Science Clock Series: Part IX

Today's number comes from chemistry, and is given by:

This one is quite straightforward; it's just the atomic number of the element fluorine.

As you probably remember from chemistry class, each element is uniquely described by something called its atomic number. This number is simply the number of protons in the nuclei of the atoms of that element. The number of protons determines how many electrons make an electrically balanced atom, and also how many electrons the atom will strive to attain (which are only the same number in the case of the noble gases, and hydrogen, somewhat).

You see, due to some slightly complicated features in quantum mechanics that I should talk about some day, not all electron configurations are created equal in energy. Many atoms, especially of the lighter elements, try to reach the same number of electrons as the noble gas closest in atomic number to them because that represents the lowest possible energy state for them.

Fluorine is a fairly light element. Its atomic number, nine, is just above oxygen (eight), and just below the noble gas neon (ten). It therefore wants to gain one more electron to bring it into a lower energy state. Now, if you know your chemistry, you know that oxygen is a highly, highly reactive element. Fires and explosions? Caused pretty much entirely by extremely rapid reaction of oxygen in the air with certain materials. From a biological standpoint, oxygen is pretty dangerous stuff if it gets where it isn't supposed to be because it can react with so much delicate biological machinery (which makes it even more amazing that our bodies run on the stuff). Single oxygen atoms that get loose in the body are classified as free-radicals, and can contribute to cell mutations and cancer if they react with DNA. Even in bulk, diatomic oxygen (the kind we breathe) can be dangerous if you get too much of it, in a condition known as oxygen toxicity.

Now, as bad as oxygen is, fluorine is worse. Much worse. It is the most reactive and electronegative element in the universe. Oxygen can actually exist in the Earth's atmosphere for a fairly long time as a diatomic molecule. Fluorine is much too unstable for that. Although it can (and does) form a diatomic molecule when isolated by itself, once free from containment it vigorously sets to work oxidizing  anything it comes into contact with  extremely rapidly. Quite a lot of substances will burn frighteningly quickly upon exposure to pure fluorine, including things we don't normally think of as flammable, such as iron wool. Or water, which will burn upon exposure to fluorine.

This video, from the Periodic Videos collection, shows some reactions of fluorine with things such as iron wool, charcoal briquettes, and a lump of sulfur:

Now, fluorine may go about obtaining its extra electron very violently, prying it out of the grasp of any other atoms it comes across if it can (barring helium and neon); however, once it has achieved the low energy state it finds itself in, it's actually quite tame. Fluorine with this extra electron is called fluoride (which is an ion since it is no longer electrically neutral), and it is quite happy to share this electron with another atom leading to exceptionally strong chemical bonds.

In fact, you probably have more exposure to fluoride than you might think if you use Teflon cookware. Teflon is the brand name for a chemical known as polytetrafluoroethylene, which is pretty much nothing but long chains of carbon atoms each with two fluorine atoms attached to it. Because these fluorine atoms bind so strongly to the carbon atoms they are highly reluctant to attract anything else, which is why Teflon is so slippery (it is the third most slippery substance known, and in fact is the only known material to which a gecko cannot stick).

Fluoride is also useful in another manner than ties back to number seven on the clock: remember how in the discussion of the Mohs scale of mineral hardness I mentioned that your teeth are made of a form of apatite and thus had a hardness of about five? Well, it turns out that tooth enamel is vulnerable to decay in acidic environments, such as your mouth after drinking anything acidic (like soda) or consuming sugar (bacteria in your mouth create various acids as they consume the sugar, again lowering the pH around your teeth).

Now, it turns out that if you have fluoride ions in your mouth, they will tend to bind themselves over time to the surface of your teeth, changing the enamel from hydroxyapatite to fluorapatite, which is much more resistant to attack by acids (in essence, the fluorine atoms bind tightly to the tooth enamel, making it harder for other materials such as acids to reach the enamel). Many toothpastes now contain fluoride, and it is often added in small amounts to drinking water. The nice thing about fluoride ions is that they're inert enough that they really don't interact much with the rest of you, passing through harmlessly.

So there you go. The universe's most reactive (and one of the more dangerous) elements, serving double duty to make our dish-cleaning less tedious and our teeth better protected from cavities. All around a fascinating element, and I highly recommend you read up on it yourself – there's much more to be said about it than I can fit in this post.

Anyway, check back next time for a number from physics and biology! Click here to jump directly to it.

Supernova Remnants, and the Pulsars that Light Them

Last week I showed you a picture of what happens to small stars (those less than ten times more massive than our Sun) when they run out of fusible hydrogen in their core. Today, let's take a look at what happens to a big star.

Upon running out of hydrogen to fuse, large stars initially proceed very similarly to their less weighty brethren. They begin to fuse the helium produced by hydrogen fusion into heavier elements such as carbon, oxygen, and neon, but unlike smaller stars they have enough mass and gravitational force in their cores to continue. Neon gets fused into heavier elements such as silicon, magnesium, and calcium, slowly working up the periodic table until the star starts making iron in its core. Iron is an interesting element because its nucleus has the highest binding energy per nucleon of any element. (Though certain isotopes of nickel have virtually the same binding-energy that iron does.) What this means, practically, is that you cannot get any energy from iron by either fusion or fission, either by fusing it into more massive elements or by splitting it apart.

Once a star arrives at iron in its core, it's done. It can't squeeze any more energy out of its core, so all that is left supporting the star against its own great weight is electron degeneracy pressure in a slowly-growing iron-nickel core. Electron degeneracy pressure is a quantum mechanical effect with no real analog in classical mechanics. Simply put, electrons exert pressure because two fermions (a class of particle of which electrons, protons, and neutrons are a member) cannot be in the same place and the same quantum mechanical state at the same time. Squeezing them together causes all the low-energy states to be taken, so the electrons vigorously resist any further compression, which would require large amounts of energy to raise electrons into high-energy states. However, there is a limit to how much pressure electrons can exert; if the mass of the core exceeds the Chandrasekhar limit of about 1.38 solar masses, electron degeneracy pressure catastrophically fails and the core collapses in on itself.

At this point, if the star is less than about 20 solar masses there is only one mechanism that can save the star from collapsing into a black hole: neutron degeneracy pressure. Similar to electron degeneracy pressure but involving neutrons, the star's core collapses into a neutron star, a sphere of neutrons packed as tightly as an atomic nucleus and about the size of a city.

As the core collapses into a neutron star, the star's outer layers fall inward tremendously fast, at speeds up to 23% of the speed of light. The neutron star at the center, however, is already packed as tightly as it possibly can be, so the infalling material rebounds off the core in a colossal shockwave to produce what we see as a supernova.

(There is a lot more going on at the same time, of course – supernovae are incredibly fascinating events where relativity and quantum mechanics are both in play, and I'm giving you merely the barest overview of all the processes happening.)

If the star in question started off heavier than about 20 solar masses, even neutron degeneracy pressure will be unable to support the core and nothing in the universe can prevent it from continuing to collapse into a black hole. However, I'm going to focus on neutron stars in this post because the picture I have for you today contains one. The story behind this particular supernova is ancient and varied, so settle in...

The story starts about a thousand years ago, in Anno Domini 1054, when a new star appeared in the constellation Taurus. As was customary, Chinese and Japanese astronomers noted the appearance of a "guest star" and recorded its location. It was also apparently observed by as least one person in the Arabic world. This star was apparently bright enough to be seen in the daytime for a period of several weeks, after which it slowly faded over a period of about two years and finally disappeared, whereupon it dropped out of history.

In 1731 a mysterious nebula (one of the first discovered telescopically) was discovered just off the tip of one of the horns of Taurus by one John Bevis. In 1758, while searching for the return of Halley's comet, Charles Messier stumbled upon this nebula and initially mistook it for his quarry. After watching it for a few weeks he realized that it wasn't moving, and came up with the brilliant idea to publish a catalog of objects that looked like comets but weren't, so that other amateur comet hunters wouldn't be fooled as he had. Thus, this nebula became the first object on what would become his now-famous list of not-comets: Messier 1.

In 1844, almost a hundred years later, the nebula was sketched for the first time by William Parsons, 3rd Earl of Rosse, whose love of astronomy and independently wealthy nature led him to build the largest telescope in the world at that time ("the Leviathan of Parsontown", 6 feet in diameter). His sketch reminded him of a crab, and so he gave our nebula the whimsical name the Crab Nebula. It remained a popular object of observation with astronomers, both amateur and professional.

In 1921 the American astronomer Carl Lampland noted changes in the Crab Nebula which implied a small size for it. In the same year, another astronomer demonstrated that the nebula was expanding. Several astronomers noticed its proximity to the "guest star" of 1054, but nothing was made of it until 1928 when the venerable Edwin Hubble definitively proposed that the nebula be associated with the star. However, it wasn't until later, when the theory behind supernovae had been worked out, that Nicholas Mayall showed that the Crab Nebula was nothing less than the remains of the supernova that exploded into the sky nearly 900 years earlier.

Although the association of the nebula with a supernova was now clear, it wasn't until the 1960's that neutron stars were first predicted by Franco Pacini. A few short years later, in 1968, a neutron star was detected in the center of the Crab Nebula, which made it both the first neutron star ever known and a shining confirmation of Pacini's hypothesis. It also explained why the nebula was so much brighter than a 900-year-old supernova remnant was expected to be.

The neutron star that lurks at the center of the Crab Nebula – known as the Crab Pulsar – is a fascinating beast by terrestrial standards. It is about 25 kilometers (about 15.5 miles) across, and makes a complete rotation every 33.08471603 milliseconds – 30 times a second! As the neutron star spins, it sends out a constant stream of electromagnetic radiation all across the electromagnetic spectrum (including visible light) from both poles of its extremely powerful magnetic field. The axis of its magnetic field is not the same as its rotational axis (much like the Earth, though a bigger offset), and as it spins around it sends off a powerful beam that appears to "blink" on and off as seen from Earth, much like the beam from a lighthouse. The neutron star is thus known as a pulsar, a portmanteau of pulsating star.

This electromagnetic energy being given off comes from the rotational energy of the Crab Pulsar, which is slowly slowing down by 38 nanoseconds per day. The energy being given off along with the star's powerful magnetic field (thousands of times more powerful than the Earth's) being spun through the Crab Nebula 30 times a second causes it to light up. Electrons are accelerated to nearly half the speed of light and spiral along the magnetic field lines of the pulsar, giving off synchrotron radiation as they do, which create a blueish glow visible in the center of the Crab Nebula in long exposures.

Now, after all this background, I suppose I should show you the picture you no doubt read this post for. I hope you can now better appreciate just how amazing this object is, even if my picture cannot do it justice. Here it is, the Crab Nebula:

The Crab Nebula, Messier 1, in Taurus.

While the Crab Pulsar itself is quite invisible in a small telescope such as the imaging telescope, you can easily see the remnant of that titanic explosion 958 years ago. Within that shell of gas exist fantastic filaments and mysterious structures, brought about by electrons powerfully accelerated to relativistic speeds by the pulsar's magnetic field. Despite being around 6,000 light-years away it is persistently the strongest source of X-rays and gamma rays from outside the solar system. The central neutron star itself is a sphere of ultra-dense matter more massive than the Sun compressed into an area smaller than New York city and spinning over 30 times a second. This matter (commonly called "neutronium") is so dense that a single thimble-full would weigh over 100 million tons. I could go on and on at length about how fascinating this single member of the group of objects know as pulsars are, but you get the idea.

I hope this post has given you a sense of the wonder and excitement I get when I study astronomy and physics. Learning more about the incredibly varied denizens of our universe never fails to amaze and astound me, and I enjoy nothing more than bringing that feeling to others. If this post made you stop and think at all, then I will feel I have succeeded. A hui hou!

Planetary Nebulae, and the White-Hot Dwarfs that Light Them

Monday night I was able to get some images with the imager, and conditions were great nearly the entire evening. One of the objects I imaged was Messier 46 (or, rather, I walked a volunteer through imaging it, as I was doing some training). M46 is an open cluster in the constellation Puppis the Poop Deck not too far away from Sirius, the brightest star in the night sky.

Messier 46 in Puppis, with the planetary nebula NGC 2438 visible near the top.

M46 by itself is a fairly unremarkable (if fairly rich) open cluster of perhaps up to 500 stars about 30 light-years across, located about 4,500 light-years from us and receding at a speed of 41.4 km/s. What's interesting is that at first glance it appears to have a planetary nebula (cataloged as NGC 2438, visible above the middle of the cluster) embedded in it. However, the nebula does not appear to share the motion of the cluster, and is just a chance alignment along our line of sight.

The nebula itself is turns out to be pretty interesting. It is located closer to us than M46 at about 2,900 light-years, and has the amazing property that its central white dwarf star is one of the hottest stars known, measuring in at an astounding 75,000 K. (That's about 74,700 °C, or 134,672 °F.)

To put those number in perspective, our Sun's surface is about 5,778 K (or about 9,940 °F), while the very hottest O-type main-sequence stars (ones that are in their hydrogen-fusing phase, about 90% of the lifetime of all stars) might get up to 30,000 K (53,540 °F). Note that all these temperatures refer to the star's surface temperature – even in our fairly low-on-the-temperature-scale Sun the core temperature is over 15 million K (about 27 million °F). That still means that the Earth-sized ultra-dense sphere of electron-degenerate carbon and oxygen at the center of NGC 2438 is over two-and-a-half times hotter than the hottest stars operating under fusion power alone, and nearly thirteen times hotter than our Sun.

Planetary nebulae are so-named not because they have anything to do with planets, but because in the early telescopes through which they were discovered they looked somewhat like them (sort of a faint, fuzzy disc-shape). They come about when a star less than ten times the mass of the Sun runs out of hydrogen in its core to fuse, at which point it begins to work its way up the periodic table fusing heavier and heavier elements in its core while its outer layers puff up and the star becomes a red giant. The outer layers slowly puff off into space to form the nebula, while the core eventually stalls at carbon and oxygen (or maybe neon, if the star has between eight and ten times the Sun's mass) and forms a white dwarf, which is basically just the exposed core of a former star.

At this point no new energy is being released, so the white dwarf enters a long period of cool down. Due to their composition of electron-degenerate matter, temperatures inside the white dwarf are a fairly steady 1,000,000 K or so (about 18 million °F), while the outside cools off as it radiates energy into space, lighting up the outer layers it puffed off earlier and making the planetary nebula visible. Eventually the nebula disperses into the interstellar medium, and the white dwarf is left to gradually get cooler, and cooler, and cooler...

To end this post, I thought I'd mention that the hottest white dwarf found to date had a surface temperature of an incredible 200,000 K (360,000 °F) – almost three times hotter than the white dwarf in NGC 2438, nearly seven times hotter than the hottest O-type stars, and a whopping thirty-five times hotter than the surface of our own star. Pretty amazing, huh? On the flip side, the coolest white dwarfs found have surface temperatures of just about 4,000 K (6,740 °F) – they've already cooled off to less than the Sun's surface temperature.

Check back tomorrow, when I'll show you a picture of what happens when a star with more than ten times the mass of the Sun runs out hydrogen in its core. Hint: it's a lot more explosive!

Edit (3/24/12): As you can probably tell, it's way past tomorrow and I still haven't gotten that post up. That's because it turned out to be much longer than I anticipated when I started it, and then I went to work for two days. I will have it up in the next few days, though.