Strictly Astronomy

Diagram showing the ‘radiant’ of the Perseid shower (marked with the red cross). Graphic by the blogger.

Sunday night (12th August) marks the peak of the year’s best meteor shower, and this year the circumstances for viewing them are the best they have been in over a decade.

Each August, the Earth’s orbit carries our planet through streams of debris shed by Comet Swift-Tuttle, which last flew through the inner solar system in 1992. The orbit of this comet is arranged just right such that it intersects our orbit, although fortunately the Earth has never found itself in the same place as the comet at the same time (although that outcome is not impossible!)

Comet Swift-Tuttle, 1992. Credit: NASA

With an orbital period of 130 years, no one who saw its last apparition will likely be alive to see its next. But on each trip around the sun, Swift-Tuttle leaves a trail of dust in its wake. As the Earth plows through this part of its orbit, some of those dust grains are intercepted by our atmosphere, and we see them as swarms of bright shooting stars across our night skies.

From a dark location, up to 100 meteors per hour may be seen, with occasional outbursts to even higher hourly rates. The meteor show is known as the “Perseids” because the tracks of the meteors appear to converge toward the constellation of Perseus.

2018 is an especially good year for the Perseids because the maximum intensity of the shower occurs a little more than 24 hours after the new Moon, so that there is effectively no moonlight in the night sky for the meteors to compete against. That enables good visibility of fainter Perseids, which would otherwise be lost in the glare of the Moon in other years. That means that hourly rates reported by observers in 2018 are likely to be higher than in other years.

Like other meteors, Perseids begin as pieces of cosmic dust little bigger than sand grains. After leaving the surface of Comet Swift-Tuttle, they may orbit the sun for hundreds or thousands of years before approaching the Earth. Impacting the upper atmosphere at relative speeds approaching 60 kilometers per second (134,000 miles per hour), friction with air molecules heats the dust to temperatures of over 1,000 degrees. Elements in the disintegrating dust grain, such as iron, magnesium, and sodium, are energized by the heating and radiate away some of the energy as light of particular colors like green and yellow. Some 4.5 billion years after their creation, these grains end their existence in mere seconds.

The best views of the Perseids come after local midnight, when the direction of the Earth’s rotation takes us toward the forward direction of the Earth’s orbit around the Sun; whereas earlier in the evening, meteors must catch up to Earth’s motion to reach the atmosphere, after midnight the Earth is more directly moving through the stream of dust particles.

To see the Perseids, find an outdoor space with a relatively clear horizon toward the northeast. Activity will begin to pick up by about 10pm local time, and steadily increase until after midnight. Here are some tips to improve the experience:

Safety first! Carry a flashlight to help navigate in the dark, especially in unfamiliar places. Red or brown paper placed over the front lens of the flashlight will help preserve your eyes’ sensitivity to faint light.

Use a reclining lawn chair or chaise lounge to relieve fatigue while looking up at the sky for long periods of time.

Bring plenty of water, too, because it’s possible to dehydrate in warm temperatures on summer nights.

If you’re observing after midnight, remember to dress warmly as the night may become quite cool in the pre-dawn hours.

7.5 billion light-years

Photo of the Andromeda Galaxy by Kees Scherer

What is the oldest light a person can see using nothing but the unaided eye? By “oldest”, we mean how long ago, from today, the light came to be after its origin, including time required to transit from the source to the Earth where it can be observed. The answer to this question turns out to involve the ways in which astrophysical light sources make and emit light, how sensitive the human eye-brain system is to light, and a remarkable amount of luck. 

There are relatively few ways that light is produced in the universe, all of which involve the conversion of energy from other forms into light. For light at visible wavelengths, meaning those to which the human eye is sensitive, the dominant source in the universe is stars. Starlight is created deep within the interiors of stars, where pressures, temperatures and densities are so high that the nuclei of atoms are squeezed closely together. The tiny difference in mass between the nuclei that go into and come out of the reactions leaves the scene in the form of light, and some of it is at the right wavelengths for the eye to see.

But that light doesn’t leave the scene immediately. The exotic conditions inside stars keeps the light effectively “trapped” for some time. The light particles, or ‘photons’, bounce around, scattering off of electrons, other photons, and atomic nuclei. Since there is no preferred direction for the photons’ travel after these scattering events, they “random walk” outward from the interior of the star. For a star with the mass and radius of the Sun, this process takes about one million years.

Eventually a photon generated near the core of the star random-walks its way far enough outward in radius that it encounters a region called the “photosphere”. The photosphere is a region of constant radius from the center of the star at which the probability is larger that a photon will be scattered away from the star — and will keep going once it does — than it will be scattered back down into the star. It is therefore the ‘visible surface’ of a star like the Sun that we can see through telescopes on Earth. We can’t see any further “into” the star because at depths below the photosphere, the star’s atmosphere becomes opaque. The random walk of light out of the core of a star means that the sunlight we see today was actually forged a million years ago, deep within the Sun’s core.

It is often written that when we see the Sun, we actually see what it looked like eight minutes in the past, given the time required for its light to transit the 150 million kilometers, or about 93 million miles, between its surface and the Earth. Light travels through the vacuum of space at about 300,000 kilometers, or 186,282 miles, per second. Dividing the distance between the source and the observer by this number yields the amount of time needed for light to traverse the distance.

So, putting this all together, we can infer that the oldest light visible to the naked eye, if we’re talking about starlight, is about a million years plus the light travel time from the source to the Earth. The next question is therefore: what is the most distant source of starlight that the unaided human eye can see? We know the limit of sensitivity of the eye to light, which corresponds to stars who have a brightness of about +6.5 or +7 in units called magnitudes. Imagine being under a very dark night sky, far from city lights; the very faintest star visible in the sky would have about such a brightness.

But all of those individual stars we see in the night sky are all inside our own Milky Way, and within a distance of only a few hundred light years, where one light year is the distance light travels in one year, or about 9,500 billion kilometers (5,900 billion miles). The light travel time doesn’t add much to the million years since that light was forged in the interiors of those faint stars.

However, the human eye can see further when all of the starlight in a distance source is combined. For most people, the most distant object observable with only the unaided eye is the Andromeda Galaxy, an object with a similar number of stars as our own Milky Way. Various ways of estimating the distance to Andromeda give a value of around 2.5 million light years. But if individual stars in the Milky Way are only visible to a distance of few hundred light years, why can we see Andromeda from so far away?

The answer has to do with concentrating all of the light of an entire galaxy into a piece of the night sky that is relatively small, and a concept called surface brightness. Even though Andromeda is relatively far away, the collective light of all of it stars yield a glow that is strong enough to be visible to our eyes from Earth. Most other galaxies are much further away than Andromeda, concentrating their light into even smaller regions of the sky, which put them below the threshold of visual detection.

Of course, stars aren’t the only source of light in the universe. Some stars reach the end of their lives through colossal explosions known as supernovae. Sometimes this happens when a massive star runs out of fuel in its core, causing the core to collapse under the weight of the star and blowing off all but the compact remnant of the core into space and releasing an incredibly amount of energy.

Other stars evolve in pairs and experience runaway thermonuclear detonations; the steady brightness of these explosions helped astronomers figure out that the expansion of the universe seems to be accelerating. Then there are other events, like the merging of pairs of white dwarf stars, neutron stars, or even black holes, that may complete destroy both partners in the gravitational dance and briefly become the most luminous objects in the universe.

The brightest such object ever seen to date was given the generic-sounding catalog name "ASASSN-15lh”. At its brightest in June 2015, it reached a luminosity more than 50 times that of the entire Milky Way galaxy, at which point it was radiating 570 billion times as much energy, per second, as the Sun. Knowing that rate, and taking into account the inverse square law of light, we can work out the distance to the object that emitted the light. It was far too faint to be seen with the naked eye, at a distance of 3.8 billion light years from Earth. But what if ASASSN-15lh had been closer?

Supernova ASASSN-15lh (right) and its host galaxy (left). Images: The Dark Energy Survey, B. Shappee, Las Cumbres Observatory, and the ASASSN team

If we take +6.5 magnitudes as the faintest light the unaided eye can see from Earth, we can calculate the distance to the brightest objects nature produces, seen at the very threshold of human vision. Running through this calculation, we get a value of about 32.6 million light years. While we don’t know exactly what caused the explosion of ASASSN-15lh(*), and the light it emitted, we can work out how far away something with its intrinsic brightness would be if it were visible to the unaided eye in our night sky.

Maybe someday, when people look up at the night sky and see a new star that wasn’t there the night before, it will actually be an extremely rare object like ASASSN-15lh. If that new star is right at the edge of visual detection, the people who see it with their eyes alone will witness 30 million year-old light that left its source long before there were ever humans on Earth(**).

(*Some readers might know that quasars and gamma ray bursts are potentially more energetic than supernovae and therefore could be the source of light visible to the naked eye. But since these objects were more common in the early universe and are very rare now, they’re (probably) mostly too far away to yield a glow that reaches naked-eye visibility.) 

Look low in the west-southwest tonight in the half-hour after sunset, and you’ll see an unusual sight: both inner planets very close to each other in the twilight sky. This phenomenon is known as an appulse.

Venus and Mercury are called “inner” (or, sometimes, “inferior”) planets because they orbit the Sun inside the Earth’s orbit. Consequently, we only ever see them relatively close to the Sun, near sunrise or sunset. Venus has been known to humanity as either the “morning star” or “evening star” for thousands of years for this reason, never straying beyond about 45 degrees east or west of the Sun’s position in the sky. Mercury’s smaller orbit means it keeps to an even smaller region around the Sun, always inside about 30 degrees’ separation.

So why are we seeing both so close together right now? It’s an accident of geometry. The image below shows the same view as above, but with the outlines of the planets’ orbits superimposed as a set of lines. 

Both planets are situated in their orbits in such a way that they appear close together in our night sky, along the same line of sight, even though they are separated in space by tens of millions of kilometers. 

The other factor that makes both planets relatively easy to see right now is, perhaps surprisingly, the tilt of the Earth’s rotation axis with respect to the ‘plane of the solar system’ in which all the major planets appear to move, more or less.  In March, the Earth is near an equinox, during which both the northern and southern hemispheres of our planet are equally illuminated by the Sun. 

A consequence of this arrangement is that, seen from mid-northern latitudes on Earth just after sunset, the other planets’ orbits projected onto our night sky is almost perpendicular to the local horizon. For a given ‘elongation’ of Venus and Mercury, meaning their angular separation from the Sun, this puts the planets highest above the horizon for the year. As a result, they are seen against a darker night sky before they set, about an hour after sunset. This isn’t so important for Venus, the brightest planet in the night sky over most of its orbit, but is helpful for spotting much dimmer Mercury.

An unusual event happens tonight, one that is an accident of the way we set up our calendars: a total lunar eclipse, a supermoon, and a blue moon all happen at the same time. This particular total lunar eclipse is happening when the full Moon occurs near its closest point to Earth (a 'supermoon') and during the second full Moon in a single calendar month (a 'blue moon'). That combination of circumstances hasn't happened since the total lunar eclipse of 31st March 1866, and won't happen again until New Year's Eve 2028.

Observers in central and western U.S. states will see a total eclipse of the moon near sunrise on the morning of Wednesday, 31st January. Partial phases begin at 3:48am Pacific time, and the eclipse is total between 4:51am and 6:07am. The Moon sets for all observers in the Lower 48 before the eclipse ends. Look low in the west in the hour or so before sunrise. (Eclipse graphics by Sky & Telescope)

Brave the cold and head out tonight to see what may well turn out to be the year’s best display of bright meteors: the Geminids! Little bits of dusty debris from the oddball ‘rock comet’ 3200 Phaeton slam into the upper atmosphere of our planet at some 35 km/s (79,000 miles per hour), leaving glowing trails that appear to ‘radiate’ away from the constellation Gemini (the small red cross in the figure above). Like train tracks converging in the distance, these bright trails are actually more or less parallel to each other, and their apparent origin at a common ‘vanishing point’ is an illusion. As they encounter our atmosphere at high speed, they are heated by friction with air molecules until they briefly glow at a temperature of several thousand degrees, being almost completely vaporized while still dozens of kilometers above the ground. The meteor debris is ionized, emitting bright colors with the characteristic fingerprints of their constituent atoms.

(AccuWeather graphic)

Photo credit: Jim Hendrickson

Saturday, 28th October, is International Observe The Moon Night! This annual event encourages people to get outside in the early evening and take a gander at Earth’s nearest neighbor in space. Here’s how the Moon will look this evening, using the Scientific Visualization Studio's Dial-a-Moon online app: 

Here are a few things to notice about the Moon if you view it through binoculars or a telescope. First, the Moon is in the waxing gibbous phase tonight, between first quarter and full. This means it is a little more than one-fourth the way along its orbit, taking the start to be when the Moon is “new” (seen roughly in the direction of the Sun in the sky). The vertical line running down the middle of the Moon’s disk is the terminator, representing where the Sun is just coming up over the horizon on the Moon. Terrain is seen is sharpest relief near the line, since shadows cast by tall objects tend to be longest there. 

Look for the lunar “seas”: the dark spots that form the familiar “Man in the Moon” pattern are remnants of giant impact basins that were filled in by lava early in the Moon’s history. Some of these are ringed by tall mountains, forming the rims of craters hundreds of miles across. And of course, much of the Moon’s ancient “southern highlands” terrain is covered in countless impact craters. These preserve a record of billions of years of lunar history shaped by the rain of comets and asteroids from above. Also, look for subtle shades of color on the lunar surface -- they’re real!

Lunar Reconnaissance Orbiter true-color image of the Moon.

If you don’t have access to a telescope or a pair of binoculars, there is still plenty to see with just your own eyes. And don’t forget the Earth around you. How bright is moonlight? We still don’t quite know exactly. Look at how the illumination changes in the evening, as the glow of twilight fades in the west and the Moon becomes the dominant source of light in the night sky. Some animals are more active at night when the moon is brightest.

Lastly, think about the human element. How do people relate to the Moon? Are they more or less likely to notice it during some parts of the lunar cycle? Consider, too, that we may owe our very existence to the fact that the Earth has a large moon at all. 

In a first, astronomers this morning announced the discovery of flashes of light emitted by what is thought to be the merger of two neutron stars -- extremely dense objects left over after the deaths of massive stars -- following the detection of gravitational waves emitted as the pair ended their “death spiral” by merging together.

The object, known as “GW170817″, was first detected on 17th August through the gravitational waves emitted by the neutron stars as they moved so close together that they eventually merged. These objects are ultradense, packing the mass of a star into a space about as large as the average city is wide; as the analogy goes, a spoonful of neutron star matter would weigh millions of tons on Earth. Because such large mass is packed into a small space, neutron stars severely warp the spacetime around them. As the orbit of the two objects ‘decayed’ into an ever-smaller radius, the rapid spinning of the system whipped up waves traveling outward at the speed of light. 

So we can think of gravitational waves -- a kind of ringing in spacetime -- as something like the waves in air or water that we sense as sound. As sensed by detectors on Earth, it was as if the pitch of a sound increased steadily, then abruptly ended in a “chirp”. The Laser Interferometer Gravitational-wave Observatory (LIGO) saw just that, in this plot of frequency versus time; the ‘chirp’ is at the far right end:

Humanity only recently achieved the ability to detect these extremely weak waves, resulting in this year’s Nobel Prize in Physics. But before we could directly sense gravitational waves, people began to speculate on whether their detection might forewarn us of a flash of light that would arrive some number of seconds later: if gravitational waves are the ‘sound’ of titanic forces in the universe being unleashed, the electromagnetic waves that follow are the ‘vision’ that tell us more about how the sound was made. Combined, the sound and vision tell us more about what made the waves than either by itself.

After GW170817 was detected, astronomers sprang into action. The precision of gravitational wave measurements is now high enough, and detectors are sufficiently scattered around the globe, that we can more or less pinpoint from where on the sky the waves appear to radiate. The LIGO scientists communicated the celestial coordinates to other astronomers, who trained an army of telescopes on them within minutes. What they saw was the afterglow of the neutron star merger, some of whose collective energy was emitted as light.

Today’s announcement is a massive feat of collaboration, timing, and telescope automation: some 70 different facilities participated in the followup observations. The graphic below shows the detection and event timeline for GW170817 across the spectrum, from the arrival of the gravitational waves (upper left) to the gamma-ray trigger, to the discovery of the optical counterpart. They’re shown as a timeline reading from left to right, with an irregular scale that ranges from seconds leading up to the event to days after it ended:

The images in two rows at the bottom of the figure show the host galaxy as the dark blob in the center (inverted so more light means blacker pixels), with the location of GW170817 marked with perpendicular lines. In the lower left corner of each image, the time after the gravitational wave detection, from hours to days, is listed. Light from the event was still visible over two weeks after the merger!

Being able to sense and measure the flash from GW170817 allows astronomers to estimate of the speed at which the universe itself is expanding, one that wouldn’t depend on assumptions about how far away the source of the waves is. Unlike finding the distance to stars and galaxies using the ‘distance ladder,’ gravitational wave strength tells us more directly how far away the thing is that made the waves.

Their estimate landed almost exactly on the consensus value obtained through many different means over about the past 25 years; the authors of the Nature paper describing the results write that the number is “consistent with existing measurements, while being completely independent of them,” a powerful validation of our current understanding of cosmology.

How is our galaxy built? What does it look like? (And how do we know?)

These questions have persistence since before we properly understood what galaxies are. Once upon a time, it was thought that what we know now as the ‘Milky Way’ was everything that was. The glowing band of light across the night sky was revealed as a swarm of faint stars when Galileo Galilei first turned a telescope on the Milky Way in 1610. The Milky Way was seen as a great system of stars, which themselves must be pretty far away if a telescopes required to see them, but it was viewed as the only such system until the twentieth century.

In the eighteenth century, people began trying to systematically map those stars in order to deduce something about the structure of the system. The Anglo-German astronomer William Herschel published a map of the Milky Way in 1785, drawn from his years of meticulous observations of its stars: William Herschel’s 1785 map proposing an overall shape of the Milky Way, based on his observations of its constituent stars.

William Herschel’s 1785 map proposing an overall shape of the Milky Way.

He also correctly identified the Sun as occupying a place within the system (the prominent “star” just to the right of center on his map) rather than lying outside of or beyond it. His logic was simple. If the Sun and its attendant planets, including Earth, resided within the system,

“the heavens will not only be richly scattered over with brilliant constellations, but a shining zone or milky way will be perceived to surround the whole sphere of the heavens, owing to the combined light of those stars which are too small, that is, too remote to be seen. Our observer’s sight will be so confined, that he will imagine this single collection of stars, of which he does not even perceive the thousandth part, to be the whole contents of the heavens."

In the generation before Herschel, thinkers began to speculate on the existence of other systems like the Milky Way. Thomas Wright suggested in 1750 that the Milky Way was really a single object, a large, flattened (and spinning) disk consisting of countless stars. The German philosopher Immanuel Kant read Wright’s musings and further developed the notion into what he called the 'island universe':

"Their analogy with the stellar system in which we find ourselves, their shape, which is just what it ought to be according to our theory, the feebleness of their light which demands a presupposed infinite distance: all this is in perfect harmony with the view that these elliptical figures are just universes and, so to speak, Milky Ways, like those whose constitution we have just unfolded."

By the turn of the twentieth century, astronomers were slowly coming to understand the the ‘spiral nebulae’ seen in their telescopes were systems like the Milky Way seen at great distances. However, these objects stubbornly refused to resolve into individual stars, even in the world’s largest telescopes, and some scientists held fast to the notion that the spiral nebulae were little more than gas clouds, perhaps illuminated by unseen stars embedded deeply within. 

The disagreement culminated in 1920’s “Great Debate” between American astronomers Harlow Shapley and Heber Curtis. Shapley argued the conventional position that spiral nebulae were small, nearby interstellar clouds, while Curtis advanced the notion that they were large and far away, and thus their constituent members must be stars. Curtis based his argument in part on his 1917 observations of a kind of stellar cataclysm called a “nova” in the direction of what we now know as the Andromeda Galaxy. Novae are very intrinsically bright, like extra-bright light bulbs, meaning that they can be seen at large distances within the Milky Way. Curtis looked back over the historical record and found 11 more novae in Andromeda on old plate photographs taken through telescopes. Based on these 12 objects, he computed a distance to Andromeda of half a million light years, much further away than the most remote known star.

Curtis was ultimately proven right before the end of the decade. The debate was properly resolved in the spring of 1929 when Edwin Hubble (of telescope fame) published a paper with a radical conclusion: the ‘Andromeda Nebula’ was certifiably far away. By monitoring a kind of variable star seen in the outer reaches of Andromeda’s spiral arms, Hubble was able to infer its luminosity; comparing this against the star’s apparent brightness revealed its distance: nearly 2 million light years. The controversy was settled: Andromeda must be something like the Milky Way itself. 

Edwin Hubble’s famous “VAR!” photograph marking the position of the Cepheid variable star he discovered in the Andromeda Galaxy. Carnegie Observatories photo.

What about Andromeda’s apparent shape? From images of it, and other objects that came to be known as “galaxies”, we find a variety of shapes and sizes that tell us something about their origins and histories. Hubble tried classifying galaxies by shape, arranging them into what became known as the “tuning fork diagram”:

Hubble’s tuning fork doesn’t sound like much.

There are basically two kinds of galaxies: spirals and ellipticals. A subset of spiral galaxies are known as “barred spirals” because of how their arms unwind from the center: the arms of ‘ordinary’ spirals curve all the way into the galaxies’ centers, while the arms of barrier spirals project off the ends of a linear feature running through their centers. Clearly Andromeda was some kind of spiral, seen at a fairly high inclination (whereas all of the model galaxies shown in the Hubble diagram are conveniently viewed face-on).

If the Milky Way was more or less like Andromeda, was it, too, a spiral of some sort? That seemed likely, because of how the Milky Way appears in our night sky. Keep in mind that we’re embedded within the galaxy, not flying above it somewhere in space. Rather than resembling the oval blobs of elliptical galaxies, the Milky Way looks more like a flattened disk seen along its edge. In certain cases, when we look out toward other nearby spiral galaxies, we see them in the same orientation, as in this image of the galaxy NGC 891:

A color image of the galaxy NGC 891 taken by the author.

We even have ways of deducing the presence of a bar in the Milky Way, so we know it properly belongs on the arm of Hubble’s tuning fork with the other barred spiral galaxies.

So by the turn of the twenty-first century, humanity had a pretty good idea of the shape and extent of our home galaxy based on observations, inferences, and some crafty detective work. Here’s what we think it would look like, if we could fly high above the disk and look back: 

A model of the Milky Way galaxy, viewed from over one of its poles. Illustration by Robert Hurt, IPAC; Bill Saxton, NRAO/AUI/NSF.

But how can we be sure? What’s happening in the parts of the Milky Way we can’t see directly?

In the image of NGC 891 above, note the dark stripe running down the middle of the galaxy, appearing to split it into two halves. This is a ‘dust lane’, composed of many individual clouds of interstellar dust that tend to form and settle in the galaxy’s mid plane. Dust particles are really good at scattering and absorbing light, so they transmit very little of the starlight coming from “behind” them as seen in the direction of Earth.  The Milky Way has its own dust lane, just like this one:

The “Great Rift” in the Milky Way, seen during northern hemisphere summer on Earth. NASA image.

With all that dust blotting things out, it seems like we could never know what’s going in really distant parts of the Milky Way. But some kinds of light can pass through the dust relatively unscathed — long wavelength varieties of light, like infrared. Here’s a Spitzer Space Telescope view toward the center Milky Way in the infrared part of the spectrum:

Staring at the Milky Way’s not-so-dark heart. Image: NASA/JPL-Caltech/S. Stolovy (Spitzer Science Center/Caltech).

There’s still some dust, but much less than in the visible light image shown previously. To look more deeply into our galaxy, we have to resort to longer wavelengths of light: radio waves. And that brings us to this week’s news about structures seen all the way through on our galaxy’s far side.

As reported last week in Science, Alberto Sanna and co-workers used a group of radio telescopes called the Very Long Baseline Array (VLBA) to measure the distance to a star-forming region on the side of the Milky Way opposite the Sun. VLBA uses a principle called interferometry to achieve extremely high resolution of astronomical radio sources. By distributing ten radio antennas, each with a diameter of 25 meters, across the western hemisphere from Hawaii to the Virgin Islands, VLBA achieves the same resolution as a single radio dish with a radius of thousands of kilometers. VLBA does this by very careful timing the arrival of the radio signals received by each dish. Software then reconstructs the incoming signal at very high spatial resolution.

The 10 VLBA dishes and their locations. Image: SeaWiFS Project NASA/GSFC and ORBIMAGE.

The authors then used a geometric principle known as parallax to determine the distance to the source of the radio waves, finding they originated some 66,500 light years away from Earth. That’s most of the width of the galaxy itself, thought to be around 100,000 light years. And with that they found, at exactly the expected distance, the continuation of one of the Milky Way’s spiral arms known as the ‘Scutum-Centaurus Arm’.

So why is this work important? Doesn’t it just confirm what we already thought was the case?

For one thing, it’s a demonstration of a very powerful technique for teasing out details of the structure of the Milky Way. The group’s parallax estimate blows the old record of about 36,000 light years right out of the water. As Sanna commented, "Most of the stars and gas in our Galaxy are within this newly-measured distance from the Sun. With the VLBA, we now have the capability to measure enough distances to accurately trace the Galaxy’s spiral arms and learn their true shapes.” 

"This means that, using the VLBA, we now can accurately map the whole extent of our Galaxy,” he added.

One of the most head-scratching stars in the night sky (at the moment) may be starting to finally give up some of its apparent mystery. The unceremonious-sounding KIC 8462852 is better known these days as “Tabby's Star” or “Boyajian's Star” after Tabetha S. Boyajian, professor of astronomy at Louisiana State University, who lead the team that first described the star in a 2015 paper. The paper is provocatively subtitled “Where’s The Flux?”, leading to the star’s other occasional nickname: the “WTF Star”. 

KIC 8462852 is an otherwise unremarkable 12th-magnitude star in the constellation Cygnus. What makes it remarkable is the recent history of its light output, which has baffled nearly every stellar astronomer who has examined its light curve. A light curve is a graphical representation of the light output of some astronomical thing over time. Most stars’ light curves are flat and boring: they emit essentially the same amount of light at all times. Other stars are known as “variables”. Their light curves show all manner of variation, from the highly regular to the highly irregular. In all those cases, however, we understand at least something about the underlying physics that causes these stars to vary.

Tabby’s Star, however, is in a league of its own. The first indications that something is up with this star came from the Kepler spacecraft mission in 2011. Kepler’s M.O. is pretty simple: in its primary mission, lasting from 2009-2013, it stared at a region of sky containing Tabby’s Star continuously, recording the brightness of every object in the field. Unlike previous instruments used in this manner, it achieved an unprecedented level of measurement precision, and was used to discover thousands of potential exoplanets. It also provided new information on the inner workings of stars themselves.

And it showed us that something about one particular star in its field of view is very, very weird.

Kepler light curves of Tabby’s Star (above) give some idea about its odd behavior. Long periods of stability (i.e., when the light curve is very flat) give way to inconsistent drops in the light received from the star. The star was seen to dim for many days at a time, losing up to 20% of its brightness, before stabilizing again at apparently full power. It didn’t at all resemble the light curve of a star hosting one or more exoplanets. Nor did it look like any other stellar light curve seen before.

Boyajian’s team wrestled with the cause of the star’s behavior. It considered explanations like a very young star prone to erratic light output as it settled onto the Main Sequence (nope: too old and mainstream for this kind of display). Or maybe it had a dusty disk around it that periodically blocked some of the light (nope: such a dust disk would re-radiate starlight in the infrared, leading to an obvious signal). After discarding some ideas like this for lack of evidence, the team proposed a radical idea: a chunky dust cloud made up of many dissolved comets (artist impression, below). 

The comets would probably tend to cluster, it was argued, if they originated as some larger object around the star that broke apart under the influence of the star’s gravity.Lots of other explanations have been proposed, and mostly dismissed, for want of supporting evidence, including:     

• Burps from a star that had actually eaten one of its planets;     

• A big planet with a set of rings tilted with respect to its orbit, periodically blocking some starlight from view;

• Chaotic stellar physics manifesting through something called “avalanche statistics”; and

• Some previously unobserved physical effect at or near the star’s surface.

The most exotic theory of all, and the one that got the most press, was that Tabby’s Star was being dimmed by a “Dyson swarm” (below): a ring or other structure around the star, composed of artificially designed and built objects, indicative of intelligent life.

Needless to say, it wasn’t the simplest explanation for the observations, and probably isn’t the right one.

New results seem to discount most of those theories and return to one of the original thoughts: the thing dimming Tabby’s Star is nothing more than garden-variety dust. New observations from NASA's Spitzer and Swift spacecraft, as well as the Belgian AstroLAB IRIS observatory, show that whatever is doing the light absorbing is better at it in the ultraviolet part of the spectrum rather than the longer-wavelength infrared.

That’s an indication that the absorbing particles are fairly tiny compared to the wavelengths of light illuminating them. But how do we know that? It turns out to be the same reason that the sky is blue: light interacts more strongly with particles when its wavelength is about the same size as the particles themselves. It’s for this same reason that the daytime sky on Earth is blue. So whatever is causing the dimming of Tabby’s Star can’t be made of particles much larger than the wavelength of light. Such particle sizes are consistent with the known properties of cosmic dust.

Lead study author Huan Meng (University of Arizona) says: “This pretty much rules out the alien megastructure theory, as that could not explain the wavelength-dependent dimming.” Instead, Meng says, the most likely reason for the observations is boring, old dust. “We suspect, instead, there is a cloud of dust orbiting the star with a roughly 700-day orbital period.”

However interesting Meng’s findings, they don’t completely explain what’s up with the “WTF Star”. There is no known mechanism to produce a dust ring or disk around a star that matches the observed properties of the KIC 8462852 system over the length of time we have seen its light varying. Other processes could be combining with the dust dimming to yield the observed variations. For instance, says AstroLAB volunteer Siegfried Vanaverbeke, “Tabby's Star could have something like a solar activity cycle.”

A Super-Duper Supermoon

Tonight (14 November 2016) sees a special occurrence of the “Supermoon”, a phenomenon much hyped by the media but not all that impressive to the casual observer. Yet it’s still a good chance to go out and see a big, bright full Moon — and to notice something important about its light. 

What is a ‘Supermoon’?

To understand how the Moon can be “super”, consider this NASA picture that shows the orbit of the Moon around the Earth: 

Although the shape of the orbit is greatly exaggerated for illustrative purposes, that shape really is not much like a circle and more like an ellipse, or an oval. Since the Earth isn’t at the center of the ellipse (according to Kepler’s 1st Law of Planetary Motion), that means sometimes the moon is a little closer to the Earth, and sometimes a little further away. The extremes of these two situations are marked on the figure as “perigee” (closest to Earth) and “apogee” (farthest from Earth). However, the orientation of the ellipse isn’t fixed in space. Instead it “precesses” over the course of about nine years, so the location in space of perigee and apogee keep changing.

Now add to this a simplified picture of the phases of the Moon, explained by orienting its orbit around us with respect to the (distant) Sun:

(Wikimedia Commons) 

Put all of this together, and two facts emerge: First, the phases of the Moon happen at different relative distances over the period of the precession of the Moon’s orbit around Earth. And second, at different intervals, any particular Moon phase will happen at some particular point in the orbit.

One possibility, therefore, is that the full Moon occurs at perigee, at which point it would be as apparently large in the night sky, and apparently as bright, as it can ever be. Full Moons that occur within a day or so of perigee have in recent years been commonly called “supermoons”. It’s also possible that the full Moon happens around apogee, making it apparently as small and relatively as dim as it can ever be. These full Moons, popularly known as “micromoons”, receive a lot less media attention. But the differences are noticeable: compare a supermoon (left, below) to a micromoon (right).

(Wikimedia Commons) 

A 12% difference in apparent size isn’t tiny, but it’s also not something that most casual lunar observers will notice. They may be more likely to note the different in apparent brightness, however, which is around 30%. Compared to a typical full Moon, that happens around the average Earth-Moon distance, the difference in brightness is more like about 15%. Still, these differences are small and only become readily noticeable when making the kind of comparison in the figure above. What is different about the November 2016 Supermoon? This month’s full Moon happens unusually close to the exact location of perigee, meaning that the Moon is both at its full phase and about as near to the Earth as it can possibly get. Perigee happened at around 11am GMT on Monday, just a couple of hours before the full Moon arrived at 1:52pm. At that time the distance to the Moon will be about 356,000 kilometers and its disc will span some 33.5 arcminutes across the night sky. The last time that full Moon occurred so near perigee in time was 26 January 1948, and it won’t be so close again until 25 November 2034. What will it look like?

The Moon may look a little bigger and a little brighter tonight, but odds are any such impressions are colored by the media hype. Since we’re a little more than a month out from the winter solstice in the northern hemisphere, tonight’s full Moon will be high in the sky around solar midnight, which is slightly after local midnight this time of year.  

If you go out tonight to see the Supermoon, look at the intensity of moonlight on the ground. That light provides about 0.3 lux of illumination, or about one-third of the brightness of twilight. By comparison, full sunlight is about 100,000 lux, and an overcast night away from sources of light pollution can be as dim as 0.0001 lux. Street lighting is around 20 lux, or roughly 60 times as bright as moonlight. Try reading a newspaper under full moonlight, however. You may be surprised at how easy it is.  

Notice, too, what colors look like under moonlight. The Moon is a nearly perfect reflector of sunlight in terms of its color, although it only reflects about 10% of the light it receives. The Moon’s surface is surprisingly dark, but it’s largely gray to white in color. Moonlight is a lot like dim sunlight, but you’ll notice that it doesn’t show some colors particularly well, such as red. As the human eye adapts to low light levels, its sensitivity to color shifts away from red and toward blue, something known as the Purkinje Effect. Why does this matter?

There’s some extra relevance to astronomy here that has to do with the ability to do astronomy from the ground. Not only is that hampered by the blurring effect of the Earth’s atmosphere, but also by the use of artificial light at night. Light pollution in the form of skyglow, caused by artificial light either emitted directly into or reflected upward into the night sky, forms a “fog” of light that makes it difficult to see the stars beyond. As the world converts its lighting from old technologies to new ones like white LED, with it comes a shift in the spectrum of that light toward shorter wavelengths that scatter efficiently in the Earth’s atmosphere and can make skyglow worse.

Some proponents of LED lighting make claims in the media that LED lights are “good for the night sky” or that they “reduce light pollution”, but that’s not strictly true. Even more confusing, sometimes they say that the color quality of white LED lighting is “just like moonlight”. Here’s why that’s not true at all:

This chart shows the spectrum of full moonlight (blue) with the spectrum of a white LED overplotted in black. Both sources have about the same color temperature, a metric that expresses something about how the intensity of a light source is distributed across the spectrum. Notice the huge spike of blue light in the LED spectrum that is not present in the moonlight spectrum; while the Moon reflects much of the blue light it receives from the Sun, much of that light is scattered away by our atmosphere. That, in part, explains why the night sky around full Moon is so bright.  

The result is that white LED light is considerably bluer than moonlight, if the two sources are matched for intensity. Unless cities that install these new lights also dim them, the night sky over those cities will get brighter. The failure to adjust light brightnesses to match the sensitivity of the eye and night has led to complaints that new LED lighting is glary and distracting. We’re still waiting for measurements that show how much the lighting actually affects night sky visibility, but we do know from extensive research that the additional blue light is potentially harmful to urban wildlife. The future of the nighttime environment in cities, and our ability to continue doing astronomy from the ground, depends on how these lighting conversions are managed, now and in the future.

And while you’re out tonight, have a look to the southwest where three bright planets (Saturn, Venus and Mars) are visible. The view below shows the positions of the planets just after dusk in the northern hemisphere.

A cluster of young stars in the Triangulum Galaxy, one of the nearer large galaxies to our own Milky Way, recently revealed a surprise to researchers: evidence for three neatly concentric supernova remnants piled one atop the other. Like the Russian nesting dolls known as matryoshki, the concentric shells of gas and dust ejected by members of the star cluster that ended their lives in titanic supernova explosions fit neatly together, as in the artist conception above, and offer a new view of the interstellar medium.

(Image by Wikimedia Commons user Fanghong, licensed under CC BY-SA 3.0)

The cluster and its shells, dubbed BCLMP17-d, is too far away to image directly in any detail, but astronomers inferred the picture in the artist’s drawing by measuring the speed at which the gas in the bubbles is expanding. To do this, researchers at the Instituto de Astrofísica de Canarias in Spain used the 4.2 m William Herschel Telescope on the island of La Palma to look at emission from excited hydrogen atoms in the bubbles. Noting very carefully the wavelengths of the hydrogen emission feature in the object’s spectrum, the researchers built up maps of the object (below), where the colors indicate the speed of the gas. The motion of the gas tends to move the hydrogen feature around in the spectrum of the object; using the Doppler principle, the speed can be computed. 

(Figure 2 from Camps-Fariña et al.)

Researchers found three distinct speeds of the gas, with strong signatures around 40 km/s, 100 km/s, and 150 km/s. These are the speeds along the line of sight to the object, which is far enough away that the variation of speed across the apparent size of the star cluster is fairly low. The black contours show the brightness of the light from the bubbles across all velocities, ranging from the least amount of light at the outermost contour, and the most light (in the densest part of the star cluster) at the innermost contour. That the colors match up pretty well with the contours suggests the three speed components are at least roughly spherical, concentric, and centered on the cluster. In other words, they are like three nested soap bubbles, each expanding into space at a different speed.

"This phenomenon allows to explore the interstellar medium in a unique way, we can measure how much matter there is in a shell, approximately a couple of hundred times the mass of the sun in each of the shells,” said John Beckman, one of the co-authors of the paper describing the result.

But we have good reason to think that a typical supernova only ejects about 10 times the mass of the Sun, so how did the shells get to be so massive? The answer has to do with the space around the cluster, into which the bubbles are expanding. It’s not empty space after all, but ‘clumpy’, filled with bits and pieces of the leftovers of star formation and previous generations of stars that themselves ended life in supernova explosions. In the case of BCLMP17-d, the first supernova of the group that went off blasted away some of this clumpy material, evaporating it into the space immediately around the cluster. The second and third supernovae that went off then had something to push against, forming the additional bubbles. 

A Celestial Pas De Deux

If you have been watching the western sky after sunset in the past few weeks, you may have noticed the curious dance of three bright planets: Mercury, Jupiter and Venus. Jupiter’s steady apparent slide to the west each night has brought it near the two inner planets, whose orbits interior to Earth’s keep them forever near the Sun in our skies. Mercury was more prominent in the night sky a couple of weekends ago, but has now begun its dive toward the Sun and inferior conjunction on 13 September, when it will pass nearest the Sun from our vantage point, reappearing in the pre-dawn hours a few days later. Meanwhile, Venus has just begun its long climb into the evening sky. It will be a regular feature of the hour or two after dusk throughout the fall of 2016, reaching its apex twelve days after the new year when the “Evening Star” will stand about 47 degrees east of the Sun in our skies.  

Tonight (27 August), Venus meets her chief rival in the night sky — at least in terms of apparent brightness. Most of the world will see the two planets pass a mere half-degree apart, or roughly the apparent width of the full Moon. By the time evening falls in the Americas, the two bright planets will be separated by only 0.1°, meaning they will fit neatly into the field of view of most telescopes. The image above shows the pairing in the west thirty minutes after local sunset, with Mercury more distantly off to the south and west. The inset shows the view expected in an 8-inh (20 cm) telescope at a magnification of 300 times. Venus will be so close to Jupiter tonight that it will nearly mingle among the latter’s bright moons. Such a close approach, called an appulse, only happens every few decades, and these two planets will not appear so close together in Earth’s skies again until the year 2065. 

In reality, of course, the two planets are very far from each other. Venus shines at apparent magnitude -3.3 and is roughly 1.5 times the average distance between the Earth and the Sun, known as an astronomical unit, or AU. Jupiter is much more distant at 6.4 AU from Earth. It’s a much larger planet than Venus, but the intensity of sunlight at Jupiter’s distance is only about 1/50 of its intensity at Venus. The faint light it reflects back to Earth wins over the large reflecting surface of the planet, meaning that Jupiter shines more dimly at magnitude -1.2. 

If you have a clear sky from the south to the west, tonight (6th August) is a great opportunity to see an unusual sight in the evening sky: all five planets bright enough to be easily visible to the unaided eye, plus a special guest appearance by the four-day-old crescent Moon. In order to see all five, begin looking shortly after sunset; Venus is the brightest of the lot at apparent magnitude -3.3, but it is also closest to the Sun and therefore will be the first to set. Saturn, the most distant of the planets visible, sets last and glows dimmest at magnitude +0.6.

Notice how the positions of the planets and the Moon trace out roughly a straight line across the sky. This apparent alignment does not result simply by chance, but tells us something fundamental about how the Solar System is constructed. The apparent path of the planets and the Moon as they move across Earth’s night sky delimit a line called the ecliptic. Related to the English word eclipse, it derives from its conceptual notion: formally, the ecliptic is the apparent path of the Sun on the celestial sphere. Eclipses of the Sun and Moon therefore take place only when the Moon is near the point in its orbit around the Earth where its apparent path crosses the ecliptic. 

(The ecliptic as traced out on the celestial sphere. Credit: Wikipedia) 

Since the planets all appear to nearly follow the ecliptic as they move across our sky, that means that their orbits must be more or less in line with Earth’s orbit around the Sun. The ecliptic therefore defines a plane of the Solar System in which the orbits of the major planets lie. Again, this isn’t chance, but an artifact of the way that the Solar System formed. According to current scientific thinking, the planets, asteroids, and other solar system bodies (except for long-period comets) formed from a rotating disk of material surrounding the forming Sun, some 4.6 billion years ago. Initially the cloud was more or less spherical, but as it began to collapse it started to rotate, forming a flattened disk, as in this artist’s concept: 

(A protoplanetary disk forming planets. Credit: Luís Calçada) 

Biggest and Brightest: Mars Approaches Opposition

This weekend is a great time to head outside in the early evening and see Mars at its very best for 2016. On Sunday night, 22nd May, the famed Red Planet comes to opposition, meaning that it rises as the Sun sets, and is above the horizon all night long. However, on Saturday night, 21st May, the near-full Moon guides the way to finding Mars low in the southeastern sky just as nightfall begins. The image above shows a view toward the southeast from the U.S. city of Chicago at 10:30pm local time on Saturday night. Mars and the Moon are both in the constellation Scorpius tonight, whose brightest star, Antares, is situated south and east of Mars. The ringed planet Saturn joins the group a little further to the east.   

Tonight we are separated from Mars by about half an astronomical unit (AU), which is itself the average distance between the Earth and the Sun (150 million kilometers, or about 93 million miles). It’s not the closest that Mars ever gets to the Earth; that’s roughly 50 million miles, or about one third of an AU. The last time Mars approached that close to our planet was in 2003, the closest approach to Earth in some 50,000 years, when we were separated by only 56 million kilometers. An event like that won’t happen again until the year 20178, when our worlds will again by only 57 million miles apart. Tonight, although it’s not quite that close, it shines brightly enough to be the brightest object in the sky after the Moon, outshining even Jupiter (visible in the western sky). It won’t be this close again until around the time of the next Mars opposition in July 2018. 

What can you expect to see? With nothing more than your eyes, note the distinct salmon color of Mars, the result of its surface composition. The dust coating the planet’s surface is rich in iron oxides — better known as rust — whose red-orange color dominates the view. Compare the color of Mars to nearby Antares, a bright red supergiant star, whose name itself is a reference to the Red Planet. In ancient Greece, the planet was known as Ares, the Greek god of war. Because of the similarity of color, the Greeks called this star Άντάρης (‘rival of Mars’ or ‘equal to Ares’). Then compare both to the creamy hue of Saturn, whose colors are dominated by the warm tones of sunlight reflected from ices in the rings and ammonia crystals and clouds in its atmosphere. 

A telescope will show even more. Mars is the only planet in the Solar System that unambiguously shows telescopic viewers true surface features (some people have seen markings on Mercury, but these are ephemeral and difficult to observe). Depending on the size of the telescope and the turbulence in the Earth’s atmosphere overhead, observers will see bright basins, dark shadings, and even brilliant white clouds of water ice and carbon dioxide ice. The Martian polar ice caps, too, often show up as bright white spots, where layers of the same ices have accumulated to depths of 2-3 kilometers.  

Just in time for this weekend’s opposition, NASA recently released a brand new Hubble Space Telescope view of Mars taken on 12th May:

Today is Spring Astronomy Day 2016!

See Mercury Race Across The Sun On Monday

Skywatchers in the western hemisphere will see a rare sight on Monday: over the course of several hours, the silhouette of the planet Mercury will appear to cross the face of the Sun. The “transit” of Mercury results from the precise alignment of the orbits of Mercury and Earth that only happens either 13 or 14 times per century; usually the orbital alignment is weak, and as seen from our planet Mercury “misses” the Sun’s disk as it orbits once every 88 days. But on Monday, the view through a properly-shielded telescope will reveal the innermost planet as a dark, perfectly circular spot that moves completely across the Sun in exactly seven and a half hours.

Because of the specifics of our respective orbits, Mercury transits only happen in either the months of May or November, with average dates of 8th May and 10th November. May transits happen less frequently than November transits because during May, Mercury is closer to its largest distance from the Sun, while in November the opposite is true. As a result, the range of possible angles between the Sun and Mercury, as seen from Earth, is smaller in November than May. While the interval between successive November transits can be either 7, 13 or 33 years, May transits are less common, with successive appearances in either 13- or 33-year intervals.

Observations of Mercury transits reach back to at least the seventeenth century. Observations from earlier than this are unlikely because the apparent size of Mercury’s silhouette against the Sun is too small for the unaided eye to resolve. This is why the first recorded Mercury transit — by the French astronomer Pierre Gassendi on 7 November 1631 — dates to after Galileo Galilei’s invention of the telescope in about 1609. Johannes Kepler earlier understood that Mercury’s orbit should periodically take it in front of the Sun, but he died in 1630 before being able to observe a predicted transit. 

While these events once had great scientific interest, they are now mainly curiosities that delight astronomy aficionados. Rarer still are transits of Venus across the Sun, the last of which took place in 2012. These events come in pairs separated by 113 years, meaning that most people alive now will not be around to see the next one in December 2117.

Who can see Monday’s event? That depends on the hour of day and which side of the Earth faces the Sun at the time. The map below indicates which parts of the world see either all, some, or none of the transit: 

You’ll need at least a good pair of binoculars or a telescope — properly shielded with a heavy filer to prevent eye damage — to even sense Mercury during the transit. It will look like a small, perfectly round and completely opaque black dot against the bright solar photosphere. Mercury is distinguishable in this sense from sunspots, which are irregular in shape, can be partially transparent, and of much larger sizes. This image compares Mercury during a transit (bottom-center) with a sunspot near the solar limb (upper right).

NOTE: DO NOT LOOK AT THE SUN THROUGH A TELESCOPE WITHOUT A FULL-APERTURE SOLAR FILTER! Doing so can cause permanent blindness! Instead, try projecting the image of the sun from a telescope or binoculars onto white paper. This method avoids bringing dangerous, strongly-focused sunlight anywhere near one’s eyes. 

Better still: Watch the transit live online! Find live streaming coverage from Slooh, NASA TV, Celestron telescopes, Sky and Telescope magazine, and  the Virtual Telescope.