1
UFABC – NHZ3043 – NOÇÕES DE ASTRONOMIA E COSMOLOGIA – Curso 2016.2
Prof. Germán Lugones
Capítulo 8
Meio interestelar
Nebulosa Cabeça do Cavalo
2
‣A pesar de que a maior parte da massa da Via Láctea está contida nas estrelas, o meio interestelar não é completamente vazio. Principalmente no disco da Galáxia, o meio interestelar contém gás e poeira, distribuídos na forma de nuvens individuais, e também em um meio difuso.
‣A densidade típica do meio interestelar é de um átomo de hidrogênio por cm3 e, aproximadamente, 100 grãos de poeira por km3.
‣O gás interestelar constitui, aproximadamente, 10% da massa da Via Láctea, ao passo que a poeira agrupa menos de 1% da massa em gás.
‣Raios cósmicos, que são partículas altamente energéticas, estão misturados com o gás e a poeira, e existe ainda um campo magnético galático, fraco (≃ 10 μG).
‣Atualmente, as observações mais importantes do meio interestelar são feitas em comprimentos de onda de rádio e de infravermelho, uma vez que o pico de emissão encontra-se frequentemente nestes comprimentos de onda.
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450 CHAPTER 18 The Interstellar Medium
The matter among the stars is collectively termed the interstellar medium. It is made up of two components—
gas and dust—intermixed throughout all space. The gas is made up mainly of individual atoms, of average size 10−10 m (0.1 nm) or so, and small molecules, no larger than about 10−9 m across. Interstellar dust is more complex, consisting of clumps of atoms and molecules—not unlike chalk dust or the microscopic particles that make up smoke, soot, or fog.
Apart from numerous narrow atomic and molecular absorption lines, the gas alone does not block radiation to any great extent. The obscuration that is evident in Figure 18.1 is caused by the dust. Light from distant stars cannot penetrate the densest accumulations of interstellar dust any more than a car’s headlights can illuminate the road ahead in a thick fog.
Extinction and Reddening
We can use its effect on starlight to measure both the amount and the size of interstellar dust. As a rule of thumb, a beam of light can be absorbed or scattered only by particles hav- ing diameters comparable to or larger than the wavelength of the radiation involved. Thus, a range of dust particle sizes will tend to block shorter wavelengths most effectively.
Furthermore, even for particles of a given size, the amount of obscuration (that is, absorption or scattering) produced by particles of a given size increases with decreasing wave- length. The size of a typical interstellar dust particle—or dust grain—is about 10−7 m (0.1 μm), comparable in size to the wavelength of visible light. Consequently, dusty regions of interstellar space are transparent to long- wavelength radio and infrared radiation, but opaque to shorter wave- length optical and ultraviolet radiation. The overall dim- ming of starlight by interstellar matter is called extinction.
18.1 Interstellar Matter
Figure 18.1 is a mosaic of photographs covering a much greater expanse of universal “real estate” than anything we have studied thus far. From our vantage point on Earth, the panoramic view shown here stretches all the way across the sky. On a clear night, it is visible to the naked eye as the Milky Way. In Chapter 23, we will come to recognize this band as the flattened disk, or plane, of our Galaxy.
The bright regions in this image are congregations of innumerable unresolved stars, merging together into a con- tinuous blur at the resolution of the telescope. However, the dark areas are not simply “holes” in the stellar distribution.
They are regions of space where interstellar matter obscures (blocks) the light from stars beyond, blocking from our view what would otherwise be a rather smooth distribution of bright starlight. Their very darkness means that they cannot easily be studied by the optical methods used to examine stellar matter. There is, quite simply, nothing to see!
Gas and Dust
From Figure 18.1 (see also Figure 18.4), it is evident that interstellar matter is distributed very unevenly throughout space. In some directions, the obscuring matter is largely absent, allowing astronomers to study objects literally bil- lions of parsecs from the Sun. In other directions, there are small amounts of interstellar matter, so the obscuration is moderate, preventing us from seeing objects more than a few thousand parsecs away, but still allowing us to study nearby stars. Still other regions are so heavily obscured that starlight from even relatively nearby stars is completely absorbed before reaching Earth.
▲ FIGURE 18.1 Milky Way Mosaic The Milky Way Galaxy photographed panoramically, across 360° of the entire southern and northern celestial sphere. This band, which constitutes the central plane of our Galaxy, contains high concentrations of stars, as well as interstellar gas and dust. The white box shows the field of view of Figure 18.4.
(ESO/S. Brunier)
R I V U X G
‣O gás está fortemente concentrado no plano galáctico e nos braços espirais. Assim, nessas regiões há muitos lugares onde as quantidade de massa nas estrelas e na matéria interestelar são aproximadamente iguais.
‣ Figura: panorâmica da Via Láctea (360∘) mostrando estrelas, gás e poeira.
SECTION 18.2 Emission Nebulae 453
18.2 Emission Nebulae
Figure 18.4 shows a magnified view of the central part of Figure 18.1 (the region indicated by the rectangle in the earlier figure), in the general direction of the constellation Sagittarius. The field of view is mottled with stars and interstellar matter. The patchiness of the obscuration is evident. In addition, several large fuzzy patches of light are clearly visible. These fuzzy objects, labeled M8, M16, M17, and M20, correspond to the 8th, 16th, 17th, and 20th objects in a catalog compiled by Charles Messier, an 18th-century French astronomer.* Today they are known as emission nebulae—glowing clouds of hot interstellar matter. Figure 18.5 enlarges the left side of Figure 18.4, showing the nebulae more clearly.
Observations of Emission Nebulae
Historically, astronomers have used the term nebula to refer to any “fuzzy” patch (bright or dark) on the sky—any region of space that was clearly distinguishable through a telescope, but not sharply defined, unlike a star or a planet.
We now know that many (although not all) nebulae are clouds of interstellar dust and gas.
If a cloud happens to obscure stars lying behind it, we see it as a dark patch on a bright background, as in Figures 18.1, 18.2(b), and 18.4—a dark nebula. But if something within the cloud—a group of hot young stars, for example—
causes it to glow, then we see a bright emission nebula Astronomers infer this elongated structure from the
fact that the light emitted by stars is dimmed and partially polarized, or aligned, by the intervening dust. Recall from Chapter 3 that light consists of electromagnetic waves com- posed of vibrating electric and magnetic fields. (Sec. 3.2, Fig. 3.7) Normally, these waves are randomly oriented, and the radiation is said to be unpolarized. Stars emit unpolar- ized radiation from their photospheres. However, under the right conditions, the radiation can become polarized en route to Earth, with the electric fields all vibrating in roughly the same plane. One way in which this can happen is if the radiation interacts with an elongated dust grain, which tends to absorb electric waves vibrating parallel to its length.
Thus, if the light detected by our telescope is polarized, it is because some interstellar dust lies between the emitting object and Earth. Based on this reasoning, astronomers have determined not only that interstellar dust particles must be elongated in shape, but also that they tend to be aligned over large regions of space.
The alignment of the interstellar dust is the subject of ongoing research among astronomers. The current view, accepted by most, is that the dust particles are affected by a weak interstellar magnetic field, perhaps a million times weaker than Earth’s field. Each dust particle responds to the field in much the same way that small iron filings are aligned by an ordinary bar magnet. Measurements of the blockage and polarization of starlight thus yield information about the size and shape of interstellar dust particles, as well as about magnetic fields in interstellar space.
CONCEPT Check
4 If space is a near-perfect vacuum, how can there be enough dust in it to block starlight?
*Messier was actually more concerned with making a list of celestial ob- jects that might be confused with comets, his main astronomical interest.
However, the catalog of 109 “Messier objects” is now regarded as a much more important contribution to astronomy than any comets Messier dis- covered.
M20 M8
R I V U X G
▶ FIGURE 18.4 Milky Way in Sagittarius Enlargement of the central part of Figure 18.1, showing regions of brightness (vast fields of stars) as well as regions of darkness (where interstellar matter obscures the light from more distant stars). The field of view is about 35° across. Two of the emission nebulae discussed in the text are labeled. (ESO/S. Guisard)
ANIMATION/VIDEOOrion Nebula Mosaic
4
‣ O gás interestelar é constituído, na maior parte, por hidrogênio neutro, que não é luminoso.
Também pode haver alguns átomos e moléculas.
‣ As regiões com gás são transparentes a todos os comprimentos de onda com exceção de linhas de absorção estreitas, i.e. o gás praticamente não bloqueia a radiação.
‣ Porém, perto de estrelas muito quentes e massivas, o hidrogênio é ionizado pela radiação ultravioleta provinda das estrelas e brilha por fluorescência.
‣Se existe suficiente hidrogênio ao redor dessas estrelas, ele será visível como uma nebulosa gasosa de emissão, brilhante, chamada região HII, ou nebulosa de emissão.
‣Um exemplo desse tipo de nebulosa é a Nebulosa de Órion, que se encontra a 1500 anos-luz da Terra.
Gás interestelar
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458 CHAPTER 18 The Interstellar Medium
very different from conditions on Earth and warn us of the potential difficulties involved in extending our ter- restrial experience from our laboratories to the study of interstellar space.
Some regions of interstellar space contain extremely dilute, even hotter gas than is found within emission nebu- lae. Ultraviolet observations by space-based instruments have found that these superheated interstellar “bubbles,”
making up the intercloud medium, may extend far into interstellar space beyond our local neighborhood and, con- ceivably, into the even vaster spaces among the galaxies.
This high-temperature gas is probably the result of the violent expansion of debris from stars that exploded long ago. Somewhat like the Sun’s faint corona, these regions are dark despite their high temperatures because the density of matter there is very low. (Sec. 16.3)
The Sun seems to reside in one such low-density region—a huge cavity called the “Local Bubble,” sketched in Figure 18.11. The Local Bubble contains about 200,000 stars and extends for nearly 100 pc. It was probably carved out by multiple supernova explosions (see Chapters 20 and 21) that occurred several hundred thousand years ago in the Scorpius–Centaurus association, a rich cluster of bright young stars. Perhaps our hominid ancestors may have seen these ancient events—stellar catastrophes as bright as the full Moon—that now aid modern astronomers.
R I V U X G
(b) (c)
(a)
1 light-year
▲ FIGURE 18.10 Orion Nebula (a) Lying some 1400 light-years from Earth, the Orion Nebula (M42) is visible to the naked eye as the fuzzy middle “star” of Orion’s sword. (b) Like all emission nebulae, the Orion Nebula consists of hot, glowing gas powered by a group of bright stars in the center. In addition to exhibiting red H! emission, parts of the nebula show a slight greenish tint, caused by a so-called forbidden transition in ionized oxygen. (c) A high-resolution image shows rich detail in a region about 0.5 light-year across. Structural details are visible down to a level of 0.1”, or 6 light-hours—a scale comparable to the dimensions of our solar system.
(NASA; ESO)
Hyades star cluster
Sirius Aldebaran
Sun Vega
To Galactic center Procyon
a Centauri Arcturus
Scorpius-Centaurus association
130 light-years
▲ FIGURE 18.11 Local Bubble The Sun resides in a vast low- density region of space that engulfs us nearly spherically. This cavity was likely caused by stellar explosions long ago, which then heated the nearby interstellar gas and expelled it well out of the solar neighborhood. Several prominent stars in our nighttime sky are plotted in this artist’s conception, which depicts what the
“bubble” might look like from afar.
(a) Lying some 1400 light-years from Earth, the Orion Nebula (M42) is visible to the naked eye as the fuzzy middle “star” of Orion’s sword.
(b) Like all emission nebulae, the Orion Nebula consists of hot, glowing gas powered by a group of bright stars in the center. In addition to exhibiting red H emission, parts of the nebula show a slight greenish tint, caused by a so-called forbidden transition in ionized oxygen.
(c) A high-resolution image shows rich detail in a region about 0.5 light-year across. Structural details are visible down to a level of 0.1”, or 6 light-hours—a scale comparable to the dimensions of our solar system.
6 320
15. The Interstellar Medium
Table 15.3. Element abundances in the interstellar medium towardsζOphiuchi and in the Sun. The abundances are given relative to that of hydrogen, which has been defined to be
1,000,000. An asterisk (*) means that the abundance has been determined from meteorites. The last column gives the ratio of the abundances in the interstellar medium and in the Sun
Atomic Name Chemical Interstellar Solar Abundance
number symbol abundance abundance ratio
1 Hydrogen H 1,000,000 1,000,000 1.00
2 Helium He 85,000 85,000 ≈1
3 Lithium Li 0.000051 0.00158* 0.034
4 Beryllium Be < 0.000070 0.000012 <5.8
5 Boron B 0.000074 0.0046* 0.016
6 Carbon C 74 370 0.20
7 Nitrogen N 21 110 0.19
8 Oxygen O 172 660 0.26
9 Fluorine F – 0.040 –
10 Neon Ne – 83 –
11 Sodium Na 0.22 1.7 0.13
12 Magnesium Mg 1.05 35 0.030
13 Aluminium Al 0.0013 2.5 0.00052
14 Silicon Si 0.81 35 0.023
15 Phosphorus P 0.021 0.27 0.079
16 Sulfur S 8.2 16 0.51
17 Chlorine Cl 0.099 0.45 0.22
18 Argon Ar 0.86 4.5 0.19
19 Potassium K 0.010 0.11 0.094
20 Calcium Ca 0.00046 2.1 0.00022
21 Scandium Sc – 0.0017 –
22 Titanium Ti 0.00018 0.055 0.0032
23 Vanadium V < 0.0032 0.013 <0.25
24 Chromium Cr < 0.002 0.50 <0.004
25 Manganese Mn 0.014 0.26 0.055
26 Iron Fe 0.28 25 0.011
27 Cobalt Co < 0.19 0.032 <5.8
28 Nickel Ni 0.0065 1.3 0.0050
29 Copper Cu 0.00064 0.028 0.023
30 Zinc Zn 0.014 0.026 0.53
elements. This interpretation is supported by the obser- vation that in regions where the amount of dust is smaller than usual, the element abundances in the gas are closer to normal.
Atomic Hydrogen. Ultraviolet observations have pro- vided an excellent way of studying interstellar neutral hydrogen. The strongest interstellar absorption line, as has already been mentioned, is the hydrogen Lyman α line (Fig. 15.15). This line corresponds to the transi- tion of the electron in the hydrogen atom from a state with principal quantum numbern =1 to one withn=2.
The conditions in interstellar space are such that almost all hydrogen atoms are in the ground state with n =1.
Therefore the Lymanα line is a strong absorption line, whereas the Balmer absorption lines, which arise from the excited initial state n =2, are unobservable. (The
Balmer lines are strong in stellar atmospheres with tem- peratures of about 10,000 K, where a large number of atoms are in the first excited state.)
The first observations of the interstellar Lymanαline were made from a rocket already in 1967. More exten- sive observations comprising 95 stars were obtained by the OAO 2 satellite. The distances of the observed stars are between 100 and 1000 parsecs.
Comparison of the Lyman α observations with ob- servations of the 21 cm neutral hydrogen line have been especially useful. The distribution of neutral hydrogen over the whole sky has been mapped by means of the 21 cm line. However, the distances to nearby hydrogen clouds are difficult to determine from these observa- tions. In the Lyman α observations one usually knows the distance to the star in front of which the absorbing clouds must lie.
The abundances are given relative to that of hydrogen, which has been defined to be 1,000,000. The last column gives the ratio of the abundances in the interstellar medium and in the Sun.
7
‣ A poeira interestelar tem uma composição mais complexa. Ela é feita de aglomerados de átomos e moléculas, de forma similar à fumaça, névoa, poeira de giz em grãos de vários tamanhos, mas muito menores (~1µm) do que a poeira aqui na Terra. Ela é contém principalmente grafite, silicatos e gelo de água.
‣ A luz das estrelas mais distantes não penetra a poeira densa, da mesma forma que um farol de carro não penetra neblina densa.
‣ A poeira circundando estrelas reflete a luz formando uma nebulosa de reflexão, de cor azulada. O espectro dessas nebulosas é o mesmo da estrela que a ilumina.
Poeira interestelar
452 CHAPTER 18 The Interstellar Medium
atomic or molecular hydrogen; some 9 percent is helium, and the remaining 1 percent consists of heavier elements. The abundances of some heavy elements, such as carbon, oxygen, silicon, magnesium, and iron, are much lower in interstellar gas than in our solar system or in stars. The most likely expla- nation for this finding is that substantial quantities of these elements have been used to form the interstellar dust, tak- ing them out of the gas and locking them up in a form that is much harder to observe.
In contrast to interstellar gas, the composition of inter- stellar dust is currently not very well known. We have some infrared evidence for silicates, graphite, and iron—the same elements that are underabundant in the gas—lending sup- port to the theory that interstellar dust forms out of inter- stellar gas. The dust probably also contains some “dirty ice,”
a frozen mixture of ordinary water ice contaminated with trace amounts of ammonia, methane, and other chemical compounds. This composition is quite similar to that of cometary nuclei in our own solar system. (Sec. 14.2)
Dust Shape
Curiously, astronomers know the shapes of interstellar dust particles better than their composition. Although the minute atoms in the interstellar gas are basically spheri- cal, the dust particles are not. Individual dust grains are apparently elongated or rodlike, as shown in Figure 18.3(a), although recent theoretical studies of how dust particles col- lide, stick, and break up suggest that their larger scale struc- ture may be considerably more complex (Figure 18.3b).
Overall Density
Gas and dust are found everywhere in interstellar space—no part of our Galaxy is truly devoid of matter. However, the den- sity of the interstellar medium is extremely low. Overall, the gas averages roughly 106 atoms per cubic meter—just 1 atom per cubic centimeter—although there are large variations from place to place: Densities ranging from 104 to 109 atoms/m3 have been found. Matter this diffuse is far less dense than the best vacuum—about 1010 molecules/m3—ever attained in laboratories on Earth.
Interstellar dust is even rarer. On average, there is only one dust particle for every trillion or so atoms—just 10−6 dust particles per cubic meter, or 1000 per cubic kilometer.
Some parts of interstellar space are so thinly populated that harvesting all the gas and dust in a region the size of Earth would yield barely enough matter to make a pair of dice.
How can such fantastically sparse matter diminish light radiation so effectively? The key is size—interstellar space is vast. The typical distance between stars (1 pc or so in the vicin- ity of the Sun) is much, much greater than the typical size of the stars themselves (around 10−7 pc). Stellar and planetary sizes pale in comparison to the vastness of interstellar space. Thus, matter can accumulate, regardless of how thinly it is spread.
For example, an imaginary cylinder 1 m2 in cross section and extending from Earth to Alpha Centauri would contain more than 10 billion billion dust particles. (Sec. 17.1) Over huge distances, dust particles accumulate slowly, but surely, to the point at which they can effectively block visible light and other short-wavelength radiation. Even though the den-
sity of matter is very low, interstellar space in the vicinity of the Sun contains about as much mass as exists in the form of stars.
Despite their rarity, dust particles make interstellar space a relatively dirty place. Earth’s atmosphere, by comparison, is about a million times cleaner. Our air is tainted by only one dust particle for about every billion billion (1018) atoms of atmospheric gas. If we could compress a typi- cal parcel of interstellar space to equal the density of air on Earth, this parcel would contain enough dust to make a fog so thick that we would be unable to see our hand held at arm’s length in front of us.
Composition
The composition of interstellar gas is reasonably well understood from spectroscopic studies of absorption lines formed when light from a dis- tant star interacts with gas along the observer’s line of sight (see Section 18.3). In most cases, the elemental abundances detected in interstellar gas mirror those found in other astronomical objects, such as the Sun, the stars, and the jovian planets.
Most of the gas—about 90 percent by number—is
10–7 m
(a) (b)
Grains are linear, or rodlike, on small scales, c
cbut can become tangled and twisted in complex ways on larger scales.
▲ FIGURE 18.3 Interstellar Dust (a) Typical interstellar dust particles, as inferred from polarization studies, have sizes of only about one ten-thousandth of a millimeter, yet space contains enough of them to obscure our view in certain directions. (b) This result of a computer simulation shows how grains may grow as dust particles collide, stick, and fragment in interstellar space.
8
‣As partículas de poeira, com tamanhos de 0,1 a 1 mícron, são suficientemente pequenas para espalharem (desviar a direção, sem absorver) a luz de menor comprimento de onda (luz azul, λ ≤ 0,4 µm) mais eficientemente do que as de maior comprimento de onda (luz vermelha, λ ≥ 0,7 µm).
‣De fato, fótons azuis são desviados cerca de 10 vezes mais eficientemente do que os fótons vermelhos. Quando um fóton é desviado, sua direção muda aleatoriamente.
SECTION 18.2 Emission Nebulae 455
Figure 18.9 shows enlargements of two of the nebu- lae visible in Figure 18.5. Notice again the hot, bright stars embedded within the glowing nebular gas and the pre- dominant red coloration of the emitted radiation in parts (a) and (c). The relationship between the nebulae and their dust lanes is again evident in Figures 18.9(b) and (d), where regions of gas and dust are simultaneously silhouetted against background nebular emission and illuminated by foreground nebular stars.
The interaction between stars and gas is particularly striking in Figure 18.9(b). The three dark “pillars” vis- ible in this spectacular Hubble Space Telescope image are part of the interstellar cloud from which the stars formed.
The rest of the cloud in the vicinity of the new stars has already been heated and dispersed by their radiation in a process known as photoevaporation. The fuzz around the edges of the pillars, especially at the top right and center, is the result of this ongoing process. (See also an up-close view of another such pillar in M16 in the chapter-opening photo.) As photoevaporation continues, it eats away the less dense material first, leaving behind delicate sculp- tures composed of the denser parts of the original cloud, just as wind and water create spectacular structures in Earth’s deserts and shores by eroding away the softest rock. The process is a dynamic one: The pillars will even- tually be destroyed, but probably not for another hundred thousand or so years.
Spectroscopists often refer to the ionization state of an atom by attaching a roman numeral to the chemical symbol for the atom—I for the neutral (that is, not ionized) atom, II for a singly ionized atom (an atom missing one electron), III producing huge amounts of ultraviolet light. As ultra-
violet photons travel outward from the star, they ionize the surrounding gas. As electrons recombine with nuclei, they emit visible radiation, causing the gas to fluoresce, or glow. (Sec. 4.2) The reddish hue of these nebu- lae—and, in fact, of all emission nebulae—results when hydrogen atoms emit light in the red part of the visible spectrum. Specifically, it is caused by the emission of radiation at 656.3 nm—the Hα line discussed in Chap- ter 4. (More Precisely 4-1) Other elements in the nebula also emit radiation as their electrons recombine, but because hydrogen is so plentiful, its emission usually dominates.
Woven through the glowing nebular gas, and plainly visible in Figures 18.5–18.7, are lanes of dark, obscuring dust. These dust lanes are part of the nebulae and are not just unrelated dust clouds that happen to lie along our line of sight. The bluish region visible in Figures 18.6 and 18.7 immediately above M20 is another type of neb- ula unrelated to the red emission nebula itself. Called a reflection nebula, it is caused by starlight scattered from dust particles in interstellar clouds located just off the line of sight between Earth and the bright stars within M20. Reflection nebulae appear blue for much the same reason that Earth’s daytime sky is blue: short-wavelength blue light is more easily scattered by interstellar matter back toward Earth and into our detectors. (More Pre- cisely 7-1) Figure 18.8 sketches some of the key features of emission nebulae, illustrating the connection between the central stars, the nebula itself, and the surrounding interstellar medium.
Light scattered through a dusty cloud, not along the line of sight, can look bluer.
Red light is emitted by nebulae when electrons and protons recombine to
form hydrogen atoms.
star(s)Hot
Dust lane Ultraviolet
radiation
interstellarDark cloud Visible
starlight
Re-emitted visible light
EMISSION NEBULA Dusty cloud
Ionized gas Observer
Unscattered red light
REFLECTION NEBULA Scattered
blue light
▲ FIGURE 18.8 Nebular Structure An emission nebula results when ultraviolet radiation from one or more hot stars ionizes part of an interstellar cloud. If starlight happens to encounter another dusty cloud, some of the radiation, particularly at the shorter wavelength blue end of the spectrum, may be scattered back toward Earth, forming a reflection nebula.
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460 CHAPTER 18 The Interstellar Medium
18.7 and 18.9 are good examples of this obscuration. Figure 18.14 shows another well-known, and particularly striking, example of such a cloud—the Horsehead Nebula in Orion.
This curiously shaped finger of gas and dust projects out from the much larger dark cloud (called L1630) that fills the bottom half of the image and stands out clearly against the red glow of a background emission nebula. For refer- ence, the stars and bright emission nebulae lie in front of the dark cloud; the red glow that silhouettes the Horsehead lies behind and above it.
Absorption Spectra
Astronomers first became aware of the true extent of dark interstellar clouds in the 1930s, as they studied the opti- cal spectra of distant stars. The gas in such a cloud absorbs some of the stellar radiation in a manner that depends on the cloud’s own temperature, density, and elemental abun- dance. The absorption lines thus produced contain infor- mation about dark interstellar matter, just as stellar absorp- tion lines reveal the properties of stars. (Sec. 4.1)
Because the interstellar absorption lines are produced by cold, low-density gas, astronomers can easily distinguish them from the much broader absorption lines formed in stars’ hot lower atmospheres. (Sec. 4.5) Figure 18.15(a) illustrates how light from a star may pass through several interstellar clouds on its way to Earth. These clouds need not be close to the star, and, indeed, they usually are not.
cloud clearly at radio wavelengths, providing an indispen- sible tool for the study of such objects. We will return to the subject of molecular emission from interstellar clouds in Section 18.5.
Figure 18.13 is a spectacular wide-field image of another dark dust cloud. Taking its name from a neigh- boring star system, Rho Ophiuchus, this dust cloud resides relatively nearby—about 170 pc from the Sun—making it one of the most intensely studied regions of star formation in the Milky Way. Pockets of heavy blackness mark regions where the dust and gas are especially concentrated and the light from the background stars is completely obscured.
Measuring several parsecs across, the Ophiuchus cloud is only a tiny part of the grand mosaic shown in Figure 18.1.
Note that this cloud, like most interstellar clouds, is very irregularly shaped. Note especially the long “streamers” of (relatively) dense dust and gas at upper left. By contrast, the bright patches within the dark regions are foreground objects—emission nebulae and groups of bright stars. Some of them are part of the cloud itself, where newly formed stars near the edge of the cloud have created “hot spots” in the cold, dark gas. Others have no connection to the cloud and just happen to lie along the line of sight.
Dark and dusty interstellar clouds are sprinkled throughout our Galaxy. We can study them at optical wave- lengths only if they happen to block the light emitted by more distant stars or nebulae. The dark outline of the L997 cloud in Figure 18.12(a) and the dust lanes visible in Figures
R I V U X G
Dust cloud
Antares
M4 Reflection nebula
◀ FIGURE 18.13 Dark Dust Cloud The Ophiuchus dark dust cloud resides only 550 light-years away, surrounded by colorful stars and nebulae that are actually small illuminated parts of a much bigger, and invisible, molecular cloud engulfing much of the 6-degree-wide region shown. Many stages of star formation can be seen in this spectacular four-image mosaic. The dark cloud itself is “visible” only because it blocks light coming from stars behind it. Notice the cloud’s irregular shape, and especially its long “streamers”
at upper left. The bright, giant star Antares, the (much more distant) star cluster M4, and a nearby blue reflection nebula are also noted. (R. Gendler/J.
Misti/S. Mazlin)
The Ophiuchus dark dust cloud resides only 550 light-years away, surrounded by colorful stars and nebulae that are actually small illuminated parts
of a much bigger, and invisible, molecular cloud engulfing much of the 6-degree-wide
region shown.
The dark cloud itself is
“visible” only because it blocks light coming from stars behind it.
The bright, giant star Antares, the (much more distant) star cluster M4, and a nearby blue
reflection nebula are also noted.
10
‣Temperatura do MIS: desde alguns graus Kelvin até alguns 103 K (dependendo da proximidade de uma estrela ou fonte de radiação). Tem peratura média ~100 K; i.e.
o meio interestelar é muito frio.
‣Densidade média ~ 10-24 g cm-3
Gás → 1 átomo/cm3 = 106 átomos/m3
Poeira → 1 partícula para 1012 átomos (1/1012)
‣Como então um material tão rarefeito e esparso pode bloquear a luz da estrela de modo tão eficaz? A resposta está na vastíssima extensão do meio interestelar.
11
Gás:
‣ O gás interestelar produz linhas de absorção quando a luz de uma estrela distante interage com o gás ao longo da linha de visada. A absorção de radiação pelo gás depende de T, ρ e da abundância do gás.
‣ Como distinguir estas das linhas de absorção gás interestelar do da atmosfera da estrela?
-linhas da estrela são mais largas
-linhas do gás interestelar são mais finas
‣ A abundância de alguns “metais” como C, O, Si, Mg, Fe é muito menor do que no sistema solar e nas estrelas. Razão possível: esses elementos são usados para formar poeira interestelar.
Poeira:
→ Observada no infravermelho.
→ Constituída de silicatos, C, Fe: sub-abundantes no gás!
→ Possivelmente também contém gelo “sujo” = agua congelada + amônia + metano + outras moléculas; semelhante a cauda de cometas
12
A luz pode ser absorvida ou espalhada pela poeira, desde que comprimento de onda da luz seja menor ou igual que a dimensão dos grãos:
λ ≤ dg
A dimensão dos grãos é dg ~10-7 m, i.e. da mesma ordem que o comprimento de onda da luz visível. Portanto, regiões com poeira interestelar:
➡ são transparentes a ondas de radio e infravermelho: λ ≫ dg
➡ são opacas ao UV, raios X e raios gama: λ ≪ dg
Extinção interestelar
13
Nuvens densas de poeira são melhor observáveis no infravermelho ou em rádio (10-6m a 102 m). Para λs mais curtos (óptico, ultravioleta): elas são opacas, i.e.
aparecem como nuvens escuras.
Figura: Nuvem de poeira Barnard 68 no visível e no infravermelho.
SECTION 18.1 Interstellar Matter 451
Because the interstellar medium is more opaque to short-wavelength radiation than to radiation of longer wavelengths, light from distant stars is pref- erentially robbed of its higher frequency (“blue”) components. Hence, in addition to being generally diminished in brightness, stars also tend to appear redder than they really are. This effect, known as
reddening, is conceptually similar to the process that produces spectacular red sunsets here on Earth.
(More Precisely 7-1)
As illustrated in Figure 18.2(a), extinction and reddening change a star’s apparent brightness and color. However, the patterns of absorption lines in the original stellar spectrum are still recognizable in the radiation reaching Earth, so the star’s spec- tral class can be determined. Astronomers can use this fact to study the interstellar medium. From a main-sequence star’s spectral and luminosity classes, astronomers learn the star’s true luminosity and color.
(Secs. 17.5, 17.6) They then measure thedegree to which the starlight has been affected by extinction and reddening en route to Earth, and this,
Scattered light
Dust cloud
Telescope Frequency
Intensity Intensity
Red light slightly reduced
Stellar absorption lines still detectable
Blackbody spectrum
Red Blue
Frequency
Blue light greatly reduced
Stellar
absorption lines
(b) (a)
R I V U X G
(c)
R I V U X G
Narrated FIGURE 18.2 Reddening (a) Starlight Star passing through a dusty region of space is both
dimmed and reddened, but spectral lines are still recognizable in the light that reaches Earth. (b) This dusty interstellar cloud, called Barnard 68, is opaque to visible light, except near its edges, where some light from background stars can be seen. The cloud spans about 0.5 light-year and lies about 520 light-years away. Frame (c) illustrates (in false color) how infrared radiation can penetrate Barnard 68. (ESO)
ANIMATION/VIDEOInfrared View of NebulaeANIMATION/VIDEOPillars Behind the Dust
R I V U X G
in turn, allows them to estimate both the numbers and the sizes of interstellar dust particles along the line of sight to the star. By repeating these measurements for stars in many different directions and at many different distances from Earth, astronomers have built up a picture of the distribu- tion and overall properties of the interstellar medium in the solar neighborhood.
Reddening can be seen very clearly in Figure 18.2(b), which shows a type of compact, dusty interstellar cloud called a globule. (We will discuss such clouds in more detail in Section 18.3.) The center of this cloud, called Barnard 68, is opaque to all optical wavelengths, so starlight cannot pass through it. However, near the edges, where there is less intervening cloud matter, some light does make it through.
Notice how stars seen through the cloud are both dimmed and reddened relative to those seen directly. Figure 18.2(c) shows the same cloud in the infrared part of the spectrum.
Much more of the radiation gets through, but even here red-
dening (or its infrared equivalent) can be seen.
14
SECTION 18.1 Interstellar Matter 451
Because the interstellar medium is more opaque to short-wavelength radiation than to radiation of longer wavelengths, light from distant stars is pref- erentially robbed of its higher frequency (“blue”) components. Hence, in addition to being generally diminished in brightness, stars also tend to appear redder than they really are. This effect, known as reddening, is conceptually similar to the process that produces spectacular red sunsets here on Earth.
(More Precisely 7-1)
As illustrated in Figure 18.2(a), extinction and reddening change a star’s apparent brightness and color. However, the patterns of absorption lines in the original stellar spectrum are still recognizable in the radiation reaching Earth, so the star’s spec- tral class can be determined. Astronomers can use this fact to study the interstellar medium. From a main-sequence star’s spectral and luminosity classes, astronomers learn the star’s true luminosity and color. (Secs. 17.5, 17.6) They then measure the degree to which the starlight has been affected by extinction and reddening en route to Earth, and this,
Scattered light Dust cloud
Telescope Frequency
Intensity Intensity
Red light slightly reduced
Stellar absorption lines still detectable
Blackbody spectrum
Red Blue
Frequency
Blue light greatly reduced
Stellar absorption lines
(b) (a)
R I V U X G
(c)
R I V U X G
Narrated FIGURE 18.2 Reddening (a) Starlight Star passing through a dusty region of space is both dimmed and reddened, but spectral lines are still recognizable in the light that reaches Earth. (b) This dusty interstellar cloud, called Barnard 68, is opaque to visible light, except near its edges, where some light from background stars can be seen. The cloud spans about 0.5 light-year and lies about 520 light-years away. Frame (c) illustrates (in false color) how infrared radiation can penetrate Barnard 68. (ESO)
ANIMATION/VIDEOInfrared View of NebulaeANIMATION/VIDEOPillars Behind the Dust
R I V U X G
in turn, allows them to estimate both the numbers and the sizes of interstellar dust particles along the line of sight to the star. By repeating these measurements for stars in many different directions and at many different distances from Earth, astronomers have built up a picture of the distribu- tion and overall properties of the interstellar medium in the solar neighborhood.
Reddening can be seen very clearly in Figure 18.2(b), which shows a type of compact, dusty interstellar cloud called a globule. (We will discuss such clouds in more detail in Section 18.3.) The center of this cloud, called Barnard 68, is opaque to all optical wavelengths, so starlight cannot pass through it. However, near the edges, where there is less intervening cloud matter, some light does make it through.
Notice how stars seen through the cloud are both dimmed and reddened relative to those seen directly. Figure 18.2(c) shows the same cloud in the infrared part of the spectrum.
Much more of the radiation gets through, but even here red- dening (or its infrared equivalent) can be seen.
‣ No caso da luz visível acontece o fenômeno de extinção
interestelar (por absorção ou espalhamento) da luz.
‣ O fenômeno é tanto maior quanto menor o λ.
‣ A luz de estrelas mais distantes perde λs curtos (azuis). Logo, além de ter brilho menor, as estrelas parecem mais
vermelhas.
‣ É o mesmo processo que produz o pôr do Sol
avermelhado na Terra: a poeira do horizonte absorve os λs
azuis e deixa passar os vermelhos.
‣ As linhas espectrais, no entanto, podem ser
identificadas (ver figura).
15
‣ Devido à extinção interestelar, pode aumentar magnitude aparente (m).
‣ Portanto, é necessário incluir uma modificação na relação entre as magnitudes aparente e absoluta de uma estrela:
mV - MV = 5 log d - 5 + AV onde AV é a absorção em magnitudes.
‣ Assim, se a extinção for alta, da ordem de alguns décimos de magnitude, o cálculo das distâncias será bastante afetado. Sem considerar a extinção interestelar, as
distâncias das estrelas parecem maiores do que são na realidade.
‣ Além disso, a extinção afeta mais os comprimentos de onda mais curtos, produzindo um avermelhamento. Isso modifica a índice de cor (B-V).
16
Cor: B-V = mB-mV = -2,5 log (FB / FV)
FB > FV ⇨ B < V [B-V] < 0
Estrela quente, azulada
FB < FV ⇨B > V [B-V] > 0
Estrela fria, avermelhada
Fluxo
17
Correção da índice de cor
‣ Linhas do espectro de absorção das estrelas: não são afetados pela poeira presente no caminho entre a estrela e o observador
‣ Determinando tipo espectral e classe de luminosidade (pelo espectro) ⇨ T e cor intrínseca da estrela (B-V)o.
‣ Verifica-se o quanto o valor observado [B-V] foi aumentado pela extinção.
Definimos o avermelhamento como:
‣ E(B-V)=[B-V] – (B-V)o
‣ Correção do avermelhamento: parece razoável esperar que quanto mais extinção existe, o mais avermelhamento haverá. Na verdade, os estudos mostram que a extinção e vermelhidão são relacionadas por:
) V E(B
3.1
A
V= −
18
O gás e a poeira interestelares podem se aglomerar em nuvens ou nebulosas.
Dependendo das características, podemos ter:
• Nebulosa de emissão / reflexão
• Nebulosa de Poeira
• Nuvens moleculares
Nuvens interestelares
19
Nebulosas de
reflexão: refletem a luz de estrela
próxima em nossa direção.
Nebulosa de
reflexão NGC1999, constelação de
Orion.
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Nebulosas de Emissão
• As regiões HII, nuvens de gás hidrogênio ionizado, ocorrem
principalmente em volta de estrelas O e B pois estas emitem os fótons UV com energia acima de 13,6 eV. Quando os átomos de hidrogênio a absorvem, os elétrons ganham energia suficiente para se libertarem do núcleo, e o gás fica ionizado.
• Estas regiões portanto têm muitos íons de hidrogênio (prótons) e elétrons livres.
• Quando um próton captura um elétron livre, há emissão de radiação. As linhas do hidrogênio são emitidas quando o elétron passa, subsequentemente, pelos vários níveis de energia. Desta maneira, os fótons UV da estrela são degradados
em fótons no visível pela região HII. Nebulosa de emissão de Orion, região HII
21
• A radiação emitida quando o elétron passa do nível n=3 para o n=2, em 658,3 nm, é dominante e causa a cor vermelha da região.
• Pelas medidas da largura das linhas no espectro da nebulosa obtém-se sua temperatura (nas proximidades da estrela): T ~ 8000K.
Nebulosa de emissão de Orion, região HII
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Nuvens de poeira
• Cerca de 90% do meio interestelar é composto por regiões escuras (sem estrelas ou nebulosas brilhantes) chamadas de nuvens de poeira.
• Elas são mais frias e mais densas que suas vizinhanças (103 a 106 vezes mais).
• nmax = 1000 átomos/cm3 (densidade dos melhores vácuos em laboratório)
• A maioria tem dimensões maiores que nosso sistema solar e apresentando formas irregulares. Ocupam 2 - 3% do volume total do meio interestelar.
• O principal componente dessas nuvens é o gás, como resto do meio interestelar.
Porém, há uma importante quantidade de poeira que absorve a luz visível. Por isso, as nuvens de poeira são melhor detectadas no infravermelho.
rho Oph no ótico rho Oph no IV
23
SECTION 18.3 Dark Dust Clouds 461
cases, the elemental abundances detected in interstellar clouds mirror those found in other astronomical objects—perhaps not surprising, since (as we will see in Chapter 19) interstellar clouds are the regions that spawn emission nebulae and stars.
PROCESS OF SCIENCE Check
4 How do astronomers use optical observations to probe the properties of dark dust clouds?
Each absorbs some of the stellar radiation in a manner that depends on its own temperature, density, velocity, and ele- mental abundance. Figure 18.15(b) depicts part of a typical spectrum produced in this way.
The narrow absorption lines contain information about dark interstellar clouds, just as stellar absorption lines reveal the properties of stars and nebular emission lines tell us about conditions in hot nebulae. By studying these lines, astrono- mers can probe the cold depths of interstellar space. In most
(b) (a)
R I V U X G
1 light-year
▲ FIGURE 18.14 Horsehead Nebula (a) Located in the constellation Orion, not far from the Orion Nebula, this
Horsehead Nebula is a striking example of a dark dust cloud, silhouetted against the bright background of an emission nebula.
(b) A stunning image of the Horsehead, taken at highest resolution by the Very Large Telescope (VLT) in Chile. (Sec. 5.2) This nebular region is roughly 5000 light-years from Earth, in the constellation Orion. (Royal Observatory of Belgium; ESO)
(b) Star
Stellar spectrum
Cloud 1
Cloud 2
Broad stellar absorption lines
Narrow absorption lines from cloud 1
Fainter narrow absorption lines from cloud 2 (a)
Intensity
Frequency Broad stellar lines
Narrow cloud lines
Real spectra are often messy mixtures of spectra from many objects along the line of sight.
◀ FIGURE 18.15 Absorption by Interstellar Clouds (a) Simplified diagram of some interstellar clouds between a hot star and Earth. Optical observations might show an absorption spectrum like that traced in (b). The wide, intense lines are formed in the star’s hot atmosphere;
narrower, weaker lines arise from the cold interstellar clouds. The smaller the cloud, the weaker are the lines.
The redshifts or blueshifts of the narrow absorption lines provide information on cloud velocities. The widths of all the spectral lines depicted here are greatly exaggerated for the sake of clarity.
ANIMATION/VIDEOHorsehead Nebula
Horsehead Nebula (a) Located in the constellation Orion, not far from the Orion Nebula, this Horsehead Nebula is a striking example of a dark dust cloud, silhouetted against the bright background of an emission nebula. (b) A stunning image of the Horsehead, taken at highest resolution by the Very Large Telescope (VLT) in Chile. This nebular region is roughly 5000 light- years from Earth, in the constellation Orion.
