This is not an astronomy website. Our pages look at the stars from an
earth science perspective.
Understanding the stars assists us in understanding the finite nature of
energy and heavier elements in our own solar system and particularly on
When we appreciate that the heavy elements we so much rely on here on
earth are artifacts of collapsing of stars and supernova explosions, it
makes us realise just how luck we are to be here and now.
The Sun is a star, with properties typical of those of billions of other
stars in our Galaxy. This recognition resolved thousands of years of
religious, philosophical, and scientific debate. To learn the fate of the
Sun, on which we absolutely depend for survival, we must look to the
stars. "Across the sea of
space, the stars are other suns."--- Christiaan Huygens (1692)
Stars generate their energy mainly by burning hydrogen in nuclear
fusion reactions. Hydrogen is the most abundant element in the sun
and in the universe, however, while the hydrogen supply is large; but it
is nonetheless finite. This implies that all stars must evolve, that is to
say when the hydrogen fuel supply is exhausted they will need to burn the
next heaviest element which will change the temperature, size, colour and
density of the star over time.
As a consequence, the favorable conditions in the Earth's biosphere cannot
exist for ever and humans, or their descendants, will have to leave Earth
at a predictable (if very distant) time in the future in order to survive.
An understanding of stellar evolution allows us to age-date the stars and
thus establishes the basic time scale of the universe. The universe must
be at least as old as the oldest stars.
The age of the Sun is 5 billion years. The oldest objects yet dated
(globular star clusters---see below) are 13 billion years old. No
identifications of significantly older stars have been made. The universe
is therefore very ancient but had a finite beginning.
The age-dating of stars, and therefore the systems which contain them, is
one of a handful of methods astronomers have for learning about the
history of the universe.
All of the elements heavier than helium have been synthesized by nuclear
reactions inside stars during their evolution particularly in supernovas.
The Earth and all its inhabitants are made of atoms that passed through
stars now long dead.
There could be no organic molecules without stellar nuclear-synthesis. For
instance, all the atoms of carbon, nitrogen, oxygen, and phosphorus
essential for DNA molecules are created inside stars (mostly stars at
least 5 times more massive than the Sun).
Stars continuously form out of interstellar gas. Although the Sun is
middle-aged, there are many "young" stars with ages of less than 10
million years within easy reach of your telescopes.
The Earth actually lies inside the Sun's (extended) atmosphere,
so all forms of atmospheric activity on the Sun can potentially affect us.
Apparent brightness of stars (apparent magnitude) - how bright they
are as viewed from earth
Absolute brightness of stars (absolute magnitude or luminosity) -
how bright they would be if all viewed from the same distance (10 parsecs)
A parsec is a unit of distance which is defined in terms of the size of
the Earth's orbit. It is 3.1 x 1013 km(!), 206,000 times the distance to
the Sun, or about 3.25 light years (one light year is the distance light
travels in a year
We measure the surface temperatures of stars using their electromagnetic
spectra, following experiments first done by the physicist Kirchhoff in
the 19th century
Stars at the red end of the spectrum tend to be larger and cooler, white
stars very small and hot and blue stars the hottest of all.
Our sun is an average size star in the yellow end of the spectrum.
Stars are found in groups related to their mass, colour and tempertaure
In the Hertzsprung-Russell Diagram...
stellar absolute brightness increases upwards while
temperature increases to the right
Redder stars to the left (large / cool)
Bluer stars (large / hot) are at the right side of the diagram
White stars (small / hot) are to the lower right
Yellow Main sequence stars like our sun (medium / medium) to the
lower left moving to the middle right
Most stars fall on this main sequence (MS), which runs diagonally
across the HRD. More luminous MS stars are also hotter.
Our sun is an MS star (lying at T = 6000 and AbsMag = 4.8)
A second concentration of the so-called red giants and supergiants
falls in the upper left hand quadrant of the HRD (luminous but cool).
These are "giants" both in luminosity and in diameter (they can be
over 100 times the size of the Sun).
Stars are not randomly scattered in the diagram but instead confined
to well-defined sequences
Starting as a cloud of dust a star can evolve in one of two basic way
form an average yellow star burning progressively heavier
elements starting with hydrogen and ending up with iron while becoming
larger and less dense and eventually fading into a nebula which
collapses to form a white dwarf
form a massive star which burns quickly and collapses inwards to
create a supernova and in doing so creates the physical conditions to
change lighter elements into heavier elements and eventually becomes a
high density black hole or neutron star
All main sequence stars (including the Sun) are powered by the fusion of
hydrogen (H) into helium (He). Fusion of hydrogen requires temperatures of
more than 10 million Kelvin. Above this temperature, the fusion rate is
strongly dependent on temperature: a small increase in temperature results
in a much higher fusion rate. Because fusion is so temperature-sensitive,
in a main-sequence star, fusion of hydrogen into helium occurs only in the
hot, dense central core.
All main sequence stars (including the Sun) are in hydrostatic
equilibrium. That is, the inward force of gravity, which tends to compress
the star, is balanced by the outward force due to the pressure
If you increase the mass of a star, by pouring a little extra
hydrogen gas onto it... The higher mass will lead to higher compression --> which leads to
higher central density and temperature --> which leads to much faster
fusion --> which leads to much higher luminosity.
Because of the extremely sensitive dependence of the fusion rate on
temperature, a small change in mass leads to a small change in the central
temperature, but a very large change in the luminosity.
High mass main sequence stars have a shorter lifetime than low mass main
Stars don't blow themselves to bits in a runaway fusion reaction because
of a physical relation among the pressure, temperature, and fusion rate
which creates a natural buffer which keeps the center of the Sun (and the
center of any other main sequence star) at a steady temperature.
The "buffer" works like this...if you increased the fusion rate in a
Core temperature increases
Core pressure increases
Core density & temperature decrease
Fusion rate decreases
Thus, increasing the fusion rate sets a chain of events into action whose
end result is to decrease the fusion rate again.
If you decreased the fusion rate in a star's core:
When a star exhausts the hydrogen in its core, it becomes a red giant or
supergiant. Mass of > 0.4 M sun become hot enough to fuse helium
into carbon. Mass of > 4 M sun become hot enough to fuse carbon
into heavier elements.
Once a star has used up all the hydrogen in its core, fusion of hydrogen
into helium stops. The core starts to contract again. As the core
contracts, it releases energy. This energy heats up the layer immediately
above the contracting helium core. The layer immediately above the core
becomes hot enough to initiate the fusion of hydrogen into helium.
The red giant star now has three main layers:
Helium core (inner layer): Releases energy as it shrinks in radius.
Fusion shell: Releases energy as it fuses hydrogen into helium.
Hydrogen envelope (outer layer): Absorbs energy, and swells greatly
These swollen stars, no longer on the main sequence, are now giants (if
Magnitude < 8 Msun) or supergiants (if Magnitude > 8 Msun).
A giant's outer hydrogen envelope cools as it expands.
A giant becomes very large in radius and very cool -- hence the name "red
Our sun will become a red giant in about 5 billion yearstime, its radius
will increase to nearly 100 times its present size (engulfing Mercury as
it expands) and its surface temperature will drop as low as 3000 Kelvin
(from its present value of 5800 Kelvin).
In a red giant a huge, cool, low-density hydrogen envelope (with a density
of about 0.1 kilograms/m3) encloses a small, hot, high-density helium core
(with a density of about 1,000 tons/m3).
Once a giant or supergiant begins to fuse helium in its core, it
has four main layers:
Carbon (+oxygen) core: Releases energy as it shrinks in radius
Helium fusion shell: Releases energy as it fuses helium into carbon
Hydrogen fusion shell: Releases energy as it fuses hydrogen into
Hydrogen envelope: Still swollen in size
Once a supergiant becomes hot enough to fuse carbon into heavier
elements, the carbon-oxygen core continues to shrink, becoming
denser and hotter as it is compressed by gravity.
Once the central temperature reaches T > 600,000,000 Kelvin, carbon
& oxygen can fuse into heavier elements, such as silicon, sulfur, and
iron - a new energy source.
Stars of Mass > 4 Msun become hot enough for fusion of carbon
& oxygen to occur.
Stars between Mass 0.4 Msun < and M < 4 Msun end
up as spheres of carbon & oxygen and are the potential source of
life forming carbon in the universe
Our sun will never become hot enough to fuse into more massive elements. Iron is the end of the line where fusion is concerned.
The iron nucleus (containing 26 protons) is the most tightly bound of all
nuclei. The fusion of iron nuclei absorbs energy, instead of emitting
The fusion process from hydrogen to iron emits energy, each step, as the
nuclei become more massive, is less efficient.
As the star's core becomes hotter, and the fusion reactions powering it
become less efficient, each new fusion fuel is used up in a shorter time.
Variable stars have luminosities which increase and decrease with a
Main sequence stars have a nearly constant luminosity. (During the next
five billion years, the Sun's luminosity will double; that's an increase
of only 0.02% every million years.)
Some giants and supergiants have luminosities which regularly increase and
decrease. The period of the luminosity fluctuations (that is, the time
between peaks in the brightness) can range from a few hours to a few
years. Cepheid stars and RR Lyrae stars are variable because they pulsate
in and out. The two most interesting types of variable star are Cepheid
variables and RR Lyrae variables. Cepheid variables are named after the star Delta Cephei (the fourth
brightest star in the constellation Cepheus). The luminosity of Delta
Cephei varies by a factor of two, with a period of 5 days. Polaris, the
Northern Star, is also a Cepheid.
Cepheid variables have the following properties:
period = 2 to 60 days
average temperature = about 6000 Kelvin
average luminosity = 300 to 40,000 Lsun
the fluctuations about the average luminosity can be large (as in
the case of Delta Cephei) or small (as in the case of Polaris, where
the fluctuations are too small to be detected with the naked eye).
RR Lyrae variables are named after the star RR Lyrae, in the
RR Lyrae stars have the following properties:
period = 4 hours to 1 day
average temperature = about 7000 Kelvin
average luminosity = about 80 Lsun
RR Lyrae variables thus have shorter periods and lower luminosities
than Cepheid variables.
Some simple thoughts on why stars pulsate...
when a Cepheid is compressed, it becomes opaque.
photons are trapped inside, heating the gas and increasing its
the high-pressure gas expands, becoming transparent
photons escape, the gas cools, the pressure drops
as the pressure drops, the Cepheid is compressed by gravity and so
The greater the luminosity of a Cepheid star, the longer its period of
The Period-Luminosity relation is useful because it lets you measure the
distance to Cepheid stars which are quite far away.
measure the period P of a Cepheid variable.
from the period-luminosity relation, determine the luminosity
measure the apparent brightness b.
compute the distance from the equation: L = 4 pi d2 b
Because Cepheids are so luminous, they can be seen to large distances.
Parallaxes can be used to determine distances to 500 parsecs or so.
Period-luminosity relation can be used to determine the distance of a
Cepheid about 40 million parsecs away.
This lets us find the distance to other galaxies in our neighborhood.
Imagine measuring the distance of a tree without being able to walk to it
(say because there is a river between you and the tree)
To find the distance you need to determine the shape and the dimensions of
the triangle ABC.
When you measure AB (the baseline) and the two angles a (BAC) and b (ABC),
the triangle ABC is completely known.
This area of mathematics involving working out triangles is called
trigonometry and uses terms like sin / tan / cos (which have to do with
the ratios of the lengths of the sides of a triangle and the angles)
Trigonometry is just a tool so all you need to do is plug your
measurements into the formula.
From the diagram above...when the tree is farther away (C'), the angle
at C becomes smaller and the shape of the triangle becomes different.
Stars are very far away so the angle is very very small.
To measure the distance to a star you use a huge triangle where one side
of the triangle is twice the distance to our sun.
(This is like the line A-B in the tree example...)
Take a measure of the object now and then repeat the measure six months
later when the earth is on the opposite side of the sun.
The object will appear to have moved relative to very distant stars in the
background. The angle is very small as the star is very distant so it is
measured in "seconds of arc" where 1 second of arc equals 1/3600 of a
P = 0.5 / 2 =
0.25 seconds of arc.
(1 second of arc (1") = 1 /
The distance between the Sun and the star is : d = r /
If P is 1 second of arc: d = 150 000 000 / tan 1" = 30 million million km
This distance is called one parsec and is a basic unit for
measuring astronomical distances.
Distance in parsecs = 1 / P in
seconds of arc
d = 1 / P = 1 / 0.25 = 4
Therefore the star is four parsecs away.