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The Galaxy Gallery Page 6: Comparative Morphology

This Is M33.

M33 is a High Surface Brightness Galaxy low mass spiral Galaxy which is a member of the local group. Its distance is approximately 2.5 million light years from the earth. Its an important calibrator in the extragalactic distance scale although it has somewhat high luminosity per unit mass compared to other low mass spiral galaxies. The Left Image is an exposure is of 30 seconds duration and again taken with a blue filter. It captures the central 2/3 of the total galaxy. There are lots of OB associations and other signs of active star formation here.

  • H-alpha Emission in M33.

    Star formation is often best manifested by bright H II regions.These are regions of hydrogen which have been ionized by ultraviolet photons which are emitted from massive stars (typically stars more than 20 times the mass of the Sun). As those UV photons ionize the hydrogen, the free protons and electrons will later recombine and emit a photon of wavelength 6562.8 Angstroms (this is call H-alpha). A narrowband filter image centered on this wavelength will then readily reveal these H II regions and hence the current sites of star formation in a galaxy, The luminous blobs that can be seen here are all H II regions

    The Right Image is the H-alpha Image of M33.

    This Is The Large Magellanic Cloud.

    The Large Magellanic Cloud (LMC) is the nearest other galaxy to the Milky Way. It covers an angular size on the sky of about 36 square degrees. It is best seen with the naked eye using averted vision and provides a good example of what the concept of low surface brightness is all about. This image is the only of its kind. It is a narrow band (15 angstrom) H-alpha image of the LMC taken with a single CCD exposure. No telescope is involved, just an 85-mm Nikon Lens. It is interesting to compare the morphology of this H-alpha image with the Red image of the LMC. Here the Red passband is measuring the intensity and appearance of the galaxy in the light of 5 billion year old stars whereas the H-alpha image is measuring the distribution of stars with ages less than 10 Million years. The dramatic difference in morphology between the two passbands is an important lesson.

    Images From CTIO