Uses of the Balmer Thermometer in Astronomy

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  • 0:01 Cecilia Payne
  • 1:22 The Balmer Thermometer
  • 4:02 The Strength of…
  • 5:44 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will outline for you how the temperature of a star can be determined using spectroscopy as discovered by none other than Cecilia Payne. It's a method of determining stellar temperatures called the Balmer Thermometer.

Cecilia Payne

As with all great discoveries and discovering minds, they are criticized to no end by a dinosauric establishment when they come up with a meteoric discovery that may lead to the extinction of long lived beliefs. Such was the case with none other than astronomer Cecilia Payne, whose discoveries led to the understanding that most of the mass of the visible universe is made of hydrogen and the rest mainly helium, something the establishment loathed to believe.

Consequently, Payne knew that stars, including our Sun, are all basically made of the same stuff, helium and hydrogen. This was a concept that was very difficult to grasp in the early 20th century, but like all great minds, she was ahead of her time.

But more importantly, Payne's discovery meant something else. It meant that when astronomers look at a star's spectrum, it really tells us more information about the temperature of the star producing such a spectrum, more so than its chemical composition, since they're made of the same stuff anyways. Therefore, stars with similar spectra must have similar temperatures. A spectrum is a result of the dispersion of a beam of electromagnetic radiation such that its components are spread out in order of wavelength.

The Balmer Thermometer

Cecilia Payne developed a method for determining the temperature of stars by using their spectra that is now known as the Balmer Thermometer. The Balmer Thermometer namely uses spectral lines of hydrogen that are visible to the human eye, called Balmer lines, to estimate stellar temperatures.

A spectral line is a line in a spectrum located at a specific wavelength when a specific atom absorbs or emits a characteristic amount of energy. When energy is absorbed in the form of a photon of light, a dark absorption line will form in a continuous spectrum.

Since the light that actually forms any given spectrum of a star comes from the outer layers of a star, a spectrum is thus only representative of those layers and its temperatures, and so, the strength of the Balmer lines is dependent upon the temperature of the star's surface layers.

Confusingly enough, medium-temperature stars are the ones that have the stronger Balmer lines, while hot and cold stars have weaker Balmer lines. The porridge of gases in a star needs to be just the right temperature to happily absorb photons in the Balmer series because Balmer absorption lines can only be produced by atoms that have electrons in their second energy level.

Here is why this is so.

Relatively hot stars will have way too many violent collisions between atoms. This will cause most of the atoms in such a hot gas to have their electrons in energy levels higher than the second one. Such energy levels are too high to form Balmer absorption lines, and so, these stars will have weak Balmer absorption lines in their spectrum. This porridge is too hot!

On the other hand, a relatively cold star will have too few collisions between atoms strong enough to excite an atom's electrons away from the lowest energy level, the ground state. This means such unexcited atoms won't be able to absorb photons in the Balmer series either, and so, the Balmer absorption lines in the spectra of these stars will also be weak. This porridge is too cold!

However, stars around 10,000 K have just the right amount of collisions happening between its atoms to excite the electrons in these atoms to their second energy level. At this temperature and energy level, hydrogen gas can absorb photons with wavelengths in the Balmer series really well, producing nice and strong dark (absorption) spectral lines as a result. Ahh, this porridge is just right!

The Strength of Absorption Lines

Calculations have shown astronomers how strong Balmer lines should be for a star with a given temperature. These calculations are the key to unlocking the temperature of a star from its spectrum.

Balmer lines graph
graph of strength vs temp for balmer lines

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