Before we take a quick look at some of the rest of the Universe, many people seem to be confused about the relative distance of Astronomical Phenomena. As a guide, the following table is presented:
| Moon | Earth Orbit | 300,000 miles |
| Sun, Mars, Mercury, Venus, | Inner Solar System | 10's of Millions of miles |
| Asteroid Belt | 200 - 300 Million Miles | |
| Jupiter, Saturn, Uranus, Neptune, Pluto | Outer Solar System | 400 - 4,500 million miles |
| Comets | Mainly Outer Solar System or Outside orbit of Pluto | 4,000 million miles + |
| Oort Cloud | Edge of Solar System | Approx. 1 light year |
| Nearest Star | Nearby in Galaxy | 4.2 Light Years (about 20,000 billion miles) |
| Magellanic Clouds | Nearest Galaxy | Approx. 200,000 light years |
| Andromeda Galaxy | Local Group of Galaxies | 2 million light years |
| Whirlpool Galaxy | 37 million light years | |
| Quasar | Distant Universe | 10-13 billion light years |
These are not strictly beyond our Solar System, but in some cases can exist there.
Well outside the orbit of Pluto lies the Oort cloud,
a sphere of dust and rocks, surrounding the solar system, presumably
left over from when the Solar System formed. It is thought that
this is the primary birthplace of comets.
A comet is a "dirty snowball" - a collection
of dust, grit and water ice - frozen by the cold temperatures
in the outer solar system. Perhaps the most famous comet is Halley's
comet (not Hayleigh's Comet! - Ha-lee's comet, or maybe even Hall-ee's
comet). This comet returned to our skies in 1986 and the European
Giotto probe successfully performed close up analysis of the nucleus
(See this page
for more details.). To earth-based observers though, the 1986
apparition wasn't particularly spectacular. It was the mathematician
Edmund Halley who first noticed the 76 year periodicity in the
appearance of a comet and accurately predicted the return of a
comet in the year 1785 - 43 years after his death. In March 1995,
Comet Hyakutake was brighter
than Halley in 1986, being well visible in the Northern Hemisphere.
In 1994, Shoemaker Levy got rather too close to Jupiter and was destroyed, colliding with Jupiter after fragmentation (also see this page for more details)
Comets have highly elliptic orbits, and spend most
of their time in the outer solar system, frozen and inactive.
A single orbit may take several thousand years. As a comet approaches
the Sun, it speeds up and it is heated up.
The surface layer of ice melts and the water formed
immediately evaporates into space (there being little gravity
to stop it). This forms a cloud of "steam" and dust
around the rocky nucleus. The Solar Wind (a stream of charged
particles emanating from the Sun) "blows" the cloud
of steam into a tail - which can be hundreds of thousands of miles
long - it is this that makes the comet so spectacular.
In 1997, Comet Hale-Bopp has been even more spectacular
than Hyakutake - being one of the great comets of this century,
only outshone by the great "Daylight Comet" of 1910.
Here are some pictures of comet Hale Bopp
which I took myself. For rather better pictures (!) visit the
appropriate NASA pages.
(You will find thousands of other places on the Web too).
A black hole is basically a star which has collapsed in on itself. The existence of such an object has been posited for many years, as even the "simple" theory of Newtonian Gravitation (as well as the more complete theory of relativity) implies the existence of an object which exerts immense gravitational force - so strong that even light cannot escape its influence - that is what causes a black hole to be black - it is so dense that its gravity is strong enough to stop light escaping from its surface.
Black Holes can not be seen directly, but there are
a number of observed phenomena which would seem to indicate that
they really do exist. One candidate, known as Cygnus X-1, is in
the constellation of Cygnus the Swan (sometimes called the Northern
Cross) and is seen by equipment which detects X-rays. It is also
thought that large black holes may be present at the centre of
many galaxies and therefore may strongly affect their structure.
Many theories about the nature of black holes have been postulated,
and some have been explored in the Science Fiction Genre. A more
detailed discussion can be found here
and some more here
.
Galaxies are huge collections of stars - there may
be as many as 100,000,000,000 (100 billion!) Suns in our own galaxy.
Over the last 100 years, it has been discovered that what many
night sky objects which were once thought to be "smokey"
or "cloudy" stars are in fact collections of thousands
of millions of them - galaxies. With modern equipment and ever
more ingenious methods of study, it is believed that there may
be as 100 billion other galaxies. They seem to come in many different
shapes and sizes. How they originally formed is still something
of a mystery as they represent the first level of "order"
in the Universe which came about after the big bang, some 10-15
billion years ago.
Novae are stars which flare up in brightness from time to time - maybe changing their brightness by a factor of 10,000 in only a few days. One type of Novae is produced when a small, dense star which is not very brilliant, but has a strong gravitational field, attracts material from a nearby star (typically a Binary Companion). The material is drawn to the small dense star and is heated by the intense gravity of the smaller star. The material flares in brightness caused a rapid increase in brilliance of the small star.
A supernova is a star which all but "self destructs" - the star has "burnt" all its fuel and gravity takes over, causing the stars core to collapse in an instant. The energy is then released in a cataclysmic explosion which blows the star to pieces - leaving only a small, dense core which may be a Pulsar, Neutron Star all a Black Hole
Are remnants of a supernova explosion - perhaps only
10 or 20 miles in diameter. A star which has exploded is
usually rotating before the explosion happens. There is a law
in Physics called "Conservation of Angular Momentum"
and this applies to rotating bodies. An example of this is seen
when a skater is spinning with their arms outstretched. As they
draw their arms in, their speed of rotation increases - if they
concentrate their mass around their "axis of rotation",
they spin faster. This is exactly what happens after a supernova
explosion. A good proportion of the stars rotational energy is
conserved (some is lost in the explosion) and so because there
is less matter to rotate, it rotates faster - much faster. Some
Pulsars appear to spin round 30 times per second. They are characterised
by a "flash" of radio or light energy which is picked
up as a "blip" by a radio telescope. When Pulsars were
discovered in 1967, the regular pulse they produced was, for a
short time, thought to be articficial. There is a pulsar at the
centre of the Crab Nebula which rotates at a rate of 30 times
/ second.
A Pulsar is a Neutron star - a neutron star is thought
to be made up of Neutron Matter - in the gravitational collapse
of the star, the electrons and protons are "squashed"
together in the most immense of crushing forces. It is thought
the merge to form Neutrons - neutron matter which is so dense
that a sphere about 0.5 mm in diameter would weigh about 97,000
tons!
Are not stars at all - though they were once thought
to be. They are "quasi-stellar" objects. They appear
at first glance to be stars, but they have such high red-shifts
that, according to the current Big Bang / expanding universe theory,
they are the remotest type of object detectable - some 10-13,000
million light years distant. They are though to be extremely old,
active galaxies which is what makes them so brilliant. It is thoguht
that Seyfert Galaxies and BL Lacertae objects may be different
stages of the same type of object as a Quasar - very old (to us)
object - as we view it - but in terms of the age of the universe,
very young object which has a prodigious energy output, possibly
created by a massive black hole in the centre.
This is the theory of how the universe began, though it is now accepted by most people as being correct, for most of history, our ideas have been different. Read more in Ptolemy, Hubble and the Shape of the Universe.