- Ultra-cheap trick: ``Say, that star looks a little bit redder than the Sun so
its surface temperature must be less than 5200K''
- Cheap Trick: Disperse the light from a star (``take a spectrum'') and
find the wavelength at which you have the most radiation then apply Wien's Law.
- One of the two ways it is really done - measure colors:
- For Planck spectra (from solid objects), the ratio of
the light in two different color filters unambiguously gives the
temperature of the objects.
Cooler objects will have redder colors. To the extent that Stellar
spectra look like blackbodies, the temperature of a star can also be
measured amazingly accurately by recording the brightness in two
- This leads us to a good question: What do Stellar spectra look like?
- In the 1800's the light from the Sun was dispersed and it looked
more-or-less like a Planck spectrum with
some Missing Light , or ``absorption lines'' at certain wavelengths.
It turns out that colors can still be used to measure stellar
temperatures, the relationship between color and surface temperature
is just a little more complicated and needs to be calibrated
using computer models.
To get a stellar temperature:
- Measure the brightness of a star through two
filters and compare the ratio of red to blue light
- Compare to the spectra of computer models of stellar spectra
of different temperature and develop an accurate
Now, those wavelengths with missing light in a stellar spectrum
turned out to be very interesting and important. Their importance
was realized after Emission line spectra were discovered
and investigated by chemists.
If a gas is heated to the point where it glows, the resulting
spectrum has light at discrete wavelengths that turn out to match
the wavelengths of missing light in stellar spectra.
Thu Jan 15 10:22:36 PST 1998