Astronomer Book
Solar System, our everyday units such as the mile and kilometre are
suitable; in deep space they are not. Even the nearest star beyond
the Sun is over 24 million million miles away. Trying to express
distance of this sort in miles or kilometres would be as clumsy as
giving the distance between London and New York in inches. Luckily,
nature has provided us with a much better unit. Light does not travel
instantaneously, but flashes along at the staggering rate of 186
thousand miles per second.
This means that in one year light can travel almost 6 million million
miles, and this distance we call a light year – which, please note, is a
measure of distance, and not of time.
Most of the stars you see at night with the naked eye are many light
years away. For example, one bright star prominent in the winter time,
named Rigel, is over 700 light years away, and more than 40 thousand
times more powerful than our Sun. At that distance it shrinks into a tiny
point. A further phenomenon here: stars appear to twinkle. This is solely
due to the effects of Earth’s atmosphere. Seen from space or the surface
of the Moon, the stars no longer twinkle.Introducing the Solar System
The Sun is at the centre of the Solar System, of which the main
bodies, apart from the Sun itself, are the eight planets. Our Earth is an
ordinary planet, third in order of distance from the Sun. Two planets,
Mercury and Venus, are closer to the Sun than we are, and the rest,
Mars, Jupiter, Saturn, Uranus and Neptune, are much further away.It may come as a shock to some to discover that our Sun, a huge
ball of incandescent gas which could contain a million bodies the
size of the Earth, is a normal star. The surface is at a temperature
of 5–6000 Celsius, and near the centre the temperature rises to
millions of degrees at immense pressure. The Sun is not "burning" in
the ordinary sense of the word. The most plentiful substance in the
universe is hydrogen, lightest of gases, and the Sun contains a great
deal of it. Near the core, where the temperatures and pressures are
so high, strange things are happening. Atoms of hydrogen (H) are
combining to form atoms of helium (He), the second-lightest gas. It
takes four atoms of H to make two atoms of He, and each time this
happens a tiny amount of mass is lost, being converted to energy.
This energy keeps the Sun shining, and the loss of mass amounts to
more than 4 million tonnes per second, so the Sun weighs far less
now than when you opened this book. Don’t panic! Enough hydrogen
is left to maintain the Sun for thousands of millions of years. We say
more about the Sun in chapter 4.
All the stars we see in the night sky are themselves "suns", many
of them much more luminous than our own Sun, but, of course, they
are a great deal further away. Here we must introduce a unit we will
use throughout this book – the light year. We know that the Sun is 93
million miles away from the Earth, and for the distances inside ourOur astronomical exploration of the Universe starts in the heart of our Solar
System with our own star, the Sun. From here we introduce the fascinating
and beautiful astronomical bodies that you can enjoy with the naked eye:
the Moon, the planets, comets and meteors, and the myriad of stars within
our own Milky Way Galaxy that are arranged into the familiar patterns of
the constellations.INTRODUCTION
12[1] An image of the Sun taken through a hydrogen-alpha filter.
[2] Detail of the planet Mars captured with a 14-inch amateur telescope.