LUNULARIA -features, morphology, anatomy ,reproduction etc.
What are cosmic rays and where do they comes from
1. PRESENTATION
Topic: What are cosmic rays and where do they comes from?
Group No.4
Roll No. 126, 94, 98, 120, 136, 140, 107
2. COSMIC RAYS:
A cosmic ray is a high-speed particle-
-either an atomic nucleus or an
electron--that travels throughout the
Milky Way Galaxy, including the solar
system. Some of these particles
originate from the Sun, but most
come from sources outside the solar
system and are known as galactic
cosmic rays (GCRs). Cosmic-ray
particles that arrive at the top of the
Earth's atmosphere are termed
primaries; their collisions with
atmospheric nuclei give rise to
secondaries.
3. INTRODUCTION TO C.RS
About 85 percent of the GCRs are protons (nuclei of
hydrogen atoms), with approximately 12 percent consisting
of alpha particles (helium nuclei). The remainder are
electrons and nuclei of heavier atoms. Because most
cosmic-ray primaries are strongly influenced by the Earth's
magnetic field and the interplanetary magnetic field, most of
those detected near the Earth have kinetic energies in
excess of about 1 GeV (gigaelectron volts, or one billion
electron volts). This energy corresponds to speeds greater
than about 87 percent the speed of light. The number of
particles drops rapidly with increasing energy, but individual
particles with energies as high as 1020 eV have been
detected
4. WHERE DO COSMIC RAYS COME FROM?
All the light and the heat which we receive on the Earth comes
from the sun, so it might seem a reasonable guess to suspect
that cosmic rays may also come from the sun. This however is
not the case. There are two bits of experimental evidence which
backs up this statement.
Firstly it is well known that during the day it is light and at night it
is dark, in other words a variation of light intensity seen on Earth
occurs during a day (24 hour cycle). Similarly the hottest part of
the day occurs between 11am and 3pm, the coolest part
occurring in the early hours of the morning. In other words there
is a heat variation during the day. Based on these facts, it is
reasonable to assume that if cosmic rays came from the sun, they
too would have some daily variation in intensity. For example the
intensity of cosmic rays received by the Earth would be large
around lunchtime, and much less in the middle of the night.
However no such variation is observed. Indeed the cosmic rays
received by the Earth is rather constant at all times during the
day.
5. WHERE DO COSMIC RAYS COME FROM?
The second reason why cosmic rays cannot come from the
sun, is that the sun is not able to give out particles with the
high levels of energy which cosmic rays are known to have.
Evidence also exists to suggest that cosmic rays cannot
exist outside of our galaxy. In order to understand why this
is so it is first necessary to consider what exactly our galaxy
looks like.
6. WHAT EXACTLY OUR GALAXY LOOKS LIKE
The galaxy can be modelled as a 'fried egg':
7. WHERE DO COSMIC RAYS COME FROM?
The galaxy has a large number of stars at it's centre and fewer further
out. In all our galaxy contains of the order 100,000 million stars. The
Earth is found towards the edge of the galaxy. The galaxy is of a spiral
shape and is rotating at 290km/s which is equivalent to 640,000mph.
This means that all the stars and planets within the galaxy are also
rotating. Therefore the Earth is rotating and moving through space,
relative to neighbouring galaxies, with a speed of 640,000mph.
Let us briefly stop for a moment and consider how fast 640,000mph is.
To get a rough idea, the maximum speed a car can legally travel in
Britain is 70mph, 9143 times slower than the speed with which the Earth
is moving through space. When Linford Christie broke the 100m world
record a few years ago, he ran at a speed of 0.01km/s. Faster still is the
speed at which chemicals from a chemical explosion are ejected. This
happens at the order of 7.5km/s. Even so this speed is still almost 40
times slower than the Earth is moving through space.
8. WHERE DO COSMIC RAYS COME FROM?
Returning to the issue of cosmic rays coming from outside
the galaxy, then the fact that the Earth is moving through
space implies that the intensity of the cosmic rays coming
from outside of space will be greater on the side of the Earth
facing the direction from which the cosmic rays came.
9. COSMIC RAYS FROM OUTSIDE OF OUR GALAXY?
To make this clearer,
consider the diagram seen in
figure 2. If cosmic rays come
from outside of the galaxy,
say travelling (as shown)
from right to left, then the
number of cosmic rays
received on face A must be
greater than the number
received on face B, (as the
purple cosmic ray intensity
shadow shows)
10. WHERE DO COSMIC RAYS COME FROM?
Furthermore if cosmic rays come from outside of the galaxy
the spatial position of cosmic ray intensity would change at
different times during the day, due to the fact that the Earth
is spinning on it's own axis. No such variation is observed.
Indeed the cosmic ray intensity on all points on the Earth's
surface are roughly the same, at all times during the day.
Therefore it is thought cosmic rays have no spatial or time
variation.
We conclude from these observations that the majority of
cosmic rays must originate within our galaxy. The next
question which follows is: where from within our galaxy do
cosmic rays come from?
11. WHERE DO COSMIC RAYS COME FROM?
Some cosmic rays are formed from stars, but most
come from Supernova explosions. As keen
astronomers will know, a star evolves over about 1010
years. At the end of their lives some stars explode.
These are called Supernova explosions. About 100
supernova explosions occur during one year, they are
observed mostly by professional astronomers.
When these supernova explosions occur they release
atomic nuclei. These atomic nuclei are the source
cosmic rays and are mostly hydrogen and helium
nuclei. These source cosmic rays are known as
Primary cosmic rays.
