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a guide to THE STARS
A STAR CAN HAVE TWO TYPES OF LIFE CYCLE...
RED GIANT
WHAT IS A STAR?
A star is basically a globe of hydrogen gas which shines because of nuclear reactions at its heart.
These consume hydrogen, converting it to helium and releasing energy in the process.
These reactions don’t occur in hydrogen gas in normal conditions on Earth as very high pressures
and temperatures are needed to start them. The controlling factor in the the life of a
star is its mass. Low mass stars, a tenth or so of the mass of the Sun, are very dim and red and last
for billions of years. Those of high mass (which could be up to about 100 times that of the Sun )
shine brilliantly with a strong bluish light, but last only a few million years.
Stars are categorised based on their temperature.
The sequence starts with type 0 stars, which are the
most massive, hottest and bluest, and runs through
type B and type A, then F and G (yellowish), K (orange),
and M (orange-red). Within these classifications there
are also giants and dwarfs, but these terms refer
merely to the star’s diameter compared with
others of its type, not to its actual mass.
WHITE
DWARF
SOLAR STAR
(EG-THE SUN)
Only about 6 per cent of the stars in the sky are as
bright as the Sun or brighter, yet these are the ones
that we see and, as a result, the ones that
we consider typical. About 70 per cent of the stars in the Galaxy are red dwarfs,
like Proxima Centauri, the nearest star of all, which can only be seen with a telescope.
2009 SPACE ODYSSEY
PLANETARY
NEBULA
STELLAR
NEBULA
MASSIVE
STAR
THE LIFE AND DEATH OF OUR SUN
The Sun, like all stars, is powered by hydrogen.
In about 5 billion years, when its fuel
runs low, it will expand to become a red giant.
It will then have a cooler outside temperature
with a bright core. Eventually the outer layers
are lost. When the Sun can no longer support itself
through nuclear energy, a planetary nebula is left
– a gas shell surrounding the dying star. When the gas
blows away, the core of the old star is left as a white
dwarf, an enormously dense star (around the size of
the Earth but with the mass of the Sun).
Over time the white dwarf will cool
down to leave a cold, dark object.
RED
SUPERGIANT
NEUTRON
STAR
SUPERNOVA
LIGHT YEARS AWAY...
BLACK
HOLE
Even if you could travel at the speed of light, it would
still take you 13.7 billion years to reach the edge of the known
universe. A light year is a measure of distance, not time.
It is the total distance that a beam of light, moving
in a straight line, travels in one year. As light travels
at 186,000 miles per second, a light year is a long way!
...Nearly 6 trillion miles. Knowing the distance that
light travels in one year means we can calculate the
distance between objects in the universe. Proxima
Centauri, the closest star to us, is 4.37 light years
away - around 24 trillion miles.
BLACK HOLES
A black hole is a region of space in
which gravity is so powerful not even
visible light can escape.
The term comes from the fact that
the absorption of light renders the
black hole’s interior invisible and
indistinguishable from the black
space around it.
Despite the interior being invisible,
a black hole may be detected through
interaction with matter that lies in orbit
outside an area called the event horizon,
for example, tracking stars that orbit
around its centre.
GALAXIES
THE HUBBLE TELESCOPE
STAR CLUSTERS
A galaxy is a huge assembly of stars, dust and
gas, an example of which is our own Milky Way
Galaxy. There are three main types, Elliptical
galaxies are round or elliptical systems,
showing a gradual decrease in brightness
from the centre outwards. Spiral galaxies
are flattened, disk-shaped systems in which
young stars, dust and gas are concentrated
in spiral arms coiling out from a central
bulge, the nucleus. Barred spiral galaxies
are distinguished by a bright central bar
from which the spiral arms emerge.
The Hubble Space Telescope was launched in 1990.
Because it orbits outside the Earth’s atmosphere, It can see
much further than ground based telescopes and has helped
determine the age of the universe, the identity
of quasars and the existence of dark energy.
Hubble has sent back some stunning
images. For example, the Sombrero
galaxy. 50,000 light-years
across and 28 million
light-years from Earth.
There are about 800
billion suns in this image.
Star clusters may contain anything
from several dozen to several
hundred stars. Globular clusters
are symmetrical systems containing
up to a million very old stars in
the halo of our Galaxy.
They are typically 50 to 150 light
years across and at least 20,000
light years away. The best-known
open cluster is the Pleiades, or
Seven Sisters, which is visible
to the naked eye.
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Stone Henge
Early cultures identified celestial objects with gods and spirits.
They related these objects and their movements to phenomena such as rain,
drought, seasons, and tides. It is generally believed that the first astronomers were
priests (such as the Magi), and that their understanding of the heavens was seen
as divine, hence astronomy's ancient connection to what is now called astrology.
Ancient structures with possibly astronomical alignments (such as Stonehenge)
probably fulfilled both astronomical and religious functions.
COPERNICUS
From the time of the ancient Greeks it was believed that the Sun and all the stars
and planets revolved around the earth. To suggest otherwise was against the
teachings of the church and could result in a charge of heresy. Born 1473, Nicolaus
Copernicus was a Polish astronomer and mathematician who was convinced that
the Earth and the planets were in orbit around the Sun. Wisely as it turned out,
he waited until near the end of his life to publish his findings. In 1600 one of his
followers, Giordano Bruno, was burnt at the stake. In 1601 a young German, Johannes
Kepler demonstrated that the planets do not move in perfect circles around the
Sun, but moved in elliptical orbits. His laws of planetary motion provided
the basis of all subsequent discoveries.
THE TELESCOPE
In 1608, a Flemish spectacle-maker, Hans Lipperhey applied for a
patent for the telescope (which was refused) but news of his
invention spread across Europe and the Italian astronomer Galileo
Galilei worked out the principles and produced his own
eight-power telescope. He gradually improved the power of his
telescope, grinding lenses himself, and began observing the heavens.
Although not even as powerful as a cheap pair of modern
binoculars, he was able to see the satellites of Jupiter, and the
phases of Venus. His observations proved that the Earth could
Copernicus
not possibly be at the centre of the universe. Galileo was not as
tactful as Copernicus, and was brought to trial in Rome and
condemned as a heretic. He managed to escape the stake by declaring his own
discoveries to be false. He was finally pardoned by the Vatican in 1991.
Sir Isaac Newton
According to the well-known story, it was on seeing an apple fall in his orchard at
some time during 1665 or 1666 that Newton conceived that the same force governed
the motion of the Moon and the apple. He calculated the force needed to hold the
Moon in its orbit, as compared with the force pulling an object to the ground.
He also calculated the relation between the length of a pendulum and the time of
its swing. Newton identified gravitation as the fundamental force controlling the
motions of all celestial bodies. Although not really an astronomer, Newton’s
influence on the study of the universe continues to this day.
He was buried in Westminster Abbey in 1727.
EDMUND HALLEY
Halley was born on 8 November 1656 in London. Along with Robert Hooke,
Sir Christopher Wren and Sir Isaac Newton, Halley was trying to develop a
mechanical explanation for planetary motion. Although progress had been made,
Hooke and Halley were not able to deduce a theoretical orbit that would match the
observed planetary motions. However, Newton was already there: the orbit would
be an ellipse. In 1705 Halley published 'A Synopsis of the Astronomy of Comets', in
which he described the parabolic orbits of 24 comets that had been observed from
1337 to 1698. He showed that the three historic
comets of 1531, 1607, and 1682 were so similar in
characteristics that they must have been successive
returns of the same object - now known as Halley's
Comet - and accurately predicted its return in 1758.
He never lived to see his prediction come true.
In 1720 Halley became astronomer royal at
Greenwich, a position which he held until his
death on 14 January 1742. The spacecraft Giotto
intercepted and photographed the icy nucleus of
Halley’s Comet in 1986. It will next be visible
from Earth in July 2061.
2009 SPACE ODYSSEY
Astronomy is the oldest of the natural sciences, dating back to
antiquity, with its origins in the religious, mythological, and
astrological practices of pre-history. Ancient astronomers were able
to differentiate between stars and planets, as stars remain relatively
fixed over the centuries while planets will move an appreciable
amount during a comparatively short time.
SIR ISAAC NEWTON
Born in 1643, the same year Galileo died, Newton was to
become regarded as the greatest scientist of all time.
He discovered that light from the Sun is composed of
waves of different colours of light which he demonstrated
by passing light through a glass prism and producing the
first spectrum. He also produced the first reflector
telescope, which used a concave mirror to gather light.
This gave a much better image because a mirror reflects
all colours of light equally. Modern reflector telescopes
are vastly larger than Newton’s original 1 inch version
but follow exactly the same principle.
Today’s free poster
GIANTS OF ASTRONOMY
THE STARS
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Halley’s Comet
THE MILKY WAY
The Milky Way is a member of a collection of more
than 50 galaxies called the Local Group. If you were
to approach the Milky Way edge-on, the first thing
you would notice is a luminous halo made up of gas
and stars enveloping the galaxy. The halo is about
100,000 light-years in diameter and 1,000 light-years
thick. A light-year is about 6 trillion miles.
This halo contains some 170 orbiting star clusters
and about a dozen small galaxies. Astronomers
Solar System
estimate that the Milky Way contains about 100
billion stars. Recently, however, this number was
upped by about a billion after the discovery that
very old, nearly invisible stars had escaped earlier detections.
The Milky Way is believed to contain four major spiral arms, all of which start at the
galaxy's centre, plus a number of smaller arms. Our Sun is located on a spur of the
Orion Arm. Most of the Milky Way's stars are concentrated in a main disk, which
lately has been described as a series of disks, none of which are entirely distinct, but
instead overlap one another. The largest is known as the thick disk; this disk is fairly
flat and spirals like a slow-spinning hurricane because of our galaxy's rotation.
Nestled within the thick disk is an even flatter disk of stars, known as the thin disk.
The stars in this thin disk rotate even faster around the galactic center than those in
the thick disk. Further in is yet another disk, known as the extreme disk,
where stars and clouds of gas are moving fastest of all.
Our Sun is located 26,000 light-years away from the galactic centre on one of the
spiral arms. It is a location considered more suitable than others for harboring
life, in part because the central region is too chaotic, and in part because the
concentration of metals there is too heavy, and it's too light in the galaxy's outer
fringes. The Sun makes one complete orbit around the galaxy about once every
225 million years. In contrast, stars near the galactic centre complete a lap in a
few million years or less. These stars as a group tend to be younger than the
galactic average, most ranging in age from 1 billion to 10 billion years old.
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THE GALACTIC BAR
As you near the centre of the
Milky Way you will feel a greater
pull of gravity as you approach
the densest and brightest part
of our galaxy, a spherical region
known as the central bulge.
Things are very different here.
Most of our galaxy is relatively
uncrowded-the nearest star to
our Sun, for example, is 4.2
light-years away. But roughly
10 million stars are known to
orbit within a light-year of
the galaxy's centre.
2009 SPACE ODYSSEY
Today’s free poster
THE STARS
Recent infrared surveys with
NASA's Spitzer space telescope
confirmed that the Milky Way
is not a perfect spiral galaxy but
instead sports a long bar of
stars within the central bulge.
This galactic bar is believed to
be made up of about 30 million
stars, stretching 27,000
light-years from end to end.
It consists mainly of old, red
stars, which is one reason it
stands out and can be detected.
The galactic bar is thought to spin like a propeller inside the Milky Way centre,
helping to create our galaxy's unique spiral shape. Observations of other galaxies
also suggest that galactic bars play an important role in feeding the colossal black
holes believed to lie at the heart of many galaxies, including our own.
OUR LOCAL BLACK HOLE
The Milky Way's suspected black hole is called ‘Sagittarius A’ and is thought to have
between 3.2 and 4 million times the mass of our Sun. It is thought that all of this
mass is confined, amazingly, to an area approximately 10 times smaller than Earth's
orbit around the Sun. ‘Sagittarius A’ is also probably rotating, making one full
revolution about every 11 minutes. We can tell this from the incredible speeds of the
stars around it, which move 50 times faster than Earth orbits the Sun. The gravity
required to keep these stars in such a fast, tight orbit is calculable, and the tiny
area into which it must fit indicates that it has to be a black hole.
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