Other types of galaxies

There is an additional type of galaxy that has certain features in common with spirals, but does not show spiral arms. This type is called SO ('S-zero') (Fig. 17.7). The bulge in an SO is almost as large as the rest of the disk, giving the galaxy an almost spherical appearance. Some SO galaxies also contain gas and dust, suggesting that they belong in the spiral classification. However, most

In this expression, D is called the luminosity scale length and gives a measure of the characteristic radius of the galaxy as seen in visible light. Typical values of D are about 5 kpc. This means that the luminosity of the disk of a spiral falls to 1/e of its peak value at r = 5 kpc.

M102, type S0. [NOAO/AURA/NSF]

M102, type S0. [NOAO/AURA/NSF]

This galaxy, M82, is the scene of very unusual activ-ity.At first it was thought that this galaxy was exploding. However, it just seems to have undergone a rapid wave of star formation. Galaxies like this are known as starburst galaxies, and we will discuss them more in Chapter 19. [NOAO/AURA/NSF]

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The Magellanic Clouds. (a) The Large Magellanic Cloud (LMC) is 50 kpc from us. (b) The Small Magellanic Cloud (SMC), which is 65 kpc from us. [NOAO/AURA/NSF]

This galaxy, M82, is the scene of very unusual activ-ity.At first it was thought that this galaxy was exploding. However, it just seems to have undergone a rapid wave of star formation. Galaxies like this are known as starburst galaxies, and we will discuss them more in Chapter 19. [NOAO/AURA/NSF]

reported their existence in 1943. These are spiral galaxies with a bright small nucleus. The spectra show broad emission lines, an indication of a very hot or energetic gas. Seyferts make up about 2 to 5% of all spiral galaxies. An example of a similar phenomenon in ellipticals is found in N galaxies (where the N stands for 'nucleus'). There is also a class of galaxies that give off very strong radio emission, radio galaxies. All of these active galaxies will be discussed in Chapter 19.

S0 galaxies have no detectable gas. The role of S0 galaxies is still not well understood.

Some galaxies have no regular pattern in their appearance. These are called irregular galaxies. The Magellanic Clouds, shown in Fig. 17.8, are irregular companions to our own galaxy. Irregulars make up a few percent of all galaxies. We distinguish between two types of irregulars: Irr I galaxies are resolved into stars and nebulae; Irr II galaxies just have a general amorphous appearance. Lenticular galaxies have an irregular elongated structure. Ring galaxies have prominent bright rings around their centers.

Peculiar galaxies have a general overall pattern, but also have some irregular structure indicative of unusual activity in the galaxy. An example is shown in Fig. 17.9.

There are also types of galaxies that are characterized by a very bright nucleus. Seyfert galaxies are named after their discoverer Carl Seyfert, who

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