Chapter 1 Our place in the universe
1.1 Our modern view of universe
- The sun, moon and planets appear to circle around our sky each day and we cannot feel the
constant motion of the earth as it rotates on its axis and orbits the sun
o Naturally assumed we live in an earth-centered or geocentric universe
- Earth is a planet in our solar system which consists of the sun and all the objects that orbit it: the
planet and their moons and countless smaller objects including rocky asteroids and icy comets
- A galaxy is a great island of stars in space containing from a few hundred million to a trillion or
more stars
- The milky way galaxy is relatively a large galaxy containing more than 100 billion stars
- Our solar system is located a little more than half way from the galactic center to the edge of
the galactic disk
- Some galaxies are fairly isolated but many others are found in groups
o Our milk way, for example, is one of the two largest among about 40 galaxies in the local
group.
o Groups of galaxies with more than a few dozen members are often called galaxy clusters
o The regions in which galaxies and galaxy clusters are most tightly packed are called
superclusters which are essentially clusters of galaxy clusters
o Our local group is located in the outskirts of the local supercluster
o All these structures make up the universe
- The universe is the sum of all matter and energy encompassing the superclusters and voids
everything within them
How did we become to be?
- The entire universe is expanding meaning the average distances between galaxies are increasing
with time
o Which implies that galaxies must’ve been closer together in the past and if reach the
point at which the expansion began
o We call this beginning the big bang and from the observed rate of expansion we
estimate that it occurred about 14 billion years ago
o The universe has continued to expand ever since the big bang but on a smaller scale the
force of gravity has drawn matter together
o While the universe as a whole continues to expand individual galaxies and galaxy
clusters ( as objects within them such as planets and stars) do not expand
- Within galaxies like the milky way gravity drives the collapse of clouds of gas and dust to form
stars and planets
- Stars are not living organisms but they go through life cycles
o A star is born when gravity compresses the material in a cloud to the point where the
centre becomes dense enough and hot enough to generate energy by nuclear fusion,
the process in which lightweight atomic nuclei smash together and stick or fuse to make
heavier nuclei
o The star lives as long as it can shine with energy from fusion and dies when it exhausts
its useable fuel
o When a star dies it blows much of its content back out into space
o Massive stars die in titanic explosions called supernovae Chapter 1 Our place in the universe
o The returned matter mixes with other matter floating between the stars in the galaxy
eventually becoming part of new clouds of gas and dust from which new generations of
stars can be born.
Galaxies therefore function as cosmic recycling plants, recycling material
expelled from dying stars into new generations of stars and planets
Stars manufacture the elements of earth and life
- The recycling of stellar material is connected to our existence in an even deeper way
- We learned that the early universe contained only the simplest chemical element: hydrogen and
helium and a trace of lithium
- We and earth are made primarily of other elements such as carbon, nitrogen, oxygen and iron
o Where do these chemicals come from?
o Evidence shows that they were manufactured by stars and some through nuclear fusion
the make stars shine and others through nuclear reactions accompanying the
explosions that end stellar life
o By the time our solar system formed about 4 and half billion years ago earlier
generations of stars had converted about 2 percent of our galaxy’s original hydrogen
and helium into heavier elements so the cloud that gave birth to our solar system was
made about 98 percent hydrogen and helium and 2 percent of other element
- The 2 percent maybe small but it was enough to make the small rocky planets or our solar
system including earth
- In summary most of the material from which we and our planet are made was created inside
stars that lived and died before the birth of our sun
- Astronomer carl sagan said we are star stuff
How can we know what the universe was in the past
- Its possible to know the past because we can see into the past by studying light and from distant
stars and galaxies
- The speed of light is 300 000 km/per sec a speed which at which it would be possible to circle
earth nearly 8 times in just one second
- Light takes more than 1 second to reach earth from the moon and 8 minutes to reach earth
from the sun
o We measure distances to the stars in units called light years
o One light year is the distance that light can travel in 1 year – about 10 trillion km or 6
trillion miles.
o Light year is a unit of distance and not time
o Because light takes time to travel thorough space we are led to a remarkable fact and
that is the farther away we look in distance the further back we look in time
o for example, the brightest star in the night sky is Sirius is about 8 light years away wich
means it takes about 8 years to reach us. When we look at Sirius we see it not as today
but as it was about 8 years ago
o the orion nebula I a giant cloud in which stars and planets are forming. It is about 1500
light years from earth which means we see it as 1500 years ago Chapter 1 Our place in the universe
Can we see the entire universe?
- The observed expansion implies that our universe is about 14 billion years old
- The distance of 14 billion light years therefore marks the boundary of our observable universe –
the portion of the entire universe that we can potentially observe.
- We have no hope of seeing or studying anything beyond the bounds of our observable universe
1.2 The scale of the universe
How big is earth compared to our solar system
- The voyage scale model solar system in Washington d.c shows the sun and the planets and the
distances betweeb them and one ten billionth of their actual sizes and distances
- The sun is a large grapefruit, Jupiter about the size of a marble and the earth the size of a ball
point in a pen
Chapter 1.2
How many planets are there in the solar system?
pluto is smaller than any of first eight planets and that it shares the outer system with thousand
of other icy objects
define a planet:
o as an object: (1) orbits a star (but is itself not a star)
o (2) is massive enough for its own gravity to give it a nearly round shape
o (3) has cleared the neighborhood around its orbit
Objects that meet the first two criteria but that have not cleared their orbital neighborhoods –
including Pluto, Eris, and the asteroid Ceres –are designated dwarf planets
The myriad objects that orbit the Sun but are too small to be round, such as asteroids, and
comets make up a class called small solar system bodies
How big is the Milky Way?
Our solar system is a single star system, while our galaxy is a collection of more than 100 billion
star systems –so many that it would take thousands of years just to count them
How big is the universe?
The milky way is only one of roughly 100 bullion galaxies in the observable universe. Just as it
would take thousands of years to count the stars in the Milky Way, it would take thousands of
years to count all the galaxies
How do our lifetimes compare to the age of the universe?
1.3 Spaceship Earth
How is Earth moving in our solar system?
The most basic motions of earth are its daily rotation (spin) and its yearly orbit (or revolution)
around the Sun Chapter 1 Our place in the universe
Earth rotates once each day around its axis, which is the imaginary line connecting the North
Pole to the South Pole
Earth rotates from west to east –counterclockwise as viewed from above the north pole –
which is why the sun and stars appear to rise in the east and set in the west each day
Earth also orbits the sun, completely one orbit each yea; earth’s average orbital distance is
called an astronomical unit, or AU, equivalent to about 150 million km (93 million times)
Earth’s orbital path defines a flat plane that we call the ecliptic plane
Earth’s axis is titled by 23 1/2 from a line perpendicular to the ecliptic plane
This axis tilt happens to be oriented so that it points almost directly at a star called Polaris, or
the North Star
Figure 1.13 earth’s axis remains pointed in the same direction (toward Polaris) throughout the
year. The average earth-sun distance is 1 AU or about 150 million km; earth takes 1 year to orbit
the Sun at an average speed of 107, 000 km/hr
How is our solar system moving in the Milky Way Galaxy?
Our local solar neighbourhood
We usually think of our local solar neighborhood as a region containing just a few thousand to a
few million of the nearest stars
Galactic Rotation
The entire milky way galaxy is rotating. Our solar system, located about 28,000 light years from
the galactic center completes one orbit of the galaxy in about 230 million years from the galactic
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