“It is nightfall; the clouds have vanished,
The sky is clear, pure, and cold…
Silently I watch the River of Stars…
Tonight I must enjoy life to the full,
For if I do not, next month, next year,
Who can know where I shall be?”
– Su T’ung-Po

A layer of fresh snow blankets the northern prairie, thin enough for the tops of golden wheat stubble to poke through, while a blast of arctic air from the northwest sweeps the darkening sky clean. Driving south on a secondary highway, an hour east of the city, I turn onto a back road and pull over by the side in complete darkness save for the lights of a farmhouse half a mile away. Emerging into the cold, I exhale a frosty breath and gaze upward into a bowl of black sky full of crackling stars. To the west I see Pegasus plunging towards the horizon with Andromeda in tow. The Big Dipper lies low in the north, its handle grazing the flat landscape and bowl pointed to the upper right. But the best view tonight lies overhead along the pale arc of the northern Milky Way through the bright constellations Perseus and Auriga, and down to the east skirting Orion’s eastern shoulder, passing the feet of the twins of Gemini, and into Canis Major, the Big Dog, with Sirius hovering over a snow-covered spruce tree like a Christmas star. As my eyes grow adapted to the dark, the outlines of our home galaxy begin to emerge. I grab my little telescope from the back of the car, set it securely on its mount, and get to work.

To us stargazers, the term ‘Milky Way’ refers to the pale white, hazy band of unresolved stars that encircles the sky and wheels into view mostly during the summer and winter months. Nearly all cultures have a name for it. Chinese and many Asian sky watchers called it the “Silver River” or “Star River”. The Spanish called it compostela, the “Field of Stars”. Norwegians called it Vinterbrauta, the Winter Way, since its brightest regions visible in summer are obscured by perpetual twilight of high northern latitudes. Perhaps most evocatively, the !Kung bushmen of southern Africa, who had a spectacular view of its thickest expanse, called it the “Backbone of the Night”.

All the stars along the band of the ‘Milky Way’ belong to our home galaxy which, confusingly, is also called the Milky Way. As galaxies go, it’s a beauty. A larger than average spiral galaxy, it features a bright central bulge and bar from which two large flocculent spiral arms emerge and wind around in a flat disk. These arms, and a handful of smaller spiral arms and spurs, contain most of the gas and dust that serves to form new generations of stars. In its current form, the starriest part of the entire Milky Way galaxy spans about 100,000 light years but has a thickness of just 1,000 light years. Freshly-minted blue-white stars pack the thin pinwheel-shaped disk while a spherical central bulge contains far older yellow-white stars, giving the galaxy a layout rather like a large dinner plate with a dollop of buttery mashed potatoes in the middle.

Despite its size and evident grandeur, the Milky Way emerged from humble beginnings not long after the the birth of the universe about 13.7 billion years ago. It may have started with clumps of hydrogen and helium gas and still-mysterious dark matter a few hundred million years afterwards. Within this clump formed some of the first stars, which ended as dense black holes, which in turn served as a gravitational flytrap to draw in more primordial interstellar gas, stars, and smaller galaxies over the next few billion years. Its inherent rotation eventually flattened into its current disk-like shape that revolves around a now-immense black hole at its center. Even today, the long gravitational arm of the galaxy assimilates stray clouds of cold gas, rogue stars, and dwarf galaxies from its local neighborhood. The Milky Way is not so much a single fixed thing but a dynamic process, a self-assembled celestial empire that’s grown over billions of years to a population of some 200-400 billion stars.

From our vantage point, about 25,000 light years from the center of the galaxy, we mostly see only a fraction of the Milky Way galaxy out to a radius of about 5,000 light years. Obscuring clouds of dark interstellar dust obscure the rest of the galaxy from our view, at least at visible wavelengths. In the northern summer, we look towards the galactic into the spectacular starry band of one of its two major spiral arms, the Scutm-Centaurus Arm and the smaller Carina-Sagittarius Arm. They take their names from the constellations in which they appear prominent. On this northern winter night, I’m looking away from the center into the thickest part of another major spiral arm, the Perseus Arm. In this part of the sky, I’m looking almost exactly opposite the galactic center so the view is more subdued compared to the summer Milky Way, but there’s still plenty to see.

My little telescope, about half the length of a baseball bat and twice as thick, is solid as a craftsman’s tool. While tiny by the standard of professional astronomers, it collects about two hundred times as much light as the human eye and offers a relatively expansive view of the sky about as wide as three fingers held at arm’s length. I indulge myself by aimlessly sweeping the rich star fields of the Perseus Arm and stopping to see a few favorites. I start in star-speckled Cassiopeia, pausing to consider Rho Cass, a yellow-white hypergiant that’s among the largest stars in the galaxy. I move on to clusters of relatively new stars including the sparse NGC 457 with two eyes and outstretched arm that resembles the movie alien ‘E.T’, and the Y-shaped Messier 103. I hop the border into the constellation Perseus to see the magnificent Double Cluster – a stunning piece of celestial jewellery with no equal in the sky. Located some 7,500 light years away, these twin star clusters formed about 12 million years ago out of cold gas and dust in the Perseus Arm. Were they as close as the Pleiades, these two clusters would span nearly a quarter of the northern sky and many of its 600 stars would shine as bright as Sirius. Then I sweep onward to more stars, and more stars, then into Auriga to linger on the trio of open clusters Messier 36 and 38, and especially M37, a densely packed open cluster with an age of 350 million years which makes it an elderly denizen of the Perseus Arm.

I’m not doing any science here, nor am I taking images or sketching, nor doing much of anything other than looking. I’m looking with full attention at the handiwork of a galaxy that, with the help of gravity, turns dark clouds of cold hydrogen gas made at the beginning of the universe into clusters of spanking new stars. It’s a staggering sight. While distant and faint, these star clusters and clouds and nebulae are not indistinct facsimiles, drawings, artist rendition, they are the real thing, products of a major galaxy in action. Yet it’s at least partially accessible to anyone who cares to look with a little practice and less effort than it takes to sweep snow off the front walk.

As I sweep the scope further southeast, the Perseus Arm recedes and turns a corner and the intervening and much closer Orion-Cygnus Arm comes into view. Also called the Local Arm, this minor spiral arm harbors our own solar system on its inner edge. Most of the bright stars visible to the unaided eye in this part of the sky belong to it although some, including our Sun, are just passing through. At the feet of Gemini I encounter more sprawling star clusters resident in the Orion Arm. There’s Messier 35, which resembles a tiny sugar donut. Then about twelve degrees east of Sirius I see the sparse Messier 47 in the constellation Puppis and the adjacent and far richer Messier 46, which also harbors a tiny ring-like planetary nebula ejected from a dying mid-sized star. All these jewel-like star clusters are interesting not for what they look like but for what they are: immense families of newly formed stars.

Surprisingly, the crown jewels of the northern winter sky do not lie along the Milky Way but slightly adjacent to it in the constellation Orion, the Hunter. Perhaps the most recognizable constellation, Orion presents a majestic sight as he rises above the southeastern horizon in early winter, his shield facing the fierce celestial bull of Taurus and a spangled sword hanging from his prominent tri-starred belt. The Milky Way runs past his eastern shoulder marked by the brilliant red supergiant star Betelegeuse. Blue-white Rigel marks his western foot. Plenty of star clusters and nebulous stellar nurseries fleck Orion along with coal-black clouds of coal molecules and interstellar dust that are slowly collapsing into even more star-forming regions. As we will see, the activity in Orion was triggered not by the usual gravitational tugging and kneading inside a spiral arm of the galaxy, but possibly because of a collision between the Milky Way and an external intergalactic cloud that formed an undulating wave of compressed gas and dust that spans half the sky.

After it skirts Orion and passes through Canis Major, the winter Milky Way continues below my local horizon into the constellations Vela (the Sail) and Carina (the Keel), which along with Puppis (the Stern) in antiquity comprised a single immense constellation called Argo Navis. Here, and further south into Crux or the ‘Southern Cross’, the Milky Way becomes truly spectacular for observers south of the equator. Many northern stargazers dream of heading south to see this most spectacular section of sky, and all who go agree it’s worth the trip to see the ‘other half’ of the galaxy.

Thoughts of a warm night stargazing in the southern hemisphere distract me now as the heat drains from my body, despite wearing extra layer clothing, after standing nearly still over a telescope for an hour in the icy night. So I pack up the telescope and prepare to head home. Before leaving, I set up a camera and a wide-angle lens to capture a few images of Orion and the winter Milky Way running off its eastern shoulder down to the horizon. Serious astrophotography involves gazing at computer screens more than the stars, which is not for me, but I sometimes snap a few photos to remember the night. I’ve taken dozens of photos of this part of the sky, all similar but no two exactly alike, and I’ve seen it hundreds of times since I first started serious stargazing well before my tenth birthday. It never grows tiresome. These stars change little over a brief human lifetime, but new discoveries about the Milky Way and my own efforts to better understand what’s out there and how it fits together lets me see it slightly differently each time. To paraphrase Heraclitus, no man ever steps in the same star river twice.

Reflection nebulae have an undeniable allure to visual observers and astrophotographers. These little blisters of blue-white light, though typically small and rather challenging to detect, offer a chance to glimpse the galaxy at work turning cold dust and gas into new star systems. So grab your telescope and let’s take a tour of some reflection nebula prominent in northern winter with my recent article published in the 1,000th issue of Sky & Telescope magazine. Click on the image above to download the article on PDF format (it’s about 6MB). Then enjoy the tour!

Comet C/2023 A3 (Tsuchinshan-ATLAS) has emerged in the morning sky for observers in the southern hemisphere, and so far it seems to be living up to its billing as the brightest comet in years. The comet has brightened to naked-eye visibility and reports suggest it will continue to brighten in the next few weeks. Observers in the northern hemisphere get a crack at seeing the comet in later in September and in to October in the evening sky.

Astronomers first observed this little interloper from the Oort Cloud at Purple Mountain (Zijinshan, or ‘Tsuchinshan’) Observatory in China on January 9, 2023, and at the robotic survey Asteroid Terrestrial-impact Last Alert System (ATLAS) in South Africa on February 22, 2023. At the time, it was a very dim 18th magnitude. A year later, as it approached the plane of the solar system, the comet brightened to 13th magnitude, bright enough to detect visually, with some difficulty, in a 15-inch or larger telescope. By April 2024 it reached 10th magnitude and revealed a tail as long as 15’ in photographs.

The comet is small, just one or two kilometers across. But as it warms, the outer layers of this dirty cosmic snowball sublimate and a glowing gaseous halo or ‘coma’ grows larger. The comet also ejects a lengthening tail away from the Sun. Spectral measurements show Comet C/2023 (A3) is relatively carbon-depleted and has a large dust-to-gas ratio. With such a dusty makeup and an expected close solar approach, it may grow a long and reflective dust tail as it rounds the sun and gets closer to Earth.

Through most of September, the comet was too close to the Sun and remained too dim to observe. But that will change quickly. The comet reaches perihelion on September 24, 2024 when it lies 0.39 astronomical units (AU) from the Sun, about the same distance as Mercury. On October 12, 2024 it comes nearest to Earth at a distance of 0.47 AU or about 70 million kilometers.

How bright will it get? Comets are notoriously hard to predict, has grown as bright as 2nd or 3rd magnitude, bright enough to see without optical aid. But it continues to bright and may outshine the nifty little Comet C/2020 (F3) NEOWISE in 2020. If we’re lucky, it may grow even brighter.

How To See Comet C/2023 A3 (Tsuchinshan-ATLAS)

For much of September 2024, the comet lies too close to the Sun to see from Earth. It emerges into the sky in late September and by month’s end will lie about 23° from the Sun with – we hope – an impressive tail. At this time, the comet is best seen just above the eastern horizon in the morning sky in the southern hemisphere in the constellation Sextans.

It will barely peek above the horizon for northern hemisphere observers from about Sept. 27 – Oct. 2, then becomes briefly unobservable as it moves into southern Leo from Sept. 28 to Oct. 5. The comet moves eastward into Virgo and, by about October 11, emerges into the evening sky for northern hemisphere observers and moves well above the western horizon over the coming days (southern observers will struggle to see the comet just above the western horizon just to the right of the Sun). For the last two weeks of October, the comet and its dusty tail may be visible in evening twilight without optics and become even easier to see as the sky darkens. It may grow nearly as bright as Venus – this remains to be seen – and become one of the brightest comets visible from the northern hemisphere in many decades. We will have to wait and see – fingers crossed.

While we all hope the comet will grow bright enough to see with the naked eye, it might help to bring a pair of binoculars to help pull the comet out of the twilight and get a better view of its coma and tail.

Towards the end of October, the comet dims steadily and may reach 4th or 5th magnitude and rise higher in the evening sky. It continues to fade into November below naked-eye brightness.

Whatever happens to C/2023 A3 (Tsuchinshan-ATLAS) as it approaches the Sun, it won’t be back this way soon, or possibly ever. The comet comes from the distant Oort Cloud, a spherical shell of perhaps a trillion comets outside the orbit of Neptune and extending perhaps as far as halfway to the nearest star. The comet is also entering the inner solar system in a retrograde orbit (opposite the planets) at a high inclination of about 139º. It will largely avoid interacting with the gas giants like Jupiter and Saturn so its orbit will not be perturbed by them. Current projections show a hyperbolic (open) orbit which will send the comet on a path to escape the solar system. So it’s likely a ‘one and done’ visitor from the most distant reaches of our solar system. See it while you can!

On the evening of Sept. 16, I was set up to capture some images of the Elephant Trunk Nebula in Cepheus. I balanced and aligned the mount, focused the telescope, queued up the camera, snapped a test image from inside the house, and ... what? The sky appeared washed out as if clouds rolled in. I went back outside, looked up, and WOW...! It wasn’t clouds obscuring the stars, it was a spectacular show of aurorae borealis as a solar coronal mass ejection (CME) hit Earth in the form of a G4-class geomagnetic storm.

The most spectacular outburst ran just ten minutes around 9:20 p.m. MDT. I had just enough time to grab (a different) camera and take a couple of shots and short video. To the eye, much of the aurora appeared green and white, at least from my suburban location. The camera revealed the white areas as red, a color that results from excited oxygen atoms about 150 miles up. The most intense aurorae appeared overhead and seemed to emanate eerily direction from the bright star Vega in the constellation Lyra. The video below shows the display close to its peak - the time scale here is sped up by 8x, so it packs three minutes into about 23 seconds.

It was a brief but spectacular show. Spaceweather.com reports that observers as far south as Arizona saw traces of aurorae. And, as the sun’s cyclical activity approaches a maximum in the coming months, there’s more aurorae on the way over the coming autumn and winter.

And my photos of the Elephant Trunk? It’ll have to wait until next time...

Tangled in the eastern ‘wing’ of the spectacular constellation Cygnus, the Swan, the delicate lacework of the Veil Nebula belies its origins in one of the most powerful explosions in the galaxy. This sprawling complex, just off the band of the summer Milky Way, spans the diameter of seven full moons and beguiles the observer with its complexity and ragged symmetry. It’s always worthy of a quick look and easily warrants an entire night at the telescope to see all of its rich and complex structure.

Once thought to be caused by two separate stellar detonations, the Veil Nebula, also known as the Cygnus Loop, was likely produced by a single star of about twenty solar masses that blew up as a supernova about 8,000 years ago. A supernova explosion occurs as big stars run out of fuel in their core and become unable to hold themselves up against the relentless pull of their own gravity. Their outer layers collapse, crush the star’s dense core into a neutron star or black hole, then snap back in a violent explosion that ejects as much energy in a few minutes as our sun does in its entire lifetime. At a distance of just 2,100 light years, the event must have been spectacularly bright and visible even by daylight for weeks to pre-historic stargazers, inciting much speculation about the nature of the night sky - a certainly a great deal of awe.

Upon detonation of the supernova, the shock wave expanded and smashed into an expanding bubble of gas cast off by the star before it exploded. The nebula now spans about 110 light years and it continues to expand at a rate of about 1.5 million kilometers per hour. Astronomers have directly observed the expansion of the nebula by comparing images taken by the Hubble Space Telescope between 1997 and 2015. We see nebulosity spread over about 3.5 degrees of sky or about the width of three fingers held at arm’s length. 

Since only the largest stars expire like this, and since the explosion itself plays out quickly over a few days or weeks, a supernova is a relatively rare event: the last known supernova in the Milky Way happened more than 400 years ago. Such explosions often leave a long-lasting imprint in the form of a visible nebula caused by the rapidly expanding shock wave. Dozens of these so-called supernova remnants fleck the night sky with evocative names like the Spaghetti Nebula, the Jellyfish Nebula, and the Manatee nebula. Most require a substantial amateur telescope and sensitive camera to detect. But except for the much smaller Crab Nebula, the sprawling Cygnus Loop is one of the easiest to see and photograph and one of the most intricate and intrinsically beautiful objects in our galaxy.

While it’s lovely to behold the Veil Nebula in photos as a form of abstract sculpture on a galactic scale, it’s also an object you can see for yourself. The complex lies a few degrees off the star epsilon (ε) Cygni (Gienah) at the tip of the eastern wing of Cygnus. It’s easily within reach of a small telescope in dark sky, especially with the help of a nebula filter, and even in a good pair of 10x50 binoculars. Like many sights in the deep sky, the Veil was first glimpsed by William Herschel in 1784. He noticed the western end of the nebula, now cataloged as NGC 6960, which runs through the 4^th-magnitude star 52 Cygni. The eastern arc of the Veil complex (NGC 6992 and NGC 6995) is easier to see in a telescope and arguably more intricate and appealing. Unless you have a field of view of 4^o or more (a large field for a telescope), you’ll only see one section at a time.  Even the brightest part of the Veil, NGC 6992, is roughly a full degree across. The delicate lacework becomes visible in 4” or larger telescopes with a UHC or OIII filter. Use higher magnification (and more aperture if possible) to see the finer structure. 

Between the two elongated sections lies the more tenuous section called Pickering’s Triangle named after Harvard astronomer Edward Charles Pickering, although to me it looks in images more like an interstellar tornado. Pickering captured an image of this section, but it was first noticed by his redoubtable assistant, Williamina Fleming. A single mother with little formal education, Fleming served as a maid in Pickering’s home until his wife noticed her keen intellect and recommended her to her husband for a position at the Harvard College Observatory in 1879. She soon became a keen and prolific analyzer of stellar spectra and discovered the now-famous Horsehead Nebula in Orion, also on a photographic plate made by Pickering.

Without question, my best view of the Veil, or at least one small and intricately knotted section of the eastern Veil (NGC 6992), came on a clear July night in 2011 in the Adirondacks in New York State. The biggest scope at the observing site was a 25” Dobsonian with a stepladder leading up to the eyepiece, aimed at the Veil Nebula. There was no lineup to look through the scope, only one young woman descending the ladder after seeing the nebula and wondering what all the fuss was about. She was interested, but clearly a beginner and didn’t have much to compare it to. The owner of the telescope explained patiently that it was indeed an impressive sight and that a more experienced observer would find it so. I asked the owner if I could head up to take a look. As he continued his explanation to the young woman, I looked through the eyepiece. “Oh my GOD”, I said. “See”, said the telescope’s owner, nodding towards me at the top of the ladder. “He is an experienced observer”.

Given its size, relative brightness, and complex shape, the photogenic Cygnus Loop also makes an excellent target for astrophotography. I’ve captured simple images using nothing more than a small monochrome astro-camera perched stationary on a picnic table and pointed straight up when the Veil was nearly overhead. A little more effort and gear yield spectacular results. All the images on this page were made with a modest astronomy camera and various camera lenses with less than one inch aperture (and a little practice). Of course, the Hubble Space Telescope takes pretty good pictures of the Veil Nebula, too. The video below shows you a close-up of the Veil Nebula taken with Hubble.

For observers like me who live north of the 50th parallel, the few weeks before and after summer solstice never grow dark enough for serious stargazing, and few stars at all are visible until after midnight. It’s a little frustrating. But a few years ago, impatient and hungry for starlight on an early summer evening, I happened across the words of the famous telescope builder and solar astronomer George Ellery Hale who wrote, “The Sun is a star, the only star whose phenomena can be studied in detail.” Of course, the Sun! If I was going to stargaze in the early summer months, the star I was going to gaze at would be the Sun. 

I already owned a “white-light” solar filter for one of my telescopes. It showed sunspots, the sandpapery solar photosphere, and the subtle darkening of the solar disk towards its limb. It’s a nice view, but not spectacular, which is why serious solar observers eventually turn to specialized (and pricey) ‘hydrogen alpha’ solar filters that show a far more striking view of our home star. So I bit the bullet and bought a small 60mm telescope with a dedicated H-alpha filter, turned it towards the Sun, and climbed the learning curve to get the most out of this little instrument.

The view was astonishing! The Sun looked like a blood orange suspended in blackness, with flamelike prominences and a forest of needle-like spicules emanating from the solar limb, feather-like filaments on the solar disk, and gnarled knots of plasma enveloping ever-changing sunspots. Best of all, unlike most sights in the night sky, features on the Sun changed from day to day and sometimes over just a few hours. The look is different from night-time astronomy - there’s no ‘diamond dust on a velvet background’ aesthetic that you see in the dark night sky - but the sun close up offers a unique and beautiful vista unlike any other in nature.

It’s awe inspiring - and a little unsettling. 

The Sun is big - a hundred times the Earth’s diameter and a million times its volume. In the image below, three or four earths would fit under the little looping prominence at the top of the solar disk. That’s easy enough to grasp. 

But a look at the ruby solar disk also leads an observer to consider that we live at the discretion of a nuclear-fueled stellar colossus that generates a staggering amount of power.  

The Sun produces its energy through nuclear fusion in its core where protons and electrons reach a density more than ten times that of lead and a temperature of 15 million degrees Celsius. In a multi-stage nuclear reaction discovered in the 1930s, four protons fuse together into a single helium nucleus with a tiny fraction of the mass of the protons converting into pure energy. How much energy? Just one gram of hydrogen fused into helium releases enough energy to (in principle) power an average house for two years, and the Sun has enough hydrogen to release as much energy as 10 billion hydrogen bombs every second for billions of years. Energy generated in the solar core works its way to the solar photosphere and out into space over tens of thousands of years - the light and heat that warms your bones this afternoon took only eight minutes to travel from the Sun’s outer layers, but it began its journey in the solar core back when Neanderthals still walked the Earth.

The fearsome amount of energy emanating from the sun’s core is held in check by the far weaker but more relentless force of gravity. The energy produced in the core wants to blow the Sun apart, but weight of the star’s massive outer layers pushing inward holds it together. This state of uneasy coexistence is called hydrostatic equilibrium and it keeps the Sun stable in brightness to within about 0.1%, steady enough to make life on Earth fairly pleasant despite it proximity to such an immense power source.

This happy state isn’t entirely stable, however. As the Sun burns through its allotment of hydrogen, the rate of energy production slowly decreases, which allows the outer layers to squeeze the solar core even more, which accelerates the rate of energy production and increases the Sun’s brightness by about 1% every hundred million years. That increase in brightness will surely affect the Earth’s biosphere - in half a billion years, the surface of our planet will be too hot to sustain nearly any form of life (as we currently understand it).

After that, the Sun runs out of hydrogen to burn in the core and undergoes significant changes to its inner structure that will balloon the star outwards as a red giant giving the entire Earth, or what’s left of it, a very bad sunburn indeed.

As I say, solar observing can be a little unsettling and awe inspiring - and always worth the effort. But the Sun’s unruly future is a concern for another day - and very likely for species that won’t exist on Earth for tens of millions of years. Our place now is to enjoy our powerful but placid star, to soak up its warmth on a summer day, and to exercise our imaginations and intellect to try to understand, at least a little, how nature works.

With dozens to hundreds of glittering blue-white stars, open clusters like the Pleiades and Double Cluster in Perseus are favorites of stargazers with backyard telescopes. These groups of newly-formed stars emerge from dark clouds of interstellar gas and dust and serve as laboratories for astronomers trying to understand stellar evolution. Now, in a fascinating result published in June 2024, a team of astronomers discovered that open star clusters themselves form in clusters and that more than half the clusters in our immediate neighborhood have a common origin in one of only three star forming regions about 30 million years ago. After you read this, you won’t look at star clusters quite the same way again. 

The study, led by Cameren Swiggum and João Alves of the University of Vienna, used data from the Gaia space observatory which was launched in 2013 to help astronomers understand the position and velocity of more than a billion stars in our part of the Milky Way galaxy. Earlier studies of the motion of a few star clusters suggested that some may trace their motion back to a common origin, but accurate data was hard to find. So Swiggum and his collaborators mined far more precise data from Gaia to sort out the dynamics of stars in 272 open clusters within 3,200 light years of the sun. These star clusters are currently scattered seemingly at random across the sky. But when the astronomers used computer models to wind the clock backwards to trace back the position of the clusters over the past 60 million years, they found 155 of them (about 57%) had a common origin in one of three groups (or families) each of which presumably emerged from immense clouds of gas and dust that spawned thousands of stars in a spiral arm of the Milky Way. 

The astronomers named each of these three families of star clusters after a prominent older cluster within it. 

The Collinder 135 family contains 39 clusters and appears to have formed about 30.9 million years ago. Its namesake cluster, a sparse but bright collection in the southern constellation Puppis, is anchored by the bright red supergiant star Pi Puppis. The family also includes NGC 2547 and IC 2395, a pair of small 5th-magnitude clusters in the constellation Vela easily visible in a telescope.

The M6 family contains 34 open clusters and appears to have formed about 34.7 million years ago. It takes its name from the dazzling open cluster Messier 6 near the ‘stinger’ of the constellation Scorpius, the Scorpion, and also includes the visually appealing IC 2391 (a.k.a. the Omicron Velorum Cluster) and NGC 3228, both in Vela, and NGC 2451A in nearby Puppis. 

The far larger Alpha Persei family contains 82 star clusters and dates back about 20 million years. The namesake cluster is a group of bright blue-white stars gathered around the star Mirfak (Alpha Persei) that are cataloged as Melotte 20 and informally called the ‘Attendants of Mirfak’. Its stars are easily visible in binoculars. Other family members are scattered widely across the sky and include IC 2602 (the ‘Southern Pleiades’) in Carina and IC 4665 in Ophiuchus.  

This interactive link lets you take a closer look at some of the most prominent clusters within each family.

This interactive tool gives you hands-on control of the calculations used in this study and lets you walk them backwards and forwards in time to see how the star clusters move about the galaxy from their place of formation to the present day.

And the video below gives an overview of the Gaia mission and the results from Swiggum’s study. 

These three families of star clusters may also have influenced galactic structure. Given the age of each family and estimates of the masses of their stars, Swiggum and his collaborators estimated that some 240 stars have detonated as supernovae in these 155 star clusters over the past 30 million years. The past positions of each family suggest that supernovae in the Alpha Persei family may have carved out the Local Bubble, a 500 light-year-wide region relatively free of interstellar dust which is nearly centered on the sun. The shock waves that created the Local Bubble may have led to additional star formation in the Taurus and Scorpius-Centaurus star forming regions. Supernovae from the Collinder 135 and M6 families may have formed a similar but somewhat larger nearby bubble called GSH 238+00+09.

Good tools - in the hands of skilled scientists - almost invariably lead to important discoveries, and the Gaia observatory has led to its fair share as astronomers try to sort out our part of the galaxy over space and time. Many of the researchers involved in this study of star clusters also discovered, for example, the remarkable Radcliffe Wave, a 9,000 light-year-long undulating structure made of molecular clouds that stretch from Monoceros to Cygnus and which lies along the edge of the two bubbles formed by these families of star clusters. We can expect more discoveries as astronomers digest and analyze Gaia’s firehose of data. 

Much of this spring brought cloud and blustery winds along with much-needed rain, so I contented myself with watching wildflowers bloom and peering through a telescope to watch northern flickers and bohemian waxwings get about their business. On a few nights, starlight streamed through “sucker holes”, transitory gaps in thick clouds that tempt stargazers to spend time setting up a telescope before snapping shut. I didn’t take the bait, but I paused from time to time to catch a glimpse high in the northeast of yellow-orange Arcturus, a brilliant star that’s a reliable herald of northern spring and as welcome as the face of a friend in a hostile crowd.

Arcturus ranks as the fourth brightest star in the sky and the brightest north of the celestial equator. It takes its name from the ancient Greek arktos (‘bear watcher’). The star lies directly south of Ursa Major, the Great Bear and its embedded Big Dipper asterism. Novice stargazers learn to find Arcturus by following the arc of the Dipper’s handle away from its bowl with the famous ditty:

“Follow the arc to Arcturus,

And on to Spica go;

Then turn northwest to Regulus, the foot of the Lion, Leo

Together Arcturus, Spica, and Regulus form the Spring Triangle and serve as the brightest stars in an otherwise sparse spring sky.

Arcturus and its host constellation Boötes has long figured in cultures and legends across the world. In the Odyssey, Homer writes of Odysseus sailing his ship away from the land of Ogygia where he was held captive for seven years and tells of him:

“...taking his seat artfully with the steering oar he held her

on her course, nor did sleep ever descend on his eyelids

as he kept his eye on the Pleiades and late-setting Boötes,

and the Bear, to whom men give also the name of the Wagon,

who turns about in a fixed place and looks at Orion,

and she alone is never plunged in the wash of the Ocean.”

Polynesian navigators called the star Hōkūleʻa, the "star of joy" and used it to sail their double-hulled canoes thousands of miles from Tahiti and the Marquesas Islands northwards. They followed the Pleiades and the star Aldebaran eastward, then turned north to cross the equator until Arcturus was overhead, and finally followed trade winds to sail west. By maintaining a heading that kept Arcturus at the zenith at its highest point in the sky each night they eventually arrived at the Big Island of Hawai’i. For the return trip, they made their way back southward by following the winds and currents until Sirius, the zenith star of Tahiti, lay overhead. 

Arcturus also dazzled visitors to the 1933 Century of Progress International Exposition in Chicago, more simply known as the 1933 World’s Fair. Astronomers of the day placed the star at a distance of 40 light years which meant its light left 40 years earlier during the last World’s Fair held in Chicago in 1893. So they devised a scheme to collect the star’s light with a telescope, convert it to electricity using photocells, then amplify and transmit the signal hundreds of miles through telegraph lines to switch on a powerful spotlight on the Hall of Science on opening night. It worked. On May 27, a crowd of 30,000 cheered by the Lake Michigan shoreline as the fairground was illuminated with the help of Arcturus while they glanced up to see the star itself beaming in the warm spring sky. The 1933 fair was a success with many spectacular demonstrations of advancements in science, technology, and architecture that offered a more promising future during the dark days of the Great Depression. 

A Preview of Our Solar System’s Future

More precise measurements place Arcturus at a relatively nearby distance of 36.7 light years, and the star offers a study in contrast with our own sun. Arcturus is only about 10% more massive than our sun, and it possesses a magnetic field that varies over 14 years to the sun’s 11-year cycle. It has spectrum rich with signatures of elements such as iron, sodium, and calcium, but at abundances only a third that of the sun. Arcturus rotates slowly, once every two years, to our star’s 26-day rotational period. At 4,300K, the star’s outer atmosphere is cooler the sun’s 5,700K temperature. Most importantly, Arcturus has an age of about 7 billion years, more than 2.5 billion years older than the sun, and so offers astronomers a close-up view of our solar system’s future.

When it was contented middle-aged star and burning hydrogen in its core, Arcturus shone only modestly brighter than our sun. But the star has since entirely consumed its allotment of hydrogen and, without an energy source to push back, endured the iron hand of gravity squeezing its central region until it grew hot enough to burn hydrogen in a shell around an inert helium core. The fierce energy from the hydrogen burning shell ballooned out the star’s rarefied outer layers to some 25 solar diameters and increased its brightness to more than 170 suns. As they expanded, the outer layers cooled to a temperature of about 4,300 K which gives the star a tangerine hue that’s the signature of a mid-sized red giant star. The visible layers of the yellow-white sun have a temperature of about 5,700K.

As shell burning continues in a red giant like Arcturus, more inert helium ash collects in the core which increases its mass and density until electrons get packed tight enough to experience quantum mechanical effects that resolutely push back against the star’s crushing outer layers. Physicists call matter degenerate, a term that refers not to any moral failing on the part of the electrons but rather to a state that behaves entirely differently from a gas. As heat is added to a normal gas in a star’s core, it expands and cools - a built-in self-correcting mechanism. But in a degenerate state, the star’s core simply grows hotter without expanding, more like a liquid than a gas. When the degenerate helium-rich core reaches 100 million Kelvin, it begins to burn helium into carbon nuclei through nuclear fusion. This creates more heat, which further increases the rate of nuclear fusion, which releases even more heat in a runaway effect known as the helium flash. For this brief phase of a star’s life, half an Earth mass of helium ignites in just a few minutes and releases a staggering amount of energy, perhaps as much as all the stars in the entire Milky Way. It sounds violent and spectacular, but outside observers see very little, if anything, of the helium flash. Most of its energy goes into undoing the work of gravity by expanding the star’s dense and massive core out of a state of electron degeneracy and back into a hot and dense non-degenerate gas.

Because the precise mass of Arcturus is uncertain, astronomers still debate whether it has endured its helium flash. But when a star does so, it shrinks and dims considerably compared to its red giant phase and continues for the next 100 million years to fuse helium into carbon in the core and hydrogen to helium in a shell around the core. When the helium runs out in the core, the game is up. The star isn’t sufficiently massive to squeeze the core hot enough to fuse carbon into heavier elements. Instead endures a series of coughs and sputters over tens of millions of years as it casts off its outer layers as an immense and rarified planetary nebula. The scorching-hot carbon core remains as a hot stellar cinder called a white dwarf. 

Arcturus, it appears, has no major planets to endure its erratic and menacing red giant phase, possibly because it formed at a time and place when fewer heavier elements were present in its birth cloud. But when the sun expands into a red giant like Arcturus in half a billion years, plenty of mayhem will ensue. Its tenuous outer layers will surely incinerate Mercury and Venus. Earth oceans, atmosphere, and possibly its outer crust will boil away leaving only an iron-rich core. As the sun loses mass during its final stages, the Earth’s remnants and the outer planets will shift into more distant orbits around a burnt-out stellar core where they will remain orbiting, sterile and forlorn, for tens of billions of years.

A Star on the Move

Whatever its eventual fate, Arcturus emerged from its birthplace when the universe was half its present age and it appears to have endured a long journey to arrive in our celestial neighborhood. The star shares a common motion through the galaxy - but not a sibling-like chemical composition - with a group of 52 of stars known as the Arcturus Stream that were perhaps once part of a much smaller galaxy ingested by the Milky Way billions of years ago. Other members of the stream are scattered around the sky and some are visible to the unaided eye including 27 Cancri and Anser in the constellation Vulpecula. These stars inhabit the so-called thick disk of the galaxy, a collection of older stars that bob up and down within 4,000 light years of the plane of the Milky Way’s 100,000 light-year-wide disk. By contrast, most freshly-born stars occupy the thin disk that extends just a few hundred light years above and below the galactic plane. The origin of the thick disk is an active area of research for astronomers who are trying to determine whether its ancient stars came from the accretion of dozens of smaller galaxies into the Milky Way or if they arose from internal dynamical processes.

Befitting a star that’s come a long way, Arcturus is surely a star on the move. It’s presently moving towards our solar system at a speed of 121 km/s and will reach its closest point in about 4,000 years before speeding away. In half a million years, Arcturus will lie far enough to fade from visibility to the unaided eye.

This rapid motion across the sky eventually captured the attention of astronomers. Edmund Halley noted in 1717 that Arcturus’ position changed appreciably compared to that noted by Ptolemy and Hipparchus some 1,800 years earlier. Halley concluded, as did Jacques Cassini more convincingly a few years later, that his ancient predecessors couldn’t have made such a conspicuous error. Arcturus moved, a direct refutation of Aristotle’s declaration that the heavens are fixed and unchanging. Astronomers call this type of movement perpendicular to our line of sight proper motion. Most nearby stars exhibit such motion to some degree. More precise measurements show Arcturus moves southward at about 2.3”/year, which works out to about 3 arc-minutes, a tenth the diameter of the full Moon, in an average human lifetime. Most stars have smaller proper motions, but all stars move, and measurements from the amazingly productive GAIA space observatory show the evolution of the position and motion of more than a billion stars in our part of the Milky Way galaxy as shown in this video:

Like most stars, even bright ones, Arcturus presents little detail in a backyard telescope. But it’s always worth a look. At the end of a spring night when I’m out hunting galaxies and globular clusters, I’ll swing the scope towards the star to take a long look through the eyepiece at its shimmering disk in the cold early-morning air. Before heading inside to catch a few hours of sleep, I’ll leave the scope aligned to the celestial sphere and return to the eyepiece in mid-morning to see Arcturus low in the west, glittering like a glass shard in the bright blue morning sky, a well-traveled beacon some half the age of the universe pulled into our neighborhood by the gravitational vicissitudes of the Milky Way.

“Judged by the eye alone, the Milky Way is one of the most delicately beautiful phenomena in the entire realm of nature - a shimmer of silvery gauze stretched across the sky; but studied in the light of its revelations, it is the most stupendous object presented to human ken.”

- Garrett P. Serviss in Curiosities of the Sky (1909).

If you’re seeking starlight, the band of the northern Milky Way offers a good place to start. Away from city lights on a July or August evening, its luminous band emerges slowly in the growing darkness and stretches from the southwestern horizon to the northeast and up to the zenith. City dwellers who see the Milky Way for the first time often mistake it for terrestrial clouds. When they’re told its true nature, that it’s a band of a hundred billion stars etched with inky clouds of interstellar dust they fall silent and consider that perhaps spending a little more time stargazing might be worth the effort.

Many articles in the pages of Seeking Starlight will be devoted to exploring specific deep-sky sights along the Milky Way, larger structures such as dark nebulae and star clouds, as well as the structure of the galaxy as a whole. The Milky Way is our home galaxy so it makes sense to learn more about it. It offers endless pleasures on an aesthetic and intellectual level, and it’s but one of at least a hundred billion other galaxies strewn in immense filaments across the observable universe.

I captured the above image of part of the Milky Way in 2018 on a steamy July night from C.M. Crockett Park just south of Warrenton, Virginia, one of my regular stargazing spots back when I lived in suburban Washington, D.C. I spent the first few hours of the night bent over a small refractor on the outskirts of a dewy field working my way across the sky looking at star clusters. I needed a break and stopped to chat with a few fellow stargazers who worked through their own observing lists. Then I walked in the darkness along the edge of the field through wet grass and came across an abandoned sailboat, too large it seemed to sail on the park’s tiny lake and at least 50km from tidewater on the Potomac River. It made a striking sight against the summer sky, and it was too good to pass up a picture. I returned to the car to grab a camera and wide-angle lens and tripod to take a few snapshots.

In the image above, you see the view to the southwest of the Great Sagittarius Star Cloud, the starriest part of the sky, which lies in the direction of the centre of the Milky Way. More star clouds and archipelagos of dark nebulae lie along the galactic plane that stretches to the upper left of the image into the constellations Scutum, Aquila,and Cygnus. To the left of the Milky Way you see bright Jupiter, bloated and misty because of dew that began to rapidly condense on the camera lens. The white glow along the horizon is the light dome of Fredericksburg, Virginia.

I’ve travelled the band of the summer Milky Way a thousand times and I never tire of it. As with most things in nature, the more you look and learn about it, the more there is to see.