M8-S2: Stars' Spectra

  • account for the production of emission and absorption spectra and compare these with a continuous black body spectrum (ACSPH137)

 Figure shows how black body radiation spectrum, absorption and emission spectra of a star can be obtained. 

  • Hypothetically, if we are able to capture the radiation emitted from a star’s core (without any interference within its path to Earth), it would contain all frequencies. Thus, this produces a continuous spectrum.

 

  • Radiation from a star’s core inevitably needs to pass through its outer layers of atmosphere or nebula to reach Earth. The gas molecules found in these layers can absorb certain frequencies which is the reason why dark lines are observed in a stellar absorption spectrum.

 

  • Gas molecules inside these atmospheric layers and nebula can also emit radiation of specific frequencies due to their electronic energy levels. This emission can be also captured in the form of bright lines separated by much larger dark regions.

 

 

  • investigate the key features of stellar spectra and describe how these are used to classify stars

Figure shows a raw absorption spectrum of a star. Notice major 'dips' representing absorption lines. The curve is not smooth and continuous. The peak can be used to determine the peak/dominant wavelength emitted by the star. 

Effective surface temperature

  • Stellar absorption spectrum reveals the dominant wavelength of radiation emitted by the star which is used to calculate effective surface temperature via Wien’s displacement law 

Where b is Wien’s displacement constant: 2.898 ´ 10-3 metre Kelvin (mK); and T is the object’s temperature in Kelvins (K). The wavelength derived from this equation would have units in metres (m).

 

Spectral Class

Colour

Surface temperature (K)

Elements evident in absorption lines

O

blue

over 30 000

ionised He, weak H

B

blue-white

30 000 – 15 000

neutral He, weak H

A

white

15 000 – 10 000

strong H

F

white-yellow

10 000 – 7 000

weak H, metals (Ca, Fe)

G

yellow

7000 - 5000

strong metals, esp. Ca

K

orange

5000 - 4000

strong metals; CH and CN

M

red

4000 - 3000

strong molecules (incl. TiO)

 

Chemical Composition

  • Different stars are surrounded by atmospheres composed of different elements, atoms and ions. As such, we can use the spectral information to differentiate between different stars, and more importantly deduce their atmospheric composition.
  • By comparing the spectra with absorption line spectra, we can obtain on Earth using a wide range of elements such as hydrogen, helium and all the way up to iron, we can attain a pretty good idea of what elements are found in stars.
  • The intensity of each spectral line also correlates with the relative abundance of each element.
  • However, it is important to note that the absence of spectral lines does not necessarily indicate the absence of a particular element. Excitation of electrons require specific physical conditions besides energy of light, which may not be present in certain stars or stages of a star’s life cycle.

 

Doppler’s Effect – translational and rotational velocity

  • The wavelength or frequency of a wave is influenced by the wave’s relatively velocity to the observer.
    • If the emitter or source of wave is moving towards the observer, the resultant wave has shorter wavelength and greater frequency.
    • If the emitter or source of wave is moving away from the observe, the resultant wave has longer wavelength and lower frequency.

 

  • Red and blue shifts observed in stellar spectra are caused by the Doppler’s Effect.

The following three spectra are for a hydrogen atom: 

 

  • Red and blue shifts reveal information about stars’ translational and rotational motion.

The movement of stars, whether towards or away from Earth, is demonstrated by red and blue shift effects in their spectra. The shifting effect is apparent when stellar spectra are compared with absorption spectra obtained by exciting elements on Earth.

  • When stars move away from us, spectral lines of certain elements e.g. hydrogen would be shifted towards longer wavelengths compared with a reference.
  • When stars move towards us, spectral lines of certain elements would be shifted towards shorter wavelengths compared with a reference.

 

         This effect can also be used to determine the rotational motion of stars.

 

  • Wave emitted from the side of a star that is rotating towards us has shorter wavelength
  • Wave emitted from the side of a star that is rotating away from us has longer wavelength
  • Wave emitted from the ‘middle’ of a start (region that has no relative rotation to an observer on Earth) has no change in wavelength

 

The combined effect of these three phenomena results in broadened absorption lines in the stellar spectra

Density of a star

  • Density and pressure, at the surface of a star can also broaden spectral lines, but the intensity varies across the line in different way from the effect of rotation.

 

  • In high density (small and massive) stars the increased gas pressure produces more rapid collision between atoms during the emission or absorption of radiation. These collisions cause changes in the electron orbits and hence produce a broader spectral line.

 

  • For example, main sequence stars are much denser than supergiants. As a result, their absorption spectral lines are distinctively broader than those of a supergiant.

 

 Summary Table

Stellar Properties

Spectral Information

Effective Surface Temperature

 

Peak radiation wavelength corresponds to effective temperature.

 

 

Effect surface temperature is inversely proportional to peak wavelength.

 

Chemical composition

 

·      Absorption lines reveal elements found in the star.

·      Elements present in a star may not always show up in the absorption spectrum due to insufficient energy conditions/

 

Translational velocity

 

·      Moving away from Earth: red-shift (wavelength becomes longer)

 

·      Moving towards Earth: blue-shift (wavelength becomes shorter)

 

Rotational velocity

 

Broadening effect of absorption lines is directly proportional to the rotational velocity of the star.

 

·      Side rotating away from Earth: red-shift

·      Side rotating towards Earth: blue-shift

·      Side with no relative rotational movement: no change in wavelength.

 

Density and Pressure

 

Higher pressure changes electrons’ energy levels. This changes the frequency of radiation that is absorbed.

 

Broadening effect of absorption lines is directly proportional to the density of the star.

 

 

 

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