# Hvorfor er [O III] en god densitetssonde i interstellært medium?

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Ifølge Draine i sin bog "Physics of the interstellar and intergalactic medium" (side 210/211) og Caltech [O III] linjeforhold (blandt andre) er en god densitetssonde, men jeg kan ikke rigtig forstå, hvorfor det er tilfældet. Kan nogen forklare mig, hvordan dette fungerer? Det behøver ikke at være numerisk eller med detaljerede formler, bare ideen om, hvad der foregår.

Nøglen til at forstå dette er begrebet forbudte linjer og forbudte overgange.

En forbudt overgang er en, der ikke kan forekomme strålende via en elektrisk dipolinteraktion. I stedet skal den gå enten gennem magnetisk dipol eller elektrisk kvadrupolemission med meget lavere sandsynlighed, eller overgangen kan gennemføres ved kollisional (de) excitation.

En forbudt linje er strålingen ved en bølgelængde svarende til en forbudt overgang. I laboratorieplasmaer ses disse normalt ikke - deraf udtrykket forbudte linjer - fordi overgange normalt udføres ved kollisional de-excitation på en tidsskala, der er meget kortere end den strålende levetid gennem magnetisk dipol / elektrisk kvadrupolemission. Imidlertid kan densiteterne i astrofysiske plasmaer være mange størrelsesordener lavere end selv de bedste laboratoriesuger. I dette tilfælde forbudte linjer er set og faktisk ofte er et vigtigt middel til strålingskøling.

Egenskaben ved forbudte linjer, der gør dem til et fremragende tæthedsfølsomt værktøj, er, at de kan "slukkes", hvis densiteten bliver stor nok til at gøre kollisional de-excitation mere sandsynlig end strålende de-excitation. Styrken af ​​den forbudte linie er således følsom over for elektrondensiteten (hvilket er det, der dominerer kollisionerne) mellem densiteter, hvor "quenching" begynder at blive effektiv, og højere densiteter, hvor linjen i det væsentlige bliver ikke observerbar.

Med det specifikke eksempel på OIII (dobbelt ioniseret ilt) optisk linieemission. Der er tre forbudte overgange af interesse mellem $^ 1D_2 rightarrow ^ 3P_2$ (501 nm), $^ 1D_2 rightarrow ^ 3P_1$ (496 nm) og $^ 1S_0 rightarrow ^ 1D_2$ (436 nm) stater. Disse overgange producerer optiske forbudte linjer, der standses ved forskellige karakteristiske tætheder. Måling af en linjestyrke forhold er vigtigt, fordi forholdet vil være uafhængig af overflod af OIII. Forholdet afhænger af elektrondensitet, forudsat at elektrondensiteten er $> 10 ^ {5} cm ^ {- 3}$, og temperatur (ved lavere tætheder er forholdet bare temperaturafhængig).

For at få tætheden kræves i dette tilfælde mere information - normalt leveres af et lignende linjeforhold for noget som NII, som har et lignende sæt forbudte linjer, men med forskellig tæthed og temperaturafhængighed.

En mere ligetil rute er at bruge linjeforholdet til forbudte linjer af OII eller SII, hvor der er et tæt placeret energiniveau, begge gennemgår forbudte overgange til det samme lavere niveau. I dette tilfælde er forholdene tæthedsfølsomme, men ikke temperaturfølsomme, igen over et interval af tætheder, hvor den ene eller anden af ​​overgangene er i stand til at blive quenchet (f.eks. For OII 372,6 / 372,8 nm, $10 ^ 2 ## INTERSTELLAR PROBES:EN NY TILGANG TIL SETI Interstellare transmissioner via energimarkører (fotoner) eller stofmarkører (sonder) ser ud til at være energisk uadskillelige alternativer for avancerede tekniske samfund. Da kun Type II- og Type III-civilisationer realistisk har råd til fyrtårne ​​eller stjernesondeteknologi, antyder alternative skelnenhedskriterier den mulige overlegenhed af intelligente artefakter til kontakt- og kommunikationsmissioner uden for jordkulturer. Der foreslås en afbalanceret og mere omkostningseffektiv søgning efter ekstraterrestriel intelligens (SETI) -strategi. ### 1. INTRODUKTION SETI-strategier kan også grupperes efter de fysiske kommunikationsmidler. Så vidt vi ved, skal al information bæres på markører for energi eller stof [1]. Således kan interstellar kommunikation forekomme ved at udveksle data, der bæres enten på masseløse energipakker (fx fotoner, neutrinoer) eller på massive sager (f.eks. Artefakter, sonder, skibe). De fleste former for kontakt med udenjordiske civilisationer falder inden for en af ​​de fire felter i tabel 1. Tabel 1 . Grundlæggende taksonomi for SETI-strategier og estimeret indsats til dato Fysiske midler til transmission af information Formodet fremmed motiv: Målrettet Formodet fremmed motiv: Ikke-formålstjenlig Energimarkører STRÅLE FOR STRØMME ( 10 5 timer,$ 10 6)

STRØMME TIL AFFALD
(

$10 3 timer,$ 10 5)

Materiale markører PROBE STRATEGI
(timer,)
PASSIV ARFAKTSTRATEGI
(

Lige siden Cocconi og Morrisons sædvanlige papir [2] om brintlinjesignalering blev offentliggjort i 1959, er det klassiske SETI-arbejde gået ud fra en "fyrstrategi". I denne model [3-4] antager søgere, at intelligente fremmede kulturer aktivt forsøger at få kontakt med andre lignende samfund på tværs af galaksen ved hjælp af radiofotoner, som er lette at generere, og som lider lidt i absorption ved passage gennem det interstellære medium. ET-signaler kan sendes i alle retninger eller kan være rettet mod specifikke stjerner, der mest sandsynligt har havnende levende livsformer, der ligner den transmitterende race. Omfattende timingsteknikker [5-11] og andre specifikke søgestrategier [3-4, 6, 12-20] er blevet foreslået. I hvert tilfælde kræves det af den potentielle modtager at oprette og vedligeholde passende detektionsapparater, der er indstillet til at modtage de fremmede meddelelser på sådanne frekvenser og på sådanne tidspunkter som begge løb måtte være enige om er "foretrukne" [3-4, 6, 21-24]. Hidtil har langt størstedelen af ​​den faktiske SETI-forskning koncentreret sig om fyrstrategier. Sandsynligvis er der brugt ca. $10 6 og 105 observationstimer verden over på denne tilgang. Den "aflytningsstrategi" opstod også i 1959 med Dysons diskussion af de observerbare egenskaber ved avancerede teknologiske civilisationer [25]. Dyson insisterede på, at det malthusiske tryk i sidste ende kan føre en intelligent art til at udnytte al den tilgængelige masse og energi i hele deres solsystem, hvilket resulterer i en enorm artefaktskal, der kredser om den centrale stjerne. Uanset hvor effektivt energien blev brugt, til sidst ville den skulle fremstå som spildvarme. Dyson anbefalede en søgning efter disse infrarøde emissioner på ca. 10 mikron. Udover tegn på storskala astroingeniør [6, 26-27] kan intern kommunikationslækage stråling være detekterbar over interstellære afstande [3, 12, 28-29]. Faktisk er Jordens elektromagnetiske signatur ved radiofrekvenser for nylig blevet analyseret set fra fremmede observatørers forsøg på at opdage intelligent teknologisk civilisation i vores solsystem [30-31] Til dato har måske$ 10 5 og 10 3 timers observation været blevet brugt over hele verden på aflytning uden for jorden.

De resterende SETI-strategier, der hver involverer stofmarkører, blev først foreslået i den moderne videnskabelige litteratur af Bracewell så tidligt som i 1960 [29, 32-34]. Under forudsætning af, at følelsesliv er rimeligt sjældent i hele Galaxy, mente Bracewell, at det kunne være mere økonomisk at sende meget sofistikerede messenger-sonder til andre stjernesystemer på jagt efter nye medlemmer til "Galactic Club." Både "probestrategien" og "artefaktstrategien" ville indebære at se på passende steder (måner, asteroider, trojanske point) efter bevis for fremmede enheder lige her i vores eget solsystem. Bortset fra et par beskedne undersøgelser af det nysgerrige, men eneste kvasi-SETI Long-Delayed Echo (LDE) fænomen [35-46], er der i det væsentlige og 0 observationstimer blevet investeret i disse tilgange til SETI.

Ubalancen mellem finansiering og indsats ser ud til at stamme fra de naturlige teknologiske ikke-chauvinistiske perspektiver, som mange radioastronomer, der forsker inden for dette felt, har. Da menneskeheden nu har den tekniske ekspertise til at sende radiomeddelelser, går det traditionelle argument, skal ET'er ikke så godt finde radio det optimale medium til interstellar kommunikation? Beacon-søgninger er ofte berettigede med den begrundelse, at sådanne signaler er alt, hvad vi er i stand til at lede efter på dette tidspunkt. Heldigvis er dette simpelthen ikke tilfældet.

Formålet med dette papir er at overveje spørgsmålet om interstellar kommunikation og kontakt fra det udenjordiske synspunkt. Hvordan kan en avanceret teknisk civilisation komme til at skabe målrettet kontakt med andre teknologiske kulturer i lignende eller mindre grad af udvikling? Det vil blive vist, at informationstransmission med fotoner (energimarkører) eller sonder (stofmarkører) er energisk og teknologisk uadskillelige alternativer for ethvert samfund, hvis tekniske kapacitet når teoretiske fysiske grænser. Når andre faktorer bruges som skelnenhedskriterier, går prober meget bedre end fotoner. Implikationerne for en ny, mere afbalanceret tilgang til SETI-forskning diskuteres og opsummeres i sidste afsnit.

### 2. ENERGI OG CIVILISERING

Kardashev [48] har udtænkt et særligt nyttigt klassificeringsskema, der undertegner alle tænkelige udenjordiske kulturer, baseret på bruttomål for den samlede strømudnyttelse. En fuld Type I-civilisation har adgang til alle planetariske energiressourcer (sollys, oceanisk deuterium, magmatisk varme osv.), Som mange forskere er enige om, at de sandsynligvis ikke kan frigives hurtigere end ca. 10 "joule / sekund uden at forårsage uoprettelig skade på planetariske økologier. Mennesket med den nuværende samlede teknologiske effekt

10 11 watt, er stadig men et voksende Type I samfund. En moden Type II-kultur, såsom en "Dyson-sfære", der kredser om levesteder, industrielle artefakter [49] eller "rumkolonier" [50-53], kan have adgang til hele magtproduktionen af ​​sin hjemmesol. For typiske F-K-stjerner er dette måske 10 26 watt. Endelig kan en fuldt udviklet galakse civilisation af type III, en organisation bestående af milliarder af beboede eller koloniserede stjernesystemer, muligvis styre effekt fra en hel galakse, omkring 10 37 watt. Det er på denne baggrund af utænkeligt stærke kulturer, at spørgsmål om interstellære kommunikationsteknikker stilles bedst.

Tabel 2. Resumé af Kardashev-civilisationer
Civilisation Type Samlet tilgængelig strøm Samlet tilgængelig masse
(watt) (kg)
Planetarisk jeg 10 15 10 24
Fantastisk II 10 26 10 30
Galaktisk III 10 37 10 41

Et par ord til skeptikere og teknologiske pessimister: Tabel 2 repræsenterer en samling af meget reelle, håndgribelige muligheder, uanset ens personlige syn på deres sandsynlighed målt ved begrænsningerne i den nuværende jordiske videnskab. Man behøver ikke tro på, at Type II- eller Type III-civilisationer faktisk eksisterer for at indrømme, at de kan, eller at deres eksistens kan være en nødvendig forudsætning for opnåelse af storstilet astroingeniør og interstellar kolonisering.

### 3. TEORETISK ENERGISK EFFEKTIVITET AF MARKEDSoverførsler

På dette grundlag har Drake [57-58] foreslået princippet om økonomi, der fastslår, at økonomi praktiseres universelt af alle overlevende klasser af levende systemer. Således vil teknologiske civilisationer i hele Galaxy normalt vælge de løsninger på ethvert teknisk problem, der er billigste. Naturligvis kan det tænkes, at nogle udenjordiske racer måske ikke er underlagt de samme konkurrenceprægede regler for naturlig udvælgelse som de jordiske livsformer [59]. Stadig andre arter vælger måske kortere levetid, mere overflødige livsstil end almindelig sparsommelighed ville diktere. Men generelle levende systemteoretikere er enige om, at Drakes princip bør gælde praktisk talt på alle systemiske niveauer fra celler til samfund, da det ser ud til at være en manifestation af det velkendte princip om mindst mulig indsats [60-61].

Når man foreløbigt accepterer princippet om økonomi, som det gælder for udenjordiske kulturer såvel som vores egne, følger det, at en teknisk kommunikativ civilisation vælger de kommunikationsmidler, der koster mindst at udføre jobbet. "Jobbet" i denne sammenhæng er overførsel af information og kompleksitet over det store interstellære rum.

Den kvantitative informationsenhed er det binære ciffer eller "bit". Under forudsætning af at alle data skal bæres på markører for stof-energi, er omkostningsdimensionen i joule. Endelig skal data transmitteres inden for et tidsinterval, målt i sekunder. Disse størrelser definerer et fortjensttal, der kan beregnes for enhver kommunikationsmetode. Dette tal, et mål for den energiske effektivitet af informationsoverførsel pr. tidsinterval, udtrykkes i enheder / bit / joule sekund

Overvej først muligheden for signalering i beacon-tilstand ved hjælp af fotoner. Ifølge Shannons klassiske teori [62-63] er den teoretiske maksimale informationsoverførselshastighed (med en vilkårligt lille fejlfrekvens) gennem en kanal med båndbredde Dn Dn log 2 (1 + S / N) bits / sekund, hvor S / N er effekt-forholdet mellem støj og signal. En sådan kapacitet opnås kun, når al redundans fjernes fra signalet, hvilket får den til at nærme sig hvid støj over båndet. Selvom dette måske mindsker dets genkendelighed som en elektromagnetisk artefakt til erhvervelsesformål, øger det dets anvendelighed som et middel til hurtig interstellar dataoverførsel, når kontakt med en anden civilisation er etableret. Under alle omstændigheder lægger Shannons grænse et maksimalt loft på den energiske effektivitet af enhver fotonisk kommunikation, hvad enten det er et erhvervelsesfyr eller en besked til regelmæssige korrespondenter.

Hvad er denne effektivitet? Den maksimale bithastighed Opstår, når båndbredden er lig med bærefrekvensen, D n = n. Fra grundlæggende kvantefysik husker vi, at energien pr. Bærerfoton er h n joule, hvor h er Plancks konstant. Derfor er den maksimale teoretiske fotoniske informationsoverførselseffektivitet f.eks:

 e g = [log 2 (1 + S / N)] / h bits / joule-sekund
Dernæst overvej muligheden for signalering med sonder. Oplysningerne opnået ved måling af en tilfældig variabel, der er i stand til n værdier med sandsynligheder p 1, p 2,. p n er: hvor H er Shannons informationsfunktion [62]. Denne funktion maksimeres til p 1, p 2,. p n =. 1 / n, i hvilket tilfælde H = log 2 n stofmarkør, der kan antage n forskellige tilstande (hvoraf kun en er til stede på et givet tidspunkt) kan højst repræsentere 2 n bit information.

Antag nu, at energiniveauer bruges som informationsmarkører [64] i den databærende del af stjernesonden, og at disse niveauer ligger inden for et interval (0, E), hvor E er den maksimale tilgængelige energi i materialesystemet. Hvis energiniveauer kun kan måles med en nøjagtighed DE, kan der højst skelnes mellem n = E / DE forskellige niveauer. Hvis der ikke optages mere end et energiniveau på ethvert tidspunkt. så kan maksimalt H = log 2 (n +1) bit være repræsenteret. Hvis der bruges to markører med niveauer i (0, 1/2 E), maksimalt H = 2 log 2 (1/2 n +1) bits. Hvis der anvendes n markører med niveauer i (0, DE), kan H = n log 2 (n / n +1) = n bits repræsenteres.

Derfor opnås den optimale anvendelse af en given mængde masseenergi E, når der anvendes n stofmarkører med værdier i (0, DE). Da Heisenbergs usikkerhedsprincip definerer en minimum energimålingsnøjagtighed D E = h / D t. hvor D t er usikkerheden i målingens varighed, kan i dette tilfælde i meddelelsen bestående af H = n = E / D E = E D t / h bits bæres af stjernesonden. Ethvert selvstændigt materialesystem med masse m er underlagt en absolut begrænsning af den samlede masse-energi af alle dets informationsmarkører. Denne grænse er hvilemassen af ​​systemet m c2, hvor c er vakuumhastigheden af ​​lys, så H = m c 2 D t / h bits kan repræsenteres af hele materialesystemet. Hvis disse data læses ud i et tidsinterval Dt, er den maksimale informationsoverførselshastighed for stjernesonder m c 2 / h bits / sekund.

For at finde stofmarkørers energiske effektivitet skal transmissionshastigheden divideres med materialets samlede energi (hvileenergi plus kinetisk energi). Fra særlig relativitet er denne energi m c 2 / (1- V 2 / c 2) 1/2, hvor V er sondens interstellære krydshastighed i forhold til de stationære referencerammer for de to kommunikerende civilisationer. Derfor er den maksimale teoretiske informationsoverførselseffektivitet for stof, f.eks.

e m = (1- V 2 / c 2) 1/2 / h bits / joule-sekund

Maksimal fotoneffektivitet eg og hastighedskompenseret maksimal stofmarkeringseffektivitet em for datatransmission sammenlignes i figur 1.

Fotoneffektivitet er repræsenteret af den lodrette linje ved langt kamp, ​​som viser energisk effektivitet som en funktion af signal-støj-forholdet (primært et teknologisk valg). Bemærk, at logaritmen for S / N er påfaldende ikke-lineær for S / N 1. Selv store forbedringer i S / N ved hjælp af overlegen teknologi giver kun minimale stigninger i fotonisk energisk effektivitet.

Hastighedskompenseret stof-markør effektivitetskurve ((em V / c) ligger under den fotoniske effektivitetskurve for alle S / N 1. Alligevel er fotoner kun marginalt overlegne stof-markører i interstellar kommunikation. Hvis rimelige værdier for S / N og sondehastighed vælges til sammenligning, forskellen i energisk effektivitet er knap en størrelsesorden eller to. Denne lille forskel vil sandsynligvis blive betragtet som de minimis af meget avancerede rumfartscivilisationer Type II og Type III, som skulle have adgang til praktisk talt perfekte foton- og materiehåndteringsteknologier begrænset udelukkende af teoretiske begrænsninger pålagt af de fysiske fysiske love. Stjerner og galaktiske tekniske kulturer vil sandsynligvis se signaler og stjerneprover som energisk uadskillelige alternativer til interstellar meddelelsessendelse. Valg af kommunikationstilstand bør derfor afhænge langt mindre ved transmissionseffektivitet end på andre faktorer.

### 4. ØKONOMISK OG POLITISK MULIGHED

Men vil Type I-samfund overhovedet transmittere? Seeger [66] har foreslået et kraftfuldt radiofrekvensfyr, der er i stand til at annoncere vores tilstedeværelse til andre væsentlige løb i Galaxy, som kunne konstrueres på et crashprogram-basis inden år 2000 e.Kr. ved hjælp af forudsigelig menneskelig teknologi. Denne isotrope 10 9 watt kontinuerlige sender vil blive placeret i solbane for at give en. lav Doppler-drivhastighed og for at minimere forurening af det lokale jordbaserede elektromagnetiske miljø. Er dette et fornuftigt projekt for menneskeheden?

Til nuværende priser på mere end $10-8 / joule vil de årlige omkostninger ved energien alene til Seegers fyrinstallation løbe omkring$ 10 9. Dette er tre størrelsesordener ud over nuværende globale årlige udgifter til alle SETI-bestræbelser og hele ni størrelsesordener højere end den samlede værdi af energi frigivet af menneskeheden i fyrtårnsform til dato. (Arecibo-meddelelsen den 16. november 1974 bestod af en stråle på ca. $1 energi til den målkugleformede klynge M13, 24.000 lysår væk [67].) For at give et enkelt, kontinuerligt 1 gigawatt-fyr, ville det synes at menneskeheden skal blive fra 10 3 -10 9 gange mere velhavende som en art. Da rigdom er proportional med energi [47], skal menneskeheden udvide sin kollektive industrielle-kommercielle base fra sit nuværende magtniveau på ca. 10 13 watt på verdensplan op til mindst 10 16 -10 22 watt, før interstellære fyrtårne ​​realistisk set kan betragtes økonomisk eller politisk gennemførlig. Selvfølgelig vil vi da være en kommende Type II-civilisation, og stjernesonder-missioner såvel som fyrtårnsstationer skal let være inden for rækkevidde. Baseret på det eneste eksempel på det teknologiske samfund, vi er opmærksomme på - os selv - ser det ud til, at fyrtårnsendere ikke er økonomisk eller politisk gennemførlige for type I planetkulturer. Kun Type II og Type III organisationer kan finde de enorme udgifter til kapital og materielle ressourcer, der er nødvendige for at gennemføre et omfattende program for interstellar udforskning [68-70], acceptabelt. Da stjerneprober og signaler er energisk ækvivalente øvelser til teknisk dygtige udenjordiske civilisationer, kan en eller begge bruges i interstellær kommunikation afhængigt af de særlige formål og behov hos samfund, der søger interaktion. Informationsoverførselshastighed (psol) Fig. 1. Sammenligning af den teoretiske maksimale informationsoverførselseffektivitet af fotoniske og stofmarkører. ### 5. HVORFOR ER PROBES BEDRE For det første er der fordelen ved kommunikationsfeedback. En sonde, der opdager en grusom beboet verden, kan deltage i en ægte samtale med indfødte, en næsten øjeblikkelig udveksling og sammenvævning af kulturer. Interaktiv udveksling kan kræve kun brøkdele af et sekund mellem spørgsmål og svar. Stjernesonder på stedet, måske i kredsløb omkring værtens sol eller hjemmeplanet, kan udføre uddannelsesmæssige og sproglige funktioner i realtid med en præcision, som ikke et eksternt signalsystem kunne håbe at matche. Som en ekstra fordel kunne sådanne intelligente enheder tilvejebringe en støjfri kommunikationskanal på enhver frekvens efter kontaktpersons eget valg (29). Til sammenligning fremstår de traditionelle SETI-beacon-erhvervsscenarier lidt mere end sterile data-swaps, der kræver årtusinder pr. Cyklus snarere end millisekunder. Med interstellare fotoniske transmissioner er forsinkelser uendelige, og bredt spredte følsomme arter kan aldrig rigtig konversere. Høj sonde. intelligens vil tillade en sådan samtale, som om sendeløbet havde gjort rejsen "personligt". For det andet har prober fordelen ved erhvervelseseffektivitet. Fyrtårne ​​kan udstråle ellers nyttig energi og information ud i rummet i århundreder, årtusinder eller endda længere uden at få noget svar eller få nye oplysninger til gengæld. Da denne energi blev opdaget af ingen modtager, blev denne energi i det væsentlige spildt og udgør rent økonomisk tab for det afsendende samfund. Sådan uforsigtighed afspejler en overdreven (og muligvis selektivt ufordelagtig) grad af skødesløshed eller generaliseret altruisme fra den transmitterende kulturs side, Starprobes bliver derimod uafhængige agenter, så snart de lanceres [71]. Hvis det konstrueres ordentligt, bør det transmitterende samfund ikke have behov for yderligere energiforbrug. Sofistikerede messenger-sonder vil være selvreparation, selvprogrammering, måske endda selvreproduktion [72-74] og i stand til at genopfylde eller genoplade ved enhver anløbshavn. De kan tålmodigt være designet til at flyde i kredsløb i hundreder eller endda millioner af år, mens de afventer fremkomsten af ​​en kommunikativ kultur på egnede planeter i systemet, alternativt kan de programmeres til at hoppe fra stjerne til stjerne, indtil de finder kommunikative livsformer, og derefter gå ind ind i en udveksling med dem uden yderligere omkostninger for det oprindelige transmitterende samfund. Et datterselskab, men ikke desto mindre vigtig fordel ved starprobes, er, at de kan tjene som kosmiske "pengeskabe" til det udsendende samfunds kulturarv og viden. Hvis den transmitterende civilisation ødelægges, eller kulturen forsvinder af en eller anden grund, kan sonderne, de sendte til andre verdener, stadig fortælle deres historie til enhver villig ører i måske geologiske tidsperioder derefter. For det tredje udfylder stjernesonder den overvældende fordel ved militær sikkerhed for den transmitterende race. Interstellære fyrtårne ​​er en invitation til katastrofe i hænderne på ukendte rovdyrede fremmede civilisationer. * I. enhver situation, der involverer kontakt via signaler, skal det afsendende samfund give væk positionen for sit hjemmestjernesystem med stor risiko for blotte spekulative fordele. Denne frygtelige krænkelse af militær sikkerhed [75-76] kan afhjælpes ved hjælp af sonder i stedet for fotoner. Hvis lokal teknologisk aktivitet opdages af en intelligent artefakt, der kredser om en eller anden målstjerne, kan enheden indlede kontakt med de oprindelige tekniske arter uden nogensinde at skulle afsløre identiteten eller opholdsstedet for dens skabere. Hvis det anses for nødvendigt for stjernesonden at rapportere sine fund tilbage til det transmitterende samfund fra tid til anden, kan dette let opnås på en måde, der næsten er umulig at spore eller afkode (f.eks. Rundstrålende eller "falsk sti" udsendelser til det tomme rum kodede meddelelser, der skifter relæer gennem tilfældigt spredte repeaterstationer i ubeboede solsystemer osv.) Med andre ord hjælper sonder til at beskytte sikkerheden ved at sende samfund i enhver udveksling mellem sig selv og fremmede kulturer. ### 6. EN BALANCERET TILGANG TIL SETI På nuværende tidspunkt viden, ville SETIists gøre det godt at antage så få antagelser om fremmede motiver og teknologier, som de kan. Som Stull [81] korrekt påpeger, "alt hvad vi kan gøre er at genkende muligheden for, at der findes udenjordiske teknologier, og behandle denne mulighed som et observationsproblem." Desværre var der indtil for nylig få forskere, der alvorligt overvejede muligheden for, at udenjordiske stjernesonder allerede kunne være til stede i solsystemet. Her er et relativt simpelt "observationsproblem" lige i vores egen baghave. Forfatteren opfordrer ikke til øjeblikkelig opgivelse af alle søgninger efter ekstrasolære fyr til fordel for interstellære sonder. Imidlertid synes den grove forskel i finansiering af søgninger efter transmissioner fra udenjordisk energi-markør over materiemarkeringstransmissioner (se tabel 1) dårligt anbefalet i lyset af argumenterne præsenteret tidligere i denne artikel. Prober er mindst lige så plausible for interstellar kontakt og kommunikation som foton signaler. Den mest intelligente og retfærdige fordeling af knap SETI-programfinansiering synes at være nogenlunde lige fordeling af penge og indsats mellem energimarkør- og stofmarkørstrategier. Naturligvis skal de billigste søgninger altid forsøges først og de dyrere senere (efter at de billigere ordninger er mislykkedes). De fleste lette beacon-strategisøgninger er allerede udført eksperimentelt [4, 6, 82-94], mens få sondesøgningsstrategier er alvorligt blevet udarbejdet selv i teorien [29, 44-46, 70, 95-106]. Tabel 3 inkluderer 24 foreslåede SETI-mål, der er anført i rækkefølge efter stigende omkostninger (forfatterens bedste skøn), vanskeligheder og følsomhed eller detekterbarhed uden for jorden. Mål, der vises tidligt på listen, skal forfølges frem for senere dyrere poster. (For monetære sammenligninger er det nuværende Cross World-produkt$ 1 x 10 13, og den planetariske nettoværdi er omkring 8 x 10 14.) I SETI-programplanlægningen bør de prioritetssøgninger, der kan udføres hurtigt og billigt på kort sigt, prioriteres højere end højere dyrere fyrsøgninger, der skal monteres i de kommende årtier [107]. ## Voyager 1 opdager en 'hum' af plasmabølger i det ugyldige interstellære rum Voyager 1, der har brugt over 43 år på at zoome væk fra Jorden siden lanceringen i 1977, er faktisk meget langt væk. Dens afstand fra solen er over 150 gange afstanden mellem jorden og solen. Det tager over 21 timer for transmissioner, der kører med lyshastighed, at ankomme til Jorden. Det passerede officielt heliopausen - grænsen, hvor tryk fra solvinden ikke længere er tilstrækkelig til at skubbe ind i vinden fra det interstellære rum - i 2012. Voyager 1 har forladt solsystemet - og det finder ud af, at tomrummet i rummet trods alt ikke er helt så ugyldigt. I den seneste analyse af data fra den frygtløse sonde, fra en afstand på næsten 23 milliarder kilometer (over 14 milliarder miles), har astronomer fra 2017 og frem opdaget en konstant brummen fra plasmabølger i det interstellære medium, den diffuse gas, der lurer mellem stjernerne. "Det er meget svagt og monoton, fordi det har en smal frekvensbåndbredde," sagde astronom Stella Koch Ocker fra Cornell University. "Vi registrerer den svage, vedvarende brummen af ​​interstellar gas." Vi ved selvfølgelig, at det interstellære rum ikke er fuldstændig tom, men da stjerner er så lyse, er det meget svagere, sprødt materiale, der hænger ud mellem dem, virkelig svært at se og måle. Normalt er vi nødt til at stole på, hvordan lyset ændrer sig, når det bevæger sig gennem interstellært materiale for at vide, at det er der, og at kvantificere det. Voyager-sonderne er de første menneskeskabte objekter, der kommer ind i det interstellare rum og repræsenterer derfor en unik mulighed for at prøve det interstellare medium direkte. Selv så langt fra solen er det ikke ligefrem let og endda uden for solvindens rækkevidde. Solen er stadig et lyst og støjende dyr, der udlader soludbrud, der kan drukne de omgivende forhold. "Det interstellare medium er som en stille eller mild regn," sagde astronom James Cordes fra Cornell University. "I tilfælde af soludbrud er det som at registrere et lyn, der brister i tordenvejr, og så er det tilbage til en mild regn." Den blide regn ifølge teamet antyder, at der kunne være mere aktivitet på lavt niveau i det interstellære medium, end forskere havde troet. Hvad denne aktivitet er forårsaget af, er ikke helt klart, det kan være termisk ophidsede plasmasvingninger eller kvasitermisk støj genereret af elektroners bevægelser i plasma, der producerer et lokalt elektrisk felt. Uanset hvad der forårsager det, har opdagelsen flere konsekvenser. Brummen kan bruges til at kortlægge plasmadensiteten, da begge Voyager-prober bevæger sig dybere ind i det interstellære rum (Voyager 2 krydsede heliopausen i 2018). Det kan også bruges til bedre at forstå samspillet mellem det interstellære medium og solvinden. Vi ved, at der er en stigning i elektrondensitet lige på den anden side af heliopausen - begge Voyager-prober opdagede det, da de rejste videre. At kende densiteten af ​​det interstellare medium mere præcist kan hjælpe os med at finde ud af hvorfor. Opdagelsen og vedholdenheden af ​​emissionen antyder også, at Voyager fortsat vil være i stand til at opdage det og give os løbende aflæsninger, der hjælper os med at forstå turbulens og den store struktur i det interstellære medium. "Vi har aldrig haft en chance for at evaluere det. Nu ved vi, at vi ikke har brug for en tilfældig begivenhed relateret til solen for at måle interstellært plasma," sagde astronom Shami Chatterjee fra Cornell University. "Uanset hvad Solen laver, sender Voyager detaljer tilbage. Fartøjet siger: 'Her er densiteten, jeg svømmer igennem lige nu. Og her er den nu. Og her er den nu. Og her er den nu. '' Voyager er ret fjernt og vil gøre dette kontinuerligt. " Ikke for evigt. Den radioisotop termoelektriske generator, der driver sondens instrumenter, nedbrydes lidt mere hvert år. Omkring 2025 er det muligvis ikke længere i stand til at holde dem kørende. Derfor er det så vigtigt at samle så mange data som muligt, mens der stadig er mulighed. Forskningen er offentliggjort i Naturastronomi. Voyager 1, der har brugt over 43 år på at zoome væk fra Jorden siden lanceringen i 1977, er faktisk meget langt væk. Its distance from the Sun is over 150 times the distance between Earth and the Sun. It takes over 21 hours for transmissions traveling at light speed to arrive at Earth. It officially passed the heliopause - the boundary at which pressure from the solar wind is no longer sufficient to push into the wind from interstellar space - in 2012. Voyager 1 has left the Solar System - and it's finding that the void of space is not quite so void-like, after all. In the latest analysis of data from the intrepid probe, from a distance of nearly 23 billion kilometers (over 14 billion miles), astronomers have discovered, from 2017 onwards, a constant hum from plasma waves in the interstellar medium, the diffuse gas that lurks between the stars. "It's very faint and monotone, because it is in a narrow frequency bandwidth," said astronomer Stella Koch Ocker of Cornell University. "We're detecting the faint, persistent hum of interstellar gas." Obviously, we know that interstellar space isn't fuldstændig empty, but since stars are so bright, the vastly fainter wispy material that hangs out between them is really hard to see and measure. Usually, we have to rely on the way light changes when it travels through interstellar material to know it's there, and to quantify it. The Voyager probes are the first human-made objects to enter interstellar space, and therefore represent a unique opportunity to sample the interstellar medium directly. Even so far from the Sun, though, and even beyond the reach of the solar wind, it's not exactly easy. The Sun is still a bright and noisy beast, letting out solar eruptions that can drown out the ambient conditions. "The interstellar medium is like a quiet or gentle rain," said astronomer James Cordes of Cornell University. "In the case of a solar outburst, it's like detecting a lightning burst in a thunderstorm and then it's back to a gentle rain." That gentle rain, according to the team, suggests that there could be more low-level activity in the interstellar medium than scientists had thought. What that activity is caused by is not entirely clear it could be thermally excited plasma oscillations, or quasi-thermal noise generated by the movements of electrons in plasma, producing a local electric field. Whatever is causing it, the discovery has several implications. The hum can be used to map the plasma density as both Voyager probes move deeper into interstellar space (Voyager 2 crossed the heliopause in 2018). It can also be used to better understand the interaction between the interstellar medium and the solar wind. We know there's an increase in electron density just on the other side of the heliopause - both Voyager probes detected it when they traveled on through. Knowing the density of the interstellar medium more accurately can help us figure out why. The discovery and the persistence of the emission also suggest that Voyager will continue to be able to detect it, providing us with ongoing readings that will help us understand turbulence and the large-scale structure of the interstellar medium. "We've never had a chance to evaluate it. Now we know we don't need a fortuitous event related to the Sun to measure interstellar plasma," said astronomer Shami Chatterjee of Cornell University. "Regardless of what the Sun is doing, Voyager is sending back detail. The craft is saying, 'Here's the density I'm swimming through right now. And here it is now. And here it is now. And here it is now.' Voyager is quite distant and will be doing this continuously." Not forever, though. The radioisotope thermoelectric generator powering the probe's instruments degrades a little bit more every year. By around 2025, it may no longer be able to keep them running. Which is why it is so important to glean as much data as we can, while there's still the opportunity. ## Interstellar Medium and Galactic Center At the University of Iowa, the study of the interstellar medium and the Galactic center relies on a combination of ground and space-based observations, complemented by theoretical efforts. In particular, Iowa faculty and students are frequent usersof the Very Large Array and Very Long Baseline Array radio telescopes, and work on data obtained by the NASA Great Observatories (Chandra, Spitzer and Hubble Space Telescopes). The Galactic Center Environment The center of our Milky Way galaxy harbors a 4-million solar mass black hole in addition to dense concentrations of gas, strong magnetic fields and powerful young stellar clusters. The interplay of these components gives rise to episodic and energetic activity that arises from our Galactic nucleus. At only 25,000 light years distant, the Galactic center provides an opportunity to study these processes in detail. Professor Lang has recently completed several surveys of the central environment: (1) the first comprehensive high resolution survey of neutral hydrogen in this region using the Very Large Array. This is an important study which provides fundamental insights concerning the kinematics of the central region (including the massive black hole at its center), as well as the dynamic of star formation and ionization, (2) the first Hubble Space Telescope survey of the ionized gas as traced by the Paschen alpha line (near-infrared) reveals unprecedented detail in the interstellar medium and the interaction between the stars and gas, and (3) the first radio polarimetric survey carried out by the VLA. This survey will help to clarify the configuration and organization of the magnetic field in this region of the Galaxy, which is thought to be much stronger and more well-ordered than in the Galactic disk. A recent "Great Observatories" panorama of the Galactic Center is shown at right. Caption for image: View of the central 50 pc ( 150 light years) of the Galactic center showing ionized gas (traced by Paschen alpha emission observed by Hubble Space Telescope), hot plasma (traced in the X-ray by the Chandra X-ray Observatory) and warm dust (traced by mid-infrared radiation by the Spitzer Space Telescope). The Interstellar Medium: The Impact of Massive Stellar Clusters Professor Lang also is interested in determining how many such young, massive clusters (similar to the ones found in the Galactic center) exist throughout the Galactic disk. To do this, she is carrying out a multi-wavelength effort (with collaborators at Rochester Institute of Technology) to image the interstellar medium (ionized gas and diffuse emission from warm gas traced in the radio and infrared) surrounding a large number of massive cluster candidates. The Milky Way is thought to have as many as several hundred powerful young clusters. The Interstellar Medium: The Soft X-ray Background Professor McEntaffer studies the origin and variability of the 1/4 keV soft X-ray background. The ROSAT observatory discovered that the local ISM is dominated by million degree diffuse gas. Men dette emission is variable and tied to the variability of the solar wind leading to charge exchange of the solar wind with interstellar neutrals as a significant emission mechanism for the background. Det time variability of important charge exchange lines will be studied by a suite of suborbital sounding rocket flights occurring between 2009-2012. The Interstellar Medium: Plasma Aspects The interstellar medium is not a quiescent environment but rather a turbulent one, and this interstellar turbulence significantly affects the evolution of the galaxy, in particular the galactic magnetic field and the spatial distribution of star formation. Although the driving mechanisms supporting the pervasive turbulence in the interstellar medium have not been clearly identified, leading candidates are supernovae, strong stellar winds and outflows from massive stars, and the magnetorotational instability. This turbulence is believed to play an important role in the generation of the kiloparsec-scale galactic magnetic field through dynamo action. In the denser molecular clouds, turbulent motions are believed to play a dominant role in the regulation of the rate of star formation. Improving observations of this turbulence and advancing our theoretical understanding of its driving mechanisms and the resulting effect of galactic processes is an important frontier of astrophysical research. Professor Spangler is a leading expert in the use of interstellar scintillation, sensitive to the electron density fluctuations in the turbulent interstellar medium, to probe the characteristics of the interstellar plasma turbulence. His work has identified one of the "Great Power Laws in the Sky," a spectrum of electron density fluctuations over 12 orders of magnitude in scale. The spectral index appears to be consistent with a value of -5/3, the prediction from turbulence theory. Professor Howes employs analytical models of turbulence and high-performance direct numerical simulations to study the dynamics and dissipation of turbulence in weakly collisional plasma conditions relevant to the interstellar medium, with an aim to connect numerical predictions of the plasma turbulence to the observations from interstellar scintillation. Recent Publications Chandran, B. D. G., Quataert, E., Howes, G. G., Xia, Q., and Pongkitiwanichakul, P., 2009, ## University of California, San Diego Center for Astrophysics & Space Sciences Although space is very empty and the stars in the Milky Way are very far apart, the space between the stars contains a very diffuse medium of gas and dust astronomers call the interstellar medium (ISM). This medium consists of neutral hydrogen gas (HI), molecular gas (mostly H2), ionized gas (HII), and dust grains. Although the interstellar medium is, by several orders of magnitude, a better vacuum than any physicists can create in the laboratory there is still about of 5-10 billion M of gas and dust out there, comprising approximately 5% of the mass of visible stars in the Galaxy. The Milky Way Galaxy is filled with a very diffuse distribution of neutral hydrogen gas which has a typical density of about 1 atom/cm 3 (10 -24 g/cm 3 ). The interstellar medium is far too cool to excite the UV or optical transitions of hydrogen, but there is a feature at 21 cm wavelength in the radio produced by the spins (magnetic fields) of the hydrogen atom's nuclear proton and orbiting electron. Because the proton and electron are spinning distributions of electric charge they create minute magnetic fields which interact, creating a small energy difference between the state in which the poles are aligned versus counter-aligned. This energy difference corresponds to the energy of radio waves at 21-centimeters. Every once in a while (about once per 500 years) hydrogen atoms will collide, exciting an atom into the higher energy spin-aligned configuration. It will take as long as 30 million years for the atom to jump back to the lower energy state via a spin-flip, emitting 21 cm radio emission. (Diagram courtesy of Dr. Terry Herter, Cornell University) The neutral hydrogen is distributed in clumpy fashion with cool, denser regions that astronomers call "clouds" but which are more like filaments. These regions have a typical temperature of about 100K and a density between 10--100 atoms/cm 3 . Surrounding the clouds is a warmer lower density medium with about 0.1 atom/cm 3 and T 10K) clouds of molecular hydrogen and dust, known as molecular clouds or dark clouds are the birthplaces of stars. We do not detect molecular hydrogen directly, but infer its characteristics from other molecules, most often CO. Over 50 other molecules have been detected including NH3, CH, OH, CS and molecules as complex as ethyl alcohol (C2H5OH - the stuff in whisky) have been found in Milky Way molecular clouds. The Horsehead Nebula (Messier Nebulae, Web Nebulae) to the right is produced by the incursion of a plume of dust from a molecular cloud, covering the lower half of the image, into a region of ionized hydrogen. A Giant Molecular Cloud (GMC) may have a mass of 10 6 M and a diameter of 150 l.y. Within the GMCs are warm dense corse of order 2-3 l.y. in diameter, with T 100K and densities as high as n 10 7 -10 9 molecules/cm 3 . It is in these regions where the star-formation process begins. There are thousands of GMCs in the Milky Way, mostly on the Spiral Arms and concentrated toward the Galactic Center. The total mass of molecular gas is estimated to be about equal to, or perhaps somewhat less ( Material left over from the formation of young, hot stars represents the most spectacular component of the ISM, the ionized hydrogen or HII regions like the Orion Nebula ( Messier Database, Web Nebulae), shown below in this HST mosaic. Here is the Near Infrared view of the Orion Nebula. Massive O and B stars, recently formed in molecular clouds (remember - massive stars live fast & die young!) ionize the gas left over from their formation heating it to a temperature, T 10,000K and causing it to fluoresce producing an emission-line spectrum. Ultraviolet photons from four massive stars called the Trapezium in the nebula have sufficient energy to strip the electrons completely away from - ionize - hydrogen atoms. This requires a photon of energy greater than 13.6 eV or wavelength less than 912Å in the ultraviolet. If a hydrogen atom absorbs a photon with wavelength less than 912Å the atom is ionized with the "extra" energy going into the kinetic energy of the electron. Collisions between electrons "thermalize" this energy heating the nebular gas to a temperature of about 10,000K. Collisions between electrons and ions in the gas excite the ions to higher energy levels producing emission features of O + ,O ++ ,N + , S + , etc. as shown in the above spectrum. Electrons recombining to upper level in hydrogen and helium cascade through many energy levels down to the ground state producing the emission features of H & He. The Orion Nebula is a bubble on the side of a much larger Giant Molecular Cloud complex. The GMC contains a lage cluster of newly formed stars Scroll down this page to see a selection of HST images of Orion showing circumstellar disks surrounding pre-main sequence stars of solar mass. About 1% of the mass of the ISM is in the form of tiny grains of dust about the size of particles of cigarette smoke. We have already described how this dust obscures the plane of the Milky Way from our view. We know something about the characteristics of this dust from the way that it scatters visible and ultraviolet photons. The effect of dust is to dim the light from distant objects (interstellar extinction) in the Galaxy and redden the colors (interstellar reddening) because red light is not scattered as efficiently as blue light. A graphical exhibition of the effects of dust at different wavelengths is shown by the visible and infrared images of the constellation Orion above. Dust scatters and obscures visible wavelengths where stars emit most of their light (note Betelgeuse the bright red giant at Orion's left shoulder). Dust is largely transparent in the infrared, but at temperatures of about 40K, emits strongly at wavelengths between 50-100 m. We know that dust grains are elongated, perhaps needle shaped, with sizes of about 1000Å, about the wavelength of light that the grains scatter most efficiently. Dust characteristics vary somewhat from place to place in the Galaxy, but a typical grain is believed to be composed of carbon in a graphite-like crystal structure, mixed with silicates (eg MgSi03 like olivine). Nearly all of the elements like Carbon and Silicon in the ISM are tied up in dust. In molecular clouds the grains appear to be coated with a water-ice shell. Ned Wright at UCLA has developed a fractal model for interstellar dust grains shown in this APOD. Ned Wright's Fractal Dust Model Dust is responsible for the blue haze around the Pleiades star cluster (Messier Database, (Web Nebulae) this nebulosity is called a reflection nebula resulting from blue light from the hot B-stars being scattered toward us from dust surrounding the cluster stars. The Pleiades Star Cluster Credit & Copyright: D. Malin (AAO), AATB, ROE, UKS Telescope Prof. H. E. (Gene) Smith CASS 0424 UCSD 9500 Gilman Drive La Jolla, CA 92093-0424 Last updated: 26 April 1999 ## Chemistry in the turbulent interstellar medium A multi-wavelength image of a portion of the Perseus molecular cloud, located about 850 light-years away, and its nebulae. Turbulence is pervasive in molecular clouds and plays an important role in producing small density and temperature fluctuations that in turn help determine the abundances of complex molecules in the cloud. A new set of chemical and hydrodynamical models is able to account for the effects of such turbulence and offers an improved explanation for observed chemical abundances. Credit: Agrupació Astronòmica d'Eivissa/Ibiza AAE, Alberto Prats Rodríguez Over 200 molecules have been discovered in space, some (like Buckminsterfullerene) very complex with carbon atoms. Besides being intrinsically interesting, these molecules radiate away heat, helping giant clouds of interstellar material cool and contract to form new stars. Moreover, astronomers use the radiation from these molecules to study the local conditions, for example, as planets form in disks around young stars. The relative abundance of these molecular species is an important but longstanding puzzle, dependent on many factors from the abundances of the basic elements and the strength of the ultraviolet radiation field to a cloud's density, temperature, and age. The abundances of the small molecules (those with two or three atoms) are particularly important since they form stepping stones to larger species, and among these the ones that carry a net charge are even more important since they undergo chemical reactions more readily. Current models of the diffuse interstellar medium assume uniform layers of ultraviolet illuminated gas with either a constant density or a density that varies smoothly with depth into the cloud. The problem is that the models' predictions often disagree with observations. Decades of observations have also shown, however, that the interstellar medium is not uniform but rather turbulent, with large variations in density and temperature over small distances. CfA astronomer Shmuel Bialy led a team of scientists investigating the abundances of four key molecules—H2, OH + , H2O + , and ArH + —in a supersonic (with motions exceeding the speed of sound) and turbulent medium. These particular molecules are both useful astronomical probes and highly sensitive to the density fluctuations that naturally arise in turbulent media. Building on their previous studies of the behavior of molecular hydrogen (H2) in turbulent media, the scientists performed detailed computer simulations that incorporate a wide range of chemical pathways together with models of supersonic turbulent motions under a variety of excitation scenarios driven by ultraviolet radiation and cosmic rays. Their results, when compared to extensive observations of molecules, show good agreement. The range of turbulent conditions is wide and the predictions correspondingly wide, however, so that while the new models do a better job of explaining the observed ranges, they can be ambiguous and explain a particular situation with several different combinations of parameters. The authors make a case for additional observations and a next-generation of models to constrain the conclusions more tightly. ## Interstellar Medium Interstellar medium Jump to: navigation, search The distribution of ionized hydrogen (known by astronomers as H II from old spectroscopic terminology) in the parts of the Galactic interstellar medium visible from the Earth's northern hemisphere as observed with the Wisconsin Hα Mapper (Haffner et al. 2003). Interstellar Medium Eight elements heavier than zinc have been detected in the interstellar medium: thorium, lead, gallium, germanium, krypton, tin, arsenic, and selenium. Bok Globule . Interstellar Medium and the Milky Way Chapter index in this window " " Chapter index in separate window This material (including images) is copyrighted!. See my copyright notice for fair use practices. 20.2 Interstellar Gas 20.3 Cosmic Dust 20.4 Cosmic Rays 20.5 The Life Cycle of Cosmic Material 20.6 Interstellar Matter around the Sun Key Terms Resumé For Further Exploration Collaborative Group Activities . Strangely Shaped Plasma Lenses May Lurk in Milky Way's The matter between stars, composed of two components, gas and dust, intermixed throughout all of space. The gaseous and dusty matter present in the space between a galaxy's stars. Ion An atom that has lost one or more electrons. Ions have a positive electric charge. The simplest ion is the nucleus of a hydrogen atom, which is a proton. . This may contain Neutral Hydogen, Molecular Hydrogen, ionized Hydrogen, Helium and Dust. The gas and dust that exists in open space between the stars. Ionosphere . 0.2 Lyman-break analogs A86 A. Contursi, A. J. Baker, S. Berta, B. Magnelli, D. Lutz, J. Fischer, A. Verma, M. Nielbock, J. Gr cia Carpio, S. Veilleux et al. (9 more) DOI:. : The material between the stars, consisting of gas, dust and cosmic rays (high energy charged particles, moving at nearly the speed of light). Intrusive: Refers to igneous rocks formed underground. : The matter contained in the regions between star systems in a galaxy. This matter is typically made up of gas, dust, and cosmic rays. Ion: An atom that has a net charge. The charge is the result of the atom having an unequal amount of protons and electrons. - Gas and dust located between the stars. Ion - an electrically charged atom due to the loss or gain of one or more electrons. Ionization - Process an atom gains or loses electrons. . The gas and dust between the stars that fills the plane of the galaxy. For centuries, scientists believed that the space between the stars was empty. of large spiral galaxies like the Milky Way, consists of 90% molecular hydrogen, about 10% helium and small fractions of heavier elements. (or ISM) is the material that exists in the space between the stars. This mostly includes cosmic gas and dust. is no very dense at all at its densest, it is emptier than the best vacuum we can produce on Earth. . The gas and dust that exists in open space between the stars. Intrinsic magnitude . is the gas and cosmic dust that pervade interstellar space: the matter that exists between the stars within a galaxy. or ISM. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ion Ion . Hubble Constant Nobel Prize Equinox Emission Electrons Craters Distance Gamma Rays Kepler's Law Cerenkov Radiation Irregular Galaxies Mass Spacesuits Subatomic Particles Money Jupiter Stargazing Neutrons Satellites Curious Minds Online We have 1773 guests and no members online . : Material that lies between stars. This includes dust, gas, and plasma. Often abbreviated "ISM." Ionized: When electrons are stripped from atoms. - J - . is where stars are made. Without it, we wouldn't exist. If there weren't thick and thin spots that condensed into thicker patches that eventually became stars, the whole universe would just be a cloud of boring, cold, lifeless gas. manifests itself to the astronomer in various phenomena. The most obvious perhaps are the emission nebula. Dimension (a) A geometrical axis. [F88] (b) An independent axis or direction in space or spacetime. The familiar space around us has three dimensions (left-right, back-forth, up-down) and the familiar spacetime has four (the previous three axes plus the past-future axis). Due: Homework 10 12-4 Exoplanets I: How can we know anything? - The gas and dust that fills the space between stars in a galaxy. Invertebrate - Animals that lack a backbone such as insects, mollusks, etc. Inverse Square Law - When a force obeys the Inverse Square Law, its strength drops off by the square of the distance involved. is in a gaseous state, with hydrogen making up 90% of the atoms. About half of this gas is tied up in interstellar gas clouds which have different properties depending on the temperature of the gas. ENAs form when charged particles from the solar wind travel outward and encounter atoms from the . Because the ENAs are neutral, they do not react to any magnetic fields. Some of these ENAs travel toward the inner solar system and are captured by the IBEX spacecraft. that is sufficiently cold that molecules can form. They are very cold (10-20K) with relatively high densities (trillion particles per cubic meter), and huge. Chemistry and physical conditions become quite different from those of the surrounding low-density . In the outer parts of the dark cloud, the hydrogen is neutral. Deeper within it, as dust blocks out an increasing amount of stellar ultraviolet radiation, the cloud becomes darker and colder. of gas between its stars that is chemically enriched from the elements emitted by stars that have already evolved. The diameter of M87 is bigger than Pluto's complete orbit. Galactic gamma-ray radiation will be produced mostly by cosmic rays interacting with the gas in the . The lower-energy gamma rays (below about 100 MeV) are produced by electrons, and the electron intensity is higher in the inner Galaxy than locally and in the outer Galaxy. EUVE will add greatly to our understanding of hot, young white dwarfs, cataclysmic variables, stellar coronae, and the local Because hydrogen is a major constituent of the , the 21-cm line has provided astronomers with a means of mapping the spiral structure of the Milky Way. The thermometer is provided by several species of atoms or molecules floating in clouds of gas in the I'll focus on carbon monoxide here. Carbon monoxide, since it is made of two different atoms, has an electric dipole moment. While there is mainly hydrogen floating around between the stars, in what we call the , there is also a little bit of dust also there - that's part of the 2% that makes up stars. This dust is very important for several reasons. The solar wind "blows a bubble" in the (the rarefied hydrogen and helium gas that permeates the galaxy). Molecular hydrogen is found in the where it is generated by ionization of molecular hydrogen from cosmic rays. It has also been observed in the upper atmosphere of the planet Jupiter. Molecular oxygen is a molecule that is composed of two oxygen atoms that has no color, odor, or taste. Multidimensional simulations are able to assist in predicting chemical yields and mass ejected in the (ISM), however simulations need to consider two ways in which nucleosynthesis takes place in the context of stellar evolution and SNe. At some distance from the Sun, well beyond the orbit of Pluto, this supersonic wind must slow down to meet the gases in the . It must first pass through a shock, the termination shock, to become subsonic. heliopause The point at which the solar wind meets the or solar wind from other stars. heliosphere The space within the broundary of the heliopause containing the Sun and solar system. Particles ranging from a few tens of nanometres to a few hundreds of nanometres in size, present in the to the extent of about one percent or two percent by mass. It crossed the heliopause - and into the - in 2012. From our view - using H.A. Rey's visualization of the stars - Voyager 1 sits just outside the "head" of constellation Ophiuchus and just below the head of upside-down Hercules. , neutral hydrogen emission cosmic rays Baade's Window, bulge disk galaxy, Spiral galaxy, elliptical galaxy, irregular galaxy(*), dwarf spheroidal galaxy(*), ring galaxy/Hoag's object Hubble classification system, Yerkes classification system . This presents a problem because the stars that actually release helium back into the make a lot of heavier elements too. Observations of galaxies with different helium abundances show that for every 3.2 grams of helium produced, stars produce 1 gram of heavier elements (French, 1980, ApJ, 240, 41). One topic for discussion at the workshop will be why Voyager has not yet crossed the boundary between the heliosphere and the local The nebula will eventually disperse into the surrounding , enriching it with key elements such as carbon, nitrogen and oxygen that are produced only deep in stellar interiors. This material will be recycled to form a later generation of stars, and even planets. The LIC is itself surrounded by a larger, lower density cavity in the (ISM) called the Local Bubble, that was probably formed by one or more relatively recent supernova explosions. In essence, a nebula is formed when portions of the undergo gravitational collapse. Mutual gravitational attraction causes matter to clump together, forming regions of greater and greater density. The reason for the asymmetric arrangement of the bubbles is probably due to the unequal density of the that surrounds the nebula. The impact of the shock wave with the dense causes the gas to heat to millions of Kelvins. As the gas then cools, it emits radiation in the optical wavelengths which is what the Hubble Space Telescope has recorded. Local Bubble: The local bubble is a large cavity in the (ISM) in the Orion arm of the Milky Way. It is about 300 ly across in which the Sun and other nearby stars reside. The average density of neutral hydrogen atoms is 10 times lower than that of ISM. At the outermost boundary of the heliosphere, our solar wind meets the , a plasma that permeates our Milky Way galaxy. Scientists estimate that this boundary is between 9 and 15 billion kilometers away from the Sun, far beyond the orbits of all the planets. The finding is surprising since the early Universe comprised mostly light elements with elements heavier than hydrogen and helium synthesized by reactions inside giant stars and fed into the local when they exploded in blinding supernova explosions at the end of their lives. The sun sends out a constant flow of charged particles called the solar wind, which ultimately travels past all the planets to some three times the distance to Pluto before being impeded by the . This forms a giant bubble around the sun and its planets, known as the heliosphere. The heliopause is the outermost boundary of the solar wind, where the restricts the outward flow of the solar wind and confines it within a magnetic bubble called the heliosphere. Neutrinos are the only known particles that are not significantly attenuated by their travel through the . Optical photons can be obscured or diffused by dust, gas and background radiation. extinction The dimming of starlight as it passes through the . Harvard-Smithsonian Center for Astrophysics 60 Garden Street, Cambridge, MA 02138 USA Phone: 617.496.7941 Fax: 617.495.7356 . Chapter 12, Sections 12.1 Due 10/04: Ch. 12, Probs. 12.4, Lecture 8 problems . Light from the Sun dispersed the remaining gas in the Solar Nebula gas into the . Planetary motions reflect the history of their formation. Planets formed from a thin rotating gas disk: . The position of the heliopause depends both on the strength of the solar wind and on the properties of the local Most of this material isn't visible at optical wavelengths, but it does glow in the infrared, which Spitzer investigates. This material forms the , a thin mixture that permeates the space between stars. Especially cold dust forms dark ribbons in this false-color image. The next generation of stars (Population II) would have had a small fraction of heavy elements, but their stellar evolution would have lead to ever greater additions to the heavy element content of the ★ Heliosheath The region of space where the solar wind meets the . Expected to be found somewhere beyond Pluto in the Kuiper Belt. Type I: In falling matter on a white dwarf can cause the star to explode and eject the excess matter back into the Because the star is relatively cool, the ejecta quickly assumes a solid state and collides with the . The resulting dusty nebula is invisible to the naked eye but can be detected using an infrared telescope. This bow shock is 16,295 AU from the star to the apex and 6,188 AU thick. So ends the story of Mercury's moon but at the same time a new chapter in astronomy began: extreme UV turned out not to be so completely absorbed by the Dust is a relative newcomer to the Universe. It joined the gases only after the first stars of the cosmos had matured and their more massive members had exploded. It is these supernovae explosiong that still seed the with the heavier elements, of which dust is mostly composed. Moving at about 400 km/sec (about 250 miles/sec), the wind needs about 4-5 days to reach Earth, and as many months to attain the outermost planets: its outer limits, the boundary between the space dominated by the Sun and the , is probably more distant still. At an estimated distance of 1,500 light-years from us, at the time of the explosion the supernova would have been bright enough to cast strong shadows on Earth. Over time, the energy and material that was ejected into the My forte is deep-sky work observations I am proud to include the Sculptor Dwarf Galaxy (10x70 binocular), Maffei I and Leo II (Celestron 14), and S147 (6-inch Maksutov). My interests led to a physics PhD, studying the from a spacecraft: By training I am an astrophysicist, . The remnants of our own Sun will end up becoming a planetary nebula with a white dwarf star at its centre. This will happen after it has reached the red giant phase of its life. The nebula comes from a shell of ionized gas which is expelled into the surrounding as it goes through its final death . the star pumps out the energy of 5000 Suns, the vast majority of it in the energetic ultraviolet. Highly enriched in carbon from nuclear reactions in the precursor advanced giant star, as the nebula grows and dissipates, it will add another load of this and other newly- created elements into the It provides a wealth of information for studying stellar atmospheres, including that of our Sun, as well as the ## AMP Energy Invests2 Billion In Australian Renewable Energy Hub

Amp Energy (Amp), A Toronto-based clean energy global developer, has invested over $2 billion in establishing the 1.3 GW Renewable Energy Hub of South Australia (REHSA). Amp may be stationed in Canada, but it has a growing portfolio in Australia. The new facility will integrate the large-scale wind, PV (Photovoltaics), and Battery Energy Storage System (BESS) assets and incorporate the Spencer Gulf Hydrogen Energy Ecoplex. It will be including at least three massive solar projects, two of which would supply South Australia’s green hydrogen ambitions. The PV projects include Robertstown (636 MW), Bungama (336 MW), and Yoorndoo Ilga (388 MW). There will be a total BESS capacity of up to 540MW across the portfolio. Overall, the projects will create sufficient electricity to power 230,000 homes annually. Plus, Amp plans to generate hydrogen for domestic and export markets from the Spencer facility to ports in Asia. The Bungama and Robertstown projects are anticipated to begin energization in late 2022. These two projects alone will create 550 jobs that are full-time during the construction phase. Dean Cooper, head of Amp Australia and the company’s executive vice president, said: The strategic value of the South Australian portfolio is significant in a jurisdiction which is undergoing one of the most rapid energy transitions in the world. (Credit: Amp India) Stephen Patterson, the SA Minister for Trade and Investment, welcomed the prospect of the REHSA, saying: South Australia has significant land mass and world-class wind and solar resources, with aspirations of reaching net 100% renewable energy generation by 2030. We’ve seen over$7 billion invested in projects with another \$20 billion in the pipeline. The Renewable Energy Hub of South Australia will be fundamental in integrating our state’s renewable energy storage assets and building our capability, and supporting the fast-moving energy transitions we’re experiencing.

Amp’s Australian operating company, Amp Power Australia Pty Limited, already manages a portfolio of 158MW of solar PV assets in NSW, including the 39 MW Molong Solar Farm.

Australia is also home to the largest solar farm globally: Sun Cable’s 10-gigawatt facility will even provide Singapore with 20% of its electricity needs via undersea cables.

## NASA spacecraft detects a constant 'hum' deep in the cosmos

Beyond the edge of the solar system, more than 14 billion miles from Earth, a NASA spacecraft has detected a curious and persistent "hum" in interstellar space.

The faint but constant vibrations were picked up by the Voyager 1 spacecraft, which, after more than four decades journeying deep into the cosmos, is the most distant human-made object in space. Scientists say the new discovery, published Monday in the journal Nature Astronomy, is providing a unique and never-before-seen glimpse of the interstellar environment — the frontier beyond the reaches of the sun and planets in our cosmic neighborhood.

"Voyager 1 is in an interesting region of space that is outside this thing called the heliosphere, which is the protective bubble that encases all the planets in the solar system," said Stella Ocker, a doctoral student at Cornell University in Ithaca, New York, and one of the authors of the new study. "So, it's really our only tool for directly sampling the nature of interstellar space."

Ocker and her colleagues don't yet know what's causing the "hum," but it was measured through ripples of plasma in what's known as the interstellar medium, the hodgepodge of gas, radiation and particles that make up the space between stars. While it's not an actual audio signal, the faint drone showed up as vibrations in a narrow frequency bandwidth, Ocker said.

Previously, scientists could only take fleeting measurements of the interstellar medium after periodic but isolated eruptions from the sun, which would unleash shockwaves that coursed through the solar system and beyond.