By G. O. Okeng'o
Ever since the origins of mankind the questions as to
“How big?” and “How far?” things are in the universe have
always attracted much attention and spun curiosity down the spines of
the human race. On a beautiful clear night, one is likely to see a
myriad of stars that seem innocently close and so reachable that
he/she may feel the temptation of reaching out to them or even
touching them! This then begs answers to the questions: how far are
those stars? What lies beyond/behind those stars? What is their
composition and what mechanism powers them? How large is the universe
as a whole? What is the universe composed of and how do we humans fit
in into the whole picture of the cosmos? Well, if you find these and
many related questions interesting then you are not alone! You are
only doing what the greek philosophers did several years ago that
gave birth to the well-known `scientific process or method', which
was later to be extended by great scientists amongst them; Nicolaus
Copernicus (the father of the model that put the sun at the center of
our solar system), Tycho Brahe (the first observational astronomer
to obtain the most accurate data of all time), Johannes Kepler (a
student of Tycho Brahe who used his data to propose how planets
move), Galileo Galileo (the first man point a telescope-a light
gathering and focusing machine- to the sky), Isaac Newton (the genius
who formulated laws of gravitation and invented calculus a branch of
mathematics that describes motion), Albert Einstein (the famous
clerk-cum-scientist who invented the the most well tested theory of
physics to date- general relativity) and Stephen Hawking (the
greatest theoretical physicist of our time after Newton), just to
name a few! It was on the shoulders of these and many other giants
that the laws of science sprung, and with them followed the
technology that we all enjoy today. But how do astronomers do their
thing?
Sir
Arthur Eddington, a famous british astrophysicist who lived between
1882-1944 (may God rest his soul!), once used an interesting analogy.
He imagined of a large ship sailing across the ocean carrying sacks
full of potatoes and a potato bug inside one of the potatoes trying
to understand the nature of the ocean in which the ship was moving.
He then likened the activities of the potato bug to scientists who
study the universe (astronomers and cosmologists). Whereas he might
have been reasonably spot on in terms of sizes, he was ultimately
wrong in the spirit of his comparison because as we know today, the
so-called 'potato bugs', have gathered lots of information about the
universe and our knowledge about the workings and functioning of the
universe continues to grow by the day, thanks to better technology
and cutting-edge study techniques!. Astronomers therefore, have a
simple
mission; to
understand the physical laws that govern the universe and their main
tool is
a combination of physics, chemistry, computer science and
mathematics.
The
question of size and location of things in the universe can be well
illustrated by classifying cosmic structures in terms of hierarchy;
from the smallest scales (our solar system), to the very largest
scales (superclusters of galaxies) and then assembling a form of
cosmic
distance ladder.
The universe is a big, big place and as a word of caution, in order
to be a good student of the universe you will need to get accustomed
to a new system of units that can get really large and sometimes
mind-boggling compared to what you're used to! This is the origin of
the term 'astronomical' that you may have probably heard some
politicians and ordinary folks use.
Let
us begin our feel for size with the planet Earth our `sweet home'
which is about 6,400 km in size. A small jetliner would cover this
distance in 40 hours. The Earth is the third planet from the Sun
after Mercury and Venus in our solar system. It is orbited by the
moon,
the closest cosmic object to the Earth at a distance of about 400,000
kilometers (km). It took Apollo 11, the first spacecraft to land
humans on the moon, (although some critics dispute this!)
approximately 4 days to cover this distance. The Earth moves around
the Sun at a distance of about 150 million km. Apollo 11 could have
taken 5 years to travel this distance to the Sun. The farthest object
in our solar system is the dwarf planet Pluto which sits at a
distance of about 6,000 million km, approximately 60 times the
distance between the Earth and the Sun.
At
this point, the km becomes a small unit to measure distances and
astronomers graduate to a larger unit of distance called the light
year,
defined as the distance that light travels in one year at it's known
average speed of about 300,000 km per second, and it's equal to
about 10 million million kilometers; the number 10 followed by 12
'zeros' or simply written as 1013
km
in scientific form. However, the Sun is just one star in about
100,000 million stars that light up our galaxy, the
Milky Way.
All the stars observed in the night sky belong to the Milky Way
galaxy and studies by astronomers indicate that they show properties
similar to our Sun. The nearest star to us after the Sun is called
Proxima
Centauri and
is at a distance of about 4 light years. It would have taken Apollo
11 about 1 million years to fly to Proxima Centauri! (would this have
been possible?). Examples of other stars visible on the sky are; the
brightest star in the night sky called Sirius
at
a distance of about 8 light years and the Pole
star located
at the North Pole and whose distance is about 700 light years. This
would translate to about 2 million and 18 million respectively, were
the Apollo mission to visit the two stars. The Milky Way galaxy is
only but a medium-sized spiral galaxy about 45,000 light years
across. At this point, the light year also becomes a small unit to
measure distances on scales of galaxies. An even larger unit called
the
kiloparsec (kpc)
equivalent to about 3,000 light years is introduced. This is the
standard unit for measuring galactic distances which puts the size of
our galaxy at about 15 kpc.
But
the Milky Way galaxy belongs to a group of about 30 galaxies, some of
which are considerably small in size called 'dwarf galaxies' and
contain more than one million stars. This cluster of galaxies is
called the 'Local Group' and the closest neighbour to our galaxy to
is the 'Andromeda galaxy' in the constellation of Andromeda, at a
distance of about 700 kpc. Andromeda has a size similar to the Milky
Way and also contains about 100,000 million (1011)
stars. The Local Group cluster has a size of about 1000 kpc, called a
Megaparsec
(Mpc).
Observations using powerful telescopes show that
galaxies are very social 'beings', preferring to assemble in groups
rather than existing as isolated systems and that there are over 100
million galaxies similar to our galaxy in the universe. Apart from
the Local Group, another cluster known as the 'Coma cluster' with a
membership of about 1000 galaxies has also been discovered among
others. However, further observations also point to clusters of
galaxies existing in groups to form even larger clusters called
'Superclusters' and our cluster, the Local Group is thought to belong
to the 'Virgo Supercluster' which is about 30-60 Mpc in size. This
then leads to our initial question; how big is the universe? Assuming
that the size of our universe has a scale similar to that of
superclusters, we can put the lower limit of the 'observable'
universe to be about 6,000 Mpc or 6,000,000 kpc or 18,000,000,000
light years or approximately 18,000,000,000,000,000,000,000
kilometers! And you can clearly see that the universe is indeed very
BIG!
References
- Roger A. Freedman and William J. Kaufmann, "Universe", W. H. Freeman, 8th Edition.
- www.wikipedia.org
- T. Padmanabhan, "After the first three minutes: The story of our universe" Cambdidge University Press (1998)
