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November 8, 2016November 8, 2016 by Bob King

What I Learned Writing ‘Night Sky with the Naked Eye’

The author enjoys a pretty display of the northern lights on October 23 under a starry sky. His new book, "Night Sky with the Naked Eye," explores all the amazing things you can see in the sky without special equipment including satellites, planets, meteor showers and of course, the aurora. — The author enjoys a pretty display of the northern lights on October 23, 2016 under a starry sky. His new book, “Night Sky with the Naked Eye,” explores all the amazing things you can see in the sky without special equipment including satellites, planets, meteor showers and of course, the aurora.

My book Night Sky with the Naked Eye publishes today. It would have never seen the light of day much less ever been conceived were it not for Fraser Cain, publisher of Universe Today, and Nancy Atkinson, an editor and writer for the same. Several years ago, Nancy invited me to write for UT. I hopped at the chance. Before her contact, I’d been writing a daily blog on astronomy called Astro Bob (and still do).

Fast forward to last summer when I got an email from Nancy saying Page Street Publishing had contacted her about writing a book about space missions. The publisher also wanted a book about night sky observing without fancy equipment for which she recommended me. Me? I felt like the luckiest guy on the planet!

Book writing proceeds in many stages. First, the table of contents had to be prepared and approved. Then followed a sample chapter. The publisher chose the one on artificial satellites, which I wrote in about a week. The tone was right, but he asked for changes in the organization, which I dutifully made. By November, a contract followed and the project was underway with a first draft due to my editor in about 10 weeks.

Cover of my book that publishes today. Credit: Bob King

Writing is hard work. But it’s a special place all writers come back to again and again. We can’t help but keep trying to find just the right words to capture a concept or emotion. And when we do, a quiet pleasure flows down the spine like warmth creeping into cold fingers splayed in front of a fire. Not that these moments always come easily. Writer Colson Whitehead describes the experience of writing as “crawling through glass.” I would soon become well-acquainted with that feeling, too.

Nancy wrote her book Incredible Stories from Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos at nearly the same time. We were grateful for each other’s support, and it was a kick to follow her progress as well as bounce ideas around. With a tight deadline in front of me, I set to work immediately, taking more than two weeks of vacation from my regular job to make sure the draft was done on time. No way was I going to compromise an opportunity of a lifetime.

Maybe you’ve thought of writing a book, starting a blog or hope one day to write for Universe Today or another online astronomy site. There’s plenty of good advice for writers out there. I’ll share what worked for me.

#1: Put your butt in the chair and keep it in the chair. My wife reminded me of this often, adding that the book wasn’t going to write itself. Temptations are everywhere. Answering the phone, making another cup of tea, staring out the window and my favorite, shoveling the driveway. I had the cleanest driveway in the neighborhood. Even an inch of new snow was enough to grab the shovel and happily scrape down to the gravel. So yes, I did occasionally get out of the chair, but many times it did me good, freeing up the brain to see more clearly into a topic. Or dream up a fitting photo or illustration.

Creativity comes at odd little moments. It can flow while tapping away in front of a glowing screen or sneak into consciousness when you’re bending down to feed the dog. So a mix of activities seemed the best but with extra emphasis on staying put. I rarely hiked last winter and kept my walks in the neighborhood brief. Instead of observing at night, I wrote or gathered photos. By January, I joked to my friends that I’d voluntarily put myself under house arrest.

#2: Spill your guts, worry about the details later. It’s incredibly tempting when writing to continuously edit one’s work, going back over every sentence to make each “perfect”. This is a muse-killer. Though difficult to stick to, once you let your thoughts flow onto paper without worrying about spelling, clauses and the whole lot of burdensome rules, you’ll become a wild horse running free on the prairie. Let it out, let it out and worry about the commas later. I don’t play a musical instrument, but free-flow writing — just getting the ideas out — must feel something like riffing on a jazz melody.

#3: When stuck, move on to another topic, take a walk, listen to music. Struggling to describe an important concept or connecting your thoughts in a way that flows on the page can drive you nuts, even bring you to tears. Sure, you can keep beating on the idea like a madman hammering on a bent nail, but why why torture yourself? A little distraction can be good. Move on to another part of the story or a different chapter or get up and take a short walk. Defocusing allows the ideas you’re having a tug-of-war with to come of their own accord.

To keep track of ideas, topics and the photos I'd need for the book, I kept a notebook. Credit: Bob King — To keep track of ideas, topics and the photos I’d need for the book, I kept a notebook filled to the gills with lists. Checkmarks indicate tasks accomplished. Credit: Bob King

As the February 1 deadline approached, time took on a physical dimension under the intense pressure to get everything done. I cut time into little blocks that when added up would get me to the finish line on the first draft. I made it just in time, shipped off my copy via e-mail, got in the car to go to work and turn up the music really LOUD. For a fews days I was on top of the world. Invincible.

My editor, Elizabeth, contacted me later with positive comments and then returned the manuscript with “developmental edits” or questions about descriptions and organization. We pitched the ever-refined draft back and forth over the next few months. Each time I read through the ten chapters and made both suggested changes and other refinements. I also added photos during this stage and worked via e-mail with the layout staff to place the best images and graphics at the best places in the text. I shot more images and requested photos from talented astrophotographers, prepared the acknowledgments and sought our recommendations from respected scientists and writers.

This diagram from the book uses the human face to illustrate how changing lighting angles causes the phases of the moon. Credit: Bob King

The editors at Page Street were quite generous with photo usage, a joy for me because that’s what I do for a living. I’ve been a photographer and photo editor at the Duluth News Tribune in Duluth, Minn. for many years. My favorite subjects are people, but I slip in an aurora or eclipse now and again. And that’s the irony. I never saw myself as a writer.

Like many, I started by keeping a journal of my observations through the telescope and reflections about the night sky. The Astro Bob blog took that a step further and writing for Universe Today and Sky & Telescope let me find my voice. So I maybe I have a voice, and I like to think I can be a helpful guide at your side, but writer? That still seems too lofty a term to describe what I do. But here we are.

After several edits including the final one, when I was sent a thick stack of low-res black and white pages of the book to mark up and return, I rested briefly before beginning the final phase: publicity. This is the weird part, where you tell everyone what a nice book you’ve written and how it would make a great Christmas gift for that budding astronomer in the family. When I held the first copy in my hands I couldn’t believe that all those hours of work at the computer became a physical object, a beautiful one even.

This map from the book shows Saturn's location around the time of opposition through 2021. — This map from the book shows Saturn’s location around the time of opposition through 2021. Credit: Bob King, Source: Stellarium

I’m biased of course, but I think both beginning and amateur astronomers will find the book useful. It includes lots of suggested activities – set off in separate boxes – to encourage you to get out under the stars. I make regular mention of the Web and phone apps as ways to become more familiar with the constellations, learn of newly-discovered bright comets and even find a dark sky.

Besides the easy naked eye topics like how to find the brightest constellations or see the best meteor showers of the year, the book offers visual challenges. Have you ever seen craters on the Moon without optical aid or the midnight glow of the gegenschein? You’ll find out how in my book. As a photographer, I’ve included tips on how to focus a digital camera and use it to photograph the aurora or a space station pass.

I’d be willing to bet that most books aren’t as complete as their authors would hope. I had to cut precious photos, graphics, 3 years of a sky calendar and other bits and pieces from mine. Ouch! To this day, I’m still thinking of ways to improve it with a fresh photo, new diagram or change of wording. Now it’s your turn to be the judge.

The zodiacal light punctuated by the planet Jupiter reflects off Lake Superior near Duluth, Minn. this morning (Nov. 8). The book describes nighttime lights such as the zodiacal, gegenschein, airglow and lunar halo and corona phenomena. Credit: Bob King — The zodiacal light punctuated by the planet Jupiter towers over northern Wisconsin along Lake Superior near Duluth, Minn. this morning (Nov. 8). The book describes nighttime lights such as the zodiacal, gegenschein, airglow in addition to lunar halo and corona phenomena. Credit: Bob King

Throughout, Nancy and I rooted for one another and shared our ups and downs. Incredible Stories was to publish within a week of Night Sky, but a type corruption error discovered in several chapters put the book on hold. Her new publication date is December 20, and I encourage you to pre-order a copy, so it arrives in time for Christmas. Order a copy of my book also, and I promise the two of us will keep you company on those long winter nights ahead.

Can I share one final tip? Once you’ve found your passion, say ‘yes’ to every opportunity that furthers it. You’ll be amazed at the places that one word will take you to.

*** To order a copy of Night Sky with the Naked Eye just click an icon to go to the site of your choice — Amazon, Barnes & Noble or Indiebound. It’s currently available at the first two outlets for a very nice discount. It should also be at your local B&N bookstore. And don’t forget to vote today!

November 7, 2016November 11, 2016 by Matt Williams

A Pulsar and White Dwarf Dance Together In A Surprising Orbit

Artist’s impression of the exotic double object that consists of a tiny neutron star orbited every two and a half hours by a white dwarf star. Credit: ESO/L. Calçada

Searching the Universe for strange new star systems can lead to some pretty interesting finds. And sometimes, it can turn up phenomena that contradict everything we think we know about the formation and evolution of stars. Such finds are not only fascinating and exciting, they allow us the chance to expand and refine our models of how the Universe came to be.

For instance, a recent study conducted by an international team of scientists has shown how the recent discovery of binary system – a millisecond pulsar and a low-mass white dwarf (LMWD) – has defied conventional ideas of stellar evolution. Whereas such systems were believed to have circular orbits in the past, the white dwarf in this particular binary orbits the pulsar with extreme eccentricity!

To break it down, conventional wisdom states that LMWDs are the product of binary evolution. The reason for this is because that under normal circumstances, such a star – with low mass but incredible density – would only form after it has exhausted all its nuclear fuel and lost its outer layers as a planetary nebula. Given the mass of this star, this would take about 100 billion years to happen on its own – i.e. longer than the age of the Universe.

An artist's impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry — An artist’s impression of an accreting X-ray millisecond pulsar. The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA/Goddard/Dana Berry

As such, they are generally believed to be the result of pairing with other stars – specifically, millisecond radio pulsars (MSPs). These are a distinct population of neutron stars that have fast spin periods and magnetic fields that are several orders of magnitude weaker than that of “normal” pulsars. These properties are thought to be the result of mass transfer with a companion star.

Basically, MSPs that are orbited by a star will slowly strip them of their mass, sucking off their outer layers and turning them into a white dwarf. The addition of this mass to the pulsar causes it to spin faster and buries its magnetic field, and also strips the companion star down to a white dwarf. In this scenario, the eccentricity of orbit of the LMWD around the pulsar is expected to be negligible.

However, when looking to the binary star system PSR J2234+0511, the international team noticed something entirely different. Here, they found a low-mass white dwarf paired with a millisecond pulsar which the white dwarf orbited with a period of 32 days and an extreme eccentricity (0.13). Since this defies current models of white dwarf stars, the team began looking for explanations.

As Dr. John Antoniadis – a researcher from the Dunlap Institute at University of Toronto and the lead author of the study – told Universe Today via email:

“Millisecond pulsar-LMWD binaries are very common. According to the established formation scenario, these systems evolve from low-mass X-ray binaries in which a neutron star accretes matter from a giant star. Eventually, this star evolves into a white dwarf and the neutron star becomes a millisecond pulsar. Because of the strong tidal forces during the mass-transfer episode, the orbits of these systems are extremely circular, with eccentricities of ~0.000001 or so.”

An artist's impression of a millisecond pulsar and its companion. The pulsar (seen in blue with two radiation beams) is accreting material from its bloated red companion star and increasing its rotation rate. Astronomers have measured the orbital parameters of four millisecond pulsars in the globular cluster 47 Tuc and modeled their possible formation and evolution paths. Credit: European Space Agency & Francesco Ferraro (Bologna Astronomical Observatory) — An artist’s impression of a millisecond pulsar and its companion. The pulsar (blue) is accreting material from its bloated red companion star and increasing its rotation rate. Credit: ESA/Francesco Ferraro (Bologna Astronomical Observatory)

For the sake of their study, which appeared recently in The Astrophysical Journal – titled “An Eccentric Binary Millisecond Pulsar with a Helium White Dwarf Companion in the Galactic Field” – the team relied on newly obtained optical photometry of the system provided by the Sloan Digital Sky Survey (SDSS), and spectroscopy from the Very Large Telescope from the Paranal Observatory in Chile.

In addition, they consulted recent studies that looked at other binary star systems that show this same kind of eccentric relationship. “We now know [of] 5 systems which deviate from this picture in that they have eccentricities of ~0.1 i.e. several orders of magnitude larger that what is expected in the standard scenario,” said Antoniadis. “Interestingly, they all appear to have similar eccentricities and orbital periods.”

From this, they were able to infer the temperature (8600 ± 190 K) and velocity ( km/s) of the white dwarf companion in the binary star system. Combined with constraints placed on the two body’s masses – 0.28 Solar Masses for the white dwarf and 1.4 for the pulsar – as well as their radii and surface gravity, they then tested three possible explanations for how this system came to be.

These included the possibility that neutrons stars (such as the millsecond pulsar being observed here) form through an accretion-induced collapse of a massive white dwarf. Similarly, they considered whether neutron stars undergo a transformation as they accrete material, which results in them becoming quark stars. During this process, the release of gravitational energy would be responsible for inducing the observed eccentricity.

Artist's illustration of a rotating neutron star, the remnants of a super nova explosion. Credit: NASA, Caltech-JPL — *Artist’s illustration of a rotating neutron star, the remnants of a super nova explosion. Credit: NASA, Caltech-JPL*

Second, they considered the possibility – consistent with current models of stellar evolution – that LMWDs within a certain mass range have strong stellar winds when they are very young (due to unstable hydrogen fusion). The team therefore looked at whether or not these strong stellar winds could have been what disrupted the orbit of the pulsar earlier in the system’s history.

Last, they considered the possibility that some of the material released from the white dwarf in the past (due to this same stellar wind) could have formed a short-lived circumbinary disk. This disk would then act like a third body, disturbing the system and increasing the eccentricity of the white dwarf’s orbit. In the end, they deemed that the first two scenarios were unlikely, since the mass inferred for the pulsar progenitor was not consistent with either model.

However, the third scenario, in which interaction with a circumbinary disk was responsible for the eccentricity, was consistent with their inferred parameters. What’s more, the third scenario predicts how (within a certain mass range) that there should be no circular binaries with similar orbital periods – which is consistent with all known examples of such systems. As Dr. Antoniadis explained:

“These observations show that the companion star in this system is indeed a low-mass white dwarf. In addition, the mass of the pulsar seems to be too low for #2 and a bit too high for #1. We also study the orbit of the binary in the Milky way, and it looks very similar to what we find for low-mass X-ray binaries. These pieces of evidence together favor the disk hypothesis.”

Cross-section of a neutron star. Credit: Wikipedia Commons/Robert Schulze — *Cross-section of a neutron star. Credit: Wikipedia Commons/Robert Schulz*

Of course, Dr. Antoniadis and his colleagues admit that more information is needed before their hypothesis can be deemed correct. However, should their results be borne out by future research, then they anticipate that it will be a valuable tool for future astronomers and astrophysicists looking to study the interaction between binary star systems and circumbinary disks.

In addition, the discovery of this high eccentricity binary system will make it easier to measure the masses of Low-Mass White Dwarfs with extreme precision in the coming years. This in turn should help astronomers to better understand the properties of these stars and what leads to their formation.

As history has taught us, understanding the Universe requires a serious commitment to the process of continuous discovery. And the more we discover, the stranger it seems to become, forcing us to reconsider what we think we know about it.

Further Reading: The Astrophysical Journal

October 28, 2016October 26, 2019 by Bob King

The Search Is On For Alien Signals Around Tabby’s Star

There’s a remote chance that inexplicable light variations in a star in the Northern Cross may be caused by the works of an alien civilization.

1,480 light years from Earth twinkles one of the greatest mysteries of recent times. There in the constellation Cygnus the Swan, you’ll find a dim, ordinary-looking point of light with an innocent sounding name — Tabby’s Star. Named for Louisiana State University astronomer Tabetha Boyajian, who was the lead author on a paper about its behavior, this star has so confounded astronomers with its unpredictable ups and downs in its brightness, they’ve gone to war on the object, drilling down on it with everything from the Hubble to the monster 393.7-inch (10-meter) Keck Telescope in Hawaii. Continue reading “The Search Is On For Alien Signals Around Tabby’s Star”

October 19, 2016October 20, 2016 by Evan Gough

Either Stars are Strange, or There Are 234 Aliens Trying to Contact Us

The Sloan Digital Sky Survey telescope stands out against the breaktaking backdrop of the Sacramento Mountains. 234 stars out of the Sloan's catalogue of over 2.5 million stars are producing an unexplained pulsed signal. Image: SDSS, Fermilab Visual Media Services

We all want there to be aliens. Green ones, pink ones, brown ones, Greys. Or maybe Vulcans, Klingons, even a being of pure energy. Any type will do.

That’s why whenever a mysterious signal or energetic fluctuation arrives from somewhere in the cosmos and hits one of our many telescopes, headlines erupt across the media: “Have We Finally Detected An Alien Signal?” or “Have Astronomers Discovered An Alien Megastructure?” But science-minded people know that we’re probably getting ahead of ourselves.

Skepticism still rules the day when it comes to these headlines, and the events that spawn them. That’s the way it should be, because we’ve always found a more prosaic reason for whatever signal from space we’re talking about. But, being skeptical is a balancing act; it doesn’t mean being dismissive.

What we’re talking about here is a new study from E.F. Borra and E. Trottier, two astronomers at Laval University in Canada. Their study, titled “Discovery of peculiar periodic spectral modulations in a small fraction of solar type stars” was just published at arXiv.org. ArXiv.org is a pre-print website, so the paper itself hasn’t been peer reviewed yet. But it is generating interest.

The two astronomers used data from the Sloan Digital Sky Survey, and analyzed the spectra of 2.5 million stars. Of all those stars, they found 234 stars that are producing a puzzling signal. That’s only a tiny percentage. And, they say, these signals “have exactly the shape of an ETI signal” that was predicted in a previous study by Borra.

A portion of the 234 stars that are sources of the pulsed ETI-like signal. Note that all the stars are in the narrow spectral range F2 to K1, very similar to our own Sun. Image: Ermanno F. Borra and Eric Trottier

Prediction is a key part of the scientific method. If you develop a theory, your theory looks better and better the more you can use it to correctly predict some future events based on it. Look how many times Einstein’s predictions based on Relativity have been proven correct.

The 234 stars in Borra and Trottier’s study aren’t random. They’re “overwhelmingly in the F2 to K1 spectral range” according to the abstract. That’s significant because this is a small range centred around the spectrum of our own Sun. And our own Sun is the only one we know of that has an intelligent species living near it. If ours does, maybe others do too?

The authors acknowledge five potential causes of their findings: instrumental and data reduction effects, rotational transitions in molecules, the Fourier transform of spectral lines, rapid pulsations, and finally the ETI signal predicted by Borra (2012). They dismiss molecules or pulsations as causes, and they deem it highly unlikely that the signals are caused by the Fourier analysis itself. This leaves two possible sources for the detected signals. Either they’re a result of the Sloan instrument itself and the data reduction, or they are in fact a signal from extra-terrestrial intelligences.

This graph shows the number of detected signals by Spectral Type of star. Image: Ermanno F. Borra and Eric Trottier

The detected signals are pulses of light separated by a constant time interval. These types of signals were predicted by Borra in his 2012 paper, and they are what he and Trottier set out to find in the Sloan data. It may be a bit of a red flag when scientist’s find the very thing they predicted they would find. But Trottier and Borra are circumspect about their own results.

As the authors say in their paper, “Although unlikely, there is also a possibility that the signals are due to highly peculiar chemical compositions in a small fraction of galactic halo stars.” It may be unlikely, but lots of discoveries seem unlikely at first. Maybe there is a tiny subset of stars with chemical peculiarities that make them act in this way.

To sum it all up, the two astronomers have found a tiny number of stars, very similar to our own Sun, that seem to be the source of pulsed signals. These signals are the same as predicted if a technological society was using powerful lasers to communicate with distant stars.

We all want there to be aliens, and maybe the first sign of them will be pulsed light signals from stars like our own Sun. But it’s all still very preliminary, and as the authors acknowledge, “…at this stage, this hypothesis needs to be confirmed with further work.”

That further work is already being planned by the Breakthrough Listen Initiative, a project that searches for intelligent life in the cosmos. They plan to use the Automated Planet Finder telescope at the Lick Observatory to further observe some of Borra’s 234 stars.

The Breakthrough team don’t seem that excited about Borra’s findings. They’ve already poured cold water on it, trotting out the old axiom that “Extraordinary claims require extraordinary evidence” in a statement on Borra’s paper. They also give Borra’s findings a score of 0 to 1 on the Rio Scale. The Rio Scale is something used by the international SETI community to rank detections of phenomena that could indicate advanced life beyond Earth. A rating of 0 to 1 means its insignificant.

Better reign in the headline writers.

October 6, 2016October 6, 2016 by Bob King

Celebrate the Power of Naked-Eye Observing With New Book

The cover of my new book "Night Sky with the Naked Eye". The book is currently available for pre-order on Amazon and Barnes and Noble. Publication date is November 8. — This is the cover of my new book “Night Sky with the Naked Eye”, a non-technical guide to all the great things visible with the naked eye at night. It’s published by Page Street Publishing and distributed by Macmillan and currently available for **pre-order on Amazon** and **Barnes and Noble**. Publication date is November 8. Look for it here on Universe Today soon!

If you’re like a lot of people, you don’t own a telescope but still have a passionate curiosity for what’s going on over your head. Good news! There’s lots to see up there without any equipment at all. This is the premise of my new book titled Night Sky with the Naked Eye, a guide to the wonders of the night sky that anyone can enjoy and understand whether you live in an apartment in the city or cabin 50 miles from nowhere.

This diagram from the book depicts why many satellites are visible during twilight before they're eclipsed by Earth's shadow. Credit: Gary Meader — This diagram from the book depicts why many satellites are visible during twilight before they’re eclipsed by Earth’s shadow. Credit: Gary Meader

I’ve always been amazed at how accessible the universe is. To make that personal connection to the cosmos we only need acquire the habit looking up. Total eclipses, monster auroras and rich meteor showers get a lot of coverage and rightly so, but there’s a lot of other stuff up there. Little things that stoke our sense of wonder happen all the time: Earth’s rising shadow at sunset, nightly satellite flyovers, the beauty of an earth-lit crescent moon or seeing your shadow by the light of Venus.

Skywatching not only informs and delights, it has the power to expand our perspective and sense of place in the scheme of things. Gazing up at the Milky Way on a dark summer night, we feel both humbled and fortunate to be alive. The night sky’s elixir of beauty, timelessness and possibility feeds an inner quietude that can be our strength in stressful times.

Night sky observing sometimes means pleasant surprises like seeing this rare Venus pillar and corona. The book explores both celestial and atmospheric phenomena. Credit: Bob King

While the book touches on the contemplative aspects of skywatching, the bulk of it is activity-oriented, intended to inspire you to get outside. I’ve got tips on weather-watching and making the most of online resources like Clear Dark Sky and satellite imagery to help you find clear skies for that must-see special event. And if light pollution is a problem where you live, we explore ways to make a difference in reducing it as well as using online atlases to find a dark observing site.

The book covers the basics of celestial and planetary motions, how to find the brighter constellations and naked-eye deep sky objects along with suggested night sky viewing activities to share with friends and family. There are 1o chapters in all:

Chapter 1: Wave “Hi!” to the Astronauts
Chapter 2: Anticipating the Night
Chapter 3: Rockin’ N’ Rollin’ Earth
Chapter 4: Dive Into the Dippers
Chapter 5: Four Seasons of Starlight
Chapter 6: Meet the Rabbit in the Moon
Chapter 7: Face to Face with the Planets
Chapter 8: Wish Upon a Shooting Star
Chapter 9: Awed by Aurora
Chapter 10: Curiosities of the Night

This is back cover of the Night Sky with the Naked Eye book jacket. — This is back of the *Night Sky with the Naked Eye* book jacket. My book will appear back to back with another space book, titled *Incredible Stories from Space: A Behind-the-Scenes Look at the Missions Changing Our View of the Cosmos,* by Universe Today contributing editor Nancy Atkinson. Watch for her announcement shortly.

Not everything is a billion miles away. We also take time to examine and appreciate closer-to-home phenomena that are part of the nighttime experience like lunar halos, light pillars and the aurora borealis. No observers’ guide would be complete without challenges. How about seeing craters on the moon with no optical aid or spotting the gegenschein? It’s all here.

Because the Internet has become an integral part of our lives, the book includes numerous online resources as well as useful mobile phone apps related to constellation finding and aurora tracking and tips on night sky photography.

Whether for yourself or to give as a holiday gift for a budding skywatcher, I hope you check out my book, which will be featured in a special promotion here at Universe Today. It would be my privilege to serve as your night sky guide.

September 25, 2016September 25, 2016 by Bob King

Bright Binocular Nova Discovered in Lupus

The possible nova in Lupus photographed on Sept. 25 from Australia. Credit: Joseph Brimacombe — The possible nova in Lupus photographed on Sunday, Sept. 25 from Australia. The star is now bright enough to see in binoculars for observers in the far southern U.S. and points south. Credit: **Joseph Brimacombe**

On September 20, a particular spot in the constellation Lupus the Wolf was blank of any stars brighter than 17.5 magnitude. Four nights later, as if by some magic trick, a star bright enough to be seen in binoculars popped into view. While we await official confirmation, the star’s spectrum, its tattle-tale rainbow of light, indicates it’s a nova, a sun in the throes of a thermonuclear explosion.

A bright possible nova was discovered only days ago near the 3rd magnitude star Epsilon Lupi. It shot from fainter than magnitude +17.5 to its current magnitude +6.8 in just four nights ... and it's still rising. The nova is bright enough to see in binoculars for observers in the far southern U.S., where it's visible low in the southwestern sky in late evening twilight. This map shows the sky facing southwest about an hour after sunset from Key West, Florida, latitude 24.5 degrees north. Source: Stellarium — The nova was discovered on Sept. 23 near the 3rd magnitude star Epsilon Lupi. It rose from fainter than magnitude +17.5 to its current magnitude +6.8 in just four nights … and it’s still rising. It’s visible low in the southwestern sky in late evening twilight low northern latitudes, the tropics and southern hemisphere. This map shows the sky facing southwest about an hour after sunset from Key West, Florida, latitude 24.5 degrees north. Source: Stellarium

The nova, dubbed ASASSN-16kt for now, was discovered during the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN or “Assassin”), using data from the quadruple 14-cm “Cassius” telescope in CTIO, Chile. Krzysztof Stanek and team reported the new star in Astronomical Telegram #9538. By the evening of September 23 local time, the object had risen to magnitude +9.1, and it’s currently +6.8. So let’s see — that’s about an 11-magnitude jump or a 24,000-fold increase in brightness! And it’s still on the rise.

Use this chart with binoculars to help you find the likely nova. The field of view is about 5 degrees with north up. The "new star" lies between a bright triangle of stars to the east and the naked-eye star Epsilon Lupi to the west. Stars are labeled with magnitudes. Chart: Bob King, Source: Stellarium — Use this chart with binoculars to help you find the likely nova. The field of view is about 5 degrees with north up. The “new star” lies between a bright triangle of stars to the east and the naked-eye star Epsilon Lupi to the west. Stars are labeled with magnitudes. Chart: Bob King, Source: Stellarium

The star is located at R.A. 15h 29?, –44° 49.7? in the southern constellation Lupus the Wolf. Even at this low declination, the star would clear the southern horizon from places like Chicago and further south, but in late September Lupus is low in the southwestern sky. To see the nova you’ll need a clear horizon in that direction and observe from the far southern U.S. and points south. If you’ve planned a trip to the Caribbean or Hawaii in the coming weeks, your timing couldn’t have been better!

Novae occur in close binary systems where one star is a tiny but extremely compact white dwarf star. The dwarf pulls material into a disk around itself, some of which is funneled to the surface and ignites in a nova explosion. Credit: NASA — Novae occur in close binary systems where one star is a tiny but extremely compact white dwarf star. The dwarf draws material into a disk around itself, some of which is funneled to the surface and ignites in a nova explosion. Credit: NASA

I’ve drawn the map for Key West, one of southernmost locations on the U.S. mainland, where the nova stands about 7-8° high in late twilight, but you might also see it from southern Texas and the bottom of Arizona if you stand on your tippytoes. Other locales include northern Africa, Finding a good horizon is key. Observers across Central and South America, Africa, India, s. Asia and Australia, where the star is higher up in the western sky at nightfall, are favored.

Nova means “new”, but a nova isn’t a brand new star coming to life but rather an explosion that occurs on the surface of an otherwise faint star no one’s taken notice of – until the blast causes it to brighten 50,000 to 100,000 times.

You can use this AAVSO chart to find the nova and track its changing brightness. Star magnitudes are shown to the tenth with the decimal omitted. Credit: AAVSO — You can use this AAVSO chart to find the nova and track its changing brightness. Star magnitudes are shown to the tenth with the decimal omitted. Click to enlarge. Credit: AAVSO

A nova occurs in a close binary star system, where a small but extremely dense and massive (for its size) white dwarf siphons hydrogen gas from its closely-orbiting companion. After whirling around in a flattened accretion disk around the dwarf, the material gets funneled down to the star’s 150,000 F° surface where gravity compacts and heats the gas until it detonates in a titanic thermonuclear explosion. Suddenly, a faint star that wasn’t on anyone’s radar vaults a dozen magnitudes to become a standout “new star”.

Novae are relatively rare and almost always found in the plane of the Milky Way, where the stars are most concentrated. The more stars, the greater the chances of finding one in a nova outburst. Roughly a handful a year are discovered, many of those in Scorpius and Sagittarius, in the direction of the galactic bulge.

We’ll keep tabs on this new object and report back with more information and photos as they become available. You can follow the new celebrity as well as print out finder charts on the American Association of Variable Star Observers (AAVSO) website by typing ASASSN-16kt in the info boxes.

I sure wish I wasn’t stuck in Minnesota right now or I’d be staring down the wolf’s new star!

September 24, 2016September 24, 2016 by Matt Williams

Why Are Stars Different Colors?

Artist's impression of a white dwarf star in orbit around Sirius (a white supergiant). Credit: NASA, ESA and G. Bacon (STScI)

Stars are beautiful, wondrous things. Much like planets, planetoids and other stellar bodies, they come in many sizes, shapes, and even colors. And over the course of many centuries, astronomers have come to discern several different types of stars based on these fundamental characteristics.

For instance, the color of a star – which varies from bluish-white and yellow to orange and red – is primarily due to its composition and effective temperature. And at all times, stars emit light which is a combination of several different wavelengths. On top of that, the color of a star can change over time.

Composition:

Different elements emit different wavelengths of electromagnetic radiation when heated. In the case of stars, his includes its main constituents (hydrogen and helium), but also the various trace elements that make it up. The color that we see is the combination of these different electromagnetic wavelengths, which are referred to as as a Planck’s curve.

Diagram illustrating Wein's Law (colored curves), which describes the emission of radiation from a black body. Credit: Wikipedia Commons/Darth Kule — *Diagram illustrating Wein’s Law, which describes the emission of radiation from a black body based on its peak wavelength. Credit: Wikipedia Commons/Darth*

The wavelength at which a star emits the most light is called the star’s “peak wavelength” (which known as Wien’s Law), which is the peak of its Planck curve. However, how that light appears to the human eye is also mitigated by the contributions of the other parts of its Planck curve.

In short, when the various colors of the spectrum are combined, they appear white to the naked eye. This will make the apparent color of the star appear lighter than where star’s peak wavelength falls on the color spectrum. Consider our Sun. Despite the fact that its peak emission wavelength corresponds to the green part of the spectrum, its color appears pale yellow.

A star’s composition is the result of its formation history. Ever star is born of a nebula made up of gas and dust, and each one is different. While nebulas in the interstellar medium are largely composed of hydrogen, which is the main fuel for star creation, they also carry other elements. The overall mass of the nebula, as well as the various elements that make it up, determine what kind of star will result.

The change in color these elements add to stars is not very obvious, but can be studied thanks to the method known as spectroanalysis. By examining the various wavelengths a star produces using a spectrometer, scientists are able to determine what elements are being burned inside.

Temperature and Distance:

The other major factor effecting a star’s color is its temperature. As stars increase in heat, the overall radiated energy increases, and the peak of the curve moves to shorter wavelengths. In other words, as a star becomes hotter, the light it emits is pushed further and further towards the blue end of the spectrum. As stars grow colder, the situation is reversed (see below).

A third and final factor that will effect what light a star appears to be emitting is known as the Doppler Effect. When it comes to sound, light, and other waves, the frequency can increase or decrease based on the distance between the source and the observer.

When it comes to astronomy, this effect causes the what is known as “redshift” and “blueshift” – where the visible light coming from a distant star is shifted towards the red end of the spectrum if it is moving away, and the blue end if it is moving closer.

Modern Classification:

Modern astronomy classifies stars based on their essential characteristics, which includes their spectral class (i.e. color), temperature, size, and brightness. Most stars are currently classified under the Morgan–Keenan (MK) system, which classifies stars based on temperature using the letters O, B, A, F, G, K, and M, – O being the hottest and M the coolest.

Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. O1 to M9 are the hottest to coldest stars). In the MK system, a luminosity class is added using Roman numerals. These are based on the width of certain absorption lines in the star’s spectrum (which vary with the density of the atmosphere), thus distinguishing giant stars from dwarfs.

Luminosity classes 0 and I apply to hyper- or supergiants; classes II, III and IV apply to bright, regular giants, and subgiants, respectively; class V is for main-sequence stars; and class VI and VII apply to subdwarfs and dwarf stars. There is also the Hertzsprung-Russell diagram, which relates stellar classification to absolute magnitude (i.e. intrinsic brightness), luminosity, and surface temperature.

The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude (expressed as Mv) to place them on a 2-dimensional chart (see below).

The Hertzspirg-Russel diagram, showing the relation between star's color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com — *The Hertzspirg-Russel diagram, showing the relation between star’s color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com*

On average, stars in the O-range are hotter than other classes, reaching effective temperatures of up to 30,000 K. At the same time, they are also larger and more massive, reaching sizes of over 6 and a half solar radii and up to 16 solar masses. At the lower end, K and M type stars (orange and red dwarfs) tend to be cooler (ranging from 2400 to 5700 K), measuring 0.7 to 0.96 times that of our Sun, and being anywhere from 0.08 to 0.8 as massive.

Stellar Evolution:

Stars also go through an evolutionary life cycle, during which time their sizes, temperatures and colors change. For example, when our Sun exhausts all the hydrogen in its the core, it will become unstable and collapse under its own weight. This will cause the core to heat up and get denser, causing the Sun to grow in size.

At this point, it will have left its Main Sequence phase and entered into the Red Giant Phase of its life, which (as the name would suggest) will be characterized by expansion and it becoming a deep red. When this happens, it is theorized that our Sun will expand to encompass the orbits of Mercury and even Venus.

Earth, if it survives this expansion, will be so close that it will be rendered uninhabitable. When our Sun then reaches its post-Red Giant Phase, the Sun will begin to eject mass, leaving an exposed core known as a white dwarf. This remnant will survive for trillions of years before fading to black.

This is believed to be the case with all stars that have between 0.5 to 1 Solar Mass (half, or as much mass of our Sun). The situation is slightly different when it comes to low mass stars (i.e. red dwarfs), which typically have around 0.1 Solar Masses.

It is believed that these stars can remain in their Main Sequence for some six to twelve trillion years and will not experience a Red Giant Phase. However, they will gradually increase in both temperature and luminosity, and will exist for several hundred billion more years before they eventually collapse into a white dwarf.

On the other hand, supergiant stars (up to 100 Solar Masses or more) have so much mass in their cores that they will likely experience helium ignition as soon as they exhaust their supplies of hydrogen. As such, they will likely not survive to become Red Supergiants, and will instead end their lives in a massive supernova.

To break it all down, stars vary in color depending on their chemical compositions, their respective sizes and their temperatures. Over time, as these characteristics change (as a result of them spending their fuel) many will darken and become redder, while others will explode magnificently. The more stars observe, the more we come to know about our Universe and its long, long history!

We have written many articles about stars on Universe Today. Here’s What is the Biggest Star in the Universe?, What is a Binary Star?, Do Stars Move?, What are the Most Famous Stars?, What is the Brightest Star in the Sky, Past and Future?

Want more information on stars? Here’s Hubblesite’s News Releases about Stars, and more information from NASA’s imagine the Universe.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Sources:

September 7, 2016September 11, 2016 by Bob King

Terzan 5 May Unlock Secret to Milky Way’s Past

Peering through the thick dust clouds of the galactic bulge (center of the galaxy) an international team of astronomers has revealed the unusual mix of stars in the stellar cluster known as Terzan 5. The new results indicate that Terzan 5 is in fact one of the bulge's primordial building blocks, most likely the relic of the very early days of the Milky Way. Credit: NASA/ESA/Hubble/F. Ferraro

Peering through the thick dust clouds of the galactic bulge an international team of astronomers has revealed the unusual mix of stars in the stellar cluster known as Terzan 5. The new results indicate that Terzan 5 is in fact one of the bulge's primordial building blocks, most likely the relic of the very early days of the Milky Way. Credit: NASA/ESA/Hubble/F. Ferraro — Peering through the thick dust clouds of the galactic bulge (center of the galaxy) an international team of astronomers has revealed the unusual mix of stars in the stellar cluster known as Terzan 5. The new results indicate that Terzan 5 is in fact one of the bulge’s primordial building blocks, most likely the relic of the very early days of the Milky Way. Credit: NASA/ESA/Hubble/F. Ferraro

Not many people have heard of the globular star cluster Terzan 5. It’s a big ball of stars resembling spilled sugar like so many other globular clusters. A very few globulars are bright enough to see with the naked eye; Terzan 5 is faint because it lies far away in the direction of the center of Milky Way galaxy inside its central bulge. Here, the stars that formed at the galaxy’s birth are packed together in great numbers. They are the “old ones” of the Milky Way.

Today, a team of astronomers revealed that Terzan 5 is unlike any globular cluster known. Most Milky Way globulars contain stars of just one age, about 11-12 billion years. They formed around the same time as the Milky Way itself, used up all their available gas early to build stars and then spent the remaining billions of years aging. Most orbit the galaxy’s center in a vast halo like moths whirring around a bright light. Oddball Terzan 5 has two populations aged 12 billion and 4.5 billion years old and it’s located inside the galactic bulge.

Globular clusters are distributed in a spherical halo about the core or bulge in the Milky Way galaxy. The Sun and planets are located well away from the center. From our perspective, most globular clusters appear concentrated in the direction of the galaxy's center. Credit: Science Frontiers Online — Globular clusters are distributed in a spherical halo about the star-rich core or bulge at the center of the disk of the Milky Way galaxy. Credit: Science Frontiers Online

The team used the cameras on the Hubble Space Telescope as well as a host of ground-based telescopes to find compelling evidence for the two distinct kinds of stars. Not only do they show a large gap in age, but the differ in the elements they contain. Terzan 5’s dual populations point to a star formation process that wasn’t continuous but dominated by two distinct bursts of star formation.

“This requires the Terzan 5 ancestor to have large amounts of gas for a second generation of stars and to be quite massive. At least 100 million times the mass of the Sun,” explains Davide Massari, co-author of the study.

Its unusual properties make Terzan 5 the ideal candidate for the title of “living fossil” from the early days of the Milky Way. Current theories on galaxy formation assume that vast clumps of gas and stars interacted to form the primordial bulge of the Milky Way, merging and dissolving in the process.

While the properties of Terzan 5 are uncommon for a globular cluster, they’re very similar to the stars found in the galactic bulge. Remnants of those gaseous clumps appear to have stuck around intact since the days of our galaxy’s birth, one of them evolving into the present day Terzan 5. That makes it a relic from the Milky Way’s infant days and one of the earliest galactic building blocks. Later, the cluster, which held onto some of its remaining gas, experienced a second burst of star formation.

This current model of the Milky Way galaxy shows the yellow-hued galactic bulge formed by ancient stars well along in their evolution, in contrast to the bluer, younger stars in the spiral arms. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

“Some characteristics of Terzan 5 resemble those detected in the giant clumps we see in star-forming galaxies at high-redshift (galaxies just beginning to form in the remote universe in the far distant past), suggesting that similar assembling processes occurred in the local and in the distant universe at the epoch of galaxy formation,” said Dr. Francesco Ferraro from the University of Bologna, Italy, who headed up the team.

The Milky Way on a late September night offers an opportunity to contemplate the grand form of the galaxy. Credit: Bob King

Terzan 5’s chandelier-like presence is helping astronomers understand how our galaxy was assembled. Reconstructing the past is one of the key occupations of astronomy. The present is continually departing with every passing moment. Soon enough, every piece of information slips into the past tense. In the near-past, which records humanity’s comings and goings, details are often forgotten or lost. The deep past is even worse. With no one around and only scattered clues, astronomers continually look for fragmental remains that when woven into the fabric of a theory, reveal patterns and processes before we came to be.

July 27, 2016 by Bob King

See a Wonderful Aldebaran Occultation with a Spectacular Twist

This map shows the 23% illuminated crescent Moon from the central U.S. around 10:00 UT Friday morning. Observers who begin observing earlier may also see stars in the Hyades cluster occulted. Credit: Stellarium — This map shows the 23% illuminated crescent Moon from the central U.S. around 10:00 UT Friday morning. Observers who start observing earlier may also see stars in the Hyades cluster occulted. Credit: Stellarium

We’re in for a celestial show from two of the sky’s glitterati this week. On Friday morning July 29 around 10:00 UT (5 a.m. CDT), the crescent Moon will occult the star Aldebaran from the eastern and southern U.S. south of a line from Toledo, Ohio through St. Louis, Tulsa and El Paso, Texas. North of that line, the Moon will slide just south of the star in a spectacular conjunction. But the real action lies within a half-mile of either side of the line, where lucky observers will see a grazing occultation.

Use this map to help you plan your occultation adventure. Much of North and South America will see a wonderful conjunction, while many areas will witness the occultation. If you can drive to the graze line, do it! Credit: David Dunham

As the Moon’s orbital motion carries it eastward at the rate of one lunar diameter per hour, Aldebaran will appear to approach the sunlit northern cusp and then scrape along the Moon’s northern limb. You’ll need binoculars or a small telescope to see the initial approach, but once star reaches the semi-dark, earthlit portion of the Moon, the graze will be visible with the naked eye.

This diagram shows the the grazing path of Aldebaran. As the Moon moves east, the star will appear to move to the right or west. Credit: David Dunham — This diagram shows the the grazing path of Aldebaran. As the Moon moves east, the star will appear to move to the right or west. Be sure not to miss it — the entire grazing event lasts just 3 minutes. Credit: David Dunham

The edge or limb of the Moon appears smooth to the eye, but it’s rife with polar mountain peaks. As Aldebaran creeps along the craggy limb, it will repeatedly flash in and out of view as peaks and cliffs momentarily block it from sight. And here’s the truly amazing thing. Observers along the western section of the graze line, where the event takes place in fairly dark sky, can watch the star blink in and out of view without optical aid when it reaches the dark part of the lunar disk. Wow!

Aldebaran's large size means it won't disappear instantaneously when it's covered either by the lunar limb during occultation or by mountains along the grazing path. Credit: Wikipedia — Aldebaran’s large size means it won’t disappear instantaneously when it’s covered either by the lunar limb during occultation or by mountains along the grazing path. Credit: Wikipedia

Aldebaran is no small star. An orange giant 67 light years from Earth, it’s 44 times the diameter of the Sun. That means that sometimes only a part of the star at a time will covered at a time in some cases, so the length of the flashes will vary. According to David Dunham, president of the International Occultation Timing Association (IOTA), Aldebaran will disappear for one-tenth of a second up to a second as the Moon rolls east, allowing some observers to sense the size of the star. Wow x 100!

Aldebaran, the brightest star in Taurus the Bull hangs near the edge of the moon two minutes before it was covered up. The star was easily visible through the telescope. Credit: Bob King — Aldebaran hangs near the edge of the moon two minutes before it was occulted last October in this photo taken with a smartphone. The star was easily visible through the telescope. Credit: Bob King

Skywatchers further east along the graze line and in other areas where the occultation / conjunction takes place after sunrise shouldn’t pass up the chance to see the event. During last October’s occultation of Aldebaran, I was able to see and photograph the star in my 10-inch scope in daylight no problem. The moon will be closer to the Sun this time around, but give it try anyway. This is the best grazing occultation of Aldebaran visible from North America in the current 4-year series.

For the many who live either north or south of the graze line, the views will still be fantastic. You’ll either see an occultation and subsequent reappearance of the star at the dark limb … or a fine conjunction. The forecast looks good for my city, so I plan on heading out to watch an orange giant meet the skinny Moon. I wish you clear skies and happy shooting!

For more information about the event including detailed weather forecasts and grazing maps, check out the IOTA’s public announcement page. Click here for Universal Times for the disappearance and reappearance of the star for over 1,000 cities.

May 12, 2016October 16, 2016 by Fraser Cain

What is the Biggest Star in the Universe?

This article was originally published in 2008, but has been updated several times now to keep track with our advancing knowledge of the cosmos!

My six-year old daughter is a question-asking machine. We were driving home from school a couple of days ago, and she was grilling me about the nature of the Universe. One of her zingers was, “What’s the Biggest Star in the Universe”? I had an easy answer. “The Universe is a big place,” I said, “and there’s no way we can possibly know what the biggest star is”. But that’s not a real answer.

So she refined the question. “What’s the biggest star that we know of?” Of course, I was stuck in the car, and without access to the Internet. But once I got back home, and was able to do some research, I learned the answer and thought I’d share it with the rest of you But to answer it fully, some basic background information needs to be covered first. Ready?

Solar Radius and Mass:

When talking about the size of stars, it’s important to first take a look at our own Sun for a sense of scale. Our familiar star is a mighty 1.4 million km across (870,000 miles). That’s such a huge number that it’s hard to get a sense of scale. Speaking of which, the Sun also accounts for 99.9% of all the matter in our Solar System. In fact, you could fit one million planet Earths inside the Sun.

Using these values, astronomers have created the terms “solar radius” and “solar mass”, which they use to compare stars of greater or smaller size and mass to our own. A solar radius is 690,000 km (432,000 miles) and 1 solar mass is 2 x 10³⁰ kilograms (4.3 x 10³⁰ pounds). That’s 2 nonillion kilograms, or 2,000,000,000,000,000,000,000,000,000,000 kg.

Artist's depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons — *Artist’s depiction of the Morgan-Keenan spectral diagram, showing the difference between main sequence stars. Credit: Wikipedia Commons*

Another thing worth considering is the fact that our Sun is pretty small, as stars go. As a G-type main-sequence star (specifically, a G2V star), which is commonly known as a yellow dwarf, its on the smaller end of the size chart (see above). While it is certainly larger than the most common type of star – M-type, or Red Dwarfs – it is itself dwarfed (no pun!) by the likes of blue giants and other spectral classes.

Classification:

To break it all down, stars are grouped based on their essential characteristics, which can be their spectral class (i.e. color), temperature, size, and brightness. The most common method of classification is known as the Morgan–Keenan (MK) system, which classifies stars based on temperature using the letters O, B, A, F, G, K, and M, – O being the hottest and M the coolest. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. O1 to M9 are the hottest to coldest stars).

In the MK system, a luminosity class is added using Roman numerals. These are based on the width of certain absorption lines in the star’s spectrum (which vary with the density of the atmosphere), thus distinguishing giant stars from dwarfs. Luminosity classes 0 and I apply to hyper- or supergiants; classes II, III and IV apply to bright, regular giants, and subgiants, respectively; class V is for main-sequence stars; and class VI and VII apply to subdwarfs and dwarf stars.

There is also the Hertzsprung-Russell diagram, which relates stellar classification to absolute magnitude (i.e. intrinsic brightness), luminosity, and surface temperature. The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude (expressed as Mv) to place them on a 2-dimensional chart (see above).

Based on the full of classification of our Sun (G2V), we can therefore say that it a main-sequence star with a temperature around 5,800K. Now consider another famous star system in our galaxy – Eta Carinae, a system containing at least two stars located around 7500 light-years away in the direction of the constellation Carina. The primary of this system is estimated to be 250 times the size of our Sun, a minimum of 120 solar masses, and a million times as bright – making it one of the biggest and brightest stars ever observed.

Eta Carinae, one of the most massive stars known. Image credit: NASA — *Eta Carinae, one of the most massive stars known, located in the Carina constellation. Credit: NASA*

There is some controversy over this world’s size though. Most stars blow with a solar wind, losing mass over time. But Eta Carinae is so large that it casts off 500 times the mass of the Earth every year. With so much mass lost, it’s very difficult for astronomers to accurately measure where the star ends, and its stellar wind begins. Also, it is believed that Eta Carinae will explode in the not-too-distant future, and it will be the most spectacular supernovae humans have ever seen.

In terms of sheer mass, the top spot goes to R136a1, a star located in the Large Magellanic Cloud, some 163,000 light-years away. It is believed that this star may contain as much as 315 times the mass of the Sun, which presents a conundrum to astronomers since it was believed that the largest stars could only contain 150 solar masses. The answer to this is that R136a1 was probably formed when several massive stars merged together. Needless to say, R136a1 is set to detonate as a hypernova, any day now.

In terms of large stars, Betelgeuse serves as a good (and popular) example. Located in the shoulder of Orion, this familiar red supergiant has a radius of 950-1200 times the size of the Sun, and would engulf the orbit of Jupiter if placed in our Solar System. In fact, whenever we want to put our Sun’s size into perspective, we often use Betelgeuse to do it (see below)!

Yet, even after we use this hulking Red Giant to put us in our place, we are still just scratching the surface in the game of “who’s the biggest star”. Consider WOH G64, a red supergiant star located in the Large Magellanic Cloud, approximately 168,000 light years from Earth. At 1.540 solar radii in diameter, this star is currently one of the largest in the known universe.

But there’s also RW Cephei, an orange hypergiant star in the constellation Cepheus, located 3,500 light years from Earth and measuring 1,535 solar radii in diameter. Westerlund 1-26 is also pretty huge, a red supergiant (or hypergiant) located within the Westerlund 1 super star cluster 11,500 light-years away that measures 1,530 solar radii in diameter. Meanwhile, V354 Cephei and VX Sagittarii are tied when it comes to size, with both measuring an estimated 1,520 solar radii in diameter.

The Largest Star: UY Scuti

As it stands, the title of the largest star in the Universe (that we know of) comes down to two contenders. For example, UY Scuti is currently at the top of the list. Located 9.500 light years away in the constellation Scutum, this bright red supergiant and pulsating variable star has an estimated average median radius of 1,708 solar radii – or 2.4 billion km (1.5 billion mi; 15.9 AU), thus giving it a volume 5 billion times that of the Sun.

However, this average estimate includes a margin of error of ± 192 solar radii, which means that it could be as large as 1900 solar radii or as small as 1516. This lower estimate places it beneath stars like as V354 Cephei and VX Sagittarii. Meanwhile, the second star on the list of the largest possible stars is NML Cygni, a semiregular variable red hypergiant located in the Cygnus constellation some 5,300 light-years from Earth.

*A zoomed-in picture of the red giant star UY Scuti. Credit: Rutherford Observatory/Haktarfone*

Due to the location of this star within a circumstellar nebula, it is heavily obscured by dust extinction. As a result, astronomers estimate that its size could be anywhere from 1,642 to 2,775 solar radii, which means it could either be the largest star in the known Universe (with a margin of 1000 solar radii) or indeed the second largest, ranking not far behind UY Scuti.

And up until a few years ago, the title of biggest star went to VY Canis Majoris; a red hypergiant star in the Canis Major constellation, located about 5,000 light-years from Earth. Back in 2006, professor Roberta Humphrey of the University of Minnesota calculated its upper size and estimated that it could be more than 1,540 times the size of the Sun. Its average estimated mass, however, is 1420, placing it in the no. 8 spot behind V354 Cephei and VX Sagittarii.

These are the biggest star that we know of, but the Milky way probably has dozens of stars that are even larger, obscured by gas and dust so we can’t see them. But even if we cannot find these stars, it is possible to theorize about their likely size and mass. So just how big can stars get? Once again, Professor Roberta Humphreys of the University of Minnesota provided the answer.

VY Canis Majoris. The biggest known star. — *Size comparison between the Sun and VY Canis Majoris, which once held the title of the largest known star in the Universe. Credit: Wikipedia Commons/Oona Räisänen*

As she explained when contacted, the largest stars in the Universe are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and therefore only 250 solar radii big. The largest stars, in contrast, will be cool supergiants. Case in point, VY Canis Majoris is only 3,500 Kelvin, and a really big star would be even cooler.

At 3,000 Kelvin, Humphreys estimates that cool supergiant would be as big as 2,600 times the size of the Sun. This is below the upper estimates for NML Cygni, but above the average estimates for both it and UY Scutii. Hence, this is the upper limit of a star (at least theoretically and based on all the information we have to date).

But as we continue to peer into the Universe with all of our instruments, and explore it up close through robotic spacecraft and crewed missions, we are sure to find new and exciting things that will confound us further!

And be sure to check out this great animation that shows the size of various objects in space, starting with our Solar System’s tiny planets and finally getting to UY Scuti. Enjoy!

We have written many articles about stars for Universe Today. Here’s The Sun, What’s the Brightest Star in the Sky Past and Future?, and What Is The Smallest Star?

Want to learn more about the birth and death of stars? We did a two part podcast at Astronomy Cast. Here’s part 1, Where Stars Come From, and here’s part 2, How Stars Die.