formed from the condensation of interstellar gas and dust composed mainly of
hydrogen. Shock waves or disturbances triggered the condensation of gas and dust to form a
small gas cloud called protostar. Once condensation starts, hydrogen atoms combine to form
hydrogen molecules. This process further accelerates condensation because of the greater mass
of the hydrogen molecules.
2. Contraction Phase
The protostar falls to its center because of its own gravitational pull. It has been estimated
that a protostar collapses in about 10 M years. A protostar rotates with a velocity that becomes
greater as the body becomes smaller.
As the protostar collapses, its potential energy is converted into heat and light energies.
When the surface temperature reaches 2000 to 3000 Kelvin (K, 1°C = 273.15 K), the temperature
at its center is as high as 100,000 K.
3. Convection Phase
Because of the great difference in temperature between the central and outer parts of the
protostar, the interior becomes unstable. Convection occurs. In this process, the hotter inner
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part is transported outward and the cooler outer part moves into the inner region. Contraction
continues. During this stage or phase, luminosity decreases.
4. Radiative Phase
After thousands of years (for a high-mass star) or millions of years (for a low-mass star),
convection stops at the center. Instead, energy is transported outward from the center by
radiation. This stage is called radiative phase. It occurs rapidly at a particular time. During this
period, luminosity stays nearly the same. However, the temperature of the star increases.
5. Nuclear Fusion Phase
Finally when the temperature becomes high, nuclear reaction occurs. The type of reaction
depends upon the temperature at the center. At temperatures between 5 and 15 million degrees,
hydrogen nuclei fuse to form helium. The nuclear fusion is called proton-proton cycle. For
massive stars where the temperature reaches a billion degrees, the following reactions, called the
carbon cycle, occur:
12
+
13
=
13
+
H1
proton
14
+
1
15
=
15
+
C6
Carbon – 12
N7
Nitrogen – 13
C6
Carbon – 13
N7
Nitrogen – 14
O8
Oxygen – 15
N7
Nitrogen – 15
1
H1
proton
13
C6
Carbon – 13
1
H1
proton
=
+
=
=
15
N7
+
Nitrogen – 15
1
H1
proton
=
13
N7
Nitrogen – 13
0
e+1
positron
14
N7
Nitrogen – 14
15
O8
Oxygen – 15
0
e+1
positron
12
C6
Carbon – 12
+
+
4
He2
Helium – 4
Nuclear energy from the above reactions causes the protostar to become self-luminous. It then
enters into the stable main sequence phase.
6. Stable Main Sequence Phase
In the stable main sequence phase, the inward pressure balances the outward pressure. The
inward pressure is due to gravitation. The outward pressure is due to high gas and radiation
pressure which is caused by the exceedingly high temperature in the center. At this stage,
contraction stops and the star becomes stable and constant in size.
A star spends most of its lifetime at this stage. Stars of different masses existing in this
condition are referred to as stable main sequence stars burning hydrogen to helium. Hot stars
give a blue luminous color while cool stars give a faint red color. Since the Sun is intermediate in
temperature, it gives a yellow color. A yellow star remains a main-sequence star for about 10
billion years.
SOLAR SYSTEM
What Is the Solar System? The solar system is the family of celestial bodies grouped around the (our)
sun. This system consists, first of the SUN itself and the nine bodies revolving around it known as
Planets. Our own Earth is one of these planets. The planets, in turn, have Satellites or moons
revolving around them. There are known to be thirty (30) such satellites. In addition, there are minor
bodies (members) within the solar system, including: comets, meteors, and asteroids (or planetoids).
PROPERTIES OF THE SOLAR SYSTEM
The solar system shows remarkable regularities in some of its properties. These regularities are
observed in the motion and location of the planets and their satellites. These observed facts are
important clues to the origin of the solar system.
Among these observed regularities are the following:
1. The orbits of all the planets are almost in the same plane. This means that the solar system is
flat.
2. The planetary orbits are nearly circular. The elliptical orbits depart only slightly from being a
perfect circle.
3. The orbits of the planets are nearly in the same plane as the rotation of the sun.
4. The planets rotate in the same direction as they revolve around the sun, with the exceptions of
Venus and Uranus.
5. The distances of the planets from the sun can be expressed in a simple relationship called
Bode’s Law named after the German astronomer Johann Bode (1747-1826). The calculated
distances (using Bode’s Law) and the observed distances of the planets from the sun are almost
the same with the exception of Neptune and Pluto.
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6. The satellite systems of Jupiter (≈16 moons) and Saturn (≈ 18 moons) are nearly identical in their
arrangements with the solar system. The distances of the satellites from the planets follow
Bode’s Law.
7. The satellites and planets contain almost all the rotational motions of the solar system.
THEORIES ON THE ORIGIN OF THE SOLAR SYSTEM
The theories of the solar system explain in general terms some of its observed properties. Among
these theories are the following:
Descartes Theory
Rene Descartes (1596 – 1650), a French mathematician and physicist, was one of the first
proponents of a theory on the origin of the solar system. In 1644, he proposed that the solar system
formed into bodies with nearly circular orbits because of the whirlpool-like motion in the pre-solar
materials. He explained the orbits of the planets in terms of primary whirlpool-like motion and the
satellites around the planets as secondary whirlpool-like motion.
Buffon’s Theory
George Louis Leclerc Buffon (1707-1788), a French naturalist in the 18th century proposed that the
planets were formed by the collision of the Sun with a giant comet. The resulting debris formed into
planets that rotate in the same direction as they revolve around the Sun.
Kant’s Theory
Immanuel Kant (1724-1804), a German philosopher proposed in 1755 that the solar system resulted
from the condensation of a huge pre-solar gas cloud. The cloud was rotating in the same direction as
the planets now revolve around the Sun.
Nebular Hypothesis or Theory
In 1796, a French mathematical astronomer by the name of Marquis de Laplace proposed that the
solar system originated from a slowly rotating mass of incandescent gases, the mass gradually cooled,
shrank, and became more and more spherical in shape. As it rotated more rapidly, it developed a bulge
around the equator. Eventually the bulging mass was separated and flung off into space, forming a ring
around the original mass. The ring cooled and contracted, and became a planet. More rings were
thrown off from the central mass and each developed into a planet. The central mass became the sun.
The planets themselves threw off rings which developed into satellites or moons.
Planetesimal Hypothesis or Theory
In the 1900’s, Forest Ray Moulton, an astronomer, and T.C. Chamberlain, a geologist, proposed that
a star speeding through space came very close to our sun. The approaching star pulled masses of hot
gas from our sun. Some of them followed the other star as it dashed off into space. Others, held by the
attraction of the sun, started to move around that body. As they became cooler, they changed into liquid
form and gradually became small solid masses – or planetesimals. These small fragments eventually
drew together to form the planets.
Tidal Hypothesis or Theory
In 1918, Sir James Jeans and Harold Jeffreys proposed that the planets were formed directly from a
great filament of gas pulled out of the sun by a passing star. The filament of gas was largest at the
middle section and tapered at both ends. The smaller planets – those closest to the sun and those
farthest away from it – were formed from the ends of the filament. The larger planets were formed from
the middle section of the filament.
Double Star Theory
Fred Hoyle proposed that the sun once had a companion star which exploded several billion years
ago. Most of its material was thrown into deep space, but a cloud of gas was left behind, held by the
sun’s gravitation. From this cloud of gas the various bodies of the solar system were formed.
Lyttleton Theory
R.A. Lyttleton proposed that the sun was originally a double star, with the two stars moving around a
common center of gravity. A passing star may have moved close to one of the two suns and may have
disrupted, transforming it into a vast expanse of swirling gases. The surviving star would be our sun.
The victim of the collision would in time have evolved into our planets.
Today modern astronomers are inclined to discount theories based on collision or near collision. They
are more inclined to believe that the solar system and even the universe as a whole evolved in a gradual
and orderly fashion.
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Proto-planet or Dust-Cloud Hypothesis or Theory
Gerard P. Kuiper, a Dutch-American astronomer, proposed that while the sun was in the process of
condensing from the masses of interstellar matter, outer envelopes of gases and dusts were also
condensing around the sun. Kuiper called the enlargements in the outer envelopes as protoplanets or
primitive planets.
As the sun condensed, it became hotter and hotter until it reached a point when it was hot enough to
send out great amounts of radiant energy. This radiant energy drove the lighter gases away from the
outer envelopes of the inner protoplanets. Eventually the planets nearest the sun lost most of their
gaseous envelopes. Mercury lost all of its atmosphere. The Earth lost most of the lighter gases, such as
hydrogen and helium. Only the larger planets retained the lighter gases. Heavier matter became the
cores of the planets. Lighter ones became asteroids, meteors, and satellites of planets. Because of the
great gravitational attraction of the sun, all the planets continue to revolve around it.
THE SUN
The Sun is the only star in the solar system. About 150 M km away, it is also the nearest star to the
Earth. It appears bigger and brighter than other stars because of its nearness to us. Compared to other
stars, however, the Sun is just a middle-sized yellow star.
The Sun has a diameter of 1 140 000 km, about a hundred times the diameter of the Earth. Its
volume could take in a million Earths with room to spare. It rotates once every 25 days at the equator
and once every 33 days at the poles. This difference in period of rotation of different parts is one
evidence that the Sun is not solid but rather a mass of gases. The gases are mostly hydrogen and
helium.
The sun’s interior has three main parts: the helium core, the radiative zone, and the convective
envelope. At present, the temperature at the core is about 15 000 °C. At this high temperature,
hydrogen atoms in the core combine in a series of reactions to form helium atoms. Since a helium atom
has less mass than the original four hydrogen atoms, the mass lost from the atoms during the reaction is
converted to radiant and thermal energy. This is the source of the sun’s energy. Every second, the sun
converts 4 M tons of matter into energy.
The helium gases that were formed in the sun’s core cannot escape into space because of the
pressure of overlying layers of gases. These gases are very dense, about 110x the density of water.
Only radiant energy, largely in the form of x-rays, can pass through these layers of gases. This part of
the Sun where the energy radiates through the layers of gases is called the radiative zone. The
radiative zone extends to about 90% of the sun’s radius. It takes millions of years, about 10 M years, for
energy to pass through this zone.
Radiant energy released from the core is absorbed and converted to thermal energy in the convective
envelope. The great amount of radiant and thermal energy absorbed by the convective envelope heats
the gases, causing them to rise, become turbulent, and create disturbances on the outer part called the
Photosphere.
The Sun’s atmosphere is made up of three parts: the Corona, the Chromosphere, and the
Photosphere. The corona is the outermost part of the sun. It is seen as an envelope of white light
around the sun during a total solar eclipse. The temperature in the corona is about 1 to 2 M °C. Next to
the corona is the chromospheres, so-called because it appears colored during a solar eclipse. It extends
to a height of about 5 000 km from the sun’s surface. The temperature of the chromospheres increases
with its altitude, from 3700°C at lower altitudes to about 49 700°C at the upper region where it merges
with the corona. The layer below the chromospheres is the photosphere, the only visible part of the sun.
It has a temperature of about 6000°C. Most of the Sun’s activity starts in the photosphere. The
photosphere is also the principal source of the Sun’s radiation.
Photographs of the sun reveal that its surface is covered with granules, prominences, flares, spicules,
and sunspots. The granules are bright and dark spots on the surface of the sun. They are actually
bubbles of hot gases hundreds of kilometers in diameter. They are also the main source of radiation.
Big eruptions on the surface of the sun are called solar prominences. They are often referred to as
fiery fountains, and one can be likened to the explosion of 100 000 hydrogen bombs. A spicule is
usually a small but sharp eruption which lasts for about 15 minutes to 1 hour. A solar flare is a spicule
lying flat and with a bigger and longer stem. It usually occurs near a sunspot. A sunspot is a region on
the surface of the Sun which is darker because it is several thousand degrees cooler than its
surroundings. Temperature within a sunspot is lower than around the spot.
Recent studies of sunspots show them to be huge magnetic storms which develop in the Sun’s
interior and erupt on the surface. Astronomers believed that the strong magnetic lines of force are weak.
The number of sunspots varies every year, but the heaviest concentration occurs every 11 years (the
sunspot cycle). Sunspots have some effect on the Earth. In the years when sunspots are heaviest, the
Earth’s magnetic field is more frequently disturbed by magnetic storms, and auroral displays are more
frequent. Sunspot activities are also believed to influence the growth of trees as shown by the thickness
of the growth rings of certain trees.
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THE PLANETS
Facts About the Planets. The planets, in the order of their distance from the sun, are: Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (Pluto was re-categorized in 2006). Some important
facts which apply to all the planets are as follows:
1. All the planets revolve around the sun in a counterclockwise direction as viewed from Polaris.
From an inside view, the direction of revolution is west to east.
2. All the planets, except Venus and Uranus, rotate in a counterclockwise direction as viewed from
Polaris.
3. The orbits of all the planets are ellipses, with the sun at one of the foci.
4. All the planets shine because they reflect sunlight.
5. All the planets have the shape of oblate spheroids.
Mercury

The innermost and smallest planet.

Has a very high temperature due to its nearness to the sun.

Has a very low gravity due to its small mass

Has no atmosphere…if there was then the high temperature on its surface must have caused the
atoms and molecules of the gases to move very rapidly. The low gravity allowed the fast moving
gases to escape into space.

The surface is marked with countless holes and craters due to meteors.

Is seen as an evening star just after sunset, and as a morning star just before dawn.

Period of rotation: 59 days
Period of revolution: 88 days
Venus
 Once considered as Earth’s twin due to similarity in mass, size, and having an atmosphere.
 Findings of the PIONEER (a U.S. spacecraft) probe revealed the following:
 Venus is quite round
 Has a much smoother surface than Earth (as revealed by VENERA also)
 No bulging at the equator and no flattening at the poles…retrograde rotation (i.e., east to
west direction or clockwise) versus that of Earth which is prograde (i.e., west to east
direction or counterclockwise)
 Has no moon or natural satellite; no water
 Is a very hot planet (compared to Earth and Mercury) due to its nearness to the sun and
the greenhouse effect due to its very thick atmosphere mostly clouds of sulfuric acid and
carbon dioxide. Due to the thickness of its atmosphere, it is sometimes referred to as the
“Veiled planet.”
 The atmosphere contains 96% carbon dioxide, 0.1% water (small amount of water vapor
is detected on the upper atmosphere), and sulfur dioxide (which has been singled out to
be largely responsible for the rise in temperature).
 Other surface features:
 Aphrodite Terra – the largest continent on the surface of Venus
 Esthar Terra – a volcano on the surface of Venus
 Eve – name of the crater on Venus’ surface.
 M. Lomosov – discovered that Venus has an atmosphere which is about 48-58 km thick.
 Period of rotation: 243 days
Period of revolution: 225 days
Mars
o Has a thin atmosphere which offers very little protection against Sun’s rays thus Mars
experiences extremities of temperature
o VIKING space probe reveals the following on Martian surface and atmosphere:
 Soil…contains peroxide
 Atmosphere…carbon dioxide, carbon monoxide, nitrogen, argon, oxygen, ozone, krypton,
xenon, clouds and fogs
 No liquid water…water in other forms in atmosphere and beneath the surface.
 Frozen carbon dioxide – found in thin mist atmosphere and polar caps.
o MARINER 9 which orbited Mars revealed the following:
 Mar’s surface is heavily cratered in some parts with ridges
 Bright circular plains and deserts in the remaining parts
 Craters are shallow with very flat bottoms
 Canals are actually chains of dark-floored craters
 Plains are result of the leveling effect of winds and dust storms
o In the 1890’s, Percival Lowel, a US astronomer, built an observatory at Flagstaff, Arizona to
specially observe Mars and its surface.
o Olympus Mons – a volcano on Mars; the largest in the solar system.
o Moons of Mars: Phobos (irregular in shape) and Deimos (believed to be a captured asteroid)
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o
o
o
Is a red planet due to the presence of carbon suboxide, a foul smelling compound which when
struck by ultraviolet light imparts an orange or reddish-brown color. The bright pink sky of Mars is
due to iron (III) oxide or rust.
Mars exhibits changes in seasons due to its angle of inclination of 25 degrees. The great
ellipticity causes more unequal heating of the northern and southern hemispheres.
Period of rotation: 1.37 days
Period of revolution: 1.88 years.
Jupiter
The largest planet with a diameter eleven times that of earth and a mass of 2.5 times that of all
planets put together.
Rotates in less than 10 hours (2.4 times faster than Earth). The fast rotation causes the equator
to bulge and its poles to flatten more than any other planets.
Surface is marked with light and dark bands, and spots, streaks, plumes, swirls, loops and
irregular patches. These are caused by high wind speeds and powerful coriolis force.
 Zones – light bands of the Jovian surface. Varies from white to pale yellow in color and
are regions of rising gases.
 Belts – dark bands in various shades of reddish brown and are regions of descending
gases.
 Both the zones and belts are produced by heat from the planet’s core.
Coloration of band is due to ammonium compounds and the organic and inorganic compounds
present in the atmosphere.
The atmosphere contains hydrogen, helium, methane and ammonia.
The Great Red Spot is the most prominent feature on Jupiter’s surface with a length of 30 000 km
a width of 12 000 km and a height of 8 – 10 km. it is located along the south tropical zone known
as the “hurricane belt.” It has been seen for 300 years and is a huge and violent storm.
Jupiter’s interior is believed to be made of a solid iron-silicate core, an inner layer of liquid metallic
hydrogen, and an outer layer of liquid molecular hydrogen. The inner layer of hydrogen is
believed to be the main source of Jupiter’s large and powerful magnetic field. Convection
currents within are deflected by rotation, generating electric current and giving rise to a magnetic
field. Strong magnetic field gives off strong radio signals into space. Jupiter’s magnetic field is
about 20 to 30 times stronger and larger than that of the Earth.
Jupiter has a thin ring believed to come from volcanic eruptions in one of its natural satellites
named Io.
Moons or natural satellites of Jupiter: (19)
 Io – has a diameter of 3 130 km; circles Jupiter at an average distance of 421 600 km;
probable structure is: sulfur and sulfur dioxide crust, molten silicate interior, and possible
solid core. It has active volcanoes. The surface is the youngest known in the solar
system.
 Europa – has a diameter of 3 130 km; circles Jupiter at an average distance of 670 900
km; probable structure: ice crust, silicate interior and possible solid core. Is rocky, but
most is covered with frozen water.
 Ganymede – has a diameter of 5 270 km; orbit Jupiter at an average distance of 1 070
000 km; probable structure: ice crust, convecting water or soft ice mantle surrounding
silicate core. The largest moon of Jupiter and the second largest in the solar system.
 Callisto – has a diameter of 4 850 km; orbit Jupiter at an average distance of 1 880 000
km; probable structure: thick ic crust, convecting water or soft ice mantle surrounding
silicate core. The oldest of Jupiter’s moon and has a highly cratered surface.
Io, Europa, Ganymede and Callisto are known as the Galilean moons.
 8 outer moons – are much smaller, circle in a retrograde direction of the inner (or
Galilean) moons; orbits are inclined 25 degrees to 28 degrees from Jupiter’s equatorial
plane; could have been asteroids captured by Jupiter’s gravitational field, or fragments of
two larger satellites that collided with comets or asteroids.
 3 other moons – were discovered during the Voyager 2 probe in 1979; are tiny satellites
found closest to Jupiter; were named 1979 J1 or Adrastea, 1979 J2, and 1979 J.
 The rest of the moons of Jupiter were discovered by the Smithsonian Astrophysical
Society through their observatory.
Period of rotation: 9.55 hrs
Period of revolution: 12 years
Saturn
 A ringed planet; a pale yellow planet; another gas giant; second largest planet but the least dense
 Composed mostly of hydrogen and helium
 Rotates faster at the equator than at the poles
 Rings – are the most distinguishing feature of Saturn. There are seven (7) major rings with
thousands of ringlets. It extends over a diameter of 25 000 km and each rotate separately of the
other. The ring particles are held in place by Saturn’s gravity and that of the moons. For
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example, the F ring is kept in place by S13 and S14 (S = shepherd; shepherd moons). The A
ring is held in place by S 15. These rings are made of ice-cooled rocks.
 Moons of Saturn (23 of them) – measures between 200 to 1 500 km; are heavily cratered; nearly
spherical; composed mainly of dust and water ice (the same material found in comets).
 Titan – the largest moon of Saturn and of the solar system. Measures 5 140 km in
diameter; the atmosphere is thicker than that of Earth; has liquid on its surface.
 Iapetus - the second largest moon of Saturn; shows two faces (one is six times brighter
than the other side).
 Tethys – has a long branching trench about 3 km deep and 65 km wide; least dense
among Saturn’s moon; contains 80% or more water ice.
 Mimas – noted for its very big crater caused probably by the impact of another moon; the
crater’s diameter is about 130 km.
 Enceladus – the only moon with no craters; reflects nearly 100% of the light striking it,
thus making it the brightest.
 Rhea – most heavily cratered moon
 Phoebe – is small; has an inclined orbit and a retrograde orbital movement.
 Saturn has a turbulent atmosphere like Jupiter. White spots, smaller versions of the Great Red
Spot and bands o flight and dark clouds are seen. Wind speeds is three times (3x) stronger than
those of Jupiter (1 600 km/hr) or ten times (10x) more than the wind speed of storms on Earth.
 Period of rotation: 10.39 hrs
Period of revolution: 29.5 years.
Uranus (Georguim Sidus or George’s Star)

Is 2 billion km from earth

Accidentally discovered by William Herschel in 1781 while he was examining portion of the sky
along the constellation Gemini.

Seen as a bluish-green planet with rings circling it. The rings are thinner and contain much less
materials than those of Saturn. Rings are dark and could have been made of materials from
meteorites.

Tipped at an angle of nearly 90% to the plane of its orbit. Tilting may have been caused by a
collision with a large body in space and the impact may have also triggered the formation of the
moons.

Poles points almost directly toward the sun.

It has 21 years each of summer, winter, autumn, and spring.

Displays faint bands which indicate that it has an atmosphere composed mostly of methane gas.

Is denser than Jupiter and Saturn.

Probably an icy rather than a gaseous planet.

Has 5 moons namely: Miranda (discovered by G. Kuiper in 1948), Ariel (discovered by W.
Lassel in 1851), Umbriel (discovered also by W. Lassel), Titania and Oberon (both discovered
by W. Herschell in 1787). The largest of the moons is 1 000 km in diameter and the moon move
at nearly right angles to the planet’s orbit.

Uranus has a retrograde rotation.

Period of rotation: approx. 23.9 hrs
Period of revolution: 84 years
Neptune
o Known as Uranus’ twins because it is about the same size but is denser and more massive.
o A bluish-green planet with a clear atmosphere.
o Its existence was first detected by John Couch Adams (1841) when his calculations
demonstrated that an unknown planet was pulling Uranus from its expected orbit. In 1845,
Urbain Leverrier, a French mathematician calculated the position of this unknown planet (now
Neptune) and Johann Gottfried Galle (1946) found the planet.
o It has two (2) moons, Triton, the inner satellite with a diameter of about 3 700 km, the only large
satellite in the solar system with a retrograde motion, and Neried, the smaller satellite.
o Period of rotation: approx. 18 hrs.
Period of revolution: 164 years
THE MINOR MEMBERS OF THE SOLAR SYSTEM
ASTEROIDS/PLANETOIDS
 Are found between the orbits of Mars and Jupiter
 There are billions of icy irregularly-shaped bodies traveling around the Sun in the same direction
as that of the planets.
 200 pieces are larger than 100 km in diameter and 500 pieces have diameters between 50 to 100
km and the rest are smaller than 50 km. Ceres is the largest asteroid with a diameter of 955 km.
 Explanation of origin of asteroids: the irregular shapes of asteroids suggest that they never
formed into planets or that they are broken pieces of a planet or that they are fragments left after
the formation of Jupiter.
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 3 types of asteroids
 1st Type – those that revolve in orbits between Mars and Jupiter such as Ceres, Pallas,
Juno and Vesta.
 2nd Type or Apollo Asteroids – asteroids with highly elliptical orbits and are probably
controlled by Jupiter. They constantly cross the Earth’s orbit, and it is possible that these
can collide with the Earth. If such a collision occur, the destructive power is approximately
equivalent to 500 000 megatons or bomb blast. Examples of this type of asteroids are:
Eros, Icarus, Geographus.
 3rd Type or Trojan Asteroids – asteroids that lie beyond the main asteroid belt. Some
are stationed along the orbit of Jupiter, 60 degrees east and 60 degrees west of Jupiter
and the Sun. They are equal in distances from Jupiter and the Sun. They seemed to be
trapped in place and will remain in their position until disturbed by other planets.
COMETS
 Usually consists of a luminous head, the coma, and a thin veil-like tail (although some comets
are without tails). The head is about 48 000 km to 1 600 000 km in diameter. The coma is the
nucleus where the mass of the comet is concentrated. The nucleus emits mostly vapors of water,
ammonia and methane. This mass of glowing gases surrounds the coma and forms the halo of
the comet.
 Tail-less comets become with tails as they approach the Sun.
 Comets are composed of solidified gases and frozen water mostly the size of dust particles. As a
comet nears the Sun, its frozen materials are changed into gases by the sun’s radiation. The
gases rush out by rapid vaporization in the form of jets carrying solid particles with them. This
forms the dust tail of the comet. The tail always spreads out from the head in the direction away
from the Sun. Comets become brighter as they near the Sun. Thus the brightest comets are
those that get closest to the sun.
 Every time a comet passes close to the sun, it loses materials. Sometimes the comet breaks into
two or more pieces. After several passages, the comet will completely evaporate.
 Comets follow a regular path around the Sun. Their orbits are long and narrow.
 Example: Halley’s comet that reappear after approximately 75 years.
METEORS, METEORITES, METEOROIDS
A meteoroid is a solid body in interplanetary space before it reaches the Earth's atmosphere. A meteor
denotes the fiery streak or "shooting star" which appears when a tiny meteoroid strikes the Earth's
atmosphere and burns up. Most meteors result from meteoroids no more than a few centimeters in
diameter. A larger meteoroid which survives the fiery passage through the Earth's atmosphere as a
meteor and strikes the Earth's surface is called a meteorite. Meteoric material less than a tenth of a
millimeter in diameter is called cosmic dust.
Meteors
Meteors appear when centimeter-sized meteoroids travelling at least 11
km/sec, but more usually 30 to 55 km/sec, strike the Earth's atmosphere.
The maximum speed is 72 km/sec. The meteoroid's kinetic energy of
motion converts into heat, vaporizing the meteoroid at heights above 60
km. The hot vapor trail is what we see as a meteor. The vaporized
material may reach temperatures of 1,000 to 2,000 Kelvins. The period
during which the meteor flashes is called incandescent flight. The
period after the light phenomena cease is called dark flight. Meteorites
the size of a golf ball (two or three centimeters) or larger vaporize in
exceptionally brilliant flashes called fireballs or bolides. These may also
produce a variety of sounds. Theodor Abrahamsen's photo at the right
captures a Perseid meteor on August 12, 1986.
A Perseid meteor on August
12, 1986.
Meteor Showers
You can see five or six meteors each hour from any given vantage point on Earth when atmospheric
conditions allow. Up to 25 million meteors arrive each day, dropping about 100 tons of material. Most
meteors are composed of debris left behind by comets as they orbit the Sun. A meteor shower occurs
when the Earth intersects a comet's path and moves through the stream of debris and dust emitted by
the comet. The meteors in a shower appear to originate from one area of the sky called the radiant. The
meteor shower is usually named after the constellation in which the radiant lies. Meteor showers occurs
at the same time each year. Common meteor showers result in ten to fifty meteors per hour. Typically the
best time to observe is in the early morning.
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The following table lists a few of the more prominent meteor showers.
Some Prominent Meteor Showers
Date of
Maximum
Radiant
Maximum
Meteors
Per Hour
Draconids
Comet
PonsWinnecke
June 30
Draco,
near
handle of
Big
Dipper
10-100
No
Geminids
3200
Phaeton
December
14
Gemini
58
No
Leonids
P/TempleTuttle
November
17
Leo
10, except
for storms
About
every 33
years
Lyrids
Comet
Thatcher
April 22
Lyra,
near
Vega
15
No
Orionids
P/Halley
October
21
Orion
30
No
Perseids
Comet
1862 III
August 12
Perseus
50-100
No
Quadrantids
(unknown)
January 4
Bootes
110
No
Taurids
Comet
Encke
November
5
Taurus,
near
Pleiades
10, many
fireballs
Yes,
irregularly
Name
Parent
Storms
Meteor Storms
Occasionally the Earth passes through an unusually heavy
concentration of cometary debris resulting in a meteor storm. Hundreds
or even thousands of meteors may flash each hour. One of the
historically most prominent meteor storms, the Leonid storm, occurs at
about thirty-three year intervals. The Leonid shower normally produces
about ten meteors per hour. When they storm, the Leonids can produce
the equivalent of over one hundred thousand meteors per hour for a
short period. The woodcut at the right by artist Adolf Vollmy, based upon
an original painting by the Swiss artist Karl Jauslin, portrays the great
Leonid meteor storm of November 12-13, 1833. Victorian era astronomy
writer Agnes Clerke described that storm as follows:
"On the night of November 12-13, 1833, a tempest of falling stars broke
Leonid Storm of November,
over the earth.... the sky was scored in every direction with shining
1833
tracks and illuminated with majestic fireballs. At Boston, the frequency of
meteors was estimated to be about half that of flakes of snow in an average snowstorm. Their numbers
... were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed,
on the basis of that much-diminished rate, that 240,000 must have been visible during the nine hours
they continued to fall."
Periodic comet Temple-Tuttle is the parent (originating object) of the debris for the Leonid meteor
shower. Leonids are among the fastest known meteors, striking the Earth's atmosphere at speeds of 71
km/sec on average.
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Curiously, no meteorite fall has ever been associated with a meteor shower. So far all meteorites
observed to fall during a meteor shower followed different orbits than the material forming the meteor
shower. Most meteoroids large and solid enough to reach the Earth's surface as meteorites are asteroid
fragments. A few meteorites appear to be debris lifted off the Moon and Mars by large impact events. A
handful may be fragments of comets.
Meteorites
Meteoroids the size of a fist or larger may survive the trip through the atmosphere to land on the Earth's
surface. They are then known as meteorites. A meteorite located after a witnessed descent is called a
fall. A meteorite from an unwitnessed descent is called a find. Meteorites are usually named for a post
office or another geographic landmark close to the place where the meteorite was found. The name of
the meteorite can refer to either a specimen of the meteorite itself or to the locality in which it was found.
Meteorites include some of the oldest and most primitive solar system material. Radiometric dates
suggest some meteorites are as much as 4.54 billion years old. Some even include cosmic material
formed before the solar system itself was born. Because many meteorites have changed so little in the
intervening eons, they offer a window into the early history of the solar system. Meteorites also represent
some of the rarest material on Earth. Until the advent of the space age meteorites were the only
extraterrestrial material available for study here on Earth. Both the scarcity and the scientific importance
of meteorites leads collectors and researchers alike to seek them out.
Types Of Meteorites
Meteorites form three main groups based upon their composition.
Types of Meteorites
Type
Composition
Example
Achondrite
Similar to
terrestrial basalts.
Some may be
fragments of the
Moon or Mars.
Mount
Egerton,
Australia
(AUB)
Carbonaceous
Chondrite
Similar to the Sun
with some
volatiles depleted.
Similar to type C
asteroids. Some
may be cometary
fragments.
Murchison,
Australia
(CM2)
Chondrite
Similar to the
mantles and
crusts of the
terrestrial planets.
Most meteorites
are chondrites.
Salaices
(H4)
Stony
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Iron
Stony Iron
Primarily iron and
nickel. Similar to
type M asteroids.
SikhoteAlin, Russia
(IIB)
Mixture of iron
and stony
material. Similar
to type S
asteroids.
Vaca
Muerta,
Altacama,
Chile (MES)
Iron Meteorites
Iron meteorites are probably what most people picture as "typical" meteorites. Iron meteorites consist
almost entirely of a mixture of metallic nickel and iron. They are easier to spot on the ground because
their highly unoxidized iron content stands out from background rocks. The outer surface of iron
meteorites often melts during their passage through the atmosphere resulting in a dark fusion crust.
Primary fusion crust forms while the meteoroid is incandescent. Secondary fusion crust forms on the
broken surfaces of fragments which break free from the main mass during incandescent flight. They may
also exhibit flow markings and interesting molten metal shapes. The interior of some iron meteorites
displays a criss-cross pattern of different iron-nickel minerals.
Iron meteorites may originate in the cores of differentiated parent bodies at least 100 km in diameter. The
composition of some main-belt asteroids called M-type asteroids resembles that of iron meteorites.
These M-type asteroids may be the source of iron meteorites. Iron meteorites with weights of 50 to 100
kg are not uncommon. The Hoba meteorite, at 60 tons, is the largest known iron meteorite to have
landed without exploding. It still lies where it was found.
Stony Meteorites
Stony meteorites are the most common, making up about 94% of observed falls. They are composed of
75-90% rocky silicates including familiar minerals such as pyroxene, olivine, and plagioclase, and 1025% nickel-iron metal and iron sulfide. (Silicates are minerals containing silicon, oxygen, and one or
more metals.)
Stony meteorites are difficult to find because they look like terrestrial rocks. The best places to find stony
meteorites are in deserts or on the ice sheet of Antarctica. The meteorites stand out against the
background of ice or sand. Like iron meteorites, stony meteorites often exhibit a dark fusion crust. There
are three major subgroups of stony meteorites, Chondrites, Carbonaceous Chondrites, and
Achondrites.
Chondrites are the most common type of stony meteorite. About 86% of all recovered stony meteorites
are chondrites. Chondrites are composed of small spherical chondrules. Chondrules are millimeter to
centimeter sized glassy mineral spheres. Chondrules are composed of silicate material that has melted
and then resolidified. Chondrules formed early in solar system history. They were the most primitive
"building blocks" of the solar system. Over time chondrules accreted to form larger and larger objects
including asteroids, moons, and planets. In some chondrites the chondrules are separated by patches of
iron metal. Different types of chondritic meteorites contain different amounts of metal. They have been
heated to varying degrees.
Chondrites are called primitive because they have changed very little since their initial formation early in
the history of the solar system. Their composition resembles that of the Sun except that the lightest
gases Hydrogen and Helium are missing from the meteorites.
Ordinary chondrites are the most common type of meteorite, representing 87% of all recovered
specimens. The parent body or bodies are unknown, but asteroids 6 Hebe and 3628 Boznemcová have
been suggested as possible sources.
Enstatite chondrites are metal rich meteorites in which the primary mineral is Enstatite. Enstatite
chondrites may be fragments of asteroid 16 Psyche. Some scientists have suggested Mercury as the
originating body.
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Carbonaceous Chondrites are essentially just pieces of chondrules stuck together. They are very black
because of their high carbon content. Some of their mineral grains predate the solar system -- probably
fragments blown out from distant stars that became supernovae. Carbonaceous chondrites also contain
water and amino acids. Some types of carbonaceous chondrites may be cometary material. The building
blocks of life on Earth may have been seeded by comets and carbonaceous meteorites early in Earth's
history. For example, the Murchison meteorite, a fragment of which appears in the table above, was
found in 1969. This carbonaceous chondrite contains 16 amino acids, 11 of which are rare on Earth. It
may represent the type of cosmic visitor which early in the Earth's history brought the raw materials
needed to jump start life.
In some stony meteorites called achondrites the chondrules have been partially or completely destroyed
by metamorphic processes. This took substantial time and pressure. Such meteorites must be fragments
of the interior of larger bodies on which the weight of the overlying rock created enough pressure to
obliterate the chondrules. About 7% of recovered stony meteorites are achondrites.
Some achondrites resemble terrestrial igneous rocks and formed during volcanic eruptions on planets
and asteroids. Some asteroids like Vesta heated up enough that their interiors melted and erupted lava
onto their surfaces. The lava hardened into a rock called basalt.
The Mt. Egerton meteorite (see table above) is a type of achondrite known as an
aubrite.
Some achondrites are composed of rock fragments broken and fused back together
during an impact event. Meteorites believed to originate from the Moon and Mars
are achondrites that formed during impact events. The achondrite Dar al Gani 476
(see photo of fragments at right) is a type known as a Shergottite. It probably
originated as a fragment of the planet Mars blasted off the surface during a large
impact event.
The heaviest known stony meteorite was Jilin which weighed 1.8 tons. It fell in Jilin,
China on March 8, 1976 as part of a meteorite shower (see below) which produced
about four tons of meteoric material altogether. Witnesses of Jilin report a
spectacular daytime fireball and several explosions. Jilin is classfied as an olivinebronzite chondrite (H5). Because the Chinese leader Mao Zedung died three days
after this fall, many Chinese took Jilin as an omen.
Fragments of DAG
476 Mars Rock
Stony Iron Meteorites
Stony Iron meteorites are the rarest type of meteorite, making up about 1 to 2% of all recovered
meteorites. Stony Iron meteorites consist of a mixture of rocky silicates and metallic nickel/iron. There
are two main groups of stony iron meteorites. Pallasites are composed of olivine crystals set in a nickeliron matrix. They probably formed in the boundary layer between the iron core and the stony mantle of
an asteroid. Pallasites are very popular as jewelry when cut and polished. Mesosiderites are conposed
of pyroxene, olivine, plagioclase, and metal grains. They probably formed when a metal-rich asteroid
collided with a silicate-rich asteroid.
Cosmic Dust
A tiny meteoroid of whatever composition which is smaller than 0.1 mm in diameter is called a cosmic
dust particle. The frictional heating which occurs during the descent through the Earth's atmosphere
does not melt cosmic dust particles because of the large surface to mass ratio of such particles. Cosmic
dust radiates away the heat which burns up larger meteorites. The Earth accumulates about 10,000 tons
of cosmic dust each year.
The Zodiacal Light, a faint pyramid-shaped glow extending away from the Sun along the plane of the
ecliptic, is caused by sunlight scattered off cosmic dust particles. The gegenschein or counterglow
which appears as a faint spot of light opposite the Sun is also caused by cosmic dust. Both are most
visible early in the morning a couple of hours before sunrise. The cosmic dust particles causing the
Zodiacal Light and the Gegenschein form a very low density cloud in the same plane as the planets.
These particles slowly spiral into the Sun over time and are replaced by new particles emitted from
comets and asteroid collisions.
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