Astronomers refer to Jupiter’s four largest moons as the Galilean satellites, since they were first observed by the great Galileo in 1609. Each of Jupiter’s moons is a distinct world in its own right, and each is influenced and influenced by its proximity to Jupiter itself.

From nearest to Jupiter to farthest, the four Galilean moons are:

Io, a red-orange sulphuric hell-hole of a world, where volcanos spray molten lava high into space. Io shouldn’t have a molten core– it’s too small– but the gravitational push and pull of Jupiter kneads the core of this small world, and keeps it perpetually active.

Europa is slightly smaller than our Moon, but it’s much lighter. The surface is smooth and free of craters, but long cracks criss-cross the surface. Fly-bys of NASA satellites suggest Europa has a liquid-water ocean miles under its icy surface, and some planetary scientists think the moon’s hot core may furnish enough energy and minerals to stimulate the formation of simple life forms.

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Jupiter's moon Europa (courtesy NASA)

Ganymede also has a smooth, glassy surface with patches of older, cratered material. The moon is the largest in the solar system, outsizing even Mercury and Pluto. The geology of this moon is not well understood.

Callisto, the most distant of the four moons, is geologically dead as a doornail. Like Mercury and our own Moon, its surface is strewn with craters, which means not much has happened here since the early days of the solar system.

When you look at the four Galiliean moons through optics, it’s not always apparent which is which. Callisto may appear closer to Jupiter’s disc than Io as it prepares to pass behind the planet, for example. Sky and Telescope magazine provides a useful and free tool to help you find the positions of all four moons at any time:

http://www.skyandtelescope.com/observing/objects/planets/38135094.html

You can clearly see the moons of Jupiter move over the course of an hour or less. It’s great fun to track them during an evening, especially when the moons pass in front or behind Jupiter, or when they cast a shadow on the big planet. The above link also gives times of such events.

At 100x or more, you can resolve the discs of each of the moons, which are all brighter than 5th magnitude and would be visible without optics if not for the glare of Jupiter. With large, high-quality telescopes and dead-steady seeing, some amateurs have even reported seeing markings on the moons!

Philosophers have debated the question for centuries. Clerics claim nature is the work of a divine hand. And modern scientists use a mix of logic, mathematics, and experiment seek the fundamental physical principles to explain why the universe behaves as it does.

But what if there’s a simple explanation for it all?

What if the universe is as it is because if it was different, mankind (and any other intelligent life) wouldn’t be here to think about it?

This apparently obvious and seemingly absurd observation is known as the anthropic principle.

It has some basis in scientific fact. Scientists have calculated that if the physical laws in our universe were just a tiny bit different, we (or any other intelligent life) could not exist. For example, if the force that holds atomic nuclei together were just a few percent stronger, the hydrogen atoms created after the Big Bang would have fused into helium atoms, and no hydrogen would remain. No hydrogen means no water, no organic molecules, and no life.

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On the other hand, if the nuclear force was a tiny bit weaker, larger atoms like carbon and iron which are essential to complex life could not hold together. Again, no life. So it seems the nuclear force (and other forces of nature) are finely balanced in a way to allow the formation of galaxies, stars, planets, and people (and perhaps other forms of intelligent life).

The anthropic principle is an interesting idea and makes for good after-dinner debate. But it is– so far– a concept of philosophy, not science.

To scientifically test the anthropic principle, we would have to examine the conditions in other universes, a feat which may, by definition, be impossible.

But those other universes may well exist. Recent cosmological theories suggest our universe is not unique, but just one of a nearly infinite number of other universes called the multiverse (current theories suggest some 10⁵⁰⁰ universes may exist, give or take). Of these many universes, some collapse after a fraction of a second, others have different physical laws not friendly to intelligent life, and yet others may hold other forms of matter not yet imagined.

According to this view, our universe– completely by chance– is “just right”, with physical laws balanced enough to allow the formation of complex material systems like atoms and molecules, stars and galaxies. And intelligent life, like us.

The anthropic principle is not a used to predict any aspect of our universe.  As a stand-alone idea, it’s a dead end.  Indeed, Steven Hawking called the anthropic principle “a counsel of despair”.   But it may simply be a natural consequence of the concept of the “multiverse”, that we happen to live in a habitable universe that allows intelligent life to form.  If our universe was otherwise, we wouldn’t be here to observe it.  But the idea that our universe takes its properties by chance, and not from any fundamental physical principle, is a depressing thought for many physicists who hope for a more satisfactory and elegant explanation of physical law.

The idea of the multiverse, that our universe is just one of countless others, is largely speculation, of course. But it puts the petty toils of everyday life into perspective. Because the fact that you, I, and all humanity are here at all to observe the universe, to look up into the night sky or enjoy a silent snowfall or the first blossoms of spring may simply be amazingly good luck.  We won the “multiverse lottery”, where our elementary particles get to inhabit an orderly and long-lived universe conducive to intelligent life.  This is surely a state of affairs to be celebrated and embraced, is it not?

As a giant cloud of gas and dust coalesces in the arms of a spiral galaxy, tiny clumps in the cloud collapse and ignite into groups of hundreds to thousands of young stars. The hottest of these stars light up the remaining gas and dust, resulting in glowing gas clouds like the Carina and Orion nebulae.

After several million years, the hottest stars of the new cluster blow off the remaining gas and dust. What remains is a group of dazzling new stars loosely held together by gravity.

Over many tens of millions of years, as the cluster revolves around the galaxy, it encounters other stars and dust clouds that disrupt the cluster and eject its members into the spiral arms of the galaxy. There, they continue to revolve about the galactic center by themselves or in loose “stellar associations”. Some of the stars of Ursa Major are part of an association and were once members of an open cluster.

Open star clusters are only found nears the arms of spiral and irregular galaxies, where there is abundant gas and dust. Elliptical galaxies no longer have enough gas and dust to sustain the creation of new stars.

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The Double Cluster in Perseus

The greatest concentration of open clusters in our skies lies along the Milky Way in Cygnus, Scutum, Scorpius, and Sagittarius. For that reason, open clusters are sometimes called “galactic clusters”. There are nearly 1000 known open clusters in our skies, and likely 10,000 more hidden behind the disk of our galaxy.

Most open clusters stay together for a few tens of millions of years. The famous Pleiades, easily visible in the northern winter sky, is about 100 million years old. The double cluster is about 5 million years old. Some are massive enough to hold together much longer; the cluster NGC 188 is an amazing 5 billion years of age.

Since all the stars in a cluster are about the same distance from us, their relative brightness is proportional to their true brightness, which in turn is proportional to their mass and chemical composition. So open clusters makes it possible for astronomers to learn more about the evolution of stars.

First, a bit of perspective.

Just before 1940, astronomers applied the newly-discovered principles of nuclear physics to determine the secret of how stars shine. This work led to a flurry of research over the 1940′s and early 1950′s about how stars are born, how they evolve as they burn through their fuel, and how they might come to an end.

One thing became clear from this work: the life span of a star depends almost entirely on its initial mass. Heavier stars burn much hotter, bluer, and faster than lighter stars. And after a few hundred million years, not long for a star, the big blue stars stop burning fuel and eventually wink out as dim white dwarfs or explode as supernovae and disappear forever. Yellow and red stars, which are much lighter and cooler, last for billions of years. So when clusters get old, that’s the only kind of stars that should remain.

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Blue stragglers in the ancient open cluster NGC 188 (Credit: K. Garmany, F. Haas NOAO/AURA)

But in 1953, the great astronomer Allan Sandage discovered hot blue stars in the ancient globular star cluster M3, a cluster we now know to be at least 10 billion years old. Such stars have since been seen in many other globulars, as well as in ancient open clusters like NGC 188. Because they seem to have stayed around much longer than theories predict, these stars got the name “blue stragglers”.

Did these blue stars simply form much later than other stars of the cluster? Astronomers think not. Nor do they believe these are simply foreground stars that do not belong to the cluster

Perhaps a blue straggler is formed when two old, smaller red stars collide or closely interact with each other to create a single higher mass star that shines blue? Possibly. This might explain why there are more stragglers in the dense core of a cluster, where collisions are more likely. But the chances of two lone stars colliding are too small to explain the full population of these stragglers.

Another likely explanation, as it turns out, is that material transfers from one star to another in a close binary star system. The bigger star in the system evolves into a red giant, swells, and loses its outer layers to a smaller red companion star which absorbs the extra mass, squeezes itself tighter, and burns hotter and brighter as a blue star. If this theory is true, many blue stragglers should have small a tiny white dwarf companion, which is all that would remain of the red giant “donor” star. Studies are underway with the Hubble Space Telescope to test this theory.

You won’t see blue stragglers in your backyard telescope. But enjoy images of them from the big scopes, and keep in mind the unexpected dynamical processes that created them.

Such distances within our galaxy are still a little staggering. Now let’s scale up to understand the size of the universe in terms of a big galaxy. On these terms, the size of the universe becomes much easier to understand…

Imagine the Milky Way and our neighbouring Andromeda galaxy were the size of dinner plates at opposite ends of a table 20 feet long. On this scale, the centre of the nearest major cluster of galaxies, the Virgo Cluster, is about 500 feet away, just a few city blocks over. And the 10,000 galaxies of the Virgo Supercluster, of which the Milky Way is an outlying member, comfortably fill a football stadium.

The Coma Wall of Galaxies lies about 1500 feet from the “plate” of the Milky Way, and the the most distant object visible with a backyard telescope, the quasar 3C273, is about 3.5 miles away. The entire universe on this scale has a radius of 20 miles, about the size of the city of Chicago.

On the scale of galaxies, as you can see, the universe becomes surprisingly intimate.

(Hat tip to the great Timothy Ferris for this analogy, which appears in a similar form in his book Seeing in the Dark).

But there’s a happy numerical coincidence that makes it much easier to contemplate such distances.

Here’s how it works…

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A light year, the distance light travels in one year, works out to be about 5.88 trillion miles. An astronomical unit, the distance from the Earth to the Sun, is about 93 million miles, a much smaller distance. A quick calculation shows there’s about 63,200 astronomical units (AU) in a light year.

So far so good.

Now it turns out, completely by chance, there’s also about 63,200 inches in a mile (I’ll let you do the math). So an inch to a mile is the same as an AU to a light year.

Using this compressed scale, where one inch is the Earth-Sun distance, you can get a better feel for the stupendous size of our galaxy…

If the Sun was a grain of sand, and the Earth a microscopic speck one inch away, then Jupiter would lie 5.2 inches away and Pluto an average of 40 inches away. Next stop… the nearest star, about 4.3 miles away, with mostly empty space between it and the Sun. The star Vega would be 26 miles away, the Orion Nebula 1340 miles away, and the globular cluster M15 some 25,000 miles distant (about three times the diameter of the Earth).

And even on this massively compressed scale, the diameter of the Milky Way Galaxy itself would be about 100,000 miles.

This little trick of scale helps you wrap your mind around relative distances and appreciate the immense scale of a big galaxy like our Milky Way, where even a light year doesn’t count for much…

Uranus was discovered by the great William Herschel in 1781, an accomplishment that cemented his reputation as the greatest amateur astronomer of his day and launched his professional career with the patronage of England’s King George III, himself a dedicated amateur astronomer.

Politics dominated the debate over the name of the new planet.  Possible names included “George’s Planet” and “George’s Star”, after the English king.  But the neutral name of Uranus was chosen by Johann Bode in honor of the Greek god of the sky.  Uranus remains the only major planet not directly named after a Roman god.

As an ice giant, Uranus, along with Neptune, consists mostly of icy water, ammonia and methane, unlike the larger “gas giants” Jupiter and Saturn which are mostly hydrogen and helium.  The blue green color of these ice giants comes from the thin layer of hydrogen and helium floating above the heavier ices, and there are scant lighter cloud bands of methane and trace hydrocarbons.

With just 1/400th the sunlight of Earth, Uranus is a dim, cold world.  Its cloud-top temperature of just 50 degrees Kelvin (-223 Celsius) is the coldest of all major planets– Neptune generates more internal heat than Uranus.

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The five largest moons of Uranus

Another odd feature… Uranus rotates on its side.  Its otational axis is inclined 98 degrees to its orbital plane, which suggests it received a mighty thump in the early days of the solar system that knocked the planet on it side.  As a result, the northern and southern hemispheres of the planet remain in darkness for decades at a time. The thin, coal-black rings of the planet likely formed after the planet was knocked over.  Same with the little clutch of small moons of Uranus, the largest of which are named after the Shakespearean characters Titania, Oberon, Miranda, Umbriel, and Ariel.

At magnitude 5.8, Uranus is visible, barely, without a telescope.  But the planet did not make a sufficient impression upon ancient stargazers to be noticed as a planet.  Even in the early telescopic age, the astronomers John Flamsteed and Pierre Lemonnier observed Uranus on dozens of occasions before Herschel’s discovery.

Grote Reber was a ham radio operator, studied radio engineering, and worked for several radio manufacturers in Chicago from 1933 to 1947.

Reber learned of Jansky’s discovery of “cosmic radio waves” in the newspapers. He was captivated by the news, and applied to work with Jansky at Bell Labs in Holdmel, New Jersey. But the Great Depression prevented Bell from hiring new staff.

But Reber didn’t give up.  He built his own radio telescope, which was the world’s first, at his own expense while working full time as a radio engineer.

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Grote Reber’s Backyard Radio Telescope… and The World’s First!

He made the reflector from sheet metal 30 feet in diameter. Just like an optical telescope, Reber’s construction used a parabolic mirror to focus all wavelengths to a single point. There, 20 feet above the dish, he mounted his radio receiver to amplify the faint cosmic signals by million of times, making them strong enough to record on a strip chart.

From his backyard radio telescope, Reber confirmed Jansky’s discovery of radio waves from the Milky Way, and found radio emission from the sun, and mysterious radio sources in Cassiopeia and Cygnus.

From 1938 to 1943, Reber made the first surveys of radio waves from the sky and published his results widely. His work ensured radio astronomy became a major field of research following World War II.

When other researchers began covering more and more of the radio spectrum, Reber turned to longer-wavelength radio waves in the 1950′s. Such signals penetrate the Earth’s atmosphere in only a few places, one of which is Tasmania. Reber spent his last years there, and died in 2002 at the age of 90.

Reber’s work was the end of golden age of astronomy, when an accomplished amateur could make groundbreaking discoveries. This often happens in a new and young science, where professionals, by definition, don’t yet exist.

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To search for static that might interfere with radio transmission, Jansky built an antenna to detect radio waves at a frequency of 20.5 MHz (a wavelength of 14.5 meters). He mounted the rotatable antenna on four Ford Model-T tires to determine the direction of any radio signal he might find. The antenna was jokingly called “Jansky’s merry-go-round” (see image above).

After recording signals for several months, Jansky found static from nearby and distant thunderstorms.

But he also found a faint steady radio hiss of unknown origin. The intensity of the hiss rose and fell once a day. At first, he thought the unknown static might be radio waves from the Sun.

After a few months of following the signal, the brightest point moved away from the Sun. The signal repeated not every 24 hours, but every 23 hours and 56 minutes, the same period in which the stars rise and set.

Jansky eventually figured out the radiation was strongest in the direction of the center of our Milky Way galaxy, in the constellation of Sagittarius. The discovery of radio waves from the center of the galaxy was widely publicized, appearing in the New York Times on May 5, 1933.

Though he wanted to learn more about the radio waves, Jansky was assigned to another project by Bell Labs managers and did not pursue the subject further.

Many scientists were fascinated by Jansky’s discovery. But no one followed up on it for several years until a modest radio engineer from Chicago became the world’s first true radio astronomer.  We continue that story next week…

Footnote: In honor of Karl Jansky, the unit used by radio astronomers for the strength (or flux density) of radio sources is the Jansky.

NASA and the European Space Agency released today a stunning image of NGC 5128.  It shows dark clouds of dust and gas flecked by pink star-forming nebulae, all imaged in visible, ultraviolet, and near-infrared light.  According to NASA, it looks like “looming rain clouds on a stormy day”.  But these clouds are thousands of light years across.

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Here’s the image:

A close-up view of the galaxy Centaurus A (NGC 5128)

This galaxy also has huge jets of matter shooting out a million light years into space, probably caused by a supermassive black hole at the centre.  Radio astronomers call the galaxy Centaurus A.  It’s one of the brightest sources of radio waves in the heavens.

NGC 5128 is the 5th brightest galaxy in our sky, easily visible with binoculars as a fuzzy star just 4 degrees north of omega Centauri. Get out and see it yourself if you can.  It’s visible in the constellation Centaurus, although you have to be south of 30 N latitude.  You can see the galaxy’s massive dust lane in a small telescope.  Although you won’t see it like this!

Position of galaxy NGC 5128 in Centaurus (click to enlarge)

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