Seeds: Chapter 13
Next Lecture - Active Galaxies
We are not alone in the Universe, there are many other galaxies visible in the sky, and they come in a variety of shapes and sizes. Working from the Mount Wilson observatory in the years after World War I, he took many images of galaxies, and classified them by their appearance, or morphology. He put them into three main classes: spiral, elliptical, and irregular. This is the classification we use today, and like many empirical relations (eg. the H-R diagram) the Hubble sequence has profound implications for the formation process of galaxies.
Spiral galaxies are galaxies like our own Milky Way and M31 (the Andromeda galaxy). Spiral galaxies have prominent spiral arms (hence their names) and a well-defined galactic disk. They also, to varying degrees, have a galactic bulge. The spirals are sub-classes as a,b,c by how prominent the bulge is compared to the disk: Sa (large bulge, small disk), Sb (medium bulge, medium disk), and Sc (small bulge, large disk). M31 is an Sb galaxy.
There are two types of spirals: normal spirals S and barred spirals SB. Barred spirals have a prominent bar in the center, from the ends of which the spiral arms trail outward. Like the normal spirals, the barred spirals are sub-classified by the bulge/disk ratio: SBa, SBb, SBc. In light of the recent microlensing discoveries, the Milky Way is most likely a barred spiral galaxy of class SBb.
Elliptical galaxies have no disk, and they are like a giant galactic bulge and halo. They do have many globular clusters like the halo of our galaxy. They are almost entirely made of Pop II stars, and have little gas, and thus no ongoing star formation. Like our halo, there is little net rotation and the stars are on highly elliptical orbits. Elliptical galaxies are sub-classified by how "elliptical" in shape they are, ranging from E0 (round) to E7 (highly elongated). Although we see their dimensions only in projection on the sky, elliptical galaxies do seem to be prolate, that is football or cigar shaped (two short axes and one long one), rather than oblate, like a squashed sphere or saucer (two long axes and one short one), though there may be a whole range in the relative dimensions of the three axes (ie. triaxial).
There are lenticular galaxies, S0, which appear to be intermediate between spirals and ellipticals. They have a very large bulge and halo, and a very small disk with no trace of spiral arms.
The irregular galaxies, Irr, are those with amorphous shapes, like the Large and Small Magellanic Clouds. Irregulars tend to be much smaller and fainter than the spirals and ellipticals on average, though there do seem to be a great number of them out there.
The traditional "tuning fork" diagram devised by Hubble showing E - S0 - S,SB is meant only as a morphological classification, and is not meant to denote any evolutionary scheme. Later on we will discuss the evolutionary possibilites between the various types of galaxies.
To determine the physical properties of galaxies, we first need to find the distance to any given galaxy we wish to study. We use distance indicators to find the distance. Galaxies are so far away that there is no hope of using a direct method such as parallax. We must resort to other distance indicators, many of which are approximate. To quote distances to galaxies, we use the unit megaparsec, Mpc, which is 10^6 pc.
The primary distance indicators for nearby galaxies are variable stars such as Cepheids and RR Lyrae stars. The period-luminosity relation for these stars can be calibrated from observations of variable stars in our own galaxy. Unfortunately, these stars are relatively faint, with absolute visual magnitudes of -6 or fainter. Thus, with a powerful telescope with a limit of apparent magnitude 24, we can see out to
Supernovae are also used as distance indicators. These can be seen much further away since they have absolute magnitudes from -16 to -20. Both Type I supernovae and the fainter Type II have been used. There is a general relation between the time it takes a supernova to fade a given number of magnitudes and its intrinsic luminosity, which makes these objects useful as distance indicators.
Other objects or approximately known intrinsic brightness are also used, and the distance deduced from the distance modulus. These indicators are called standard candles. Examples of bright objects used as standard candles are planetary nebulae, H II regions, the brightest O and B stars in galaxies, and surface brightness fluctuations in galaxies caused by the brightest stars, and the brightness of globular clusters. These have not very high accuracies because there is no good way to get the intrinsic luminosities of these things, and they come in a range of luminosities.
Other methods which sort of involve standard candles are the relationships between the maximum rotation velocity of a spiral galaxy disk and the luminosity of the galaxy (the Tully-Fisher relation) and between the central velocity dispersion and the central surface brightness of an elliptical galaxy (the Faber-Jackson relation). When calibrated on nearby galaxies with Cepheid distances, these two relations are the main distance indicators for galaxies from 10 to 100 Mpc away.
The intrinsic luminosities of galaxies themselves make poor standard candles, but when there are many galaxies in a group or cluster all at the same distance, then the brightest galaxies do have approximately the same luminosity. This is a distance indicator of the last resort, due to its large error bar.
Object of a known size can be used as distance indicators through the small angle formula for the apparent angular size versus distance. These are called standard yardsticks (I guess metersticks would be more appropriate). Examples of these are the angular diameters of globular clusters or galaxies.
The final distance indicator is the Doppler velocity of a distant galaxy. As we see in the next section, the discover by Hubble in 1929 really set the stage for cosmology.
Go to Previous Lecture ---- Go to Next Lecture
Back to the Lecture Notes Index
Back to the ASTR001/Sec3 Page
Steven T. Myers - Last revised 24Apr96