A supernova explodes

The Crab Nebula is a supernova remnant. It’s what’s left of an exploded star. It’s a vast expanding cloud of gas and dust surrounding one of the densest objects in the universe, a neutron star.

Chinese astronomers noticed the sudden appearance of a star blazing in the daytime sky on July 4, 1054 CE. It likely outshone the brightest planet, Venus, and was temporarily the 3rd-brightest object in the sky, after the sun and moon. This “guest star” – the exploding supernova – remained visible in daylight for some 23 days. At night it shone near Tianguan – a star we now call Zeta Tauri, in the constellation Taurus the Bull – for nearly two years. Then it faded from view.

The supernova erupted – and the Crab Nebula formed – about 6,500 light-years away.

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The Crab Nebula and supernova in history

The ancestral Puebloan people in the American Southwest may have viewed the bright new star in 1054. A crescent moon was in the sky near the new star on the morning of July 5, the day following the observations by the Chinese. So the pictograph below, from Chaco Canyon in New Mexico, may depict the event. The multi-spiked star to the left represents the supernova near the crescent moon. Furthermore, the handprint above may signify the importance of the event or may be the artist’s “signature.”

After exploding onto the scene in 1054 and shining brightly for two years, there are no reports of anything unusual in this spot in the sky until 1731. Then in that year, English amateur astronomer John Bevis recorded an observation of a faint nebulosity. In 1758, French comet-hunter Charles Messier spotted the hazy patch. It became the first entry in his catalog of objects that were fuzzy but not comets, now known as the Messier Catalog. Thus, the Crab Nebula has the name M1.

In 1844, astronomer William Parsons – the 3rd Earl of Rosse – observed M1 through his large telescope in Ireland. Because he described it as having a shape resembling a crab, that became its familiar nickname.

Yet it wasn’t until 1921 that people made the association between the Crab Nebula and Chinese records of the 1054 “guest star.”

How to see the Crab Nebula

Since this beautiful nebula shines at magnitude 8.4, it requires magnification to see. Fortunately, it’s relatively easy to find with binoculars or a telescope due to its location near several bright stars. Plus, it’s near several recognizable constellations. Although you can see it at some time of night all year except – from roughly May through July when it’s too close to the sun – the best observing is late the Northern Hemisphere fall through early spring.

To find the Crab Nebula, first draw an imaginary line from bright Betelgeuse in Orion to Capella in Auriga. About halfway along that line is the star Beta Tauri (or Elnath) on the Taurus-Auriga border.

Having identified Beta Tauri, backtrack a little more than a 3rd of the way back to Betelgeuse to find the fainter star Zeta Tauri. Scanning the area around Zeta Tauri should reveal a tiny, faint smudge. It’s about a degree (twice the width of the full moon) from Zeta Tauri and more or less in the direction of Beta Tauri.

Views through binoculars or a telescope

Binoculars and small telescopes are useful for finding the object and showing its roughly oblong shape. However, they won’t show the filamentary structure or any of its internal detail. Here are two examples showing what to expect in binoculars or through a telescope.

First, the eyepiece view, above, simulates a 7-degree field of view centered around Zeta Tauri. This is what you might expect from a 7 X 50 pair of binoculars. Of course, the exact orientation and visibility will range widely depending on time of observation, sky conditions and so on. Scan around Zeta Tauri for the faint nebulosity.

Then the second image, above, simulates an approximately 3.5-degree view that you might see through a small telescope or finder scope. To give you a clear idea of scale, two full moons would fit with room to spare in the space between Zeta Tauri and the Crab Nebula in this chart.

Keep in mind that exact conditions will vary.

Science of the Crab Nebula

The Crab Nebula is an oval gaseous nebula with fine filamentary (thread-like) structures, expanding at around 930 miles (1,500 km) per second. In its heart is a neutron star about the mass of the sun but only about 12 miles (19 km) in diameter. This neutron star is also a pulsar that spins about 30 times per second. The neutron star’s powerful magnetic field concentrates radiation emitted by the star as two beams that appear to flash periodically as the beams sweep into view.

Views from the Hubble and Webb space telescopes

The Crab Nebula may be from a new type of supernova

For a long time scientists thought the Crab Nebula was the remnant of a type II supernova. But in June 2021, scientists announced they’d finally found evidence for a new type of supernova, an electron-capture supernova. Consequently, they now believe the Crab Nebula may be this type of supernova. Read more about this exciting discovery.

The center of the Crab Nebula is approximately RA: 5h 34m 32s; Dec: +22° 0′ 52″

Bottom line: The Crab Nebula is visible with binoculars and small telescopes, and relatively easy to find since it’s near bright stars in prominent constellations. Although astronomers long thought that it was the remnant of a type II supernova, there’s increasing evidence that it may have been a new type of supernova called an electron capture supernova.

The post Meet the Crab Nebula, remnant of an exploding star first appeared on EarthSky.

The Audubon Society’s Christmas Bird Count is one of the longest-running citizen science projects. It had a modest beginning on Christmas Day in 1900. And it’s since become a strong data-gathering project to study bird population trends. This year’s count – the 124th – runs from December 14, 2023, to January 5, 2024. You have to sign up in advance, and the signup has already begun. Go here to sign up for the Christmas Bird Count 2023.

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Audubon Christmas Bird Count – how it’s done

The Christmas Bird Count is a carefully run event. Each count site is a 15-mile (24-km) wide circle; you can see what it looks like by zooming in on this map to inspect a region near you. Counts for each circle are organized by a “circle compiler.” On the day of the count (set by the circle’s compiler), people head out to designated routes within a circle to count every species and number of birds that they see and hear during the day. And, if you live within the range of a count site, you can also tally the birds you see in your yard and at the feeder.

To participate in the count – it’s free – you need to sign up with a local circle compiler at the Audubon’s website. If you’re a beginning birder, you’ll be matched up with a more experienced birder. Make sure you register early, because the compiler will need time to organize the event.

In addition, you can share your bird count photographs and experiences on social media with the hashtag #ChristmasBirdCount. We here at EarthSky would love to have you send us your photographs, too!

Audubon Christmas Bird Count history

In some parts of the U.S., there used to be bird-hunting competitions on Christmas Day. However, Frank M. Chapman, an ornithologist at the American Museum of Natural History, came up with an alternative, an activity to count birds in a given area each Christmas to build up a record of their numbers.

That first count was in 1900. Overall, 27 birders conducted counts at 25 sites, tallying about 89 bird species.

Since then, the Christmas Bird Count has come a long way. It’s continued annually since the inaugural event, growing in volunteers and census sites. For instance, the 121st Christmas Bird Count took place from December 14, 2020, to January 5, 2021. That count occurred at 2,459 locations, with 72,815 volunteers in the U.S., Canada, Latin America, the Caribbean and Pacific Islands. Altogether, volunteers observed a total of 2,355 bird species.

What have we learned from these counts?

Additionally, Audubon and other research groups use Christmas Bird Count data to monitor population trends that will help guide conservation efforts. To date, scientists have published more than 300 peer-reviewed studies based on this data. The data is also used by federal agencies to craft policy on bird conservation.

Each annual count provides a snapshot of the birds at a given time and place. It’s hard to draw conclusions from one year to the next, because changes happen gradually. To understand trends, scientists do a statistical analysis of data taken over several years.

Warning signs of environmental degradation show up in declines of bird populations in some types of habitats. For instance, the sharpest declines in bird populations have been in grassland habitats, followed by coastal habitats.

Bird census data also informs scientists about the effects of climate change on wildlife. In a 2014 report, National Audubon predicted how the ranges of 588 species of birds in North America could be affected by climate change. They concluded that more than 314 species could lose over 50% of their current climatic range by 2080.

Bottom line: Audubon’s 124th Christmas Bird Count will take place from December 14, 2023, to January 5, 2024. You can join in to help collect important data about birds. Find out how to join in the Audubon Christmas Bird Count.

The post Audubon Christmas Bird Count signup has begun first appeared on EarthSky.

Delta Cephei is a pulsating star

Delta Cephei, in the constellation Cepheus the King, is a variable star that changes in brightness with clock-like precision. In fact, it doubles in brightness and fades back to minimum brightness every 5.366 days. So with careful observation under a dark sky, you can see this star change in brightness over several days. This star, and others like it, are important players in establishing the distance scale of our galaxy … and our universe.

Delta Cephei itself looms large in the history of astronomy. An entire class of supergiant stars – called Cepheid variables – is named in this star’s honor.

#TBT to 1912 when Henrietta Swan Leavitt published ground-breaking observations of Cepheid variable stars. They were key to the discovery by Edwin Hubble that the Andromeda “nebula” is a galaxy like our own Milky Way. Learn more: https://t.co/5oDsiPWtiJ pic.twitter.com/KXeP6XHgin

— NASA Universe (@NASAUniverse) March 3, 2022

They were discovered by Henrietta Swift

Cepheid variable stars, also called Cepheids, dependably change their brightnesses over regular intervals ranging from a few days to a few weeks. In 1912, astronomer Henrietta Leavitt discovered that the star’s periodic change in brightness was directly related to its intrinsic brightness (or actual luminosity). She found that the longer the brightness pulsation cycle, the greater the intrinsic brightness of the star. This Cepheid period-luminosity relationship is now sometimes called the Leavitt law.

Why are these stars varying in brightness? It’s thought that these stars vary because they expand (get brighter) and then contract (get fainter) in a regular way.

Cepheids help measure cosmic distances

As a matter of fact, the regularity of Cepheids’ brightening and dimming is a powerful tool in astronomy. It lets astronomers probe distances across vast space. Of course, the surest way to measure star distances is with stellar parallax. But, for the parallax method to work, the stars have to be relatively nearby (within about 1000 light-years). Luckily, in recent years, astronomers have been able to make direct parallax measurements of more distant stars, thanks to space-based telescopes such as Gaia.

Still, the problem remains. How can we find the distance to stars that are too faraway to give us a reliable distance measurement using parallax? Suppose you measured the distance to a nearby Cepheid star using the parallax method. Then suppose you watched its pulsations, which you know are correlated with the star’s intrinsic – real – brightness. Then you know both its distance and how bright the star looks at that distance.

Armed with this information, you can then look farther out in the universe, toward more distant Cepheids, those too far for parallax measurements. You can measure the apparent brightness – which is fainter – and pulsation rate of such a star. With a few simple steps of math, you can then find the distance to it.

Astronomers use Cepheid variable stars to measure distances across space. For this reason, they’re known as standard candles by astronomers.

Edwin Hubble used Cepheids to expand our known universe

In 1923, the astronomer Edwin Hubble used Cepheids to determine that the then-called Andromeda nebula is actually not a nebula but a giant galaxy lying beyond our Milky Way. It released us from the confines of a single galaxy and gave us the vast universe we know today. This work in understanding the size of the universe is sometimes called the cosmic distance ladder.

The work continues today, not just with Cepheids but also with other astronomical objects and phenomena.

Cepheids in other galaxies

Distance determinations using Cepheids in other galaxies, as well as other techniques, is an active area of research in astronomy. Astronomers are constantly improving distance accuracies to further constrain the value of the Hubble Constant that indicates the expansion rate of the universe.

Cepheids have been observed as far away as 100 million light-years in the galaxy NGC 4603, by the Hubble Space Telescope. However, measuring them at distances of 30 million light-years and farther is difficult because it’s hard to isolate Cepheids from their neighboring stars. At such distances, astronomers transition to other methods to determine distances, such as observing type 1a supernovae.

How to spot Delta Cephei in the night sky

The original Cepheid, Delta Cephei, is circumpolar – always above the horizon – in the northern half of the United States.

Even so, Delta Cephei is much easier to see when it’s high in the northern sky on autumn and winter evenings. If you’re far enough north, you can find the constellation Cepheus by way of the Big Dipper. First, use the Big Dipper “pointer stars” to locate Polaris, the North Star. Then jump beyond Polaris by a fist-width to land on Cepheus.

You’ll see the constellation Cepheus the King close to his wife, Cassiopeia the Queen, her signature W or M-shaped figure of stars making her the flashier of the two constellations. They’re high in your northern sky on November and December evenings.

How to watch Delta Cephei vary in brightness

The real answer to that question is: time and patience. But two stars lodging near Delta Cephei on the sky’s dome – Epsilon Cephei and Zeta Cephei – match the low and high ends of Delta Cephei’s brightness scale. So, those two stars should help you watch Delta Cephei change.

So look at the charts above, and locate the stars Epsilon and Zeta Cephei. At its faintest, Delta Cephei is as dim as the fainter star, Epsilon Cephei. At its brightest, Delta Cephei matches the brightness of the brighter star, Zeta Cephei.

Have fun!

Delta Cephei a stunning binary star 887 light years distant. #Astrophotography #stargazing pic.twitter.com/lhVQjw9RMM

— Scott MacNeill (@scottiemacneill) January 16, 2016

Bottom line: Cepheid variables are a famous class of stars, used in establishing the distance scale of the universe. They’re helpful in this way because their brightness pulsation rate is correlated to their intrinsic brightnesses. So we can see how bright they look, and determine their distance. The stars are named for Delta Cephei in the constellation Cepheus, the first of its type to be identified, in 1784.

The post Delta Cephei helps measure cosmic distances first appeared on EarthSky.

Alpha Cephei, also known as Alderamin, is the brightest star in the constellation Cepheus the King. Astronomers are intrigued by this star because it spins at such a fast rate. Unlike our almost-round sun, Alderamin is distorted by its rapid spin into an oblate form, like a partly deflated beach ball.

This star isn’t the brightest one around, but it’s pretty bright. It shines at about magnitude 2.5.

Its faint constellation, Cepheus, is surprisingly easy to spot in a dark sky because the stars of Cepheus trace out the shape of a child’s stick house.

Science of Alpha Cephei

Alderamin, 49 light-years away, is a white star that is twice the mass of the sun and about 17 times its luminosity. It is considered a Class A star, which is now evolving off the main sequence into a subgiant. It’s thought that this star is now on its way to becoming a red giant as its internal supply of hydrogen fuel runs low.

Alpha Cephei rotates rapidly; observations suggest it could be as fast as 152 miles a second (246 km/s). In comparison, the sun’s rotation speed at the equator is not quite 1.2 miles a second (2 km/s). As a result of its rapid rotation, Alpha Cephei appears oblate. That’s because the star’s surface rotation speed gets progressively faster as you move away from its rotation axis towards its equator. And, as surface rotation speed increases, the star’s surface is increasingly pushed out. As a result, Alpha Cephei bulges along the equator.

A closer look at Alpha Cephei

The image below is a model of Alpha Cephei based on data from the CHARA telescope array. It shows Alpha Cephei’s oblate shape. It also shows a darker equatorial region, shaded to indicate that these are cooler regions of the star. Meanwhile, a section near the pole appears brighter. That’s because the surface gravity is higher around the “flattened” poles compared to the bulging equator, and as a result, higher temperatures and pressure are needed to maintain an equilibrium. These differences in brightness across rapidly rotating stars result in a phenomenon scientists call gravity darkening.

Astronomer Jim Kaler wrote about this rapidly spinning star:

The spin may also be related to the star’s activity. [Our] sun is magnetically active in broad part because its outer third is churning up and down in huge convective currents, the movement helping to generate a magnetic field. Such outer zones are supposed to disappear in class A stars like Alderamin. Yet Alderamin emits about the same amount of X-ray radiation as does the sun and has other features that together suggest considerable magnetic activity. No one really knows why. Such anomalies, of course, drive the science. Understanding Alderamin will someday help us understand our own star, on which we depend for life.

How to find Alpha Cephei

On a dark night, Alpha Cephei is easily visible and relatively easy to find. Look northward for this star. It is circumpolar throughout all of Europe, northern Asia, Canada and American cities as far south as San Diego, California. Its constellation, Cepheus, has the shape of the stick house we all drew as children. Or you might prefer to see the shape of Cepheus as a point on the King’s crown. Cepheus is a rather faint constellation, but Alpha Cephei is by far its brightest star and is easily observable to the unaided eye, even in cities.

If you know the W or M-shaped constellation Cassiopeia the Queen, you can use the Cassiopeia stars Schedar and Caph to star-hop to Alderamin.

A once and future pole star

Alpha Cephei has been a pole star in the past, that is, a star close to the sky’s north pole. The last time was in 18,000 BCE. It will again be a pole star some 5,500 years from now. No matter what is going on then on Earth, the heavens will pursue their long and predictable cycles. And Alpha Cephei will lie some 3 degrees from the sky’s north pole around the year 7500 CE. That means it won’t be as good a pole star as our present-day Polaris, which will be 0.4525 degrees from the north celestial pole on March 24, 2100. But it’ll be pretty good.

This star’s proper name, Alderamin, is from Arabic and means “the right arm,” presumably of Cepheus the King, who played a role in Greek mythology.

Bottom line: Alpha Cephei, or Alderamin, is the brightest star in the faint constellation Cepheus the King. It’s spinning so fast that it appears like a slightly flattened beach ball.

The post Alderamin, or Alpha Cephei, is a fast-spinning star first appeared on EarthSky.

They call it the Flying Star

61 Cygni is a double star in the constellation Cygnus the Swan. It’s not a standout in brightness. Why go to the trouble of finding it? Stars are individuals, and there’s something interesting about each one. But 61 Cygni is particularly cool because it has one of the highest proper motions of any visible star. That’s its sideways movement across the dome of the sky.

If you took photos of 61 Cygni over the course of several years, you’d see it shift position in the sky with respect to the more distant stars around it.

This unusual motion across our sky earned 61 Cygni the nickname the Flying Star.

The movement of the double star 61 Cygni over 8 years. pic.twitter.com/2HujtmYayl

— Dave Eagle FRAS ? Keep Looking Up. (@Dave_StarGeezer) June 14, 2023

61 Cygni has a high ‘proper motion’

So why does this star have such a high proper motion? Think of two people who are running, one near you, and the other farther away. In relation to the more distant landscape, the person closer to you would appear to cover more ground – more objects would pass behind them – than the person farther away.

Then in a similar way, very distant stars appear “fixed” in relationship to each other. However, they’re actually all moving through space in their various journeys around the center of our Milky Way galaxy. But most are so far away that we can’t easily detect their proper motions. On the other hand, 61 Cygni is different. It moves relatively rapidly in front of the fixed stars because 61 Cygni is relatively near Earth.

While not the closest star to the sun (that honor goes to the Alpha Centauri system), 61 Cygni is just 11.4 light-years distant. That makes it the fourth-closest star visible to the unaided eye, after Alpha Centauri, Sirius, and Epsilon Eridani. And it’s the 15th nearest known star system to the Earth.

Science of 61 Cygni

First, 61 Cygni isn’t just one star. In fact, it’s a binary system with an orbital period of about 659 years. So, to the unaided eye and through most binoculars, it appears as one star. However, if you look at it through a modest-sized telescope, you’ll see it resolved as two stars. They have apparent magnitudes of 5.21 and 6.03.

The 61 Cygni binary system is the 15th-nearest known star system to us. Both are K-type dwarf stars in the main sequence, thought to have formed 6 billion years ago (the sun, in comparison, is 4.6 billion years old). The more massive star of the pair has 70% of the sun’s mass and puts out 15% of the sun’s total electromagnetic energy. Its companion has 63% of the sun’s mass and shines at just 8.5% of the sun’s luminosity. Both are a bit over half the size of the sun. They’re also variable stars, exhibiting small changes in brightness over time.

The history of 61 Cygni

61 Cygni has no role in classical mythology. Of course, since it’s barely visible to the eye, the ancients apparently left no written reference to it at all. But its role in the history of astronomy is assured.

The motion of 61 Cygni across our sky, while large compared to other stars, can’t be easily detected with the eye alone over the span of a human lifetime. It was with the arrival of telescopes and through meticulous observations that astronomers discovered high proper motions of stars.

Astronomer Giuseppe Piazzi, in 1792, first noticed that 61 Cygni had a high proper motion when he compared his observations to those taken by another astronomer 40 years earlier. By 1804, he had gathered enough information to be the first to publish about this extraordinary star that he nicknamed the Flying Star.

Piazzi correctly noted that this high proper motion indicated that 61 Cygni was a nearby star, and that parallax measurements could be used to figure out its distance. German astronomer F. W. Bessel was the first to get reliable measurements of 61 Cygni stars’ parallaxes that gave a distance of 10.4 light-years, which is pretty close to the actual distance we know today, 11.4 light-years. It’s also the first time a star’s distance was reliably measured.

How to see it

As a matter of fact, 61 Cygni is roughly halfway between two other stars that you can probably identify. First, the brighter one is Deneb, the brightest star in the constellation Cygnus the Swan. And the other star is Zeta Cygni, at one end of the Swan’s wing. You’ll find 61 Cygni between these two. Several other similarly dim stars are located nearby, so you’ll need a detailed finder star chart to properly identify 61 Cygni.

61 Cygni’s position is RA: 21h 06m 55s, Dec: +38° 44′ 57″
Proper motion: 4″ in Right Ascension, 3″ in Declination
Parallax: 0.286″

Bottom line: 61 Cygni, while faint to the unaided eye, is one of the closest stars to Earth. It exhibits a high proper motion – or motion across the sky – compared to other stars.

The post 61 Cygni – a double star – is nicknamed Flying Star first appeared on EarthSky.

Scorpion’s Crown and associates

Scorpius the Scorpion is one of the easiest constellations to see in the sky. It is, in fact, a large J-shaped figure. The bright red star Antares lies at the Scorpion’s heart. Likewise, a curved arc of three stars – Acrab, Dschubba and Fang – mark the Scorpion’s head. They’re known as the Crown of the Scorpion. The upper part of Scorpion – Antares at the heart, and the three stars at the Crown – are beautiful to look at and fascinating to contemplate. Also, Antares and these three stars are part of a nearby grouping of young stars known as the Scorpius–Centaurus Association.

Young stars born from the same cloud

The Scorpius–Centaurus Association is an OB association. Generally speaking, that’s a term astronomers use to describe a collection of young stars formed from the same giant cloud of dust and gas. Indeed, most stars in our galaxy formed this way, without much in the way of gravitational bonds to each other. This is in contrast to stars that form in more compact gravitationally-bound open or globular star clusters. Observations of these associations provide scientists with a deeper understanding of how stars form and evolve. However, they can be a challenge to study because member stars, at least the ones relatively close to us, cover a large area in the sky.

Members of the Scorpius–Centaurus Association are, on average, about 420 light-years away. Moreover, they appear in several southern constellations, including the upper part of Scorpius, Centaurus the Centaur, Lupus the Wolf and Crux the Southern Cross. This association is of great interest to astronomers because it’s the nearest OB association to us. In Scorpius, members of the association, known as Upper Scorpius, may be just 11 million years old (this is very young in the range of stellar lifespans) while other members of the Scorpius–Centaurus Association range to as much as 15 million years in age.

The stars and their distances and temperatures

Astronomers have directly measured distances for over 400 brighter members of the Scorpius–Centaurus Association. In addition, they have identified much fainter, lower mass, stars from spectroscopic surveys. Overall, the exact number of stars in this association remains unknown but it’s likely in the few thousands.

Most stars visible to us are massive hot blue stars, like those in the Scorpion’s Crown. For example, the largest star in the Upper Scorpius sector is Antares, with a mass almost 15 times that of our sun. However, the masses of stars in the association run the gamut from very massive stars to very low mass brown dwarf stars.

Generations of stars: Dead stars cause new star birth

Much larger stars once existed throughout the Scorpius–Centaurus Association. They’re now long gone, having exploded as supernovae. Still, they continue to be important players in subsequent star formation. Indeed, these supernovae left ghostly traces of their presence – bubble-like cavities within the cloud complexes – when powerful shockwaves from the explosions initially swept through the massive molecular cloud. Farther away from the supernovae, the shockwaves, their power a bit muted from traveling large distances, passed through some cloud regions. Thus, triggering new rounds of star formation as their energy compressed dust and gas in their paths.

On his website, astronomer Thomas Preibisch wrote about the Scorpius-Centaurus Association’s possible history. (Some dates written here are modified based on new research). About 15 to 17 million years ago, star formation began in a region of the massive cloud located in what is today’s constellations of Centaurus and Lupus. Then, around 12 million years ago, a very massive star in that region exploded as a supernova, creating a tremendous shockwave. Finally, about 11 million years ago, energy from that shockwave reached molecular clouds in the upper part of Scorpius, triggering star formation. That’s how the Scorpion Crown stars and Antares were born.

Supernova in Scorpion’s crown triggered a shockwave

Massive stars in this new family emitted powerful ultraviolet radiation and stellar winds that cleared out much of the remaining cloud material, putting a stop to further star formation. However, the most massive star of that family exploded as a supernova, sending out another shockwave. Now, that shockwave is moving through a neighboring cloud complex, called Rho Ophiuchi, triggering another cycle of star formation.

Bottom line: The three stars that make up the head of Scorpius the Scorpion are part of a young collection of stars formed from the same cloud of gas and dust, called the Scorpius-Centaurus Association.

The post The Scorpion’s Crown and its stellar neighborhood first appeared on EarthSky.

Most of Earth’s volcanoes and earthquakes occur in regions that skirt the Pacific Ocean, known as the Ring of Fire. It’s not really a ring, though, but more of a horseshoe-shaped swath – 24,900 miles (40,000 km) long – dotted with seismically active locations.

If you could view it from space, the Ring of Fire would appear as a strip that runs up the western coasts of South America and North America, continuing across the Alaskan Aleutian Islands to Russia’s Kamchatka Peninsula. Then, it heads south, off the coast of Eastern Asia, passing through Japan. At Southeast Asia, it jogs eastward through the Indonesian islands of Sumatra and Java, past Papua New Guinea, then southward again to New Zealand.

Volcanoes

Geologists have found evidence of nearly 1,000 prehistoric volcanoes active along the Ring of Fire in the past 12,000 years. The January 15, 2002, Hunga Tonga–Hunga Ha’apai eruption occurred on the Ring of Fire. It was the largest explosion recorded in the atmosphere by modern instrumentation. It blasted winds to the edge of space and may weaken the ozone layer.

Some other memorable volcanic eruptions along the Ring of Fire include the 1991 eruption of Mount Pinatubo in the Philippines. Pinatubo’s eruption caused global cooling for about a year. And in the United States, the 1980 Mount St. Helen eruption resulted in the collapse of half the mountainside. It was the most significant volcanic eruption in U.S. history, causing the death of 57 people. The explosion obliterated the surrounding 230 square miles in minutes.

In 2022, Netflix came out with a documentary called The Volcano: Rescue from Whakaari. The documentary heartbreakingly recounts the 2019 eruption of Whakaari (White Island) in New Zealand. There were 47 tourists and tour guides on the island when the volcano suddenly erupted, ultimately killing 22.

But volcanic activity along the Ring of Fire also births new land. For example, a young volcano is making itself known off the coast of Japan. The Nishinoshima volcano has been growing since 2013 and underwent a growth spurt in June 2020.

Earthquakes

About 90% of all earthquakes, and 80% of the largest ones, occur in the Ring of Fire. Since the invention of equipment to measure earthquake intensity in the 1930s, four of the most powerful earthquakes have occurred in the Ring of Fire. In 1952, a magnitude 9.0 earthquake struck the Kamchatka Peninsula of Russia. Chile’s Valdivia earthquake in 1960 was a stunning 9.4 to 9.6 on the Richter scale. Four years later, a magnitude 9.2 earthquake hit Alaska.

And, more recently, the 2011 9.1-magnitude earthquake Tohoku Earthquake in Japan has become seared in our memories. The world watched, horrified, as large stretches of coastal land and communities near Sendai, Japan, became engulfed in a tsunami triggered by an offshore earthquake. It caused widespread death and destruction and lead to the serious breach of a nuclear reactor. More than 18,000 people died.

Why is the Ring of Fire so seismically active?

Volcanoes and earthquakes most often occur along the borders of tectonic plates. These plates are layers of the Earth’s crust that float – moving very slowly – on molten rock in Earth’s mantle.

The Ring of Fire is not a single geological feature. In actuality, it’s separate adjoining features caused by interactions of several different tectonic plates. Where the two plates meet – in a subduction zone – the heavier plate sinks below the lighter plate and melts in the Earth’s molten mantle.

There are several plates causing seismic activity along the Ring of Fire. In most locations, the plates are colliding, causing subduction zones. (An exception: in a section of western North America, the plates are rubbing against each other laterally. This builds up tension that occasionally releases as earthquakes.)

Some of these subduction zones are also in the deepest parts of the ocean. The Marianas Trench, 36,037 feet (10,984 m) – that’s 6.8 miles (11 km) – below sea level at its deepest known point, is in a subduction zone.

When tension from a bending plate releases, earthquakes happen. At some locations, molten rock is able to infiltrate through the crust to create volcanoes. Most of these seismic activities happen undersea.

Bottom line: The Ring of Fire is home to the majority of Earth’s volcanoes and earthquakes. This region rings the Pacific Ocean where tectonic plates interact.

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Albireo is 2 stars

Albireo, also known as Beta Cygni, is the 2nd-brightest star in the constellation Cygnus the Swan. At first glance, it doesn’t particularly stand out. But viewing this star through a small telescope can take your breath away. It resolves into a striking double, with one component a lovely gold star and the other a dimmer blue close by. From our perspective, the two stars appear close in the sky, but astronomers don’t believe they’re gravitationally bound to each other. Regardless, the color contrast between the two is so striking that Albireo is considered the most beautiful double star in the heavens.

How to find Albireo

How can you spot Albireo in the night sky? It’s easy to find, if you can locate Cygnus the Swan. Cygnus has an easy-to-recognize shape, that of a cross, and the constellation is also known as the Northern Cross. The brightest star in Cygnus – Deneb – marks the head of the Cross or the Tail of the Swan. Albireo marks the base of the Cross or the Head of Cygnus.

And how can you see Albireo as two stars? They’re best viewed at 30X (“30 power” or a magnification of 30). Unless you have exceedingly powerful binoculars, mounted on a tripod, binoculars won’t show you Albireo as two stars. But any small telescope will. When you do see Albireo as two stars, be sure to notice the striking color contrast between the two.

Science of Albireo

The brighter, golden star – Albireo A – is about 430 light-years away. Albireo B, the dimmer blue star, is around 400 light-years distant.

In 2022, NASA said:

Albireo A and B most likely represent an optical double star and not a physical binary system because the two components have clearly different measured motions through space.

However, Albireo A is a binary star, with two stars so close together that you can’t see them as separate. The Albireo A binary star system has an orbital period of 121.6 years. The brighter star is responsible for the gold color you see through a telescope. It’s a red supergiant star, about 5 times the mass of the sun. It outshines its fainter companion, a hot main sequence star that’s 2.7 times the sun’s mass. However, in a recent analysis of the Albireo A binary system, astronomers were surprised to find that there may be another yet-undetected star in the mix, possibly making Albireo A a triple star system.

Albireo B, the fainter blue star of the pair when viewed through a small telescope, appears just 34 arc seconds away from gold-colored Albireo A. It’s a hot blue star, about 3.7 times the sun’s mass.

Bottom line: Through a telescope, Albireo, a seemingly nondescript star in the constellation Cygnus, pops as a stunning gold and blue double star. Astronomers still don’t know if Albireo A is a triple star.

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Antares is an eye-catching star, shining with a distinctive bright red sparkle on northern summer evenings. Indeed, in the Southern Hemisphere, it’s a red beacon in winter evening skies. This star, also known as Alpha Scorpii, lies about 550 light-years away. It’s the brightest star in the zodiacal constellation Scorpius the Scorpion, which has figured prominently in the sky lore of ancient cultures. Antares’s nickname is the Scorpion’s Heart. Its magnitude varies between 0.6 and 1.6.

Today, we know that Antares is a massive star – a red supergiant – in the final stages of its life.

The Heart of the Scorpion is a red supergiant

Antares holds the stellar classification of a M1 red supergiant star.

The M1 designation means that Antares is reddish in color and much cooler than many other stars. Its surface temperature is about 6,100 degrees F (3,400 degrees C). That’s in contrast to our sun’s surface temperature of about 10,000 degrees F (5,500 degrees C).

So, Antares is relatively cool, and its surface temperature is relatively low. Yet the star appears very bright to us. That’s because Antares is a truly enormous star. Its surface area – the surface from which light can escape this star – is gigantic. As an illustration, if you could place our sun and Antares side by side, you’d find Antares more than 10,000 times brighter than our sun. Plus, it is about 700 times the sun’s diameter!

And that’s just in visible light. In addition, when you consider all the various wavelengths of electromagnetic radiation, Antares pumps out about 75,900 times the energy of our sun.

For Antares, the end is nigh

Like all M-type giants and supergiants, Antares is close to the end of its lifetime. In fact, someday soon (astronomically speaking), it will effectively run out of fuel and collapse. The resulting rapid collapse of its enormous mass – some 11-14 times the mass of our sun – will cause an immense supernova explosion. This will ultimately leaving a tiny neutron star or possibly a black hole. To be sure, this explosion, which could be tomorrow or millions of years from now, will be spectacular as seen from Earth, but we are far enough away that there likely is no danger to our planet.

In the meantime, astronomers love to explore huge Antares. For example, in 2017, the European Southern Observatory released a detailed image, taken in infrared wavelengths, of features on Antares’ surface. In addition, they also found that there was a lot of turbulence in the star’s atmosphere and that the star was expelling gases further away than they expected. See video below.

Just how large is Antares?

Antares is a truly enormous star with between 680 to 800 times the sun’s radius. That’s more than three astronomical units (AU). One AU is the Earth’s average distance from the sun. So, if by some bit of magic, we could substitute Antares for our sun, Antares’ surface would extend well past the orbit of Mars and into the asteroid belt.

Recently, astronomers discovered more details about Antares’ outer surface. In 2020, a study of data from radio telescopes showed that Antares’s chromosphere (that’s the layer above the star’s surface) extended out by 2.5 times the star’s radius, far more than previously thought. Conversely, in comparison, our sun’s chromosphere is only 1/200th of its radius.

They also saw that its companion star, Antares B, was lighting up some of the gaseous material that Antares was ejecting.

Antares and Antares B

Antares isn’t alone, either. It has a companion, Antares B. However, it’s hard to see Antares B next to its much brighter companion.

The companion is a blue-white main sequence star with a magnitude of just 5.5. This is near the edge of what you can see with the unaided eye. Antares itself varies in brightness, and its visual magnitude ranges from 0.6 to 1.6. Antares B is also a big star, bigger than our sun. It’s about seven times the sun’s mass and five times the sun’s size. But Antares B is no match for the size of mighty Antares.

Amateur astronomers can spot Antares B on a steady night with a telescope of at least 8-inch aperture and 200 power. The secondary star is about 2.5 arcseconds due west of Antares.

How to see the Heart of the Scorpion

From the Northern Hemisphere, look southward in the early evening from late spring to early fall to find the fishhook pattern of Scorpius the Scorpion, with ruby Antares at its heart. With the eye alone, and with binoculars, you should notice its reddish color. Also, if you have binoculars and a dark sky, scan just to the right of Antares. Indeed, you should see a little globular star cluster, M4.

Antares is the 15th brightest star in the sky. From our northerly latitudes, we see it arc across the south. Because we’re sometimes looking at it through a greater thickness of Earth’s atmosphere near the horizon, we see Antares twinkle fiercely. In the Northern Hemisphere, anyone south of 63° north latitude can – at one time or another – see Antares. (Helsinki, yes, Fairbanks, no.)

Conversely, from the Southern Hemisphere, Antares appears higher in the sky. Your chance of seeing this star on any given night increases as you go farther southward on Earth’s globe. So, if you traveled to about 63° south latitude, you’d find that Antares is circumpolar. That means that, from Earth’s southernmost regions, Antares never sets and is visible every night of the year.

The midnight culmination of Antares is in mid-June. That’s when Antares is highest in the sky at midnight (midway between sunset and sunrise). It is highest in the sky at about dawn in late March and at about evening twilight in late July.

Antares in history and skylore

Both the Arabic and Latin names for the star Antares mean heart of the Scorpion. If you see this constellation in the sky, you’ll find that Antares does indeed seem to reside at the Scorpion’s heart.

Antares is Greek for rival of Ares, meaning rival of Mars. Antares is sometimes said to be the anti-Mars due to its competing red color. For a few months every couple of years Mars is much brighter than Antares. Also, every couple of years Mars passes near Antares, as if taunting the star. Mars moves rapidly through the heavens and Antares is fixed to the starry firmament.

The most well-known story of Scorpius, home to Antares, is that the Earth goddess, Gaia, sent him to sting arrogant Orion, who had claimed his intent to kill all animals on the planet. Scorpius killed Orion, and now both reside in the sky on opposite sides of the heavens.

In Polynesia, Scorpius represents a fishhook, with some stories describing it as the magic fishhook used by the demigod Maui to pull up land from the ocean floor that became the Hawaiian islands. According to the University of Hawaii’s Institute for Astronomy, the Hawaiian name for Antares, Lehua-kona, seems to have little to do with the constellation. It means “southern lehua blossom.”

Antares’ position is RA:16h 29m 24s, dec: -26° 25′ 55″.

Bottom line: Antares is a brilliant ruby red star in summer for the Northern Hemisphere (winter for the Southern Hemisphere). It’s an enormous red supergiant star, whose constellation – Scorpius the Scorpion – has a rich history in skylore.

The post Massive ruby red Antares is the Scorpion’s Heart first appeared on EarthSky.

Mira the Wonderful

Although stars appear to shine at a constant brilliance, many are variable stars. They brighten and dim over many different timescales. Their changes in brightness are often too small to be perceptible to the unaided eye. But the star Mira, aka Omicron Ceti, is different. Its brightness changes are large and distinctly noticeable to the eye.

Depending on when you look for Mira, this reddish star in the constellation Cetus the Whale might or might not be visible. It goes through its bright-to-faint-to-bright cycle about every 332 days.

Mira is not visible from late March to June for observers in mid-northern latitudes because it is too close to the Sun. Near the expected date of maximum brightness it 2023, it does rise before sunrise (your local time) – but not by much – and it will be challenging to spot in the brightening morning sky. Luckily, it’ll rise four minutes earlier each day. Generally, you can see it with the unaided eye for about six weeks before it reaches maximum brightness and over two months afterwards. Of course that depends on when Mira reached its maximum brightness.

And Mira has a predicted brightness peak coming up. It should be brightest on or near June 13, 2023. If you’d like to see this unusual star in 2023, now’s your chance.

Early astronomers noticed this star’s dramatic and regular changes in brightness. Mira sparkles in the sky, getting progressively dimmer, and a few months later, it’s gone! Then, after some months, it’s back again. Its brightness changes led the 17th century astronomer Johannes Hevelius to name the star Mira, from the Latin word for wonderful or astonishing.

So Mira is on track to hit another brightness peak around June 13, 2023. How bright will it get? That’s a question many variable star observers are eagerly waiting to find out.

Now you see it, now you don’t

Mira has an average peak brightness of magnitude 3.5. It’s not one of the sky’s brightest stars, even when brightest. It gradually fades to around magnitude 9 (too faint to see with the eye; for reference, in a dark sky, the unaided eye can barely detect a magnitude 6 star). Then it rebounds back to its peak brightness. So Mira undergoes about a 159-fold change in brightness, as it moves through its 332-day brightness cycle.

It’s impossible to predict exactly how bright or faint Mira will become at each maximum. Have a look at the graph below, called a light curve. Mira-watchers contribute their observations to the American Association of Variable Star Observers (AAVSO). The AAVSO creates an ongoing light curve for Mira, using its Light Curve Generator tool. The light curve below covers the last 10 years. Parts of the plot with no data were when Mira was close to or behind the sun. In 2019 and 2022, Mira was as bright as magnitude 2. That’s almost as bright as Polaris, the North Star, not the sky’s brightest star, but a respectably bright star.

How to see Mira

Catch Mira while it’s at its brightest! Then watch it as it fades away. Its peak brightness for 2023 comes in June. At that time, Mira rises around 4:00 a.m. (your local time) and climbs above the horizon before sunrise. However, it’s in the constellation of Cetus which isn’t a prominent constellation. It’s faint. You’ll want a dark sky. If you have a dark sky, you can pick out the Whale’s lopsided pentagon of a Head. Check Stellarium for a view from your location.

Here is a list of upcoming predicted maximum brightnesses for Mira, via SEDS:

2023: June 13
2024: May 10

Look for Mira around these dates! That’s when, according to predictions, it should be at its brightest.

Also, look at the chart below. Notice that the distinctive nearby V-shaped Hyades star cluster in Taurus the Bull points to Cetus and its star Mira.

Mira science

Early astronomers marveled at Mira’s brightness changes and considered them a great mystery. But modern astronomers know Mira as a red giant star. It’s slightly more massive than our sun but at least 330 times larger in size. Its huge surface area makes it more than 8,000 times more luminous. Mira is some 300 light-years away. It’s thought to be around 6 billion years old. Mira has a faint white dwarf companion star.

There are many types of pulsating variable stars known today. But Mira was the first of its type discovered. And so, astronomers named an entire class of variable stars after it. Mira variables are stars that have one to a few times the mass of our sun. They’re near the end of their stellar lifetime, at the red giant stage. Mira variables have pulsation periods from 80 to 1,000 days, brightness variations from 2.5 to 10 visual magnitudes, and tend to shed material from their outer layers.

So Mira’s brightness changes aren’t due, for example, to some external factor (such as a disk around the star). They’re caused by the actual expansion and contraction of the entire star, every 332 days. This expansion-contraction oscillation is a complex phenomenon related to changes in the rate that radiation escapes from the star.

Mira’s story is of special interest since our sun will someday follow the same stellar evolutionary path. About 5 billion years from now, our sun will become a Mira variable.

Why Mira’s brightness changes

For much of its existence, Mira converted hydrogen to helium at its core as a main sequence star. When that fuel was exhausted, its core contracted, causing it to heat up. That heating triggered a new round of hydrogen-to-helium nuclear fusion in a shell around the core, causing Mira to balloon in size into a red giant star. Meanwhile, the collapsing core continued to heat up until it became hot enough for the fusion of helium to carbon, and some oxygen.

Mira is currently at a stage in its stellar evolution called the asymptotic giant branch. Its core of carbon and oxygen is inert. However, the star is still actively “burning” a layer of helium around the core, converting it to carbon. And just outside it, a shell of hydrogen is being converted to helium.

The outer layers of Mira are weakly held by gravity and are starting to waft away. Mira will eventually shed its material to form a planetary nebula, with its exposed hot core — a white dwarf star — left behind.

Mira’s 13-light-year-long tail

In 2006, the Galaxy Evolution Explorer telescope obtained ultraviolet images of Mira that surprised scientists. They revealed a long comet-like tail of material trailing the star as it sped through ambient galactic gas. Mira moves through space at about 290,000 miles per hour (466,709 km/h). The tail, about 13 light-years long, is composed of gases and dust released by Mira over the last 30,000 years. The amount of gases and dust in Mira’s tail equal about 3,000 times the Earth’s mass.

Mira in history

Did the earliest stargazers notice Mira as it appeared disappeared and reappeared? If they did, they left no records of this star. The star’s earliest known history begins only 400 years ago, when Dutch astronomer David Fabricius first noticed Mira. That was in the year 1596. He assumed Mira was a nova because, as novae do, the star faded away after a few months. However, Fabricus relocated the star 13 years later. It must have surprised him!

Another Dutch astronomer, Johannes Holwarda, was the first to identify Mira as a variable star, and determined a period of 11 months. That value was refined in 1667 by French astronomer Ismael Bouillaud to 333 days, very close to the currently accepted value of 332 days.

Mira got its name, meaning wonderful or astonishing in Latin, from Johannes Hevelius in 1642.

The position of Mira is RA: 02h 19m 21s, Dec: -02° 58′ 39″.

Latest observations of Mira from AAVSO

Bottom line: Mira is a variable star that undergoes periodic changes in brightness every 332 days, ranging from a maximum brightness of around 3.5 visual magnitudes to a minimum brightness of about 9 magnitudes. It’s expected to be brightest around June 13, 2023.

Read more: Mira Revisited, from the AAVSO

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