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Omega Centauri is the Milky Way’s largest star cluster

Sphere of multicolored stars, less dense toward edges.
Omega Centauri in infrared light captured by the Spitzer Space Telescope. Image via Wikimedia Commons.

Omega Centauri is a monster globular star cluster

Omega Centauri, the largest globular star cluster of the Milky Way, contains about 10 million stars. This behemoth, with a diameter of about 150 light-years, is 10 times more massive than a typical globular cluster. However, despite all these stars, scientists released a study in 2018 that said Omega Centauri probably is not home to life.

Stars pack so tightly inside Omega Centauri that the average distance between stars in the cluster’s core is 0.1 light-years. That’s much closer than the sun’s nearest neighbor, Proxima Centauri, at 4.25 light-years. So scientists concluded that stars in Omega Centauri would gravitationally interact with each other too frequently to harbor stable habitable planets.

Although it’s not only Omega Centauri’s largest size that sets it apart from other globular clusters. Most globular star cluster generally have stars of similar age and composition. However, studies of Omega Centauri reveal that it has different stellar populations that formed at varying periods of time. In fact, it may be that Omega Centauri is something other than a globular cluster. It might be a remnant core of a small galaxy that was absorbed by the Milky Way galaxy in the distant past!

The difference between an open and a globular star cluster

The symmetrical, round appearance of Omega Centauri distinguishes it from star clusters such as the Pleiades and Hyades, which are open star clusters.

An open star cluster is a loose gathering of dozens to hundreds of young stars within the disk of the Milky Way galaxy. Open clusters are weakly held together by gravity, and tend to disperse after several hundreds of millions of years.

Globular clusters, on the other hand, orbit the Milky Way outside the galactic disk. They harbor tens of thousands to millions of stars. Tightly bound by gravity, globular clusters remain intact after 12 billion years.

A large, round, symmetrical star cluster.
The globular cluster Omega Centauri — with as many as 10 million stars — is shown in all its splendor in this image captured with ESO’s La Silla Observatory. Image via Wikimedia Commons.

How to see Omega Centauri.

Omega Centauri – the most luminous of all globular star clusters – is far to the south on the sky’s dome. It’s visible from the southern half of the United States, or south of 40 degrees north latitude (the latitude of Denver, Colorado). Canadians hasten to remind us that they can spot Omega Centauri from as far north as Point Pelee in Canada (42 degrees latitude). When seeing conditions are just right, they “… can catch the Omega Centauri star cluster skimming along the surface of Lake Erie,” they say.

From the Southern Hemisphere, Omega Centauri appears much higher in the sky and is a glorious sight.

Chart of Centaurus constellation and Crux, with Omega Centauri and several stars labeled.
Use the bright constellation Crux as a guide to find Centaurus and Omega Centauri.

Finding Omega Centauri from the Northern Hemisphere

If you’re in the Northern Hemisphere and want to spot this cluster, know that Omega Centauri can only be seen at certain times of the year. It’s best seen in the evening sky from the Northern Hemisphere late on April, May and June evenings.

So around mid-May, this wondrous star cluster is highest up and due south around 11 p.m. local DST.

Then, by mid-June, Omega Centauri is highest up and due south around 10 p.m. local DST.

Northern Hemisphere residents can see Omega Centauri from January through April as well, but they must be willing to stay up past midnight or get up before dawn.

Star map with constellations and a white arrow pointing from Spica to Omega Centauri.
Use the bright blue-white star Spica to locate the large Omega Centauri star cluster on Northern Hemisphere spring evenings. This chart shows the view from 35 degrees N latitude. Image via Stellarium.

Use the Big Dipper to find Spica

For those in the Northern Hemisphere, Spica, the brightest star in the constellation Virgo, serves as your guide star to Omega Centauri. When Spica and Omega Centauri transit – appear due south and reach the highest point in the sky – they do so in unison. However, Omega Centauri transits about 35 degrees south of (or below) sparkling blue-white Spica. For reference, your fist at arm’s length approximates 10 degrees of sky. Find Spica by following the arc in the handle of the Big Dipper.

Chart showing Big Dipper with long magenta arrows from its handle to Arcturus and Spica.
Use the Big Dipper to locate the stars Arcturus and Spica.

Use the search function in to locate sky objects as viewed from your location

It’s visible to the unaided eye

Generally, open clusters visible to the unaided eye are hundreds to a few thousand light-years away. In contrast, globular clusters are generally tens of thousands of light-years distant.

At about 16,000 light-years, Omega Centauri is one of the few of our galaxy’s 200 or so globular clusters that is visible to the unaided eye. It shines at +3.9 magnitude. It looks like a faint, fuzzy star, but Omega Centauri’s mere presence testifies to its size and brilliance. Like any globular cluster, Omega Centauri is best viewed with a telescope.

Omega Centauri’s position is at Right Ascension: 13h 26.8m; Declination: 47 degrees 29′ south.

Spherical cluster of countless stars, growing more diffuse from the center outward.
View at EarthSky Community Photos. | Prabhakaran A in Jebel Jais, United Arab Emirates (UAE), captured this telescopic view of the star cluster Omega Centauri on April 1, 2022, and wrote “Globular star cluster Omega Centauri, also known as NGC 5139. It’s the largest and brightest of 150 or so known globular clusters that roam the halo of our Milky Way galaxy.” Thank you, Prabhakaran!

Bottom line: The Milky Way’s largest globular star cluster, Omega Centauri, contains about 10 million stars. And it’s visible from parts of the Northern Hemisphere.

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Virgo the Maiden in northern spring skies

Lines and dots forming polygon with arms and legs, labeled with Spica and Arcturus above.
The constellation Virgo the Maiden is easy to find by using the handle of the Big Dipper to arc to Arcturus in Boötes, then speeding on (spiking down) to Spica, Virgo’s brightest star. Image via Chelynne Campion.

The constellation of Virgo the Maiden

From the Northern Hemisphere, Virgo the Maiden appears high above the southern horizon on May evenings. And this is the best time of year to view this constellation, which is the largest of the zodiac. As a matter of fact, Virgo is also the 2nd-largest constellation overall, after Hydra. Plus, thanks to its brightest star, Spica, there’s an easy trick to finding this constellation.

So, to find Virgo, remember this handy mnemonic device: Arc to Arcturus and spike to Spica. What does that mean? Using the readily identifiable Big Dipper, you can follow the curve of its handle as you arc to a bright orangish star named Arcturus in the constellation Boötes. Then “drive a spike” (or sometimes the saying is “speed on down”) to Spica.

Spica is a blue-white, 1st-magnitude star near the center of Virgo.

Big dipper with arrows to Arcturus and Spica, with 4 stars connected with lines nearby.
First, follow the curve in the Big Dipper’s handle to the bright orange star Arcturus. Then extend that line to Spica. To be sure you’ve found Spica, look for a lopsided square pattern nearby: that’s Corvus the Crow. Image via Chelynne Campion.

The stars of the Maiden

Spica, which marks a bundle of wheat that the Maiden is holding, is the 15th brightest star in the sky. Spica is a magnitude 1.04 star that lies 250 light-years from Earth.

Then the next brightest star in Virgo is the binary star Gamma Virginis, or Porrima. Porrima is magnitude 2.74 and lies near the center of the constellation, above (northwest of) Spica. It lies 38 light-years away. Next, the third brightest star is at the northern reaches of the constellation. Vindemiatrix is a magnitude 2.82 star located 109 light-years away.

Star chart: Virgo, stars in black on white, and a blue line crossing the constellation.
Virgo the Maiden and its stars. Image via the International Astronomical Union (IAU).
Virgo: Antique colored etching of young woman in long dress with labeled scattered stars.
Here’s a classical illustration of the constellation Virgo the Maiden, via Urania’s Mirror/ Wikipedia.

The Virgo Cluster

Without a doubt, Virgo is famous for its thousands of galaxies. One grouping – the Virgo Cluster – is near the border with Coma Berenices, west of Vindemiatrix. The Virgo Cluster is the nearest large group of galaxies to the Milky Way. The Virgo Cluster lies at the center of the Local Supercluster, a massive group of clusters of galaxies. Plus, the Local Group of galaxies, which includes the Milky Way, is also part of the Local Supercluster.

Additionally, the gravitational pull from the Virgo Cluster in the Local Supercluster is slowing the escape velocity of the Milky Way and our Local Group. So the Virgo cluster is one of the few places in the universe we are speeding toward. Therefore, the galaxies of the Virgo Cluster are some of the few we see with a blueshift instead of a redshift. One day, these many galaxies will merge into one huge conglomeration.

In fact, the galaxy with one of the highest blueshifts lies right on the border of Virgo and Coma Berenices. This galaxy, M90, is moving rapidly among the other objects in the Virgo Cluster. That’s because it’s also being stripped of gas and dust due to its close quarters with the other galaxies. At magnitude 9.5, you can see this galaxy in a telescope across the 60 million light-year span.

In addition, other galaxies between 8th and 9th magnitude in this location are M49, M58, M59, M60, M84, M86, M87, and M89. Even more galaxies come into view if you scan along the line between Virgo and Coma Berenices.

Star field with circles around extremely many labeled galaxies.
View larger. | The Virgo Cluster. Image via Wikimedia Commons.

M87, or Virgo A

M87 is a special galaxy that deserves to be singled out from the Virgo Cluster. It shines at magnitude 8.6 and is therefore easy to detect in any telescope and even in some binoculars. M87 lies about 60 million light-years away. Its potato-shaped clump of stars extends well over a half million light-years across, and thought to be five times the size of the Milky Way’s diameter. However, the diameter of the galaxy’s halo is about a million light-years, and while that is large, astronomers expected it to be even larger. They believe something cut the halo off early on in its formation.

As a matter of fact, M87 is home to the largest known number of globular clusters. For comparison, the Milky Way has about 200 globulars, while M87 has thousands. These clusters may be dwarf galaxies that M87’s gravitation sucked in.

Another amazing feature of M87 is its jet that extends outward from the core for thousands of light-years. A monster black hole at the galaxy’s core is the source of the jet. In fact, M87’s black hole was the first ever imaged, in 2019. Then, recently that image was enhanced and released with more detail in April 2023. Plus, its black hole and its jet were imaged together for 1st time ever in April 2023.

Long broken beam of light coming from a bright spot in space.
View larger. | An optical light image of the jet erupting from the black hole at the core of galaxy Messier 87 (M87 or NGC 4486). The Hubble Space Telescope took this image on July 6, 2000. Image via NASA/ The Hubble Heritage Team (STScI/AURA)/ Wikimedia Commons.

The Sombrero Galaxy

Not to be overlooked is another bright and notable galaxy that’s apart from the large cluster is M104, or the Sombrero Galaxy. It’s located on the southeastern border of the constellation with Corvus the Crow. Without a doubt, M104 is a stunning galaxy in photographs. Even better, at magnitude 8.3, you can see it in small telescopes. It’s an edge-on, dusty spiral galaxy with a bright core. M104 lies approximately 55 million light-years away.

Edge-on galaxy with dark lane of dust and diffuse, bright central area.
M104, or the Sombrero Galaxy, lies in the constellation Virgo the Maiden. Image via ESA/ Hubblesite/ Wikimedia Commons.

Virgo in mythology

Virgo personifies Persephone, daughter of Demeter, the harvest goddess. According to a Greek myth, it once was always springtime on Earth. But then the god of the underworld, Hades, kidnapped Persephone.

Demeter, overcome with grief, abandoned her role as an Earth goddess. Thus, the world’s fruitfulness and fertility suffered. Then, according to the myth, Earth wouldn’t become fruitful again until Persephone returned home. So, Zeus insisted that Hades return Persephone to Demeter. Also, Zeus said that Persephone must eat nothing until her return. Alas, Hades purposely gave Persephone a pomegranate.

Thus, Persephone was given back to her mother, but Persephone – because of the pomegranate – has to return to the underworld for four months every year. To this day, spring returns to the Northern Hemisphere when Persephone reunites with Demeter. Then the winter season reigns when Persephone dwells in the underworld. Considering this, from the perspective of the Northern Hemisphere, Virgo is absent from early evening sky in late autumn, winter and early spring. Virgo’s return to the sky at nightfall – in the months of April and May – coincides with the season of spring.

Woman in Greek garb greeting a young woman ascending from the dark underground.
The Return of Persephone by Frederic Leighton. Image via Wikipedia.

Bottom line: Virgo the Maiden is the largest of the zodiac constellations. A handy mnemonic device – using the Big Dipper and its bright star Spica – make it easy to find.

The constellations of the zodiac

Meet Taurus the Bull in the evening sky
Gemini the Twins, home to 2 bright stars
Meet Cancer the Crab and its Beehive Cluster
Leo the Lion and its backward question mark
Virgo the Maiden in northern spring skies
Meet Libra the Scales, a zodiacal constellation
Scorpius the Scorpion is a summertime delight
Sagittarius the Archer and its famous Teapot
Capricornus the Sea-goat has an arrowhead shape
Meet Aquarius the Water Bearer and its stars
Meet Pisces the Fish, 1st constellation of the zodiac
Say hello to Aries the Ram
Born under the sign of Ophiuchus?

Find the Keystone in Hercules, and M13

Star chart of man-shaped constellation with bent arms and legs.
The fact is, Hercules is a faint constellation. But its mid-section contains the easy-to-see Keystone asterism. You can find Hercules between the bright stars Vega in Lyra the Harp, and Arcturus in Boötes the Herdsman. And once you find the Keystone, you can easily locate M13, the Hercules cluster. Chart via Chelynne Campion/ EarthSky.

Use Vega to locate the Keystone in Hercules

In late spring, from mid-northern latitudes, you can easily find the brilliant star Vega in the eastern sky at dusk and nightfall. The brilliant blue-white star Vega acts as your guide star to the Keystone, a wedge-shaped pattern of four stars in the constellation Hercules.

Look for the Keystone asterism – star pattern – to the upper right of Vega. Or hold your fist at arm’s length, it’ll easily fit between Vega and the Keystone.

Also, you can locate the Keystone by using Vega in conjunction with the brilliant yellow-orange star Arcturus. The Keystone is found about 1/3 of the way from Vega to Arcturus, the two brightest stars to grace the Northern Hemisphere’s spring and summertime sky. From mid-northern latitudes this time of year, Arcturus is found quite high in the eastern sky at nightfall and evening. Then, by late evening, Arcturus will have moved high overhead.

Star chart of man-shaped constellation with bent arms and legs.
Before you can find M13, you need to find the Keystone in Hercules, a pattern of four stars. So as darkness falls, look for the Keystone to the upper right of the brilliant star Vega. Chart via Chelynne Campion/ EarthSky.

Use the Keystone to find M13

Furthermore, the Keystone is your ticket to find a famous globular star cluster in Hercules, otherwise known as the Hercules cluster, aka Messier 13 or M13.

Most likely, you’ll need binoculars to see the Hercules cluster. Although sharp-eyed people can see it with the unaided eye in a dark, transparent sky. But through binoculars, this cluster looks like a dim smudge or a somewhat fuzzy star. However, a telescope begins to resolve this faint fuzzy object into what it really is, a great big, globe-shaped stellar city populated with hundreds of thousands of stars!

Then, later in the evening, the Keystone and the Hercules cluster swing high overhead after midnight, and are found in the western sky before dawn.

White star chart with black dots and lines showing keystone shape and lines radiating outward.
Can you find the Keystone on this chart? See the compact grouping of 4 stars at the center of Hercules? That’s it. Note the whereabouts of Messier 13 within the Keystone pattern. Also, above the Keystone is another globular cluster, M92, it’s a bit smaller and dimmer than M13, but also easy to pick up in binoculars or a telescope. Image via International Astronomical Union/ Sky & Telescope/ Wikimedia Commons. Used with permission.

Photos of M13 from EarthSky Community Photos

Spherical ball of stars in the center of a dark sky with multiple stars around it.
View at EarthSky Community Photos. | Jeremy Likness captured this image on April 28, 2023, in Florissant, Colorado. He wrote: I was really just testing tracking and focus between shots, but this cluster (Messier 13) came out so clean with only 20 minutes total exposure time I decided it was a keeper!” It’s certainly a keeper. Thank you, Jeremy!
Tight cluster of dots of white light at center, diffusing out into blackness at the edges.
View at EarthSky Community Photos. | This isn’t an open star cluster. It’s a globular star cluster, the Hercules cluster, captured by Randall Kayfes in Tucson, Arizona, on June 3, 2022. Randall wrote: “The Hercules star cluster M13 is one giant ball of stars and a favorite go-to star cluster in the summer.” Thank you, Randall!
Bright white cluster of thousands of stars at center with smattering surrounding in black sky.
View at EarthSky Community Photos. | Ron Haggett in Yuma, Arizona, took this image of a globular cluster on January 5, 2022. Ron wrote: “Messier 13 or the great globular cluster in Hercules. Fortunately for me it is viewable around 5 in the morning!” Thank you, Ron!

Bottom line: Let the bright star Vega guide you to a famous star pattern in Hercules – called the Keystone – and then to the Hercules cluster, aka M13, a famous globular star cluster.

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Eta Aquariid meteors are richer from the Southern Hemisphere

Eta Aquariid: Lines marking constellation with radial arrows near middle of it.
The radiant point of the Eta Aquariid meteor shower is near the star Eta Aquarii in the constellation Aquarius the Water Bearer. The radiant rises in the wee hours after midnight and is still climbing toward its highest point at dawn. That highest point is in the south as viewed from the Northern Hemisphere, and closer to overhead for the Southern Hemisphere. That’s why the Southern Hemisphere sees more meteors (the radiant is higher up), and it’s why – for all of us around the globe – the hours before dawn are best for this shower. However, in 2023, a bright moon will compete with the meteor shower.

The famous Eta Aquariid meteor shower – one of the year’s major meteor showers – peaks every year in early May. In 2023, the peak centers around May 6. This shower is known to be richer as seen from Earth’s Southern Hemisphere than from the Northern Hemisphere. Why?

Water Jar in the constellation Aquarius

If you traced the paths of Eta Aquariid meteors backward on the sky’s dome, you’d find that these meteors appear to stream from an asterism, or recognizable pattern of stars, known as the Water Jar in the constellation Aquarius. See chart at the top of this post.

This spot in the sky is the radiant point of the Eta Aquariid meteor shower. The meteors seem to emanate from the vicinity of the Water Jar, before spreading out and appearing in all parts of the sky.

Water Jar rises about the same time worldwide…

Because the Water Jar is on the celestial equator – an imaginary great circle directly above the Earth’s equator – the radiant of the Eta Aquariid shower rises due east as seen from all over the world. Moreover, the radiant rises at about the same time worldwide, around 1:40 a.m. local time (2:40 a.m. daylight-saving-time) in early May, around the shower’s typical peak date.

So you’d think the shower would be about the same as seen from around the globe. But it’s not.

The sun rises later in the Southern Hemisphere

The reason it’s not is that sunrise comes later to the Southern Hemisphere (where it’s autumn in May) and earlier to the Northern Hemisphere (where it’s spring in May).

Later sunrise means more dark time to watch meteors. And it also means the radiant point of the Eta Aquariid shower has a chance to climb higher into the predawn sky as seen from more southerly latitudes. That’s why the tropics and southern temperate latitudes tend to see more Eta Aquariid meteors than we do at mid-northern latitudes.

Cruise to a southerly latitude, anyone?

Everything you need to know: Eta Aquariid meteor shower

Bottom line: Everyone around the globe can enjoy the Eta Aquariid meteor shower in early May. Best for the Southern Hemisphere! The peak in 2023 is on the morning of May 6.

Read more: EarthSky’s annual meteor shower guide

‘Not just a problem of science’: how the environmental crisis is also cultural

“I want people to understand that the environmental crisis is bound up inextricably in our cultural world and how we relate to each other,” Dr Wendy Nālani E Ikemoto is discussing Nature, Crisis, Consequence, which she curated for the New-York…

How far is a light-year? Plus, distances in space

Schematic portraying a light-year: A large yellow shell outside smaller shell with arrows pointing from center to each shell.
The large yellow shell depicts a light-year; the smaller yellow shell depicts a light-month. Read more about this image at Wikimedia Commons.

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How far is a light-year?

Objects in our universe are extremely far away. In fact, they’re so far away that kilometers or miles aren’t a useful measure of their distance. So, with this in mind, we speak of space objects in terms of light-years, the distance light travels in a year. Light is the fastest-moving stuff in our universe. It travels at 186,000 miles per second (300,000 km/sec). So, a light-year is 5.88 trillion miles (9.46 trillion km).

However, stars and nebulae – not to mention distant galaxies – are vastly farther than one light-year away. And, if we try to express a star’s distance in miles or kilometers, we soon end up with impossibly huge numbers. Yet miles and kilometers are what most of us use to comprehend the distance from one place on Earth to another. In the late 20th century astronomer Robert Burnham, Jr. – author of Burnham’s Celestial Handbook – devised an ingenious way to portray the distance of light-years in terms of miles and kilometers.

Keep reading, for a way to comprehend the vastness of the universe, using units of distance we know and use every day.

Let’s start with astronomical units

Burnham started by relating the light-year to the astronomical unit – the Earth-sun distance.

One astronomical unit, or AU, equals about 93 million miles (150 million km).

Also, another way of looking at it is this: the astronomical unit is a bit more than 8 light-minutes in distance.

Diagram of sun, Earth and moon, labeled. There is a line between the sun and Earth labeled 150,000,000 km. The moon is to the right side of Earth.
A light beam takes 8 minutes to travel the 93 million miles (150 million km) from the sun to the Earth. Image via Brews Ohare/ Wikimedia Commons.

A light-year, pictured as a mile

Robert Burnham noticed that, quite by coincidence, the number of astronomical units in one light-year and the number of inches in one mile are virtually the same.

Generally speaking, there are 63,000 astronomical units in one light-year, and 63,360 inches (160,000 cm) in one mile (1.6 km).

This wonderful coincidence enables us to bring the light-year down to Earth. So, if we scale the astronomical unit – the Earth-sun distance – at one inch, then the light-year on this scale represents one mile (1.6 km).

The closest star to Earth, other than the sun, is Alpha Centauri at some 4.4 light-years away. So, scaling the Earth-sun distance at one inch places this star at 4.4 miles (7 km) distant.


Extremely dense star field with 2 bright stars and a small red circle at bottom left.
The very faint Proxima Centauri, which is gravitationally bound to Alpha Centauri, is indicated by a red circle in this image. Proxima Centauri, is our sun’s nearest neighbor among the stars. A beam of light from this star takes about 4 years to travel to Earth. The two bright stars are Alpha Centauri and Beta Centauri. Image via Skatebiker / Wikimedia Commons (CC BY-SA 3.0).

Familiar space objects, conceptualized

Scaling the astronomical unit at one inch (2.5 cm), here are distances to various bright stars, star clusters and galaxies:

Alpha Centauri: 4.4 miles (7.0 km)

Sirius: 8.6 miles (14 km)

Vega: 25 miles (40 km)

Pleiades open star cluster: 444 miles (715 km)

Antares: 550 miles (885 km)

Hercules globular star cluster (aka M13): 25,000 miles (40,233 km)

Center of our Milky Way galaxy: 26,100 miles (42,000 km)

Great Andromeda galaxy (M31): 2,540,000 miles (4,087,000 km)

Sombrero galaxy (M104): 28,000,000 miles (45,000,000 km)

Whirlpool galaxy (M51): 31,000,000 miles (50,000,000 km)

And so on, back to approximately 13 billion+ light-years to the farthest galaxies: 13,000,000,000 miles (21,000,000,000 km)

Okay, the numbers are still pretty big! But hopefully they can help you see that our universe is very vast. And this video helps puts it in perspective.


The fastest-moving stuff in the universe

As mentioned above, light travels at an incredible 186,000 miles per second (300,000 km/sec). That’s very fast, indeed. In fact, if you could travel at the speed of light, you would be able to circle the Earth’s equator about 7.5 times in just one second!

In other words, a light-second is the distance light travels in one second, or 7.5 times the distance around Earth’s equator. So, a light-year is the distance light travels in one year.

How far is that? Multiply the number of seconds in one year by the number of miles or kilometers that light travels in one second, and there you have it: one light-year. It’s about 5.9 trillion miles (9.5 trillion km).

Chart with 6 different objects and their light years from Earth.
This chart shows objects close to home and takes us all the way out to the Andromeda Galaxy, the most distant object most people can see with the unaided eye. And then, it shows one of the oldest galaxies ever found. Image via NASA Space Place.

Bottom line: Here’s a way to understand the scale of light-years in miles and kilometers.

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