Vortex formation using a free jet

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In a previous post I shared with you an image of milk vortices created in a hot coffee cup. Today I share with you a demonstration of vortex formation using a free jet of air of a speed higher than that (stagnant) surrounding it.

This is one case of fluid instability  called the Kelvin-Helmholtz instabilities (KHI) which occurs when two fluids have different velocities. A very popular and practical example of it are sea surface waves which are created due to the velocity difference between the water and wind.

More accurately, the shear force applied by one fluid (air in this case) on the other (water) creates a shear stress in the other fluid which if greater than the surface tension of the second fluid results in the KHI.

It is also common in clouds and on gas planets like Saturn and Jupiter as the following image of Saturn show.

Image credit: NASA/JPL/Space Science Institute

Four Days at Saturn – Cassini full-frame video

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Update on the Rosetta (comet) mission

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The Rosetta orbiter is continuing its science until the end of the extended Rosetta mission in September 2016. The lander’s future is less certain. This film covers some of what we’ve learnt from Philae about comet 67P/Churyumov-Gerasimenko so far.

This includes information about the comet’s surface structure from the ROsetta Lander Imaging System – or ROLIS camera – a copy of which can be found at the German Space Agency, DLR, in Berlin.

Data from all Philae’s instruments has informed the work of the other scientific teams. Rosetta scientists have analysed grains from the comet and discovered that it contains carbon rich molecules from the early formation of our solar system.

The video also contains footage from the Max Planck Institute for Solar System Research in Germany – where a flight replica of Philae’s COSAC instrument is maintained in a vacuum chamber to test commands. COSAC has already detected over a dozen molecules containing carbon, hydrogen, nitrogen and oxygen from the dust cloud kicked up from landing.

Mettalicity of stars

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As any student who studied a bit of chemistry knows that metals in the periodic table are various, after all the periodic table is usually taught to students as including group 1 and group 2 corresponding to alkali metals and alkaline earth metals, respectively.

Since the universe is mostly made up of hydrogen (74%) and helium (24%) [1], astronomers often refer to all the other elements we know of as “metals”. What we know about the “metallicity” is that it is tightly related to the age of the star. In other words, astronomers categorize stars into populations (I, II, III) based on their metallicities. Initially a star is born with mainly hydrogen and some helium (i.e; low metallicity). As the star burns the hydrogen, the helium gets more abundant and as the helium and hydrogen are consumed new elements (metals) are formed. So logically, young stars have low metallicities (these belong to population II) and old stars have high metallicities (population I)! Population III stars are those that have no metallicities and are yet to be discovered.

The metallicity is denoted by the letter Z as well as [Fe/He], the latter of which is read “the ratio of iron to helium” (i.e; abundance of Fe / abundance of He). For example, one of the brightest stars in our sky is Sirius, a main sequence star. It has a [Fe/He] = 0.5 [2]. It should be noted here that Z and [Fe/He] are calculated differently so they have different but correlated values, where [Fe/He] is logarithmic in that [Fe/He] = log(abundance of Fe / abundance of He)!

Below is an example of a plot of the logarithmic (base 10) abundance versus the atomic number, Z, of the elements, the latter not to be confused with metallicity, in the solar system!

Abundance of elements in the Solar system

Footnotes

[1] of all baryonic matter!
[2] The Abundance Patterns of Sirius and Vega [H. M. Qiu et al. 2001] doi:10.1086/319000