Chapter 9: Measuring the Stars About 10 11 (100,000,000,000) stars in a galaxy; also about 10 11 galaxies in the universe Stars have various major characteristics, the majority of which fall into several simple types. These are related to their stages of the life cycles, just like people. The INITIAL MASS is a very important aspect that determines a star s future:
Life Sequence of a Star Life Cycles of the Hot and Massive Young stellar objects (YSOs) The normal life cycle of stars with mass about that of the Sun is as follows: 1) Gas and dust in a cool nebula condense, forming a young stellar object (YSO) 2) Shrinking, the YSO dispels its remaining birth cloud, and its hydrogen fire ignites 3) As the hydrogen burns steadily, the star joins the main sequence. 4) When the star uses up all of the hydrogen in its core, the hydrogen in the shell (a larger region around the core) ignites. 5) The energy released by the burning of the hydrogen shell makes the star brighter and it expands, which makes the surface larger, cooler, and redder. The star has become a red giant. 6) Stellar winds blowing off the star gradually expel its outer layers, which form planetary nebula around the remaining hot stellar core. 7) The nebula expands and dissipates into space, leaving just the hot core. 8) The core, now a white dwarf star, cools and fades forever. Note that after most H is converted to He in the core, the star becomes a Red Giant. If the star is much more massive, it burns elements in the core all the way up to Iron (Fe), and becomes a super Red Giant, then explodes as a SUPERNOVA, and the inner region forms a neutron star or black hole. The Sun will last about 10 billion years, but a much more massive star may last only a few million years after its birth. Less massive starts may be red dwarfs, and remain so for as very long time. RULE: the bigger (more massive) the star, the shorter is its lifetime.
Part II: Chapter 9 This chapter is heavy on the various methods used to measure cosmic distances. This has been a major and constant theme in astronomy, and slowly a great variety of methods have ben found to extend accurate measurements to objects at ever greater distances (see illustration below).
PARALAX: The oldest method is parallax which has already been mentioned in past chapters. This is how humans have a since of distance: two eyes separated by several centimeters. Our brain gets information from both eyes, and using tis separation of the eyes and the different respective from both eyes gets an estimate of how far away objects are located. All done by the brain automatically and subconsciously. Inverse Square Law: As light spreads out from a source its intensity decreases according to the inverse-square law, 1/r 2. Thus when we observe and object in the night sky there are: the apparent magnitude and the absolute magnitude The apparent magnitude is just what we measure, where as the absolute magnitude is the intensity that the star is actually emitting. To know this, we must know the distance to the star. Ancient Greeks categorized the brightest objects as magnitude 1, the next group as magnitude 2 etc. BUT this was by the naked eye, and thus not very sensitive to measuring the actual amount of light being received. Based on good energy measurements to days, and still using the old system: Sun, -26.7; full moon, -12.5; Venus, -4.4; Sirius, -1.5; Alpha Centauri, 0; Polaris, 2.5; naked-eye limit, 6; binocular limit, 10; 1-meter telescope, 18-20; Hubble, Keck, 30) See book for Table (p. 153).
STELLAR TEMPERATURE: It is also possible to record the spectra of a star, and from this to estimate the absolute magnitude at the surface. By knowing BOTH th absolute magnitude and relative magnetite it is possible to calculate the distance. Only the LARGEST objects that are not too far away an be measured by parallax, like their size.
Hertzsprung-Russel (HR) diagram: Estimating stellar mass by observing interaction with some nearby object:
Diagramming Star Color, Brightness, Mass Based on OBSERVATIONS can make a graph: Vertical axis = Luminosity, amount of energy (can also use the Magnitude system) Horizontal axis = Star spectral type (COLOR; = temperature) Called Color-Luminosity diagram or Hertzsprung-Russell (HR) Diagram The Absolute Magnitude was derived from the ancient system of Greek astronomer Hipparchus (190-120 B.C.). He set the brightest star in the night sky as 1, and then next as 2, and so on-----6. Later when it became possible to measure more accurately, some stars set at 0, and even negative numbers. Today the Absolute Magnitude is defined as the Apparent Magnitude if that star were located at 32.6 light-years from Earth. In this system the Sun is 4.8. Spectral type: What color is my star? The spectral type represents the spectrum of a star. Based on blackbody curves (below), can get information about the star temperature Early designated as (OBAFGKM = Oh Be A Fine Girl, Kiss Me) O is hottest and M is coolest
A black body is a perfect absorber and emitter of light. One very rood example is the pupil of your eye, a window of light into your brain. Evolution found that this black spot is excellent for getting the most light inside. * A heated body also emits radiation with a black body CONTINUOUS distribution of wavelengths based on temperature. Below illustrates various stars, their color, and blackbody curve.
Star light, star bright: Classifying luminosity Each spectral class has subdivisions (not important for us) V is the luminosity class; Sun is designated as V (5) ; Roman numeral Super-giants are I and II, and giants are III The brighter they burn, the bigger they swell: Mass determines class GREATER MASS means a hotter star with faster core burning Two factors determine a star s brightness: temperature and surface area Brighter star has more surface area Also, the hotter the star, the more energy is released per area of surface (E = T 4 ) Most stars are located on the diagonal band, called Main Sequence There is also a RED GIANT SEQUENCE in the upper right Supergiant; blue supergiant sequence also, above and to left Bottom left is white dwarf cluster or sequence; these dwarfs move to the right as the cool, become dimmer and change to emit longer wavelengths
Spectra of stars After light is emitted from the photosphere with a blackbody spectra associated with a certain temperature, it passes through the outer atmosphere of the star, which can be hundreds of thousands of kilometers thick. While propagating through this region it interacts with the various atoms existing there. Each type of atom (hydrogen, helium, carbon, cesium etc. etc.) has very specific energy levels. Those wavelengths that match these energy levels will be absorbed, while causing electrons to jump to higher levels. This action produced absorption spectra, which can be seen as black vertical absorption lines on the continuous blackbody spectra. Hotter stars show strong Hydrogen lines, whereas cooler stars have strong metallic lines.
Eternal Partners: Binary and Multiple Stars If two or more stars orbit a common center of mass, this system is called binary stars or multiple stars, respectively. Binary stars and Doppler Effect About ½ of all stars are binary Can observe by the Doppler Shift Two stars are binary, but three s a crowd: Multiple stars Sometimes two stars APPEAR to be near each other, but are actually far apart and unrelated. Three stars are unstable; so might be a true binary associated with a third star.
Change IS Good: Variable Stars Some stars change in brightness over time Pulsating stars Flare stars Exploding stars Going the distance: Pulsating stars Cepheid variables: period & luminosity related; greater period = greater average brightness; can measure the apparent brightness with time, get the period, and then determine the actual brightness. Distance proportional to 1/r 2 Can be used to look at starts and get distances in far away galaxies
Among the major topics in astronomy, and one that has been hugely controversial throughout the history of this field, is how to accurately determine distances to the great variety of objects. Over much time a surprisingly large variety of techniques have come to be understand that cover vastly different distant scales. Below is an illustration of the major techniques now available to astronomers:
The name parsec is "an abbreviated form of 'a distance corresponding to a parallax of one arcsecond'." [1] It was coined in 1913 at the suggestion of British astronomer Herbert Hall Turner. A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond ( 1 3,600 of a degree). In other words, imagine three straight lines forming a right triangle between the Earth, the Sun and a distant object, as follows: line 1 connects the Earth and the Sun, line 2, perpendicular to the first line, connects the Sun and the object, and line 3 connects the object to the Earth. Now, if the angle at the object between lines 2 and 3 is exactly one arcsecond, then the object's distance from the Sun would be exactly one parsec.