1 Astro Section 1 Professor Brandt
Monday February 22nd 1999
Announcements: Lecture notes: Class 19. One issue to review from the test. The mass of a star does NOT greatly increase when it becomes a red giant. Mass represents amount of matter present and cannot change arbitrarily. You need to know this for the rest of the course. Constellations Canis Major - great dog. Contains Sirius but most other stars in this constellation are faint. The star Sirius called the dog star. It's the brightest star in the sky - 8.5 ly distant and 26 times as luminous as the sun. Canis Minor - little dog contains Procyon - yellow white. Only 10.5 ly away. 5 times as luminous as the Sun. Greek for "before the dog" Today the lecture is titled "People discover the Galaxy" In this lecture we will move out to larger size scales and will start talking about galaxies. Many of you have probably seen the Galaxy - go out at night in a rural place for a dark sky. The faint band of light running across the sky. Greeks and Romans called this the Milky Way. This band is actually made up of many stars and is part of our Galaxy. People had seen galaxies outside our own for a long time too but were not sure of their nature. For example the Andromeda Galaxy can just be seen by naked eye on a dark night. Messier was a French scientist in 18th century who was interested in comets. Comets appear as little blurs in the sky when far away. They move over time. Messier kept finding other blurry objects that did not move - made a list of them so he could avoid them - called Messier objects. Star clusters (globular clusters) Planetary nebulae. Spiral nebulae = galaxies Slides were shown illustrating.
M57 globular cluster M5 globular cluster N6C6397 globular cluster M31 galaxy M65 galaxy M66 galaxy M33 galaxy M51 galaxy We've only gained a proper appreciation for our Galaxy in the last 100 years. Today we will start to tell this story but first we need to talk about distance measurements. There are 3 units astronomers often use to describe distances. light year ly - unit of distance NOT time. Distance light travels in one year 9.46 x1015 meters. Parsec - unit useful for people who measure parallax - 3.26 light years 3.08x1016 meters. Dpc=1/parallax (in arcsec) Kiloparsec - 1000 parsecs - 3.08x1019 meters. We are 8.5 Kpc from galactic center. And a general principle you need to know - call it the 2 of 3 principle. If you know 2 of the following things 3 you can get the other. 1 - luminosity 2 - apparent brightness 3 - distance Consider driving a car at night. There is a red stoplight up ahead - how do we know when to stop the car? have an idea of luminosity and watch brightness. Like the inverse square law for apparent brightness. Have an equation that relates these 3 things F=L/(4pi d2) Distance measurement methods - much of astronomy is dedicated to finding better ways to measure distances. parallax - works out to about 100 light years if you exploit the orbit of the Earth about the sun spectroscopic parallax - suppose we take a spectrum of a star and determine its type OBAFGKM. If we know that the star is on main sequence from spectral type we can get luminosity from HR diagram. Also can measure apparent brightness. Use (1) luminosity and (2) apparent
brightness and get the distance. Works out to a few thousand pc. At larger distances it's hard to get spectra and colors. Note above we have assumed star is main sequence - what if this were wrong? would get distance wrong. Astronomers have developed clever ways to test if a star is on main sequence. Use broadening of absorption lines in star spectrum. broad - main sequence. giant - narrower supergiant - narrowest Also can use stars not on main sequence other distance determination methods use the fact that some stars in our galaxy have regular pulsations. Certain stars when leaving the main sequence develop small instabilities that cause them to pulsate - not pulsars. Normal fusion powered stars that just have a small instability inside. Pulsations are due to the fact that size and temperature of the star expands and then falls over and over again. Cepheid variables - a type of pulsating star (high mass) 1-60 day periods. Henrietta Lenvitt discovered in 1912 that longer period Cepheid variables are more luminous than shorter period variables. Period - luminosity relation. if you know the period by watching over time you can get luminosity.... then again use 2 of 3 principle to get the distance. Good Method. Cepheids are high luminosity (100-10000 times more than the sun) - can use Cepheids to get distances throughout our Galaxy and even to nearby galaxies. Only disadvantage is that they are fairly rare. Also RR Lyrae - another type of pulsating star. Periods about .5-1 day. A bit less than 100 L(.) - all have same luminosity. Common so while can't be used as far away as Cepheids they are very useful.