How hot are the stars?
We typically think of stars as bright white pinpoints of light in our night sky.
However, if you look carefully at the stars, even without binoculars or a
telescope, you will see a range of colours from red through yellow to blue.
For example, Betelgeuse (Orion’s armpit) looks red, Pollux (in Gemini) is
similar to the Sun and is yellow, and Rigel has a blue tint.
A star’s colour depends on its surface temperature. Dark red stars have
surface temperatures of about 2500 K. The surfac e temperature of brighter
red stars is approximately 3500 K, yellow stars, like our Sun, are roughly
5500 K, whilst blue stars range from about 10,000 to 50,000 K in surface
temperature.
Stars appear to the naked eye to be
only one colour but they actually
emit a broad spectrum of colours.
You can see that starlight consists
of many colours when using a prism
to separate and spread the colours of
the light of the Sun, a yellow star.
These colours range from red,
produced by the photons (particles
of light) with the least energy, to
violet, produced by the most
energetic photons.
A spectacular explosion on the star
Eta Carinae about 150 years ago
produced three huge clouds of gas and
dust – two puffy lobes and a thin disk.
Astronomers call Eta Carinae a
luminous blue variable star because of
its colour and because it often
becomes very bright – as it did when
the explosion occurred. © NASA
Astronomers are able to make
accurate measurements of surface
temperature by comparing the star’s
apparent brightness through
different filters. The thermal
radiation spectra have very distinct
shapes; the difference in apparent
brightness allows astronomers to match the light emitted to surface
temperature.
Visible light is one of six bands of electromagnetic radiation. These range
from the least energetic, radio waves, to the most energetic, gamma rays. All
six bands can be emitted by stars, but most individual stars do not emit all of
them.
Astronomers study a star’s spectrum by separating it, spreading it out and
displaying it. The display itself is also known as a spectru m. The scientists
study thin gaps in the spectrum. When the spectrum is spread out from left to
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right, the gaps appear as vertical lines. The spectra of stars have dark
absorption lines where radiation of specific energies is weak. In a few special
cases in the visible spectrum stars have bright emission lines where the
radiation of specific energies is especially strong.
An absorption line appears when a chemical element or compound absorbs
radiation that has the amount of energy corresponding to the l ine. For
example, the spectrum of the visible light coming from the Sun has a group of
absorption lines in the green part of the spectrum. Calcium in an outer layer
of the Sun absorbs light rays that would have produced the corresponding
green colours.
Although all stars have absorption lines in the visible band of the
electromagnetic spectrum, emission lines are more common in other parts of
the spectrum. For instance, nitrogen in the Sun’s atmosphere emits powerful
radiation that produces emission lines in the ultraviolet part of the spectrum.
The following webpage has an activity on this subject:
http://www.astrosociety.org/education/publications/tnl/32/starsci ence3.html.
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