Chapter 1
LAU Beirut – Spring 2021
Dr. Elie Matar
Prepared by Dr. Nelly Mouawad
Introductory lecture
How does the
Solar System fit
into the
Universe?
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Earth Belongs to the Solar System
In our solar system, there
exist:
Stars (1 star: our sun)
Planets (8 planets)
Moons/satellites (>100)
Comets (millions/billions)
Asteroids (billions)
Meteorites/meteoroids
Dwarf planets
Trans-Neptunian objects
(TNOs)
The Kuiper Belt
The Oort Cloud
Interplanetary Dust
Space junk
And more
Solar System Rotation is dominantly
counterclockwise as seen from top
Every week we discover about 40 near-Earth objects (NEOs).
There are now over 18,000 known NEOs. Image credits: NASA/JPL-Caltech
To Scale: the solar system
On a dry lakebed in Nevada, a group of friends
build the first scale model of the solar system
with complete planetary orbits: a true
illustration of our place in the universe.
https://www.youtube.com/watch?v=zR3Igc3R
hfg
Earth and the other planets
Pluto was demoted, it is not defined as a planet anymore but as a
dwarf planet or a trans-Neptunian object.
Earth and the other planets
Pluto was demoted, it is not defined as a planet anymore but as a
dwarf planet or a trans-Neptunian object.
Terrestrial Planets
Let’s take a look at the most
fascinating worlds of our solar
system
Enceladus – Saturn‘s moon
Enceladus – Saturn‘s moon
An artist's concept of what it might look like
from the surface of Saturn's giant moon Titan.
We believe that it rains methane on Titan!
Enceladus – Saturn‘s moon
An artist's concept of what it might look like
Real image of Titan taken 25
km above the
from the surface of Saturn's giant moon Titan.
surface with the Huygens probe in 2005!
We believe that it rains methane on Titan!
Pluto – in real color
Image taken with the spacecraft Horizons- NASA
Enceladus – Saturn‘s moon
Our moon is fascinating on its own
Did you ever wonder…
…where did all the craters on Moon come from?
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Small chunks of rock and ice also orbit the Sun.
433 Eros, only 33 km
(21 mi.) across is one
of hundreds of
thousands of small
asteroids, or minor
planets, orbiting
between the orbits of
Mars and Jupiter.
Anatomy of the solar system
Comet 67P
Also called comet
Churyumov–Gerasimenko is about 4.3 x 4.1 km in size
It was visited by the spacecraft Rosetta:
http://www.esa.int/Our_Activities/Space_Science/Rosetta
Comets are small chunks of rock and ice that orbit the
Sun.
Asteroids and comets can fall on Earth: the piece of rock found on Earth
is called meteorite
The Meteor Crater in Arizona
Mercury like the moon has its surface covered
with craters Gibran Crater
5 July 2010 Edde Club
Gibran Crater
I named the crater Gibran when I was working on the space mission MESSENGER, which
went in orbit around Mercury.
https://en.wikipedia.org/wiki/Gibran_(crater)
Image of satellites and debris around Earth
(source www.esa.org and www.nasa.gov )
Interesting article: https://www.nasa.gov/mission_pages/station/news/orbital_debris.html.
There are more than 500,000 pieces of debris the size of a marble or larger that are tracked
from Earth. There are many millions of pieces of debris that are so small they can’t be tracked.
On Feb. 10, 2009, a defunct Russian satellite collided with and destroyed a functioning U.S.
Iridium commercial satellite. The collision added more than 2,000 pieces of trackable debris to
the inventory of space junk.China's 2007 anti-satellite test, which used a missile to destroy an
old weather satellite, added more than 3,000 pieces to the debris problem.
The Solar Neighborhood
The edge of the solar system
If we zoom out of the solar system,
we will find out that…
Star are organized in pairs, star clusters, galaxies
Stars like Grains of Sand
For more infos:
https://www.universetoday.com/1075
98/what-is-the-kuiper-belt/
Our Solar System belongs to our Galaxy –
the Milky Way
This is not a picture of our Galaxy
We cannot take such a picture of our own Galaxy.
We are stuck in the disk.
Fun Facts
Our Sun takes about 225 million
years to make 1 turn around the
center of our Galaxy.
It moves with a speed of 220 km/s
Since its creation 4.5 billion years
ago, it has done 20 turns only!
There are different types of Galaxies, sizes, colors,
and shapes
What is an a Galaxy?
.Millions or Billions of
Stars
with all the different
objects that can orbit
around them (planets,
moons, asteroids…)
.Clusters: Stars could be
formed in clusters of
stars of 1,000s to
100,000s of stars. These
orbit the center of
Galaxies too.
Most fascinating features Dead Stars: Interstellar
.Clouds .Neutron Stars, pulsars Matter:
.Nebulae – birth place of stars .Black Holes – of different sizes .Dust
.Supernovae – Exploding stars .White dwarves .Gas
and more…. .Dark matter
M 13: Globular Star Cluster – LAU Astronomy Club
The Orion Nebula – Birthplace of Stars
The Crab Nebula – Wreckage of an Exploded
Star
Galaxies are present in all different orientations
Credits: Space Telescope Science Institute
Credits: Space Telescope Science Institute
About 85% of the total in
the universe is dark matter,
only 15% is regular matter. It
is called dark matter
because it is a type of
matter that does not
interact with light. So it is
unseen matter, but its
gravitational pull can be
measured.
We will cover dark matter
when we talk about the
large structure of the
universe toward the end of
the course.
Galaxies in space as seen with the Hubble telescope.
Our Galaxy belongs to a cluster of galaxies called the Local Group
The Local Group is the
galaxy group that
includes the Milky
Way. It has about 54
galaxies, most of them
dwarf galaxies.
Its gravitational center
is located somewhere
between the Milky
Way and the
Andromeda Galaxy.
The three largest
members of the group
are the Andromeda
Galaxy, the Milky Way
and the Triangulum
Galaxy. The larger two
of these spiral galaxies
each have their own
system of satellite
galaxies.
Local group with clusters around it
The Local Group is surrounded by other galaxy clusters.
Local Super-cluster
Clusters themselves are grouped together in huge associations called super-clusters. A
typical super-cluster contains dozens of individual clusters spread over a region of space
up to 150 million light-years across.
Regular Matter is structured in
filaments
Large Scale structure of the nearby universe
This is a composite infrared image from the 2MASS (Two-Micron-All-Sky-Survey) project that
shows the light from 1.6 million galaxies. The galaxies form a lacy filamentary structure.
super-clusters of galaxies are not spread uniformly across the universe, but are found in vast
sheets separated by truly immense voids.
Sizes in the Universe
A Thermonuclear Explosion
A hydrogen bomb uses the same physical principle as the thermonuclear reactions at the Suns center: the
conversion of matter into energy by nuclear reactions. This thermonuclear detonation on October 31, 1952,
had an energy output equivalent to 10.4 million tons of TNT (almost 1000 times greater than the nuclear
bomb that detonated over Hiroshima in World War II). This is a mere ten-billionth of the amount of energy
released by the Sun in one second. (Defense Nuclear Agency)
1-4: By observing galaxies, astronomers learn about the
origin and fate of the universe
A Galaxy
Animation
• https://www.youtube.com/watch?v=vRjGarIC
al4
Describe the objects you see in the
night sky
Apart from the moon
Ever seen the big dipper?
Ever seen Orion?
Notice the
different colors
of the stars
The
constellation
Orion
Eighty-eight
constellations
cover the
entire sky.
Constellation
names are derived
from the myths and
legends of
antiquity.
The views of Orion – notice the names of the stars
Three Views of Orion
88 constellations cover the entire sky.
6000 stars visible to unaided eye (only half are above the horizon).
88 semi-rectangular groups of stars called constellations.
The Sky in spring 2016
Comet Lovejoy, glowing green in the center of the image.
The star cluster known as the Pleiades, or the Seven Sisters, a little above Comet Lovejoy to the
right.
The California Nebula, the red arc of gas directly to the right of Lovejoy.
A meteor, streaking along the left of the sky.
A haze of glowing green light collecting along the horizon, as a result of oxygen in the upper
atmosphere.
A pretty shroud of low altitude clouds on the plain below the observatory.
Oh and that streak of light on the plain? That's the Pan-American Highway.
In a dark sky you can see the disk of the Milky Way
The Milky Way from Faraya – LAU astronomy club
Did you ever wonder…
Sun Mon Tues Wed Thurs Fri Sat
…why a week is 7 days long?
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Astronomy Permeates Daily Life
• 7 objects (‘wanderers’) move among the stars from our perspective
Saturnus was a God an ancient Roman religion. In Latin Saturnus.
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The planets are wanderers in the sky
They seem to wander among the stars
Jupiter and Saturn pas de deux
http://apod.nasa.gov/apod/ap011220.html
Check out what is available in the skies tonight
http://www.space.com/16149-night-sky.html
Sizes, distances and units
Angular Measure
• Basic unit of angular measure is the degree º
– Full circle measures 360º
– Right angle measures 90º
• Angular distance is the number of degrees across
the sky between two points.
• Angular diameter or angular size is the number of
degrees from one side of an object to the other
side.
FORMAL: The angular size of the Moon is 0.5º
INFORMAL: The Moon subtends an angle of ½º
Astronomers use angles to denote the positions and
apparent sizes of objects in the sky.
Angular Measure for Small Angles
1º = 60 arcminutes = 60’
1’ = 60 arcseconds = 60“
1º = 60’ × 60”/1’ = 3600 “
e.g., On January 1, 2001, the planet Saturn had an
angular diameter of 19.7” as viewed from Earth.
Angular sizes
D = linear size of object ;
α = angular size of object (in arcsec)
d = distance to the object
Angular sizes
Same
angular size
Same
linear size
The Small-Angle Formula
α (rad) = D/d ; units of α are in radians, with 1π rad = 180°
α (“) = D/d × 206,265
here α is in arcsecs, because 1 rad = 206,265”
Astronomical distances
Astronomical distances are often measured in astronomical
units, light-years or parsecs
By definition:
• 1 astronomical Unit (AU) is the average Earth-Sun distance
1 A.U. = 1.5 × 1011 m
used mainly to measure distances in the solar system
• 1 light-year (ly) is the distance that light travels in a year
d (ly) = v (m/s) × t(s)
= 3×108(m/s) × 365.25days × 24 (hr/day) × (3600s/hr)
1 ly = 9.46 × 1015 m.
1 ly = 6.32 × 104 A.U
used mainly to measure distances to stars and galaxies
Astronomical distances (The Parsec)
By definition, one parsec (pc)
is the distance from which
Earth would appear to be one
arcsecond from the Sun.
α (“) = (D × 206,265 )/d
d (pc) = 1 AU × 206,265
1 pc = 3.26 ly
used mainly to measure to
Galaxies
Powers-of-ten notation is a useful shorthand system for
writing numbers
Examples of Powers-of-Ten Notation
12,000 years
from now, the
bright star
Vega will be
the “new
North Star”
because of
precession.
Which planet(s) in our solar system has (have) been
visited by spacecraft?
A. Venus and Mars
B. All of the terrestrial planets
C. All
D. Mercury, Venus, Neptune, and Mars
E. Only Mars
Q1.3
Which planet(s) in our solar system has (have) been
visited by spacecraft?
A. Venus and Mars
B. All of the terrestrial planets
C. All
D. Mercury, Venus, Neptune, and Mars
E. Only Mars
A1.3
Which of the following statements is true?
A. A solar system may contain billions of stars.
B. Stars are spread out more or less uniformly
throughout the Universe.
C. The Milky Way contains a variety of solar systems,
stars, and galaxies.
D. A galaxy contains billions of stars.
E. A solar system contains billions of galaxies.
Q1.12
Which of the following statements is true?
A. A solar system may contain billions of stars.
B. Stars are spread out more or less uniformly
throughout the Universe.
C. The Milky Way contains a variety of solar systems,
stars, and galaxies.
D. A galaxy contains billions of stars.
E. A solar system contains billions of galaxies.
Q1.12
Interesting links
What is a star, a planet, a moon? – FYI.
• https://testtube.com/dnews/temp-slug-episode-
1696?utm_source=FB&utm_medium=DNews&utm_campaign=DNewsSocial
• Scientific Notation and SI prefixes
You will often encounter very large or very small numbers. In that situation we will
use scientific notation by using powers of 10:
Exp1: Earth’s mean radius is equal to 6,371,000 m or 6.371×106 m.
Exp2: The radius of a hydrogen atom is about 0.000 000 000 053 m or 5.3×10-11 m.
You can practice using this website:
http://janus.astro.umd.edu/cgi-bin/astro/scinote.pl .
Alternatively, one can use SI prefixes:
Exp1: Earth’s radius can be written as 6,371 km or 6.371 Mm.
Exp2: The radius of the H atom is about 0.53 pm (or Picometer)
1 pm = 10-12 m. (see table 1.4 for more prefixes).
What lengths are most conveniently measured in
astronomical units (AU)?
A. Distances on the Earth
B. Distances within the solar system
C. Distances between stars in our Galaxy
D. Distances between galaxies
E. Only the largest distances in the universe
Q1.5
What lengths are most conveniently measured in
astronomical units (AU)?
A. Distances on the Earth
B. Distances within the solar system
C. Distances between stars in our Galaxy
D. Distances between galaxies
E. Only the largest distances in the universe
A1.5
The Cosmic Calendar
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We did more 50 years ago!
Scale of the Solar System - FYI
Let’s build a 1 : 10,000,000,000 solar system
Mercury - salt/sand grain
3 m away
Venus - peppercorn
12 m away
Earth - peppercorn
16 m away
Mars - big salt / small pepper
25 m away
Jupiter - marble
~80 m away
Pluto - head of pin Check out this website:
~540m away http://janus.astro.umd.edu/AW/awtools.html#viewers
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