# Betelgeuse, Betelgeuse, Betelgeuse

In principle, measuring the distance of a nearby star is fairly simple.  It uses a method known as parallax.  It is the same method we use to have depth perception every day.  If you cover one eye and hold your thumb in front of a distant object, when you switch eyes your thumb will appear to shift against the background.  This shift is known as a parallax shift, and it is due to a shift in your point of view.  The bigger the parallax, the closer an object is.

In astronomy, we can use the Earth’s changing position in its orbit to shift our point of view.  We can observe a star relative to more distant stars over the course of a year, and measure its parallax.  Some simple trigonometry then allows us to determine its distance.  Parallax is actually the origin of the word “parsec,” which is short for “parallax arc second.”  If a star has a parallax of one arc second (or 1/3,600 of a degree), then it is one parsec away (about 3.26 light years).

For more distant stars, however, this can get a bit tricky.  It is particularly difficult for large stars such as red and blue supergiants.  Take for example the star Betelgeuse.  Betelgeuse is a red supergiant with an apparent size  of about 45 milliarcseconds.  However its parallax is only about 5 – 10 milliarcseconds.  As a result, the distance to Betelgeuse has been relatively uncertain.

This is problematic because, without an accurate measure of distance, we can’t be sure about other properties.  We know the star’s apparent diameter, and its apparent motion, but without a precise distance we can’t be sure of its actual diameter and motion.  Knowing those would be useful to determine the age and origin of Betelgeuse.

Earlier this year (doi:10.1088/0004-6256/135/4/1430), a team used radio interferometry to make a precise measurement of the parallax of Betelgeuse. Radio interferometry allows you to combine signals from multiple radio telescopes to create an image as if they were all one huge telescope.  This means you can make much more precise measurements than possible with a single telescope. They determined that Betelgeuse is 197 parsecs away (about 640 light years).

With an accurate distance, the team could then use Betelgeuse’s apparent motion to determine its actual motion through the galaxy.  You can see this in the figure below (where Betelgeuse is labeled as α Ori).  One thing clear from the figure is that the motion of Betelgeuse is very different from the Orion Nebula Cluster (ONC), so it is not likely that Betelgeuse originated in the Orion Nebula. Instead, it seems Betelgeuse originated from the Ori OB1 cluster.

# Swiftly Tilting Planet

As I write this, it is the end of a long day.  It’s the last week of classes at RIT, so things are rather hectic.  Usually my day ends with a calm drive home, when I can think about topics to write about.  But this evening has seen a winter storm move in, so instead of a calm drive, it was white knuckle time while I drove through thick slushy snow in the dark.  So I had plenty of time to contemplate the fact that winter storms occur because I live on a rock.

This particular rock speeds around its parent star at about 100,000 kilometers per hour.  The rock has a roughly circular orbit, but its axis of rotation is tilted about 23 degrees from its orbital plane.  This means that for about half the year the northern hemisphere on which I live is tilted toward the Sun, which makes for longer days where the Sun has a higher path in the sky.  For the other half the northern hemisphere is tilted slightly away from the Sun, making for shorter days where the Sun is lower in the sky.  It just so happens that it is near the time when the northern hemisphere will experience its shortest period of daylight.

Having shorter periods of daylight with the Sun lower in the sky means that the surface of this rock receives less radiant heat from the Sun, thus making the temperature in my part of the world colder.

Of course it isn’t quite that simple.  The relatively high rotation rate of this rock (about 24 hours) means that the surface undergoes fairly swift periods of heating and cooling.  The rock also has a fairly copious atmosphere with large quantities of water vapor.  This means that the surface temperature does not merely rise and fall as the planet rotates, but instead has regions of warmer and cooler air that can swirl around, collide and do all sorts of interesting things.

This rock also has high concentrations of surface water, with about 70% of its surface covered with oceans and lakes.  This means there is a ready source of water that can evaporate into the atmosphere.  This can create clouds and rain, but it turns out that the surface temperatures of this planet are near the freezing point of water at typical atmospheric pressure, so sometimes instead of falling as droplets of water, it freezes into thin crystals of ice and floats to the ground.

If enough of it floats to the ground you get a winter storm.  Fortunately, my sistren and brethren on this rock are familiar with this kind of thing, and can generally deal with it.  It just takes some slow and careful driving to get home sometimes.

Living on a rock hurtling through space is never dull, but I wouldn’t have it any other way.

Image: My yard.

# Dark Matter Junction

One of the big mysteries of astrophysics concerns the nature of dark matter.  We have solid evidence that dark matter is real.  You can read a 5-part series I wrote on just how we know it’s real here: http://goo.gl/8TySFi.  What we don’t know is just what dark matter is made of.

The most common idea is that dark matter is some kind of weakly interacting massive particle (WIMP), that is not regular matter (electrons, protons, neutrons, etc.) but is somewhat similar.  “Weakly interacting” in this case means that it doesn’t interact strongly with light, which would explain why it is basically invisible or “dark” matter.   Other ideas such as tiny black holes or modified gravity simply don’t agree with what we observe (again, note the series above).  So the race has been on to directly detect some kind of dark matter WIMPs.  That is, some kind of low mass, chargeless particle that doesn’t interact strongly with light.  So far we haven’t been successful.

There is an alternative idea for dark matter that isn’t as popular, known as an axion.  The axion is a hypothetical particle that was first proposed in 1977 to address certain issues in quantum chromodynamics (QCD), which describes the behavior of quarks and such that make up protons and electrons (among other things).  I won’t try to go into the details of that here.  What makes the axion interesting for dark matter is that if axions exist, the would be low mass, chargeless particles that don’t interact strongly with light.

That seems like a perfect candidate for dark matter!  So why isn’t the idea more popular?  It has to do with two reasons.  First, while axions are an interesting solution to the QCD problem, they aren’t the only possible solution.  Second, if axions exist, they would be very different from other massive particles.  All the known regular matter (electrons, quarks, protons, etc.) are fermions, but axions are bosons.  This means that axion dark matter would behave differently than WIMP dark matter.  WIMP dark matter describes what we observe rather well, so they are generally favored over axion dark matter.  But axion dark matter hasn’t been ruled out as a possibility.

Of course the proof either way will be to detect a dark matter particle directly.  Now a new paper in Physical Review Letters (paywalled, but arxiv here:http://goo.gl/hwg8KU) argues that there might be a simple way to detect axionic dark matter.  The idea uses a relatively simple device known as a Josephson junction.  You can see an image of one below.

Basically, a Josephson junction consists of two superconducting materials separated by a thin layer of non-superconducting material.    A superconductor is a material through which a current can flow without any resistance.  Within a superconductor the electrons flowing through the material pair up (known as Cooper pairs).  This pairing is an effect of quantum mechanics. If you have two superconductors separated by a thin layer of regular material, then the paired electrons can flow through the regular material at a constant rate, known as a supercurrent.  This supercurrent is very sensitive to changes in voltage, so Josephson junctions are used in devices to make extremely sensitive measurements of voltage and magnetic fields.

If dark matter is made of axions (and that’s still a big if) then there should be lots of them flowing through the Earth all the time.  Some of them would also interact with Josephson junctions, and that interaction should be observable as a fluctuation in the supercurrent.  The hard part will be to make sure that the junctions are shielded from other interference, such as radio and microwave radiation, but that doesn’t seem to be an insurmountable problem.

Image: National Institute of Standards and Technology (http://goo.gl/aYxEPO)

Paper:  Beck C. Possible Resonance Effect of Axionic Dark Matter in Josephson Junctions. Phys. Rev. Lett. 2013;111(23)

# Peanut Gallery

You’re probably familiar with the general shape of our Milky Way galaxy, that is, a circular disk of stars with a thicker bulge in the center.  While that is a good description of the overall shape, there is actually some interesting structure within the center of our galaxy.

This past Fall the first detailed 3D map of the central bulge of the Milky Way.  The results were published in the Monthly Notices of the Royal Astronomical Society or MNRAS (paywalled, but arxived here: http://goo.gl/c1nnV3).  The authors created their map by looking for bright red giant stars at infrared wavelengths.

You might remember a few days ago I mentioned how a method called TRGB (Tip of the Red Giant Branch) that could be used to test the accuracy of Cepheid variable stars (http://goo.gl/3mXWRB).  This method is useful because these bright red giants have a fairly uniform brightness (absolute magnitude) so by observing their apparent brightness at infrared wavelengths we can determine their distance.

Near the central region of our galaxy there are lots of these stars, so a survey of the region provides a good measure of the distribution of stars near the center of our galaxy.  The team used data from the Vista Variables in the Via Lactea (VVV) infrared survey, which has been mapping the central region of our galaxy since 2010.

What they found was that central bulge of our galaxy is not shaped like a simple oval, but rather has a kind of peanut shape.  You can get an idea of the shape in the artist rendering of our galaxy seen below.  What’s particularly interesting about this is that it implies that stars in the central bulge are not moving in simple circular paths.

While that might seem odd, it isn’t entirely unexpected.  For stars in the disk of our galaxy, most of the mass concentrated at the center, much like the Sun is at the center of our solar system.  So those stars have fairly circular paths.  But for stars actually in the central bulge, the mass of the galaxy is all around them.  So their orbits are more strongly affected by the distribution of other stars around them.

This month another paper was published in MNRAS (again, arxiv to the rescuehttp://goo.gl/vOJ1Rx) which ran computer simulations of the central bulge to see what type of stellar motion could give rise to a peanut-shaped central bulge.  What they found in their simulations is that stars in the central region can gradually develop a peanut or figure-8 shaped orbit.  This is due to a resonance between the stars and the overall gravity of the galactic plane.

Of course this is just a computer simulation, but it makes a very clear prediction.  If such a resonance does drive the peanut shape of our central bulge, then the stars within the bulge should have speeds perpendicular to the galactic plane that are much higher than stars outside the bulge.  We can put this to the test when we get data from the Gaia spacecraft, which is set to launch later this month.

Image:  ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt (http://goo.gl/UyWnH4)

Paper 1:  Wegg C, Gerhard O. Mapping the three-dimensional density of the Galactic bulge with VVV red clump stars. Monthly Notices of the Royal Astronomical Society. 435(3):1874-1887.

Paper 2:  Quillen AC, Minchev I, Sharma S, Qin Y, Di Matteo P. A vertical resonance heating model for X- or peanut-shaped galactic bulges. Monthly Notices of the Royal Astronomical Society.

# Wormholes, Entanglement and Holograms, Oh My!

You may have heard about how quantum particles are wormholes, or holographic somethings.  Whatever.  It’s quantum wormhole things that, well, do something.  So what’s all the hype really about?

It all stems from a recent paper in Physical Review Letters (paywalled, but arxiv to the rescue: http://goo.gl/Z0gez3) dealing with entangled particles and their connection of their holographic duals to wormhole geometries.  So let’s start with a little background.

Entanglement is a well-known property of quantum mechanics.  It is perhaps most famously demonstrated through the Einstein-Podolsky-Rosen (EPR) experiment.  Suppose we have a mischievous mutual friend.  She decides to prank us by sending sending each of us one member of a pair of gloves.  She packs each glove in a box and mails one to each of us.  We find out about the prank, so we both know that we’re getting one glove of a pair.  But until either of us open our respective box, neither of us know which glove we have.  Once the box arrives at your door, you open it up, and find you have the left glove.  At that moment you know I must have the right glove.

This is the basic idea of the EPR experiment.  For gloves it isn’t a big deal, because from the get-go the left glove was heading your way.  You just didn’t know you were getting the left glove.  That’s because gloves are not quantum things.  In the quantum regime, things get much more strange.  In quantum theory, things can be in an indefinite state until you observe them.  It would be as if our boxes contained a pair of something (gloves, shoes, salt and pepper shakers, etc.) but it is impossible to know what specific something until one of us opens their box.

In quantum theory we would say the boxes contain a superposition of possible things, and the outcome only becomes definite when the outcome is observed.  Now even though you can’t know what specific object you have, you know that I must have its pair.  So if you open the box to find a red right shoe, you know immediately that I must have a red left shoe.  We both know this without opening the box, so we can say that the outcomes of opening our boxes are entangled.  Knowing the contents of one box tells us the contents of the other.  We’ve actually done this experiment with photons, atoms and the like, and it really works.

Of course this is really hard to wrap your head around.  If I’m thousands of miles away from you, and I open my box to find a salt shaker, I know you must have a pepper shaker.  But your box couldn’t have known that until I opened the box.  How is that possible?  How can the opening of my box instantly affect your box thousands of miles away?  Do the boxes communicate faster than light? (No.)  Is there some secret (hidden variable) so that the boxes know what they will become when observed? (No.)  That is part of what makes entanglement so strange, and the EPR experiment so popular.  The one thing we can say is that entanglement is a very real physical effect in quantum mechanics.  There isn’t anything magical going on, just something we humans find strange.

Wormholes come from general relativity.  Unlike entanglement, there is no experimental evidence for wormholes.  Instead, they are a hypothetical connection between two locations in space.  Normally when people think of wormholes, they think of something out of science fiction (http://goo.gl/nz5SV2) where people use wormholes to travel to distant stars, but the hypothetical wormholes in general relativity aren’t traversible, nor do they have to be large.

This particular paper is looking at how there might be a connection between wormholes and quantum particles.  This idea isn’t new, in fact the idea that fundamental particles could be wormholes dates back to the 1950s, when John Wheeler proposed a model known as geometrodynamics, where everything was empty space and charged particles were the mouths of wormholes.  Wheeler was an excellent physicist known for coming up with a lot of wild ideas, some of which worked, and some of which didn’t.  In the case of geometrodynamics, it never really worked, and after a while interest faded.

But with the rise of string theory, different versions of the idea have gained some popularity.  Hence this new paper.  What the authors did was to look at a specific case of the EPR experiment, dealing with two quark particles.  What they were able to show is that the entangled quarks can be described in two ways.  The first is the standard way in which entanglement is described in quantum theory, but the second (dual) way is as two particles connected by a wormhole.  Both of these descriptions are equivalent.

Does this mean that entangled particles are wormholes?  No.  What it means is that there is an interesting connection between the mathematics of entanglement and the mathematics of quantum wormholes.  Just to be clear, this has nothing to do with any new experimental evidence.   But it is interesting, because it shows a connection between quantum entanglement and general relativity, and that may lead the way toward a better understanding of quantum gravity.

Image:  “Doge Meme” from Know Your Meme (http://goo.gl/Wv2n4l), with text added.  Originally posted on Reddit by papajohn56 (http://goo.gl/Q7MFhE).

Paper:  Jensen K, Karch A. Holographic Dual of an Einstein-Podolsky-Rosen Pair has a Wormhole. Phys. Rev. Lett. 2013;111(21)

# Dyanamo

Vesta is one of the larger asteroids in the asteroid belt.  It is the third largest (about 500 kilometers wide), but the second most massive (after Ceres).  It is also the only large asteroid for which we have high resolution data, when the Dawn spacecraft orbited it for a while in 2012.  You can see a view of Vesta in the image below.

Since we know something of the chemical composition of Vesta, we can compare that with the composition of various meteorites.  From this comparison we have identified about 1200 meteorites that are likely to have originated from Vesta.  From the variety of these meteorites we know that Vesta is differentiated, with an iron-nickel core, magnesium rich mantle, and rocky crust.  Given its size and composition, it likely had a molten core and mantle for the first few million years of its existence.

One of the striking features of Vesta is its surface brightness.  Vesta is the brightest major asteroid, and this seems to be due to its basaltic crust.  Still, its brightness has been a bit of a mystery, because interactions with the solar wind would tend to darken the surface over time.  But this would be mitigated if Vesta had a magnetic field.

Unfortunately, the Dawn probe doesn’t have a magnetometer, so it can’t detect whether Vesta has a magnetic field.  But Vesta observations do indicate that the core of Vesta is about 200 kilometers wide, which does reinforce the idea that Vesta once had a melted core.  With a melted core, it would be possible for Vesta to have had a magnetic dynamo within its core.

A dynamo occurs when a melted iron core undergoes rotation and convection.  The motion of the conductive metal generates a strong magnetic field.  The melted iron core of the Earth maintains our planet’s strong magnetic field, which helps to shield us from cosmic rays and solar flares.  In contrast, Venus does not have such a dynamo, and its magnetic field is significantly weaker.  Mars also lacks a dynamo, and has a weak magnetic field.

One of the mysteries about Vesta and other planetoids is whether it is possible for something so small to have had a magnetic dynamo in its youth.  Evidence of Vesta’s large iron core suggests that it might have been possible, and now a paper in Science (paywalled, but a free pdf is available from the authors http://goo.gl/nzehTB) strengthens that idea.

The authors looked at a particular Vesta meteorite, and found it had a remnant magnetic field.  From the strength of this field they gauged that when the meteorite was still part of Vesta, the asteroid had a magnetic field between 2 and 10 microteslas.  By comparison, Earth’s current magnetic field is about 30 – 60 microteslas.  They also looked at argon isotopes from the meteorite, and determined its age to be about 3.7 billion years old.

This doesn’t mean that Vesta still had a strong magnetic field at that time.  Vesta formed about 4.5 billion years ago, so 900 million years later any dynamo would have likely cooled and faded. The magnetic imprint on this particular asteroid is a secondary effect.  The early dynamo would leave a strong imprint within the crust as it formed, and when chunks of the crust were scattered due to an impact, the magnetic imprint of the fragments imprinted a lesser magnetic field on the resulting meteorite.

So it seems likely that Vesta had a dynamo core in its youth.  But to be sure we’d need to look at the magnetic field of its current crust.  Hopefully that will be planned for a future mission.

Image: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA (http://goo.gl/sJlGq)

Paper:  Fu RR, Weiss BP, Shuster DL, et al. An ancient core dynamo in asteroid Vesta. Science. 2012;338(6104):238-41.

# Cosmic Corn Syrup

Previously I showed how you can use polarizing filters and corn syrup to demonstrate the rotation of polarized light.  So what does corn syrup have to do with astrophysics?

It turns out that visible light isn’t the only thing that can be polarized.  Radio waves can have polarization as well.  It also happens that the radio pulses generated by pulsars can be polarized.  That means we have lots of polarized radio sources throughout the galaxy.

Deep space in the galaxy is not entirely empty.  There is a small amount of ionized gas (plasma) that exists between the stars.  The radio signals from pulsars must therefore pass through this plasma to reach us.  That means we can use pulsar signals to learn about the plasma between us and the pulsar.

By itself, the plasma simply spreads out the signal by frequency.  This is similar to the way glass spreads out a pulse of light by frequency.  The speed of light passing through glass (or the index of refraction) is slightly different for each color, so the pulse is spread out a bit.  For light and glass, the effect is small, so we don’t normally notice it, however it’s this same effect that lets prisms break light into colors.  For pulsar pulses and galactic plasma, the effect is more dramatic. The radio pulse travels for light years, so there is much more time for it to spread by frequency.  So instead of a simple radio pulse, we see the higher frequencies reach us a bit sooner than the lower frequencies.  This spread is known as the dispersion measure (DM), and  it depends on the amount plasma between the radio source and us.  So the DM of radio signals lets us determine how much ionized gas there is in the galaxy.

When there is a magnetic field present in the plasma the radio pulses not only spread by frequency, but their polarization rotates by frequency.  Just like the corn syrup, a magnetized plasma rotates polarized radio signals by frequency. This twisting of the polarization is known as the rotation measure (RM).  The RM of a radio pulse depends on the amount of plasma between us and the radio source (which we know through the DM) and the strength of the magnetic field. By measuring the DM and RM of a polarized pulsar signal, we can determine the strength of the magnetic field.

There are lots of pulsars through our Milky Way Galaxy.  By observing the DM and RM of these pulsars we can create a map of the magnetic field within our galaxy.  The result can be seen in the figure below.  You can see the magnetic field isn’t uniform, but it has strong regions and weak regions.

So even though we can’t see the galactic magnetic field directly, we know what it looks like.  Just by using cosmic corn syrup and polarized light.

# Rock On

You might think that meteorites are all very similar, simply being rocks from space, but actually meteorites are quite varied.  Their physical and chemical makeups tell a distinct story about their origin and history, and we can even identify some as originating from Mars or the Moon.  Meteorites are divided into four broad categories: chondrites, achondrites, pallasites and iron meteorites.

Chondrites are by far the most common type of meteorites.  They were formed in the early solar system when dust grains began coalescing into small asteroids.  Unlike other meteorites, they haven’t experienced enough heat to cause melting, and they haven’t been part of a large enough asteroid for differentiation to occur.  Because of this, they provide the best source to understanding the conditions of the early universe.

Achondrites are chondrites that have reached high enough temperatures to melt, or at least differentiate significantly.  The process by which they formed is similar to the way basalt forms on Earth, so they can be difficult to distinguish from terrestrial rocks (unlike chondrites, which are quite distinct).  Asteroids from Mars and the Moon are achondrites.

Iron meteorites are pretty much what they sound like.  While chondrites and achondrites are stony in nature, iron meteorites are dense iron-nickel.  They originated from the cores of large asteroids or planetesimals that were shattered by a collision.  Although they only make up about 5% of all meteorites, they are very distinctive, and are typically what people think of when they think of meteorites.

Pallasites are perhaps the most unusual type of meteorites.  They are noted from their mottled mixture of olivine crystals embedded in iron-nickel.  You can see a slice of one in the image below.

We aren’t entirely sure how they formed.  One idea has been that, like iron meteorites, pallasites come from the destruction of large asteroids.  But rather than coming from their interior, they came from the boundary region of their iron core and stony mantle.  Thus the mixture of (stony) olivine and iron-nickel.

Another idea is that they formed when large meteors collided with larger asteroids.  Such an impact would create a mix of iron and stony material we see in pallasites.  Recently a paper in Science (paywalled, but one of the authors has a free pdf here http://goo.gl/t2yzUu) has found evidence to support the collision theory.

The team looked at the metallic portions of several pallasites using a superconducting quantum interference device (SQUID), capable of making very sensitive measurements of magnetic fields.  What they found is that the metal in these pallasites were once highly magnetized.

This is very interesting, because if pallasites had formed in the core/mantle boundary of a planetesimal, the iron wouldn’t have gotten cool enough to become magnetized before the planetesimal was shattered.  If, however, pallasites were formed by an impact of a meteorite with a planetesimal, then the iron could cool while still in the presence of the planetesimal’s magnetic field.  So the fact that the metallic regions of a pallasite were magnetized would seem to rule out the core/mantle idea.

This would also imply that planetesimals in the early solar system likely had strong magnetic fields caused by a magnetic dynamo, similar to the way Earth still has a strong magnetic field.  Modern asteroids don’t have strong magnetic fields, but there is some evidence that the asteroid Vesta once did.

But that’s a story for another time.

Image: A slice of pallasite.  Doug Bowman (http://goo.gl/UL2rX)

Paper: Tarduno JA, Cottrell RD, Nimmo F, et al. Evidence for a dynamo in the main group pallasite parent body. Science. 2012;338(6109):939-42.

# Color and Light

I’ve already discussed polarization of light and how it can be used to determine the position of the Sun, even on a cloudy day.  This works because sunlight is polarized when it scatters in the air.

There are lots of materials that can affect the orientation of light.  One of the more interesting effects can be seen with corn syrup or other kind of sugar water. Because sugar molecules have a handedness to them, polarized light changes orientation as it moves through the corn syrup.  The polarization twists as it moves through the corn syrup.  However the rate at which the light twists depends on the color of the light.  This means the polarization is spread apart by color, similar to how a prism spreads out the colors of light by direction.  You can’t see this effect normally, but if you look at the light through a polarizing filter you can see the colors vary by orientation, as shown in the video below.

If you did a careful measurement of the spread of color, you could measure how much corn syrup the light has passed through.  The more corn syrup it passes through, the greater the spread of colors.

We can use the same effect effect in radio astronomy, but I’ll talk about that later.

# Variable Variables

Yesterday I talked about how Henrietta Leavitt discovered the period-luminosity relation for Cepheid variable stars, and how that allows us to determine the distances of galaxies.  You can see this relation in the image below, which plots the brightness of several Cepheid variables versus their period.  As you can see, there is a simple relation between them.

Since Leavitt’s discovery of this relation in the early 1900s, Cepheid variables have been used to determine cosmic distances.  It is one of the “standard candles” that we use to determine things like the expansion of the universe.  Of course you might notice that the stars don’t follow this relation exactly.  Some are a bit brighter than expected, and some a bit dimmer.  This means when we observe a Cepheid variable in a distant galaxy there’s a bit of uncertainty to the galaxy’s distance.

This uncertainty limits our ability to understand things like dark energy.  Our understanding of dark energy depends on an accurate determination of what is known as the Hubble constant.  One way we can determine this constant is through the cosmic microwave background, while another uses Cepheid variables and supernovae observations.  These two methods agree to within 5%, but that 5% difference is too large to distinguish between different models for dark energy.  So our observations are precise enough to say that dark energy is real, but not precise enough to determine its exact nature.

We now know that the period-luminosity relation for Cepheid variables is a bit more complex than originally thought.  While the general relation works really well, there are small variations that depend on certain characteristics of a star.  One of these characteristics is its metallicity.  As I’ve written about before  (http://goo.gl/Zjo6tv), the metallicity of a star is a measure of how much metal (which in astronomy means anything other than hydrogen and helium) a star contains.

Recently, a paper in Astrophysical Journal (http://goo.gl/xkJ9kY) looked at the dependence of Cepheid variables on their metallicity.  To study the metallicity dependence on the variables, the team couldn’t use the period-luminosity relation to determine galactic distances, so they used a different method known as Tip of the Red Giant Branch (TRGB).  The TRGB method looks at the brightness of the brightest red giants in a galaxy at infrared wavelengths.  It turns out that bright red giant stars have an upper brightness limit at infrared wavelengths, so it is an alternative to Cepheids for measuring galactic distances.

The team compared the TRGB distances for several galaxies with the Cepheid distances, and then looked at the metallicity of the Cepheids in comparison.  What they found is that the brightness of a Cepheid variable decreases slightly with higher metallicity.  Knowing this will help us more accurately determine galactic distances, which may help us understand more about dark energy.

Image:  NASA/JPL-Caltech/Carnegie (http://goo.gl/npgP6)

Paper:  Sakai S, Ferrarese L, Kennicutt, Jr. RC, Saha A. The Effect of Metallicity on Cepheid‐based Distances. ApJ. 2004;608(1):42-61.