Tag Archives: the palomars

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

Stream fanning

streamfanning In my first year at Columbia I worked with grad student Sarah Pearson on an idea that Kathryn Johnston had while trying to find an orbit for Palomar 5 in a Law & Majewski potential. Wait what? Who’s Sarah, who’s Palomar 5, and what is a Law & Majewski potential?

Palomar 5 is a globular cluster in the halo of our Galaxy, the Milky Way. It is about 12 billion years old and consists of roughly 30,000 stars. The star cluster can be seen within the footprint of the Sloan Digital Sky Survey. But even more fascinating is that we can also see a stream, consisting of at least as many stars, stretching out from the cluster along its orbit. This stream – there are actually two, one in the leading direction and one in the trailing direction – spans about 23 degrees on the sky, while being on average half a degree wide. That’s about the size of 50 full moons!  Continue reading Stream fanning

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

How Mass Segregation affects the Expansion of Star Clusters

Orange points show the observed radii (r_h) of Milky Way globular clusters plotted against their distance from the Galactic center (R_G). Globular clusters in the outer halo of the Galaxy tend to be significantly more extended than the ones nearby. Lines show different models for the sizes, star clusters can expand to at a given radius within the age of the Universe.

Recently, my Iranian collaborators and I published another paper on mass segregation in outer-halo globular clusters. This time we looked at the effect that primordial mass segregation can have on the size evolution of these clusters (i.e., what happens if heavy stars are preferentially born closer to a star cluster’s center). The problem is the following: if you look at the globular clusters in the outer halo of our Galaxy, you find them to be significantly more extended (i.e., with a radius larger than 5-6 pc) than their counterparts that are closer to the Galactic center (orange points in the Figure above; but see also my previous posts [1] [2]).  Continue reading How Mass Segregation affects the Expansion of Star Clusters

Mass segregation in Palomar 14

The mass function slope is shown as it changes with radius of Palomar 14 (measured from the cluster center). A low value of this slope means that we detect more massive stars at this radius than further out, where the slope is larger. Such a signature is called mass segregation, and is usually a consequence of dynamical evolution of a star cluster.

Matthias Frank, Eva Grebel (both in Heidelberg), and I have recently published another paper on one of our Milky Way’s outer-halo globular clusters. Using archival data from the Hubble Space Telescope and Matthias’ sophisticated photometry tools, we measured the masses of stars within one of the most controversial globular clusters known to us: Palomar 14.  Continue reading Mass segregation in Palomar 14


Boat on the Neckar

Last week I’ve been to Heidelberg to visit collaborators and friends (or both). Matthias (definitely both) was kind enough to host me for the week at the Observatory (Landessternwarte), which is situated on top of the Königsstuhl high above Heidelberg Castle.  Continue reading Heidelberg

Palomar 4

This website was never intended to be a science blog, but since science is an essential part of my life – with all its ups and downs, kinks and quirks, bores and funzies – I shouldn’t neglect it here! Today, a paper of me and my dear collaborators from Iran, Elham Hasani Zonoozi and Hosein Haghi appeared on astro-ph.

Mass function slope versus cluster radius
Slope of the stellar mass function versus projected radius from the cluster center for the observations of Palomar 4 (red) and for our best-fitting model (black). The goal was to find a model that reproduces the observed trend of the massive cluster stars being more centrally concentrated, which can be seen from the slope being below its nominal value (dashed line) in the center, while being above that line in the outer parts of the cluster.

Continue reading Palomar 4