Spectroscopy and Stars – Learn


Isaac Newton was the first to observe that a prism could disperse sunlight into a spectrum of colours. The spectroscope was invented in an effort to explore the spectrum in more detail. It was discovered that the solar spectrum was not continuous but was crossed by a number of black lines.

Spectroscopes disperse light into its constituent colours and wavelengths by passing it through a prism. Light entering the prism is refracted towards the normal to the surface. Different wavelengths of light are refracted in different amounts, resulting in the light being dispersed, revealing its colours. Violet refracts the most and red the least.


Observing Spectra of Discharge Tubes, Reflected Sunlight and Incandescent Filaments

Incandescent Filaments:

An incandescent light bulb produces light by heating a metal filament to a very high temperature. This results in the production of electromagnetic radiation at a range of wavelengths. This range of wavelengths from the incandescent globe is a continuous spectrum. Some of the light produced is in the infrared part of the spectrum that is invisible to human beings but is detected as heat. The visible spectrum is between about 400 nm and 750 nm, so radiation longer than 750nm is invisible infrared EMR. There is a sharp drop of light in the ultraviolet, at wavelengths shorter than 400nm.

 

 

Incandescent globes produce a ‘warm’ light which has a yellow tone to it due to the spectrum emitted from it being ‘heavy’ in the shorter wavelengths of yellow/orange/red. Compare this with the ‘white’ light from an LED globe which is more evenly distributed across the visible spectrum.

 

 

 

Discharge Tubes:

Discharge tubes contain a low-pressure gas through which a current is passed. The current excites electrons in the gas and as they return to their ground state they emit light in the ultraviolet range. The inside of the glass is coated with a material called a phosphor. The ultraviolet light excites the phosphor, which then emits light over the entire visible spectrum. This is the basis on which Fluorescent lights operate.

Fluorescent lights are more efficient than incandescent lights because they emit less energy in the infrared range. This enables them to convert more of the electrical energy into light energy and still remain cool to touch. They are more expensive than an incandescent globe, but this is offset by lower running costs and a longer operating life. In household use, fluorescent lights operate on 15% of the power for an equivalent incandescent light.

The spectra of incandescent and fluorescent lights differ. Incandescent sources present a continuous spectrum, with bands greatly broadened on account of the higher temperatures, whereas fluorescent sources are ‘spiky’ with discrete emissions at narrow wavelength bands.

 

Reflected Sunlight

Sunlight arrives as a nearly white light to our eyes. What we see when we look at an object is due to sunlight reflecting off a surface into our eyes. When it reflects, it shows the colours that we see when we look at the object. Other colours that we do not see are absorbed by the object. Often what we see in reflected light is the combination of colours that reflect. For example, you may look at a red book, however, what you are observing is a combination of some specific wavelengths which combine to ‘appear’ red. Some of the wavelengths are absorbed by molecules in the material we are looking at. The molecules absorb some frequencies and reflect others. This results in some parts of the spectrum being more intense than others in producing the colours that we see.


Spectroscopy and Identifying Elements

Spectroscopy is the study of the electromagnetic spectrum and how electromagnetic waves interact with matter. Gustav Kirchhoff and Robert Bunsen used Bunsen’s burner to burn elements and clearly describe the cause of these spectral lines in 1859. They discovered the following:

  • A continuous colour spectrum is produced by glowing solids or dense gaseous bodies (a continuous black body spectrum)
  • If a gas exists between the light source and the spectroscope, light is absorbed from the continuous spectrum at wavelengths or colours characteristic of the chemical components of the gas (an absorption spectrum)
  • A glowing gas produces bright lines on a dark background at wavelengths or colours characteristic of the chemical components of the gas (an emission spectrum).

Further investigation by Kirchoff and Bunsen allowed them to conclude that the lines produced in the absorption and emission spectra were characteristic of the atoms that were heated or that the light passed through. Every atom of a particular element produced the same spectrum but no two elements produced the same spectrum. Their early work with spectroscopy led to a better understanding of the elements that the stars contained. It also further developed our understanding of the structure of the atom and introduce a whole new understanding of physics: quantum mechanics.

An element is in its ground state when the electrons are in their lowest possible energy state. Each electron in an atom has an exact energy level. When elements are heated to high temperatures or have an electrical current passed through them, they produce light. This light is produced because atoms within the element absorb energy and become ‘excited’. Eventually this ‘excited’ electron will return to the ‘unexcited’ or ground state. When this happens, the energy that had been absorbed is released. The colour of light that is emitted corresponds to a specific wavelength and frequency which will depend on the amount of energy it has released. Since atoms can usually have many different excited states, they can produce many different colours along the visible spectrum. The combination of colours produced by a particular element is distinctive to that element and is known as its emission spectrum.


Data from the Spectra of Stars

Observing the spectra of stars has allowed scientists to gather a lot of information, including:

  • The surface temperature of a star
  • The rotational and translational velocity of a star
  • The density of a star
  • The chemical composition of a star

Surface Temperature

A star emits energy across a wide range of wavelengths on the electromagnetic spectrum. However, the intensity of any particular wavelength varies. The colour of a star indicates the area of the spectrum of the star that is most intense. Physicists and astronomers have found that hot stars radiate more energy at short wavelengths than cooler stars. Short wavelengths correspond to the blue end of the visible spectrum, while longer wavelengths correspond to the red end of the spectrum. The temperature of a star’s outer layers determines its colour. The core of the star is much hotter than the outer layers, due to fusion reactions and gravitational energy. The following diagram illustrates the variety of colours that can be observed when good quality images of the stars are obtained:

 

Some stars are so hot that their peak wavelength is in the ultraviolet part of the spectrum. This makes them impossible for the eye to detect. UV telescopes in space are needed to observe these stars as UV light is mostly absorbed by the atmosphere.

The relationship between the wavelength at which the peak intensity occurs and the surface temperature of a star is known as Wien’s Law.

The diagram below illustrates emission at different wavelengths for objects of different temperatures:

The Rotational and Translational Velocity of a Star

Each element has a known absorption or emission spectra which is likened to a ‘fingerprint’ for that element. No two elements have the same spectra. The position of known absorption or emission lines are frequently shifted from where they are expected to be, however, their relative positions remain constant. This means that the element can still be identified. More importantly, the change in spectra can indicate if a star is moving away or toward Earth. If the wavelengths are slightly longer – they have been moved toward the red end of the spectrum – this is known as redshift. The opposite, whilst less common, also applies – If the wavelengths are slightly shorter – they have been moved toward the blue end of the spectrum – this is known as blueshift. This effect is often referred to as the Doppler effect. The extent of the redshift reveals how fast the star is moving.

The rotation of a star broadens the observed atomic absorption bands. Due to rotation, light emitted from the side of the star rotating away from the observer is redshifted and light from the side of the star rotating towards the observer is equally blueshifted meaning the observed band smears out over a range of wavelengths. The degree of broadening in the star’s spectrum reveals the rate of rotation.

Density

The absorption lines of stars are also affected by the density of the gases in the star’s outer layers. In lower density gases, particles need to travel further before they collide with other particles. Lower density stellar atmospheres produce sharper, narrower spectra lines. Denser gases and stellar atmospheres produce wider spectral lines with a lower intensity. The degree to which this widening or narrowing occurs allows scientists to estimate the density of a stars atmosphere. The density of stars varies greatly.

The Chemical Composition of a Star

A continuous spectrum of light is emitted from a stars core. As these photons move through the star they are absorbed by the electrons of elements in the outer atmosphere. These photons are then reemitted in all directions as an absorption spectra characteristic of the element they interacted with. Each element produces its own characteristic spectral line known as Fraunhofer lines, named after Jospeh von Fraunhofer who studied the Sun’s absorption lines in the 1800s. Matching the absorption lines in the spectrum of the Sun (below) or any other star’s spectrum with absorption lines produced by an element in the laboratory, identifies the elements present in that particular star. The intensity of the absorption lines indicates the abundance of that element.

 

 

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