This image might not look like much, but it’s actually an amazing step forward for solar astronomy. It captures the image of a large sunspot not in visible light, but in microwaves. 

The difference is important because different wavelengths of light are emitted by different layers of the Sun’s surface. The visible light we see everyday mostly originates from the photosphere, which is the lowest or deepest part of the Sun that we can directly observe. Microwaves are emitted by the chromosphere, which is the next layer above the photosphere. The chromosphere is much less dense than the photosphere, and has lots of interesting phenomena such as filaments and prominences, formed by a complex dance of thermodynamics, magnetic fields, and plasma. The chromosphere is also unusual because it’s actually hotter than the photosphere. You might expect the Sun is hottest at its interior, and the farther out you go, the cooler things become. That’s true for the photosphere, but not the chromosphere. The chromosphere is coolest near the photosphere with a temperature of about 4,000 K, but heats up as you move outward, reaching a temperature of 25,000 K. Just how the chromosphere gets so hot remains a bit of a mystery.

The mysterious heating of the chromosphere is also what typically allows us to see it. Although it’s very diffuse, parts of it emit visible light. We typically have to look at the edges of the Sun to see it (or during a solar eclipse) since the brilliant light of the photosphere is so bright. But in microwaves the chromosphere is brighter than the photosphere. The problem has always been that microwaves are observed with radio telescopes, which typically have a low resolution. This new image is from the ALMA observatory, which can capture microwave images with a resolution rivaling that of Hubble. These new images have enough detail that we can start to see some of the complex behavior of the chromosphere.

And that may help us understand just how the chromosphere gets so hot.

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Sunspots are one way we can track the activity of the Sun. There have been fewer sunspots than usual in recent years, and that may point toward an historic solar minimum. 

Sunspots are dark patches that occasionally appear on the surface of the Sun. They aren’t actually dark. If you could see a sunspot by itself it would appear bright red, but since sunspots are about a quarter as bright as the rest of the Sun, they appear as dark regions. Since the early 1600s astronomers have counted the number of sunspots over the years, and we’ve noticed a few patterns. One is that sunspot counts vary between maximum and minimum over an 11-year period. There are other patterns as well, such as the Gleisberg Cycle, which lasts 80 – 90 years.

Directly recorded sunspot counts over the years. Credit: Robert A. Rohde (CC BY-SA 3.0)

There are times when the pattern seems to break down, and the Sun can enter into an extended period of little sunspot activity. The most famous is the Maunder minimum of the 1600s. While we don’t have direct sunspot counts before the early 1600s, we can look at the levels of carbon-14 as measured from tree rings. Since carbon-14 levels have a good correlation to sunspot counts, we can get a handle on a much longer history of sunspots. It turns out there have been other periods of minimum activity, such as the Wolf minimum of the 1300s. In general, the sunspot activity of the Sun in recent centuries is somewhat higher than most, except for a period during the middle ages known as the Medieval maximum.

For the past couple of cycles the sunspot maximums have been lower than usual. The pattern is similar to the early stages of the Dalton minimum in the early 1800s, which has raised the question of whether we are entering a period of reduced sunspot activity. This may also have some effect on global temperatures. The Dalton minimum saw a brief period of colder temperatures, and the Maunder minimum was marked by the “little ice age” where Europe and North America experienced a colder period. It should be stressed that connections between sunspot activity and global temperatures is still not clear. The Dalton cold period for example, saw the explosion of Mount Tambora, which would also contribute to cooler temperatures.

What is clear is that periods of minimal sunspot activity are notoriously difficult to predict. While the pattern of the past few cycles has similarities with the early Dalton minimum, it could also be a small fluke before a return to cycles as normal.

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Our Sun emits solar flares on a regular basis, but what would happen if it released a flare 100,000 times more powerful than the largest flare in recorded history? Could it destroy our power grid, irreversibly change life on Earth, or even strip the atmosphere from our planet? Let the fear-mongering begin! 

Superflares are in the news, and with it comes wild claims about their possible threat to human civilization. As with most hyped stories, they focus on just how powerful one could be, and how it might disrupt human civilization rather than the actual science, which is unfortunate because the science is pretty interesting.  It’s based on recent work that looked at the flare activity of more than 5,000 Sun-like stars. What they found was that stars where superflares occur tend to have stronger magnetic fields than stars where superflares are less likely. This correlation between superflares and magnetic field strength suggests that superflares are produced by a mechanism similar to regular solar flares.

From solar observations we know that solar flares occur when magnetic field lines near the Sun’s surface “snap” into realignment. The Sun actually rotates differentially, meaning that its equator rotates a bit faster than its poles. As a result, over time the magnetic field at its surface wraps around the Sun a bit. When the magnetic field is twisted up, it can realign quickly, producing a solar flare. For stars with stronger magnetic fields, the resulting realignment has a tendency to be more severe, and thus large solar flares (superflares) are more likely. This work shows that superflares are not a different phenomena, but are solar flares on a larger scale. They also found that superflares can occur on stars with magnetic field strengths similar to the Sun. While superflares are much more likely on stars with strong magnetic fields, the team found that about 10% of the superflares they observed occurred on stars with magnetic fields on par or weaker than that of our Sun. From their observations, they estimate that a superflare could occur on the Sun about once every thousand years or so.

That said, if such a superflare were to occur today it wouldn’t end civilization as we know it. There is some evidence in tree ring data that small superflares might have occurred in 775 and 993 AD, with little effect on human society. In our modern world solar flares do pose some risk to satellites and our electrical power grid, but we have ways of limiting their effect. In a bad-case scenario we might have some infrastructure to rebuild, but it wouldn’t be an extinction-level event.

So there’s no need to worry. Just keep calm and science on.

Paper: Christoffer Karoff, et al. Observational evidence for enhanced magnetic activity of superflare stars. Nature Communications 7, Article number: 11058, doi:10.1038/ncomms11058 (2016)

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The Sun is getting hotter. Not significantly on the scale of human lifetimes, and not even enough to account for global warming. But on a geological scale it’s happening, and it has dire consequences for life on Earth. 

The Sun’s changing luminosity, radius and temperature.

The Sun gets hotter over time because of the gradual transformation of elements over time in the Sun’s core. As hydrogen is fused into helium, the core becomes more dense, and thus gravity squeezes it a bit more and the core gets hotter. As the core temperature continues to rise, hydrogen fusion (p-p chain) becomes more efficient, and a secondary fusion reaction known as the CNO cycle also starts to kick in. This heats the core even further. As a result the outer layer of the Sun swells, making the Sun slightly larger. But it also gets brighter, so the end result is that the Sun produces more energy as it ages.

For most of its lifetime this increase is pretty gradual. Over a hundred million years the luminosity of the Sun will increase by about 1%. But over billions of years this is significant. When life first appeared on Earth about 3.5 billion years ago, the Sun was about 75% as bright as it is now. In a couple billion years it will be about 20% brighter than it is now.

The changing habitable zone of the Sun.

Because of Sun’s increase in energy over time, the habitable zone of the solar system is gradually thinning and getting further from the Sun. A few billion years ago both Earth and Mars were in a reasonable zone of habitability, but now only Earth remains. In another billion years or so even the Earth will leave the habitable zone and will likely be too hot to sustain life. This has interesting consequences for the possibility of life on other worlds. While many young stars may have planets conducive to life, as the stars age the range changes. The universe may be littered with planets that once had life, but are now barren. Planets where life survives long enough to develop civilization and technology may be rare, even if life is common throughout the universe.

But of course the days are numbered even for civilizations like ours. Either we become extinct like so many species before us, or we adapt and change, eventually leaving our Sun for greener pastures. To do otherwise is to face our end of days.

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Yesterday’s post about Earth’s changing mass raised similar questions about the Sun. In the Sun’s case we know that it’s losing mass, and at a rate fast enough that it’s forced us to change the way we measure astronomical distances.

The Sun loses mass in two major ways. The first is through solar wind. The surface of the Sun is hot enough that electrons and protons boil off its surface and stream away from the Sun, generating a “wind” of ionized particles. When those particles strike Earth’s upper atmosphere they can produce aurora. The solar wind varies a bit in intensity, but from satellite observations we know that the Sun loses about 1.5 million tonnes of material each second due to solar wind.

The second way the Sun loses mass is through nuclear fusion. The Sun fuses hydrogen into helium in its core, producing its life-giving glow over billions of years. The production of helium transforms some of the hydrogen’s mass into energy, which radiates away from the Sun in the form of light and neutrinos. By observing just how much energy the Sun radiates, and using Einstein’s equation relating mass and energy, we find the Sun loses about 4 million tonnes of mass each second due to fusion.

So the Sun loses about 5.5 million tonnes of mass every second, or about 174 trillion tonnes of mass every year. That’s a lot of mass, but compared to the total mass of the Sun it’s negligible. The Sun will keep shining for another 5 billion years, and by that time it will have lost only about 0.034% of its current mass.

While the amount of mass loss is negligible, it isn’t zero, and it has an effect on Earth’s orbit. As the Sun loses mass its gravitational pull on the Earth weakens over time. As a result, Earth is receding slightly from the Sun. Because of solar mass loss the Earth’s distance from the Sun increases by about 1.6 centimeters per year. In astronomy, one of the ways we measure distance is through the astronomical unit, which has traditionally been defined as the distance from the Sun to the Earth. For most of astronomical history the changing distance of Earth was too small to consider, and so the astronomical unit could be considered a constant. But over time our measurement of this distance has become astoundingly accurate, and currently has a precision of about 3 parts per billion. This is accurate enough to observe the gradual increase in distance. So in 2012 the astronomical unit was defined as fixed constant. As a result the Earth is slightly more than 1 astronomical unit away from the Sun.

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There’s news of a large coronal hole on the Sun, which just goes to show how good astronomers are at making the mundane sound terrifying. In turns out coronal holes are neither dangerous nor uncommon, but what exactly are they?

The corona is an outer region of diffuse plasma surrounding the Sun. At a basic level, the Sun can be grouped into three parts: the interior, the photosphere, and the corona. If you you think of the photosphere as the surface of the Sun, then you can think of the corona as the Sun’s upper atmosphere. It was first discovered due to its halo or “crown” effect during a total solar eclipse, hence it’s name. When we finally had space-based x-ray telescopes in the 1970s it was noticed that there were sometimes dark regions in the corona, similar to the way sunspots appear in the photosphere. These “holes” in the corona seemed to be caused by a lack of hot ionized material in the region.

We now know that these coronal holes are due to an interaction between the charged plasma of the corona and the Sun’s magnetic field. Typically the solar magnetic field loops back on itself near the Sun’s surface. The plasma tends to follow along magnetic field lines, so the material of the corona tend to stay trapped near the Sun’s surface. But sometimes the magnetic field tends to weaken in some areas, and the field lines tend to be directed away from the Sun. When that happens the material of the corona is pushed outward by solar wind, creating a coronal hole.

Like sunspots, coronal holes tend to follow an 11 year cycle. If a coronal hole happens to face Earth we generally get more aurora, but that’s about it. So the appearance of a large coronal hole on the Sun is somewhat interesting for astronomers, but it’s nothing we haven’t seen before.

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Quantum theory is often viewed as a strange and mysterious model where objects behave in illogical ways. While it’s true that quantum objects behave in ways that are counterintuitive, we actually understand the behavior quite well. In fact, many of these strange behaviors are used in modern astronomy. Take, for example, the quantization of magnetic moments.

Most atoms have a small magnetic field. This magnetic field can be approximated as a small magnet, just as the Earth’s magnetic field is sometimes treated as a magnetic. The strength of that imaginary magnet is given by the magnetic moment of the atom. With this in mind, the orientation of an atom’s magnetic field can be represented by the orientation of the magnet.

The basic setup of a Stern-Gerlach experiment. Credit: Wikipedia

Suppose, then, that we were to toss atoms through an inhomogeneous magnetic field. Individually the atoms have no particular orientation, so we would expect that the orientation of their magnetic moments are entirely random. As a result, some of the atoms would be more strongly attracted toward the north direction of the magnetic field, while others would be more attracted to the south, and everywhere in between. If the atoms really did act like tiny magnets, we would expect to see the beam of atoms spread out evenly by the magnetic field. In fact, what we see is that the atoms either move toward the north or south, and that’s it. Instead of spreading out evenly, the atoms lock into specific orientations. This experiment is named the Stern-Gerlach experiment, after the physicists who first performed it in 1922, and it demonstrates one of the basic aspects of quantum theory. When you try to measure the state of a quantum system, the results you get are often snapped to discrete results. It would be like measuring the height of a random collection of people, and finding they are all exactly either 5 ft or 6 ft tall.

The Zeeman effect.

As strange as this is, we actually use a similar effect to measure the strength of magnetic fields in the Sun. Since electrons also have magnetic moments, strong magnetic fields can cause their energy levels in an atom to shift. As a result, the emission lines an atom gives off can be shifted by magnetic fields. Emission lines can even be split slightly, which is known as the Zeeman effect. We see this effect near sunspots, which is how we know that sunspots are cooled by magnetic dampening.

That’s part of the real power of astrophysics. Once we understand a phenomena, however strange, we can use it has a tool to study the stars.

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NASA’s Solar Dynamics Observatory has been observing the Sun for five years. It’s goal is to study the dynamic variations of the Sun and how they affect our planet. It’s gathered 2.6 petabytes of information, and its data is used for a range of scientific work, from helioseismology and studies of the corona to solar flares and sunspots. It’s also gathered some stunning visuals showing the complex dance of our closest star. Some of the visuals from year five are presented in this wonderful video.

Throughout the video you can see prominences that burst out from the solar surface and wisps of plasma flowing along magnetic field lines. There are filaments looking like cracks in the Sun, and bubbling granules as material from the warmer interior churns toward the Sun’s surface. You can watch coronal mass ejections, and see how the limb of the Sun appears cooler and dimmer. You can even see a transit of Venus.

Often in astronomy, it is the brief moments of fame that get the greatest attention. The landing on a comet, or on Titan. The mission to Ceres, or the upcoming Pluto flyby. These missions deserve the recognition they get, but there are also missions such as the SDO, which quietly gather data, year after year. Though not nearly as sexy, they are just as important as the missions that dance in the Sun.

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The Sun is so intensely bright that it’s difficult to look at (and you shouldn’t try). When observing the Sun with scientific instruments, we often use filters to diminish the light so that we can observe surface features of the Sun in detail, such as sunspots and the churning of granules due to convection near the surface. But how do we study the interior of the Sun?

One way is through neutrinos generated in the Sun’s core. Unlike light, which can take 20,000 to 150,000 years to travel from the Sun’s core to its surface, neutrinos leave the Sun soon after they are produced. We’ve been able to detect solar neutrinos since the 1960s, but these were neutrinos due to secondary reactions in the core. More recently we’ve been able to observe neutrinos from the principle fusion mechanism known as the pp-chain. From these observations we know the rate at which fusion occurs in the Sun, as well as its central pressure, temperature and density.

Between the core and surface things get a bit more tricky. Surrounding the core is a radiative zone, where the heat of the core moves toward the surface mainly through photon radiation. Surrounding that is a convection zone, where stellar material churns in a cycle. Heated by the interior, the material rises toward the surface. It then cools and sinks toward the interior where the process happens all over again. We know of these levels through helioseismology, which is the study of sound waves traveling through the Sun’s interior. While light takes thousands of years to travel from the Sun’s core to its surface, the solar interior is relatively transparent to acoustic waves, which means they can travel through the Sun at the speed of sound.

As the methods of helioseismology have gotten more sophisticated, we’ve been able to determine some of the characteristics of the convection flow, and what we’ve found is that it’s much more turbulent than originally supposed. This means that while our surface and deep interior models are pretty good, our mid-range models aren’t. This isn’t particularly surprising, since the complex transition between the radiative and convective regions is notoriously difficult to model.

But what’s amazing is that we can use sound waves to actually test these models. With methods such as neutrino physics and helioseismology, we can really see the complex beauty of the Sun’s interior.

Paper: Laurent Gizona & Aaron C. Bircha. Helioseismology challenges models of solar convection. PNAS, vol. 109, no. 30, 11896–11897 (2012).

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In the early 1900s the general view of stars was that they contained about the same ratio of elements as Earth. That is, they were made largely of things like iron, silicon, oxygen and the like, and not much in the way of hydrogen and helium. That’s in direct contradiction with what we know today, but it was a reasonable assumption at the time. At that point there was no understanding of nuclear fusion and the generation of elements, and a safe assumption was that the distribution of elements was fairly even across the solar system.

There was evidence to support this idea as well. By observing the spectrum of the Sun, astronomers were able to identify certain elements such as carbon, oxygen and iron, so we know the Sun contained the same type of elements as Earth. Determining the ratio of elements, however, was a different story. It was thought that the darker the elemental absorption lines in the solar spectrum, the more of that particular element there must be. But the results were inconsistent, so it was difficult to pin down clear ratios.

Cecilia Payne at work. Credit: Harvard Observatory

Enter Cecilia Payne-Gaposchkin. In the 1920s she was a graduate student of Harlow Shapley (famous for determining the shape of the Milky Way). Her dissertation focused on solar spectra, and she was able to demonstrate that the strength of an absorption line didn’t just depend on the amount of an element, but also on the ionization of the element (which depended upon a material’s temperature). From this she was able to prove that the ratios of silicon and carbon were similar to the ratios found on Earth.

But she also found that the Sun contained massive amounts of Hydrogen. Her calculated levels were about a million times higher than expected. This is the type of result that makes you question your abilities, but Payne confirmed her work and clearly showed high levels of hydrogen.

Payne successfully defended her dissertation, but the result was controversial. Some astronomers such as Henry Russell actively opposed the idea. But Payne’s results were right, and later evidence confirmed her results. It was one of the first steps toward our modern understanding of the Sun and other stars.

Paper: Payne, C. H. Stellar Atmospheres; A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars (PhD Thesis). Ratcliffe College. (1925)

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When you look at an image of the Sun, you might notice that it’s edge appears slightly dimmer than its center. This is an effect known as limb darkening, and its actually quite useful to astronomers.

Our line of sight means we see light from different depths. Credit: Wikipedia

It all has to do with our line of sight to the Sun. When we view the middle of the Sun, we are looking directly into the Sun. When we view the edge of the Sun, we see light coming at a glancing edge through the Sun. Because of the density of the Sun’s atmosphere, the light we observe always comes from about the same distance through the upper layer of the Sun. So in the middle we see to a certain depth into the Sun, but on the edge we see light from a more shallow depth. The depth we observe can be determined by simple geometry, so by looking across the Sun, we are actually seeing to a varying depth at different points.

The reason the Sun appears dimmer near the edge is because the Sun gets hotter the deeper you go. This makes sense, but with limb darkening we can prove it. In fact, with detailed measurements of limb darkening, we can determine just how the temperature of the Sun varies with depth.

Limb darkening seen on Betelgeuse. Credit: NASA/Hubble

A similar effect occurs with other stars. There are only a few stars where we can resolve this directly, but Betelgeuse is a good example. In the case of Betelgeuse, the darkened region is much wider than that of the Sun, which indicates Betelgeuse has a much thicker atmosphere. This is typical of red giant stars, which, while larger than the Sun, are much cooler and less dense.

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Yesterday I talked about how water can form on the Moon. It might seem a bit surprising that water exists on the Moon, but it doesn’t sound like a crazy idea. What does sound crazy is the idea that there is water on the Sun’s surface, and yet we know that there is.

The surface of the Sun (specifically the photosphere) has a temperature of about 6000 K. It is so hot that hydrogen atoms are ionized, and molecules can be ripped apart. It’s pretty inhospitable for a molecule such as water. Despite this, the potential for water is there. Oxygen is produced in stars through the CNO fusion cycle, and we have observed quantities of oxygen in the Sun’s spectra. Hydrogen is the most abundant element in the universe, and most of the Sun’s mass is hydrogen. All that’s needed is a cool enough temperature for the hydrogen and oxygen to come together to form water.

It turns out there is just such a place in sunspots. We normally think of sunspots as dark regions on the Sun. They aren’t actually dark, but they are cooler and dimmer than the rest of the Sun, which is why they appear dark in solar images. Within a large sunspot, the temperature can be as cool as 3500 K, which is cool enough for water to form. Naturally it only exists as water vapor, but there really is water on the Sun’s surface.

Spectra for hot water. The red line is the theoretical spectra for water at 3000K. The blue line is the observed curve in sunspots. Credit: Polyansky, et al.

The presence of water on the Sun has long been suspected, but proving it has been a real challenge. That’s because water has a complex spectra with millions of absorption lines. These lines also vary with temperature, making it even more challenging. Experimentally measuring the line spectra of water vapor at 3500 K isn’t feasible, so you need to calculate the expected spectra using computer simulations.

In 1997, a team did just that. They were able to calculate more than 6 million absorption lines for very hot water, and then compared the results to observed spectra deep within sunspots. They found a clear match, showing that water does indeed form within “cool” sunspots.

So yes, there really is water on the Sun’s surface. It’s a fact worth remembering if you ever want to win a bar bet.

 Paper: Polyansky et al. Water on the Sun: Line Assignments Based on Variational Calculations. Science 277 (5324): 346-348 (2014)

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