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What Tool Do Astronomers Use To Figure Out A Stars Chemical Makeup

Written By Thursday, April 21, 2022 Add Comment Edit

Learning Objectives

By the end of this section, you lot volition be able to:

  • Understand how astronomers can learn about a star'due south radius and composition past studying its spectrum
  • Explain how astronomers can measure the movement and rotation of a star using the Doppler effect
  • Draw the proper motion of a star and how it relates to a star'south infinite velocity

Analyzing the spectrum of a star can teach the states all kinds of things in addition to its temperature. We can mensurate its detailed chemic limerick equally well as the pressure level in its atmosphere. From the pressure, we get clues about its size. We can also measure its motion toward or away from united states and judge its rotation.

Clues to the Size of a Star

As we shall see in The Stars: A Celestial Census, stars come up in a broad variety of sizes. At some periods in their lives, stars can aggrandize 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 every bit our Sun).

Suppose you want to make up one's mind whether a star is a giant. A giant star has a big, extended photosphere. Because it is so big, a giant star's atoms are spread over a great book, which ways that the density of particles in the star's photosphere is depression. As a effect, the pressure in a behemothic star's photosphere is also low. This low pressure affects the spectrum in two ways. Beginning, a star with a lower-pressure photosphere shows narrower spectral lines than a star of the same temperature with a higher-pressure photosphere (Figure 1). The divergence is big plenty that conscientious study of spectra tin can tell which of two stars at the same temperature has a higher pressure (and is thus more than compressed) and which has a lower force per unit area (and thus must be extended). This result is due to collisions between particles in the star's photosphere—more collisions lead to broader spectral lines. Collisions will, of course, be more frequent in a higher-density environment. Think about it similar traffic—collisions are much more likely during rush hour, when the density of cars is high.

Second, more than atoms are ionized in a behemothic star than in a star like the Sun with the same temperature. The ionization of atoms in a star'due south outer layers is caused mainly by photons, and the amount of free energy carried by photons is determined by temperature. Merely how long atoms stay ionized depends in part on pressure. Compared with what happens in the Lord's day (with its relatively dense photosphere), ionized atoms in a behemothic star's photosphere are less probable to pass close enough to electrons to collaborate and combine with 1 or more of them, thereby condign neutral again. Ionized atoms, as nosotros discussed earlier, take unlike spectra from atoms that are neutral.

Illustration showing the difference between spectra of stars at the same temperature but different pressures. At top left is a small white dot representing a white dwarf star. To its right is its spectrum, with a wavelength scale in nanometers (nm) running from 300 nm on the left to 800 nm on the right. Crossing the white dwarf spectrum are very broad, fuzzy vertical black absorption lines, which remove a great deal of light from the band of color. At bottom left is shown the partial disk of a blue giant, vastly larger than the white dot representing the white dwarf. Its spectrum, shown to the same scale, has very narrow and very sharp vertical black absorption lines. The blue giant lines are much narrower than the broad, fuzzy lines of the white dwarf.

Figure 1: Spectral Lines. This figure illustrates one difference in the spectral lines from stars of the same temperature but unlike pressures. A behemothic star with a very-low-force per unit area photosphere shows very narrow spectral lines (bottom), whereas a smaller star with a higher-pressure photosphere shows much broader spectral lines (top). (credit: modification of work by NASA, ESA, A. Field, and J. Kalirai (STScI))

Abundances of the Elements

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

Notation that the absence of an element'southward spectral lines does not necessarily mean that the element itself is absent-minded. As we saw, the temperature and pressure in a star's atmosphere will decide what types of atoms are able to produce absorption lines. Only if the physical weather condition in a star'southward photosphere are such that lines of an chemical element should (co-ordinate to calculations) be at that place can nosotros conclude that the absence of observable spectral lines implies depression abundance of the element.

Suppose 2 stars take identical temperatures and pressures, simply the lines of, say, sodium are stronger in one than in the other. Stronger lines mean that there are more atoms in the stellar photosphere absorbing calorie-free. Therefore, we know immediately that the star with stronger sodium lines contains more sodium. Complex calculations are required to determine exactly how much more, just those calculations tin can be done for whatever element observed in any star with any temperature and pressure.

Of course, astronomy textbooks such as ours ever make these things audio a flake easier than they really are. If you await at the stellar spectra such as those in Figure 3 of The Spectra of Stars (and Brown Dwarfs), you may get some feeling for how difficult it is to decode all of the information independent in the thousands of absorption lines. First of all, information technology has taken many years of careful laboratory piece of work on Earth to determine the precise wavelengths at which hot gases of each element have their spectral lines. Long books and figurer databases have been compiled to show the lines of each element that can exist seen at each temperature. Second, stellar spectra unremarkably take many lines from a number of elements, and we must be conscientious to sort them out correctly. Sometimes nature is unhelpful, and lines of different elements have identical wavelengths, thereby adding to the confusion. And third, equally nosotros saw in the chapter on Radiation and Spectra, the motion of the star can change the observed wavelength of each of the lines. So, the observed wavelengths may non match laboratory measurements exactly. In practice, analyzing stellar spectra is a enervating, sometimes frustrating task that requires both training and skill.

Studies of stellar spectra have shown that hydrogen makes up about iii-quarters of the mass of about stars. Helium is the 2nd-most abundant chemical element, making up almost 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%. Among the 4% 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 atomic weight are more arable 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 upwards water); add together carbon and nitrogen and you are starting to write the prescription for the chemistry of an astronomy educatee. Nosotros are made of elements that are common in the universe—just mixed together in a far more sophisticated course (and a much libation surround) than in a star.

As 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 every bit the star'south metallicity. The metallicity of the Sun, for instance, is 0.02, since two% of the Sun's 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 Lord's day, which is a typical star. Some very rare elements, nevertheless, have non been detected in the Sun. Estimates of the amounts of these elements in the universe are based on laboratory measurements of their abundance in primitive meteorites, which are considered representative of unaltered material condensed from the solar nebula (run across the Cosmic Samples and the Origin of the Solar Organization chapter).

Radial Velocity

When we measure the spectrum of a star, we determine the wavelength of each of its lines. If the star is not moving with respect to the Dominicus, then the wavelength corresponding to each chemical element volition be the aforementioned equally those nosotros mensurate in a laboratory here on Earth. But if stars are moving toward or away from u.s.a., we must consider the Doppler effect (see The Doppler Effect). We should run across all the spectral lines of moving stars shifted toward the red cease of the spectrum if the star is moving away from us, or toward the blue (violet) end if information technology is moving toward us (Effigy 2). The greater the shift, the faster the star is moving. Such motion, forth the line of sight betwixt the star and the observer, is called radial velocity and is usually measured in kilometers per 2nd.

Diagram illustrating the Doppler Shift. At bottom is the wavelength scale in nanometers (nm), starting at 400 nm on the left and progressing to 750 nm at right. Above the scale are three spectra, one above the other. The spectrum in the center shows a stationary object, with five hypothetical spectral lines shown at their rest positions. At top red-shift is illustrated with the same five lines each equally moved slightly to the right, or to the red part of the spectrum. At bottom blue-shift is illustrated with the same five lines each equally moved slightly to the left, or to the blue part of the spectrum. This image is for illustrative purposes, and no exact red- or blue-shift value is given.

Figure 2: Doppler-Shifted Stars. When the spectral lines of a moving star shift toward the ruby-red end of the spectrum, we know that the star is moving away from united states. If they shift toward the blue end, the star is moving toward united states.

William Huggins, pioneering withal again, in 1868 fabricated the first radial velocity decision of a star. He observed the Doppler shift in ane of the hydrogen lines in the spectrum of Sirius and plant that this star is moving toward the solar organization. Today, radial velocity can be measured for whatsoever star brilliant enough for its spectrum to be observed. Equally we will run into in The Stars: A Celestial Census, radial velocity measurements of double stars are crucial in deriving stellar masses.

Proper Motility

There is another type of movement stars can have that cannot be detected with stellar spectra. Unlike radial motion, which is along our line of sight (i.east., toward or away from Earth), this motion, called proper move, is transverse: that is, across our line of sight. We come across it as a change in the relative positions of the stars on the angelic sphere (Figure 3). These changes are very ho-hum. Even the star with the largest proper motion takes 200 years to change its position in the sky past an corporeality equal to the width of the total Moon, and the motions of other stars are smaller even so.

Photographs of Barnard's Star demonstrating its large proper motion. At left (a) the star is seen in the center of an image taken in 1985, along with several background stars. At center (b) is the same field as photographed in 1995. The background stars have not moved, but Barnard's Star has moved downward from the center of the image (where is was seen in 1985). At right (c) is the same field in 2005. The background stars have again not moved, and Barnard's Star is now near the bottom of the image.

Effigy 3: Big Proper Motion. Iii photographs of Barnard'due south star, the star with the largest known proper move, show how this faint star has moved over a menstruum of 20 years. (modification of work by Steve Quirk)

For this reason, with our naked eyes, we do not notice any change in the positions of the bright stars during the grade of a human lifetime. If we could alive long enough, notwithstanding, the changes would go obvious. For example, some 50,000 years from at present, terrestrial observers will find the handle of the Large Dipper unmistakably more aptitude than information technology is now (Figure iv).

Illustrations of changes in the Big Dipper as a result of proper motion. The upper panel shows the seven stars of the Big Dipper as they appeared 50,000 years ago. The central panel shows how the asterism appears today, with an arrow attached to each star pointing in the direction of its proper motion across the sky. The bottom panel shows how the Big Dipper will appear in 50,000 years.

Effigy 4: Changes in the Large Dipper. This effigy shows changes in the appearance of the Big Dipper due to proper motion of the stars over 100,000 years.

Nosotros measure out the proper motion of a star in arcseconds (1/3600 of a degree) per yr. That is, the measurement of proper motion tells united states simply by how much of an angle a star has changed its position on the angelic sphere. If two stars at different distances are moving at the same velocity perpendicular to our line of sight, the closer one will prove a larger shift in its position on the celestial sphere in a year's fourth dimension. Every bit an analogy, imagine you are standing at the side of a thruway. Cars volition appear to whiz past you lot. If you then picket the traffic from a vantage point half a mile away, the cars will move much more slowly across your field of vision. In lodge to convert this angular movement to a velocity, nosotros need to know how far abroad the star is.

To know the true space velocity of a star—that is, its total speed and the management in which it is moving through space relative to the Sun—nosotros must know its radial velocity, proper movement, and distance (Effigy v). A star'south space velocity can also, over time, cause its distance from the Sun to change significantly. Over several hundred grand years, these changes can exist large plenty to touch on the apparent brightnesses of nearby stars. Today, Sirius, in the constellation Canis Major (the Big Dog) is the brightest star in the sky, but 100,000 years ago, the star Canopus in the constellation Carina (the Keel) was the brightest i. A little over 200,000 years from now, Sirius volition accept moved away and faded somewhat, and Vega, the vivid blue star in Lyra, will have over its place of honor as the brightest star in Earth'due south skies.

Diagram illustrating the radial velocity, proper motion, and space velocity of a star. At bottom left is a yellow disk representing the Sun. On the upper right is a smaller orange disk representing a distant star. A dashed, straight line connects the centers of the Sun and the star. (Above, to the left and parallel to this dashed line is a solid line with arrows at each end terminating at what would be the centers of both stars. This line is the total distance, d, separating the Sun and this hypothetical star.) Another dashed, straight line is drawn from the Sun, below and at an angle (shown as the Greek letter mu), from the dashed line that connects the Sun and star. The angle, mu, between these dashed lines is the measured proper motion of the star as seen from the Sun. In this case the star is moving to the upper left in the diagram. Three arrows are drawn from the center of the distant star. Each arrow represents the components of the star's motion through space that contributes to its measured proper motion. The first arrow points directly away from the Sun toward the right, along the projected path of the dashed line connecting the Sun and star. This represents the radial velocity, i.e. the velocity along our line of sight. At a right angle to this arrow, and pointing up and to the left from the star, is the arrow for the transverse velocity. The transverse velocity is perpendicular to our line of sight, and is what we see as proper motion. Between the two arrows is a third, in this case pointing straight up in the diagram, that represents the total space velocity of the star. It is the combination of the transverse and radial velocities.

Effigy 5: Infinite 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 Sun to a star. The transverse velocity is a component of the space velocity projected on the sky. What astronomers measure is proper motion (μ), which is the modify in the credible direction on the heaven measured in fractions of a caste. To convert this alter in direction to a speed in, say, kilometers per second, it is necessary to also know the distance (d) from the Sun to the star.

Rotation

We tin can too use the Doppler effect to measure how fast a star rotates. If an object is rotating, so one of its sides is budgeted us while the other is receding (unless its centrality of rotation happens to be pointed exactly toward us). This is conspicuously the case for the Dominicus or a planet; we can observe the light from either the budgeted or receding border of these nearby objects and directly measure the Doppler shifts that arise from the rotation.

Stars, however, are so far away that they all appear equally unresolved points. The best we can do is to analyze the light from the entire star at once. Due to the Doppler consequence, the lines in the light that come from the side of the star rotating toward us are shifted to shorter wavelengths and the lines in the light from the opposite border of the star are shifted to longer wavelengths. Y'all can call up of each spectral line that we notice as the sum or blended of spectral lines originating from different speeds with respect to us. Each bespeak on the star has its own Doppler shift, then the absorption line we run across from the whole star is actually much wider than it would be if the star were non rotating. If a star is rotating quickly, there volition be a greater spread of Doppler shifts and all its spectral lines should be quite broad. In fact, astronomers phone call this effect line broadening, and the amount of broadening can tell us the speed at which the star rotates (Effigy 6).

Diagram illustrating the use of spectra to determine stellar rotation. At top left is a white disk representing a non-rotating star as seen from above one of its poles. Three equally wavy arrows point downward, representing light emitted from this star, headed toward Earth. Immediately below the wavy arrows is a spectrum with one narrow absorption line in the middle. Below the spectrum a graph is shown, with luminosity on the vertical axis and wavelength on the horizontal. A curve is plotted which begins as a horizontal line about 3/4 of the way up the luminosity scale then dips sharply downward to near zero luminosity and then back up again to the original horizontal level. This sharp, narrow, and deep line is indicative of no or very slow rotation. On the top right another white disk is shown, with a circular arrow within, indicating its rotation. The left side of the rotating star is moving toward the observer, and the right hand side is moving away. The three wavy arrows are different than those for the non-rotating star. The rotating star's left-most arrow has many waves representing short (blue) wavelengths, its central arrow has fewer waves, and the right-most arrow has the least waves representing long (red) wavelengths. The spectrum of the rotating star has a much broader absorption line. The rotating star's graph also plots luminosity versus wavelength, but its curve is much broader and less deep than the non-rotating star.

Figure 6: Using a Spectrum to Determine Stellar Rotation. A rotating star volition show broader spectral lines than a nonrotating star.

Measurements of the widths of spectral lines show that many stars rotate faster than the Dominicus, some with periods of less than a day! These rapid rotators spin so fast that their shapes are "flattened" into what we call oblate spheroids. An example of this is the star Vega, which rotates one time every 12.5 hours. Vega's rotation flattens its shape so much that its bore at the equator is 23% wider than its diameter at the poles (Figure 7). The Sun, with its rotation period of most a month, rotates rather slowly. Studies have shown that stars decrease their rotational speed equally they age. Young stars rotate very quickly, with rotational periods of days or less. Very old stars can have rotation periods of several months.

Diagram comparing stars with different rates of rotation. At left the star Altair is shown as seen looking at its equator. The rotation period is given as 6.5 hours. The star appears flattened from top to bottom and bulging outward along the equator, somewhat like an American football viewed lengthwise. At right the Sun is shown, with the rotation period given as 24-30 days. The Sun appears nearly circular.

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

As you can see, spectroscopy is an extremely powerful technique that helps us learn all kinds of information about stars that we simply could non gather any other way. We will meet in later chapters that these same techniques tin also teach united states of america most galaxies, which are the most distant objects that can we discover. Without spectroscopy, we would know next to nothing virtually the universe beyond the solar system.

Astronomy and Philanthropy

Throughout the history of astronomy, contributions from wealthy patrons of the scientific discipline take fabricated an enormous difference in building new instruments and carrying out long-term research projects. Edward Pickering'southward stellar classification project, which was to stretch over several decades, was made possible by major donations from Anna Draper. She was the widow of Henry Draper, a md who was one of the almost accomplished apprentice astronomers of the nineteenth century and the first person to successfully photograph the spectrum of a star. Anna Draper gave several hundred thousand dollars to Harvard Observatory. As a issue, the bang-up spectroscopic survey is still known equally the Henry Draper Memorial, and many stars are nonetheless referred to by their "Hard disk" numbers in that catalog (such as HD 209458).

In the 1870s, the eccentric piano architect and real estate magnate James Lick (Effigy 8) decided to exit some of his fortune to build the globe's largest telescope. When, in 1887, the pier to firm the telescope was finished, Lick's body was entombed in information technology. Atop the foundation rose a 36-inch refractor, which for many years was the master musical instrument at the Lick Observatory nigh San Jose.

Photographs of: left (a) Henry Draper, and right (b) James Lick.

Figure eight: Henry Draper (1837–1882) and James Lick (1796–1876). (a) Draper stands next 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 not only as a memorial to himself but too to assist in further astronomical research.

The Lick telescope remained the largest in the earth until 1897, when George Ellery Hale persuaded railroad millionaire Charles Yerkes to finance the construction of a 40-inch telescope near Chicago. More recently, Howard Keck, whose family unit made its fortune in the oil industry, gave $70 million from his family foundation to the California Institute of Technology to assistance build the world'due south largest telescope atop the 14,000-foot peak of Mauna Kea in Hawaii (see the chapter on Astronomical Instruments to learn more nigh these telescopes). The Keck Foundation was so pleased with what is now chosen the Keck telescope that they gave $74 one thousand thousand more than to build Keck II, some other 10-meter reflector on the same volcanic tiptop.

Now, if any of you become millionaires or billionaires, and astronomy has sparked your involvement, do keep an astronomical instrument or project in mind as you plan your estate. But bluntly, private philanthropy could not perhaps back up the full enterprise of scientific research in astronomy. Much of our exploration of the universe is financed past federal agencies such every bit the National Science Foundation and NASA in the U.s., and by similar government agencies in the other countries. In this manner, all of us, through a very small share of our taxation dollars, are philanthropists for astronomy.

Key concepts and summary

Spectra of stars of the aforementioned temperature but different atmospheric pressures take subtle differences, so spectra tin can be used to decide whether a star has a large radius and low atmospheric pressure (a giant star) or a small-scale radius and loftier atmospheric pressure. Stellar spectra can as well be used to determine the chemical composition of stars; hydrogen and helium make up most of the mass of all stars. Measurements of line shifts produced by the Doppler effect signal the radial velocity of a star. Broadening of spectral lines by the Doppler effect is a measure of rotational velocity. A star can too 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 twelvemonth in the direction of a star equally seen from the Lord's day

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

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

What Tool Do Astronomers Use To Figure Out A Stars Chemical Makeup,

Source: https://courses.lumenlearning.com/astronomy/chapter/using-spectra-to-measure-stellar-radius-composition-and-motion/

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