Fundamental Astronomy

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416 20. Astrobiology According to this scheme viruses are not alive although they have certain properties common to living beings. There are also some other molecules, such as viroids and prions, that are not classified as living; yet they are not quite inanimate. If even the definition of terrestrial life leads to such borderline cases, a more general definition of all possible kinds of life is truly challenging. Facing this problem we have to restrict our discussion to life that, at least to some extent, resembles our own. 20.2 Chemistry of life The set of really important elements is relatively small; it includes hydrogen (H), oxygen (O), nitrogen (N), carbon (C), sulphur (S) and phosphor (P). The heavier elements can be remembered by the mnemonic SPONC. The importance of carbon is in its ability to make lots of different very complex molecules, which are essential for life. There are three basic types of molecules that function as common building blocks to all life: lipids for membranes, nucleotides and amino acids. Amino acids consist of three different components, a carboxyl (COOH), an amine part (NH2) and a side chain, which can be just a single hydrogen or a more complex structure. Altogether there are dozens of different amino acids, but only 20 of them are used in genetically coded proteins. Amino acids can join to form more complex molecules, proteins. Typically, up to several hundred amino acids are needed for a protein molecule. Proteins have numerous functions: they support structures, they act as catalysts in nearly all biological reactions, in which case they are called enzymes, they carry messages as hormons, and so on. Nucleotides are basic building blocks of the genetic material, DNA and RNA. A nucleotide has three components, a sugar, a phosphate group and a base. The phosphate parts are always the same. All DNA molecules have the same sugar part; also all RNA molecules have the same sugar part, which, however, differs from the sugar of the DNA molecules by having one additional oxygen atom. The base can be one of five different types, adenine (A), guanine (G), cytosine (C), thymine (T), or uracil (U). DNA molecules contain bases A, G, C and T, and RNA molecules A, G, C and U. Nucleotides join to form long chains. The deoxyribonucleic acid or DNA (Fig. 20.1) consists of two such chains that are bound together as two intertwined helices. The corresponding bases join together by hydrogen bonds. The bases come always in matching pairs, AT, TA, CG, or GC. A helix is like a screw that can be either left- or righthanded. All terrestrial DNA molecules have the same handedness or chirality: they are all of L-type, or lefthanded. The reason for the asymmetry is not quite understood. The DNA molecule contains information on how to make proteins. Three consequtive base pairs form a code, called a codon, that specifies one amino acid. Usually thousands of such triplets together contain instructions for building a protein. The basic units of heredity are called genes. They are regions of DNA whose final products are either proteins or RNA molecules. Human cells contain some 25 000 genes, and the DNA consists of 3 × 10 9 base pairs. In plants the numbers can be much higher, but the simplest known bacterial genomes have only a few hundred genes. It has been estimated that the minimum number of genes needed for a living being is about 200–300. Actually, only a part of the DNA contains genetic information. The rest is called junk-DNA since it has no known function. The fraction of the junk-DNA is highly variable; in some bacteria the amount is very small. The DNA is the storage of the genetic code, but the code is not functional directly from the DNA. Instead a complex molecular translation machinery is needed to execute the instructions. First, the instructions encoded in the genetic sequence are copied (or transcribed) into another type of nucleic acid. This ribonucleic acid or RNA resembles the DNA but has one more oxygen atom in its sugar. Different RNA molecules serve several roles in the translation process. Messenger RNA (mRNA) carries the information in the DNA to an organ called ribosome, composed of several RNA’s and multiple proteins. There another RNA, the transfer RNA (tRNA), brings amino acids into the reaction site, and still another RNA molecule, or one of the ribosomal RNA’s (rRNA) forms the linkages (peptide bonds) between the adjacent amino acids.
RNA molecules have to carry instructions for making just one protein, and thus they are much shorter than DNA molecules. Still they contain similar information, and in some simple life forms like viruses they can act as the storage of the genetic code. 20.3 Prerequisites of life Fig. 20.1. (a) The DNA molecule consists of two strands of alternating phosphate groups and sugars connected by matching base pairs. When the information stored in a fragment of the DNA is needed, the strands separate and the bases are used to produce an RNA molecule that will contain the same information. (b) The strands coil to form a double helix. (Webb: Where is everybody?) 20.3 Prerequisites of life Just after the big bang there were hardly any other elements than hydrogen and helium. Heavier elements are needed, both for life itself and for solid planets on which life can evolve. Thus at least some of the earliest stars had to explode as supernovas and eject their fusion products to the interstellar space. Most stars in elliptic galaxies and globular clusters are very old and have a low metallicity. Therefore they are not very probable locations for life. The most suitable places for life seem to be the disk populations 417
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Fundamental Astronomy
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Dr. Hannu Karttunen University of T
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VI Preface to the First Edition The
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VIII Contents 5.6 Continuous Spectr
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X Contents 16. Star Clusters and As
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1. Introduction 1.1 TheRoleofAstron
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1.2 Astronomical Objects of Researc
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theory of relativity must be used t
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of the gas may be 10 −21 kg m −
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12 2. Spherical Astronomy angles co
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14 or 2. Spherical Astronomy sin B
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16 2. Spherical Astronomy defined t
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18 2. Spherical Astronomy the verna
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20 2. Spherical Astronomy pole. The
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22 2. Spherical Astronomy Fig. 2.16
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26 2. Spherical Astronomy
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30 2. Spherical Astronomy numbers,
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32 2. Spherical Astronomy In the 19
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34 2. Spherical Astronomy mid-Septe
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36 2. Spherical Astronomy correspon
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38 2. Spherical Astronomy second of
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2. Spherical Astronomy 40 Since ∆
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42 2. Spherical Astronomy For the s
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44 2. Spherical Astronomy red. What
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3. Observations and Instruments Up
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the long, oblique path through the
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Fig. 3.6a-e. Diffraction and resolv
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3.2 Optical Telescopes Fig. 3.10. T
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3.2 Optical Telescopes Fig. 3.13. T
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so thin that it absorbs very little
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3.2 Optical Telescopes 59
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3.2 Optical Telescopes 61
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3.2 Optical Telescopes ◭ Fig. 3.1
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and CCD-cameras have largely replac
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Fig. 3.22a-e. The principle of read
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than a prism. The dispersion can be
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3.4 Radio Telescopes Fig. 3.25. The
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Fig. 3.27. The Atacama Large Millim
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Fig. 3.30. The VLA at Socorro, New
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Fig. 3.31. (a) X-rays are not refle
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Fig. 3.33. Refractors are not suita
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tored by laser interferometers. If
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4. Photometric Concepts and Magnitu
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If we are outside the source, where
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magnitudes can be equal, the flux d
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flux L will now decrease with incre
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this line is extrapolated to X = 0,
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since due to extinction, radiation
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96 5. Radiation Mechanisms Fig. 5.1
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98 5. Radiation Mechanisms From (5.
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100 5. Radiation Mechanisms Fig. 5.
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104 5. Radiation Mechanisms We now
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5. Radiation Mechanisms 106 Again F
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110 5. Radiation Mechanisms We can
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112 5. Radiation Mechanisms differe
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114 6. Celestial Mechanics The solu
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116 6. Celestial Mechanics prove (6
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118 6. Celestial Mechanics in the d
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120 6. Celestial Mechanics But cons
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122 6. Celestial Mechanics by the e
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124 6. Celestial Mechanics Substitu
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126 6. Celestial Mechanics a cloud
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128 6. Celestial Mechanics Since th
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130 6. Celestial Mechanics Exercise
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132 7. The Solar System Fig. 7.1. (
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134 7. The Solar System for each ob
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136 7. The Solar System Fig. 7.5. L
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138 7. The Solar System Inserting t
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140 7. The Solar System Therefore o
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142 7. The Solar System Fig. 7.10.
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144 7. The Solar System behaves mor
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146 7. The Solar System Table 7.1.
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150 7. The Solar System of the plan
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152 7. The Solar System If we denot
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154 7. The Solar System where F⊥
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158 7. The Solar System larized fea
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162 7. The Solar System The outer c
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168 7. The Solar System weak and co
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170 7. The Solar System amount of w
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176 7. The Solar System ter the rin
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178 7. The Solar System face, most
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180 7. The Solar System The F ring,
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182 7. The Solar System Herschel hi
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188 7. The Solar System liding, sin
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196 7. The Solar System of this mat
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202 7. The Solar System Near the po
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204 7. The Solar System be solved f
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8. Stellar Spectra All our informat
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which appears as irregular fluctuat
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8.2 The Harvard Spectral Classifica
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ate of ionization is essentially de
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lines of titanium, scandium and van
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efore τ = 1, i. e. the radiation i
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yielding Iν(0,θ)= Sν(τ ∗ ) +
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222 9. Binary Stars and Stellar Mas
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10. Stellar Structure The stars are
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where a = 4σ/c = 7.564 × 10−16
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where me is the electron mass and N
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is produced by the proton-proton (p
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oxygen, which in turn reacts to for
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According to (4.7), (4.4) and (5.16
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Example 10.6 The Radiation Pressure
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11. Stellar Evolution In the preced
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increases. The stellar surface temp
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Since stars are most likely to be f
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Fig. 11.4a-c. Energy transport in t
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Fig. 11.5. The structure of a massi
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tract to planetlike brown dwarfs. S
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Fig. 11.10a-f. Evolution of a low-m
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Fig. 11.12. The Wolf-Rayet star WR
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In a neutron capture, a nucleus wit
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This is several orders of magnitude
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264 12.TheSun Fig. 12.2. (a) The ro
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266 12.TheSun according to the heli
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268 12.TheSun shines into view for
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270 12.TheSun was established that
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272 12.TheSun Fig. 12.11. In pairs
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274 12.TheSun Fig. 12.15. (a) Quies
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276 12.TheSun Fig. 12.17. An X-ray
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13. Variable Stars Stars with chang
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Fig. 13.3. The variation of brightn
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Fig. 13.5. The lightcurve of a long
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duced when the carbon condenses int
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13.3 Eruptive Variables Fig. 13.12.
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Fig. 13.15. Supernova 1987A in the
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14. Compact Stars In astrophysics t
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about 1.4 M⊙, which is thus the u
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1968. In the following year it was
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ditions required for hypernova expl
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This is greater than the speed of l
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Fig. 14.12. Scale drawings of 16 bl
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are (or, in the radio case, have on
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14.6 Exercises Exercise 14.1 The ma
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308 15. The Interstellar Medium Fig
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310 15. The Interstellar Medium R d
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312 15. The Interstellar Medium pol
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318 15. The Interstellar Medium Tab
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322 15. The Interstellar Medium met
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324 15. The Interstellar Medium
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326 15. The Interstellar Medium He
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330 15. The Interstellar Medium up
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332 15. The Interstellar Medium hav
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334 15. The Interstellar Medium len
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336 15. The Interstellar Medium dro
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338 15. The Interstellar Medium the
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340 16. Star Clusters and Associati
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17. The Milky Way On clear, moonles
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Fig. 17.3. The directions to the ga
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classes of stars are main sequence
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Fig. 17.9. The size of the volume e
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lar material similarly moves in the
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Fig. 17.14. In order to derive Oort
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Fig. 17.17. The radial velocity as
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a suitable mass distribution, such
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esting because it may be a small-sc
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Page 368 and 369: 18. Galaxies The galaxies are the f
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Page 394 and 395: 394 19. Cosmology tographs form an
Page 396 and 397: 396 19. Cosmology Fig. 19.3. Hubble
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Page 412 and 413: 412 19. Cosmology where Ω0 = ρ0/
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Page 418 and 419: 418 20. Astrobiology of spiral gala
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Page 430 and 431: Appendices 431
Page 432 and 433: Ellipse. Equation in rectangular co
Page 434 and 435: A and B can be determined graphical
Page 436 and 437: When using matrix formalism it is c
Page 438 and 439: Similarly, a volume integral I = f
Page 440 and 441: B. Theory of Relativity Albert Eins
Page 442 and 443: time interval between the events is
Page 444 and 445: C. Tables Table C.1. SI basic units
Page 446 and 447: Table C.6. The Sun Property Symbol
Page 448 and 449: Table C.11. Osculating elements of
Page 450 and 451: Discoverer Year of a Psid e i r M
Page 452 and 453: Table C.16. Periodic comets with se
Page 454 and 455: Table C.17 (continued) C. Tables Na
Page 456 and 457: C. Tables Table C.19. Some double s
Page 458 and 459: Table C.22. Optically brightest gal
Page 460 and 461: Table C.23 (continued) Abbreviation
Page 462 and 463: C. Tables Table C.26. Millimetre an
Page 464 and 465: Table C.27 (continued) Satellite La
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468 Answers to Exercises 6.2 a = 1.
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470 Answers to Exercises Chapter 17
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472 Further Reading Morrison (ed.):
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474 Further Reading Maps and Catalo
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Name and Subject Index A aberration
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coherent radiation 95 Cold Dark Mat
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forbidden transition 101, 332 four-
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Kellman, Edith 212 Kepler’s equat
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nautical mile 15 nautical triangle
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Reber, Grote 69 recombination 95, 9
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synchronous rotation 135 synchrotro
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Colour Supplement 491
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Plate 14. The first view from the s
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Plate 1 Plate 2 Colour Supplement 4
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Plate 6 Plate 5 Colour Supplement 4
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Plate 9 Colour Supplement Plate 10
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Plate 23 Plate 24 Colour Supplement
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Colour Supplement Plate 29 507
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Fundamental Astronomy

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