Tag Archives: halo

How to weigh the Milky Way

The Northern Hemisphere of the sky as seen by the Sloan Digital Sky Survey. Stellar streams stick out from the vast number of stars in this view, of which most lie within the Milky Way disk. The Palomar 5 stream is the densest of the stellar streams discovered so far and turned out to be a perfect scale and yardstick for our understanding of the Milky Way. (Credit: Ana Bonaca, Marla Geha and Nitya Kallivayalil with data from the Sloan Digital Sky Survey.)

The Milky Way consists of roughly 100 billion stars like our Sun, which form a huge stellar disk with a diameter of 100-200 thousand light years. The Sun is also part of this structure, hence, when we look into the sky, we look right into this gigantic disk of stars. The vast number of stars and the huge extent on the sky make it hard to measure fundamental quantities for the Milky Way – such as its weight.  Continue reading How to weigh the Milky Way


Streams, streams, streams


For my presentation at the Gaia Challenge in Heidelberg, I made a quick ADS search for publications on tidal stream observations since their first discovery in 1995 by Carl Grillmair. Although tidal streams had been theoretically predicted before 1995, Grillmair showed for the first time that some star clusters have significant amounts of stars outside their tidal radii. At about the same time, the Sagittarius (Sgr) dwarf galaxy was discovered by Rodrigo Ibata, and people started finding patches of stars that belong to the stream emanating from this galactic satellite everywhere in the halo of the Milky Way. It took 6 more years until Michael Odenkirchen showed with commissioning data of the Sloan Digital Sky Survey that the globular cluster Palomar 5 has a coherent stream emanating from its Lagrange points out into the tidal field of the Milky Way. This publication also marked the onset of survey since in astronomy. Since 2000/2001 the rate of papers presenting new observational results on streams in the Galactic halo or around other galaxies grows exponentially. While this growth was initially driven by studies on the Sgr stream (red cumulative curve above), the focus is now shifting towards fainter streams like Palomar 5, NGC 5466 or GD-1 (blue curve). It’s a really exciting time to work in this field!

Gaia Challenge 2014


End of October, I was in Heidelberg to attend the Gaia Challenge workshop at the Max Planck Institute for Astronomy (MPIA). This workshop series was initiated last year by Justin Read, Mark Gieles and Daisuke Kawata at the University of Surrey (Guilford, south of London). This year we came together again, but this time the meeting was organized by Glenn van de Ven, who is a research-group leader at the MPIA. Glenn booked the Haus der Astronomie (house of astronomy) for us, which is a public outreach building on the premises of MPIA. The cool thing about it is that it’s shaped like a spiral galaxy (see photo above). The bulge of the Haus der Astronomie is a large auditorium and the spiral arms consist of seminar rooms and a child day care center. It’s quite a unique place!  Continue reading Gaia Challenge 2014