A “What Am I?” Puzzle

1. If it wasn’t for my atypical stellar atmosphere, I would be of no value to astronomers. I have a layer of ionized hydrogen and fully ionized helium, neither of which can lose energy to outer space. So these ions keep absorbing energy from my interior until I expand. This eventually lowers my density and allows electrons to recombine with ions and neutralize them. Now excitation can lead to loss of energy to outer space, eventually decreasing radiative pressure. This makes me shrink. Hydrogen and helium then reionize, and the cycle repeats itself.
2. Since temperature is related to luminosity, there are more ionized atoms associated with more luminous stars of my type. And the more there are, the longer it takes to go through a cycle. In chemistry, a correlation of concentration and color becomes more practical if it relies on prepared standard solutions of known concentrations. Similarly if it wasn’t for parallax measurements for nearby relatives, the relationship between luminosity and the logarithm of periods could not be calibrated.
3. A technique known as spatial scanning has allowed the Hubble Space telescope to extend the range of measurements for my type of star.
   

   By applying spatial scanning to astronomical parallax,  NASA’s Hubble Space Telescope can be used to make precision distance measurements 10 times farther into our galaxy than previously possible. Image source: NASA/ESA, A.Feild/STScI

4.  Spectroscopy lets astronomers determine whether I am metal-rich or not. Each of the two types has a different linear relationship.
   

   The same period can correspond to different luminosities, depending on whether this type of variable star is metal- rich(Type I) or metal poor. A metal-rich star is a star that has a higher proportion of any element other than hydrogen or helium, even though the elements may not necessarily be metals from a chemical standpoint.

5. So thanks to my star-type, astronomers have a way of measuring the distance of  galaxies. Using these distances from spatial scanning and thanks to Hubble’s infrared camera, astronomers could correct the apparent brightness of my star-type to the values that would be observed if they all were located at a standard distance of 10 parsecs (1 parsec = 3.26 light years). When the observed brightness is plotted against the periods of various variable stars of my type, , the ratio of the brightness for the corrected curve to that of the galaxy in question is determined. The reduction in brightness factor is then square rooted because of the inverse square relationship between the distance of a star and its brightness. If the denominator of that result is then multiplied by ten parsecs, we get the distance to that galaxy or star cluster. Here’s an example involving the Large Magellanic Cloud.
   

   The blue data is for a group of variable stars whose brightness has been adjusted for a distance of 10 parsecs from the earth. The red data represents data for the brightness of variable stars in the Large Magellanic Cloud, which are 4940^2 dimmer. From this we can conclude that the Large Magellanic Cloud is 4940 *10 parsecs from the earth or 49400*3.26 =160 000 light years away. Source: http://hubblesite.org/hubble_discoveries/science_year_in_review/pdf/2006/cepheid_calibration.pdf

6. Probably the best known star of my type is Polaris. It has a period of only four days. In contrast, the period of RS Puppis is about 40 days.
   

   RS Puppis is one of the brightest stars of its type in the Milky Way Galaxy. Picture source: https://www.spacetelescope.org/news/heic1323/ The source describes how a light echo was used to determine its distance.

7. In the Hertzsprung-Russell diagram, I am on an instability strip.

For more What Am I blogs, see:

uvachemistry.com/2014/08/07/what-chemical-compound-am-i/

uvachemistry.com/2016/07/11/again-guess-whats-being-described

uvachemistry.com/2016/06/22/guess-whats-being-described

Sources:

Universe. William J. Kaufmann II. W.H. Freeman

NASA Cepheid Calibration http://hubblesite.org/hubble_discoveries/science_year_in_review/pdf/2006/cepheid_calibration.pdf

An 1842 Photo Technique Still Has Its Charms

Decades ago, when my brother was first hired out of graduate school, one of their Swiss clients was kind enough to send him a hands-on science-gift every Christmas. And my sibling was in turn generous enough to give them to me. When he no longer worked for the company that was linked to that benefactor, I wrote to the Swiss Santa, explaining how my students had benefited from the educational experience made possible by their offbeat, well-thought-out gifts. They happily obliged, and until the financial crisis of 2008, every December, they continued to send us science-related kits and toys .

One of my favorites was a cyanotype kit. Cyanotype can be used as a printing process but it’s also one of the oldest photography techniques, dating back to the 1840s. Through serendipity, the astronomer John Herschel first created the necessary emulsion of ammonium iron (III) citrate and potassium ferricyanide, which supply the ions needed to produce a beautiful blue precipitate. Before it can be used, the emulsion has to absorbed onto paper or cloth and then allowed to to dry in the absence of sunlight or any other source of ultraviolet.

Recently I dug out some cyanotype paper from the Swiss kit to review the chemistry involved and to witness another happy marriage of art and science. If the paper wasn’t over 10 years old and not slightly oxidized or affected by humidity, it would look greenish, the color of ammonium iron (III) citrate. But mine was a very pale blue. Yet because the changes are reversible, the paper still turned out to be functional.

In the upper left photo, the compass has been sitting on the emulsion paper for about 3 minutes and the original light bluish color is fading. The paper has gone almost completely grey after 8 minutes of exposure, as seen in the photo in the upper-right-hand corner. Indoors, I removed the compass and the parts not exposed to light are now light blue (picture at 3 o clock position). But when placed in water for about 2 to three minutes, the bit of blue disappears (not shown). Finally as seen in the final photo, a deep blue appears in areas that were exposed to UV.

How do we account for all the color changes?

Pictures 1 and 2

When ultraviolet strikes ammonium iron (III) citrate, (oxalates are also used and preserve better) the citrate converts to acetone dicarboxylic acid by donating an electron to the ferric ion Fe³⁺, reducing it to the ferrous ion, Fe²⁺. The Fe²⁺ in turn reacts with potassium ferricyanide( K₃[Fe^III(CN)₆] · 3H₂O) to create Prussian blue (containing ferric ferrocyanide Fe^III [Fe^II(CN)⁶]+. But in bright sunlight, the reaction does not stop there. Instead the Prussian blue is converted into Berlin white (ferrous ferrocyanide = Fe^II₂[Fe^II(CN)₆] ).

Old nomenclature persists, and it sometimes turns off the uninitiated. Ferric ions and ferric-complexes contain the oxidized state of iron, Fe⁺³. Ferrous ions and ferro-complexes have Fe⁺². The chemical properties of the different oxidation states differ more significantly than the slight changes in the suffix might suggest.

Picture 3

Initially the only light-bluish parts are the ones that still have the compounds of the original solution, the parts that did not get exposed to ultraviolet light. Notice that because the sun was hitting the compass circle at an angle, it cast a shadow, and that shadow is light blue at this stage.

Unshown Picture

The reason that the light blue (or green if you have fresh paper) disappears is that the original soluble emulsion is dissolved away in the water.

Picture 4

As the paper dries and gets exposed to oxygen in the air, the Fe²⁺ that is in the grey compound(outside of the cyano-complex) of the light-exposed areas is oxidized back to Fe³⁺. Prussian blue is generated. It’s the same blue end-product of the invisible ink trick that was part of children’s old chemistry sets. Peak absorption of light in the 690 nm region of the spectrum is used to move an electron from Prussian blue’s Fe²⁺, part of the cyano-complex, to Fe³⁺ ions , resulting in us perceiving a deep blue.

To use it as a printing technique, instead of placing an object on the emulsion paper, you can either draw on an acetate and then proceed as usual. Or you can print a black and white photo on a transparency and place it on the paper to create a cyanotype or a “blueprint”, as old architects would say. Here are my blueprints of both suggestions, some loves of my life outside of science.

Sources:

The Swiss Santa’s Cyanotype Kit Bioengineering http://bioengineering.ch/company/history/

Chemistry and Light http://www.chemistryandlight.eu/index.php/cyanotype-process/

Prussian Blue: Artists’ Pigment and Chemists’ Sponge

Mike Ware Buxton, Derbyshire, United Kingdom. J. Chem. Educ., 2008, 85 (5), p 612