Archaeology of the Milky Way

Captivating shimmer: The Milky Way is part of the system that is home to our Sun and an estimated 200 billion other stars. On a dark night, it shimmers like a nebulous ribbon on the firmament. PHYSICS & ASTRONOMY_Galaxy Archaeology of the Milky Way The universe has billions and billions of galaxies, but only one that we can explore star by star in all its dimensions: our Milky Way. It can be thought of as a “model organism” for the formation and evolution of galaxies and is thus a key research topic in cosmology, and the research focus of the team working with Hans-Walter Rix, Director at the Max Planck Institute for Astronomy in Heidelberg. The researchers recently found indications that quite a number of earlier ideas about our galaxy have to be revised. TEXT THOMAS BÜHRKE visitor approaching the en- that world-class research can be carried astronomers has now even analyzed trance to the institute on out even with relatively small instru- data from almost a billion stars. This, Königstuhl hill near Heidel- ments. The decisive factor here is that too, has led the researchers to enter the berg first notices hexagons the astronomers have been using them business of big data mining. But what is chalked on the ground and to chart the entire sky continuously the point of all this? A joined together to form a huge honey- over many years. “If you want to investigate the evo- comb. These were left over from the lution of galaxies such as the Milky last open house day and symbolize DATA MINING WITH Way, there are two possibilities,” says segments of the 39-meter-diameter 800 MILLION STARS Rix. “One is to observe galaxies that are main mirror that is to collect the light increasingly far away from us.” Because of distant stars and galaxies in the Eu- This treasure trove of sky survey data the speed of light is finite, looking into ropean Extremely Large Telescope in contains more information than can the distance is always looking into the Chile from the next decade on. Astron- currently be analyzed and modeled. “In past. It is indeed now possible to look omers at the institute are involved in the very recent past, the quantity and over a billion years into the past direct- developing two cameras for what will quality of the data has doubled every ly at galaxies that formed one billion become the largest telescope on Earth. one to two years,” says Hans-Walter Rix. years after the Big Bang. But until it has been built, the ex- “Ten years ago, we had good spectra Galaxies at different distances from plorers of the cosmos must be content from around 8,000 stars; today, it’s four us are therefore at different stages of with the telescopes that are currently million.” For a study of the spatial dis- evolution. However, it is never possible available. This isn’t necessarily a disad- tribution of the dust in the Milky Way, to see one and the same galaxy at dif- Photo: Corbis vantage, as recent years have shown an international team led by Max Planck ferent times. Moreover, these galaxies 4 | 15 MaxPlanckResearch 55 PHYSICS & ASTRONOMY_Galaxy Two views of the galaxy: The graphic on Thick disk the left is an edge-on view of our Milky Way system looking from an angle. Two components can be seen in addition to Thin disk the central bulge: a thin disk of stars close to the center plane and a thick disk that stretches farther into the outer ~ 27 ,000 light-years region. Measurements indicate that the thick disk probably doesn’t exist at all, but is based on a misinterpretation of data. The illustration on the right depicts a schematic plan view of the galaxy with several spiral arms. Our Sun is around Sun Bulge Galactic center 27,000 light-years away from the center. are so far away that it is generally pos- some memory of their birth orbit. And the center plane – also belongs to this sible to make statements only about the they have distinctive chemical compo- largest population. system as a whole, since it isn’t possi- sitions, which can serve as chemical Around 15 percent of the stars are ble to recognize individual stars. fingerprints. The idea now is to deter- found in the central bulge. This is a Rix went down a different path mine the properties of as many stars as spherical region around the center with years ago. He investigates the galaxy possible and to deduce their past histo- a diameter of around 16,000 light- that is closest to us: our Milky Way. “It ry with the aid of computer models. years. The central body itself is invisi- is only in our own galactic home that This method is similar to the one ble. It is very likely a black hole with a we are able to observe the properties of used by scientists wanting to under- mass of roughly four million solar individual stars in detail, in large num- stand the migration of a demographic masses. The remaining 5 percent of the bers and in three dimensions,” he says. group over many thousands of years by stars move far above or far below the Fortunately, our Milky Way is a typical analyzing genetic material. Insiders call disk in the so-called halo around the galaxy, and what we learn about it can it galactic archaeology, and it is now central region. be generalized. providing completely new insights. Moreover, the view that the disk has Around half the stars in today’s uni- two components – a thin one made up verse are found in galaxies that are sim- A MASS MONSTER IS of stars close to the center galaxy, and ilar to our Milky Way in terms of size, HIDING IN THE GALAXY a thicker one – has become textbook mass and chemical composition. “It is knowledge. The thicker component something like the Rosetta Stone of gal- The Milky Way is a spiral galaxy with contains around 10 percent of all stars axy research,” says Rix. But how can a diameter of around 100,000 light- – including all the old stars – and ex- the evolutionary history of our cosmic years. It is subdivided into three re- tends far into the outer regions. homeland be reconstructed if we can gions. The very flat disk, just 2,000 How the thick disk formed, howev- see it only in its present state? light-years thick, is home to around 80 er, remains a mystery. The favored the- The Milky Way is a dynamic system: percent of all stars, and has a total ory assumed that the Milky Way collid- stars have formed throughout its histo- mass of about 50 billion solar masses, ed with another large galaxy a long ry. They move on different orbits along with clouds of dust and gas. Our time ago, swirling up all the stars that around the galactic center, bearing Sun – only 80 light-years away from were already in existence at that time Graphic: MPG, based on original material from Gaba p/CC-BY-SA 3.0 56 MaxPlanckResearch 4 | 15 PHYSIK & ASTRONOMIE_Galaxis Sun and leading to the formation of the A crucial result of the SDS survey is that tute for Astronomy, plays a crucial role thick disk component. no strict subdivision into the two here. Bergemann was able to deter- All stars born after this postulated groups – the thin disk and the thick mine the ages of stars and show that galactic cataclysm are now in the thin disk – is evident. The old school of stars become increasingly younger the disk. Our Sun is one of them. Howev- thought that there was a collision with farther they are from the center of the er, the new measurements now indi- another large galaxy that is supposed to Milky Way. cate that the existence of such a clear- have formed the thick disk will proba- ly defined thick disk is probably a bly have to be discarded. In the mean- MIGRATION IN THE INTERPLAY misinterpretation based on very limit- time, it seems most plausible that, dur- OF THE SPIRAL ARMS ed data material. ing the turbulent early phase of the Hans-Walter Rix and his colleagues galaxy, stars were simply born in a disk “This supports a scenario for the forma- had evaluated spectroscopic data from that wasn’t quite so thin. tion of the Milky Way that says that the the Sloan Digital Sky Survey (SDSS). This Years later, Rix and his colleagues star birth progressed from the inside sky survey is being undertaken with a still occasionally come under fire at outwards over the course of many bil- 2.5-meter telescope at Apache Point Ob- conferences for their new findings: lions of years,” explains the young re- servatory in New Mexico (USA), to many researchers don’t like throwing searcher. Galaxies like the Milky Way which the Max Planck Institute in Hei- accepted teachings overboard – espe- therefore start from an old center and delberg makes a 25 percent contribution. cially not those that they themselves grow gradually toward the outside – The spectra make it possible to de- have developed. “similar to cities,” adds Rix. In this re- termine the chemical composition of However, the treasure trove of data spect, galactic archaeology has already around 15,000 stars and group them provided a great many more insights. borne many fruits. into populations with the same abun- Maria Bergemann, who completed her However, nature came up with one dances.

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