- A star can’t simply be measured by a thermometer (temperature) or a ruler (distance and radius)

- The majority of the electromagnetic spectrum (other than radio and visible light) is absorbed and deflected by Earth’s atmosphere. Therefore, in order to make observations, satellites need to be placed above Earth’s atmosphere.

Visible Light lies in between infrared and ultraviolet

Importance of Spectroscopy

Visible light from the stars can be passed through a prism to produce a spectrum. Each star will produce a unique spectrum as seen from the three examples of stellar spectra shown to the right. Astronomers use the spectra to determine the element’s motion, composition, temperature, and atmospheric density.

Properties of Stars

1. Composition of a Star

Every element produces a unique pattern of bands:

The absorption spectrum is a continuous spectrum with dark bands that are produced when light from a hot source passes through a cooler gas. The elements in the gas absorb light at specific wavelengths. The examples in the diagram above are emission spectra.
The emission spectrum is a series of unevenly spaced coloured lines (the opposite of the absorption spectra). The bright lines indicate what elements are present (see below).

A key point is that the absence of a spectral line does not necessarily mean that an element is not present. It could be that the conditions in the star are not able to produce the emission or absorption lines for a particular element.

2. Motion of a Star (Doppler effect)

The apparent shift in colour when a star is moving away or toward Earth can be detected with a shift in absorption or emission lines. If the star is moving away from Earth then the spectral lines will shift to the longer wavelengths, a redshift. If the star is moving toward Earth then the spectral lines will shift to the shorter wavelengths, a blueshift.

3. Atmosphere of a Star (high or low pressure)

When attempting to classify stars, astronomers found that some stars contained the same spectral lines; however, when comparing two of the stars it was found that some lines were “smeared” out and others were broader. The explanation:

High atmospheric densities in the outer atmosphere layers of stars cause the lines to be smeared out.

Low atmospheric density is generally found in giant stars, and these stars generally emit spectra with narrow lines

4. Temperature of a Star (Wien’s Law)

Most stars behave like black-body radiators, indicating that they emit energy at different wavelengths, and that they have a peak maximum wavelength in which they emit most of their energy.

In the 1890s Wilhem Wien found that the maximum wavelength emitted by a black body radiator was inversely proportional to the temperature.
If the maximum peak wavelength can be measured using a spectroscopy, and the temperature of the star can be determined using Wien’s Law:

The Sun emits energy over a wide range of wavelengths. Calculate the surface temperature of the Sun when the maximum wavelength is 5 x10-7m (500nm).

Solution:

Therefore the temperature for the surface of the Sun is 5800K.

	Created by Mobeen Ramzan over 5 years ago

Grade 12 Earth and Space Science Note on Classifying and Measuring Stars, created by Mobeen Ramzan on 09/02/2019.

Classifying and Measuring Stars