Application Exercise: Spectral Classification of Stars
Objectives
To learn the basic techniques and criteria of the spectral classification sequence by:
examining and classifying spectra according to the strength of
...
Application Exercise: Spectral Classification of Stars
Objectives
To learn the basic techniques and criteria of the spectral classification sequence by:
examining and classifying spectra according to the strength of the hydrogen Balmer absorption lines
examining and classifying spectra according to temperature using Wien's Law
comparing the two schemes by identifying the temperature where the Balmer lines are strongest and
recognizing the corresponding spectral class
summarizing the physical reason why both very cool stars and very hot stars have weak Balmer lines.
Background and Theory
Classification lies at the foundation of nearly every science. Scientists develop classification systems based on
perceived patterns and relationships. Biologists classify plants and animals into subgroups called genus and species.
Geologists have an elaborate system of classification for rocks and minerals. Astronomers are no different. We classify
planets according to their composition (terrestrial or Jovian), galaxies according to their shape (spiral, elliptical, or
irregular), and stars according to their spectra.
In this exercise you will classify six stars by repeating the process that was developed by the women at Harvard around
the turn of the 20th century. The resulting classification was a key step in elucidating the underlying physics that
produces stellar spectra. Thus, in astronomy as well as biology, the relatively mundane step of classification eventually
yields critical insights that allow us to understand our world.
The spectrum of a star is composed primarily of blackbody radiation--radiation that produces a continuous
spectrum (the continuum). The star emits light over the entire electromagnetic spectrum, from the x-ray to the radio.
However, stars do not emit the same amount of energy at all wavelengths. The peak emission of their blackbody
radiation comes at a wavelength determined by their surface temperature, the relationship known as Wien's Law. Most
stars put out the maximum amount of radiation in and around the optical part of the electromagnetic spectrum. The
ae 23diagram at the left shows three blackbody curves for stars of different temperatures. As the temperature drops, the
relative flux decreases, and the peak moves from the blue (hot) to the red (cool) wavelength regions of the spectrum.
In addition to the continuous spectrum, a star's spectrum will feature a number of either emission or absorption lines.
Emission lines are produced by atoms when electrons drop from high energy levels to lower ones, emitting photons at
specific frequencies in the process. This process adds radiation to the star's spectrum; emission lines are brighter than
the region of the spectrum around them. Absorption lines are produced by atoms when their electrons absorb radiation
at a specific frequency, thereby causing the electrons to move from a lower energy level to a higher one. This process
removes some of the continuum being produced by the star and results in dark features in the spectrum. These lines
are dimmer than the wavelength region around them
[Show More]
.png)
.png)
.png)
