CHAPTER 12
Measuring the Properties of Stars
CHAPTER OUTLINE
12-1 Stellar Luminosity
1. When we discuss a star’s brightness we must be careful to distinguish between how bright the star looks to us from how bright it really is.
2. Two stars may differ in brightness due to any combination of the following:
(a) One star is inherently brighter than the other (it has a greater luminosity).
(b) One star is much closer to us than the other, making it appear brighter.
(c) There is more interstellar material absorbing light from one star than from the other.
3. When we refer to a star’s brightness,we will refer to its apparent brightness as seen from Earth. When we refer to a star’s luminosity, we will refer to the star’s inherent brightness (the rate at which it emits energy).
Apparent Magnitude
1. Hipparchus created the first star catalog with corresponding brightness determinations (apparent magnitudes) in the second century B.C.
2. Apparent magnitude is a measure of the amount of light received from a celestial object. Hipparchus assigned an apparent magnitude of 1 for the brightest stars and 6 for the dimmest.
3. The modern magnitude scale is set up so that a 5-magnitude difference corresponds to a ratio of 100 in the amount of light received. Also, a one magnitude difference corresponds to a ratio of 1001/5 2.512 in the amount of light received.
4. Modern measuring devices allow astronomers to determine magnitudes to an accuracy of 0.001 or better.
5. A few stars (e.g., Sirius) are so bright that they have negative magnitudes. Sirius’s apparent magnitude is –1.43 (it is about 10 times brighter than an average first-magnitude star).
6. Modern, large telescopes equipped with CCD devices can image objects as dim as 25th magnitude or better.
12-2 Measuring Distances to Stars
1. The absence of observable stellar parallax was an argument against the heliocentric system. Stellar parallaxes were finally observed in 1838.
2. Parallax angle is half the maximum angle that a star appears to be displaced due to the Earth’s motion around the Sun.The maximum parallax angle of the nearest star is only 0.76 arcseconds.
3. A parsec is the distance from the Sun to an object that has a parallax angle of one arcsecond. One parsec corresponds to 3.26 ly or 206,265 AU.
4. Parallax distance formula: Distance to star (in parsecs) = 1/parallax angle (in arcseconds).
5. Only stars within about 120 parsecs (400 light-years) have parallax angles great enough to allow accurate calculations of their distances from Earth.
6. The satellite Hipparcos measured positions and parallaxes to an accuracy of 0.001 arcsecond. Its successor, Gaia (to be launched in 2013), will give us precise information for the billion brightest objects in the sky and create a 3-D map of the Milky Way.
7. Accurate stellar distances help to determine other quantities about celestial objects. They also help to determine the distance scale of the universe more accurately.
Absolute Magnitude
1. if we know any two of the following three factors, we can calculate the third: apparent brightness (or, in astronomy, apparent magnitude), luminosity, and distance
2. The intrinsic luminosity of a star is usually given as its absolute magnitude, which is defined as the apparent magnitude a star would have if it were at a distance of 10 parsecs.
Tools of Astronomy: Calculating Absolute Magnitude
1. The difference between a star’s apparent (m) and absolute (M) magnitudes is called the distance modulus: mM = 5 log(d) 5, where d is the star’s distance from us in parsecs.
2. Sirius’ apparent brightness (–1.43) is due to its closeness (2.6 parsecs from Earth). Its absolute magnitude is +1.47.
Tools of Astronomy: Naming Stars
1. Most of the names of stars are of Arabic origin.
2. Only the brightest stars have popular names. The brightest stars in each constellation are given a Greek letter according to their brightness. Dimmer stars are given English letters followed by their constellation names. Less distinguished stars don’t have popular names and don’t have letter designations.
3. Only the International Astronomical Union has the authority to assign designations to celestial bodies and any surface features on them; thus, do not be fooled by companies that let you pick a name for a celestial object for a fee.
12-3 Motions of Stars
1. In 1718 Halley discovered that stars do move with respect to one another. Thus, constellations do gradually change shapes.
2. A star’s proper motion is expressed as the angle through which a star moves each year—its angular velocity—as seen from the Sun.
3. Barnard’s star, the second closest star to the Sun, exhibits the greatest proper motion of any star—10.3 arcseconds/year.
4. A star’s tangential velocity—speed across our line of sight—can be calculated using its proper motion and distance. This is difficult to determine for stars. It is easier to detect a star’s radial velocity —its velocity toward or away from Earth, using the Doppler effect.
5. A star’s space velocity relative to the Sun corresponds to its actual motion relative to the Sun and combines its radial and tangential velocities.
12-4 Spectral Types
1. A star’s color is determined by its temperature. A star’s blackbody curve can be used to determine its temperature.
2. A star’s absorption spectrum—the absorption of radiation at various wavelengths—can also be used to determine its temperature.
3. Annie Jump Cannon devised a system for classifying stellar spectra and applied this system to several hundred thousand stars. The different spectral classes were arranged alphabetically, based on the strength of the hydrogen lines.
4. The work of Cecelia Payne-Gaposchkin resulted in a reclassification based on temperature. The spectral classes used today (from hottest to coolest) are designated as O B A F G K M.
5. The temperature range of O stars is 30,000– 60,000 K. The coolest stars are M stars with temperatures less than 3500 K.
6. Within each spectral class, stars are subdivided into 10 categories (spectral types) by number. The Sun is listed as a G2 star.
The Hertzsprung-Russell Diagram
1. The Hertzsprung-Russell (H-R) diagram is a plot of luminosity (absolute magnitude) versus temperature (or spectral class) for stars.
2. About 90% of all stars fall into a group running diagonally across the diagram; this diagonal band is called the main sequence.
3. Stars on the H-R diagram fall into categories such as main sequence stars, white dwarfs, giants, and supergiants.
Spectroscopic Parallax
1. Spectroscopic parallax is the method of measuring the distance to a star by comparing its absolute magnitude to its apparent magnitude.
2. The H-R diagram can be used to determine the absolute magnitude of a star if we know its temperature and category.
3. Because of limits in the technique, spectroscopic parallax is most useful when applied to groups of stars that are nearly the same distance from Earth.
Luminosity Classes
1. In the 1880s Antonia Maury discovered that absorption lines are subject to a smearing effect, which has become valuable in classifying stars.
2. The greater the density in a star’s atmosphere, the more frequent the collisions among atoms, the broader the corresponding absorption lines.
3. The extent of line broadening allows us to classify stars in different luminosity classes, denoted by roman numerals I, II, III, IV and V; supergiants (I), bright giants (II), giants (III), subgiants (IV), and main sequence stars (V).
4. The classification of stars in luminosity classes allows spectroscopic parallax to be used with any star.
Analyzing the Spectroscopic Parallax Procedure
1. Knowing the luminosity class of a star and its temperature allows us to determine its absolute magnitude (and thus luminosity). Knowing its absolute magnitude and its apparent magnitude allows us to find the star’s distance.
2. This procedure can be used to find distances to stars that are too far for parallax measurements.
3. The basic procedure outlined here is often used in science. By observing familiar objects (nearby stars in this case), we see patterns that are then applied to other objects of the same type (more distant stars in this case).
Luminosity and the Sizes of Stars
1. White dwarfs are hot but dim stars because they are small. They may be 0.01 the Sun’s size.
2. A giant star is one of great luminosity and radius (10 to 100 times the Sun’s radius). A supergiant is a star of very great luminosity and radius (more than 100 times the Sun’s radius).
3. The sizes of a few very large stars have been measured directly by interferometry.
4. Knowing the temperature of a star gives us its energy emitted per square meter. Knowing the total energy emitted (from the absolute magnitude or luminosity) we can then calculate the surface area of the star.From that the diameter of the star can be determined.
Tools of Astronomy: Determining the Spectral Type of a Star
1. A hydrogen atom near a cool star has most of its electrons in the ground state. When this atom absorbs a photon, the photon’s energy is such that the electron moves from the ground state to one of the other energy levels. These lines are in the ultraviolet region of the spectrum and don’t appear in the visible part of the spectrum.
2. A hydrogen atom near a hotter star may have electrons in the second energy level. For this atom to absorb a photon, that photon must be of the right energy to move the electron from level 2 to higher levels. These photons are part of the Balmer series and are in the visible range.
3. A complete analysis would require examining a number of lines of several different chemical elements. The spectral type of a star is determined by the relative intensities of these lines.
12-5 Multiple Star Systems
1. More than half of what appear as single stars are, in fact, multiple star systems.
2. Optical doubles are two stars that have small angular separation as seen from Earth but are not gravitationally linked.
3. A binary star system is a system of two stars that are gravitationally linked so that they orbit their common center of mass.
4. In 1802 Herschel obtained the first evidence that some double stars orbit one another.
5. Binary star systems are classified into several categories according to how they are detected.
Visual Binaries
1. A visual binary is an orbiting pair of stars that can be resolved (normally with a telescope) as two stars.
2. If one uses large telescopes, about 10% of the stars in the sky are visual binaries.
3. Visual binaries can be confirmed by observing the system over time and looking for signs of revolution.
Spectroscopic Binaries
1. A spectroscopic binary is an orbiting pair of stars that can be distinguished as two due to the changing Doppler shifts in their spectra.
Eclipsing Binaries
1. Algol, discovered by John Goodricke in 1783, is an eclipsing binary, in which one star moves in front of the other as viewed from Earth.
2. Algol’s light curve—a graph of the numerical measure of the light received from a star versus time—shows peaks and dips that indicate an unseen companion.
Other Binary Classifications
1. An astrometric binary is an orbiting pair of stars in which the motion of one of the stars reveals the presence of the other.
2. A composite spectrum binary is a binary star system with stars having spectra different enough to distinguish them from one another.
12-6 Stellar Masses and Sizes from Binary Star Data
1. Binary stars are important because they allow us to measure stellar massesusing Kepler’s third law as modified by Newton.
2. Knowledge of the size of one of the star’s ellipses, along with knowledge of the period of its motion, permits calculation of the total mass of the two stars.
3. To determine how the system’s total mass is distributed between the two stars, we need only consider the ratio of the two stars’ distances to the center of mass.
4. Because the inclinations of spectroscopic binary orbits are usually not known, exact mass calculations cannot be done. However, assuming an average inclination we can obtain valuable information about average masses of spectroscopic binary stars.
5. Eclipsing binaries that are also spectroscopic binaries provide us with a way of measuring not only the masses of the two stars but also their sizes.We derive this information using measurements of their Doppler shifts.
12-7 The Mass-Luminosity Relationship
1. The mass-luminosity diagram shows the relationship between the luminosity and mass of main-sequence stars. More massive stars are more luminous (luminosity mass3.5).
2. The mass-luminosity relationship is valuable in investigating less accessible stars and in constructing and evaluating hypotheses on the life cycle of stars.
12-8 Cepheid Variables as Distance Indicators
1. In 1784, John Goodricke discovered that the star Cephei varies in luminosity in a regular way. Soon after, other stars were seen that exhibit delta Cephei’s characteristic light curve—rapid brightening followed by slow dimming.
2. Doppler-effect data show that these stars are pulsating in rhythm with their changes in luminosity.
3. It is fairly easy to identify this class of stars, now named Cepheid variables or Cepheids.Each Cepheid has a very constant period of variation, ranging from about 1 day to about 3 months for different Cepheids.
4. In 1908 Henrietta Leavitt discovered that for Cepheids in the Magellanic Clouds, the brighter variables have the longer periods. Even though the stars in these clouds do not have the same intrinsic luminosity, their apparent magnitudes are directly related to their absolute magnitudes because they are all at about the same distance from us.
5. Cepheids are important because a Cepheid’s period, which is easy to determine, allows us to determine its absolute magnitude. This in turn allows us to find its distance by using the distance modulus.
6. Cepheids could be valuable distance indicators if the distance to one could be determined. None are close enough to be measured by parallax, but beginning in 1917, Shapley worked out a complex statistical method to determine distances to Cepheids in our Galaxy.
7. Shapley’s work led to a period-luminosity diagram for Cepheid variables.
8. It took until the late 1950s to determine the correct relationship between Cepheids’ periods and absolute magnitudes accurately because there are actually two different types of Cepheid variables. Cepheid I stars are brighter than Cepheid II stars (for the same period) by about a factor of 4 (or 1.5 magnitudes). Cepheid I stars are younger and metal richer than Cepheid II stars.
9. Another group of variables, the RR Lyrae stars, are also used as distance candles. Their periods range from a few hours to one day and their luminosity is approximately constant. They are less luminous than the Cepheids and thus can only be seen to smaller distances.
10. Though only a few thousand Cepheids have been detected, they are extremely useful in giving us a method of measuring distances not only to distant parts in our Galaxy but also to faraway galaxies.
Historical Note: Henrietta Leavitt
1. Henrietta Swan Leavitt began work at the Harvard College Observatory in Cambridge, Massachusetts in 1895. At that time it was unusual for women to work outside the home.
2. Leavitt’s primary work was with variable stars, and she discovered many of them. In 1908 she published a list of 1777 variable stars in the Magellanic Clouds; the list included a table of 16 Cepheid variables on which she had very precise data.