City of the stars
[5] Supernova 1987a is seen in the left-
hand photograph, and, at right, after
it had subsided. The stellar explosion
that occurred here was due to a star
undergoing core collapse – a Type II
supernova. A Type Ia supernova occurs
when a white dwarf in a binary system
accretes matter from the other star,
re-kindling nuclear fusion in the white
dwarf. Anglo-Australian Observatory,
photograph by David Malin.
[6] The Crab Nebula, seen by the Very
Large Telescope (VLT). ESO.
[7] Artist’s impression of the double
pulsar. Michael Kramer, Jodrell Bank
Observatory, University of Manchester.The spectrum of a star can be obtained by passing its light through
a diffraction grating, an optical device which splits light by the physical
process of diffraction. In this case incoming light encounters a regular
pattern of parallel rulings on an optical surface. Gratings may work by
allowing light to pass through them (transmission grating) or by reflecting
light off a surface (reflective grating). A simple yet effective way to see the
effects of a reflective grating is to reflect light in a CD or DVD disc. If the
disc is held in a certain way, the light is seen spread into a spectrum of
bright colours.
In addition to being able to tell us the chemical make up of a star,
spectral analysis can also be used to measure magnetic field strength
by monitoring how spectral lines appear to split due to a phenomenon
known as the Zeeman Effect. Shifts in the position of a star’s spectrum
can also indicate the speed a star is moving relative to the Earth, due
to the Doppler Effect. It’s this effect which can be used to determine the
presence of pairs of stars too close to be separated visually. As the stars
orbit each another, the two sets of spectral lines can be seen to undergo
a periodic shift relative to one another. This shows what visually looks
like a single star, to be a pair. Such a system is commonly known as a
spectroscopic binary.Double and Binary Stars
On the subject of paired stars, gravitationally bound stars which orbit one
another are abundant in our Galaxy. In fact our Sun is unusual in being
alone. A telescope shows many examples of multiple star systems in
the night sky, some of which simply look as if they are close by a line of
sight effect. In reality, they could be many billions of miles apart and not
connected at all. Such arrangements are known as optical double stars.
In addition, some stars are gravitationally bound to one another often
taking many years to complete their orbital dance. Such systems are
called binary stars. One of the most beautiful binary star systems visible in
the northern hemisphere is that known as Albireo which marks the beak
of Cygnus the Swan. Seen with a small telescope this seemingly single
star, splits into a yellow primary with a bluish companion close by. It's not
known whether this is a true binary system, but if it is, both components
will take in the order of 100,000 years to complete one orbit.luminous star. At the other end of the classification scheme, most M
class stars are relatively cool red stars with temperatures less than 3,700
degrees, masses less than 0.45 solar masses and low luminosities, less
than 0.08 times that of the Sun. The M classification also covers some
rather large red-supergiant stars. Examples of M-class stars are the
closest stellar neighbour to the Sun, the red dwarf star Proxima Centauri,
and Betelgeux, the red supergiant that marks the north-eastern corner
of the main pattern of Orion. One of the largest stars known, VY Canis
Majoris, a red hypergiant star in the constellation of the Great Dog, also
comes under the M classification.
Each class letter is subdivided further by the numerals 0 to 9, the
lowest number representing the hottest member of that particular class.
For example, an A0 star will be hotter than an A1 star, and so on, down
to A9.
The MKK system has a further subdivision to indicate the luminosity
class of the star, indicated by the Roman numerals I to V. I represents
supergiants while V are dwarf or main-sequence stars. The Sun has a
classification of G2V indicating that it’s a G-type main sequence star.
Modern research has revealed further types of star that merit spectral
classes of their own. To cater for this need, extra identifiers have been
created and include amongst others; L, T and Y to cover cool red and
brown dwarf stars, C to cover stars which are rich in carbon and D to
cover white dwarf stars.
Stellar Spectra
The light we receive from the stars is rich in information. This can
be extracted by looking at the spectrum of the star, essentially taking
its light and spreading it out to reveal its constituent colours. When
this is done, a rainbow spectrum of colour is revealed which is
crossed by dark, so-called absorption lines. These lines reveal the
chemical fingerprint of the star – which atoms are present and their
relative abundance.
It’s the pattern of lines in a star's spectrum that ultimately gives it its
place in the spectral classification scheme and the multitude of different
"fingerprints" discovered, which has created such an extensive range of
class-types.
7