Focusing on the universe, one spectrum at a time.
Embedded Files
What sparked my Interest for spectroscopy
What sparked my Interest for spectroscopy
In the winter of 2024, I came in touch with introductory literature in the field of astrophysics for the first time. It was then that I first read the book „Searching for the Oldest Stars“ by Anna Frebel, which introduced me to some basic knowledge of stellar spectroscopy.
Spectroscopy is the process of splitting light into its wavelengths and analysing their intensities. The fact that white light is composed of different colours was first demonstrated by Sir Isaac Newton in the 1660s. When a beam of light passes through a medium (such as air or glass), its speed changes. This is described by the refractive index n, with v = c/n. The change in velocity depends on the wavelength: longer wavelengths, such as infrared, bend less strongly than shorter wavelengths. Therefore, blue light is deflected more strongly than red light.
The next major breakthrough came around 1815, when Joseph von Fraunhofer noticed multiple dark lines in the solar spectrum. These are now known as absorption lines. Absorption lines appear when light from a hot, dense source passes through a cooler gas: atoms in the gas absorb photons at specific energies, leaving dark lines in the spectrum. The opposite effect is emission, where a hot, thin gas emits light at particular wavelengths as electrons drop back to lower energy levels, producing bright lines against a dark background. Since each element absorbs or emits a unique set of wavelengths, spectroscopy allows us to determine the chemical composition of unknown gases. Thanks to this discovery, we can now identify the precise chemical make-up of stars and nebulae.
During his research, Fraunhofer also improved early versions of the so-called diffraction grating. Like a prism, a diffraction grating separates light into its component wavelengths, but with the crucial advantage that the dispersion is linear, making accurate measurements much easier. A diffraction grating consists of many very fine parallel lines that split light into spectra of different orders (m = 1, 2, 3, …). The diffraction angle is given by sinθ = mλ/d, where d = 1/N is the groove spacing for a grating with N lines per millimetre.
When a new star is born in a cloud of dense gas, the rest of its development mostly depends on two factors. Its mass, which is measured in solar masses (M☉) and its Temperature. These factors as well as the stars composition define the spectral class of the star (O, B, A, F, G, K, M). The spectral class and absolute magnitude (M; luminosity of a celestial object) allow for classification in a Herzsprung-Russel-diagram (HRD).
The HRD is a two-dimensional plot of luminosity versus surface temperature (or spectral type) that reveals clear patterns among stars. Most stars lie along the main sequence, where they spend the majority of their lifetimes fusing hydrogen into helium in their cores. Stars above the main sequence, such as giants and supergiants, are more luminous for their temperature and have expanded outer layers. White dwarfs occupy the lower-left region, representing hot but faint stars with very small radii. By plotting stars on the HRD, astronomers can visualize their current evolutionary stage and predict their future development.
Richard Powell, CC BY-SA 2.5, via Wikimedia Commons
When astronomers observe the light coming from a star, one of the first things they can determine is its temperature, which reveals a great deal about the star’s physical properties and stage of life. The temperature can be estimated from the overall distribution of light across different wavelengths, using Wien’s displacement law. This law states that the wavelength at which the star emits most intensely, λmax, is inversely proportional to the temperature, T, following the relation λmax= b / T, where b is Wien’s displacement constant, approximately 2.897 x 10-3 meter-Kelvin. By examining the star’s spectrum and identifying the peak of its emitted light, astronomers can fit a blackbody curve to the observed continuum and determine the effective temperature. Hotter stars emit more strongly at shorter, bluer wavelengths, while cooler stars peak at longer, redder wavelengths, providing a direct link between the observed light and the thermal conditions on the star’s surface.
The motion of a star causes observable changes in its spectrum through the Doppler shift. When a star moves away from the observer, its spectral lines are displaced toward longer wavelengths, producing what is known as redshift. By comparing the observed wavelength of a line to its rest wavelength, astronomers can determine the star’s radial velocity along the line of sight. In addition, a star’s rotation leads to rotational broadening of its spectral lines, as one side of the star moves toward the observer while the opposite side moves away. These opposite Doppler shifts widen the lines instead of shifting them uniformly. Measuring this broadening allows astronomers to estimate how fast the star is spinning, while the redshift provides information about its overall motion relative to Earth.
Since I have always been fascinated by our night sky, I immediately knew that I wanted to replicate some of Fraunhofer’s experiments with my own telescope. However it turned out to be a tad more complex that what I originally expected.
Sounds like a topic for you?
Sounds like a topic for you?
Don´t miss out on my next blog post explaining my goals and how I want to achieve them.
Don´t miss out on my next blog post explaining my goals and how I want to achieve them.
Sources
Sources
„Searching for the Oldest Stars“ by Anna Frebel
https://www.britannica.com/science/spectroscopy
https://en.wikipedia.org/wiki/Absolute_magnitude
https://en.wikipedia.org/wiki/Black-body_radiation
Page updated
Google Sites
Report abuse