100 Years – 100 Objects: Phosphophyllite

The Houston Museum of Natural Science was founded in 1909 – meaning that the curators of the Houston Museum of Natural Science have been collecting and preserving natural and cultural treasures for a hundred years now. For this yearlong series, our current curators have chosen one hundred exceptional objects from the Museum’s immense storehouse of specimens and artifacts—one for each year of our history. Check back here frequently to learn more about this diverse selection of behind-the-scenes curiosities—we will post the image and description of a new object every few days.that we’ll be sharing here – and at 100.hmns.org– throughout the year.

This description is from Joel, the Museum’s President and Curator of Gems and Minerals. He’s chosen spectacular objects from the Museum’s mineralogy collection, which includes some of the most rare and fascinating mineral specimens in the world,


Unificada Mine, Cerro Rico de Potosi, Bolivia

Phosphophyllite crystals from Potosi, with their beautiful bluish green color, brilliant luster and attractive transparency, are among the most highly desired treasures in the mineral world. They are rare today because most crystals were destroyed by mining before their identity was even understood. Any size crystal larger than one centimeter is highly valued, and this 6.8-cm twinned pair of gem crystals, the second largest known, should probably be considered priceless.

Marvel at the world’s most spectacular collection of natural mineral crystals in the Cullen Hall of Gems and Minerals at the Houston Museum of Natural Science.

You can see more images of this fascinating artifact – as well as the others we’ve posted so far this year – in the 100 Objects section at 100.hmns.org

June Flickr Photo of the Month: Bananagrams

This month’s featured photographer is Sulla55.

On Valentine’s weekend we had several of our photographer friends from Flickr come to the museum to participate in Wikipedia Loves Art, a contest aimed at illustrating Wikipedia articles. We had over 40 photographers arrive and split into teams to see who could get the most and best shots of our artifacts. Sulla55 created this shot to depict the event. Here’s what Sulla55 had to say about the image.

I created this shot in honor of the ‘Wikipedia Loves Art’ event at the Houston Museum of Natural Science on February 15, 2009. Many thanks to Erin for arranging this opportunity, and for the Museum for being so photographer-friendly. I used Bananagrams tiles (similar to Scrabble) and my HO scale miniature photographers. Not a very complicated set-up: I used an Ott light and the top of my microwave. 🙂

Creative Commons License Photo Credit: Sulla55

The photos submitted from the Wikipedia Loves Art event were amazing. I only wish we could show every photo on our blog – but you can check them all out here. Erin and I want to give a big thanks to everyone who came and made this event such a success.

The winning team was Assignmenthoustonone. Thank you to Sulla55, Stephaniedancer, Mockbird, Kinjotx, Skarsol and Jjsala for submitting and sharing such beautiful photos. Each member won a yearlong free family membership and four tickets to see our Terra Cotta Warriors exhibit.

If you’d like to be invited to future photography events at HMNS, join our HMNS group on Flickr.

Where have all the sunspots gone?

We are now just past the summer solstice, which occurred at 12:45am CDT on June 21. Anyone willing to brave the almost 100 degree heat can go out to our sundial and project a real image of the Sun – which would reveal if any sunspots are present. Lately, however, this activity has been slightly less exciting because in the past three years, the Sun has been largely without sunspots.

02 Sun Structure
Creative Commons License photo credit: Image Editor

Sunspots are slightly cooler regions on the photosphere (the Sun’s surface).   Most of the Sun is at about 10,000o Fahrenheit, while sunspots are only about 6,000oF.  To understand why they form, let’s consider some basic facts about the Sun.  First, the Sun is not solid but is instead made of burning, ionized gases, or plasma (the ‘surface’ of the Sun is the layer where this gas becomes opaque.)  When a solid body, such as a ball, rotates, it does so rigidly; every part of the ball takes the same amount of time to rotate once.  For example, no matter where you are on the Earth, one day is 24 hours.  Since the Sun is fluid, however, it has whats called differential rotation; different parts of the sun rotate at different rates.  At its equator, the Sun rotates once in about 24.5 days.  Near the poles, this period is much longer: up to 35 days. 

Secondly, the  Sun has a magnetic field.  Any current causes a magnetic field; any moving charge is a current. Among the layers of the Sun is a convective zone in which hotter gases from the center of the Sun rise while cooler gases fall back towards the Sun’s center.  The Sun, as a plasma, is made of ionized gases.  The motion of ionized gas particles in convective loops forms currents in the Sun; which in turn generate a magnetic field.  Due to differential rotation, magnetic field lines can become distorted or twisted. 

Where these twisted magnetic field lines puncture the Sun’s surface, the transport of heat via convection from inside the Sun is blocked.  This results in a cooler, darker region on the Sun’s surface–a sunspot.  The lower temperature of a sunspot means that it emits light at a much, much lower intensity than the rest of the Sun’s surface.  Further, the cooler temperature shifts more of its radiation into the infrared portion of the spectrum.  This is why the spot appears much dimmer than the rest of the Sun.  Sunspots appear dark only because of the contrast with the much brighter solar disk. If you could separate a sunspot from the Sun, you would discover that it is quite bright in and of itself.  When we observe the Sun at the H-alpha wavelength (a particular wavelength of red light), sunspots appear brighter than the Sun’s disk.

Once formed, sunspots exist for about two weeks.  They also vary in size, with the biggest sunspots being up to 50,000 miles across.  Compare that with the Earth, which is less than 8,000 miles across.   

Sunspot 923
Creative Commons License photo credit: fdecomite

It turns out that the biggest sunspots are noticeable to the naked eye when the Sun is low to the horizon or seen through mist or clouds.  You should never try this however; always observe the Sun by projecting its image or by looking through a filter expressly designed for this purpose.  There is evidence, though, that before modern understanding and technology, early astronomers risked eye damage by looking at the Sun when it seemed dimmer than usual.  For example, ancient Chinese astronomers may have made reference to sunspots in 28 BC.  (This may have supported the legend that a raven lived in the Sun). 

europa 606
Creative Commons License photo credit: dizarillo

Early telescope users Galileo Galilei and Thomas Harriot were among the first western astronomers to observe sunspots.   David Fabricius and his son Johannes were the first to publish a description of sunspots in June 1611.  In 1843, German astronomer Heinrich Schwabe discovered that the number of sunspots varies in a cycle of about 11 years.  Swiss astronomer Rudolf Wolfthen used data from Schwabe and others to reconstruct solar cycles back to about 1745.  Wolf designated the cycle from 1755-1765 as Cycle 1, and we still use that count today.  Accordingly, the last cycle which peaked in 2001 was Cycle 23, and the next cycle expected to begin now and peak in 2012 will be Cycle 24.  Also, solar astronomers use the ‘Wolf number’ to describe the number of sunspots on the Sun.   It was George Ellery Hale who first associated sunspots with magnetism.

This graph shows that not all solar cycles are the same.  Peaks in the early 19th century were much smaller than those of the 20th century, for example.  Towards the left of the graph, covering about 70 years including the last half of the 17th century, is a period which seems to have no peaks.  This is the Maunder Minimum, noted by Edward R. Maunder.  The decades of few sunspots coincided with decades of unusually cold winters in Europe and North America.  This is also a time when few aurorae were observed.  In fact, the 11 year cycle of minima and maxima continued in this time as well, it’s just that the peaks were very, very small compared to later periods. 

For much of 2009, we’ve been past due for the start of the next solar cycle–Cycle 24.  Since the peak of Cycle 23 occurred in 2001 and the next peak was expected in 2012, scientists expected to begin seeing many Cycle 24 sunspots in late 2007 and especially by 2008.  Instead, 266 of the 366 days of 2008 were spotless.  The dearth of sunspots continued into early 2009, where 134 days (78% of all days through June 22) have been spotless.  Solar scientists were a bit baffled by the late start to the new cycle; a few wondered if the Maunder minimum might be recurring.  On June 17, however, researchers Rachel Howe and Frank Hill of the National Solar Observatory in Tucson, Arizona, put forward an explanation.   Far below the surface of the Sun is a stream of plasma analogous to the jet stream on Earth.  The Sun generates these ‘jet streams’ once about every 11 years.  Once formed, they then gradually shift from the polar region towards the equator.  Using helioseismology, Howe and Hill were able to determine that this time around, the solar jet stream is shifting more slowly than usual, resulting in a delayed Cycle 24.  However, they reported, that jet stream has just now gotten to a low enough latitude–22 degrees from the sun’s equator, to allow frequent sunspot formation.  If  this explanation holds, we should expect  to see many more sunspots as we approach 2012. 

As if on cue, two sunspots of the new cycle were visible on the Sun as of June 22. Does this mark the long awaited increase in sunspot activity?  You can surf to http://www.spaceweather.com to view an image of the Sun each day.  Or, if you can stand the heat, come out to our sundial and make your own image of the Sun.  Near the solstice, the sun’s apparent height in our sky does not change all that much.  For quite a few days after the solstice, the Sun will shine through the lenses in our gnomon

Nature rarely allows us the comfort of feeling that we’ve got it all figured out.  Even when we understand something well, such as the 11-year solar cycle in this case, it often turns out that we understand it only partially.  Constant observation and discovery is always required.  In this context, the spotless Sun of the past few years reminds us that scientific knowledge is not a series of decrees from on high, but is a process that we can all participate in.

Mastodons in Manhattan? Or, Pachyderms of the Pleistocene

South Street Seaport...
Creative Commons License photo credit: 708718

It’s hard to imagine it now, but the world’s largest cities – the places where humanity’s impact on the Earth can be seen most vividly – were once overrun with prehistoric wildlife so large and fascinatingly diverse as to boggle the mind. Depending on where you live, your own backyard may have been home to mastodons, giant beavers, a herd of fearsome Postosuchus, or Ice Age bears.

A new Discovery Channel series profiles six cities across the United States, each with their own unique story to tell about the richness of prehistoric life. With stories ranging from the Triassic to the Pleistocene, the variety of life that came before us is truly amazing.

Mastodon Skeleton, from the side
Creative Commons License photo credit: The_Gut

Check out this video to see the HMNS visiting curator of paleontology, Dr. Robert Bakker, discuss the “hairy monsters” that once roamed the boggy forests that would be transformed – by both natural and human forces – into modern-day Manhattan. Watching the cityscape melt away into verdant wetlands of the Pleistocene is pretty amazing, as is seeing the ancient plant life still growing through modern sidewalks.

You can see more when “Prehistoric New York” airs locally, this Sunday, June 28, at 8 p.m. on the Discovery Channel.