Globular Clusters

The Globular Star Cluster Fornax5

Credit: NASA, ESA,
S. Larsen (Radboud University, the Netherlands)A globular cluster is a
spherical collection of many tens of thousands of stars that are tightly bound (tied)
together by gravity.
The stars are all orbiting around the middle of the cluster, and often remind people
of bees swarming around their hive.

They contain many more stars and are much older thanopen clusters, and are
considered the pensioners of the universe as it seems they contain some of the first
stars to be produced.
Globular clusters are found in a large sphere (called the halo)
around spiral galaxies, but are located well away from the spiral arms of the
galaxies they surround. They are also found in the outer reaches of
large elliptical galaxies.
Our own galaxy, the Milky Way is known to have around 150 globular clusters.
Open Clusters

Pleiades Star Cluster

Credit: HST/NASA
An open cluster is a group of up to a few thousand stars of about the same age that
were formed from the same cloud of gas and dust, and are still loosely bound (or
tied) to each other by gravity.
In contrast, globular clusters are very tightly bound (tied together) by gravity.
Open clusters are found only in spiral galaxies and irregular galaxies, in which star
formation is taking place.
The clusters are usually less than a few hundred million years old (so very young as
far as the universe is concerned) and are slowly drifting apart. Young open clusters
may still be contained within the gas cloud from which they formed; radiation from
the stars can illuminate it to create a glowing nebula such as the Horsehead Nebula
in the constellation Orion.
Star Formation

Star Forming Region NGC 3582

Credit: T.A. Rector/University of Alaska Anchorage,

T. Abbott and NOAO/AURA/NSF
Stars are formed, or are "born", in large clouds of gas and dust. The cloud slowly
shrinks and then starts to collapse onto a number of points (or cores) within the
cloud, all due to the pull of gravity.
Right in the middle of these cores, it can get very hot and dense. When this
happens, nuclear fusion can start and a star is born. This is called stellar ignition.
The sudden burst of light made by the new star blows away much of the nearby gas
cloud, but it can leave just enough material behind to form a number of planets later
on.

You can see what is happening in a bit more detail in this simulation.
At this point the star becomes relatively stable, with the outward pressure from
nuclear reactions balancing the inward pull of gravity.

A typical star like the Sun will live for around 10 billion years, until it eventually
runs out of fuel. All stars go through a life cycle in the same way that we do - they
just live longer. When they eventually run out of fuel, they will end their days in
spectacular fashion.
Life Cycle of a Star

Stars are formed in clouds of gas and dust, known as nebulae. Nuclear reactions at
the centre (or core) of stars provides enough energy to make them shine brightly for
many years. The exact lifetime of a star depends very much on its size. Very large,
massive stars burn their fuel much faster than smaller stars and may only last a few
hundred thousand years. Smaller stars, however, will last for several billion years,
because they burn their fuel much more slowly.

Life Cycle of a Star

Credit: NASA
Eventually, however, the hydrogen fuel that powers the nuclear reactions within
stars will begin to run out, and they will enter the final phases of their lifetime. Over
time, they will expand, cool and change colour to become red giants. The path they
follow beyond that depends on the mass of the star.
Small stars, like the Sun, will undergo a relatively peaceful and beautiful death that
sees them pass through a planetary nebula phase to become a white dwarf, this
eventually cools down over time leaving a brown dwarf. Massive stars, on the other
hand, will experience a most energetic and violent end, which will see their remains
scattered about the cosmos in a enormous explosion, called a supernova. Once the
dust clears, the only thing remaining will be a very dense star known as a neutron
star, these can often be rapidly spinning and are known as pulsars. If the star which
explodes is especially large, it can even form a black hole.
Red Giant

Drawing of a Red Giant

Credit: ESA/JAXA

When hydrogen fuel at the centre of a star is exhausted, nuclear reactions will
start move outwards into its atmosphere and burn the hydrogen that’s in a shell
surrounding the core. As a result, the outside of the star starts to expand and cool,
turning much redder. Over time, the star will change into a red giant and grow to
more than 400 times its original size.
As they expand, red giants engulf some of their close-orbiting planets. In
the Sun's case, this will mean the fiery end of all the inner planets of our Solar
System, which might also include the Earth; but don't worry, this won't happen for
another 5,000,000,000 years.
While the atmosphere of the star grows, its core shrinks due to gravity.
Temperatures and pressures in the middle increase until the conditions are right
for nuclear fusion to start again, but this time using helium as a fuel, rather than
hydrogen.
With the star being powered by helium, its outer layers return to normal for a while
and it starts to shrink, get hotter and turn a little more blue. However, this stage
only lasts for a million years or so, as the helium quickly runs out. When it does, the
core shrinks again and this time the helium starts burn in a shell around the core
and hydrogen may start burning in a shell around that! The outer layers of the star
starts to grow, cool and turn red again as it enters its second red giant phase.
What happens next depends on the mass of the star. Small sun-like stars move into
a planetary nebula phase, whilst stars greater than about 8 times the mass of the
Sun are likely to end their days as a supernova.

Planetary Nebula

Helix Nebula Credit: HST/NASA

A planetary nebula is an expanding, glowing shell of hot gas (plasma) that is cast
off towards the end of a low-mass star's life. Despite the name, they have nothing to
do with planets, and were so named because early astronomers thought they
looked a bit like planets through a small telescope.
Low-mass stars turn into planetary nebulae towards the end of their red
giant phase. At that point the star becomes highly unstable and starts to pulsate.
The outer layers are ejected by the resulting stellar winds. Planetary nebula are
relatively short-lived, and last just a few tens of thousands of years.
As the outer layers drift away from the star, the remaining core shines brightly and
is very hot (100,000°C+) - the core is now a white dwarf star. The ultraviolet
radiation pumped out by the white dwarf causes the ejected outer layers to glow -
the planetary nebula.
Over time, the enriched material from the planetary nebula is scattered into space
and will be used for future generations of stars.
White Dwarf

White dwarf in comparison to Earth

Credit: NASA, S. Charbinet

A white dwarf is the remaining compact core of a low-mass star that has come to
the end of its lifetime following a planetary nebula event. They are thought to
make up roughly 6% of all known stars in the Sun's neighbourhood.
White dwarfs are made of highly compressed carbon and oxygen material, and are
so dense that their mass is comparable to that of the Sun, even though their size is
similar to that of the Earth's. A matchbox of white dwarf material would weigh the
same as fifteen elephants.
Newly created white dwarfs have some of the hottest surface temperatures of any
star, at over 100,000°C, but because of their small size, they appear quite faint from
a distance.

As nuclear reactions no longer occur in white dwarfs, they have to rely on their
thermal store of energy for all heat and light. Over time this will gradually radiate
away, allowing them to cool down and change colour. Eventually, they will
disappear from sight to become cold black dwarfs.
Brown Dwarf

An artist's impression of a Brown Dwarf

Credit: NASA/JPL-Caltech
Brown dwarfs are very dim, glowing objects that never gained enough mass to
become fully fledged stars. In other words, the temperatures and pressures at their
centres never got high enough for nuclear fusion to start.
Brown dwarfs occupy the mass range between that of large gas giant planets and
the lowest mass stars, i.e. somewhere between 12 and 75 times the mass of the
planet Jupiter. Whilst most have been found orbiting stars, some have been found
roaming alone in the cosmos.
These sub-stellar objects will have formed in the same way as stars, but glow a cool
brownish colour due to thermal energy (or heat) created by the collisions of
material within them.
Supernova

Supernova near a distant galaxy

Credit: NASA, ESA, The Hubble Key Project Team,
and The High-Z Supernova Search Team

A supernova is the spectacular explosion of a high-mass star that has come to the
end of its life. For a brief time, a supernova can shine as brightly as an entire galaxy,
but will fade again over a matter of days.
The explosion occurs when a high-mass star finally runs out of nuclear fuel.
Without any outward pressure to balance the inward force of gravity, the outer
layers of the star collapse onto the core, and are then dramatically expelled in a
nuclear explosion, at a velocity of up to 30,000 km/s. The resulting shock wave
creates an expanding shell of gas and dust called a supernova remnant.
What remains of the star's core becomes a neutron star or a black hole if it is a
very massive star (greater than 40 times the mass of our Sun).
After many millions of years, the material in the supernova remnant will be
scattered into nearby gas clouds and may eventually be used in the birth of a new
star. The majority of elements in the universe were created by nuclear
reactions at the centre of stars. It is safe to say that we are all made from the
stardust of long-dead stars.
Neutron Stars

Artist's impression of a neutron star

Credit: Casey Reed/Penn State University

A neutron star is the incredibly compact core that remains after

a supernova event.
When a high-mass star comes to the end of its lifetime, its outer layers collapse
onto the core, compressing material to the point where the atoms are smashed
apart, leaving only neutrons - sub-atomic particles with no electric charge.
The outer layers are then ejected in a super-massive explosion, leaving a rapidly
spinning neutron star behind. Some neutron stars have been found to rotate at
several hundred times a second.

A neutron star can weigh the same as one or two Suns and yet will only be about 20
km across. For comparison, a matchbox of neutron star material would weigh the
same as the country of Wales. A house of it would weigh the same as the Moon.
Pulsar

Artist's impression of a Pulsar

Credit: Science@NASA
A pulsar is a rapidly spinning neutron star that emits repeating pulses of energy
towards Earth. They appear following a supernova, when the highly compressed
core of a recently exploded, massive star is left spinning rapidly and with a very
strong magnetic field.
Pulsars emit beams of electromagnetic radiation (light) that can only be detected
if they are pointing towards Earth. Due to the rotation of the neutron star, the beams
sweep past the Earth at regular intervals, or in pulses, hence the name. It's a bit like
seeing the flashes of light from a lighthouse.
The time separation between the pulses of a pulsar range from between 0.0014
seconds to 8.5 seconds. In other words, these objects are spinning incredibly fast
when compared to the 24 hours it takes the Earth to rotate. In extreme cases,
pulsars rotate at more than 500 times per second, and remember, these objects are
around 30km across and can weigh as much as the Sun.
The first pulsar was observed in 1967 by Jocelyn Bell Burnell and Antony Hewish
but due to the regular nature of the pulses observed, they wondered if they were a
signal from an alien civilisation. This led to the first pulsar being named LGM-1 (or
Little Green Men 1). We now know this is not the case.
Black Holes

Simulation of a black hole distorting an image

of a galaxy
Credit: Alain r, Wikimedia

Black Holes are very strange objects. They are formed when very massive stars
come to the end of their lifetime, in a supernova event.
Everything that remains of the star is crushed down into an incredibly small, dense
object. Close to the object, gravity is so strong that nothingcan get away, not even
light. This means that we cannot see anything within that region - hence the name
black hole.
However, it is possible to see the effects of a black hole on the stars and material
around it. Gas, dust and other stars close to a black hole can be sucked in by gravity -
a bit like water going down a plughole. As material swirls around the black hole it
crashes into each other, producing heat and light. Because this happens away from
the black hole, the light can escape so that we can observe the activity.
 Black hole at a Galaxy centre
Credit: : ESA V. Beckmann (NASA-GSFC)
Black holes can also distort the image of galaxies they pass in front of. The gravity
of the black hole will bend the light we receive from the distant galaxy even though
it is too far away for any material to be sucked into the black hole. This is
called gravitational lensing (see the simulation on the right).
Once established, black holes can grow by consuming material, stars and even other
black holes around them. Over time, super-massive black holes can develop, and it
is thought that these lurk at the centre of galaxies.
Stars

Pleiades Star Cluster

Credit: HST/NASA
Stars are massive, luminous balls of hot gas (plasma), which are held together
by gravity.
Although they look small, they are actually large bright objects like the Sun. They
look so small because they are very far away - the nearest star, Alpha Centauri is
4.3 light years away, this is approximately 41,000,000,000,000 km away!
All the energy in a star is produced in its centre, or core, by nuclear fusion. This
energy emerges from the star as heat and light so that the star appears to glow, i.e. it
is luminous.
Stars come in lots of different sizes and colours, which can tell us a lot about what
type of star they are.
Stars exist because of a balance between gravity trying to make the star shrink and
all the heat from the middle trying to make it grow.
Stars "live" for many millions of years, but they do not change much during most of
that time. However, amazing things can happen when they are born or when they
run out of nuclear fuel and die. Explore this more in the life cycle section below.