This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: Chapter 7 Galaxies The Hubble Ultra Deep Field (Figure 7.1) is the result of 800 exposures of a single field in Fornax, summing to a total exposure of over 11 days. The limiting magnitude in the V band is m V = 29. Within the 3 arcmin 3 arcmin field of view of the Ultra Deep Field, there are 10 , 000 galaxies. If you Figure 7.1: Hubble Ultra Deep Field. multiply the 1100 galaxies per square arcminute within the Hubble Ultra Deep Field by the 150 million square arcminutes on the celestial sphere, that implies that there are 170 billion galaxies potentially observable by our 156 7.1. CLASSIFICATION 157 telescopes. The universe is as full of galaxies as a pomegranate is of pips. Despite this fact, it wasnt until the 1920s that astronomers were convinced that large galaxies other than the Milky Way galaxy definitely existed. Edwin Hub- ble discovered Cepheid stars in the Andromeda Nebula (M31), and showed that the Andromeda Nebula is actually the Andromeda Galaxy , comparable in size to our own galaxy. Modern distance measures put the Andromeda Galaxy at a distance of 700 kpc from our own galaxy. 7.1 Galaxy Classification Edwin Hubble, in addition to determining the true nature of the Andromeda Galaxy, also devised the classification scheme for galaxies that we use to- day. (Classification is an important first step in understanding. The purely empirical classification of stellar spectra, for instance, led to the physical understanding that the OBAFGKM sequence of spectral types is a temper- ature sequence.) Galaxies, unlike stars, are not customarily classified by their spectra. In practice, Hubble found it was most useful to classify galax- ies by their shapes. The Hubble classification scheme for galaxies is thus a morphological classification. 1 The Hubble scheme divides galaxies into three main classes: elliptical , spiral , and irregular galaxies. Our galaxy is an example of a spiral galaxy. As a useful mnemonic device, the different types of galaxies are laid out in whats generally called a tuning fork diagram, as shown in Figure 7.2. In the tuning fork diagram, elliptical galaxies are on the forks handle, the two types of spiral galaxies (with and without central bars) provide the two tines of the fork, and irregular galaxies are dumped off to one side. Hubble erroneously though that the sequence shown in the tuning fork diagram was an evolutionary sequence, with galaxies moving from left to right on the diagram as they evolved. We now know that Hubble was wrong on this point: elliptical galaxies do not evolve into spiral galaxies. Nevertheless, the tuning fork is still appears in astronomy textbooks, as a convenient visual aid to remembering the different classes of galaxies....
View Full Document