Our Sun is adrift among the stars. As our home star moves through the galaxy, so to do the other stars. This means that the apparent positions of the stars change over time. Because of the great distances of stars this shift is minuscule and difficult to measure. For years we have only been able to measure the motion of a few close stars. But that’s beginning to change. 

From 1989 to 1993, the Hipparcos spacecraft made high precision measurements of more than 100,000 stars, cataloging their positions and distances, as well as a measure of their proper motion across the sky. This data was compiled in the Hipparcos Catalog in 1997. A less precise catalog of more than 2 million stars, known as the Tycho catalog was also published. While the accuracy of the Hipparcos data for some stellar clusters has been debated, it has proved to be quite accurate for most stars.

Then in 2013 the Gaia spacecraft was launched, with the goal of measuring the position and motion of more than a billion stars. In 2014 the Gaia team published its initial data, including measurements of more than 2 million Hipparcos stars. This gave us the opportunity to see just how far these stars had moved over the course of 25 years.

Fortunately the data from both Hipparcos and Gaia are freely available. So the United States Naval Observatory (USNO) analyzed the data to calculate both the location and motion of these 2 million stars, giving the most accurate proper motions thus far. They then went one step further, and compared the positions of these 2 millions stars with the positions of million that had been measured by the USNO in 1998 and 2004, and were able to determine the proper motions of millions more stars. A new catalog containing this data will be released soon.

We’ve long known the stars moved over time, but we are now able to determine this motion accurately for millions of stars. This will help us understand not only the dynamics and evolution of our Milky Way galaxy, it could also provide clues to things like dark matter.

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This is an image of a black hole. Actually, the black hole is in the center and can’t be directly seen. What we see are two large jets streaming away from the black hole. When matter accretes around a black hole, some of it is captured, but some of it is pushed away into long jets. In the case of this image, the jets stream out for about 1.5 million light years. It’s known as Hercules A, and it’s one of the brightest radio galaxies.

The 3C 348 galaxy in the visible range.

The purple lobes in this image show radio emissions, not visible light. It was taken using the Very Large Array (VLA). Since the jets are made of plasma, they are bright at radio wavelengths, but not very bright at optical wavelengths. The supermassive black hole driving Hercules A is at the center of a rather bland elliptical galaxy known as 3C 348. The image above combines an image of the galaxy in the visible spectrum with the radio image of the lobes.

Since the VLA is an array of radio antennas, it can produce detailed radio images. We can see, for example how the jets stream out in a narrow beam at nearly the speed of light, eventually slowing and interacting to create wide turbulent lobes. By studying images such as this we can better understand how high energy plasma interacts in space.

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Astronomers have discovered what seems to be the most distant galaxy yet discovered. The galaxy known as EGS-zs8-1 has a redshift of z = 7.73, trumping the previous record for a galaxy at z = 7.51. Several articles are giving a distance of this galaxy as about 13 billion light years away, but that’s not really an accurate measure, and it’s part of the reason we usually talk about redshift rather than distance.

The redshift of a galaxy is typically given by a number known as z, which is the difference between the observed wavelength of a particular emission line and the standard wavelength as measured here on Earth, divided by the standard wavelength. In this way, an object with no redshift would have a z = 0, and the greater the redshift the greater the z. The nice thing about redshift is that it’s purely an observational result.

From this z number we can infer a distance based upon Hubble’s law, which states that the greater the redshift the more distant the galaxy. But since Hubble’s law also means the universe is expanding, we have to be careful about which distance we mean. Do we want the distance of the galaxy when the light left it? The distance the light traveled to get to us? Or the distance of the galaxy now? These are all different.

With a z of about 7.73, that means the galaxy was about 3.4 billion light years away when the light left it. Because of the expansion of the space between us and the galaxy, it took the light about 13 billion years to reach us. But since then the galaxy has moved away from us at an ever greater rate, so it is now about 29.5 billion light years away from us.

That last distance might seem odd given that the universe is only 13.7 billion years old, but it’s important to keep in mind  that the galaxy hasn’t traveled 26 billion light years in 13 billion years. In fact any actual motion away from us through space would be in addition to the 26 billion light years. That last distance is entirely due to the expansion of space between us. In an expanding universe, distance is always changing.

Which is why astronomers tend to stick with redshift.

Paper: P. A. Oesch et al. A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z=7.730 using Keck/MOSFIRE. ApJ 804 L30 (2015)

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One of the most basic skills in astronomy is know how to find objects in the night sky. That means you need a way to navigate the sky. The simplest way is known as altitude and azimuth. Starting at due north, rotate clockwise along the horizon until you are directly under the star you want, then move above the horizon to reach your star. It is a simple coordinate system, since it is just so many degrees clockwise (azimuth) and so many degrees upward (altitude).

The downside of this coordinate system is that the Earth’s rotation means the positions of the stars are constantly changing relative to your position. This means a star’s altitude and azimuth change moment by moment. For this reason, astronomers often use a different coordinate system, known as declination and right ascension. This system is based upon the latitude and longitude of Earth.

If you extend the equator outward to the sky, it traces a celestial equator. Extending the Earth’s rotational axis into the sky gives the north and south celestial poles. Tracing angles from the celestial equator to the celestial poles gives you the declination, just as it would give you your latitude on Earth. The celestial longitude is known as right ascension, and instead of being measured in degrees it is measured in hours (24) minutes and seconds. This is to account for the rotation of the Earth. The movement of the stars can be used as a measurement of time, known as sidereal time.

Credit: NASA/JPL-Caltech/R. Hurt

Declination and right ascension have the advantage that they don’t change moment by moment for a particular star. If your telescope is mounted to align with declination and right ascension, then you only need to account for the sidereal time to track a star. Of course the Earth not only rotates, it also precesses (wobbles slightly) over the years, so orientation of these coordinates do slowly change over time. For this reason astronomers usually pick a standard date to fix the coordinates. The most popular is known as J2000, which defines 1 January 2000 at noon as the standard.

While this system is useful for the positions of stars and other objects in our local neighborhood, it isn’t as useful for locating objects on a cosmic scale. For this we use the galactic coordinate system seen above. For this coordinate, the “equator” is defined as the plane of the Milky Way, with zero degrees longitude defined as the direction of galactic center. The Sun is the center of this coordinate system.

In each of these coordinate systems we are the center of the universe, whether it is us personally, the center of the Earth, or the location of our Sun. Of course that makes sense, since the only view of the universe we have is ours.

The post Where Are You? appeared first on One Universe at a Time.

While we’re quite familiar with our side of the Milky Way galaxy, the far side of our galaxy is still a bit of a mystery.  The reason for this is that the center of the Milky Way is filled with gas, dust and stars, so it is very difficult to see the other side of our galaxy.  The central region is so cluttered with material that it sometimes referred to as the Zone of Avoidance, since we have to exclude that region from observations beyond our galaxy.  

Map of neutral hydrogen in our galaxy.

There are some things we can observe about the far side.  While the central region blocks most of the visible light from the far side, it doesn’t block as much of the radio waves, infrared and x-ray wavelengths.  So we have been able to make some broad observations.  We know, for example, that our galaxy really is a barred spiral galaxy because we can map the distribution of hydrogen throughout most of the far side.  We can also see some of the spiral arms that exist on the far side.

Another way to observe to far side is to focus on the outer region where material flares out from the central plane of the Milky Way.  It is sometimes referred to as the flared edge of the galaxy. We’ve mapped gas and dust in that region, but now a new paper in Nature has announced the observation of Cepheid variable stars in this region.

Cepheid variables are a particular type of star that oscillate in brightness proportional to its absolute magnitude. They are extremely useful because you can use their observed variation to determine their actual distance.  So by observing these Cepheid variable stars, the authors could show conclusively that they are, in fact, within the flared region of the far side.  This is important because observing their location and motion can help us better determine the overall motion of stars in our galaxy.

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As part of an “Ask an Astrophysicist” project, I was asked whether I’m in the Dark Matter or MOND camp.  Before answering that, let’s talk about about what these two camps are.

Both MOND and dark matter were introduced to address a problem with the way galaxies behave.  At a basic level, the stars in a galaxy such as ours orbit the galactic center in roughly circular orbits.  The speed of a star in its orbit should be governed by Newton’s law of gravity.  So, using Newton’s gravitational theory, we can predict a star’s speed given its distance from the center and the distribution of matter in our galaxy.  The dotted line in the figure below gives the result of that prediction.  We can also measure the speeds of stars in our galaxy and plot them as a function of their distance from the center.  The result of those measurements are given by the dots on the figure below.

As you can see, Newton’s beautiful theory doesn’t agree with the experimental data.  The two are not even close. We see the same type of discrepancy in other galaxies, so it isn’t just that our galaxy is weird.  Our theory is somehow very, very wrong.

One solution to this problem is that somehow we haven’t accounted for all the mass in our galaxy.  We can measure the mass in the galaxy by looking at all the stars and nebulae in it.  This is relatively easy because they give off light.  Of course there is also dust and gas in the galaxy that doesn’t give off light.  This “dark” material is harder to measure, so it is quite possible that we’ve underestimated the amount of matter in a galaxy.  The problem with this idea is that our prediction isn’t just a little off, it’s way off.  To make our theory agree with observation, most of the matter in a galaxy would have to be “dark.”  If there were that much dust and gas in the galaxy we would see it, and we don’t.  So we might be a little off in our measurement of “dark” material, but not off that much.  The missing mass can’t be regular gas and dust.

Perhaps the missing matter could be something like neutrinos, which don’t interact with light and would therefore be invisible?  Nope.  Neutrinos move too fast to be held by the regular matter of the galaxy, so that can’t be it.  Maybe there’s some still unobserved type of particle similar to neutrinos, but with more mass so they could be held by regular matter.  We’ll call them WIMPs, or Weakly Interacting Massive Particles.  Maybe there are very tiny black holes swarming the galaxy, or some other strange mass.

It seems like we’re really reaching here, so let’s just lump all these possible solutions into a broad term we’ll call “dark matter.”  Given our experimental data, dark matter must have two properties:  1) it can’t interact with light very much, otherwise we would be able to see it directly.  2) it must have mass and interact with regular matter gravitationally.  Any candidate for dark matter must have these properties to agree both with observation and Newton’s theory of gravity.  There’s one more fact we know from astronomical observations, and that is if dark matter exists, it must make up about 95% of the mass in our galaxy.

To make a long story short, to make Newtonian gravity match our observational data, we have to introduce a new type of invisible material, and this invisible stuff must make up the majority of our galaxy’s mass.  There is, of course, an alternative.  Our theory could be wrong.  This seems much more reasonable. After all, we know that Newtonian gravity doesn’t work for large masses and high speeds.  Even the orbit of Mercury deviates a bit from Newton’s predictions, so we already know it’s “wrong.”  It could also be wrong on galactic scales.  If it is, then maybe we don’t need to make up invisible undetected “dark matter.”

Enter MOND, or MOdified Newtonian Dynamics.  MOND proposes a correction to Newton’s Laws of motion by noting that for stars in a galaxy the force of gravity is very tiny.  Newton’s Laws of motion state that the rate at which an object speeds up or slows down (its acceleration) is directly proportional to the force of gravity applied to the object.  MOND proposes that the force of gravity is proportional to a function of acceleration.  On the scales we normally see on Earth, this function is about equal to the acceleration itself, so motion would be just as Newton predicts, but for really small forces the function levels off to a small constant (about 10 trillionths of gravity on Earth).  By making this modification to gravity, we have a theory that agrees with observation.

There are two big downsides to MOND however.  The first is that MOND violates conservation of momentum, which is one of the fundamental principles of physics.  So if MOND is correct, then it isn’t just gravity but most of physics that has to be modified.  The second is that, unlike general relativity, MOND is not derived from a fundamental theoretical concept, but it is introduced specifically to make predictions agree with experiment.  An analogy would is that MOND is much like epicycles added to the beautiful theory of circular orbits, rather than Kepler’s revolutionary proposal that planets move in elliptical orbits.

It seems we have two pretty pathetic alternatives:  propose that our galaxy is mostly made up some kind of yet undetected, unknown, invisible stuff, or propose a kludgy new theory of gravity that would require us to rewrite most of physics.  Neither one seems particularly appealing.  The only good news in all this is that our observational data is solid.  We know what is happening, even if we don’t know why.

Having said all that, which of these camps am I in?  I’m in the dark matter camp. To learn why, you’ll have to wait for my next post.

In the meantime feel free to ask more questions.

The post Beautiful Theory, Ugly Data appeared first on One Universe at a Time.

It’s time to solve our quasar mystery today.  If you’ve been following my posts you know that early observed quasars were both visually bright and “loud” in the radio spectrum.  Later, “quiet” radio quasars were also observed.  This was interesting because it meant there wasn’t a simple energy source just producing energy randomly, otherwise brightness and radio loudness should go hand in hand.

Then in another post I wrote about how the brightness of quasars can vary a bit over time, and how that tells us something about the size of these things.  What we found was that although quasars are as bright as an entire galaxy, the source of their energy is a volume only a few light years across, which is tiny given its intensity.  That means we can rule out things like supernovae and nuclear fusion as an energy source.

We’re now ready for our third clue.  This comes from the observation that there are other bright objects out in the universe.  One of these objects is known as a radio galaxy.  These are radio loud galaxies that aren’t abnormally bright in the visual spectrum.  Quasars are visually bright, and sometimes radio loud,  sometimes radio quiet.  Then there are other objects such as blazars, which are very, very bright in a wide range of spectra.  While these objects initially appear quite different, they all stem from galaxies, and they all have an energy source with a small (on the galactic scale) volume.

Of course we do know of one very powerful energy source with a small volume: black holes.  As I’ve written earlier, we have a great deal of evidence that a supermassive black hole exists in the center of our galaxy, and in most other galaxies.  So it seems reasonable that a black hole could be the source of these very bright energy sources.

But how could we prove this to be the case?  The key is recognizing that quasars, radio galaxies and blazars are all related.  When a supermassive black hole eats nearby gas, dust and stars, it is known as an active galactic nucleus (AGN).  These active black holes have superheated material swirling around them in a circular disk, known as an accretion disk.  They also have jets of gas and dust shooting out from their polar regions at a large fraction of the speed of light.  The jets are formed by a fairly complex process, but basically some of the material from the accretion disk gets so much energy from the heat, magnetic fields, and such that instead of falling into the black hole it shoots away and escapes.  You can see a basic model of all this in the figure below.

The material in the accretion disk is swirling around the black hole in a tight circle, which means all the charged particles in the disk are being accelerated.  When you accelerate charges, they give off radio waves along their direction of motion, known as synchrotron radiation.  So if you were to look at an AGN along the edge of its accretion disk, you wouldn’t see much of the disk in the visible spectrum, but you would see a great deal of intense radio energy.  In other words you would see a radio galaxy.

If you look at an AGN from a bit of an angle, you would see intense visible light from the superheated accretion disk, and depending on your angle you might (or might not) also see intense radio waves, which makes it a quasar.  Finally if you viewed an AGN down the barrel of a jet, you would see the very intense energy source of a blazar.

So now we know that quasars are active black holes in the centers of galaxies.  Since we see quasars at very high redshifts, we know that galactic black holes existed even in the earliest galaxies.  We also know that quasars are part of a diverse family of active black holes.

It all depends on your point of view.

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Even with the presence of dark matter, the stars closer to the center of a galaxy orbit faster than the stars farther away.  This raises a bit of a mystery. Many galaxies (including our own) have a spiral shape to them.  Since the central stars of a galaxy move faster than stars near the edge, the  spiral should gradually twist tighter and tighter until the spiral arms all blended together in a uniform disk.

If that were true, then we would expect to see very few spiral galaxies, and instead see lots of galaxies that look like uniform disks.  This isn’t what we see, in fact spiral galaxies are quite common.  There must be some mechanism that maintains a galaxy’s spiral shape.

This mechanism turns out to be an effect known as a density wave.  We also have such density waves here on Earth.  We call them traffic jams.  If you’ve ever been in a trafic jam you’ve noticed that while you slowly make your way through it the overall traffic pattern remains the same.  Usually when you’re caught in a traffic jam you eventually find the source (construction, minor accident, etc.) but sometimes you enter a traffic jam and go slowly through it without ever seeing a cause.  At some time earlier something started it, but now there is just the traffic.  The jam itself is now the cause of the jam.

A similar effect occurs with spiral arms in a galaxy.  Individual stars are not locked in a particular spiral arm, rather they move around the galactic center passing through one spiral arm after another.  But as a star moves toward a spiral arm, that arm’s mass gives it a little gravitational boost to edge them into the arm.  When a star begins to leave an arm, the gravitational pull of the arm slows it down just a bit.  As a result stars tend to cluster into the traffic jam of spiral arms, and the arm patterns sustain themselves even while individual stars move through them.

You can see this effect in the animation below.  The density waves of the spiral arms keep their shape, but you can see stars move in and out of them.

So the next time you’re caught in traffic, don’t get frustrated.  Take heart in the fact that you are participating in Earth’s rendition of the dance of the stars.

The post Traffic Jam appeared first on One Universe at a Time.

A recurring theme in computational astrophysics (and physics in general) is the concept of chaos.  While many aspects of the universe are ordered and predictable, other aspects are quite chaotic.  Often things lie at a fine line between the two.

A good example of this can be seen in galactic motion.  On the one hand things are quite regular.  At a broad level stars move in a generally circular path around the galactic center.  This is analogous to our solar system, where planets move in (roughly) circular orbits around the sun.  This makes it easy to make a rough model of our galaxy as a fairly uniform disk of stars.

Of course when we look more closely things are not so simple.  For one our galaxy is not a uniform disk of stars, but rather lies mainly in spiral arms.  (Why this is the case is a topic for a future post.)  Then there is the motion of individual stars and star clusters themselves.

It turns out the motion of stars can be approximately described by a simple differential equation called the Henon-Heiles equation.  Unfortunately the solution to this equation is chaotic.  In other words the solution is very dependent on a star’s initial velocity and position.  Determining precise measurements of a star’s position and velocity can be quite a challenge.  So usually we have to look at general properties of the solution rather than finding a particular solution.

The good news is that the Henon-Heiles equation as long been studied by mathematicians, so we actually know a great deal about it.  One common way to look at general solutions is to plot what is known as a Poincare map of solutions.  I’ve plotted one for the Henon-Heiles here.  A Poincare map help you determine certain aspects of a star’s motion.  For example, in the figure below you can see that the range is bounded to a particular region.  So we know the star won’t just wander off.  We can also see regions where the motion tends to cluster.  So even though we don’t know the exact motion of a star, we know its general motion.

Problems like this can’t be solved well analytically, so it is an area where computational methods shine.

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Yesterday I mentioned how we could use the motion of hydrogen gas in our galaxy to determine the distribution of mass in our galaxy.  Since that calculation on is based on gravity, what we really measure is the total amount of gravitational mass there is in our galaxy.  When we do this we find something very strange: our galaxy (and other galaxies) have more mass than they should.

There are other ways we can measure the amount of matter in our galaxy.  In particular we can measure the amount of stars, gas and dust we can see, which gives us a good estimate.  However when we do that we find the mass we measure gravitationally is much higher than the directly observed mass.  If we subtract the observed mass from the total gravitational mass, what we have is a different kind of mass.  A dark, unseen kind known as dark matter.  In the figure below I’ve plotted a comparison of the the amount of regular visible matter vs the amount of dark matter in our galaxy.

We’re not yet sure what dark matter is, though we have some ideas.  What we do know is that it has gravitational mass and it is not any kind of regular matter such as gas, dust, planets or stars.  Dark matter is something different, and there is about four times more dark matter than regular matter in the universe.  What dark matter actually is remains to be seen.

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Previously I noted how the ionized gas between stars can slow down radio signals from pulsars and how the amount of slowdown depends on the frequency.  Basically, higher frequencies travel a bit faster than lower frequencies.  As a result, a radio pulse from a pulsar is widened in frequency by this ionized gas.  The amount of pulse widening is known as the dispersion measure (DM).  Since this pulse broadening only occurs when radio waves travel through ionized gas, you can use the dispersion measure to determine how much interstellar gas is between you and the pulsar.  The bigger the DM, the more interstellar plasma.

We have dispersion measures for about 2,000 pulsars in our galaxy, so we can use them to make a map of how interstellar gas is distributed.  The result is plotted below.  In the graph I’ve used galactic coordinates, which means zero degrees latitude is the plane of the Milky Way, and zero degrees longitude lies in the direction of galactic center.  The result is pretty much what you would expect.  Most of the interstellar gas lies in the same region as the stars and dust in our galaxy.

Plasma density in the Milky Way

While the result is just what we would expect, the nice thing about this type of observation is that proves that there isn’t a halo of transparent interstellar gas surrounding our galaxy.  So when we measure the odd behavior of galactic rotation we know we can’t use interstellar gas as an explanation.  Instead we have to look to dark matter.

Even expected answers can prove useful in unexpected ways.

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