Astronomer Book
Stellar Classification
The light that we receive from a star contains a vast amount of
information about its make-up and its characteristics. Just by looking at a
star’s colour we can tell what the temperature of the star is, how big it is
and how luminous it is.
Stars are categorized by a spectral classification scheme known as
the Morgan-Keenan classification system or MKK system for short. The
main class types are divided by letters of the alphabet in the sequence
O, B, A, F, G, K and M. The stars identified by this classification scheme
get progressively cooler along the scheme. Stars of class O are hot blue
stars with temperatures in excess of 33,000 degrees, masses in excess
of 16 solar masses and luminosities greater than 30,000 times that of our
Sun. The apparently faint star Lambda Orionis which marks the head of
Orion the Hunter is, in reality, nothing of the sort, being an O-type highlySupernova
However, consider a star say 10 times as massive as our Sun. It
is created inside a nebula as before, heats up and begins to shine.
Everything happens at an accelerated rate. The supply of hydrogen will
not last for nearly so long as with a star such as the Sun, and when the
crisis comes it is of a very different order. The star collapses. There is an
implosion followed by an explosion and the star blows most of its mass
away into space. This is called a supernova outburst. For a brief period the
luminosity can be over a 100 million times that of the Sun but dies down
fairly quickly and you are left with a very small super dense object known
as a neutron star.
A neutron star is a very strange object which has all of the space inside
its atoms removed. Consequently the star is quite small, typically having
a diameter about the same size as a large city. A teaspoon of neutron star
material would weigh more than Mount Everest!
Neutron stars have intense magnetic fields and material that is caught
by these fields radiates brightly as it is dragged down to the surface of the
star. Neutron stars can rotate very rapidly indeed and if one happens to
rotate in such a way that we get to see its glowing magnetic poles pass
our line of sight, the star is seen to pulse. The most famous example
of this type of object, which is known as a pulsar, is that which sits at
the heart of the supernova remnant known as the Crab Nebula. This
particular example rotates 30 times per second and its flashes can
be seen across a whole range of wavelengths including visible light.
The fastest pulsar currently shows pulses at regular intervals of 1.4
milliseconds. Pulsar PSR-J0737-3039 is a very unusual object consisting
of two pulsars in orbit around one another.
If the original star is large enough, the resulting neutron star may
actually be too heavy to exist as a such an object. If the central object has
a mass over 3-4 solar masses, then its gravity will cause it to collapse
completely into an object known as a black hole. This is a very exotic
object which has such an intense gravitational field that light itself cannot
escape from its surface.5 56