The truth is, the cluster and our Sun had a falling out.

Once upon a time, somewhere in our galaxy, our Sun’s atoms were part of a giant molecular cloud. Approximately 7 billion years ago, that molecular cloud was bumped. Exactly what did the bumping no one knows. That anonymous bump so shocked the dark molecular cloud that in recoiled and collapsed in on itself. At first this inward spiral wasn’t at all dramatic, and an imaginary space traveler looking at this shocked cloud with her imaginary eyes might not have perceived the motion. Over time, however, momentum built up, and the collapse gained speed, with the densest parts of the cloud pulling themselves into fragments, as more ethereal parts were left behind to collapse more slowly. In one of these collapsing regions a womb of gas and dust that was neither too big nor too small began to glow as a single star exhaled its first breath of heat. As it grew and began to illuminate its surroundings, a disk formed; a disk containing just enough stardust to someday form 8 planets and a lot of harder to categorize smaller bits.

While this star, which would come to be called “The Sun,” was busy forming, its nursery mates were similarly busy growing, glowing, and in some cases even going an extra step and exploding. This stellar nursery was filled with screaming stars that wept radio waves and threw off high energy jets as they tried to find their way onto the main sequence. While these stars wailed and grabbed at matter, they also traveled as a pack around the galaxy. While we can’t do more than guess at the Sun’s original orbital position, we know that today it takes about 135 million years for the Sun to orbit the galaxy. Let’s assume for a minute that the Sun emerged from the center of of that cluster. This would put it in a position to watch some of its nursery mates race ahead around the galaxy, take less time to orbit, while other of its nursery mates slowly fell behind, taking longer to orbit (and a few just explode themselves into oblivion as supernovae). After a few orbits and a few hundreds of millions of years, these differences in speed caused the fastest (and slowest) stars to fall out of the cluster, as their positions no longer made it possible for the casual observer to match them up with their cluster of origin. Over time, differences in orbital velocities drew more and more of the stars away from their siblings. Eventually, it became impossible to tell exactly which stars made up those sibling stars to the Sun.

The Sun, like its sisters and brothers, simply fell out of the cluster as it raced around the galaxy, just as a runner might fall away from the pack.

We are an orphan system, alone in the galaxy. Unlike the majority of stars, our Sun has no companion. Having escaped the chaos of our home, we are now simply alone.

While observed and documented for about 2000 years, only for the last 100 years have we known that novae and supernovae are different objects, and that supernovae are stars blowing themselves to bits. Only in my lifetime have we known Novae are white dwarfs surrounded by accretion disks that periodically blow (some of) themselves to bits.

These dynamic objects change dramatically in brightness. For scale, human vision allows us to see objects from magnitude 0 (the brightest stars) to magnitude 6 (the things you only see in the middle of no where). With novae, stars often can increase in brightness by as much as 10 magnitudes in 24 hours. They don’t stay bright for long, however, and in the vastness of the sky it is easy to missing these pin prick flare ups.

To help try and find these things, in 1973 a Sky Patrol was started (although negatives weren’t even checked all the time). These images were used to examine pre-outburst appearances of objects and to search for every novae brighter then 8th magnitude. In 1976, visual searches for novae were added to photographic searches.

A “team plan” was designed, with small numbers of zones allocated to different observers around the globe – there was a very strong collaboration with the AAVSO, with different fields being assigned systematically on both sides of the Atlantic. Each clear night people went out, looked up, and watched for cosmic explosions. The biggest question was: can visual observers with binoculars find these new stars in fields rich with stars? For some it was an easy yes. With 10×50 binoculars it is possible to memorize a field down to 8th magnitude, and these observers were able to affectively find new objects. The biggest issue in these objects was the need for a solid master image to compare the sky against. No atlas exists that is entirely accurate, and only a photo can give a true view of the sky. This, photos (which could be used for master images) provided the needed check on observations.

Today, Guy Hurst is the Patrol Coordinator, and he organizes many observers searching many many sections of the sky. Hundreds of novae and supernovae have been discovered and data across the stars’ rises and falls, have been collected. This data helps us understand the way the outbursts evolve over time (like discovering how different woods burn in a camp fire).

This is interesting work, and new observers are always welcome to join. Interested in playing along? Email guy at tahq dot demon dot co dot uk.

No matter what form they take, binaries that are lined up with one star in front of the other on the sky have variations in brightness that can be observed easily, in some cases even with the unaided eye. This means you, no matter who you are, if you can read this page with your eyes, you can observe variable stars and contribute to our understanding of the universe.

Here’s how it works: As two stars line up side by side, we are able to see the light from both. If they are close enough together (which is most of them), their light blends together and we see them as a single bright object.  When they line up, one in front of the other, we only see the light from one of the stars, and if the bigger and brighter one is in front, the system appears fainter but not necessarily a lot fainter. Now, when the smaller star goes in front, the system appears a lot fainted because the smaller star blocks the brighter light (think of a large van with it’s headlights on parked in front of a larger spot light announcing a mall opening. The van is emitting light, but it blocks more then it gives off). As we watch the light of these stars change, we see a flat bright line when the stars are side by side, and then two different sized dips as the pass in front of each other.

Looking at this, we can somewhat understand the geometry of the system based on how long the dips – the eclipses – take, and how much time there is between eclipses. If you have two systems that each consist of identical pairs of stars, you might get short eclipses that are equally spaced (bright bright deep-dip bright bright little-dip) as the stars go round round in a circular orbit that is at a slightly angle, such that the little star grazes across the bottom of the star on the front and dips across the top of the star as it passes behind. At the same time, you might see, from identical stars in a different system, long eclipses that are closely spaced in time, followed by long gaps (l-i-t-t-l-e-d-i-p, brgt, b-i-g-d-i-p, bright, bright, bright, bright) if it is an elliptical system with the smaller star crossing directly in front of the middle of the bigger star (wider area to cross) and directly behind the middle of the bigger star. We also use a bunch of math and theory to measure stellar masses based on this data combined with spectra (a topic for another time).

Today, several people are showing their light curves of various binary systems, ranging from white dwarfs stripping mass of their companions, to fairly close, fairly fast orbiting regular stars. Everyone is asking for help. For instance, the stars DW UMa and SW Sex (for Sextantis) both are looking for people to help them observe in detail. These two white dwarf binary systems have changing orbits and it is only possible to understand them if we hand the stars off from observer to observer around the world around the year. Interested? You can actually find out more about the DW UMa program (they’d love it if you had a CCD camera and filters), through their google group. My initial attempt to quickly copy their group URL failed, but I’ll try and get a URL later today.

Want to get involved in general, check out The AAVSO Mentoring program to find mentor to walk you through your first steps of celestial exploration.

WhiteDwarf.org has movies of these stars.

There about 12 of these stars known and 7 of them have been found in the Sloan Digital Sky Survey.

Part of understanding these systems requires ultra-violet spectra – Images that send the star’s UV-light through an optical system  that spreads it out such then individual atomic lines can be observed. This light doesn’t make it through Earth’s atmosphere, so the Hubble Space Telescope is used.  Ground based data are required to support these observations. A white dwarf binary can periodically flare up in brightness (called a nova or a CV outburst) when materials build up between the stars and undergo run away thermal nuclear reactions. During these outburst, stars can become bright enough to destroy HST’s UV detectors. To make sure this never happens, amateur astronomers (perhaps some of you!) monitor the stars with backyard telescopes. HST has to have ground-based observations within 24 hours of the space-based observation to confirm the star is not outbursting. Without these observations, they cancel the HST time. So far, no cancellation has occurred thanks to the hard work of amateurs.

Using these hard one observations (both ground and space), they are working to understand the masses of the white dwarfs that pulsate as a function of their temperature – This means they’ll look at pulsating stars of a couple different temperatures and see how things vary.

One of the first (and sadest) things for them to discover was accretions pulsations do not pulsate forever – and apparently they’re shy of HST. Three of the 9 stars they looked at opted to stop pulsating just as they got HST time.  In the other 6 stars, periods were seen to change in some cases, partially go away in other cases, and stay exactly the same for 1 star.

White dwarfs are quickly cooling and quickly evolving as they cool, consume matter from their companion star, and reheat during outbursts. These changes cause periods to come and go on not just human time scales, but on decadal time scales. Understanding how things change as a function of specific observables should be possible, but we need more stars and we need more time, but because of the short evolution time, someone like Paula can hope to see the answer come over her research lifetime as the stars she works on evolve.

This means one person can watch one star fundamentally change in one life time. How cool is that!

This paper caught my attention for one simple, stupid reason.  Every read Ringworld by Niven? It depicts a bunch of stars all going Nova at once in the center of the galaxy. Well, O-stars that are sufficiently large enough can do just that!

The physics here is also pretty amazing. The supermassive black hole in the center of the Milky Way is  ~4×10^6 Solar Masses. This means its event horizon is ~200,000 AU from the center of the galaxy. Around this massive compact object there are pockets of mind boggling star formation. It appears that the Milky Way’s bar may be in part to blame – 13E appears to be embedded in the bar and 13N is in or near it. Nevertheless, we hadn’t thought this type of star formation is possible. It was thought that stars in the galactic center had formed in waves 7 million and 100 million years ago. Now, we thin we know better, and we think that can (in cosmic terms) still be forming today.

And someday, someone may look to the center and see a Ring World like Galactic Center supernova.

Tau Boo A’s flip might not be entirely identical to Sol’s back flipping behavior, however. While the Sun is observed to flip fully every 22 years, Tau Boo A’s flips may be faster paced. This particular star has a GIANT planet in a very close orbit. The planet is 6.5 Jupiter Masses in size and orbits just 0.046 AU from Tau Boo A. (Mercury’s orbit is 0.39 AU in size!) As this large star flies around every 3.3 days it carries the star’s surface with it. According to the press release, “It is possible that the giant planet that has already managed to speed up the surface of tau Bootis is also spinning up the magnetic engine of its host star.”

Here’s how that works. As far as we understand it, stellar magnetic fields are created in or near the boundary layer between the part of the star that transfers energy via radiative transfer (like heat transferring up a pot handle) and the part that transfers it via convection (like a lava lamp). In this boundary zone are ionized (charged) particles. As they rotate around the star like so many particles flowing through so many circular wires, their motion generates magnetic fields. Since the inside of the star doesn’t rotate like a solid, all these moving, magnet making charged particles can have a myriad of interactions that lead to all sorts of magnetic behavior (and misbehavior).

It is possible that the gravitational pull of the planet Tau Boo b (lower case is the planet), on Tau Boo A tugs that layer of the star, carrying it around faster and faster, and possibly accelerating the process that leads to the magnetic pole flip.

That said, magnetic fields are very very hard to understand. Lots of modeling needs to be done. And even if the models show that the planet should accelerate the field flip, the result won’t be fully trusted until we observationally see the flip happen a few more times. Astronomers will be watching to see what happens. They have some hope – if the planet had a 11ish year cycle between half flip (north goes to south), we would have had a less than 1 in 5 ish chance of seeing the flip in the past 2 years of looking. The fact that we saw it is just enough improbable that it hints (and I won’t saw more than hints) that the cycle is maybe shorter.

Maybe.

It’s just kind of cool to imagine a planet twisting up a star.

(And if you want to know how to name a star, check out Fraser’s site)

Image credit: Karen Teramura (UH IfA)

Here is what has me excited. In a new paper in Astronomy and Astrophysics (which my Uni doesn’t get) with Pierre Kervella as lead author,  the distance to a Cepheid variable has finally been accurately measured in a method so simple I can’t believe it wasn’t done before. The binocular-bright Cepheid RS Pup is embedded in a nebula. As it’s light varies, it causes the dust and gas to also vary in brightness. By measuring how long after the star varies in brightness the blob of gas and dust varies in brightness, it is possible to tell how far apart the star and blob are located (sort of like measuring the distance between two cities based on how long it takes to drive between them going 100 km/hr). The next step is to measure the angle on the sky between the two. This gives us one angle and one side on a triangle. Everything else is than calculatable – including the distance from us to them.

Beautiful. Clean. Simple. I wish I knew why no one did this before. (Hopefully that’s addressed in the paper.)

The other thing the press release doesn’t do is tell me if these new results significantly changed our understanding of our place in space. Cepheid variable stars are one of the standard  candles used to measure the distances to other galaxies and to calibrate the supernovae distance scale. If it turns out that we misplaced the Cepheids it will rescale things a bit. We shouldn’t be off by more than a few percent (we have some not totally accurate ways to measure distances today with bad parallax measurements), but still… It will be interesting to know how close we got by averaging a whole bunch of imperfect measurements.

Once I get my hands on the paper, I’ll let you know.

The two sets of binaries are very tight. One pair orbits a point between the two stars (their center of mass) with a separation of 0.06 AU, and the other set has a maximum separation of 0.56 AU. For perspective, Callista orbits Jupiter at a distance of 0.01 AU and Mercury orbits the Sun at a distance of 0.46 AU.

This is a system that is neat to model. While brown dwarfs live a long long long long time, as these stars (someday many 10s of billions of years in the future) evolve, the orbits in the system will change, material may exchange between stars, and who knows what else. This is a discovery announcement, and I can’t wait to see what the theorists do with their knowledge that these things can form!

Needless to say, not everyone embraced the change.

These results rested solely on our understanding of type 1a supernova, which is an uncomfortable place to be (but that understanding seems to be solid, despite a mis-leading press-release earlier today.

There has always been the active question: Are the results real or is there something going on with the supernovae that we don’t understand that is making it look like the acceleration is there. Astronomers have taken two tactics in dealing with this problem: 1) Look for other lines of evidence that confirm the supernovae result, and 2) try and figure out supernovae better.

The new evidence hasn’t been easy to find, but since the supernovae results were announced in the late 1990s, other evidence has been found in the cosmic microwave background, in the large scale structure of the universe, and also in gravitational lenses. So… Even if we don’t fully understand supernovae, everyone (well, almost everyone at least) grudgingly acknowledges the universe is accelerating apart.

But the degree to which it is accelerating apart at what period of time (so far) can only be easily measured with supernovae. This means we really need to understand supernovae.

Now, at the most simplistic level (where accuracy is given up for clarity), Type 1a Supernovae are formed when a white dwarf star gravitationally consumes too much mass from a companion star and gravitationally collapses and explodes in one violent step. Since all white dwarfs become explosive (to first order) at the same mass, they all have the same amount of material to contribute to the explosion and they all create an explosion of the same size. Thanks to the luminosity-distance relationship, we can measure how bright a supernova appears with a telescope and compare that to how luminous it actually is to calculate how far away it is (This is the same thing you do when you estimate the distance to a motorcycle on how bright its headlight appears). Once you know how far away a supernova is, you can measure its recession rate with a spectroscope. (These are actually very hard to do technically, but that’s what graduate students are for).

At the next level of complexity, we know that Type 1a actually show some variation. Some take a longer period of time to fade away and give off more energy. Others fade faster and are fainter. Now, since we know how the supernovae’s total light output changes as a function of their lightcurves’ shapes (with error bars), we can correct for this second order effect. No big deal. This is like knowing that when you put giant wheels on your car you have to fix your odometer to compensate for the car go farther for every one turn of the tires. Like I said, no big deal, this is something astronomers just have to be more aware of. We even think we understand why this is happening (see link). Okay, nothing to worry about, move along.

But there is still another problem. It a fact that there are more metals in the universe today (defined by astronomers as anything heavier than Helium). There are Sun’s creating carbon, and iron and everything in between all across the cosmos, and every exploding star releases a variety of everything and anything nuclear reactions can create. This means the stars that are forming today are forming out of materials that where just a twinkle in a young giant star’s eye some day in the past. The first stars were almost pure hydrogen and helium. Those stars have very different physics from today’s stars. Metals moderate the formation of stars, making stars form smaller and burn in a more controlled way. When white dwarf stars first started forming, they had fewer metals than modern white dwarfs and that could have effected how supernovae explode, causing supernovae to vary as a function of time in ways that we don’t know about.

Earlier today a press released crossed-my inbox that said, “distant supernovae were an average of 12 per cent brighter. The distant supernovae were brighter because they were younger.” On first read, it would seem to imply that supernovae have gotten brighter as a function of going back in time. This would imply that supernovae luminocities are a function of lightcurve shape and when the supernovae exploded. Eek! Things got much more complicated! But, the press release goes on to say, (words of my former classmate at U-Texas and now U-Toronto Post Doctoral Fellow, Andy Howell) “We found that the early-universe type 1a supernovae had a higher wattage, but as long as we can figure out the wattage, we should be able to correct for that. Learning more about Dark Energy is going to take very precise corrections though, and we aren’t sure how well we can do that yet.” This seems to imply that yes, things are getting complex but we can indeed cope, but there will be error bars.

Not liking this new reality, I went and found the actual paper (subscription required to get beyond the abstract, but please read the abstract). Ummm, if I’m reading this right, the average luminosity of supernovae is changing, because the ratio of brighter ones to fainter ones is changing, but the type 1a supernovae themselves are still totally predictable within error bars. The faint ones act the same (but there are fewer), the bright ones act the same (but there are more of them), and the average changes while the physics (within error bars) is respectably well (within error bars) understood.

No, type 1a supernovae aren’t totally standard candles. They don’t all give off the exact same amount of light and they don’t all explode in the same way. They are a family of candles and we know how bright they all are, each in their own unique way (with error bars).

That said, what the papers lack it number, the make up for in titles. For instance, consider the following:

Ruthenium and hafnium abundances in giant and dwarf barium stars by D.M. Allen and G.F. Porto de Mello.

So imagine with me if you will: Before you expands a waiting room of sick dwarves and sicker giants who all happen to be movie stars. Some of them have had a bit to much Ruthi and others got into the hafnium before they all landed at the ER. Now some nice nurse has handed them a swig of barium to fill them up before they go in for testing.

This is what lept to my mind when I read that title.

All joking aside, this was a very serious paper that did some very difficult research. All very heavy elements – those heavier than lead – are produced during supernovae explosions. Some of these elements are produced by simply bombarding atomic nuclei with neutrons under extreme conditions. During a core-collapse supernova, the type created during the death of a single giant star, 10^22 neutrons per square centimeter can be emitted (that is a 1 followed by 22 zeros of neutrons flowing thru the area of a typical adult big toe toenail). With that many neutrons flowing, they will actually hit atomic nuclei at a rapid rate. When too many neutrons build up in just the right conditions, they decay into a proton, electron and neutrino.

This process can happen over and over as things build and decay at different rates as they constantly get bombarded. This process also has a slower cousin that takes place in stars that sometimes leads to low level production of heavy elements (a process that generally gets swept under the rug when discussing the creation of elements with the public).

Back in graduate school, while I spent a part of my life studying the isotopic ratios of Magnesium Hydride in stars, I had this fabulous chart that you could use to trace out the rapid neutron capture and other processes that can lead to different isotopes. It was a giant poster, and a quick google didn’t allow me to find it to share with you, but I did find this applet you can use to watch the growth and decay of elements. It lacks the excitement of a supernova blast, but it’s kind of neat none the less.

So, in making that poster I had and this applet I just shared, scientists spent countless hours calculating decays and atomic cross sections, and all sorts of crazy particle physics and quantum mechanical things. They think they’re right. We experimentalists think we can believe them, but… Well, data helps all of sleep at night. To prove them right stellar spectroscopists like Allen and de Mello have to pain stakingly measure the ratios of rare and hard to study elements in stars and test those ratios against theory. In this case, they were testing the ratios of the heavy element Ruthenium (made mostly but not entirely in the rapid process) and hafnium (which is made mostly but not entirely in the slow process). They found that while most of the time theory and experiment matched (woot!), there were several cases where these two elements where produced in stars (AGB stars to be exact) in higher levels than theory can explain.

This means the theories aren’t completely right yet, but they’re getting there. It means there are new projects for students, and new things to learn about stars and supernovae. But we’re getting there. We’re way more right than we are wrong.

This paper will never have a press release written about it. I only read it because its title made me think of sick giants and dwarves undergoing nasty medical procedures. None the less, this paper represents where the incremental advances in science are coming from. Slow and study, careful and thorough, building our understanding one isotopic abundance at a time.

So far so good.

The problem is, as you then move away from the surface of the Sun, you enter regions where the temperatures again go up – A lot – like back to millions of degrees hot levels of a lot!

And no one fully knows why. This is a very counter intuitive situation. Imagine that the surface of a lava lamp was 23C and the air half an inch away was 200C! In a press conference Wednesday, astronomers announced that they think they may have found a starting point for understanding what is going on in this bizarre situation.

In a pair of presentations given by Bart De Pontieu (Lockhead Martin Solar and Astrophysics Laboratory) and Scott McIntosh (Southwest Research Institute) it was shown that a combination of sound waves and magnetic fields can channel energy (and heat is a form of energy) into the Sun’s Chromosphere. In their models, they find that sound waves propagate through the convection zone, and the energy within the sound waves can escape in locations where broken magnetic field lines form solar spicules (a time of flame shaped thing). The sound waves trigger shocks that super heat fountains of material that is ejected into the chromosphere. When they compared their models to actual high-speed images they found excellent correspondence between modeled expectations and reality.

During the press conference’s questions session, one of the journalists asked, (to paraphrase), “Why should we think that 10 years from now we’ll be saying that the question of ‘Why is the Chromosphere so hot’ was definitively answered in 2007?” While that may sound like a really obnoxious question for a generally well-behaved room full of science writers, it was actually a really honest question. We still don’t fully understand the Sun’s magnetic field or exactly what causes the field lines to break and reconnect is a bit hairy to try and understand and model. We still don’t fully understand how convection works in the Sun either. We are incrementally building better and better tools for modeling what we observe, but our theoretical models include lots of assumptions. To say we can definitively announce anything that includes both magnetic fields and convection is, um, optimistic.

But this is a start. I honestly think that 10 years from now, as we continue to build a fully refined understanding of what is going on, the papers written on the results shown in this press conference will be cited. Tomorrow’s understanding builds on yesterdays results. Sometimes science goes in leaps of ingenuity. This is not one of those times, but it is still solid science.

Next Up: Tidal Streams…

Not being a good humorist, I will not try to fill in the blanks. I will simply work to explain how two such very different objects can announce their formation via the same physical process.

Astrophysical jets are a by product of magnetic fields, and magnetic fields are generally a by product of charged particles moving. For instance, if you wrap a bunch of wire into a loop, and connect it to a battery, the current through the loop (the charges moving though the loop) will turn your loop of wire into a magnet strong enough to deflect a compass needle, and maybe even to make a small engine.

In astrophysical situations, charged particles (for instance, ionized atoms), move in different situations and create their own magnetic fields. For instance, in accretion disks, lots of particles are confined to orbits in a flat pancake of material, and these orbital paths act (in a college physics without calculus, cows are spheres, approximation) like the current in your coiled wire.

If you use a compass to explore the direction of the magnetic field associated with a coil, you’ll find that the N-S axis goes though the center of the loop perpendicular to the loop. This is the same orientation as astrophysical magnetic fields.

So where do jets come in? Well, as some of my students demonstrated for me, that coil can get things moving if it wants to! If you take large diameter wire and wrap it around small diameter PVC pipe, you can build something called a Coil Gun. I’d recommend something like 50 ft of 10 gauge insulated wire around a 3/4-inch PVC. This worked really well for some of my students (see Video). Now find yourself the largest power source you can that is Direct Current (I used the portable battery for jump starting cars that was in my trunk, my students daisy chained drill batteries). Drop a magnet into the PVC and flick the connection on and off (you want to create a burst of magnetic field). If all goes well, the magnets will get flung out one end of the PVC or the other (it all depends on which way is the fields North, and which way is the magnet’s North.

So, the accretion disk acts like your coil and material trying to get into the center can gets flung out by the magnetic field just like your magnet got flung out. To first approximation.

To make your coil gun more dangerous, you need only increase the current (bigger battery, or preferably a bank of capacitors you can charge in parallel and discharge in series) and increase the number of coils. By maximizing both these things you can create a fairly good weapon.

The universe can do the same thing by increasing the size of the disk and increasing the orbital velocities of the charged particles. Small, slow disk = Small sad jets. Large, high velocity disk = jets to be feared.

Today’s press feed offered both the scenarios in two separate stories. Astronomers lead by Emma Whelan used the Very Large Telescope in Chile to image a very low mass brown dwarf with small jets. This object, named 2MASS1207-3932, is about 24 Jupiter masses and has a 5 Jupiter mass companion (I say companion because I don’t know if their formation resembled that of two binary stars or a planet and star, so I have no idea would nouns to use). This little almost-a-star’s jets are about 1 billion kilometers long and the material in the jets is moving at a few kilometres per second (a speed man-made rockets can attain). This is only the second brown dwarf found to have jets, it is the smallest star found to have jets. The fact that such tiny failed stars have jets while forming raises the possibility that gas giant planets may also go through a phase of having jets during their formation.

On the other end of the size scale, astronomers lead by Hans Krimm have used the Swift satellite to figure out that long duration gamma-ray bursts (those associated with a special class of supernovae) have continued X-Ray flare activity for several minutes to hours after the initial gamma ray burst. The physical picture of the event looks something like this: A giant star runs out of material to fuse into heavy elements (namely Iron in the core), and when the fusion shuts off, there is no longer sufficient radiation pressure (pressure from the light created in the nuclear reactions) to support the out layers of the star. The star had been rotating, and when the outer layers – now unsupported – collapse inward, they collapse somewhat into a disk. Magnetic fields channel some of the material in this collapsing disk out in jets. The whole process is short lived, but it sputters out rather then cleanly coming to an end. Initially, when the disk would have the most food to offer the forming forming black hole that is where the star’s core used to be, the jets are emitting in a few seconds as much energy as the Sun will emit in its entire lifetime. When this jet material hits shells of ejected material, gamma rays are given off. The jets in this case are moving at near light speed! This initial gamma ray burst will general last a few 10s of seconds to in rare cases a few hundreds of seconds. Over the subsequent several minutes or hours, additional X-ray flares may also be observed. By studying the X-ray flares associated with GRB 060714, Krimm and his team determined they were likely caused by subsequent infall of material and associated emission as the star’s material continues to build the black hole.

In both cases the jets were related to formation of a new object, and the physics in both situations was similar. The differences come out of the details. As with almost everything in astronomy, it all comes down to the mass. Small objects have small jets, and giant objects have dangerous jets. And luckily, none of these jets are pointed at us.

Seriously though, trying to determine the formation date of a stellar object is tricky business, and the best direct method we have involves studying the ratios of different nuclear isotopes in stellar atmospheres. Called nucleo-chronometry, this process first asks “In what ratio where all the elements in this star formed?”, and then looks to see in what ratios those elements are actually observed. In a perfect universe, there will be a baseline distribution of stable elements that appear in textbook perfect ratios side by side with unstable elements with long but varied half-lifes. It is this combination of different decay rates that allow the star’s age to be determined. For instance, if a star was expected to form with some amount A of element Fo* and some amount B of element Fi* (where Fo has a half life of 1 billion years, and Fi has a half life of 3 billion years), than after 3 billion years, we’d expect to see only only 1/2^3 A= 1/8 A of element Fo and 1/2 B of element Fi. Only one element is required to get a rough estimate of how old a star is – in fact carbon dating uses just the element Carbon-14 to measure the age of old organic materials – but more reliable results come from looking at more then one element.

This technique was recently used to identify a population III star likely from about the first generation of low mass stars, that was born roughly 13.2 billion years. This star, named HE 1523-0901, is perhaps the oldest known star in our galaxy. At first glance, this is just another story of someone going, “Oh neat, an extreme,” but the reality is, determining the age of a star is a real bear, and, in many cases, it just isn’t possible. This piece of research, lead by A. Frebel of my graduate alma mata the University of Texas, and including T. Beers, my undergraduate advisor at M.S.U., required a lot of hard work, and 7.5 hours on the ESO’s Very Large Telescope in Chile.

On paper (or at least on the computer screen), this process sounds pretty straight forward, but in reality it is a messy problem. Observationally, nucleo-chronometry is challenging because it requires high quality (specifically high signal-to-noise) high resolution spectroscopy. In plain English, the light from the star has to get spread into an extremely long and bright rainbow that allows astronomers to study the specific shades of color that correspond to the electron energy levels in different isotopes of atoms. This can be done with the brightest stars without a lot of pain using couple-meter telescopes. To look at faint stars (in other words, most stars), many-meter behemoth telescopes and long exposure times are required. This is hard work, requires good observing conditions, and the data reduction is something known to make graduate students (including myself) want to throw things. The results are worth it, but it can take as long to acquire and reduce the data as it does to measure the elements in the data.

Obtaining and quantifying the data aren’t the only difficulties. Stars are created out of recycled materials, and one star may have had a dozen different types of supernovae, each with their own element distributions, in its ancestry. Star’s atmospheres can also be altered, either (in the case of extremely low mass stars) through mixing of internally enriched materials, or via mass transfer from a nearby star (where mass transfer may be just a nearby red supergiant undergoing normal mass loss that falls onto the star being observed). When looking at the atmosphere to determine age, one must have some sense of what is original, what should have been their originally, and what if anything ended up there after the fact.

This is good science that only comes from whatever one calls the typing equivalent of elbow grease.

But, this is also the type of work that Frebel and her team did. They were able to determine age estimations using 7 different atomic ratios. Having done my own painful share of isotopic measurements, my hat is totally off to these guys for this clean bit of science. They successfully dated the oldest star around – no bling required.

I could insert a joke about how this is harder than getting a date with Paris Hilton or Britney Spears, but… That would just be google fodder.

*Elements Fo and Fi are just made up

As I understand it, there are a couple different models. In one, a molecular cloud will very slowly, over lots and lots of time collapse due to gravity (some clouds formed with our galaxy still haven’t collapsed all the way into stars!). Higher density regions will collapse faster, and lower density regions will either get sucked into higher density regions, or just collapse very very slowly. Pretty much everything in the galaxy has some angular momentum due to inherent rotation. As the densities within a giant molecular cloud collapse, they begin to spin and flatten. There is a period of time during which gravity is pulling material into the center of the density while the radiation pressure from the warm gas is ejecting the material in jets. Luckily, the system is able to not blow itself apart in the process, and gravity wins. When a star turns on – when nuclear reactions start up in the center, the light from the star creates so much pressure on surrounding material that the inflow of mass stops and the star clears out the area around it.

Now, if all the stars in the universe where formed simply through the very very slow gravitational collapse and fragmentation of molecular clouds, we would live in a very boring universe. Shocks (such as those from supernovae, spiral density waves, and collisions) can speed up the collapse of gas by pushing stuff together (condensing it). In this scenario, only the highest density regions survive to form stars, and the lower density gas dispersed. Here is a way to picture it: Imagine you have a rake with very flexible light wieght tines. Thanks to the help of a squirrel, you have one small patch of lawn with an over density of leaves. When you rack that one section of leaves, leaves that aren’t part of the original clump get pushed into it, and the force from the rake condences the pile. If the clump gets big enough, with a lot of large friction with the ground, the tines may bend and leave that clump behind. In a similar way, the shock wave can push together a large density of material, and if the material is dense enough gravity will hold it together and it will grow into a star.

In colliding galaxies, massive amounts of star formation will be triggered by shockwaves from the collapse, but the material that doesn’t get turned into stars may get strewn through space or pushed into the central black holes. No matter its fate, after the collision, the two galaxies will be dead, and star formation will have ceased.

image credit goes to HST.