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Teachers Notes Booklet 1: Introduction to the Universe
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The European Space Agency
The European Space Agency (ESA) was formed on 31 May 1975. It currently has 17
Member States: Austria, Belgium, Denmark, Finland, France, Germany, Greece, Ireland,
Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland &
United Kingdom.
The ESA Science Programme currently contains the following active missions:
Venus Express – an exploration of our
Cluster – a four spacecraft mission to
sister planet.
investigate
Rosetta – first mission to fly alongside
Sun and the Earth's magnetosphere
and land on a comet
XMM-Newton – an X-ray telescope
Double Star – joint mission with the
helping to solve cosmic mysteries
Chinese to study the effect of the Sun
Cassini-Huygens – a joint ESA/NASA
on the Earth’s environment
mission to investigate Saturn and its
SMART-1 – Europe’s first mission to
moon Titan, with ESA's Huygens probe
the Moon, which will test solar-electric
SOHO
propulsion in flight, a key technology for
atmosphere and interior
future deep-space missions
Hubble Space Telescope – world's
Mars Express - Europe's first mission
most important and successful orbital
to Mars consisting of an orbital platform
observatory
searching for water and life on the
Ulysses
planet
investigate the polar regions around the
INTEGRAL – first space observatory to
Sun
-
interactions
new
–
views
the
first
between
of
the
the
Sun's
spacecraft
to
simultaneously observe celestial objects
in gamma rays, X-rays and visible light
Details on all these missions and others can be found at - http://sci.esa.int.
Prepared by
Anne Brumfitt
Content Advisor
Chris Lawton
Science Editor, Content Advisor, Web Integration & Booklet Design
Karen O'Flaherty
Science Editor & Content Advisor
Jo Turner
Content Writer
© 2005 European Space Agency
Teachers Notes Booklet 1: Introduction to the Universe
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Booklet 1 – Introduction to the Universe
Contents
1.1
Solar System................................................................. 4
1.2
Planets and Moons ......................................................... 6
1.3
Asteroids and Comets..................................................... 9
1.4
Bodies of the Universe .................................................. 11
1.5
Stellar Clusters and Constellations.................................. 14
1.6
Relative Distances to Objects......................................... 15
1.7
Stellar Motions ............................................................ 16
1.8
Other Materials............................................................ 18
Tables
1.1
Distance and Orbital Parameters for the Planets ................. 7
1.2
Observational Characteristics of the Planets....................... 7
1.3
Dates of Primary Meteor Showers................................... 10
1.4
Brightest Open and Globular Star Clusters ....................... 14
Figures
1.1
Barringer Meteor Crater ................................................ 10
1.2
View of Orion and Actual Distance to Stars ...................... 14
1.3
Diurnal Stellar Motion ................................................... 16
Teachers Notes Booklet 1: Introduction to the Universe
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1.1 Solar System
The Solar System is a collection of planets, moons, asteroids and comets and other
rocky objects orbiting the Sun. The Solar System is believed to extend out to at least
150 000 million km from the Sun, although the planets are all found within about 6000
million km.
Our Solar System is thought to have formed 4.6 x 109 years ago from a vast, rotating
cloud of gas and dust known as the solar nebula. As the solar nebula rotated, its gravity
began to attract gas and dust towards the centre, eventually forming our Sun.
The Sun
The Sun is the powerhouse of the Solar System. Without it, life on Earth simply would
not exist. Despite burning its hydrogen fuel for the best part of 5 billion years, the Sun
is still only half way through its life cycle.
The study of the Sun, its environment and how the material it discharges interacts with
other worlds in the Solar System is of great interest to us all. ESA has a number of
missions that analyse various aspects of this solar emission.
Solar Missions
SOHO is a space-based observatory viewing and investigating
the Sun from its deep core, through to its outer atmosphere
(the corona) and the powerful solar wind. This joint mission
with NASA is being conducted from the inner Lagrange point
allowing continuous monitoring of the Sun.
The main scientific goal of the joint ESA-NASA Ulysses deepspace mission is to make the first-ever measurements of the
unexplored region of space above the Sun's poles.
Teachers Notes Booklet 1: Introduction to the Universe
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The Cluster mission is currently investigating the small-scale
structure (in three dimensions) of the Earth's plasma
environment, such as those involved in the interaction between
the solar wind and the magnetospheric plasma, by using four
spacecraft flying in formation.
The Double Star mission is a joint venture with the Chinese and
like the Cluster mission it is studying the effects of the Sun on
the Earth's environment. The mission consists of two spacecraft,
one in a polar orbit and one in an equatorial orbit, which are
based on the Cluster design.
Teachers Notes Booklet 1: Introduction to the Universe
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1.2 Planets and Moons
The formation of the Sun had a dramatic effect on the temperatures across the solar
nebula, introducing a temperature range that stretched from about 2000K near the Sun
to less than 50K at the outer regions. The heat in the inner Solar System only allowed
materials with high condensation temperatures to remain solid. These particles
eventually gathered to form the four terrestrial planets: Mercury, Venus, Earth and
Mars.
A similar process formed the outer planets of the Solar System: Jupiter, Saturn,
Uranus, and Neptune. Yet, they are different because icy materials such as frozen
water, carbon dioxide and methane were also available. Consequently, these outer
Jovian planets are much larger than the terrestrial planets. In addition these giant
planets were able to enhance their atmosphere by capturing gas atoms moving more
slowly due to the colder temperatures.
Pluto, the ninth planet, is unlike its neighbours. It is a tiny rocky world just 2300 km in
diameter (65% of the size of our own Moon). Up to now, its existence has baffled
astronomers. Recent observations of these outer regions, however, have shown that
there are other similar-sized objects, known collectively as transneptunian objects, in
this area of the Solar System. It is possible that Pluto belongs to this group of objects.
Each planet travels around the Sun in an elliptical orbit that is held in place by the
gravitational attraction between the Sun and the planet. Some of the planets, including,
of course, Earth, have moons orbiting them. Pluto has just one moon in orbit around it,
while Jupiter has 63 moons known to be orbiting it. Kepler's three laws of planetary
motion define the motion of the planets around the Sun, and the movement of moons
around their parent planet.
Teachers Notes Booklet 1: Introduction to the Universe
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Distance
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
AU
0.387
0.723
1.000
1.524
5.203
9.539
19.182
30.058
39.440
106 km
57.9
108.2
149.6
227.9
778.3
1427.0
2869.6
4496.6
5900.1
Year
Day
87.969d
224.701d
365.365d
686.980d
11.862y
29.457y
84.010y
164.793y
248.5y
58.65d
243.01dR
23.934h
24.623h
9.824h
10.233h
17.24hR
18.4h
6.39h
d and y are multiples of Earth day and year.
stands for retrograde, that is the spin of the planet is in
the opposite direction to its orbital motion.
*
R
Table 1.1: Distance and Orbital Parameters for the Planets
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Observational
Parameters
Mean Angular Mean Visual
Diameter (")
Magnitude
7.8
+0.0
25.2
-4.4
17.9
-2.0
46.8
-2.7
19.4
+0.7
3.9
+5.5
2.3
+7.8
0.1
+14.0
Moons
0
0
1
2
63
47
27
13
1
Physical Data
Radius
Mass
Density
Gravity
0.38
0.95
1.00
0.53
11.19
9.41
3.98
3.81
0.18
0.06
0.86
1.00
0.15
1323
752
64
54
0.01
0.98
0.95
1.00
0.71
0.24
0.13
0.24
0.32
0.36
0.37
0.90
1.00
0.38
2.69
1.19
0.93
1.22
0.03
*Limiting visual magnitude of human eye is around +6
*Physical Data is relative to the Earth: Radius – 6400 km; Mass – 6 x 1024 kg;
Density - 5.5 kgm-3; Gravity – 9.81 ms-2
* No of moons correct at June 2005
Table 1.2: Observational Characteristics of the Planets
BepiColombo is a joint mission with the Japanese Space
Agency to explore Mercury. The mission consists of two
orbital spacecraft: MMO – Mercury Magnetospheric Orbiter
and MPO – Mercury Planetary Orbiter.
Teachers Notes Booklet 1: Introduction to the Universe
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Venus Express is heading for Venus to provide detailed
analysis of its atmosphere. The mission is reusing technology
already developed for the Mars Express and Rosetta
missions.
SMART-1 is a technology proving mission to test a new form
of propulsion and miniaturisation technologies. This compact
spacecraft's journey to the Moon took around 15 months and
utilised an ion drive. Now in orbit around the Moon the
spacecraft is providing detailed analysis of the surface.
Mars Express, so called because of the rapid and streamlined
development time, represents ESA's first visit to another planet
in the Solar System. Borrowing technology from the failed Mars
96 mission and Rosetta, Mars Express is answering fundamental
questions about the geology, atmosphere, surface environment,
history of water and potential for life on Mars.
A NASA/ESA/ASI mission to explore the Saturnian system.
The ESA component consists largely of the Huygens probe,
which entered the atmosphere of Saturn's largest moon,
Titan, and descended under parachute down to the surface.
Teachers Notes Booklet 1: Introduction to the Universe
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1.3 Asteroids and Comets
There were some materials left over from the solar nebula once the Sun and the planets
had formed. Some of this debris remains in our Solar System in the form of asteroids
and comets.
Asteroids
Asteroids, which are sometimes called minor planets, are rocky bodies mostly found in
the planetary region between Mars and Jupiter. This region is known as the asteroid
belt, and it stretches from about 250 million km to about 600 million km from the Sun.
The largest known asteroid is Ceres with a diameter of roughly 1000 km. Only around a
dozen are more than 250 km across. Over 100 000 asteroids larger than one kilometre
in diameter are known to exist, with more being discovered all the time.
We often hear of asteroids on the news, when near-Earth asteroids pass close enough
to our planet to cause concern of a potential impact either now, or in the future. These
near-Earth objects have highly elliptical orbits, which bring them into the inner Solar
System, crossing the orbit of Mars and occasionally coming close to Earth.
Comets
Comets are often referred to as 'dirty snowballs', as they are made up of ice and dust.
The ones we can see travel around the Sun in highly elliptical orbits taking from a few
years to thousands of years to return to the inner Solar System. Typically comets are
just a few kilometres across, which makes them very difficult to spot for most of their
orbit. As they approach the Sun, however, solar radiation vaporizes the gases in the
comet and the characteristic comet 'tail' is formed.
The tail of a comet consists of two
parts: a whiter part made of dust, which always points away from the Sun, and a blue
part consisting of ionised gas. Comets are mainly found in two regions of the Solar
System: the Kuiper belt, a region that extends from around the orbit of Pluto to about
500 AU from the Sun, and the Oort Cloud (from the Kuiper Belt to about 50 000 AU
from the Sun).
The Rosetta mission will track the comet 67 P/ChuryumovGerasimenko and deploy a lander onto its surface. On its 10year journey to rendezvous with the comet, the spacecraft will
hopefully pass by at least one asteroid.
Teachers Notes Booklet 1: Introduction to the Universe
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Meteors
Occasionally small rocks or dust particles enter the Earth's atmosphere. The dust
particles and small rocks burn up in the atmosphere leaving behind brief trails in the
sky witnessed as meteor showers. It is estimated that more than 200 million kg of
meteoritic material is swept up by the Earth each year, with around 10% reaching the
ground.
Much of this material orbits the Sun in distinct streams, usually as debris from different
comets. At various times throughout the year the Earth crosses these streams and for
a few nights an observers can witness a meteor shower.
Shower
Quadrantids
Lyrids
Eta Aquarids
Delta Aquarids
Perseids
Orionids
Taurids
Leonids
Geminids
Activity Dates
Jan 1-6
April 16-25
April 19 – May 28
July 12 – Aug 19
July 17 – Aug 24
Oct 16-27
Oct 20 – Nov 30
Nov 14-21
Dec 7-17
Maximum
Jan 3
April 22
May 5/6
July 29
Aug 12/13
Oct 21
Nov 5
Nov 17
Dec 14
Rate
60
15
35
20
75
25
10
15
75
Parent Object
2003 EH1A
C/1861 GI Thatcher C
1P/Halley C
pos. 96P/Macholz C
109P/Swift-Tuttle C
1P/Halley C
2P/Encke C
55P/Tempel-Tuttle C
3200 - Pantheon A
Parent Object: A – Asteroid, C – Comet
Rate – typical number per hour at maximum
Table 1.3: Dates of Primary Meteor Showers
Sometimes larger fragments survive their passage through the atmosphere and impact
the surface, where they become known as meteorites. Most impacting fragments are
tiny and cause little or no damage. Historically, however, there have been several
major impacts, which may be responsible for changes in climate and the mass
extinction of species.
The image alongside shows the Barringer
Meteor Crater in the United States. It was
created around 50 000 years ago when a 50 m
wide rock hit the ground at an estimated speed
of 65 000 kilometres per hour. The resulting
crater is 175 m deep, 1500 m wide and
scattered debris in a 15 km radius.
Figure 1.1: Barringer Meteor Crater (credit: NASA)
Teachers Notes Booklet 1: Introduction to the Universe
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1.4 Bodies of the Universe
It is hard to comprehend the enormity of our Universe. Our Sun is only one of billions of
stars in our galaxy, which is known as the Milky Way. But beyond the Milky Way, there
are billions of other galaxies, too. Collectively, all these galaxies, along with the vast
amount of space found in between them, are called the Universe.
Solar System
The Sun
The star located at the centre of the Solar System, with a diameter of 1.39 million km
and a visible surface temperature of 5780 K.
Planets
Nine planets orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus,
Neptune and Pluto.
Moons
There are now over 100 known moons in the orbit around the planets of the Solar
System. Some, like Earth and Pluto, have just one companion while Jupiter has over
60.
Comets
Rocky-icy bodies in elliptical orbits travel through the Solar System developing a large
tail of gas and dust in their orbit as they draw closer to the Sun. There are at least 150
periodic comets.
Asteroids
Thousands of rocky bodies, the largest being around 1000 km across, orbit the Sun.
The majority are found between the orbits of Mars and Jupiter in a region known as the
asteroid belt. In recent years observations have begun to probe the extent of another
source of asteroid type objects (transneptunian objects) in the Kuiper Belt stretching 30
to maybe 100 AU.
Teachers Notes Booklet 1: Introduction to the Universe
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Our Galaxy
Stars
•
Variables - stars whose brightness varies periodically
•
Binaries - 70% of all stars in our galaxy exist in multiple star systems
•
Neutron stars - a star with around 2 solar masses that is made up entirely
from neutrons - remnants of a supernova explosion.
•
Pulsars - a rapidly spinning neutron star emitting pulses of radiation like a
lighthouse
•
White dwarfs - a star with up to 1.44 solar mass that is made up entirely of
electrons.
•
Novae - a star that undergoes a sudden increase in brightness by around 10
magnitudes and then declines over a period of months
•
Supernovae - a massive star that has exploded
•
Young hot blue-white stars
•
Old cool red star
Star Clusters
•
Younger open, or galactic, clusters
•
Older globular clusters
Nebulae
•
Planetary nebulae - remains of an old dying star
•
Emission - glows in the presence of UV light
•
Reflection - shines by reflected starlight
•
Absorption - dark, seen only as a silhouette
•
Supernova remnants
•
Gas Clouds and Interstellar Medium (ISM)
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Extragalactic
Galaxies
•
Spiral
•
Barred spiral
•
Elliptical
•
Lenticular
•
Irregular
Clusters and Superclusters of Galaxies
Quasars
Active Galactic Nuclei
Gamma-Ray Burst
Other Components
Dark Matter
Dark Energy
Teachers Notes Booklet 1: Introduction to the Universe
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1.5 Stellar Clusters & Constellations
We are familiar with the constellations that we see regularly in the night sky - a
distinctive pattern of stars. Although these stars may form shapes that are recognisable
to us here on Earth, they do not usually have any real link to each other, as they are
often at different distances from the Earth, and are in fact very far away from each
other.
Figure 1.2: View of Orion and Actual Distance to Stars
Stellar clusters, on the other hand, are systems of stars that are held together by the
gravity of their members. Eventually these clusters slowly evaporate. After a few billion
years, the relatively loose collections of stars known as open clusters will no longer be
held together by gravity and the cluster will stop existing. More highly compacted stellar
clusters, known as globular clusters, which are typically about 15 billion years old, have
not yet evaporated. Due to their relatively well-known distances, and the similarities
that tend to exist among their stars, stellar clusters play an important role in
astrophysics. Some of the nearest stellar clusters are visible with the naked eye. The
most visible open clusters are the Pleiades and Hyades, both to be found in the
constellation of Taurus.
Name
Pleiades
Hyades
Beehive
M35
M47
NGC4755
M13
M4
ω Centauri
47 Tucanae
Type
Open
Open
Open
Open
Open
Open
Globular
Globular
Globular
Globular
RA
03h47
04h29
08h40
06h09
07h37
12h54
16h42
16h24
13h27
00h24
DEC
+24°07
+16°30
+20°00
+24°20
-14°30
-60°20
+36°28
-26°32
-47°29
-72°05
North/South
North
North
North
North
South
South
North
South
South
South
Table 1.4: Brightest Open and Globular Star Clusters
Teachers Notes Booklet 1: Introduction to the Universe
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1.6 Relative Distances to Objects
A light year is the distance light travels through empty space in the course of one year.
1 light year = 9.461 x 1012 km = 5.878 x 1012 miles
In order to comprehend the enormity of space, astronomers use a variety of methods to
measure the distances between stars and between galaxies.
Our own galaxy, the Milky Way, is around 120 000 light years across and the Sun
occupies a position roughly 28 000 light years from the centre. Within the Milky Way,
the nearest star to the Sun is Proxima Centauri, which is about 4.4 light years away.
But most of our nearest stars are between 100 and 1000 light years away from Earth.
From any given location on Earth it is possible to view around 7000 stars with the
naked eye and countless more with a telescope. In all, our galaxy contains over 1
billion stars.
The distance to stars in our galaxy is obtained using a technique called parallax. By
identifying certain stellar properties it is then possible to calibrate a distance scale out
to our galactic neighbours.
The nearest galactic objects are the Magellanic Clouds. The Large Magellanic Cloud is
170 000 light years away, while the Small Magellanic Cloud is at a distance of 210 000
light years. The next nearest galaxy is Andromeda (M31 in the Messier catalogue), at a
distance of 2.3 million light years.
Galaxies are usually part of a larger group of galaxies. The group of galaxies that
includes the Milky Way and Andromeda, plus several other smaller companion galaxies,
is known as the Local Group. The other galaxies in the Local Group are between 80 000
to three million light years away from the Milky Way.
The next nearest rich cluster of galaxies, the Virgo cluster, is around 60 million light
years away. It is believed that the Milky Way-Andromeda cluster is part of an even
bigger supercluster along with Virgo-Coma cluster.
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1.7 Stellar Motions
Diurnal Effects
During the course of one night, the constellations appear to move across the sky. Stars
rise above the eastern horizon and set below the western horizon. The stars appear to
rotate around one point in the sky. This optical effect occurs because the Earth itself is
rotating about axis.
Figure 1.3: The motion of star around the north celestial pole during the course of one
evening.
Annual Effects
If you observe the night sky regularly over the course of one year, you will notice that
the constellations appear to change their position slightly from one night to the next at
any given time, only returning to their original positions once a year. This is due to the
difference between a calendar day (24 hours) and a sidereal day (23 hours 56
minutes), or the time the Earth actually takes to spin once on its axis.
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Annual Parallax
Annual Parallax is the difference between the position of a star observed from the Earth
and by a hypothetical observer at the Sun. The effect is a tiny shift in the positions of
relatively close stars against the background of distant stars. If the position of a nearby
star is plotted during the course of a year it sweeps out an ellipse, called the parallactic
ellipse, across the sky. This change in position is very small and requires high precision
instruments to make the observation.
Precession
This effect, which can be observed for example with a spinning top, is caused by the
gravitational pull from the Sun and Moon on the Earth's equatorial bulge. (Note - if the
Earth were a perfect sphere precession would not occur.)
Precession causes the Earth's rotation axis to sweep out a circle on the sky with an
angular radius of 23° 27' (this value corresponds to the axial tilt of the Earth). The
circle is traced out at the rate of 1° every 71.6 years, taking 25 800 years to complete
a full circle.
This means that the celestial pole, which currently points at the star Polaris, changes
with time. Careful examination of Figure 1.3 (on the previous page) shows the Pole
Star also leaving a star trail since it is ¾ of a degree away from the celestial pole.
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1.8 Other Materials
This is booklet one in a series of six booklets currently available. The full range of titles
is:
Booklet 1
Introduction to the Universe
Booklet 2
Stellar Radiation and Stellar Types
Booklet 3
Stellar Distances
Booklet 4
Cosmology
Booklet 5
Stellar Processes and Evolution
Booklet 6
Galaxies and the Expanding Universe
Each booklet can be used to cover a topic on its own, or as part of a series. Booklets 5
and 6 expand on the material covered in the other booklets and there is, therefore,
some overlap in content.
All the booklets can be accessed via the ESA Science and Technology at:
http://sci.esa.int/teachernotes
For other educational resources visit the ESA Science and Technology Educational
Support website at:
http://sci.esa.int/education
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