stars

Introduction
All stars form in nebulae, huge clouds of gas and dust.
Though they shine for thousands of years, stars do not last forever. The changes that occur in
a star over time and the final stage of its life depends on a star's size.
The exact lifetime of a star depends very much on its size. Very massive stars use up their
fuel quickly. This means they may only last a few hundred thousand years.
Smaller stars use fuel more slowly and will shine for several billion years.
Eventually, the hydrogen that powers a star's nuclear reactions begins to run out. The star
then enters the final phases of its lifetime. All stars will expand, cool and change colour to
become a red giant or red supergiant.
What happens next depends on how massive the star is.
A smaller star, like the Sun, will gradually cool down and stop glowing. During these
changes, it will go through the planetary nebula phase and white dwarf phase. After many
thousands of millions of years, it will stop glowing and become a black dwarf.
A massive star experiences a much more energetic and violent end. It explodes as
a supernova. This scatters materials from inside the star across space. This material can be
collected in nebulae and form the next generation of stars.
After the dust clears, a very dense neutron star is left behind. These spin rapidly and can give
off streams of radiation, known as pulsars. If the star is especially massive, it forms a black
hole when it explodes.

Formation of stars:-
Star form in huge clouds of gas and dust called nebulae. These areas of space are sometimes
known as 'stellar nurseries' or 'star forming regions'.
The gravity of the gas and dust in the clouds pulls everything inwards. The clouds slowly
collapse onto a number of points (or cores).
Deep in the centre of these cores, there is lots of dense material squashed together, and it is
very hot. Eventually it is hot enough for nuclear fusion to start.
Nuclear fusion is the process that powers a star. This point is called stellar ignition because it
is when a star starts to shine.
Stars are not true stars until they can fuse hydrogen into helium. Before that point, they are
called protostars.
Star formation takes place in swirling clouds of gas and dust that are many times larger than a
typical solar system.
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Over time, a region within the cloud becomes denser than its surroundings. At this
point, gravity kicks in and the cloud starts to collapse in on itself. As the cloud shrinks, its
centre (or core) becomes very hot and dense. The cloud will start to spin a little as it
collapses, forming a disk.
Eventually, the star's core becomes so hot and dense that nuclear fusion begins. This is the
actual birth of the star. The light, heat and particles produced by the new star blow away most
of the remaining cloud.
The star is left alone in space to slowly use up its nuclear fuel. After several million or billion
years, it runs out of fuel and goes through the final stages of its life.

The sudden burst of light made by a new star blows away much of the nearby gas cloud.
However, it can leave enough material behind to form a number of planets.
After stellar ignition, the star becomes relatively stable. All the energy in a star is made in its
centre (its core), by a process called nuclear fusion. The star releases the energy as heat and
light. This is what makes a star shine.
The outward force generated by the fusion process is balanced by the inward pull of the
star's gravity. It is the balance between the two which stops the star from collapsing or
expanding.

In that case, a brown dwarf might form!

Brown dwarfs are very dim, glowing objects. We think the temperatures and pressures deep
within brown dwarfs were never high enough for nuclear fusion to start. In other words, they
 stars
are too big to be a planet, but not massive enough to shine like a star. We sometimes call
these failed stars 'sub-stellar objects'.
The first brown dwarf was discovered in 1995. Like other stars, they form from a cloud of
dust and gas. However, unlike the Sun and other stars, they don't release energy by nuclear
fusion. Instead, their glow comes from leftover heat.
Brown dwarfs are usually bigger than gas giant planets but smaller than the lowest-mass
stars. They have masses between 12 to 75 times the mass of the planet Jupiter.
Most brown dwarfs are in orbit around a parent star. However, some have been found alone
in the cosmos.

Stellar Classification: -
Astronomers began to categorise stars, based on their mass and temperature, hundreds of
years ago. As scientists have learned more about stars, this classification scheme has had to
evolve.
Stars are grouped into 7 main categories (also called classes), created by astronomer Annie
Jump Cannon. The classes are called O, B, A, F, G, K and M.
Stars in the 'O' class are the most massive and hottest, stars in the 'M' class are the smallest
and coolest.
If you look closely at stars in the sky, you notice they are not all the same colour. Some
appear redder, and some appear bluer. The colour of light a star gives off is controlled by its
temperature. Hotter 'O' stars glow bluer and cooler 'M' stars glow redder.
This is similar to what happens when you heat up metal to very high temperatures. As the
metal heats up, it will start to glow red. As it gets hotter, that red becomes more yellow and
then white. Eventually, the metal will be hot enough to glow a bright blue colour.
Our closest star, the Sun, shines with a yellow light. The Sun is classed as 'G' star and has a
temperature of about 5,800 K. (When talking about the temperature of stars, we usually use
the unit 'Kelvin' -& 5,800 K is about 5,500 °C.)
Blue supergiants can have temperatures of 12,000 to 15,000 K, making them the hottest stars
in the universe. The star Rigel in Orion is a blue supergiant.
The hotter stars are usually much less common than the cooler, redder ones. For example, 'O'
type stars make up only 1 in every 3 million stars we see. The next hottest 'B' stars are more
common, making up 1 in 800. Then, each cooler subtype becomes more and more common.
The coolest 'M' type stars make up 75% of all of the stars we see.
We can be even more accurate when we categorise stars by splitting each class into 10
smaller sub-classes. These sub-classes are numbered 0 - 9, with 0 being hotter than 1. For
example, the Sun is a G2 star. This is hotter than a G7 star but cooler than a G0 star.
Similarly, a B9 star is cooler than a B4 star
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The table below lists the temperature ranges and colours for each class of star:

Example
Class Temperature (K) Colour
Star
O 30,000 - 50,000 Blue Alnitak
B 10 000 - 30,000 Blue-White Rigel
A 7,500 - 10,000 White Vega
F 6,000 - 7,500 Yellow-White Procyon
G 5,200 - 6,000 Yellow The Sun
K 3,700 - 5,200 Orange Pollux
M < 3,700 Red Betelgeuse

Stellar Evolution: -
All stars form in nebulae, huge clouds of gas and dust.
Though they shine for thousands of years, stars do not last forever. The changes that occur in
a star over time and the final stage of its life depends on a star's size.
 stars
The exact lifetime of a star depends very much on its size. Very massive stars use up their
fuel quickly. This means they may only last a few hundred thousand years.
Smaller stars use fuel more slowly and will shine for several billion years.
Eventually, the hydrogen that powers a star's nuclear reactions begins to run out. The star
then enters the final phases of its lifetime. All stars will expand, cool and change colour to
become a red giant or red supergiant.
What happens next depends on how massive the star is.
A smaller star, like the Sun, will gradually cool down and stop glowing. During these
changes, it will go through the planetary nebula phase and white dwarf phase. After many
thousands of millions of years, it will stop glowing and become a black dwarf.
A massive star experiences a much more energetic and violent end. It explodes as
a supernova. This scatters materials from inside the star across space. This material can be
collected in nebulae and form the next generation of stars.
After the dust clears, a very dense neutron star is left behind. These spin rapidly and can give
off streams of radiation, known as pulsars. If the star is especially massive, it forms a black
hole when it explodes.
Eventually, the hydrogen that powers a star's nuclear reactions begins to run out. The star
then enters the final phases of its lifetime. All stars will expand, cool and change colour to
become a red giant or red supergiant.
What happens next depends on how massive the star is.
A smaller star, like the Sun, will gradually cool down and stop glowing. During these
changes, it will go through the planetary nebula phase and white dwarf phase. After many
thousands of millions of years, it will stop glowing and become a black dwarf.
A massive star experiences a much more energetic and violent end. It explodes as
a supernova. This scatters materials from inside the star across space. This material can be
collected in nebulae and form the next generation of stars.
After the dust clears, a very dense neutron star is left behind. These spin rapidly and can give
off streams of radiation, known as pulsars. If the star is especially massive, it forms a black
hole when it explodes.

Red giant: -
Stars spend most of their lives in the main-sequence stage. When the hydrogen in the centre
of a star runs out, the star begins to use hydrogen further out from its core.
This causes it to start to grow. Its radius can reach up to 400 times its original size. As the star
expands it also cools. The change in temperature causes the star to glow redder. The star is
now a red giant.
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Red giants can be 20 and 100 times the size of the Sun though only contain 0.25 to 8 times
the mass of the Sun. They are also very bright stars. The surface temperature of a red giant is
less than 4,000 - 5,000 K.

Over time, as the outer layers of the star expand, gravity causes its core to shrink and
contract. The temperature and pressure in the centre increase until nuclear fusion can start
again. Now the core is fusing helium, rather than hydrogen.
The star, now powered by helium, starts to shrink, get hotter and turn blue. However, the
star's supply of helium quickly runs out, so this stage only lasts for about a million years.
When the helium runs out, the core shrinks again. This time the star begins to use helium
further out from its core. At the same time, it may start fusing hydrogen in a shell around the
helium fusion!
The outer layers of the star expand, cool and turn red again. It has entered its second red giant
phase.
Red giants can swallow up planets as they expand. The Sun will reach its red giant stage in
about 5,000 million years’ time. During this phase, it will probably engulf the inner planets of
our Solar System which could include the Earth. But don't worry! This won't happen for a
very long time.
Planetary nebulae: -
The planetary nebula phase is a final stage in a low-mass star's life. During this phase,
the star sheds its outer layers. This creates an expanding, glowing shell of very hot gas.
 stars
Despite the name, they have nothing to do with planets. They got this name because
astronomers using small telescopes long ago, and they thought they looked a bit like planets.
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. This produces strong stellar
winds, which throw off the outer layers of the star.

The outer layers drift away from the star, leaving a small, hot, bright core behind, called a
white dwarf. The white dwarf gives of ultraviolet radiation which lights up the layers of gas
around the star.
Over time, the material from the planetary nebula is scattered into space. Eventually it will
form part of the clouds of dust and gas where new stars form.
Planetary nebula last just a few tens of thousands of years. This is short compared to the
thousands of millions of years which low-mass stars shine for.
Our Sun is a low-mass star and will produce a planetary nebula in about 5,000 million years’
time.

white dwarf: -
About 6 % of all known stars in our part of the Milky Way are white dwarfs. When a small
star runs out of fuel, it produces a planetary nebula. The outer layers of the nebula drift away
from the star, leaving a white dwarf.
A white dwarf is a bright, hot, compact star. They are about the same size, in terms of
volume, as the Earth. However, they contain about as much mass as the Sun.
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Because of their small radius, they are very faint. A typical white dwarf shines with only
0.1& - 1 % of the brightness of the Sun. Our nearest white dwarf is Sirius B, but it is too faint
to see with an optical telescope.
White dwarfs do not release energy through nuclear fusion reactions. The light and heat they
emit are left over from previous stages of their evolution. Despite this, white dwarfs have
some of the hottest surface temperatures of any star. They can be over 100,000 °C!
The material within a white dwarf was created by its parent star during its main sequence and
red giant phases. This material is compacted into a relatively small space, which makes white
dwarfs very dense.
The density of a white dwarf is about 1 million tonnes per cubic metre which is 200,000
times as dense as the Earth! Imagine the mass of the Sun, squashed to the size of the Earth! A
matchbox of white dwarf material would weigh the same as fifteen elephants!
The material is so compact it reaches a state known as neutron degeneracy. The normal
relationships between temperature, pressure and density do not hold for degenerate matter. As
the mass of a white dwarf increases, its radius decreases. There is a maximum mass beyond
which a white dwarf becomes unstable and collapses to form a black hole. This limit (known
as the Chandrasekhar Limit) is about 1.44 solar masses.