Download S T A R S

STARS
Revised June 2009
By the
Queensland Wilderness Adventure Club
(Sponsored by the Youth Ministries of the South Queensland Conference of SDA Church)
Edited by Dr Bruce Hands
M Astron [JCU]
MB BS [U of Q]
CONTENTS
1
a
What is the earth's nearest celestial neighbour ?
Page 3
b
What is the distance from the earth ?
3
c
What governs the tide ?
4
d
What causes an eclipse ?
5
e
What is a shooting star ?
6
f
How fast does light travel ?
6
2
Make a diagram showing relative positions and movements of the earth, sun and
moon. Show positions and movements for eclipses of the sun and moon.
6
3
Make a diagram of our solar system and be able to name the planets in order from
the sun.
8
4
What is the difference between planets and stars ?
5
What is a constellation? Name and point out six.
visible throughout the year.
6
Draw a chart of the Southern Cross, Orion and Scorpio.
35
7
What is the Milky Way ?
36
8
What is the morning and evening star ?
9
Explain zenith and Nadir.
10
What is the Aurora Australis?
Identify in the sky eight stars.
11
Name two constellations
12
Observe the Milky Way in the night sky.
Why does it carry both names ?
37
38
What causes it ?
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39
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STARS
1.a
What is the earth's nearest celestial neighbour ?
Apart from the components in our solar system, the nearest celestial neighbour is
commonly quoted as Alpha Centauri. Alpha Centauri is that one of the two pointers to
the Southern Cross that is furthest from the Southern Cross and is also the brighter of the
two pointers.
Alpha Centauri is a binary star which means there are two stars orbiting
around each other. To complicate matters, Proxima Centauri has a third star that orbits
around the binary. Which of the three stars in the complex is closest to earth varies with
the orbital positions in the complex. Currently, Proxima Centauri is closest to earth and is
about 4.2 light years from earth.
1.b
What is the distance from the earth ?
There are different ways of measuring interstellar distances. A light year is the distance
that light travels in one year. In light years, Alpha Centuri is 4.2 light years away. The
speed of light is about 3 x 10 to the 8th power m/s or 186 000 miles per second. That
means that the distance that light travels in one year is 2.99792458 x 100 000 000 x 60 x
60 x 24 x 365.25636 or about 1 followed by 16 zeros with the answer in meters. A parsec
is another unit of stellar distance.
It is measured by the apparent movement of a star
against its background when measured from earth by two measurements taken from
opposite sides of earths orbit. That is, by two measurements taken six months apart. In
those units, Proxima Centauri is 1.3 pc away.
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1.c
What governs the tide?
The major influencing factor is the moon.
The sun has a smaller influence.
Moons
Gravitational
Pull on
oceans
Moon
Sun
Earth
High tide
12 hours
apart
When sun & moon
pull in the same
alignment
King tide
as sun &
moon pull
in same
alignment
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1.d
What causes an eclipse ?
Sun’s radiation
Moon In
Earth’s
shadow
Sun
Earth
Solar eclipse
As seen by
someone on earth’s
surface
in the moons
shadow
looking at the
sun
Lunar eclipse
as seen by
someone on
earth’s surface
at night
looking at the
moon
Moon’s
shadow is
on part of
earth’s
surface
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1.e
What is a shooting star?
Meteoroids are dust particles left in the orbit of a comet. They are believed to originate
from comets.
When meteoroids intersect with the earth’s atmosphere, they produce
visible streaks of light as they heat, burn and vaporise. Then they are called meteors.
This streak of burning debris is commonly called a shooting star.
Asteroids are larger
and generally follow a different orbit.
Asteroids (or small planets) are small, naturally
formed solid bodies that orbit the sun. They are airless and no outflow of gas or dust can
be detected.
1.f
How fast does light travel?
The speed of light is 2.99792458 x 108 m/s – generally taken as 3 x 108 m/s or 186 000
miles per second. This is roughly 660 million miles per hour.
2.
Make a diagram showing relative positions and movements of the earth, sun
and moon. Show positions and movements for eclipses of the sun and moon.
Earth’s
axis
Earth’s
North
pole
-
S
moon
E
Earth’s
South
pole
Earth’s
rotation
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During an eclipse:
Sun’s radiation
Moon In
Earth’s
shadow
Sun
Earth
Solar eclipse
As seen by
someone on earth’s
surface
in the moons
shadow
looking at the
sun
Lunar eclipse
as seen by
someone on
earth’s surface
at night
looking at the
moon
Moon’s
shadow is
on part of
earth’s
surface
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3.
Make a diagram of our solar system and be able to name the planets in order
from the sun.
The planets in order from the sun are:
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
The relative sizes of the
http://antwrp.gfsc.nasa.gov/apod
planets are illustrated
28 Aug 2006
Stars 1 Honour Trainer's Notes_Extra Info.doc
on
the
Page 8 of 39
web
site
The relative sizes of the orbits are illustrated on the web site www.nineplanets.org
The small print names in this diagram are Venus, Earth, Mars, Jupiter and Saturn.
innermost orbit of Mercury is red and not labelled.]
The orbits named below are Jupiter, Saturn, Uranus, Neptune and Pluto.
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[The
If we use the distance from the sun to earth as one unit [= one astronomical unit = 1 AU]
then the relative distances of the planets from the sun can be compared by studying the
table below.
Semi major axis
AU
Eccentricity
Inclination
degrees
Period
years
Mercury
0.38710
0.205631
7.0048
0.2408
Venus
0.72333
0.006773
3.3947
0.6152 =225 days
Earth
1.00000
0.016710
0.0000
1.0000
Mars
1.52366
0.093412
1.8506
1.8807
Jupiter
5.20336
0.048393
1.3053
11.856
Saturn
9.53707
0.054151
2.4845
29.424
Uranus
19.1913
0.047168
0.7699
83.747
Neptune
30.0690
0.008586
1.7692
163.723
Pluto
39.4817
0.248808
17.1417
2488.02
For years, Pluto irritated Astronomers because it did not seem to fit into the order of things
very well.
This was because of its size, orbit shape and inclination and the unusually
large moon that it possessed.
Various suggestions were proposed but now the truth is
becoming apparent. British astronomer Kenneth Edgeworth and Dutch-American Gerard
Kuiper suggested during 1943 to 1950 something that has only been seen to exist in the
1990s.
Pluto is the King of millions of Edgeworth Kuiper Objects (EKO), also known as
Kuiper Belt Objects (KBO].
These constitute a whole band of objects that have been
detected. Most are in the size range of 100-500km. About 90% are in a roughly circular
orbit beyond Neptune.
Another scattered band exists in an inclined elliptical orbit and
extends out to about 200AU. These are more than another asteroid belt as is between
Mars and Jupiter. EKOs have a combined mass in the order of 300 times the total mass
of all asteroids.
They have a very diverse range of colours, surfaces, sizes, surface
activities and some possess satellites and atmospheres. Several binary pairs are known
to exist. The short period comets are believed to come from this band. Pluto is 1485
miles [2390 km] in diameter. The next known largest EKO is Quaoar. [“kwa-whar”] which
has a diameter of 800 miles [1300 km].
Pluto’s moon Charon has a diameter of 737
miles [1186 km]. Quaoar is about 10 AU beyond Pluto. Varuna is another EKO and
has a diameter of 560 miles [900 km]. Beyond the EKOs may be more planets – even
more giant planets like Jupiter. Such planets are beginning to become within detectable
range with the advent of giant telescopes. Dark giants way beyond the present imagined
limit of our solar system would be difficult to see as they creep ever so slowly around our
sun. They are suspected to exist because of distortions in the paths of comets. There
are also planetary mass objects [PMO].
These are not easily categorized.
PMOs are
planet sized objects beyond the reach of nearby stars.
They are bigger than Jupiter but
too small to be a star.
To be a star the object has to be at least 75 Jupiter masses.
This is the mass required to squeeze the core to maintain hydrogen fusion. If deuterium
fusion is the energy source, 13 Jupiter masses would be sufficient to maintain a star for
awhile.
PMOs so far detected are less than 12 Jupiter masses. Mass is not easily
calculated when the object is being gravitationally influenced by nearby objects.
But if
they are a planet, what are they doing drifting around beyond the reach of any star?
An
alternative name to PMO is “planetar”.
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4.
What is the difference between planets and stars?
stars.
Identify in the sky eight
To be able to distinguish between a planet and a star: Be familiar with the constellations in the zodiac (the path through which the sun &
moon travel) so as to be able to recognise a “star” that is out of place. That works for
the brighter planets like Jupiter, Venus, Mars and Saturn but is a bit hard for the fainter
planets.
 The planets give off a more steady light and the stars give off a twinkling light. That is
because the light from the stars travel a far greater distance and will be distorted and
bent a little somewhere in its travels to earth.
 When viewed over several nights, a planet will be noticed to steadily migrate through
the stars in the constellation where the planet is observed. The word “planet” comes
from a Greek word that means “wanderer” because the Greeks observed that some
“stars” were not fixed in position with the rest of the stars but wandered along
independently.
 Planets are found only in the band of sky where the sun and moon are seen.
The twenty brightest stars are:
Sirius
Alpha Canis Majoris
Canopus
Alpha Carinae
Rigel Kent
Alpha Centauri
Arcturus
Alpha Bootis
Vega
Alpha Lyrae
Capella
Alpha Aurigue
Rigel
Beta Orionis
Procyon
Alpha Canis Minoris
Achernar
Alpha Eridani
Hadar
Beta Centauri
Altair
Alpha Aquilae
Betelgeuse
Alpha Orionis
Aldebaran
Alpha Tauri
Acrux
Alpha Crucis
Spica
Alpha Virginis
Antares
Alpha Scorpio
Pollux
Beta Geminorium
Fomalhaut
Alpha Pisces Austrini
Denub
Alpha Cygni
Mimosa
Beta Crucis
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The stars are named by use of the Greek alphabet. The brightest star in each
constellation is named alpha followed by the constellation name. Then the next brightest
star is called beta followed by the constellation name. Thus Sirius is the common name
for the brightest star in the constellation of Canis Major and the alternative name for the
star Sirius is Alpha Canis Majoris.
You need to be able to identify at least eight stars.
star constellation charts that follow will do.
Any of the stars above or any in the
The Greek alphabet is [diagram from Wikipedia]:
5.
What is a constellation ?
visible throughout the year.
Name and point out six.
Name two constellations
The word “constellation” means in Latin ”set with stars”. A constellation is a grouping of
stars that have been arbitrarily considered to be a group and usually named after some
object, animal or mythological being.
Some significant constellations are illustrated
below.
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Crux
This is the smallest constellation. Acrux is a triple system and the brightest pair
of the three are about 3000 and 1900 times more luminous than our sun. The distance
between the pair is about 6 times the distance across our solar system. Alpha, Beta,
Delta, Zeta plus some stars in Scorpius and Centaurus all form a group moving in similar
motion. Dcrux is a variable star. The coal sack is a dark nebula filled with inert dust &
gas. The jewel box (NGC 4755 or KCrux) is an open cluster of many & coloured stars,
the most luminous of which is about 80 000 times that of our sun.
Gamma Crucis
Delta Crucis
Beta Crucis
Epsilon Crucis
The Jewel Box
The Coalsack
Alpha Crucis
& Zeta Crucis
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Centaurus
Alpha Centauri is the closest binary to our sun and Proxima Centauri orbits this binary.
Omega Centauri (NGC 5139) is globular cluster. The two pointers Apha & Beta Centauri
point to Crux. (NGC is the abbreviation for New General Catalogue which is a listing of
star clusters, nebulae and galaxies.) Omega Centauri was named as a fourth magnitude
star almost 2000 years ago. In 1677 Halley observed it as a cluster and not as a star.
Now there are known to be over a million stars in the cluster with some 1000 times more
bright than our sun. The cluster is about 17 000 light years away. (Remember that the
Milky Way Galaxy is about 100 000 light years in diameter and about 3 000 light years
thick.) NGC 5128 is a galaxy with a dark band through it that gives the impression of a
galaxy with one very big explosion blowing it into two pieces – or possibly two separate
galaxies colliding. Measurements place it about 25 million light years away.
NGC 5128
Omega Centauri
Beta Centauri
Alpha Centauri
Crux
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Musca
Crux
Beta Muscae
Alpha Muscae
Alpha Muscae means the Southern Fly. Beta Muscae is a slowly rotating binary of about
400 or 500 years.
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The twelve constellations in the Zodiac are Taurus, Aries, Pisces, Aquarius, Capricornus,
Sagittarius, Scorpio, Libra, Virgo, Leo, Cancer and Gemini.
Taurus
Zeta Tauri
Nath or Beta Tauri
M1
Crab Nebula
Aldebaran
Alpha Tauri
Pleiades or M45
Ecliptic (the projection of earths orbit onto the celestial
Sphere = the path of the sun across the sky)
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Aldebaran is Alpha Tauri. It means the Follower as it follows the Pleiades across the sky.
It was involved in numerous Roman, Greek , Persian and Eastern stories. Arcturus in the
book of Job is thought to be Aldebaran. It is about 40 times our sun’s diameter and is
about 68 light years away. Aldebaran is one of the few first magnitude stars that may be
occulted by the moon. The disappearance of the star is startlingly abrupt, particularly so
at the moons dark edge. The V shaped central group is the Hyades star cluster group
and represents the bull’s head – Aldebaran being the eye and Zeta and Beta being the
horns. (Aldebaran is not actually in the star cluster.) Numerous old stories associate
the Hyades with rainy weather. The whole of the Hyades group is moving towards Orion
and there are several hundred stars in the cluster. Zeta Tauri has no English common
name but translation of it’s Chinese name means the Gate of Heaven. It is a closely
paired binary. Zeta is a shell star, having a permanent shell of gas around it. The Crab
Nebula is about 6 light years in diameter and expanding at 600 miles per second (= 50
million miles per day). This is believed to be the supernova explosion of July 4 1054 AD.
The explosion would have been giving the light of about 400 million suns. If the law of
averages prevales, we are over due for another supernova.
The Pleiades or Seven
Sisters is the most famous star cluster in the heavens.
In Greek this means To Sail.
The rising of the Pleiades was the sign of the opening of the navigational season in the
Mediterranean.
There are many legends associated with the Pleiades. Eta Tauri or
Alcyone is the brightest star in the Pleiades. The whole group is leisurely drifting along at
about 25 miles per second. The entire cluster is enveloped in a vast faint nebulosity.
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Taurus
Pleiades
Aries
The Ram is named in Greek mythology
as having golden fleece.
Alpha Arietis or Hamar
Ecliptic
Beta Arietis or Sheratan
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Beta Arietis
Pisces
Alrisha
Alpha
Piscium
M74
Eta Piscium
Ecliptic
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Alrisha means the cord in Arabic. This is the cord that ties the two fish together. It is fine
binary of strange colours that are described differently by different observers. M74 or
NGC 628 is a face on spiral galaxy very close to Eta Piscium and about the same size as
our galaxy (80 000 light years in diameter) but quite a lot less luminous.
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Aquarius
Pisces
Zeta
Pi
Gamma
Sadalmelik (alpha)
Helix Nebula
NGC 7293
M2 NGC 7089
Sadalsud (beta)
M73
Ecliptic
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Gamma, Eta, Zeta and Pi make the water carrier or jug of Aquarius.
M2 is a globular
cluster easily seen with a small telescope, say 2 to 10 inch. It is about 50 000 light years
away. There are more than 100 000 stars in the cluster. The cluster shines with about
half a million times the luminosity of our sun. If we were at M2 and looking at our sun, we
would be able to see our sun with only the greatest of telescopes.
M72 is a globular
cluster about 60 000 light years away and about 85 light years in diameter.
A 10 inch
telescope will see the cluster but a much larger one is needed to see it’s interesting
characteristics. With better than a 10 inch telescope, the Saturn nebula is seen with a
fluorescent green tint. It’s shape is reminescent of Saturn. Binoculars will pick up the
Helix nebula as a circular hazy spot. Estimations on it’s distance vary a lot but if a
distance of 450 light years is accepted, then the diameter of the bluish green nebula is
about 1.75 light years across.
The helix nebular is so named because the nebula is
made of two rings easily seen forming a “two ring coil”.
It is made of tenuous gas
illuminated by a tiny but hot central star.
Although the dwarf star is only about 2% the
diameter of our sun, it has a temperature of about 100 000 degrees Kelvin.
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Capricornus
Alpha
Aquarius
Delta
M30
Gamma
Beta
M2
M73
Beta
Alpha
Ecliptic
Alpha Capricorni is made of two stars Alpha 1 and Alpha 2. They are close together but
not a binary. Each has a small companion star. Beta Capricorni is a binary and Beta B
is a close double itself. Close to Delta Capricorni is where Neptune was first observed in
1846.
M30 is a globular cluster about 40 000 light years away. Capricornus was
associated with the lowest point reached by the sun below the celestial equator (= 23.5
degrees). It gave it’s name to the Tropic of Capricorn for latitude 23.5 degrees south.
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Sagitarrius
Archer
Beta Capricorni
Alpha Capricorni
M55
M22
M25
Kaus Australis
M24
M17
Gamma Sagittarii
M8
M20(Trifid)
M23
Ecliptic
(the dark line part is the bow and gamma is the tip of the arrow being shot at the scorpion)
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Sagittarius contains a wide range of objects.
Also, the Greek lettering is quite out of
order. The brightest star is Epsilon (Kaus Australis). Behind Gamma Sagittarii are some
bright star clouds in the Milky Way and this region marks the central hub of our galaxy.
There are some curious winding lanes of dust in that area. Gamma has a yellow tint.
Connected by a solid line in the diagram above is the “milk dipper”, the end of the handle
being Epsilon. Below Epsilon is M8, the Lagoon Nebula – so named because of a dark
swirl running through the middle of the nebula. It is about 3 000 light years away and can
be easily seen with the naked eye. It is possible that M20 (the Trifid nebula) is also part
of the same nebulosity.
M17 is also known as the Swan nebula, Omega nebula and
Horseshoe nebula. Although it is very faint, it is a large nebula being about 40 light years
across. M24 is also visible to the naked eye.
M55 can be seen by binoculars. It is a
globular star cluster about 20 000 light years away and about 80 light years in diameter.
The luminosity is about 100 000 that of our sun.
As we look towards this area of the
Milky way, we are looking towards the Galactic centre and there are millions of stars and
associated structures. We are about 30 000 light years from the galactic centre. The
Milky way disc is about 80 000 (100 000 if some far flung traces are counted) in diameter
and about 10 00 light years thick at the hub. The disc is about 3 light years thick. The
overall shape is a barred spiralled galaxy.
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Scorpius
Scorpion
Sagitarrius
M7
M23
Antares
(alpha)
M4
M80
delta
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beta
Antares (alpha) in Greek means “the rival of Mars”, so called because of its red color. In
Roman the translation means the “heart of the scorpion” so called because of its position
in the constellation. It is the 15th brightest star in the sky. Some think that “the chambers
of the south” in the book of Job refer to this constellation.
In ancient times, this
constellation was given a larger area of the night sky and what is now alpha and beta
Librae were previously in Scorpio. The translation of the two common names for alpha
and beta Librae is the northern claw and the southern claw.
In the Middle Ages, the
scorpion became the symbol of unyielding fortitude, from the legend that if this creature
was surrounded by enemies, it would sting itself to death rather than to submit to capture.
Lord Byron refers to this when he wrote
“The mind that broods o’er guilty woes
Is like the scorpion girt by fire“
Antares is a supergiant star about 600 million miles in diameter or about 700 times the size
of our sun. It is somewhat egg shaped and is about 9 000 more luminous than our sun.
The density of Antares is about a millionth that of the sun and therefore is only about 10 to
15 more in mass than our sun.
Antares varies in both its diameter and luminosity.
There is a small green companion star to Antars. This companion star can be seen with
a 6 inch telescope when conditions are right. There are about 100 stars in what is called
the “Scorpio-Centaurus Association”.
These stars are in Scorpio, Crux and Centaurus
and are a part of the Local Star Cloud which shows a common space motion. They form
a part of one of the spiral arms of our galaxy. Beta Scorpii has two small companions,
one of which is itself a double. M4 (NGC6121) is a globular cluster that is visible to the
naked eye. A 4 inch refractor starts to show the individual stars. M6 and M7 are open
star clusters seen by the naked eye.
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Beta Scorpio
Libra
Delta Scorpii
5897
Beta
Alpha
Beta Librae is the Northern Claw of Scorpius and some observers say it is greenish. Star
colours are strangely elusive and vary from observer to observer.
If one sees it to be
greenish, then it is the only star to be green without a red companion.
Alpha Librae is
the Southern Claw of Scorpius.
48 Librae is the classic example of a shell star. The
magnitude is 4.85.
The star is surrounded by an outer shell of turbulent gases which
occasionally show violent expansion velocities up to 60 miles per second.
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Alpha Librae
Virgo
Spica
M104
Sombrero
Virgo galaxy cluster
Beta Virginis
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Spica is Alpha Virginis and is a close, massive binary. The brighter of the two is also a
pulsating variable. Antares and Spica are the 15th and 16th brightest star in the heavens
and have magnititudes very close to 1. Spica is about 2 300 times more luminous than
our sun. It is about 275 light years away from us. In ancient writings this constellation
represented a girl holding an ear of wheat because of its timing to harvest season. In
other customs, it represented a girl standing for justice.
With greater than a six inch
telescope, thousands of galaxies can be seen in the Virgo galaxy cluster.
M104 is
probably part of the Virgo galaxy cluster but is seen as a galaxy edge on and has a dark
ring of equatorial dust thus giving it the name of Sombrero.
A ten inch telescope is
advised to see the band.
It is a massive galaxy about 130 000 light years in diameter
(counting the faint outer portions). It has a mass of about 1.3 trillion suns. The speed of
this galaxy (red shift = 700 miles per second) has been heavily implicated in calculations in
the expanding nature of the universe.
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Beta Virginis
Beta Leonis
Leo
Regulus
Gamma Leonis
Regulus lies at the base of the sickle of Leo and represents the head and mane of the lion.
Beta or Denebola is the lions tail. Close to Gamma Leonis is the radiant point of the 33
year cycling Leonid Meteor showers, one of which was very spectacular on Nov 13 1833.
It is due on Nov 17 or 18 1999. Towards Virgo, many galaxies are seen as part of the
Virgo Cluster.
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Regulus
Cancer
Alpha
M44
Beta
Several thousand years ago, this constellation was background of the sun during the
summer solstice, its maximun elevation above the celestial equator. At that time, the sun
would be directly overhead if the observer was at a latitude of 23.5 degrees north. This
line around the earth is the Tropic of Cancer (appropriate from the point of view of causing
skin cancer - Cancer is really the “Crab”). Currently, the solstice lies between Gemini and
Taurus.
The brightest stars in Cancer are 4th magnitude stars.
M44 is known as
Praesepe or the Beehive Cluster or the Manger. It is one of the largest, nearest and
brightest clusters, being easily seen with the naked eye.
Prior to the invention of
tetescopes, it was called the Cloudy One or Little Cloud or Little Mist. Galileo counted 36
stars.
Over the 14th magnitude, there are now known to be 350 stars in the cluster.
This cluster and the Hyades Cluster in Taurus are moving with almost equal speed (25
miles per second) and parallel to each other.
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Beta Cancri
Gemini
Pollux
Castor
Orion
M35
These are the tips of the two horns in Taurus
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Castor is alpha although it is (currently) fainter visually (photographically it is brighter). It
is made up of three binaries
Castor is also known as the Horseman.
The pair of
twinkling stars have been known for centuries as the heavenly twins. Castor was the first
object beyond our solar system in which the force of gravitation was shown to be in
operation.
The main pair of stars in Castor are only about the distance across our solar
system apart. Neptune and Pluto were discovered when passing through Gemini.
The constellations around the south celestial pole are visible throughout the year (for those
who live in the southern hemisphere)
Orion
The hunters head
Betelgeuse
Bellatrix
The hunters shoulders
The hunters belt
Zeta
The hunters sword
Sword of Orion
M42 & M43
Rigel
The hunters legs
Saiph
(the solid line section is commonly called the “saucepan”)
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Stars around the Southern Celestial Pole.
Crux
Pointers
Musca
Triangulum Australe
South Celestial Pole
Canopus
(in Carina)
Larger Magellanic Cloud
Smaller Magellanic Cloud
Hydrus
Archernar
6.
Draw a chart of the Southern Cross, Orion and Scorpio.
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7.
What is the Milky Way ?
Observe the Milky Way in the night sky.
The Milky Way is the galaxy in which our solar system exists. It is a barred spiral galaxy
about 100 000 light years across and about 3 000 light years thick at the centre. We are
about 25 000 light years from the centre.
When we look towards Sagittarius, we are
looking towards the centre.
It seems that there are countless galaxies in the universe.
The Milky Way has a spherical bulge at the centre.
Young stars are in that bulge. A
flat pancake about 600 light years thick of young stars surrounds the central bulge and is
known as the thin disk. The Milky Way that we see at night is in the thin disk.
Older
stars are in a thick disk that is about three times thicker than the thin disk. A sphere of
old spherical star clusters surrounds the flat disks. The spiral arms of the galaxy revolve
around the galaxy at a different speed to our galaxy.
That means our sun is slowly
crossing the spiral arms.
The speed of crossing those arms determines how many
comets are disturbed from the Oort cloud and sent in towards our solar system.
Solar
systems further out than the Milky Way cross the spiral arms at a faster rate and are not
protected as well from cosmic collisions. The centre of the Milky Way contains a tangle
of strong magnetic fields. The galaxy that is closest to the Milky Way is a dwarf, irregular
galaxy in the constellation Canis Major and is 42,000 light years away.
The Larger
Magellanic cloud [at 170 000 light years away] and the Smaller Magellanic cloud [close to
200 000 light years away] are next closest. All three galaxies appear to be being sucked
into the Milky Way.
Considering the diameter of the Milky Way, all these three galaxies
are very close. There are 12 dwarf galaxies within 500,000 ly of the Milky Way.
Halo of globular clusters
Bulge
Thin disk
Thick disk
Milky Way
Moving groups are collections of stars that share the same motion through space.
An
example of a moving group is the group of about 40 stars that includes Sirius, Alpha
Coronae Borealis, Beta Eridani and Zeta Leonis. An open cluster such as the Pleiades is
more compact than a moving group but not as compact as a globular cluster.
Calculations of the energy required to sustain the known visible constituents of the
universe tell us that something in excess of 50% of what is immediately around us cannot
be detected. That is called dark matter.
It seems that something in excess of 50% of
the universe is invisible.
Furthermore, the number of stars in space increases by the
cubed power of the distance from us but the luminosity decreases by the squared power
as distance is increased.
The reason that the night sky is not brighter than the day sky
is that the light from distant stars has not yet reached us.
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The Milky Way is that blurry, hazey white smudge that extends roughly from east to west
through the night sky. As you study more carefully, you notice that the Milky Way does
not follow the course of the moon and planets. When looking at a star map that shows
the Milky Way and the course of the sun through the stars, you will notice that there is a
significantly different course. This is because the disc of our galaxy is is not parallel to
the disc of our solar system. The constellations that the sun passes through are those
constellations that are in the same plain as our earth and they are the twelve signs of the
zodiac. The mass of stars in the disc of our galaxy will be above the path of the sun
(ecliptic) for six months and below the ecliptic for the other six months.
8.
What is the morning and evening Star ?
Why does carry both names ?
Venus is called both the morning and the evening star. Venus is cloud covered and
therefore reflects a lot of the sun’s radiation. That makes Venus shine very brightly.
When Venus is near the western horizon, it is called the evening star. At sunset it is
commonly the first “star” to be seen in the semi darkened sky as it is commonly the
brightest object in that part of the twilight sky. Likewise at sunrise, when Venus is in the
eastern sky it is the last “star” to disappear into the brightening sky as it is commonly the
brightest object in the predawn sky.
Because the orbit of Venus is inside earths orbit,
Venus has to be seen between the sun and the tangent at earths orbit
Sun
Venus Orbit
Earth Orbit
To an observer on earth,
Venus has to be seen to the left
of the tangent. This means Venus
has to be seen towards the east in the hours between midnight and dawn or towards the
west in the hours between sunset and midnight. Very rarely [every eight years], the
morning and evening star can be seen in the same 24 hour period.
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9.
Explain zenith and nadir.
As defined on the web http://en.wikipedia.org ,
Zenith is the vertical direction pointing away from the direction of the force of gravity.
Nadir is the direction pointing in the same direction as gravity.
10.
What is the Aurora Australis?
Energetic solar flares eject streams of charged particles into space as part of the solar
wind. These interect with the region governed by the earth’s magnetic field and give rise
to showers of energetic particles that follow the earths masgnetic field down towards the
earths surface. In the atmosphere between 100-300km these excite atoms of gas which
emit light and this is seen as curtains and bands of coloured light. They are seen more
vividly closer to midnight.
In the southern hemisphere these are called the Aurora
Australia. They are seen usually in latitudes over 50 degrees.
In the northern hemisphere, the same phenomena is called the Aurora borealis and below
is a sample of the aurora borealis as recorded on the web site
http://antwrp.gfsc.nasa.gov/apod
9 April 2007.
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