Dark nebula Barnard 68 (from European Southern Observatory)

Dark nebulae appear dark because of tiny ice-covered dust grains that scatter background star light.  The grains are cold, just 10K (or -263 C), and less than 1/1000 of a millimeter across.  Within the cloud there may be just 100 dust grains per cubic centimeter, still nearly a vacuum by earthly standards.  But these clouds are tens of light years thick, so light from background stars is slowly but surely blocked and scattered in all directions.

Dark nebulae are a little tricky to see because you’re looking for, well… nothing… an absence of stars.  But in time, you learn to see these clouds as they protrude irregularly into the starry background.  The regions from Scorpius through Crux, and the band of Milky Way that cuts through the constellation Cygnus hold dozens of dark nebulae.  To get the best view, slowly sweep these parts of the sky with binoculars and linger on each patch for a time until your eyes and brain learn to look at voids in the starry background..  Dark sky is essential.

A warning… as you see dark nebulae, you may at first find the lack of stars quite unsettling.  In his 1882 novel Two on a Tower, Thomas Hardy called dark nebulae “deep wells for the human mind to let itself down into”.

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The Collinder list was developed by Per Collinder, a Swedish graduate student in astronomy whose 1931 doctoral work involved the study of the structure and distribution of open star clusters in the Milky Way. An early version of his list appeared in his Ph.D. thesis, and eventually worked its way into mid-20th-century star maps like those in Norton’s Star Atlas.

Hundreds of these Collinder clusters are worth a look through binoculars or a telescope. You’ve likely seen quite a few of them without knowing it, including…

* The Double Cluster in Perseus (Cr24 and Cr25)

* The “Coathanger Cluster” (pictured above), also called Brocchi’s cluster (Cr399)

* The Ursa Major Moving Group (Cr285), which includes most of the stars of the Big Dipper

* The Hyades (Cr50) in Taurus

* The Beehive or Praesepe (Cr189), also known as M44.

* Orion’s Belt (Cr70)

Most of the objects in Collinder’s list are true star clusters. Though some, like the Coathanger, are simply asterisms, chance alignments of unrelated stars. And a few like Collinder 409 (a.k.a M71) are loosely-packed globular clusters. It wasn’t Collinder’s intent to map new star clusters, so the majority of the Collinder clusters also appear in the New General Catalog or Messier catalog.

We’ll point you towards some of the most appealing Collinder objects over the coming months, starting with a few pretty sights in the southern region of the constellation Canis Major, the Big Dog. And if you’re looking for things to see, check out some of these “Cr” objects for yourself with the help of a good star atlas. They make for good stargazing for those who tire of the same old Messier objects.

The video is not too hard to follow. But it runs about 15 minutes. So if you’re busy, just dip into it a bit at a time. By the end, you’ll have a good idea about how studying how spectroscopy may reveal insights into the chemical composition of stars, planets, and perhaps, indirectly, extraterrestrial civilizations.

Thanks to subscriber Suzane for pointing out this and many other astro-related videos. You can see links to more videos here…

With the stars, of course, it’s a little different.  They’re a lot further away.  So closing one eye and then other won’t cut it: you need a much bigger side-to-side displacement.  In fact, astronomers use the baseline between the Earth and the Sun… a distance of some 300 million kilometers… to detect the parallax of nearby stars.

The image below shows the basic idea.  When the Earth is on one side of the Sun, say, in July, a nearby star appears in a slightly different place relative to the background stars than it does when the Earth is one the other side of the Sun in January.  The tiny angular change, along with the known distance from the Earth to the Sun and a little basic trigonometry, directly gives the distance to the star.

But even with this large baseline, the apparent movement, or parallax, of a nearby star relative to more distant stars is much smaller and harder to measure.

These little angular shifts are extremely small, even for nearby stars.  The nearest star, Proxima Centauri shows a parallax of 0.77 arc-seconds (an arc-second is 1/3600 of a degree).  That’s about the angular size of a penny at a distance of 5 km.  Because of the difficulty in measuring such small angles, astronomers can only measure a parallax of about 0.01 arc-seconds.  That’s enough to accurately measure the distance to stars out to a few hundred light years… a tiny distance compared to the 50,000 light-year radius of our galaxy.

While ancient astronomers understood the concept of parallax, no one was able to detect these tiny angular shifts.  Some, including the great Tycho Brahe in the late 16th century, believed the lack of parallax was evidence the Earth was fixed in space and was likely the center of motion of the heavens.  The first measurement of parallax was made by Frederick Bessel in 1838.  He saw the shift of the star 61 Cygni (shown at the top of this article).

Space satellites can measure stellar parallax quite accurately.  The amazingly productive Hipparcos satellite, launched by the European Space Agency in 1989, was designed to make much more accurate measurements of stellar parallax than Earth-bound telescopes.  Hipparcos measured the parallax of more than 120,000 stars out to a distance of 1,600 light years.  The upcoming Gaia satellite will do even better, directly measuring the distance to stars tens of thousands of light years away.

By the way, if a star was at a distance which produced a parallax of 1 arc-second as seen from Earth, it would have a distance of 1 parsec.  A parsec, which works out to about 3.3 light years, is the unit of distance used by most professional astronomers.  Light-years are for us amateurs.

That’s it for today.

The composition of stars is one example.  In 1835, the French scientist Auguste Comte declared the composition of stars was an example of knowledge forever beyond human understanding.  Just a few years after Comte’s death, 19th century astronomers carefully measured starlight with prisms and spectroscopes and discovered that stars are made of the same material found on Earth… hydrogen and carbon and oxygen, among other elements.

The distance to the stars and nearby galaxies is another example of the “unknowable” becoming known to determined and patient observers.  Until the mid-19th century, no one knew the distance to the stars, and no one knew for sure if anyone would ever know how to find the distance to the stars.  Then, careful measurement of the parallax of a few nearby stars as the Earth moved around the Sun revealed the distance to a few closeby stars.  But the true scale of our galaxy was a complete mystery.  No one knew for sure whether the Milky Way was all there was to the universe, and whether it was a hundred light years across, or a thousand, or a trillion.

This changed in 1912.  That’s when an obscure and underpaid astronomer named Henrietta Leavitt discovered a particular type of bright variable star pulsated with a period directly proportional to its true brightness.  She studied these variable stars in the Large Magellanic Cloud, and noticed that brighter stars had longer pulsation periods.  Since all these stars were roughly the same distance from Earth, she was able to use the period of these variable stars to determine their true brightness.  And their true brightness could be compared to their apparent brightness to figure out the true distance to these stars… and the other star clusters and galaxies to which these stars belonged.

This was a revolution.  Edwin Hubble took Leavitt’s discovery to measure these variable stars in the Andromeda “Nebula” and determined it was not a nebula at all, but a galaxy in its own right lying more than 2 million light years away, some 20 times the span of our Milky Way.  This discovery exploded the size the known universe, and was one of the most stunning and famous scientific discoveries of the past 100 years.

For her effort, Henrietta Leavitt was paid just $10.50 a week.  She died in obscurity in 1921 at the age of 53, nearly forgotten.  Though to his credit, Hubble often said she deserved the Nobel Prize for her work.

Henrietta Leavitt

The stars discovered by Leavitt are called “Cepheid variables”, after the prototype star delta Cephei in the constellation Cepheus, not far from the star clusters we looked at in our last article.  There are more than 700 known Cepheid variables in our galaxy, and thousands more in most galaxies visible out to a distance of 100 million light years.

You can see a few Cepheid variables yourself.  Bright stars such as eta Aquilae and Polaris, the north star, are Cepheid variables, not to mention delta Cephei itself (see below).  These three stars are ideal targets for even the most casual stargazer armed with a modest pair of binoculars.  If you’re keen, you can track for yourself the change in brightness of some of these stars.

The location of delta (δ) Cephei (upper left), the namesake of Cepheid variable stars (click to enlarge)

The inspiring but melancholy story of Henrietta Leavitt is told well by George Johnson in his short book, “Miss Leavitt’s Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe”.  It’s a delightful read.

As a writer, Clarke was no Hemingway.  But the strength of his writing came not from his elegant style or complex character development, but from his thought-provoking ideas, most of which were based on scientific fact.

Perhaps his most unexpected plot twist came in his sequel to 2001, called 2010.  Towards the end of the novel, the mysterious black monoliths of an advanced and meddlesome race began infesting and multiplying in the atmosphere of Jupiter, adding mass to the big planet in a matter of days.  As its mass grew, the planet shrunk, then finally collapsed and ignited as a new star (I won’t tell you why this happened.  You can read the book or see the movie if you’re interested; as usual, the book is better).

Here’s a view from the movie of 2010 that shows the collapse of Jupiter into a star.  The key moment happens at 3:15 into this clip.

Clarke’s idea is almost entirely accurate.  As it happens, Jupiter is not just the largest planet by size in the solar system.  Given its composition, structure, and mass, it’s as large as it can possibly be.

If Jupiter was less massive, it would be smaller like Saturn or Uranus.  But if you could add 2x, 5x, 10x or more mass to Jupiter, perhaps with the help of Clarke’s magic monoliths, the planet would not grow larger.  It would shrink.  (Don’t you wish your waistline worked like that).

As it is now, the dense core of Jupiter generates heat which radiates into space.  If the planet’s mass increased by 50x to 60x, the core would get hotter and denser until the planet turned into a “brown dwarf”, a type of failed star.  At that mass, the planet would start to grow again back to its present size.  In fact, all brown dwarfs, whether 10x or 60x Jupiter’s mass, all have roughly the same diameter as Jupiter.

If Jupiter grew to 75x its current mass, its core would get hot enough to fuse hydrogen into helium in its core.  It would become small main sequence red dwarf star, and burn steadily for billions of years.  Though even then, its diameter as a red dwarf star would only be 30% larger than the planet’s current diameter.

By comparison, our sun, which is bigger than most stars, has a diameter 10x that of Jupiter, but a mass 1000x as great.

You’ll find Jupiter this month in the constellation Capricorn, hovering fat and bright in the southern sky at an impressive magnitude -2.6.  It’s the brightest object in the southern night sky, save the moon.  Aside from Jupiter, Capricorn has quite a few sights for binoculars and small telescopes, including a lovely double star just west of the biggest planet (see page 59-60 of Stargazing for Beginners for more about what to see in this part of the sky this time of year).

Next time, we’ll give you a few tips on how to observe  Jupiter.  No monoliths required.

And some of you asked if it’s possible to photograph zodiacal light. In fact, this sight is not too hard to photograph with a simple camera and lens, although taking good astrophotos is a little different than daytime shots. If you’re interested in basic astrophotography with a digital camera, here’s a resource to help you get up to speed quickly…

Now, to today’s business… this one is a little longer than usual, but I think you’ll find it interesting…

Images of the sun from a space satellite, showing the increase and decrease in activity from 1996 to 2006

In our last article on sunspots, we mentioned the strange period from 1645-1715 when sunspots seemed to disappear. Called the Maunder Minimum, this period coincided with the “Little Ice Age”, when Northern Europe and other parts of the world were plunged into a long period of cool summers and long winters when crop yields fell and rivers, harbors, and canals froze.

This quiet period of solar activity was closely followed by another, called the Dalton Minimum, from 1795-1825. It also matches up well with a period of cooler climate, though the eruption of the Tambora volcano in 1816 made some contribution as well.

The sunspot cycles since 1600, showing the Maunder and Dalton minima

So you may wonder… do periods of little sunspot activity lead to cooler climate on Earth? And do periods of increased sunspot activity, such as occurred from 1900-1950 account for periods of higher temperatures on Earth? Can sunspots explain the rise in temperature during the 20th century, perhaps, rather than greenhouse gases produced by human activity?

In fact, the sun does get hotter when there are more sunspots. Because although the spots are cooler, they’re accompanied by hotter, brighter patches called faculae that cause the overall brightness of the sun to increase by 0.1% at visible wavelengths, and more at ultraviolet wavelengths.

Such increases in solar brightness are included in climate models. It seems the 11-year sunspot cycle as well as the increase in solar activity earlier in the 20th century lead to an increase in average global temperature of 0.1 to 0.2 Celsius… which is only about 20% of the observed increase of 0.5 to1.0 degree.

So… case closed, right? It is greenhouse gases, and not solar activity, that are the main cause of climate changes this past century?

Well, not so fast. Because when sunspot numbers rise and fall, there’s more going on than simply changes in solar brightness. Periods of reduced sunspot activity correspond to periods of reduced magnetic activity on the sun, and reduced outflows of charges particles from the sun (the so-called solar wind). The solar wind whizzes past the Earth and deflects cosmic rays from deep space from hitting our atmosphere.

A recent proposal from Danish scientists suggest that when cosmic rays strike our atmosphere, they create tiny aerosol particles that lead to increased cloud formation and less sunlight hitting the Earth. So it’s a double whammy… fewer sunspots mean a dimmer sun, which also means more cosmic rays into the atmosphere and more cloud cover which further cools the Earth. And vice-versa when there is more solar activity.

Another recent theory suggests increased UV light from the sun drives energy flow from the upper to lower atmosphere by disrupting a layer of ozone high in the atmosphere. How this affects climate is unclear.

As it turns out (as far as we know), computer models of the climate do not take these indirect effects of solar activity into account when calculating the change in global climate. And while human activity counts for only 5% of carbon dioxide emitted into the atmosphere each year, the sun accounts for ALL the energy striking the Earth and driving its dynamic and enormously complex ocean currents and atmosphere.

So you see, despite what you hear in the media, there is still much uncertainty about how the Earth’s climate really operates and changes over time, and how changes in solar activity drive climate change. Healthy and open skepticism, as always, is appropriate.

And remember… the Earth is so complex that even the best computer model in the world can’t tell you with any certainty whatsoever whether you’ll need an umbrella when you head out the door to go the office a week from today.

But there is another type of supernova with a completely different physical cause.  This is the Type Ia supernovae, which also turns out to be indispensable to astronomers for measuring the size of the universe.

Like their Type II counterparts, Type Ia supernovae show up in our galaxy and in others.  Like Type II, they are immensely bright… about 5 billion times as bright as the Sun.  Many Type Ia supernovae are discovered each year, some by amateur astronomers.

Type II supernova occur in a single massive star that’s collapsed after running out of fuel in its core.  But Type Ia supernovae occur in a double or multiple-star system.  One star in the system is a white dwarf, which pulls gas from its companion main sequence or red giant star onto its surface.   When the white dwarf collects enough new mass to approach the Chandresekhar limit of 1.4 solar masses (see White Dwarfs) the star collapses and explodes as a supernova.  Ka-boom!

Supernova 1994D in galaxy NGC 4526

A more recent view suggests the white dwarf doesn’t collapse, but gets hot enough to burn all its carbon in an immense runaway nuclear fusion reaction that we see as a supernova explosion.

But here’s the important thing… all white dwarfs collapse or explode at about 1.4 solar masses, which means they make the same kind of explosion with the same intrinsic brightness.  And if all Type Ia supernova have roughly the same intrinsic brightness, they serve as excellent distance markers to the galaxies in which they occur.  The fainter a Type Ia appears, the farther away it must be.  In this way, astronomers can measure the distances to far away galaxies for which there is no other practical distance marker.

How can astronomers tell a Type Ia from a Type II?  Type Ia supernova have an optical spectrum that shows almost no hydrogen in the explosion… this is very different from Type II blasts, which contain a lot of leftover hydrogen from the exploded star.  Type Ia supernova also have a distinct “light curve, a way of brightening and fading over time.

So it might be worth knowing a little more about these spectacular explosions…

As you learned before, when a big star has burned through its stock of hydrogen, helium, carbon, etc., it’s eventually left with iron and nickel in the core.  But iron and nickel can’t fuse together to release any more energy.  So the game is finally up… and when burning stops, there is no more energy to hold the star up against its own gravity, and the star’s hot core suddenly collapses into a neutron star or a black hole.  This releases a huge amount of gravitational energy.

At the same time, the gas in the star’s outer layers collapse, hits the dense core and bounces back outwards at high speed, releasing the energy of the supernovae.  The expanding shell of gas that bounces creates a shock wave in the interstellar medium that emits visible light; many such remnants of supernovae exist in our galaxy.  You can see one tonight in the constellation Cygnus.  It’s called the Veil Nebula.

The Veil Nebula in Cygnus, created by a supernova some 18,000 years ago.

During this violent physical event, there are particles and energy flying everywhere… atoms, gamma rays, neutrons, everything.  Many neutrons get captured by atomic nuclei in what’s called the r-process.  This is how most of the elements heavier than oxygen get made, right up to and including uranium.  The silicon in the chips of your computer and the iron in your blood were likely made in a supernova explosion.

The most recent nearby supernova was SN1987A in the Large Magellanic Cloud  (I confess that when I was an astronomy graduate student, I was at the observatory in Chile where this supernova was discovered, but I missed seeing it by 4 days; I used to catch rides to the telescope with the guy who discovered it).  Other famous supernovae include the supernova of 1054, which created the Crab Nebula you can see in Taurus, and Tycho’s star that blew in 1572 leaving a strong source of radio emission.

And there will be more supernovae in our skies.  Betelgeuse, Spica, Antares, and Eta Carina are well-known stars likely to end as supernovae in the next few thousand to few million years.  When they go, they’ll be the brightest object in our sky except for the Sun and Moon.  These exploding stars will be easily visible during the day and will cast shadows at night.  It will be quite a show.

What we’ve just described is called Type II supernova, the type caused by the sudden collapse of a massive star.  There is another type of supernova that’s immensely useful for for figuring out the distance scale of the universe.  But that’s for another issue…

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The Basics

• To review: for mid-sized star like our sun, the end comes after helium burns by nuclear fusion into carbon in the core.  When the helium runs out, the star can’t compress itself any further to get hot enough to burn carbon into heavier elements.  The core settles down to become a carbon-rich white dwarf… a large glowing diamond in space.

• But for larger stars, the core gets hot enough for carbon to fuse itself in more complex reactions into heavier elements like oxygen, neon, and magnesium.  The star expands into a red supergiant… it gets cooler and redder, but not much brighter than it already was.  So it moves to the right in the HR diagram.

• What happens next depends very much on the star’s mass.  If the core is not too large, the neon or magnesium becomes dense and holds itself up with the pressure of electrons.  When it gets squashed by the surrounding layers of star, the core ignites it does so violently, like the “helium flash” we mentioned in the last article.  But an oxygen or neon flash is catastrophic… it blows the star apart, and may leave behind a white dwarf made of oxygen, magnesium, or neon.

The “onion-like” layers of nuclear fusion of light elements into heavy elements in a massive star

A Deeper Look

• For more massive stars (say more than 5 solar masses), the core is so hot that it never gets dense enough to flash and blow itself apart.  Every time light elements run out, heavier elements ignite and hold the star up.  So neon, magnesium, silicon, and other elements are created in the core.  In some cases, heavier elements burn in the center, and lighter elements burn in shells around the core, like layers of an onion (see the above drawing).

• This complex nuclear dance comes to an end when lighter elements fuse into iron and nickel, because these elements cannot burn into heavier elements.  At this point, the game is up: there’s no more energy to hold up the star.  The core collapses to become a neutron star or black hole, and the outer layers are violently ejected in a supernova explosion.

• How massive does a star have to be to burn all the way to iron in the core and die as a supernova?  It all depends on mass loss.  The outer layers of a star of are ejected into space in mechanisms that aren’t well understood.  So a star 5x the mass of the sun may eject enough material to end up as a white dwarf rather than blow up as a supernova.  These details occupy professional astronomers today.

Good To Know

The fusion of light elements into heavier elements in the core of the star is a key source of such elements in the galaxy.  Some of the oxygen you are breathing right now, and most of the iron in your blood, were created in the cores of heavy stars that blew up as supernovae.