The Syrian Regime’s Barrel Bombs Kill More Civilians Than ISIS And Al Qaeda Combined

I first saw this graph when Yasmeen (y2es) schooled someone, who responded to horrific images of the abandoned truck in Austria that contained the corpses of 70 refugees by trying to emphasize that “…thousands of refugees are trying to escape the war in Syria, women are trying to escape the rape and abuse of IS and men are running away out of fear of IS.”

Aside from the rampant regime apologia I’ve sadly gotten used to seeing, it’s so damn cruel to see so many people in my daily interactions, who actually understand that Assad is a monster, still harbor such a skewed perception of the regime’s brutality.

Yasmeen said it all, but I thought this graph was really telling so I wanted it on my blog, front and center.

The Cayley graph of the free group on two generators (in red and blue), embedded in the Poincaré hyperbolic disk model.

Since the free group has no relations, its Cayley graph has no cycles. The resulting structure, an infinite tree where every node is connected to a fixed number of neighbours, is also called a Bethe lattice.

I absolutely adore this picture: it combines so many interesting mathematical ideas (group theory, graph theory, hyperbolic geometry) into a single image!

Chemical composition of a hypergiant star with a diameter 400 times the Sun’s, on your screen

In the first part of my internship at the observatory I promised to write about those lines in the spectrum we see after the data reduction. (The water vapour lines are so big because we observed the star in Estonia.)

The spectrum you see above is a yellow hypergiant‘s called V509 Cassiopeiae (V509 Cas or HR 8752) around 4,500 light years from Earth. It has a mass of 25 and diameter 400 times the Sun’s (imagine how huge it is by thinking that a million Earths fit into the Sun). But there are other reasons I picked it out.

Hypergiants are the most luminous stars known in the Universe. They are very rare objects that can reach a luminosity millions times that of the Sun. In the project Tartu Observatory’s involved, the variability and pulsations of them are observed. The pulsation frequencies provide the chance to learn the otherwise unobservable internal structure of stars.

About the spectrum itself: the lines you see in a star’s spectrum act like thermometers. Some compounds only appear in the spectra of very cool stars. Others, like helium, appear only in the spectra of very hot stars.

The surface temperature of V509 Cas has risen rapidly from 5,000 to 8,000 K during the years 1973-2005. This result marks the discovery of a fundamental missing link in the evolution of hypergiant stars.

In an atmosphere there can only be an absorption. The wavelengths that get absorbed depend on the chemical make up of the gas in the stellar atmosphere. But above we see hydrogen (H-alpha) emission lines. Emission is when an object emits radiation, meaning that these lines must come from the circumstellar environment, not from the star itself.

So V509 Cas is surrounded by an envelope or a shell around it. These shells are a fossil record of the star’s mass-loss history: V509 Cas ejected around the Sun’s mass of material in 20 years so we can observe its crazy changes during our life time.

Sources: x, x, x, x