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Asteroid spectral types

From Wikipedia, the free encyclopedia

An asteroid spectral type is assigned to asteroids based on their emission spectrum, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies that are not internally differentiated, the surface and internal compositions are presumably similar, while large bodies such as Ceres and Vesta are known to have internal structure. Over the years, there has been a number of surveys that resulted in a set of different taxonomic systems such as the Tholen, SMASS and Bus–DeMeo classification.[1]

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Transcription

Twinkle, twinkle little star. Oh. I know who you are. At first glance, stars pretty much all look alike. Twinkly dots, scattered across the sky. But as I talked about in episode 2, when you look more closely you see differences. The most obvious is that some look bright and some faint. As I said then, sometimes that’s due to them being at different distances, but it’s also true that stars emit different amounts of light, too. If you look through binoculars or take pictures of them, you’ll see that they’re all different colors, too. Some appear white, some red, orange, blue, and for a long time, the reason for this was a mystery. In the waning years of the 19th century, astrophotography was becoming an important scientific tool. Being able to hook a camera up to a telescope and take long exposures meant being able to see fainter objects, revealing previously hidden details. It also meant that spectroscopy became a force, say it with me now, for science. A spectrum is the result when you divide the incoming light from an object into individual colors, or wavelengths. This reveals a vast amount of physical data about the object. But in the late 1800s, we were only just starting to figure that out. Interpreting stellar spectra was a tough problem. The spectrum we measure from a star is a combination of two different kinds of spectra. Stars are hot, dense balls of gas, so they give off a continuous spectrum; that is, they emit light at all wavelengths. However, stars also have atmospheres, thinner layers of gas above the denser inner layers. These gases absorb light at specific wavelengths from the light below depending on the elements in them. The result is that the continuous spectrum of a star has gaps in it, darker bands where different elements absorb different colors. At first, stars were classified by the strengths of their hydrogen lines. The strongest were called A stars, the next strongest B, then C, and so on. But in 1901, a new system was introduced by spectroscopist Annie Jump Cannon, who dropped or merged a few of the old classifications, and then rearranged them into one that classified stars by the strengths and appearances of many different absorption lines in their spectra. A few years later, physicist Max Planck solved a thorny problem in physics, showing how objects like stars give off light of different colors based on their temperature. Hotter stars put out more light at the blue end of the spectrum, while cooler ones peaked in the red. Around the same time, Bengali physicist Meghnad Saha solved another tough problem: how atoms give off light at different temperatures. Two decades later, the brilliant astronomer Cecelia Payne-Gaposchkin put all these pieces together. She showed that the spectra of stars depended on the temperature and elements in their atmospheres. This unlocked the secrets of the stars, allowing astronomers to understand not just their composition but also many other physical traits. For example, at the time, it was thought that stars had roughly the same composition as the Earth, but Payne-Gaposchkin showed that stars were overwhelmingly composed of hydrogen, with helium as the second most abundant element. The classification scheme proposed by Cannon and decoded by Payne-Gaposchkin is still used today, and arranges stars by their temperature, assigning each a letter. Because they were rearranged from an older system the letters aren’t alphabetical: So the hottest are O-type stars, slightly cooler are B, followed by A, F, G, K, and M. It’s a little weird, but many people use the mnemonic, “Oh Be A Fine Guy, Kiss Me,” or “Oh Be A Fine Girl, Kiss Me,” to remember it — which was dreamed up by Annie Jump Cannon herself! Each letter grouping is divided into 10 subgroups, again according to temperature. We’ve also discovered even cooler stars in the past few decades, and these are assigned the letters L, T, and Y. The Sun has a surface temperature of about 5500° Celsius, and is a G2 star. A slightly hotter star would be a G1, and a slightly cooler star a G3. Sirius, the brightest star in the night sky, is much hotter than the Sun, and is classified as an A0. Betelgeuse, which is red and cool, is an M2. Stars come in almost every color of the rainbow. Hot stars are blue, cool stars red. In between there are orange and even some yellow stars. But there are no green stars. Look as much as you want, and you won’t find any. It’s because of the way our eyes see color. A star can put out lots of green light, but if it does it’ll also emit red, blue, and orange. And our eyes mix those together to form other colors. A star can actually emit more green light than any other color, but we’ll wind up seeing it as white! How do I know? Because if you look at the sun’s spectrum, it actually peaks in the green! Isn’t that weird? The Sun puts out more green light than any other color, but our eyes see all the mixed colors together as white. Wait, what? White? You may be thinking the Sun is actually yellow. Not really. The light from the Sun is white, but some of the shorter wavelengths like purple and blue and some green get scattered away by molecules of nitrogen in our air. Those appear to be coming from every direction but the Sun, which is why the sky looks blue. The Sun doesn’t emit much purple, so the sky doesn’t look purple, and the green doesn’t scatter as well as blue. That gives the Sun a yellowish tint to our eyes, and looking at the Sun is painful anyway, so it’s hard to accurately gauge what color it appears. That’s also why sunsets look orange or red: You’re looking through more air on the horizon to see the Sun, so all the bluer light is scattered away. So we can use spectra to determine a lot about a star. But if you combine that with knowing a star’s distance, things get amazing. You can measure how bright the star appears to be in your telescope, and by using the distance you can calculate how much energy it’s actually giving off — what astronomers call its luminosity. An intrinsically faint light looks bright if it’s nearby, but so does a very luminous light far away. By knowing the distance, you can correct for that, and figure out how luminous the objects actually are. This was, no exaggeration, the key to understanding stars. A lot of a star’s physical characteristics are related: Its luminosity depends on its size and temperature. If two stars are the same size, but one is hotter, the hotter one will be more luminous. If two stars are the same temperature, but one is bigger, the bigger one will be more luminous. Knowing the temperature and distance means knowing the stars themselves. Still, it’s a lot of data. A century ago, spectra were taken of hundreds of thousands of stars! How do you even start looking at all that? The best way to understand a large group of objects is to look for trends. Is there a relationship between color and distance? How about temperature and size? You compare and contrast them in as many ways possible and see what pops up. I’ll spare you the work. A century ago astronomers Ejnar Hertzsprung and Henry Norris Russell made a graph, in which they plotted a star’s luminosity versus its temperature. When they did, they got a surprise: a VERY strong trend. This is called an HR Diagram, after Hertzsprung and Russell. It’s not an exaggeration to call it the single most important graph in all of astronomy! In this graph, really bright stars are near the top, fainter ones near the bottom. Hot, blue stars are on the left, and cool, red stars on the right. The groups are pretty obvious! There’s that thick line running diagonally down the middle, the clump to the upper right, and the smaller clump to the lower left. This took a long time to fully understand, but now we know this diagram is showing us how stars live their lives. Most stars fall into that thick line, and that’s why astronomers call it the Main Sequence. The term is a little misleading; it’s not really a sequence per se, but as usual in astronomy it’s an old term and we got stuck with it. The reason the main sequence is a broad, long line has to do with how stars make energy. Like the Sun, stars generate energy by fusing hydrogen into helium in their cores. A star that fuses hydrogen faster will be hotter, because it’s making more energy. The rate of fusion depends on the pressure in a star’s core. More massive stars can squeeze their cores harder, so they fuse faster and get hotter than low mass stars. It’s pretty much that simple. And that explains the main sequence! Stars spend most of their lives fusing hydrogen into helium, which is why the main sequence has most of the stars on it; those are the ones merrily going about their starry business of making energy. Massive stars are hotter and more luminous, so they fall on the upper left of the main sequence. Stars with lower mass are cooler and redder, so they fall a little lower to the right, and so on. The Sun is there, too, more or less in the middle. What about the other groups? Well, the stars on the lower left are hot, blue-white, but very faint. That means they must be small and we call them white dwarfs. They’re the result of a star like the Sun eventually running out of hydrogen fuel. We’ll get back to them in a future episode. The stars on the upper right are luminous but cool. They must therefore be huge. These are red giants, also part of the dying process of stars like the Sun. Above them are red SUPERgiants, massive stars beginning their death stage. You can see some stars that are also that luminous but at the upper left; those are blue supergiants. They’re more rare, but they too are the end stage for some stars, and again we’ll get to them soon enough in a future episode. But I’ll just say here that it, um, doesn’t end well for them. But, on a brighter note, we literally owe our existence to them. And this implies something very nifty about the HR diagram: Stars can change position on it. Not only that, but massive stars versus low mass stars age differently, and go to different parts of the HR diagram as they die. In many ways, the diagram allows us to tell at a glance just what a star is doing with itself. This difference between the way low mass stars like the Sun and higher mass stars age is actually critical to understanding a lot more about what we see in the sky… so much so that they’ll be handled separately in later episodes. I’m sorry to tease so much about what’s to come, but this aspect of stars — finally understanding them physically — was a MAJOR step in astronomy, leading to understanding so much more. And don’t you worry: we’ll get to all that. Today you learned that stars can be categorized using their spectra. Together with their distance, this provides a wealth of information about them including their luminosity, size, and temperature. The HR diagram plots stars’ luminosity versus temperature, and most stars fall along the main sequence, where they live most of their lives. Crash Course Astronomy is produced in association with PBS Digital Studios. Head over to their YouTube channel and catch even more awesome videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought Café.

Contents

Taxonomic systems

In 1973, astronomers Clark R. Chapman, David Morrison, and Ben Zellner developed a simple taxonomic system for asteroids based on color, albedo, and spectral shape. The three categories were labelled "C" for dark carbonaceous objects, "S" for stony (silicaceous) objects, and "U" for those that did not fit into either C or S.[2] This basic division of asteroid spectra has since been expanded and clarified.[3] A number of classification schemes are currently in existence,[4] and while they strive to retain some mutual consistency, quite a few asteroids are sorted into different classes depending on the particular scheme. This is due to the use of different criteria for each approach. The two most widely used classifications are described below:

Overview of Tholen and SMASS

Summary of asteroid taxonomic classes[5]
Tholen Class SMASSII
(Bus Class)
Albedo Spectral Features
A A moderate Very steep red slope shortward of 0.75 μm; moderately deep absorption feature longward of 0.75 μm.
B, F B low Linear, generally featureless spectra. Differences in UV absorption features and presence/absence of narrow absorption feature near 0.7 μm.
C, G C, Cb, Ch, Cg, Chg low Linear, generally featureless spectra. Differences in UV absorption features and presence/absence of narrow absorption feature near 0.7 μm.
D D low Relatively featureless spectrum with very steep red slope.
E, M, P X, Xc, Xe, Xk from low (P)
to very high (E)
Generally featureless spectrum with reddish slope; differences in subtle absorption features and/or spectral curvature and/or peak relative reflectance.
Q Q moderate Reddish slope shortward of 0.7 μm; deep, rounded absorption feature longward of 0.75 μm.
R R moderate Moderate reddish slope downward of 0.7 μm; deep absorption longward of 0.75 μm.
S S, Sa, Sk, Sl, Sq, Sr moderate Moderately steep reddish slope downward of 0.7 μm; moderate to steep absorption longward of 0.75 μm; peak of reflectance at 0.73 μm. Bus subgroups intermediate between S and A, K, L, Q, R classes.
T T low Moderately reddish shortward of 0.75 μm; flat afterward.
V V moderate Reddish shortward of 0.7 μm; extremely deep absorption longward of 0.75 μm.
K moderate Moderately steep red slope shortward of 0.75 μm; smoothly angled maximum and flat to blueish longward of 0.75 μm, with little or no curvature.
L, Ld moderate Very steep red slope shortward of 0.75 μm; flat longward of 0.75 μm; differences in peak level.
O Peculiar trend, known so far for very few asteroids.

S3OS2 classification

The Small Solar System Objects Spectroscopic Survey (S3OS2 or S3OS2, also known as the Lazzaro classification) observed 820 asteroids, using the former ESO 1.52-metre telescope at La Silla Observatory during 1996–2001.[1] This survey applied both the Tholen and Bus–Binzel (SMASS) taxonomy to the observed objects, many of which had previously not been classified. For the Tholen-like classification, the survey introduced a new "Caa-type", which shows a broad absorption band associated indicating an aqueous alteration of the body's surface. The Caa class corresponds to Tholen's C-type and to the SMASS' hydrated Ch-type (including some Cgh-, Cg-, and C-types), and was assigned to 106 bodies or 13% of the surveyed objects. In addition, S3OS2 uses the K-class for both classification schemes, a type which does not exist in the original Tholen taxonomy.[1]

Bus–DeMeo classification

The Bus-DeMeo classification is an asteroid taxonomic system designed by Francesca DeMeo, S. J. "Bobby" Bus and Stephen M. Slivan in 2009.[6] It is based on reflectance spectrum characteristics for 371 asteroids measured over the wavelength 0.45–2.45 micrometers. This system of 24 classes introduces a new "Sv"-type and is based upon a principal component analysis, in accordance with the SMASS taxonomy, which itself is based upon the Tholen classification.[6]

Tholen classification

The most widely used taxonomy for over a decade has been that of David J. Tholen, first proposed in 1984. This classification was developed from broad band spectra (between 0.31 μm and 1.06 μm) obtained during the Eight-Color Asteroid Survey (ECAS) in the 1980s, in combination with albedo measurements.[7] The original formulation was based on 978 asteroids. The Tholen scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types (also see § Overview of Tholen and SMASS above). The types are, with their largest exemplars in parenthesis:

C-group

Asteroids in the C-group are dark, carbonaceous objects. Most bodies in this group belong to the standard C-type (10 Hygiea), and the somewhat "brighter" B-type (2 Pallas). The F-type (704 Interamnia) and G-type (1 Ceres) are much rarer. Other low-albedo classes are the D-types (624 Hektor), typically seen in the outer asteroid belt and among the Jupiter trojans, as well as the rare T-type asteroids (96 Aegle) from the inner main-belt.

S-group

Asteroids with an S-type (15 Eunomia, 3 Juno) are silicaceous (or "stony") objects. Another large group are the stony-like V-type (4 Vesta), also known as "vestoids" most common among the members of the large Vesta family, thought to have originated from a large impact crater on Vesta. Other small classes include the A-type (246 Asporina), Q-type (1862 Apollo), and R-type asteroids (349 Dembowska).

X-group

The umbrella group of X-type asteroid can be further divided into three subgroups, depending on the degree of the object's reflectivity (dark, intermediate, bright). The darkest ones are related to the C-group, with an albedo below 0.1. These are the "primitive" P-type (259 Aletheia, 190 Ismene), which differ from the "metallic" M-type (16 Psyche) with an intermediate albedo of 0.10 to 0.30, and from the bright "enstatite" E-type asteroid, mostly seen among the members of the Hungaria family in the innermost region of the asteroid belt.

Taxonomic features

The Tholen taxonomy may encompass up to four letters (e.g. "SCTU"). The classification scheme uses the letter "I" for "inconsistent" spectral data, and should not be confused with a spectral type. An example is the Themistian asteroid 515 Athalia, which, at the time of classification was inconsistent, as the body's spectrum and albedo was that of a stony and carbonaceous asteroid, respectively.[8] When the underlying numerical color analysis was ambiguous, objects were assigned two or three types rather than just one (e.g. "CG" or "SCT"), whereby the sequence of types reflects the order of increasing numerical standard deviation, with the best fitting spectral type mentioned first.[8] The Tholen taxonomy also has additional notations, appended to the spectral type. The letter "U" is a qualifying flag, used for asteroids with an "unusual" spectrum, that falls far from the determined cluster center in the numerical analysis. The notation ":" (single colon) and "::" (two colons) are appended when the spectral data is "noisy" or "very noisy", respectively. For example, the Mars-crosser 1747 Wright has an "AU:" class, which means that it is an A-type asteroid, though with an unusual and noisy spectrum.[8]

SMASS classification

This is a more recent taxonomy introduced by American astronomers Schelte Bus and Richard Binzel in 2002, based on the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) of 1,447 asteroids.[9] This survey produced spectra of a far higher resolution than ECAS (see Tholen classification above), and was able to resolve a variety of narrow spectral features. However, a somewhat smaller range of wavelengths (0.44 μm to 0.92 μm) was observed. Also, albedos were not considered. Attempting to keep to the Tholen taxonomy as much as possible given the differing data, asteroids were sorted into the 26 types given below. As for the Tholen taxonomy, the majority of bodies fall into the three broad C, S, and X categories, with a few unusual bodies categorized into several smaller types (also see § Overview of Tholen and SMASS above):

  • C-group of carbonaceous objects includes the C-type asteroid, the most "standard" of the non-B carbonaceous objects, the "brighter" B-type asteroid largely overlapping with the Tholen B- and F types, the Cb-type that transition between the plain C- and B-type objects, and the Cg, Ch, and Cgh-types that are somewhat related to the Tholen G-type. The "h" stands for "hydrated".
  • S-group of silicaceous (stony) objects includes the most common S-type asteroid, as well as the A-, Q-, and R-types. New classes include the K-type (181 Eucharis, 221 Eos) and L-type (83 Beatrix) asteroids. There are also five classes, Sa, Sq, Sr, Sk, and Sl that transition between plain the S-type and the other corresponding types in this group.
  • X-group of mostly metallic objects. This includes the most common X-type asteroids as well as the M, E, or P-type as classified by Tholen. The Xe, Xc, and Xk are transitional types between the plain X- and the corresponding E, C and K classes.
  • Other spectral classes include the T-, D-, and V-types (4 Vesta). The Ld-type is a new class and has more extreme spectral features than the L-type asteroid. The new class of O-type asteroids has since only been assigned to the asteroid 3628 Božněmcová.

A significant number of small asteroids were found to fall in the Q, R, and V types, which were represented by only a single body in the Tholen scheme. In the Bus and Binzel SMASS scheme only a single type was assigned to any particular asteroid.[citation needed]

Color indices

The characterization of an asteroid includes the measurement of its color indices derived from a photometric system. This is done by measuring the object's brightness through a set of different, wavelength-specific filters, so-called passbands. In the UBV photometric system, which is also used to characterize distant objects in addition to classical asteroids, the three basic filters are:

  • U: passband for the ultraviolet light
  • B: passband for the blue light
  • V: passband sensitive to visible light, more specifically the green-yellow portion of the visible light
Wavelengths of the visible light
Colors violet blue green yellow orange red
Wavelengths 380–450 nm 450–495 nm 495–570 nm 570–590 nm 590–620 nm 620–750 nm

In an observation, the brightness of an object is measured twice through a different filter. The resulting difference in magnitude is called the color index. For asteroids, the U–B or B–V color indices are the most common ones. In addition, the V–R, V–I and R–I indices, where the photometric letters stand for visible (V), red (R) and infrared (I), are also used. A photometric sequence such as V–R–B–I can be obtained from observations within a few minutes.[10]

Mean-color indices of dynamical groups in the outer Solar System[10]:35
Color Plutinos Cubewanos Centaurs SDOs Comets Jupiter trojans
B–V 0.895±0.190 0.973±0.174 0.886±0.213 0.875±0.159 0.795±0.035 0.777±0.091
V–R 0.568±0.106 0.622±0.126 0.573±0.127 0.553±0.132 0.441±0.122 0.445±0.048
V–I 1.095±0.201 1.181±0.237 1.104±0.245 1.070±0.220 0.935±0.141 0.861±0.090
R–I 0.536±0.135 0.586±0.148 0.548±0.150 0.517±0.102 0.451±0.059 0.416±0.057

Appraisal

These classification schemes are expected to be refined and/or replaced as further research progresses. However, for now the spectral classification based on the two above coarse resolution spectroscopic surveys from the 1990s is still the standard. Scientists have been unable to agree on a better taxonomic system, largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful).

Some groupings of asteroids have been correlated with meteorite types:

See also

References

  1. ^ a b c Lazzaro, D.; Angeli, C. A.; Carvano, J. M.; Mothé-Diniz, T.; Duffard, R.; Florczak, M. (November 2004). "S3OS2: the visible spectroscopic survey of 820 asteroids" (PDF). Icarus. 172 (1): 179–220. Bibcode:2004Icar..172..179L. doi:10.1016/j.icarus.2004.06.006. Retrieved 22 December 2017.
  2. ^ Chapman, C. R.; Morrison, D.; Zellner, B. (May 1975). "Surface properties of asteroids - A synthesis of polarimetry, radiometry, and spectrophotometry". Icarus. 25 (1): 104–130. Bibcode:1975Icar...25..104C. doi:10.1016/0019-1035(75)90191-8. Retrieved 11 October 2018.
  3. ^ Thomas H. Burbine: Asteroids – Astronomical and Geological Bodies. Cambridge University Press, Cambridge 2016, ISBN 978-1-10-709684-4, p.163, Asteroid Taxonomy
  4. ^ Bus, S. J.; Vilas, F.; Barucci, M. A. (2002). "Visible-wavelength spectroscopy of asteroids". Asteroids III. Tucson: University of Arizona Press. p. 169. ISBN 978-0-8165-2281-1.
  5. ^ Cellino, A.; Bus, S. J.; Doressoundiram, A.; Lazzaro, D. (March 2002). "Spectroscopic Properties of Asteroid Families" (PDF). Asteroids III: 633–643. Bibcode:2002aste.book..633C. Retrieved 27 October 2017.
  6. ^ a b DeMeo, Francesca E.; Binzel, Richard P.; Slivan, Stephen M.; Bus, Schelte J. (July 2009). "An extension of the Bus asteroid taxonomy into the near-infrared" (PDF). Icarus. 202 (1): 160–180. Bibcode:2009Icar..202..160D. doi:10.1016/j.icarus.2009.02.005. Archived from the original on 17 March 2014. Retrieved 28 March 2018. (Catalog Archived 2018-03-29 at the Wayback Machine at PDS)
  7. ^ Tholen, D. J. (1989). "Asteroid taxonomic classifications". Asteroids II. Tucson: University of Arizona Press. pp. 1139–1150. ISBN 978-0-8165-1123-5.
  8. ^ a b c David J. Tholen. "Taxonomic Classifications Of Asteroids – Notes". Retrieved 6 January 2019.
  9. ^ Bus, Schelte J.; Binzel, Richard P. (July 2002). "Phase II of the Small Main-Belt Asteroid Spectroscopic Survey. A Feature-Based Taxonomy". Icarus. 158 (1): 146–177. Bibcode:2002Icar..158..146B. doi:10.1006/icar.2002.6856. Retrieved 11 October 2018.
  10. ^ a b Fornasier, S.; Dotto, E.; Hainaut, O.; Marzari, F.; Boehnhardt, H.; De Luise, F.; et al. (October 2007). "Visible spectroscopic and photometric survey of Jupiter Trojans: Final results on dynamical families" (PDF). Icarus. 190 (2): 622–642. arXiv:0704.0350. Bibcode:2007Icar..190..622F. doi:10.1016/j.icarus.2007.03.033.

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