Download Measuring colour in astronomy

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Measuring colour
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Measuring colour in astronomy
How we see colour
Different colours in the spectrum correspond to different wavelengths of light. It is the cones in
the retina of the eye which respond to colour. Because there are three different types of cone,
sensitive in different parts of the spectrum – red, green and blue, we are able to see the world in
colour.
We see ‘white’ when all three types of cone are being stimulated equally, and this occurs when
there are the same proportions of red, green and blue in the light as there are in light from the
Sun. As the following graph shows, the Sun emits fairly equally across the visible spectrum so
objects such as clouds or white pieces of paper appear white to us.
The spectrum of the Sun is close to that of a perfect black body at a temperature of 5777 K
The spectrum of the Sun shows many absorption lines due to elements in its outer atmosphere.
If we ignore these, the Sun’s spectrum is remarkably close to that of a perfect ‘black body’ at the
same temperature. See the illustration above.
By the term 'black body', scientists mean an object which would absorb completely all light and
other electromagnetic radiation that fell on it, reflecting none, so that it would appear black.
However, like all objects, a black body also emits radiation as well as absorbing it (the two
processes always going on simultaneously).
The range of wavelengths emitted by an object depends on its temperature. Objects at room
temperature emit radiation at long infrared wavelengths which we neither see nor feel as 'heat'.
An object as hot as the Sun (about 5800 K) emits most of its radiation energy at visible
wavelengths (that we see), and at short infrared wavelengths (that we feel as 'heat'), with a
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Richard Beare, 15th August, 2007
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AstroProjects
Measuring colour
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reasonable quantity of energy also being emitted in the near ultraviolet as well. A perfect black
body emits more radiation at any given temperature than any other object. (In reality there is no
such thing as a perfectly black body, but there are objects which come very close to this
theoretical ideal.)
Other stars have spectra which are also close to being black body spectra. On a dark night it is
easy to see (especially through binoculars or a telescope) that stars have different colours, and
this is because they have different surface temperatures. The coolest appear red, those about
the same temperature as the Sun appear white, and the hottest appear blue. As the next three
graphs show this is because a black body spectrum peaks at shorter wavelengths the hotter the
temperature.
Black body spectra for surface temperatures of 11600 K (twice as hot as the Sun), 5800 K (the
Sun), and 2900 K (half the temperature of the Sun). The ‘traffic lights’ on the left show the
relative amounts of red, green and blue light, and the larger circle underneath shows the colour
seen by the human eye.
Try this The images above are screenshots from an on-line interactive java simulation on
the Physlets website at http://webphysics.davidson.edu/Applets/java11_Archive.html . If
your browser is java enabled, have a look at this, and see if you can predict for yourself
how the spectrum and colours will change as you change the temperature.
Measuring colours in astronomy
Astronomers are very interested in the colour of a star because it indicates how hot the outer
surface of its atmosphere is, and this tells them a lot about the type of star.
To measure the brightness at different wavelengths, filters are used, each of which only lets
through light of a certain colour, usually red, green or blue. The particular range of wavelengths
transmitted by a filter is referred to as a waveband. Sometimes ultra-violet and infra-red filters are
used as well.
Astronomers have a quick way of measuring a star’s colour: they look at the ratio of how much
brighter it is in one waveband as compared with another (often blue compared with green). This
is called a colour index. Because the magnitude scale is logarithmic, the ratio of brightness is
found by subtracting the corresponding magnitudes, one from the other. For example, if a star
has magnitude B in blue light and V in green light, it is said to have a BV colour of B - V.
A hot star is brighter at the blue end of the visible spectrum than in the middle, so it will have a
smaller B magnitude than V magnitude, (remember that greater brightness means smaller
magnitude). The B - V colour will therefore be negative for a hot, blue star. By contrast a cool,
red star will be brighter at green wavelengths than at blue, so V will be larger than B, and the
B - V colour will be positive.
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Richard Beare, 15th August, 2007
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AstroProjects
Measuring colour
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Sloan Digital Sky Survey colours
The Sloan Digital Sky Survey (SDSS) used rather different filters to the usual red, green and blue.
As the following diagram shows the wavebands used corresponded to near ultra-violet, violetblue-green, green-yellow-orange-red, red-very near infrared, and near infrared. The five filters
are referred to as u, g, r, i and z for short. (Note: g is not green and z has nothing to do with
redshift!)
The five filters used by the Sloan Digital Sky Survey. The scale at the top shows wavelength in
nanometres (nm).
The simplest colour index that you can calculate using SDSS data is g - r, where g and r are the
magnitudes using the g and r filters. This directly compares the blue end of the spectrum with the
red end.
Galaxy colours from SDSS data
Many of the galaxies imaged by SDSS are too far away to make out individual features, so one
has to measure the spectrum and colour of a whole galaxy. Since the light from a galaxy is just
the sum of all the light from the individual stars of which it is made, its spectrum – and hence its
colour – is equivalent to the spectra of all the individual stars added together (except for
corrections due to redshift and due to absorption and scattering by dust).
Sc spiral galaxies with many bright blue stars in their spiral arms typically have a g - r colour index
of 0.6, as compared with 0.8 for elliptical galaxies (although there is a lot of variation, especially
for spiral and irregular galaxies).
It turns out that, however, that u - i rather than g - r gives a more useful 'colour' index for galaxies,
even though the actual u and i wavebands are invisible to the human eye. This is because u – i
differentiates much better between different types of galaxy. Sc galaxies typically have a u - i
colour index of 3.2, as compared with 2.5 for elliptical galaxies (although, as with g – r, there is a
lot of variation).
By looking in more detail at a galaxy's spectrum it is possible to draw further conclusions about it,
particularly in regard to the amount of star formation taking place. A separate information sheet
Specimen spectra for bright galaxies shows how to do this.
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