Sterrekunde

Is galaktiese botsings met 'n hoë spoed oorleefbaar?

Is galaktiese botsings met 'n hoë spoed oorleefbaar?


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Dit is 'n hipotetiese vraag, maar dit word eintlik bedoel as 'n lewendige manier om te vra na die gevolge van hoëspoedige galaktiese botsings. Die APOD van 27 November het die Cheshire Cat-sterrestelselgroep getoon, en die onderskrif sê dat die "oë" twee groot elliptiese sterrestelsels is wat mekaar teen meer as 1000 km / s toemaak en in 'n gloed van röntgenstrale gebad het van die verhitting van gepaardgaande interstellêre media.

Hoe intens is hierdie gloed? Ek vermoed dit is baie flou, maar ek weet nie hoe om dit te kwantifiseer nie. Sou dit X-straal-sterrekunde vir inwoners van die sterrestelsels belemmer? Sou dit sterk genoeg wees om die lewe te beïnvloed? Het iemand kwantitatiewe inligting en weet hy hoe om dit te interpreteer? (Ek besef wel dat elliptiese sterrestelsels bestaan ​​uit ou sterre met 'n lae metaalagtigheid wat nie goeie kandidate vir bewoonbare planete is nie, maar laat ons dit ignoreer.)


Interessante vraag. Ek het bietjie gelees en daaroor nagedink en ek kan 'n antwoord gee, alhoewel ek regstellings en insette van iemand wat meer kundig as ek is, uitnooi.

Eerstens is die twee sterrestelsels enorm. Ek het nie spesifieke groottes gelys nie, maar volgens hierdie artikel:

1,2-1,5 x 10 ^ 14 fossielgroep vir sonmassa

Dit is ongeveer 100 Andromeda-sterrestelsels. Aangesien 4,6 miljard jaar oud is, is dit redelik om aan te neem dat hierdie twee sterrestelsels waarskynlik meer vrye stof en gas het en dat die swaartekrag-botsing teen groter spoed groter is.

Dit is regtig die vraag met die hoeveelheid x-straalstraling op die agtergrond as twee sterrestelsels bots. hoeveel vrye gas en stof en hoe vinnig die sterrestelsels in mekaar beweeg. As ek groter en jonger is, gloei die twee sterrestelsels baie helderder as die Melkweg, en Andromeda sal oor 4 miljard jaar wees.

Oor hoe intens die gloed is, stem ek saam met u, dit is waarskynlik nie baie helder as u daar is nie. Die flou gloed en die twee helder oë wat u op die foto sien, is lang blootstelling. Hierdie webwerf (blaai na onder) sê dit is 19 uur en 30 minute blootstelling. Andromeda is byvoorbeeld skaars met die blote oog sigbaar, maar as dit gefotografeer word, word dit baie keer helderder gemaak. Ek is nie slim genoeg om 100% seker te kan sê nie, maar my raaiskoot is dat die flou pers gloed skaars sigbaar sou wees, as dit hoegenaamd sigbaar was, as ek aanvaar dat dit selfs in die sigbare spektrum bereik, wat X-straal nie doen nie. Chandra stel dalk die kleur in op 'n sigbare spektrum in die prent.

Hoe gevaarlik is dit? Weereens, raai ek, maar ek dink in die Milky-way-Andromeda-botsing sal ons waarskynlik meer UV van die son kry as van hoë botsings teen gas en stof, maar dit is net 'n raaiskoot. Dit sal nogal lekker wees as die hele naghemel egter 'n bietjie gloei.

Wat die Milky-way-Andromeda-botsing betref, sê Wikipedia dat hier nie veel gratis gas in die botsing sal wees nie, dus is dit 'n veilige weddenskap dat daar geen sigbare gloed in die lug sal wees nie en selfs nie soveel van 'n versnelling van nuwe stervorming, alhoewel daar sommige moet wees. Die Melkweg vorm tans elke jaar ongeveer 7 nuwe sterre, en ek dink ek sal geleidelik vertraag gedurende die volgende 4 miljard jaar, en dan aansienlik versnel namate die botsing aan die gang kom, maar vanuit die oogpunt van die Aarde weet ek nie ' Ek dink daar is geen waarborg dat ons nuwe sterformasies van naderby sal sien nie.

Hier is 'n prettige artikel / onderhoud oor die botsing tussen die Melkweg en Andromeda, alhoewel ek dink dat Roeland van der Marel miskien die nuwe sterreformasie 'n bietjie oordryf as ons Wikipedia wil glo waar daar staan ​​dat daar nie veel gratis gas sal wees nie .

Wat die snelheid van ons botsing betref, is Andromeda tans op ons af (of ons is op pad, afhangend van wat u verkies) teen ongeveer 110 km / s bron. en dit is tans ongeveer 2,5 miljoen ligjare weg. Bron.

Om 2,5 miljoen ligjare (ongeveer 24 miljoen triljoen km) in 4 miljard jaar (126 miljoen miljard sekondes) te beslaan, werk 'n gemiddelde snelheid van 190 km / s, sodat ons ongeveer kan skat dat die spoed van die impak tussen ons twee sterrestelsels bereik 'n hoogtepunt van ongeveer 270 km / s. Maar (sien video, jy moet ook die rotasiesnelheid in ag neem en dit blyk (video hierbo) dat die rotasies in teenoorgestelde rigtings sal beweeg as die sterrestelsels bots. Ons wentel ongeveer 250 km / s oor die Melkweg en Andromeda, omdat dit groter is , waarskynlik 'n bietjie vinniger wentelsnelheid. As ons die 2 snelhede in ons nek van die bos bymekaar optel, sien ons dalk 'n relatiewe snelheid en 'n paar botsings teen stof en stof (raai hier, want ek weet nie of die rotasies sal vertraag nie soos wat die sterrestelsels naderkom), maar laat ons sê 500 tot 600 km / s of so. Vir enige nabygeleë sterre uit Andromeda kan dit vinnig genoeg wees vir sigbare veranderinge aan 'n paar nabygeleë Andromeda-sterre in konstellasies gedurende 'n menslike leeftyd.

Miskien sal ons elke paar duisend jaar of so deur 'n oortwolk van 'n ander ster gaan, miskien selfs die af en toe 'n kuipergordelekwivalent, elke tien of 50 miljoen jaar - miskien. Ons kon 'n paar baie indrukwekkende meteorietreën sien en miskien 'n af en toe 'n bietjie meer gereelde komeet of 'n meteoor-impak van die dinosourusse - maar ek bespiegel net. Daar kan tussen 4 en 5 miljard jaar van nou af baie interessante dinge wees.

Wanneer sterre vorm, kan hulle (volgens hierdie webwerf) 100 tot 200 keer helderder wees as gedurende hul hoofreeks, dus as ons sonnestelsel naby 'n botsende gaswolk en 'n nuwe vorming van sterre kom, kan dit interessant wees. As ons ook naby die kern van Andromeda gaan, sal ons 'n rukkie 'n baie helder naghemel hê.


'A movie camera to watch the whole universe' 'n kunstenaar se opvatting van die LSST binne sy koepel. Met dank aan Wikimedia

Die verre 'hemelse sfeer' is vir die grootste deel van die mensegeskiedenis as volmaak en onveranderlik beskou. Sterre het op hul plek gebly, planete het voorspelbaar beweeg, en die paar skelm komete is as atmosferiese verskynsels beskou. Dit het begin verander met die waarneming van die Deense sterrekundige Tycho Brahe van die supernova van 1572 - blykbaar 'n nuwe ster - en sy studies van die Groot Komeet van 1577, wat hy bewys eintlik 'n verre voorwerp was. Desondanks is die indruk van permanensie sterk. Daar is baie min astronomiese voorwerpe wat opvallend met die blote oog verskil: net die helderste komete, novae en supernovas. Vir waarnemers in die noordelike halfrond was die laaste supernova met blote oog in 1604.

Moderne teleskopiese studies vertel 'n heel ander verhaal. Vandag weet ons van ongeveer 'n halfmiljoen veranderlike sterre in ons sterrestelsel, en ons identifiseer jaarliks ​​duisende verbygaande voorwerpe. Alhoewel baie sterre op voorspelbare maniere wissel, is die heelal ook vol onvoorspelbare geweld. Wanneer twee sterre naby mekaar wentel, kan massa van die een na die ander vloei. As een van die sterre 'n ou, ineengestorte wit dwerg is, kan die gas wat dit van sy metgesel trek, ophoop totdat die dwerg 'n skielike termonukleêre ontploffing ondergaan - 'n supernova soos die wat Tycho gesien het. Daar is ook 'n meer algemene tipe supernova wat geproduseer word deur die dood van alleensterre wat meer as tien keer die massa van die son is.

Supernovae toon 'n wye verskeidenheid gedrag wat afhang van die gedetailleerde eienskappe van die stelsel ten tye van die finale, noodlottige ramp. Die atome wat uit supernova-ontploffings ontstaan, het die grondstof vir alle planete verskaf, ook ons ​​eie. Sterrekundiges is verstaanbaar gretig om meer hieroor te leer, maar die twee klasse supernovas gesamentlik kom net een keer per eeu in ons sterrestelsel voor.

Dit is natuurlik nie baie winsgewend om na gebeure op tydskale van 'n eeu te soek nie, net in ons sterrestelsel. Gelukkig is ons sterrestelsel slegs een van ongeveer 'n triljoen sterrestelsels in die sigbare heelal. As u die hele tyd miljoene sterrestelsels monitor, is dit moontlik om elke dag baie supernovas te vind. Dit is een van die opwindendste uitdagings van moderne hoëspoed-sterrekunde.

Behalwe supernovas, is daar slegs 'n paar veranderlike bronne wat helder genoeg is om op groot afstande van ander sterrestelsels gesien te word, selfs met behulp van kragtige teleskope. Verreweg die algemeenste is die ewekansige wisselvalligheid van kwasars. Kwasars bestaan ​​uit 'n supermassiewe swart gat, miljoene tot miljarde keer die massa van ons son, wat skyn soos materiaal na die swart gat val, opwarm en energie uitstraal.

Vandag dink ons ​​dat in werklikheid elke sterrestelsel 'n supermassiewe swart gat in die middel bevat, en dat iets soos 1 persent daarvan vinnig genoeg is om die ligte kwasars te sien. Die supermassiewe swart gat in die middel van ons eie sterrestelsel is in wese 'af'. Soms kom so 'n swart gat egter vinnig aan. Die boeiendste oorsaak is 'n sogenaamde 'gety disruption event' waarin 'n ster soos die Son te naby aan die swart gat gaan en deur die getye van die swart gat uitmekaar geruk word. Van die puin val dan in die swart gat om 'n kortstondige fakkel aan te dryf. Hierdie gety-ontwrigtingsgebeurtenisse is baie skaarser as supernovas, en kom net ongeveer elke 10.000 jaar in 'n spesifieke sterrestelsel voor. In die verre heelal is die studie van veranderlikheid in wese die studie van swart gate en supernovas.

Dit gee u 'n gevoel van die merkwaardige astronomiese dieretuin van veranderlike en kortstondige voorwerpe. Die uitdaging vir die professionele sterrekundige is om al hierdie verskillende bronne te vind en te karakteriseer, nie net vir hoe hulle individueel werk nie, maar ook om hul algehele demografie en statistiek te bepaal. Om 'n groot aantal daarvan te vind, benodig u 'n groot teleskoop wat die talle verre, flou voorwerpe kan opspoor. Oor die algemeen sien groter teleskope egter net kleiner dele van die lug raak. Hierdie frustrerende reël kan slegs gebuig word deur groot bedrae geld te spandeer.

As u wetenskaplike doel is om die grootste moontlike aantal oorgange te vind en hul evolusie in die kosmiese geskiedenis van die heelal te bestudeer, wil u 'n groot teleskoop gebruik wat soveel lug bedek as wat u kan bekostig. Dit is fundamenteel die doel van die Large Synoptic Survey Telescope (LSST). LSST is in Chili geleë (effektief) 'n teleskoop met 'n deursnee van 6,7 meter wat in 2022 met die volledige wetenskaplike werking begin.

LSST sal die naaste sterrekundiges ooit wees om 'n filmkamera te skep om die hele heelal dop te hou. Dit sal ongeveer die helfte van die lug ondersoek met behulp van 'n kamera wat meer as 40 keer die oppervlakte van die volle maan strek. Maar LSST kan slegs een keer elke drie nagte 'n nuwe beeld van elke pleister van daardie lug kry. LSST kan verganklikhede 30 miljoen keer flouer opspoor as wat met die blote oog sigbaar is, wat dit 'n fenomenale projek maak om groot getalle flou verbygaande bronne in die sigbare heelal te vind - LSST behoort ongeveer 1 000 supernovas per dag te vind! Maar hierdie vermoë kos: ongeveer $ 600 miljoen net vir konstruksie, plus 'n beduidende bedryfskoste.

Aan die ander kant van LSST is 'n projek waaraan ek werk: die All-Sky Automated Survey for Supernovae (ASAS-SN). Teen die einde van hierdie jaar sal ASAS-SN bestaan ​​uit 20 diafragma-teleskope van 14 cm wat oor die hele wêreld versprei is en ongeveer $ 3,5 miljoen kos vir konstruksie en werking tot 2022. Met sulke klein teleskope - groot telefoto-lensies, regtig - ASAS-SN kan slegs helder oorgange vind, ongeveer 25 000 keer flouer as wat sigbaar is vir die menslike oog. Tog moet dit nog steeds ongeveer een supernova per dag vind. En omdat ASAS-SN uit klein teleskope bestaan, kan dit die lug baie vinniger as LSST beeld. Die gekombineerde 'beeld' van al die ASAS-SN-teleskope strek oor 1600 keer die oppervlakte van die Maan. Dit stel hulle in staat om elke aand die hele sigbare lug te besigtig.

Die twee projekte is baie komplementêr en balanseer in wese 'n kompromie tussen 'hoeveelheid' en 'kwaliteit'. LSST bied 'hoeveelheid': die groot aantal flou bronne wat nodig is vir statistiese studies van verre bronne, en om die evolusie van verbygaande bronne oor kosmiese tyd te bestudeer. Die tipiese LSST-verbygaande is egter flou en moeilik om vir lang tydperke in detail te bestudeer, selfs met die wêreld se grootste teleskope. ASAS-SN bied 'kwaliteit'. Die helder bronne wat deur ASAS-SN gevind is, is diegene wat die nabygeleë Heelal die beste ondersoek, en wat met groter teleskope in die fynste besonderhede en die langste tyd bestudeer kan word.

Een van die belangrikste instrumente vir sterrekundiges is die spektrum van 'n voorwerp: hoeveel lig as die funksie van die kleur daarvan uitgestraal word. 'N Spektrum is die beste manier om die snelhede, temperature, elementêre samestelling en tipe van 'n voorwerp te klassifiseer (bv. Watter tipe supernova? Wat was die unieke eienskappe daarvan?). Omdat u die lig in smal vakkies met kleur moet kap, het u baie meer lig nodig om 'n spektrum van 'n voorwerp te maak as om 'n beeld daarvan te kry. LSST is al 'n groot teleskoop, dus dit sal moeilik of onmoontlik wees om 'n spektrum van die tipiese, flou LSST-verbygaande te kry.

Selfs vir die minderheid LSST-bronne wat helder genoeg is om een ​​spektrum te verkry, sal die bron vinnig vervaag en te flou word om 'n ander spektrum te kry om te bestudeer hoe dit mettertyd ontwikkel. Daarom sal 'n weglaatbare fraksie van LSST-ontdekkings deur hierdie fundamenteel belangrike astronomiese instrument bestudeer word. Die ASAS-SN-transiënte is baie minder, maar baie helderder, dus 'n baie groot deel van ASAS-SN-transiënte kan spektroskopies bestudeer word, en dit kan vir lang tydperke bestudeer word, selfs al verdwyn hulle.

Projekte soos LSST en ASAS-SN gaan voort met die revolusie wat Tycho begin het, en onthul die veranderlike en soms gewelddadige gebeure wat die hoogs onvolmaakte, steeds veranderende hemelse sfeer verlig.


Kosmiese botsing spel die begin van die einde

Sterrekundiges getuig van moontlike nuwe meganismes vir sterrestelsels.

Sterrekundiges het dalk net 'n nuwe manier gesien hoe sterrestelsels kan "sterf" deur die supergevoelige Atacama Large Millimeter / submillimeter Array (ALMA) in Chili te gebruik.

Die skikking van 66 radioteleskope, uitgespreid oor die Atacama-woestyn, ongeveer 1400 km noord van die hoofstad van Chili, het gesien hoe 'n sterrestelsel nege miljard ligjare daarvandaan byna die helfte van sy stervormende gas in die ruimte uitstoot.

Belangrike navorsingspunte

  • 'N Radioteleskoopreeks het waargeneem hoe 'n sterrestelsel 46% van sy totale stervormende gas verloor
  • Sonder hierdie gas sal die sterrestelsel binne enkele tienmiljoene jare ophou om sterre te skep en 'sterf'.
  • Die uitstoot van gas is waarskynlik veroorsaak deur 'n samesmelting van sterrestelsels - wat dui op 'n nuwe meganisme waardeur sterrestelsels hul lewens beëindig

Die sterrekundiges vermoed dat hierdie rampspoedige gebeurtenis veroorsaak is deur twee sterrestelsels wat bots en saamsmelt tot 'n nuwe een - inspirerend ID2299 genoem. Hierdie massiewe ontwrigtingsgebeurtenis bied vars insig in die meganismes wat die vorming van sterre kan stop, en voeg nog 'n stuk by tot die ingewikkelde legkaart van hoe sterrestelsels ontwikkel en sterf.

Die navorsing verskyn in Natuursterrekunde.

“Dit is die eerste keer dat ons 'n tipiese massiewe stervormende sterrestelsel in die verre heelal waarneem wat gaan sterf weens 'n geweldige koue gasuitwerping,” sê hoofnavorser Annagrazia Puglisi van die Durham Universiteit, UK, en Saclay Nuclear. Navorsingsentrum (CEA-Saclay) in Frankryk.

Vorige bewyse het aangedui dat sulke uitstoot van stervormende gas veroorsaak kan word deur vinnige galaktiese winde wat uit die nuutgevormde massiewe sterre uitgeslaan is, of deur die kragtige aktiwiteit van swart gate wat in die harte van massiewe sterrestelsels draai.

Antennes van die Atacama Large Millimeter / Submillimeter Array (ALMA), op die Chajnantor-plato in die Chileense Andes. Die Groot en Klein Magellaanse wolke, twee metgeselle sterrestelsels van ons eie Melkwegstelsel, kan gesien word as helder vlekke in die naghemel, in die middel van die foto. Krediet: ESO / C. Malin

"Ons studie dui daarop dat gasuitstoot deur samesmeltings geproduseer kan word," sê mede-outeur Emanuele Daddi, van CEA-Saclay.

Die leidraad wat tot hierdie gevolgtrekking gelei het, kom van 'n "gety tail" - 'n langwerpige stroom sterre en gas wat uitstrek na die interstellêre ruimte. Gewoonlik is hierdie kenmerke flou, maar die span het daarin geslaag om ID2299's vas te lê toe dit net in die ruimte begin en dus nog relatief helder.

Daddi wys daarop dat wind en getysterte baie dieselfde kan lyk. Vorige navorsing wat waargeneem het dat winde galaktiese gas die ruimte in gedryf het, kon eerder getysterte gesien het.

"Dit kan ons daartoe lei om ons begrip van sterrestelsels te hersien," sê Daddi.

Die waargenome uitwerping maak 'n verstommende 46% van die ID2299 se gas uit, en dit spuit teen 'n merkwaardige tempo uit - gelykstaande aan 10.000 Suns se gas per jaar. Dit beteken dat die sterrestelsel vinnig die materiaal verloor wat nodig is om nuwe sterre te skep.

Die gas wat in ID2299 agterbly, sal ook nie lank hou nie: die sterrestelsel is ook besig om honderde kere vinniger sterre te vorm as ons eie Melkweg en verbruik die oorblywende materiaal.

Die span skat dat ID2299 oor slegs 'n paar tien miljoene jare sal stilstaan ​​- en wanneer sy sterre uiteindelik uitknip, sal dit vir ewig donker word.

Kollig: sterftes in sterrestelsels

  • Sterrestelsels begin "sterf" as hulle al hul stervormende gas verloor of verbruik, sodat nuwe sterre nie gebore kan word nie
  • Bestaande sterre sal uiteindelik deur hul brandstof verbrand en donker word en die sterrestelsel tot sy ware einde bring
  • Sterre soos ons son kan net ongeveer tien biljoen jaar duur, maar kleiner, koeler rooi dwergsterre kan triljoene jaar duur

Lauren Fuge

Lauren Fuge is 'n wetenskaplike joernalis by The Royal Institution of Australia.

Lees wetenskaplike feite, nie fiksie nie.

Daar was nog nooit 'n belangriker tyd om die feite te verduidelik, bewyse-gebaseerde kennis te koester en die nuutste deurbrake in wetenskaplike, tegnologiese en ingenieurswese aan te bied nie. Cosmos word uitgegee deur The Royal Institution of Australia, 'n liefdadigheidsorganisasie wat toegewy is aan die koppeling van mense met die wêreld van die wetenskap. Finansiële bydraes, hoe groot of klein ook al, help ons om toegang te gee tot betroubare wetenskaplike inligting op 'n tydstip waar die wêreld dit die nodigste het. Ondersteun ons asseblief deur vandag 'n donasie te maak of 'n intekening aan te koop.

Maak 'n donasie

Watter vinnige sterrekunde kan ons vertel van die galaktiese dieretuin

Die verre 'hemelse sfeer' is vir die grootste deel van die mensegeskiedenis as volmaak en onveranderlik beskou. Sterre het op hul plek gebly, planete het voorspelbaar beweeg, en die paar skelm komete is as atmosferiese verskynsels beskou. Dit het begin verander met die waarneming van die Deense sterrekundige Tycho Brahe van die supernova van 1572 - blykbaar 'n nuwe ster - en sy studies van die Groot Komeet van 1577, wat hy bewys eintlik 'n verre voorwerp was. Desondanks is die indruk van permanensie sterk. Daar is baie min astronomiese voorwerpe wat opvallend met die blote oog verskil: slegs die helderste komete, novae en supernovas. Vir waarnemers in die noordelike halfrond was die laaste supernova met blote oog in 1604.

Moderne teleskopiese studies vertel 'n heel ander verhaal. Vandag weet ons van ongeveer 'n halfmiljoen veranderlike sterre in ons melkweg, die Melkweg, en ons identifiseer jaarliks ​​duisende verbygaande voorwerpe. Alhoewel baie sterre op voorspelbare maniere wissel, is die heelal ook vol onvoorspelbare geweld. Wanneer twee sterre naby mekaar wentel, kan massa van die een na die ander vloei. As een van die sterre 'n ou, ineengestorte wit dwerg is, kan die gas wat dit van sy metgesel trek, ophoop totdat die dwerg 'n skielike termonukleêre ontploffing ondergaan - 'n supernova soos die wat Tycho gesien het. Daar is ook 'n meer algemene tipe supernova wat geproduseer word deur die dood van alleensterre wat meer as tien keer die massa van die son is.

Supernovae toon 'n wye verskeidenheid gedrag wat afhang van die gedetailleerde eienskappe van die stelsel ten tye van die finale, noodlottige ramp. Die atome wat uit supernova-ontploffings ontstaan, het die grondstof vir alle planete verskaf, ook ons ​​eie. Sterrekundiges is verstaanbaar gretig om meer daarvan te wete te kom, maar die twee klasse supernovas gesamentlik gebeur slegs een keer per eeu in ons sterrestelsel.

Dit is natuurlik nie baie winsgewend om na gebeure op tydskale van 'n eeu te soek nie, net in ons sterrestelsel. Gelukkig is ons sterrestelsel slegs een van ongeveer 'n triljoen sterrestelsels in die sigbare heelal. As u die hele tyd miljoene sterrestelsels monitor, is dit moontlik om elke dag baie supernovas te vind. Dit is een van die opwindendste uitdagings van moderne hoëspoed-sterrekunde.

Afgesien van supernovas, is daar slegs enkele veranderlike bronne wat helder genoeg is om op groot afstande van ander sterrestelsels gesien te word, selfs met behulp van kragtige teleskope. Verreweg die algemeenste is die ewekansige veranderlikheid van kwasars. Kwasars bestaan ​​uit 'n supermassiewe swart gat, miljoene tot miljarde kere die massa van ons son, wat skyn soos materiaal na die swart gat val, opwarm en energie uitstraal.

Vandag dink ons ​​dat in werklikheid elke sterrestelsel 'n supermassiewe swart gat in die middel bevat, en dat ongeveer 1 persent daarvan vinnig genoeg massa toeneem om as ligte kwasars gesien te word. Die supermassiewe swart gat in die middel van ons eie sterrestelsel is in wese 'af'. Soms kom so 'n swart gat egter vinnig aan. Die mees boeiende oorsaak is 'n sogenaamde 'gety disruption event' waarin 'n ster soos die Son te naby aan die swart gat gaan en deur die getye van die swart gat uitmekaar geruk word. Van die puin val dan in die swart gat om 'n kortstondige fakkel aan te dryf. Hierdie getyversteuringsgebeurtenisse is baie skaarser as supernovas, en kom slegs ongeveer een keer elke 10.000 jaar in 'n spesifieke sterrestelsel voor. In die verre heelal is die studie van veranderlikheid in wese die studie van swart gate en supernovas.

Dit gee u 'n gevoel van die merkwaardige astronomiese dieretuin van veranderlike en kortstondige voorwerpe. Die uitdaging vir die professionele sterrekundige is om al hierdie verskillende bronne te vind en te karakteriseer, nie net vir hoe hulle individueel werk nie, maar ook om hul algehele demografie en statistieke te bepaal. Om 'n groot aantal daarvan te vind, benodig u 'n groot teleskoop wat die talle verre, flou voorwerpe kan opspoor. Oor die algemeen sien groter teleskope egter net kleiner dele van die lug raak. Hierdie frustrerende reël kan slegs gebuig word deur groot bedrae geld te spandeer.

As u wetenskaplike doel is om die grootste moontlike aantal oorgange te vind en hul evolusie in die kosmiese geskiedenis van die heelal te bestudeer, wil u 'n groot teleskoop gebruik wat soveel lug bedek as wat u kan bekostig. Dit is fundamenteel die doel van die Large Synoptic Survey Telescope (LSST). LSST is in Chili geleë (effektief) 'n teleskoop met 'n deursnee van 6,7 meter wat in 2022 met die volledige wetenskaplike werking begin.

LSST sal die naaste sterrekundiges ooit wees om 'n filmkamera te skep om die hele heelal dop te hou. Dit sal ongeveer die helfte van die lug ondersoek met behulp van 'n kamera wat meer as 40 keer die oppervlakte van die volle maan strek. Maar LSST kan slegs een keer elke drie nagte 'n nuwe beeld van elke pleister van daardie lug kry. LSST kan verganklikhede 30 miljoen keer flouer opspoor as wat met die blote oog sigbaar is, wat dit 'n fenomenale projek maak om groot getalle flou verbygaande bronne in die sigbare heelal te vind - LSST behoort ongeveer 1 000 supernovas per dag te vind! Maar hierdie vermoë kos: ongeveer $ 600 miljoen net vir konstruksie, plus 'n beduidende bedryfskoste.

Aan die ander limiet van LSST is 'n projek waaraan ek werk: die All-Sky Automated Survey for Supernovae (ASAS-SN). Teen die einde van hierdie jaar sal ASAS-SN bestaan ​​uit 20 diafragma-teleskope van 14 cm wat oor die hele wêreld versprei is en ongeveer $ 3,5 miljoen kos vir konstruksie en werking tot en met 2022. Met sulke klein teleskope - groot telefoto-lense, regtig - ASAS-SN kan slegs helder oorgange vind, ongeveer 25 000 keer flouer as wat sigbaar is vir die menslike oog. Tog moet dit nog steeds ongeveer een supernova per dag vind. En omdat ASAS-SN uit klein teleskope bestaan, kan dit die lug baie vinniger as LSST beeld. Die gekombineerde 'beeld' van al die ASAS-SN-teleskope strek oor 1600 keer die oppervlakte van die Maan. Dit stel hulle in staat om elke aand die hele sigbare lug te besigtig.

Die twee projekte is baie komplementêr en balanseer in wese 'n kompromie tussen mekaar hoeveelheid en gehalte. LSST bied hoeveelheid: die groot aantal flou bronne wat nodig is vir statistiese studies van verre bronne, en om die evolusie van verbygaande bronne oor kosmiese tyd te bestudeer. Die tipiese LSST-verbygaande is egter flou en moeilik om vir lang tye in detail te bestudeer, selfs met die grootste teleskope ter wêreld. ASAS-SN bied gehalte. Die helder bronne wat deur ASAS-SN gevind is, is diegene wat die nabygeleë Heelal die beste ondersoek, en wat met groter teleskope in die fynste besonderhede en die langste tyd bestudeer kan word.

Een van die belangrikste instrumente vir sterrekundiges is die spektrum van 'n voorwerp: hoeveel lig as die funksie van die kleur daarvan uitgestraal word. 'N Spektrum is die beste manier om die snelhede, temperature, elementêre samestelling en tipe van 'n voorwerp te klassifiseer (bv. Watter tipe supernova? Wat was die unieke eienskappe daarvan?). Omdat u die lig in smal vakkies met kleur moet kap, het u baie meer lig nodig om 'n spektrum van 'n voorwerp te maak as om 'n beeld daarvan te kry. LSST is al 'n groot teleskoop, dus dit sal moeilik of onmoontlik wees om 'n spektrum van die tipiese, flou LSST-verbygaande te kry.

Selfs vir die minderheid LSST-bronne wat helder genoeg is om een ​​spektrum te verkry, sal die bron vinnig vervaag en te flou word om 'n ander spektrum te laat bestudeer hoe dit mettertyd ontwikkel. Daarom sal 'n weglaatbare fraksie van LSST-ontdekkings deur hierdie fundamenteel belangrike astronomiese instrument bestudeer word. Die ASAS-SN-transiënte is baie minder, maar baie helderder, dus 'n baie groot deel van ASAS-SN-transiënte kan spektroskopies bestudeer word, en dit kan vir lang tydperke bestudeer word, selfs al verdwyn hulle.

Projekte soos LSST en ASAS-SN gaan voort met die revolusie wat Tycho begin het, en onthul die veranderlike en soms gewelddadige gebeure wat die hoogs onvolmaakte, steeds veranderende hemelse sfeer verlig.

Christopher Kochanek is professor in sterrekunde aan die Ohio State University.


Is hoëspoed-galaktiese botsings oorleefbaar? - Sterrekunde

Revolusionêre projekte soos LSST en ASAS-SN het wetenskaplikes gehelp om nuwe mylpale te bereik in moderne hoëspoed-sterrekunde.

Deur Christopher Kochanek

Vir die grootste deel van die mensegeskiedenis is die verre 'hemelse sfeer' as volmaak en onveranderlik beskou. Sterre het op hul plek gebly, planete het voorspelbaar beweeg, en die paar skelm komete is as atmosferiese verskynsels beskou. Dit het begin verander met die waarneming van die Deense sterrekundige Tycho Brahe van die supernova van 1572 - blykbaar 'n nuwe ster - en sy studies van die Groot Komeet van 1577, wat hy bewys eintlik 'n verre voorwerp was. Desondanks is die indruk van permanensie sterk. Daar is baie min astronomiese voorwerpe wat opvallend met die blote oog verskil: net die helderste komete, novae en supernovas. Vir waarnemers in die noordelike halfrond was die laaste supernova met blote oog in 1604.

Moderne teleskopiese studies vertel 'n heel ander verhaal. Vandag weet ons ongeveer 'n halfmiljoen veranderlike sterre in ons sterrestelsel en identifiseer duisende verbygaande voorwerpe elke jaar. Alhoewel baie sterre op voorspelbare maniere wissel, is die heelal ook vol onvoorspelbare geweld. Wanneer twee sterre naby mekaar wentel, kan massa van die een na die ander vloei. As een van die sterre 'n ou, ineengestorte wit dwerg is, kan die gas wat dit van sy metgesel haal, ophoop totdat die dwerg 'n skielike termonukleêre ontploffing ondergaan - 'n supernova soos die wat Tycho gesien het. Daar is ook 'n meer algemene tipe supernova wat geproduseer word deur die dood van alleensterre wat meer as tien keer die massa van die son is.

Supernovae toon 'n wye verskeidenheid gedrag wat afhang van die gedetailleerde eienskappe van die stelsel ten tye van die finale, noodlottige ramp. Die atome wat uit supernova-ontploffings ontstaan, het die grondstof vir alle planete verskaf, ook ons ​​eie. Sterrekundiges is verstaanbaar gretig om meer hieroor te leer, maar die twee klasse supernovas gesamentlik kom net een keer per eeu in ons sterrestelsel voor.

Dit is natuurlik nie baie winsgewend om na gebeure op tydskale van 'n eeu te soek nie, net in ons sterrestelsel. Gelukkig is ons sterrestelsel slegs een van ongeveer 'n triljoen sterrestelsels in die sigbare heelal. As u miljoene sterrestelsels deurentyd monitor, is dit moontlik om elke dag baie supernovas te vind. Dit is een van die opwindendste uitdagings van moderne hoëspoed-sterrekunde.

Behalwe supernovas, is daar slegs 'n paar veranderlike bronne wat helder genoeg is om op groot afstande van ander sterrestelsels gesien te word, selfs met behulp van kragtige teleskope. Verreweg die algemeenste is die ewekansige wisselvalligheid van kwasars. Kwasars bestaan ​​uit 'n supermassiewe swart gat, miljoene tot miljarde kere die massa van ons son, wat skyn soos materiaal na die swart gat val, opwarm en energie uitstraal.

Vandag dink ons ​​dat in werklikheid elke sterrestelsel 'n supermassiewe swart gat in die middel bevat, en dat iets soos 1 persent van die massa vinnig genoeg is om as ligte kwasare gesien te word. Die supermassiewe swart gat in die middel van ons eie sterrestelsel is in wese 'af'. Soms kom so 'n swart gat egter vinnig aan. Die mees boeiende oorsaak is 'n sogenaamde 'gety disruption event' waarin 'n ster soos die Son te naby aan die swart gat gaan en deur die getye van die swart gat uitmekaar geruk word. Van die puin val dan in die swart gat om 'n kortstondige fakkel aan te dryf. Hierdie gety-ontwrigtingsgebeurtenisse is baie skaarser as supernovas, en kom net ongeveer elke 10.000 jaar in 'n spesifieke sterrestelsel voor. In die verre heelal is die studie van veranderlikheid in wese die studie van swart gate en supernovas.

Dit gee u 'n gevoel van die merkwaardige astronomiese dieretuin van veranderlike en kortstondige voorwerpe. Die uitdaging vir die professionele sterrekundige is om al hierdie verskillende bronne te vind en te karakteriseer, nie net vir hoe hulle individueel werk nie, maar ook om hul algehele demografie en statistieke te bepaal. Om 'n groot aantal daarvan te vind, benodig u 'n groot teleskoop wat die talle verre, flou voorwerpe kan opspoor. Oor die algemeen sien groter teleskope egter net kleiner dele van die lug raak. Hierdie frustrerende reël kan slegs gebuig word deur groot bedrae geld te spandeer.

As u wetenskaplike doel is om die grootste moontlike aantal oorgange te vind en hul evolusie in die kosmiese geskiedenis van die heelal te bestudeer, wil u 'n groot teleskoop gebruik wat soveel lug bedek as wat u kan bekostig. Dit is fundamenteel die doel van die Large Synoptic Survey Telescope (LSST). Located in Chile, LSST is (effectively) a 6.7-metre diameter telescope, scheduled to start full science operations in 2022.

LSST will be the closest astronomers have ever come to creating a movie camera to watch the whole universe. It will survey approximately half the sky using a camera that spans more than 40 times the area of the full Moon. But LSST can obtain a new image of each patch of that sky only once every three nights. LSST can detect transients 30 million times fainter than visible to the naked eye, making it a phenomenal project for finding huge numbers of faint transient sources across the visible Universe – LSST should find some 1,000 supernovae per day! But this capability comes at a cost: roughly $600 million just for construction, plus a significant operation cost as well.

At the other limit from LSST is a project I am working on: the All-Sky Automated Survey for Supernovae (ASAS-SN). By the end of this year, ASAS-SN will consist of 20 14-cm aperture telescopes spread across the globe, and costing roughly $3.5 million for both construction and operation through to 2022. With such small telescopes – big telephoto camera lenses, really – ASAS-SN can find only bright transients, roughly 25,000 times fainter than are visible to the human eye. Even so, it should still find about one supernova a day. And because ASAS-SN is comprised of small telescopes, it can image the sky far faster than LSST. The combined ‘image’ from all the ASAS-SN telescopes spans 1,600 times the area of the Moon. This allows them to survey the entire visible sky every night.

The two projects are highly complementary, essentially balancing a trade-off between ‘quantity’ and ‘quality’. LSST provides ‘quantity’: the large numbers of faint sources needed for statistical studies of distant sources, and for studying the evolution of transient sources across cosmic time. However, the typical LSST transient is faint and hard to study in detail for long periods of time, even with the world’s largest telescopes. ASAS-SN provides ‘quality’. The bright sources found by ASAS-SN are the ones that best survey the nearby Universe, and that can be studied in the greatest detail and for the longest periods of time using larger telescopes.

One of the most important tools for astronomers is the spectrum of an object: how much light is emitted as a function of its colour. A spectrum is the best way to classify the velocities, temperatures, elemental composition and type of an object (eg, which type of supernova? What were its unique properties?). Because you must chop up the light into narrow bins of colour, you need far more light to make a spectrum of an object than to get an image of it. LSST is already a large telescope, so it will be difficult or impossible to get a spectrum of the typical, faint LSST transient.

Even for the minority of LSST sources bright enough to obtain one spectrum, the source will quickly fade and become too faint to get another spectrum to study how it evolves with time. Therefore, a negligible fraction of LSST discoveries will be studied by this fundamentally important astronomical tool. The ASAS-SN transients are far fewer in number but are far brighter, so a very large fraction of ASAS-SN transients can be studied spectroscopically, and they can be studied for long periods of time even as they fade away.

Projects like LSST and ASAS-SN are continuing the revolution begun by Tycho, revealing the variable and sometimes violent events that light up the highly imperfect, ever-changing celestial sphere.

Christopher Kochanek is the professor of astronomy at the Ohio State University.

This article was originally published at Aeon and has been republished under Creative Commons.

Featured Image Courtesy: Visual Hunt

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Third EGRET Catalog

Click image for larger view Researchers working with the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory have cataloged the entire high-energy gamma-ray sky as we know it, from pulsars in our own Galaxy to blazars at the farthest ends of the Universe.

This Third EGRET Catalog, presented today at the 193rd American Astronomical Society Meeting in Austin, Texas, contains 271 gamma-ray sources detected from 1991-1995, including the Large Magellanic Cloud, the great solar flare of 1991, a probable radio galaxy, and 170 sources yet unidentified.

"This catalog includes all the high-energy gamma ray sources in the Universe that could be detected by EGRET," said Dr. Robert Hartman, an astrophysicist on the EGRET team at NASA's Goddard Space Flight Center. "This is a huge step for gamma-ray astronomy from the early days 25 years ago, yet in many ways the field is still in its infancy."

Dr. Hartman said that NASA's Gamma-Ray Large Area Space Telescope (GLAST), planned for a 2005 launch date, is expected to detect thousands of high-energy gamma ray sources with a 30% increase in sensitivity.

According to Dr. Neil Gehrels, the Gamma Ray and Cosmic Ray Astrophysics Branch Head at Goddard, "Gamma-ray astronomy is a window into the Universe's hottest and most cataclysmic events, and I am tremendously excited by the EGRET catalog. The field of gamma-ray astrophysics is becoming a real astronomical discipline with a significant number of objects to observe."

High-energy gamma rays are largely produced by high-speed particle collisions. Particles from a supernova explosion, for example, can accelerate to nearly the speed of light and collide with gas in the interstellar medium. We observe this interaction as gamma rays. The galactic plane, in fact, glows in gamma ray energy from high speed collisions.

EGRET covers a very wide chunk of gamma ray energies in the electromagnetic spectrum, from 30 to 20,000 MeV, which is comparable to a single telescope measuring from infrared, through ultraviolet and into X-ray wavelengths.

Because of a low photon detection rate, EGRET produces likelihood maps, not visual images. These maps depict the likelihood within a 50%, 68%, 95% and 99% probability that a gamma-ray source is at a particular point in the sky. Those sources detected at less than a certain degree of statistical probability, depending on its location relative to the galactic plane, are not included in the Third EGRET catalog. This threshold eliminated a few sources originally included in the Second EGRET catalog.

Also not included in this Third EGRET Catalog were gamma ray bursts -- brief, intense flashes of gamma rays which are thought to be the most powerful forces in the Universe other than the Big Bang. The Catalog focuses instead on permanent sources of gamma rays.

This will most likely be the last full EGRET catalog, said Dr. Hartman, for the instrument will now concentrate on narrower regions of observations and conserve its remaining neon gas supply.

EGRET was assembled at NASA's Goddard Space Flight Center by a team of scientists from Goddard, Stanford University, the Max-Planck Institute for Extraterrestrial Physics, and Grumman Corporation (now part of Northrup-Grumman). EGRET is one of the four gamma-ray instruments on the Compton Gamma Ray Observatory, the second of NASA's Great Observatories and the gamma-ray equivalent to the Hubble Space Telescope. Compton was launched aboard the Space Shuttle Atlantis in April, 1991, and at 17 tons, it is the largest astrophysical payload ever flown.

The Third EGRET Catalog will be published in an upcoming issue of Astrophysical Journal Supplements.

Obtain a copy of the Third EGRET Catalog.

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High-speed crash makes galaxies 'sterile'

A new view of two very well-known galaxies has revealed they are connected by faint, starless filaments of hydrogen gas, which trace back to a very high-speed intergalactic collision.

The smash-up between M86 and NGC4438 had not been suspected before, and may explain why M86, which is visible to the naked eye, is unable to give birth to new stars.

"Stars and gases behave very differently in collisions," says astronomer Professor Jeffrey Kenney of Yale University and lead author of a paper in the November 2008 issue of Astrofisiese joernaalbriewe.

During galactic smash-ups stars rarely collide, since there is so much space between them. But gases do slam into gases. The faster the collision, the higher the temperature the gases reach.

In the case of M86, its gases are millions of degrees in temperature and radiate x-rays.

But there has been no easy explanation for all this blistering hot gas.

The new evidence of M86's collision may solve that mystery. What's more, the super-hot gas also probably explains why M86 is unable to produce new stars.

Agitated

To make stars you need colossal clouds of frigid gas that will collapse to begin star-producing nuclear reactions.

Super-hot gases are far too agitated to clump together and collapse to form such new heavenly bodies.

As a result, the hot galactic atmosphere of M86 leaves it bereft of baby stars and dominated by older stars that formed before something - probably galactic gas collisions - turned up the heat.

"This has been an ongoing mystery for years," says Kenney of the absence of young stars in elliptical galaxies, which also happen to be the largest galaxies in the universe.

The faint streamers of hydrogen gas between the two galaxies were previously detected at the edges of images of both galaxies M86 and NGC4438.

But it wasn't until new technologies enabled a wider, deeper view of the space between them that the connection was discovered, says astronomer Dr Bill Keel of the University of Alabama.

"These galaxies have a history," says Keel.

The discovery underscores the growing realisation that no galaxy is an island, he says. "It's no longer an isolated, stable system." It's part of a larger process of collisions, mergers and near misses.

Keel hopes Kenney and his colleagues will search for more telltale gas filaments between other galaxies in the Virgo cluster, where both M86 and NGC 4438 reside.

M86 is the brightest galaxy in the Virgo cluster, a neighbour galaxy cluster about 50 million light-years away from our own Local Group cluster.

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Secrets Of The Strange Stars That Circle Our Supermassive Black Hole

High winds are the norm at the center of the Milky Way. Astronomers have now clocked suns orbiting the galactic core at a staggering 3,000 miles (4,800 kilometers) per second. At this rate, Earth would complete its orbit around the sun in a mere three days. What lurks at the galaxy’s core that can accelerate stars to such speeds?

Astronomers have considered various possibilities. Does the center of the galaxy harbor a tight cluster of superdense stellar remnants (neutron stars)? Or perhaps a huge ball of subatomic neutrino particles?

But these and other more exotic possibilities were eliminated in the spring of 2002 when a star called S2 swept down in its highly eccentric orbit and passed within 17 light-hours of the Milky Way’s center — a minuscule distance in galactic terms. In 17 hours, light travels three times the distance between Pluto and the sun.

Only one object is compact enough and has sufficient mass to accelerate stars to such a high speed: a supermassive black hole. Astronomers had suspected that a black hole must lie at the Milky Way’s core, but plotting the orbit of S2 and other stars dramatically strengthened the evidence.

Our central black hole is small by the standard of what lurks in the hearts of other galaxies. Observations of the giant elliptical galaxy M87 suggest the presence of a black hole 6 billion times more massive than the sun. The interaction of two supermassive black holes probably produces the intense X-rays streaming from the galaxy NGC 6240. The Andromeda Galaxy may harbor a black hole of 140 million solar masses.

In comparison, our galaxy’s black hole is paltry — containing about 4 million solar masses. But its nearness means we can study it in detail, including charting the orbits of dozens of stars buzzing around it like bees. The stellar-mass black holes found in some binary star systems are too small to be observed in detail by telescopes anytime soon. So, the best chance of seeing what happens in the bizarre neighborhood around a black hole is to study the one at the Milky Way’s heart. So far, it has not failed to surprise us.

Bright stars surround the supermassive black hole at the Milky Way’s center. ( Credit: NASA/CXC/M.Weiss )

The Inner Realm

The galactic center lies about 26,000 lightyears from Earth toward the constellation Sagittarius. It is a region of the sky where bright stars mingle with dark clouds of gas and dust. The actual center is too obscured to reveal much when astronomers observe it in visible light. What we know of it comes from data collected in infrared and radio wavelengths. These wavelengths can pass through the dust and gas and reach Earth-based telescopes.

Astronomers have long known that the strongest source of radio energy in the sky, after the sun, lies at the galactic center. This broad core region is called Sagittarius A, often abbreviated as Sgr A.

Sgr A hosts dozens of individual radio sources. One is called Sagittarius A*, pronounced “Sagittarius A star.” It lies at the very center of the galaxy and coincides with the position of the supermassive black hole. Everything else rotates clockwise (from Earth’s point of view) around this point, making it the dynamic center of the galaxy. And it is a very busy neighborhood.

Surrounding Sgr A* at a distance of several light-years, a shell of dust rotates counterclockwise — opposite to the galaxy’s general rotation. Lying inside the shell, and turning in the same direction, is a small spiral structure with three arms.

Each arm is a stream of hot gas set aglow by nearby stars. The gas flows toward the center of the spiral where Sgr A* lies. Radio images taken a few years apart revealed the spiral is rotating. More recently, a close-up look at Sgr A* with new imaging technology has revealed the amazingly powerful gravity of the object the spiral encircles.

Stellar Raceway

In 2002, a team of astronomers led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, published the first scientific paper announcing S2’s 17-light-hour close encounter with Sgr A*. Using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile, Genzel’s group caught S2 as it rounded Sgr A* at a fantastic speed. The VLT’s adaptive optics reduces atmospheric blurring, allowing astronomers to chart S2’s position more accurately.

For the previous decade, the astronomers had been plotting S2’s orbit, mostly with ESO’s 3.6-meter New Technology Telescope, also in Chile. The orbital positions allowed the researchers to calculate S2’s orbital period around Sgr A* as about 16 years. The orbit is quite eccentric. The star swoops in to within 17 light-hours at its closest approach to Sgr A*, but then sweeps outward to a distance of some 10 light-days at its farthest point. To produce such an orbit requires a compact black hole with about 4 million solar masses.

Genzel and his colleagues were not the only ones tracking S2 and the many other stars zipping around Sgr A*. Astronomer Andrea Ghez’s Galactic Center Group at UCLA has studied S2 and its motions with the 10-meter Keck Telescope in Hawaii. In 2000, the team reported evidence that S2’s path is curved — early evidence S2 is orbiting something at the galactic center. The UCLA team later discovered S2’s close orbital distance to Sgr A* at about the same time as Genzel and his colleagues.

Sagittarius A*, at the Milky Way’s core, lies in the constellation Sagittarius and to the east (left) of Scorpius. The region is rich with star clusters and nebulae. It’s a perfect place to explore with binoculars under dark skies. (Credit: Sky images: Gerald Rhemann Constellation outlines: Astronomy: Roen Kelly)

Stars of Mystery

Extensive observations in recent years by Genzel, Ghez, and others paint a fascinating picture of the flurry of activity around Sgr A*. One of the most challenging observations astronomers have performed on the galactic center stars is spectroscopy, or separating starlight into its component wavelengths. A spectrum reveals much about a star’s composition, age, and mass.

Gathering enough light from a distant star to take a good spectrum requires tracking the target through a narrow slit for many hours. Any small shift in the slit’s position contaminates the spectrum with light from other sources. Spectroscopy is especially challenging in the crowded star field around Sgr A*, where the density of stars is more than a million times higher than in our stellar neighborhood.

In 2003, Ghez took a spectrum of S2 with the Keck Telescope using its adaptive optics system. The slit trained on the star was only 0.04 inch (1 millimeter) wide. Keeping this narrow gap locked on S2 was like aiming a gun sight on an object the size of a basketball 1,000 miles (1,600 km) away.

The spectrum revealed S2 to be a heavyweight star some 15 times the sun’s mass. Such large stars exhaust their hydrogen supply quickly — in this case, in less than 10 million years. That means S2 must be younger than 10 million years. In addition, the star has a very hot atmosphere, as do other stars orbiting close to Sgr A*. This also indicates a relatively young age.

In short, these stars formed 3 to 6 million years ago. This raises a major problem: Why are such young stars orbiting so close to Sgr A*, a region of intense magnetic fields and strong gravitational forces that would normally prevent star formation?

Radio astronomy reveals hidden features of the Milky Way’s center, including remnants of supernova explosions and stars forming in vast clouds of gas and dust. (Credit: W.M. Goss/C. Lang/VLA/NRAO)

Stellar Masquerade

One possible explanation is that S2 and its companions may be old stars masquerading as young ones — “a phenomenon we understand quite well in Los Angeles,” Ghez once quipped to a science reporter.

In this case, what seem to be young stars are actually the cores of older suns that collided and merged. The collisions could have stripped away the suns’ cool outer layers, exposing their hot interiors. The result would be a cluster of massive stars that appear much younger than they really are.

But there’s a problem with this scenario. A collision violent enough to strip away the outer layers should also annihilate both stars and leave only a trail of hot gas. And so astronomers have proposed alternatives. For example, perhaps the stars formed elsewhere and migrated inward under the black hole’s gravitational pull.

The problem with this explanation is that most active star formation in the Milky Way occurs far from the core, in its spiral arms. It would take the stars too long to migrate as close to the center as S2.

Dense dust clouds do lie closer to Sgr A* than the spiral arms, to within a few dozen light-years. Stars are probably forming inside of them. It’s conceivable that a cluster of young stars could spiral down to within a few light-years of the center — and do so in less than 10 million years.

The problem here is that to get closer to the black hole, the stars would have to shed angular momentum — the quantity that keeps planets in nice safe orbits around stars instead of “falling” directly into them.

One way to lose angular momentum is to bump into other stars. But it’s difficult to imagine how stars could endure this process and migrate to within light-hours of Sgr A* without being destroyed. Besides, the process should leave behind a trail of stars toward Sgr A* for a long distance, something astronomers have not yet seen. Instead, the shell of stars orbiting close to Sgr A* has a definite outer edge.

Star Birth in a Disk

Another possibility is that Sgr A*’s central cluster stars formed within a rotating disk of gas and dust immediately surrounding the black hole. In fact, some observations suggest most stars in the central cluster orbit roughly in the same plane — an arrangement reminiscent of the major planets in our solar system. The planets formed in a disk of gas and dust, so perhaps S2 and its fellow travelers did, too.

However, not all astronomers agree the central cluster has a disklike structure. Another caveat: To spawn stars, the disk would need to be dense enough to withstand the black hole’s tidal forces.

It’s also conceivable that Sgr A*’s companion stars formed in dust clouds circling at high speed within a few light-years of the galactic center. Collisions between the clouds could have spawned shock waves, triggering star formation. As the result of collisions between the clouds, they and the new stars embedded within them could have shed enough momentum to settle into orbits around the black hole. The galactic core’s strong magnetic field would have gradually swept the leftover interstellar dust and gas away from the black hole. What would remain is a disk of young stars in close orbit to Sgr A*.

This scenario explains much of what astronomers see in the galactic core, although not all. UCLA astronomer Brad Hansen thinks he has a viable alternative: Hot young stars now orbit the Milky Way’s central black hole because a second smaller black hole dragged them there.

The process begins in a crowded young star cluster, dozens of light-years from the galactic center. Collisions between big stars in the cluster’s core form an intermediate-sized black hole in the range of 1,000 to 10,000 solar masses. Gradually, the black hole would migrate toward the galactic center, dragging its cargo of “hostage stars” along with it. Hansen argues this is the only way to quickly transport massive young stars into the galactic center from an outside star-birth location.

All the black-hole ferry scenario lacks is hard evidence to support it. If a second black hole orbits the primary black hole in the galactic core, its presence might be detectable. Its tug on Sgr A* might cause a detectable wiggle. Clearly, astronomers still have a lot of work left to fully understand the processes at work in the galactic core.

Dozens of young stars orbit at high speeds around the galaxy’s central black hole. By plotting the stars’ positions for years, astronomers calculated their orbits and estimated the mass of the black hole they encircle. (Credit: Astronomy: Jay Smith, after Andrea Ghez (UCLA))

Imaging The Black Hole

Fast-moving stars like S2 remain the best evidence that a black hole lies at the heart of the Milky Way. Other support includes periodic bursts of infrared light from Sgr A*. The bursts suggest the black hole spins, completing a turn every 17 minutes. Astronomers have also detected strong radio pulses coming from Sgr A*. This may indicate that packets of ultra-hot gas and dust are falling into the black hole.

But this is all still circumstantial evidence. The definitive proof might come if astronomers could actually image the black hole’s edge or “event horizon,” beyond which no light or matter can escape.

Radio energy passes through the veil of obscuring dust and gas around the galactic center, providing a way to directly image a black hole. By itself, a black hole is essentially invisible. But it would be detectable as a silhouette against the accretion disk of gas spiraling into it. The gas emits energy as it accelerates to high speeds around the black hole.

Light follows a highly curved path near a black hole, making its silhouette appear wider than it actually is. Bright rings or arcs, formed as the black hole bends or “lenses” light from background sources, might protrude from the silhouette’s edges.

In 2008, radio astronomers announced an important milestone in the study of our galaxy’s black hole. By combining the power of three radio telescopes, researchers detected features around Sgr A* as small as 31 million miles (50 million km) across. The study found that radio emission from Sgr A* is offset from the black hole, perhaps because it comes from an accretion disk. Astronomers hope the Event Horizon Telescope — a nearly Earth-sized radio observatory comprising about a dozen separate instruments — will be able to image the black hole’s silhouette in the next few years.

Whatever the result, imaging the Milky Way’s central black hole will put the existence of black holes on a firmer footing and perhaps reveal important new insights about the evolution of galactic cores. A failure to see it will bring into question what we understand about the heart of our own galaxy — including the origins of the highspeed roller derby of young stars whizzing around its center.

This story originally appeared in Astronomy‘s special issue, The Milky Way Inside And Out .


What High-Speed Astronomy Can Tell Us about the Galactic Zoo

For most of human history, the distant ‘celestial sphere’ was regarded as perfect and unchanging. Stars remained in place, planets moved predictably, and the few rogue comets were viewed as atmospheric phenomena. This began to change with the Danish astronomer Tycho Brahe’s observation of the supernova of 1572 – apparently, a new star – and his studies of the Great Comet of 1577, which he proved was actually a distant object. Nonetheless, the impression of permanence is strong. There are very few astronomical objects that noticeably vary to the naked eye: only the brightest comets, novae and supernovae. For observers in the northern hemisphere, the last naked-eye supernova was in 1604.

Modern telescopic studies tell a quite different story. Today, we know of roughly a half-million variable stars in our galaxy, and identify thousands of transient objects each year. Although many stars vary in predictable ways, the Universe is also full of unpredictable violence. When two stars orbit close to each other, mass can flow from one to the other. If one of the stars is an old, collapsed white dwarf, the gas it pulls from its companion can accumulate until the dwarf undergoes a sudden thermonuclear explosion – a supernova like the one seen by Tycho. There is also another, more common type of supernova produced by the deaths of solitary stars more than about 10 times the mass of the Sun.

Supernovae show a broad range of behaviours that depend on the detailed properties of the system at the time of the final, fatal cataclysm. The atoms that emerge from supernova explosions have provided the raw material for all planets, including our own. Astronomers are understandably eager to learn more about them, but the two classes of supernovae combined happen only about once per century in our galaxy.

Obviously, for events occurring on time scales of a century, searching for them in our galaxy alone is not terribly profitable. Fortunately, our galaxy is only one of about a trillion galaxies in the visible Universe. If you monitor millions of galaxies all the time, it is possible to find many supernovae each and every day. This is one of the most exciting challenges of modern high-speed astronomy.

Other than supernovae, there are only a few variable sources luminous enough to be seen at the great distances to other galaxies, even using powerful telescopes. By far the most common is the random variability of quasars. Quasars consist of a supermassive black hole, millions to billions of times the mass of our Sun, which shine as material falls towards the black hole, heats up and radiates energy.

Today we think that essentially every galaxy contains a supermassive black hole at its centre, and something like 1 per cent of them are accreting mass fast enough to be seen as luminous quasars. The supermassive black hole at the centre of our own galaxy is essentially ‘off’. On rare occasions, though, such a black hole rapidly turns itself ‘on’. The most fascinating cause is a so-called ‘tidal disruption event’ in which a star like the Sun passes too close to the black hole and is ripped apart by the black hole’s tides. Some of the debris then falls into the black hole to power a transient flare. These tidal disruption events are far rarer than supernovae, occurring only about once every 10,000 years in any particular galaxy. In the distant Universe, the study of variability is essentially the study of black holes and supernovae.

T his gives you some sense of the remarkable astronomical zoo of variable and transient objects. The challenge for the professional astronomer is to find and characterise all these different sources not only for how they work individually, but also to determine their overall demographics and statistics. To find large numbers of them, you need a big telescope that can detect the much more numerous distant, faint objects. In general, however, bigger telescopes see only smaller pieces of the sky. This frustrating rule can be bent only by spending large sums of money.

If your scientific goal is to find the largest possible number of transients, and to study their evolution across the cosmic history of the Universe, then you want to use a big telescope that covers as much of the sky as you can afford. This is fundamentally the goal of the Large Synoptic Survey Telescope (LSST). Located in Chile, LSST is (effectively) a 6.7-metre diameter telescope, scheduled to start full science operations in 2022.

LSST will be the closest astronomers have ever come to creating a movie camera to watch the whole universe. It will survey approximately half the sky using a camera that spans more than 40 times the area of the full Moon. But LSST can obtain a new image of each patch of that sky only once every three nights. LSST can detect transients 30 million times fainter than visible to the naked eye, making it a phenomenal project for finding huge numbers of faint transient sources across the visible Universe – LSST should find some 1,000 supernovae per day! But this capability comes at a cost: roughly $600 million just for construction, plus a significant operation cost as well.

At the other limit from LSST is a project I am working on: the All-Sky Automated Survey for Supernovae (ASAS-SN). By the end of this year, ASAS-SN will consist of 20 14-cm aperture telescopes spread across the globe, and costing roughly $3.5 million for both construction and operation through to 2022. With such small telescopes – big telephoto camera lenses, really – ASAS-SN can find only bright transients, roughly 25,000 times fainter than are visible to the human eye. Even so, it should still find about one supernova a day. And because ASAS-SN is comprised of small telescopes, it can image the sky far faster than LSST. The combined ‘image’ from all the ASAS-SN telescopes spans 1,600 times the area of the Moon. This allows them to survey the entire visible sky every night.

The two projects are highly complementary, essentially balancing a trade-off between ‘quantity’ and ‘quality’. LSST provides ‘quantity’: the large numbers of faint sources needed for statistical studies of distant sources, and for studying the evolution of transient sources across cosmic time. However, the typical LSST transient is faint and hard to study in detail for long periods of time, even with the world’s largest telescopes. ASAS-SN provides ‘quality’. The bright sources found by ASAS-SN are the ones that best survey the nearby Universe, and that can be studied in the greatest detail and for the longest periods of time using larger telescopes.

One of the most important tools for astronomers is the spectrum of an object: how much light is emitted as a function of its colour. A spectrum is the best way to classify the velocities, temperatures, elemental composition and type of an object (eg, which type of supernova? What were its unique properties?). Because you must chop up the light into narrow bins of colour, you need far more light to make a spectrum of an object than to get an image of it. LSST is already a large telescope, so it will be difficult or impossible to get a spectrum of the typical, faint LSST transient.

Even for the minority of LSST sources bright enough to obtain one spectrum, the source will quickly fade and become too faint to get another spectrum to study how it evolves with time. Therefore, a negligible fraction of LSST discoveries will be studied by this fundamentally important astronomical tool. The ASAS-SN transients are far fewer in number but are far brighter, so a very large fraction of ASAS-SN transients can be studied spectroscopically, and they can be studied for long periods of time even as they fade away.

Projects like LSST and ASAS-SN are continuing the revolution begun by Tycho, revealing the variable and sometimes violent events that light up the highly imperfect, ever-changing celestial sphere.


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