Obtaining a spectrum from a galaxy rather than a star differs in that the galaxy can often be resolved as an extended rather than a point-source object. These Hα lines have been redshifted beyond the rest frame value of 6563 Ångstroms. The other two galaxies show prominent Hα emission lines suggesting active star formation in them. The top left galaxy is at zero redshift whilst the bottom right one has a redshift of z = 0.246. Note the differing redshifts of the galaxies. These are all discussed on another page.įour different galactic spectra from the 2dF Galaxy Redshift Survey. Careful analysis of a star's spectrum provides astronomers with a wealth of detail including its effective temperature, rotational velocity, translational velocity, its density and its chemical composition and metallicity. The concept of spectral classes is discussed in more detail on the next page. The study of many thousands of stellar spectra in the late Nineteenth Century led to the development of our modern classification system for stars.
Stars of different temperatures, size and metallicities will have different spectra but most exhibit absorption lines even if they do not all show strong Balmer lines as in this star.
This can be used to determine the effective temperature of the star. The overall shape of the spectrum approximates a black body curve with a peak wavelength. Note the characteristic absorption line features including strong lines for Hα, Hβ, Hγ and Hδ - the Balmer Series. The spectrum below is an intensity plot of a star. Let us know use these basic principles to account for and compare spectra produced by different types of astronomical objects. The spectrum formed is an emission or bright line spectrum, as shown by the middle spectrum in Figure 1. As these photons can re emitted in any direction an external observer will detect light at these wavelengths. When they de-excite they emit photons of specific frequency and wavelength. If this cloud can be excited by a nearby source of energy such as hot, young stars or an active galactic nucleus then the electrons in atoms of the gas cloud can get excited. Stellar spectra typically look like this.Įmission spectrum: A third possibility occurs if an observer is not looking directly at a hot black body source but instead at a diffuse cloud of gas that is not a black body. This means that the resultant spectrum will show dark absorption lines or a decrease in intensity as shown in the dips in the absorption spectrum top right in the diagram above. The net effect of this is that the intensity of light at the wavelength of that photon will be less in the direction of an observer. The direction of this re-emission however is random so the chances of it travelling in the same path as the original incident photon is very small. Eventually the electron will de-excite and jump down to a lower energy level, emitting a new photon of specific frequency. Photons of specific frequency can be absorbed by electrons in the diffuse outer layer of gas, causing the electron to change energy levels. The photons emitted from the core cover all frequencies (and energies). If we were able to view the light from this source directly without any intervening matter then the resultant spectrum would appear to be a continuum as shown bottom left in the Figure 1 above.Ībsorption spectrum: Most stars are surrounded by outer layers of gas that are less dense than the core. Figure 1: How continuous, emission and absorption spectra can be produced from same source.Ĭontinuum spectrum: In this diagram, a dense hot object such as the core of a star acts like a black body radiator.
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