Which of the Following Properties Do You Think We Can Infer Simply by Looking at the Spectrum

Learning Objectives

By the end of this section, you will exist able to:

• Understand how astronomers tin can learn virtually a star's radius and limerick past studying its spectrum
• Explain how astronomers can measure the motion and rotation of a star using the Doppler event
• Describe the proper move of a star and how information technology relates to a star's space velocity

Analyzing the spectrum of a star tin teach u.s. all kinds of things in addition to its temperature. We tin mensurate its detailed chemic composition every bit well equally the pressure level in its atmosphere. From the pressure, we get clues near its size. We tin also mensurate its motion toward or abroad from usa and guess its rotation.

Clues to the Size of a Star

As we shall see in The Stars: A Angelic Census, stars come up in a wide multifariousness of sizes. At some periods in their lives, stars can expand to enormous dimensions. Stars of such exaggerated size are called giants. Luckily for the astronomer, stellar spectra can be used to distinguish giants from run-of-the-mill stars (such as our Sun).

Suppose you want to decide whether a star is a behemothic. A giant star has a big, extended photosphere. Because it is and then large, a giant star'south atoms are spread over a great volume, which means that the density of particles in the star'south photosphere is depression. Equally a result, the pressure in a giant star'south photosphere is also low. This depression force per unit area affects the spectrum in ii ways. Showtime, a star with a lower-pressure photosphere shows narrower spectral lines than a star of the aforementioned temperature with a college-force per unit area photosphere (Effigy 1). The difference is big enough that careful study of spectra can tell which of two stars at the same temperature has a higher pressure level (and is thus more than compressed) and which has a lower pressure level (and thus must be extended). This effect is due to collisions between particles in the star's photosphere—more collisions lead to broader spectral lines. Collisions volition, of course, exist more frequent in a higher-density environment. Recall about information technology like traffic—collisions are much more probable during rush hr, when the density of cars is loftier.

2d, more atoms are ionized in a behemothic star than in a star similar the Sunday with the same temperature. The ionization of atoms in a star'southward outer layers is caused mainly by photons, and the corporeality of energy carried past photons is determined by temperature. But how long atoms stay ionized depends in function on pressure. Compared with what happens in the Sun (with its relatively dense photosphere), ionized atoms in a behemothic star'southward photosphere are less likely to laissez passer close enough to electrons to interact and combine with i or more of them, thereby becoming neutral again. Ionized atoms, as we discussed earlier, have dissimilar spectra from atoms that are neutral.

Effigy i: Spectral Lines. This figure illustrates one difference in the spectral lines from stars of the same temperature but different pressures. A behemothic star with a very-low-pressure photosphere shows very narrow spectral lines (bottom), whereas a smaller star with a higher-pressure level photosphere shows much broader spectral lines (top). (credit: modification of piece of work past NASA, ESA, A. Field, and J. Kalirai (STScI))

Abundances of the Elements

Absorption lines of a majority of the known chemical elements accept now been identified in the spectra of the Lord's day and stars. If nosotros see lines of iron in a star's spectrum, for example, then we know immediately that the star must contain atomic number 26.

Note that the absence of an element'south spectral lines does not necessarily mean that the element itself is absent. Equally we saw, the temperature and pressure level in a star'southward atmosphere will determine what types of atoms are able to produce assimilation lines. But if the physical weather condition in a star's photosphere are such that lines of an element should (according to calculations) be at that place can nosotros conclude that the absence of appreciable spectral lines implies depression abundance of the element.

Suppose ii stars have identical temperatures and pressures, just the lines of, say, sodium are stronger in one than in the other. Stronger lines mean that in that location are more atoms in the stellar photosphere arresting calorie-free. Therefore, nosotros know immediately that the star with stronger sodium lines contains more than sodium. Complex calculations are required to determine exactly how much more, simply those calculations tin be washed for any element observed in whatsoever star with any temperature and pressure level.

Of course, astronomy textbooks such as ours always make these things sound a chip easier than they really are. If yous await at the stellar spectra such as those in Figure 3 of The Spectra of Stars (and Dark-brown Dwarfs), you may get some feeling for how hard information technology is to decode all of the data contained in the thousands of absorption lines. Commencement of all, it has taken many years of careful laboratory work on Earth to decide the precise wavelengths at which hot gases of each element have their spectral lines. Long books and computer databases have been compiled to evidence the lines of each element that can be seen at each temperature. Second, stellar spectra unremarkably accept many lines from a number of elements, and nosotros must exist careful to sort them out correctly. Sometimes nature is unhelpful, and lines of different elements accept identical wavelengths, thereby adding to the confusion. And third, equally we saw in the chapter on Radiation and Spectra, the move of the star tin alter the observed wavelength of each of the lines. So, the observed wavelengths may not friction match laboratory measurements exactly. In practice, analyzing stellar spectra is a demanding, sometimes frustrating task that requires both training and skill.

Studies of stellar spectra have shown that hydrogen makes up about three-quarters of the mass of nearly stars. Helium is the second-about abundant element, making up well-nigh a quarter of a star's mass. Together, hydrogen and helium make upwards from 96 to 99% of the mass; in some stars, they amount to more 99.9%. Amongst the four% or less of "heavy elements," oxygen, carbon, neon, iron, nitrogen, silicon, magnesium, and sulfur are among the most arable. Mostly, but not invariably, the elements of lower diminutive weight are more abundant than those of higher atomic weight.

Take a careful look at the list of elements in the preceding paragraph. Two of the most abundant are hydrogen and oxygen (which make up water); add carbon and nitrogen and you are starting to write the prescription for the chemistry of an astronomy student. We are made of elements that are mutual in the universe—just mixed together in a far more sophisticated form (and a much cooler environment) than in a star.

Every bit we mentioned in The Spectra of Stars (and Brown Dwarfs) section, astronomers use the term "metals" to refer to all elements heavier than hydrogen and helium. The fraction of a star's mass that is composed of these elements is referred to equally the star's metallicity. The metallicity of the Sunday, for example, is 0.02, since 2% of the Sun'due south mass is made of elements heavier than helium.

The Chemical Elements lists how common each element is in the universe (compared to hydrogen); these estimates are based primarily on investigation of the Sun, which is a typical star. Some very rare elements, even so, take not been detected in the Sun. Estimates of the amounts of these elements in the universe are based on laboratory measurements of their affluence in primitive meteorites, which are considered representative of unaltered material condensed from the solar nebula (meet the Cosmic Samples and the Origin of the Solar System chapter).

Radial Velocity

When we measure the spectrum of a star, we make up one's mind the wavelength of each of its lines. If the star is not moving with respect to the Sun, then the wavelength corresponding to each chemical element volition be the same every bit those we measure in a laboratory here on Earth. But if stars are moving toward or abroad from united states of america, we must consider the Doppler effect (run into The Doppler Issue). We should encounter all the spectral lines of moving stars shifted toward the blood-red end of the spectrum if the star is moving away from u.s., or toward the blue (violet) stop if it is moving toward united states (Figure ii). The greater the shift, the faster the star is moving. Such motion, along the line of sight between the star and the observer, is called radial velocity and is commonly measured in kilometers per second.

Figure ii: Doppler-Shifted Stars. When the spectral lines of a moving star shift toward the red terminate of the spectrum, we know that the star is moving away from us. If they shift toward the blue cease, the star is moving toward us.

William Huggins, pioneering yet once more, in 1868 made the first radial velocity determination of a star. He observed the Doppler shift in 1 of the hydrogen lines in the spectrum of Sirius and found that this star is moving toward the solar system. Today, radial velocity can exist measured for any star brilliant enough for its spectrum to be observed. As nosotros volition encounter in The Stars: A Angelic Census, radial velocity measurements of double stars are crucial in deriving stellar masses.

Proper Motion

At that place is some other type of motion stars can have that cannot be detected with stellar spectra. Unlike radial motility, which is forth our line of sight (i.e., toward or abroad from World), this motion, called proper motion, is transverse: that is, beyond our line of sight. We see it as a change in the relative positions of the stars on the angelic sphere (Figure 3). These changes are very boring. Even the star with the largest proper motion takes 200 years to change its position in the sky by an corporeality equal to the width of the total Moon, and the motions of other stars are smaller nevertheless.

Figure iii: Big Proper Motility. Three photographs of Barnard's star, the star with the largest known proper motion, show how this faint star has moved over a period of 20 years. (modification of work past Steve Quirk)

For this reason, with our naked optics, we do not detect any modify in the positions of the vivid stars during the course of a human lifetime. If nosotros could live long plenty, still, the changes would become obvious. For case, some l,000 years from now, terrestrial observers volition notice the handle of the Big Dipper unmistakably more aptitude than it is now (Figure 4).

Effigy iv: Changes in the Big Dipper. This figure shows changes in the appearance of the Big Dipper due to proper motion of the stars over 100,000 years.

We measure the proper movement of a star in arcseconds (1/3600 of a degree) per yr. That is, the measurement of proper movement tells usa only by how much of an angle a star has changed its position on the celestial sphere. If ii stars at dissimilar distances are moving at the aforementioned velocity perpendicular to our line of sight, the closer one will show a larger shift in its position on the angelic sphere in a year's fourth dimension. As an analogy, imagine you are standing at the side of a freeway. Cars will appear to whiz past yous. If yous and so watch the traffic from a vantage point one-half a mile abroad, the cars will move much more slowly beyond your field of vision. In order to catechumen this athwart motility to a velocity, nosotros need to know how far away the star is.

To know the true space velocity of a star—that is, its total speed and the direction in which information technology is moving through space relative to the Sunday—we must know its radial velocity, proper move, and distance (Figure 5). A star's space velocity can besides, over time, cause its altitude from the Sun to alter significantly. Over several hundred thousand years, these changes can exist large plenty to affect the apparent brightnesses of nearby stars. Today, Sirius, in the constellation Canis Major (the Big Dog) is the brightest star in the sky, just 100,000 years ago, the star Canopus in the constellation Carina (the Keel) was the brightest one. A little over 200,000 years from now, Sirius will accept moved away and faded somewhat, and Vega, the bright bluish star in Lyra, volition take over its identify of award equally the brightest star in Earth'due south skies.

Figure 5: Space Velocity and Proper Motion. This effigy shows the true space velocity of a star. The radial velocity is the component of the space velocity projected forth the line of sight from the Lord's day to a star. The transverse velocity is a component of the infinite velocity projected on the heaven. What astronomers measure is proper move (μ), which is the change in the apparent direction on the sky measured in fractions of a degree. To convert this alter in direction to a speed in, say, kilometers per 2nd, it is necessary to also know the altitude (d) from the Sun to the star.

Rotation

We tin as well utilise the Doppler effect to measure how fast a star rotates. If an object is rotating, and then one of its sides is budgeted us while the other is receding (unless its axis of rotation happens to be pointed exactly toward u.s.). This is clearly the example for the Sun or a planet; we can discover the low-cal from either the approaching or receding edge of these nearby objects and direct mensurate the Doppler shifts that ascend from the rotation.

Stars, however, are so far away that they all appear as unresolved points. The best we can exercise is to analyze the light from the entire star at once. Due to the Doppler upshot, the lines in the light that come from the side of the star rotating toward u.s. are shifted to shorter wavelengths and the lines in the light from the opposite border of the star are shifted to longer wavelengths. You tin recollect of each spectral line that we observe equally the sum or blended of spectral lines originating from different speeds with respect to us. Each point on the star has its ain Doppler shift, so the absorption line nosotros meet from the whole star is actually much wider than it would exist if the star were not rotating. If a star is rotating rapidly, there will exist a greater spread of Doppler shifts and all its spectral lines should exist quite broad. In fact, astronomers call this outcome line broadening, and the amount of broadening tin tell usa the speed at which the star rotates (Figure six).

Figure half dozen: Using a Spectrum to Determine Stellar Rotation. A rotating star will show broader spectral lines than a nonrotating star.

Measurements of the widths of spectral lines show that many stars rotate faster than the Sun, some with periods of less than a day! These rapid rotators spin then fast that their shapes are "flattened" into what we telephone call oblate spheroids. An example of this is the star Vega, which rotates once every 12.v hours. Vega's rotation flattens its shape and so much that its diameter at the equator is 23% wider than its bore at the poles (Effigy vii). The Dominicus, with its rotation period of about a month, rotates rather slowly. Studies have shown that stars subtract their rotational speed as they historic period. Young stars rotate very quickly, with rotational periods of days or less. Very erstwhile stars tin can have rotation periods of several months.

Figure 7: Comparison of Rotating Stars. This illustration compares the more rapidly rotating star Altair to the slower rotating Sun.

As you tin can see, spectroscopy is an extremely powerful technique that helps united states learn all kinds of information virtually stars that we merely could not gather any other way. Nosotros will see in later chapters that these aforementioned techniques can also teach us almost galaxies, which are the most distant objects that can nosotros observe. Without spectroscopy, we would know next to cipher near the universe beyond the solar system.

Astronomy and Philanthropy

Throughout the history of astronomy, contributions from wealthy patrons of the scientific discipline have made an enormous difference in building new instruments and conveying out long-term research projects. Edward Pickering'south stellar classification projection, which was to stretch over several decades, was made possible by major donations from Anna Draper. She was the widow of Henry Draper, a dr. who was one of the almost accomplished apprentice astronomers of the nineteenth century and the get-go person to successfully photograph the spectrum of a star. Anna Draper gave several hundred thousand dollars to Harvard Observatory. As a result, the great spectroscopic survey is still known as the Henry Draper Memorial, and many stars are all the same referred to past their "HD" numbers in that catalog (such as HD 209458).

In the 1870s, the eccentric piano builder and existent estate magnate James Lick (Figure 8) decided to leave some of his fortune to build the world's largest telescope. When, in 1887, the pier to house the telescope was finished, Lick's body was entombed in it. Atop the foundation rose a 36-inch refractor, which for many years was the primary instrument at the Lick Observatory near San Jose.

Effigy viii: Henry Draper (1837–1882) and James Lick (1796–1876). (a) Draper stands adjacent to a telescope used for photography. After his death, his widow funded further astronomy piece of work in his name. (b) Lick was a philanthropist who provided funds to build a 36-inch refractor non simply as a memorial to himself simply also to aid in further astronomical inquiry.

The Lick telescope remained the largest in the world until 1897, when George Ellery Unhurt persuaded railroad millionaire Charles Yerkes to finance the structure of a 40-inch telescope nearly Chicago. More recently, Howard Keck, whose family fabricated its fortune in the oil industry, gave $70 million from his family foundation to the California Institute of Technology to assistance build the world'south largest telescope atop the 14,000-foot top of Mauna Kea in Hawaii (see the chapter on Astronomical Instruments to learn more about these telescopes). The Keck Foundation was and then pleased with what is now called the Keck telescope that they gave $74 million more to build Keck II, another 10-meter reflector on the aforementioned volcanic elevation.

Now, if any of you become millionaires or billionaires, and astronomy has sparked your involvement, practice keep an astronomical instrument or projection in mind every bit yous plan your manor. Only frankly, individual philanthropy could not possibly support the total enterprise of scientific enquiry in astronomy. Much of our exploration of the universe is financed past federal agencies such as the National Science Foundation and NASA in the United states of america, and by like government agencies in the other countries. In this mode, all of usa, through a very small share of our tax dollars, are philanthropists for astronomy.

Key concepts and summary

Spectra of stars of the same temperature but dissimilar atmospheric pressures accept subtle differences, so spectra tin can be used to make up one's mind whether a star has a large radius and low atmospheric pressure (a behemothic star) or a small radius and high atmospheric pressure level. Stellar spectra can also be used to decide the chemic composition of stars; hydrogen and helium make up well-nigh of the mass of all stars. Measurements of line shifts produced by the Doppler effect bespeak the radial velocity of a star. Broadening of spectral lines past the Doppler effect is a measure of rotational velocity. A star can also show proper motion, due to the component of a star's space velocity across the line of sight.

Glossary

giant: a star of exaggerated size with a large, extended photosphere

proper motion: the angular change per year in the direction of a star as seen from the Sun

radial velocity: move toward or abroad from the observer; the component of relative velocity that lies in the line of sight

space velocity: the total (three-dimensional) speed and direction with which an object is moving through space relative to the Sun

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