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Unformatted text preview: life and work of Edwin Hubble on this website:
http://www.spacetelescope.org/about/history/the man behind the name/
See the essay by Hubble’s student, Alan Sandage at
http://antwrp.gsfc.nasa.gov/diamond jubilee/d 1996/sandage hubble.html
1 1 PHSC12000 V1.0 2 will ﬁnd evidence for the existence of dark matter in the universe - one of the most intriguing
mysteries of modern cosmology.
In this ﬁrst part of the labs, you will explore the morphological types of galaxies and the
overall shapes and features of their spectra, including the features in the spectra called
“spectral lines.” You will count galaxies in randomly chosen patches of the sky to see how
homogeneous their distribution over the sky really is. You will then measure the wavelengths
of spectral lines in real galaxy spectra. To determine galaxy redshifts you will compare the
measured redshifted galaxy lines to laboratory measurements of the line wavelength on Earth. Morphological types of galaxies
Hubble classiﬁed galaxies into a sequence, now called the tuning-fork Hubble diagram (due
to its resemblance to a tuning fork viewed horizontally) based on their visual appearance:
shape, smoothness, and presence of features such as bars and spiral arms. All this collectively is called “galaxy morphology” and the classiﬁcation scheme is called “morphological
classiﬁcation.” Hubble had conjectured that the sequence represented the path of galaxy
evolution. Although this was not conﬁrmed by further observations, the classiﬁcation is still
widely used by astronomers. What properties of the galaxies indicate that the classiﬁcation
scheme is not an evolutionary sequence? Consult your textbook and discuss with your TA.
Hubble’s classiﬁcation contains many sub-classes of galaxies3 , but the two main classes of
galaxies are elliptical and spiral galaxies. Ellipticals tend to be smooth in their light distribution and roundish in their appearance. Spiral galaxies tend to have features such as bars
and spiral arms.
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