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classification. This is done by adding a I through V following the Hubble classification, with I being the brightest (just as for stars). Efforts are still underway to find other properties of spirals that correlate with luminosity class. In this way, the luminosity of a galaxy can be determined without needing to know its distance. (Similarly, the luminosity class of a star can be determined from the shapes of certain spectral lines, allowing us to know the absolute magnitude of a star without knowing its distance.) Once the absolute magnitude of a galaxy is known, and its apparent magnitude is observed, its distance can be determined.

An important feature of spirals is the obvious presence of an interstellar medium — gas and dust. Even when a spiral is seen edge-on, we can tell that it is a spiral by the presence of a lane of obscuring dust in the disk of the galaxy. The light from spirals contains an important contribution from a relatively small number of young blue stars, suggesting that star formation is still

Various types of barred spirals. (a) NGC 1530, in Cameleopardalis, is type SBb. (b) NGC 1365, in Fornax, type SBc. [(a) NOAO/AURA/NSF; (b) ESO]

Various types of barred spirals. (a) NGC 1530, in Cameleopardalis, is type SBb. (b) NGC 1365, in Fornax, type SBc. [(a) NOAO/AURA/NSF; (b) ESO]

taking place in spirals. Where galaxies are found in a cluster, to be discussed in Chapter 18, approximately 80% of the galaxies are ellipticals. Outside of clusters, approximately 80% are spirals. Typical radii for the luminous part of the disk in spirals are about 10 to 30 kpc. Stellar masses of the galaxies we can see range from 107

to 1011 M,

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