If we take a small region of the Universe, such as the neighbourhood of the Sun, it does change over time as individual stars burn up their fuel and die, eventually becoming objects such as black dwarfs, neutrons stars and black holes. The theory does acknowledge that change takes place on a smaller scale. In other words, the Universe doesn’t evolve or change over time. This states that the Universe is infinite in extent, infinitely old and, taken as a whole, it is the same in all directions and at all times in the past and at all times in the future. At the heart of the Steady State theory is the Perfect Cosmological Principle. This theory was developed in 1948 by Fred Hoyle (1915-2001), Herman Bondi (1919-2005) and Thomas Gold (1920-2004) as an alternative to the Big Bang to explain the origin and expansion of the Universe. However, this hasn’t always been the case and for a while the Steady State theory was very popular. It is now generally accepted by most cosmologists. The Big Bang theory states that the Universe originated from an incredibly hot and dense state 13.7 billion years ago and has been expanding and cooling ever since. This is an elegant alternative theory to the Big Bang, which was very popular among astronomers in the 1950s, but is now obsolete. Or service.This post, the latest in my series about cosmology, talks about the Steady State theory. Given and the use cannot be construed as an endorsement of any product Otherwise used for non-commercial purposes, provided proper credit is SDSS images may be downloaded, linked to, or SDSS images may not be used for any commercial publication or otherĬommercial purpose except with explicit approval by the Astrophysical The strong absorption bands in the blue half of the spectrum are due to absorption by molecular carbon. Here is an example of a carbon star, a "rare" star which lies away from the main stellar sequence. Through sheer number of objects observed, the SDSS will help advance the field of stellar astronomy. These stars are easily identified by their low flux in the blue portion of the spectrum and clear titanium oxide absorption bands towards the red end. Many of the stars to be observed within the Survey are cool red stars such as this M star. This is a typical DA white dwarf, as identified by its very broad hydrogen absorption features. SDSS software fits the redshifts for these galaxies by identifying the various hydrogen, nitrogen II, sodium II, and oxygen III lines.Īlthough the SDSS is primarily an extragalactic survey, spectra will be obtained for some 100,000 stars. The prominent features of such a galaxy are now in emission, not absorption. This galaxy, at a redshift of 0.3762, has especially prominent magnesium and sodium absorption.Īn emission-line galaxy with z=0.0886. These galaxies are typically elliptical galaxies at a redshift of 0.25 to 0.5, targeted for spectroscopy due to their red color in the imaging portion of the Survey. This is an example of an SDSS Bright Red Galaxy (BRG). At the end of its run, the Survey will have spectra for approximately one million such galaxies. Strong absorption in hydrogen and the G band can also be seen in this spectrum. These "typical" galaxies' redshifts are usually fit by identifying the calcium H and K lines in conjunction with the strong magnesium and sodium lines at 51 Angstroms, respectively. Again, 1D correctly identified the emission features, although the broad absorption caused an error of about 0.05 in redshift.Ī galaxy at redshift 0.1234. The Survey will obtain spectra for roughly 100,000 quasars, although mostĪ quasar with broad absorption at z~3.7. Note that the software regarded much of the Lyman alpha forestĪs unidentifiable emission lines, but still calculated a correct redshift. Well as the nitrogen, silicon, oxygen and carbon emission features just The emission lineįitting algorithm correctly identified the redshifted Lyman alpha peak, as The spectrum of a quasar at a redshift (z) of 4.16. (Images and captions courtesy of Brian Wilhite, University of The lower green curve represents the error in the number of counts.Ĭlick on any of the links below to download a larger version of the The yellow curve shows a smooth continuum for the spectrum while Dotted magenta lines give the location of night sky Solid red lines show potential emission features that 1Dįailed to identify. Solid blue lines indicate the location of strong emission features locatedīy the code. Lines correspond to either emission or absorption features fit by 1D. Spectra represent features identified by the SDSS 1D code. Processed by the SDSS 2D reduction software. Spectrograph at Apache Point Observatory. The following are examples of spectra obtained by the SDSS twin
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