The way in which a star behaves
largely depends upon its mass.
Gas Giant Planets
Although Gas Giant Planets are not stars their overall composition is
similar to stars like the Sun in that they are largely composed of hydrogen
and helium with much smaller amounts of other elements. The only two
true examples in our Solar System are Jupiter and Saturn. Many Planets
ranging from Saturn Masses to Jupiter Masses and well
above have been detected orbiting other stars. Gas Giant Planets do
not reach high enough masses for temperatures to initiate thermonuclear
Brown Dwarfs (sometimes spelt Brown Dwarves)
For some time, astrophysicists had
predicted the existence of objects which have masses in between Jupiter
sized bodies and very low mass stars The first brown dwarf was
detected in 1995.
The lower mass limit of a brown dwarf (and the notional boundary
between these objects and a planet) is defined as 20 Jupiter masses (20 Jm).
High mass brown dwarfs can support fusion for about ten million years.
It is not hydrogen that is burnt but heavy hydrogen (deuterium).
Lower mass brown dwarfs reach temperatures
between 500,000 and 3,500,000 degrees Celsius in the core and are only able
to burn deuterium.
Lithium is generally present in brown dwarfs and not in low-mass
stars. Stars, which achieve the high temperature necessary for fusing
hydrogen, rapidly deplete their lithium. The temperature necessary for
this reaction is below the temperature necessary for hydrogen fusion.
Convection in low-mass stars ensures that lithium in the whole volume of the
star is depleted. Therefore, the presence of the lithium line in a candidate
brown dwarf's spectrum is a strong indicator that it is indeed a brown dwarf
rather than a low mass red dwarf star.
The use of lithium to distinguish candidate brown dwarfs from low-mass
stars is commonly referred to as the lithium test. However the
test is not infallible since the most massive brown dwarfs are hot enough to
burn lithium as well as deuterium when they are young although they are not
hot enough to burn hydrogen. Dwarfs of mass greater than 65
MJ can burn off their
lithium by the time they are half a billion years old thus this test is not
However, lithium is also seen in very young stars, which have not yet had
a chance to burn it off. Heavier stars like our Sun can retain some lithium
in their outer atmospheres, which never get hot enough for lithium
depletion, but those are obviously distinguishable from brown dwarfs by their size.
.Brown dwarfs can have a surface temperature of between 1500 K and
1000 K. Methane and water molecules can exist within this range.
Red Dwarf Stars
These are the first true stars in that the temperatures in the core can
reach up to 15,000,000 degrees which is more or less the temperature at
which hydrogen can burn to helium. These stars are known as red dwarf
stars and are also called stars of spectral class M. They range in
mass between 0.08 and 0.7 solar masses.. They take a much longer time than
Sun-like stars to reach the fusion stage to condense from a mass of dust and
gas. They spend millions of years condensing (the smaller the mass the
longer it takes).
Once their cores are hot and dense enough fusion starts and proceeds at a
leisurely pace. Fusion may continue for as long as the present lifetime of the
universe. Their indefinite lives mean that red dwarfs are now relatively
plentiful in old globular clusters and in our galaxy. Even if the Universe
lasted long enough red dwarfs could not become red giants. They do not have
the mass to create the conditions in the core to allow the fusion
of helium to carbon. Thus means they would never reach the same white
dwarf stage as our Sun but would remain at a helium stage forever.
Red dwarfs have surface temperatures of around 3500 K. The lower mass limit for a red dwarf is, by definition, 0.08 solar
mass as that was thought to be the minimum mass that is required to produce
the core temperature and pressure that will support the nuclear fusion of
Solar Mass Stars
Stars of between 0.7 and 1.2 solar masses have similar spectra. They
remain on the main sequence, fusing hydrogen in a stable manner for eight to
billion years. These stars, such as our Sun, when they run out of hydrogen,
are massive enough to burn helium to carbon and to support some burning between carbon
and helium to give oxygen. They become red giants and cast off
'planetary nebula'. The remnant hot core will slowly cool as a white dwarf.
mainly consisting of carbon and oxygen.
Although the Sun is in the top 15 % of the most massive stars there
are quite a number of stars somewhat more massive. There are however a
small number of stars far more massive than the Sun. They burn far faster
than the Sun and end their comparatively short lives in the production of
massive white dwarfs, or in the most extreme cases, stars of over 8 solar masses
undergo enormous supernova explosions which end in the production of neutron
stars or in the most extreme cases of all black holes. These
very massive stars will be discussed in detail in the next section of the
web-site dealing with thermonuclear reactions.
Hertzsprung - Russell Diagram
In the early 1900's, Ejnar
Herstzprung and Henry Norris Russell independently made the discovery
that the luminosity of a star is related to its surface temperature. (They
actually used a quantity known as the Magnitude of the star and the
Spectral Class of the star.) A schematic Hertzsprung-Russell
diagram is shown below. Courtesy NASA
Hertzsprung and Russell both used the spectral class (which is related
to the temperature) in their plots. They ordered the stars as O, B, A, F, G,
K, and M. Since the O stars are the hottest, this means that
in the HR diagram, the temperature axis is unusual in that the temperature
decreases as one moves to the right.
The Hertzsprung-Russell diagram. The diagram shows the main
sequence, red giants, supergiants, and white dwarfs. In addition, we identify
the location of the Sun, the twelve brightest northern hemisphere stars and the
white dwarf companions of Sirius and Procyon.
The diagram Courtesy NASA
Very many thanks are due to Dr Bob Gomersall
of the BTL Group. in Shipley, West Yorkshire for his kind permission to reproduce
the beautiful diagram on the left. Acknowledgement for illustration to I R
and W D Gomersall
Click illustration for enlargement
Note the main sequence stars all lie on a
clear curve. They are referred to perhaps rather inaccurately as dwarfs.
Epsilon Eridani is an orange dwarf, the Sun is a yellow dwarf, Sirius is a
white dwarf. What can not be indicated in this diagram is the fact that the majority of stars are
red dwarfs. The Sun and other orange, yellow and white stars make up most
of the rest with only a small number in the blue. Green stars have been
reported but are very rare. The white dwarfs are extremely hot but very
small. The red giants are large stars like the Sun at the end of their
live before they collapse into the white dwarf stage. The Red and Blue
Supergiants are massive stars which are close to the end of their life span and
finally explode as supernovae and yield neutron stars and a cloud of material
containing large amounts of higher elements which are added to the interstellar
medium and form part of the material of future stars and planets. A few
very massive stars collapse into black holes and explode as hypernovae.
Table of Typical Well Studied Stars
||Mass in Mo
||Diameter (Sun =1)
||Age of Star years
||Blue Super Giant
||5-12 Χ 10-5
|High Mass Brown Dwarf
||60 X Jupiter
||about same as Jupiter
|Low Mass Brown Dwarf
||20 X Jupiter
||about same as Jupiter
As the table above shows there is very little difference in
the diameters of brown dwarfs and gas giant planets. The
diagram on the left shows an artist's rendition comparing
stars, brown dwarfs, and planets to the same size scale. From
left to right is the limb of the Sun, a very low mass star (red
dwarf), a pair of brown dwarfs, and the planet Jupiter. These
objects have masses ranging from 1000 times that of Jupiter (for
the Sun) through 75, 65, 30, and 1 Jupiter mass, respectively.
Despite the range in mass, all four of the low-mass objects are
approximately the same size, ten times smaller than the diameter
of the Sun.
Diagram Courtesy UCLA/NASA/JPL
Within 26 light years from the Sun and visible from the
northern hemisphere there are 4 blue stars, 1 green-tinted star,
5 yellow stars (including the Sun), 22 orange stars, 87 red
stars, and 9 white dwarfs. There are in addition many brown
dwarfs. The brown dwarfs are composed of a few red
and redder dwarfs along with lots of magenta dwarfs (or cooler
objects). Despite the fact that there are at least as many brown
dwarfs as stars, the stars are responsible for most of the mass
off objects in the volume studied.
The mass of the Sun is approximately 1.98 X 1038
kilograms. It is used as a unit of mass in astronomy
Mo. Jupiter is only 0.001Mo
and the Sun is 300,000 times more massive than the Earth
Morgan Keenan Spectral Types
This stellar classification most commonly used. is the Morgan
Keenan. It is
also known as the Yerkes classification. The common classes are
normally listed from hottest to coldest (with mass, radius and
luminosity compared to the Sun) and are given in the following
table. The colors in this table are greatly exaggerated
for illustration. The actual colors of the listed stars are
mostly white with a faint tint of the color indicated; stars'
colors are often too subtle to notice, particularly when they
are near the horizon.
30,000 60,000 K
10,000 30,000 K
7,500 10,000 K
||White with bluish tinge ("white")
6,000 7,500 K
5,000 6,000 K
||Light yellow ("yellow")
3,500 5,000 K
||Light orange ("orange")
2,000 3,500 K
||Reddish orange ("red")
Acknowledgement for table and illustration of Morgan Keenan Classification of
Duration of the Life of a Star
The more massive a star is the faster it burns
and the shorter is it's life span in the main sequence.
There are two ways of stating the composition of the
Percentage by number of atoms 94% hydrogen, 6% helium
0.11% of oxygen, carbon and nitrogen with 0.02% other elements.
Percentage by mass 78.5% hydrogen, helium 19.7%,
oxygen 0.86%, carbon 0.4%, nitrogen, iron and other elements 0.54%
Astronomers use the word METAL in a totally different way to
chemists. The word metal in astronomy means ALL elements except hydrogen
and helium. This of course includes elements such as carbon, nitrogen,
oxygen, chlorine, sulphur, phosphorus and all the other elements that are
classified as non-metals in the chemical, physical, geological and biological
sciences. Although to call these elements METALS seems quite absurd to the
science of chemistry it has become so much part of the vocabulary of astronomy
that it is unlikely that a better term will ever be found. Even in
astronomy the term is not used for terrestrial planetology and is really only
applicable to STARS.
The term METALLICITY is used in describing the amount of
METALS (in the astronomical sense) a star contains. It is very difficult to
measure the composition of a star - even our own Sun . the photospheric
composition of the Sun given in the current Wikepedia is given below. It
does not agree closely with the figures given in above which were taken from
Nick Strobel'sAstronomy Notes web-site
http://www.astronomynotes.com/copyright.htm. Composiition figures must
of necessity be estimates only. The photospheric composition of the
Sun by mass is approximately Hydrogen 73.46%, Helium 24.85%, Oxygen 0.77%,
Carbon 0.29%, Iron 0.16%, Sulphur 0.12%, Neon 0.12%, Nitrogen 00.09%,Silicon
0.07%, Magnesium 0.05%
The metallicity of a star is hard to estimate and varies
enormously. Compared to the Sun Sirius is estimated at 190%, Procyon 110%,
Vega 63% and Epsilon Eridani 0.49-0.60.
The importance of metallicity comes into play when
considering the possibility of large terrestrial type planets like our Earth.
As a generalisation the higher the metallicity the greater the likelihood of
Binary and Multiple Star Systems
Our Star the Sun is a single star. However there are many
star systems which consist of two or more stars orbiting a common centre of
An example of a binary or perhaps a triple star system is Alpha Centauri.
It consist of two Sun-like stars - one Alpha Centauri A is a yellow white
slightly more massive than the Sun and is of the same spectral type(G2).
Alpha Centauri B is a deep yellow (K1 spectral type) and is slightly less massive
than the Sun. The two stars orbit each other and take 80 years to complete an orbit. The two stars are 4.39 light years
away from the Sun. A third star known as Proxima Centauri is nearer to the
Solar System by 0.17 light years so it is very far away from the two larger
stars. It is a very faint M5 red dwarf only 0.12 times the mass of the Sun.
Binary and multiple star systems come in all sorts of
varieties. In the case of Alpha Centauri the two main stars are both
Sun-like whilst the third star in the system is a very small red dwarf. In
the case of Sirius there is one star which is two and a half times the mass of
the Sun called the Dog Star. It is accompanied by a white dwarf Sirius B
sometimes called the Pup. Sirius B is about 42,000 kilometers in diameter
and has a density 125,000 times that of water. White dwarf stars have
already been discussed in the web-page on the Sun and the eventual fate of the
Sun after it completes the main sequence stage.
Another example of a binary was given in the web-site on the
page on Extra-solar Planets. 55 Cancri A very similar to the Sun. So far
away is the red dwarf companion that we can almost think of Cancri A as being a
single star capable of accommodating a planetary system.
It used to be believed that binary systems where the stars
were close together would be unlikely to have planets. However, recently a
large planet was detected in a triple star system It was
discovered by Maciej Konacki of the California Institute of Technology in
Pasadena. After surveying only about 20 multiple stars with the 10-metre Keck I
telescope in Hawaii, he found signs of a planet in a triple star system called
HD 188753. The system lies about 149 light years away in the constellation
new planet is at least 14% more massive than
Jupiter and orbits very close to a Sun-like
star, while two further lower mass stars circle
farther out, at roughly the distance between
Saturn and the Sun. The two other stars
would rise together as smaller discs glowing
orange and red. "The environment in which this
planet exists is quite spectacular," says
Konacki. "With three suns, the sky view must be
- literally and figuratively - out of this
According to Konacki, this
planet must witness dramatic multiple sunrises.
In its skies, the main star would look
yellow-white like the Sun, but loom much larger.
The star would look 25 times larger than our
Sun because the planet orbits very close to the
star Konacki told New Scientist.
as reported to Hazel Muir and published in the
New Scientist in 13 July 2005 issue.
In the illustration on the
left the artist's impression shows a scene from
one of the gas giant planet's moons. Watch
a multiple sunset, viewed from the hypothetical
moon around the new planet. Image artist's
impression Courtesy NASA/JPL/Caltech
Click illustration for
The next section
of the web-site 'The Magic Furnace' will deal with
the thermonuclear production of the chemical
elements in the cores of very massive stars. It
is in the process of preparation
The Solar Wind