Dark and Diffuse Nebulae

If you step out on a dark night (in clear conditions away from light pollution), as you gaze along the arc of the Milky Way, you will see small patches of diffuse light and other patches where there appear to be few stars. These are diffuse nebulae and dark nebulae, respectively.  These patches are made of gas and dust, and the are places where new stars form.

Dark Nebulae

Many of these patches are so-called dark nebulae where cool gas and interstellar dust block the view of the background stars. These nebulae are laced across the spiral arms of the Milky Way in irregular patches and rivulets without definite shape or boundaries.

Dark nebulae are part of cool, giant molecular clouds where dust and gas from old stars and gas from the earliest days of the universe slowly pull themselves together by gravity. The icy dust grains are less than 1/1,000 of a millimeter across, but they have an interesting chemistry, consisting of frozen nitrogen, carbon monoxide, ammonia, formaldehyde, and more complex organic molecules (even ethyl alcohol).

Dark Nebula in the foreground of stars along the Milky Way

Because dark nebulae are the birthplace of stars and planets, astronomers find them intensely interesting. Computer modeling shows that although the nebulae are tenuous, with only a few particles per cubic centimeter, passing stars push and pull on the particles, causing them to coalesce into denser patches that begin to fall in on themselves and heat up. Hundreds of tiny globules in a dark nebula may eventually become hot enough to start the process of nuclear fusion, where hydrogen in the center of the globule begins to burn into helium, releasing huge amounts of energy. When this happens, dense globules of gas and dust turn into clusters of new stars which light up the remaining dust and gas into what astronomers call diffuse nebulae.

Diffuse Nebulae

After a dark cloud of gas and dust collapses into dense globules that ignite into stars, the leftover material is set aglow by the intense blue and ultraviolet light from newly formed stars. The glowing hydrogen gas surrounding the stars is called an “emission nebula”.

These nebulae usually glow a reddish-pink color. That’s because the new stars excite the atoms of hydrogen gas that remain in the cloud, and the atoms relax again by emitting red light at 656 nm, a wavelength set by the structure of the hydrogen atom. Emission nebulae also have traces of ionized oxygen which also emit light at a characteristic wavelength near 500 nm (blue-green).  In a way, emission nebulae are much like the neon lights you see on buildings and billboards. The lights use electricity to make gases glow, while an emission nebula gets its energy from the light of new stars embedded within.

These nebulae also contain a fair bit of dust that reflects the blue light of the new stars. The reflective dust is called a “reflection nebula”; in many cases, the two nebulae occur in the same area of star formation (see the image of the Trifid Nebula, below).

Both emission nebulae and reflection nebulae are sometimes called diffuse nebulae.

 

The Trifid Nebula in the constellation Sagittarius. Red light comes from hydrogen gas atoms excited by the light from stars within the nebula; blue light from these stars is re-flected off fine dust particles back into our line of sight

Even a random search with a small telescope along the plane of the Milky Way reveals many diffuse nebulae which look like hazy patches of silver-white light. The sword of the constellation Orion contains one of the brightest and most famous such nebulae.  It’s often just called the Orion Nebula.  A telescope gives you an astoundingly beautiful view of the Orion Nebula; no amount of observation is enough to reveal all its detail. You can see many more such nebulae such as the Swan, Lagoon, Trifid, and eta Carina nebulae with a small telescope or pair of binoculars. Just remember, you won’t see color when you observe such nebulae visually; there isn’t enough light to stimulate the color-sensing cells in your eye. But in dark sky, these objects are still quite striking.

Diffuse nebulae don’t last long, at least on astronomical time scales. After a few million years, the hot young stars burn off the remaining gas and dust, leaving a small loose cluster of gravitationally-associated stars.

Open Star Clusters

After the hot stars near the center of an emission nebula push away the remaining gas and dust, a group of a few dozen to a few hundred young stars remain clustered together. These groups are called open star clusters, and they are often found along the Milky Way. They are beautiful to observe, especially in dark sky where they look like dazzling jewels set against the black velvet of deep space.

An open star cluster

Open star clusters are found mostly near the arms of spiral and irregular galaxies where there is abundant gas and dust for new star formation.  For that reason, they’re sometimes called “galactic star clusters”. The greatest concentration of open clusters in our sky lies along the band of the Milky Way in the constellations Cygnus, Scutum, Scorpius, Sagittarius, Crux, Centaurus, Cassiopeia, and Perseus.

There are nearly 1,000 known open clusters in our skies, and likely 10,000 more hidden behind the dusty disk of our galaxy. Some famous open clusters include the Pleiades (M45) and Hyades in Taurus, the Beehive (M44) in Cancer, and the Double Cluster in the constellation Perseus.

Since all the stars in an open cluster are about the same distance from us, their relative brightness is proportional to their true brightness, which in turn depends on their mass and chemical composition. The stars also form at about the same time. So open clusters are like laboratories that help astronomers learn more about the evolution of stars.

Over many tens of millions of years, as an open 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 associations of stars. Some of the stars of Ursa Major are part of an association and were once members of an open cluster. The Sun was likely once a member of an open cluster whose members have long scattered into the plane of the Milky Way galaxy.

Globular Clusters: Ancient Stellar Relics

Globular star clusters are different in size and nature from open star clusters. Where open clusters contain a few dozen to a few hundred new stars, globular clusters are each populated by hundreds of thousands of the oldest stars in the universe.

Globular clusters formed 12-13 billion years ago, not long after the universe began. They likely collapsed from clouds of gas too small to form a galaxy but too large to form an open star cluster. In a sense, globular clusters are a little like “micro-galaxies” left over from the formation of larger galaxies.

Our Milky Way retains some 180 globular clusters. In the early 1900’s, famed astronomer Harlow Shapley figured out most globular clusters in the Milky Way are found in a halo around the nucleus of our galaxy. He used the distance and position of globular clusters to determine the size of the Milky Way and the Sun’s position on its outskirts.

Because they’re so old, stars in globular clusters are quite different from younger stars in open clusters. Younger stars contain traces of heavier elements like calcium, silicon, iron, and carbon… what astronomers call “metals”.   But the stars in globulars formed long before such elements were formed in the innards of massive old stars.  The stars in globulars are called “metal poor” and consist almost entirely of hydrogen and helium.

Unlike open star clusters, globular clusters are strongly bound by gravity and are stable over time. Eventually, most of the stars in globular clusters will die off and fade from view. But as far as we know, globular clusters… even as they darken… will remain bound forever.

The globular cluster M5 as it might appear in a small telescope

You can see dozens of these spherical, tightly-bound clusters with binoculars or a small telescope. In the northern hemisphere, the brightest and prettiest globular clusters include the Great Cluster in Hercules, also known as M13, and the clusters M3 and M5 in the constellations Canes Venatici and Serpens, respectively.  The two brightest globular clusters in the sky, the Omega Centauri cluster and 47 Tucanae (in Centaurus and Tucana, respectively) are only visible south of the tropics.

As you learn to observe globular clusters, you might at first think they’re all the same: just fuzzy, grainy white balls in the eyepiece of your telescope. But as you look closer, you’ll see that each differs in shape and structure, as distinct as a human face.

Red Giants, Planetary Nebulae, and White Dwarfs

As you have discovered in the last sections, stars are born in emission nebulae like the Great Orion Nebula, grow up in open star clusters like the Pleiades, then disperse into the galaxy where they glow for tens of millions to tens of billions of years with energy created from atoms fusing in their hot cores. But much like living things, stars also have an end. Eventually, nearly all the hydrogen atoms in the core get fused into helium atoms. When the hydrogen in the core runs out, fusion burning slows and the core shrinks, heating up to over 100 million degrees. If it gets hot enough, helium starts fusing into heavier atoms like carbon and oxygen.  This new burning generates more energy and stops the core from contracting. The hot core pushes out the star’s outer layers. The star balloons in size by a hundred times or more and becomes a cool and luminous red giant.  The stars Arcturus, Aldebaran, and Gacrux in the constellations Bootes, Taurus, and Crux are examples of red giants (or orange giants… same idea) you can see with your unaided eye. Eventually, the helium runs out and the core shrinks again. But in small and mid-sized stars, the core does not get hot enough to burn carbon and oxygen, so fusion stops. Only a thin shell of helium around the core continues to burn for a short time. This hot shell drives the star’s outer layers into interstellar space where they escape forever. We see this glowing shell of ejected gas– heated and ionized by the star’s scorching-hot core– as a planetary nebula.

The Dumbbell Nebula in the constellation Vulpecula is a bright planetary nebula accessible in a small telescope. The faint star casting off the nebula is visible near the center of the narrow waist of the nebula.

The name “planetary nebula” came from William Herschel, the 18th-century astronomer who suggested these disk-like nebulae looked like planets, especially the blue-green disk of the planet Uranus. Herschel understood these objects weren’t planets, but he had no idea what they were at the time. A planetary nebula ejects hydrogen gas and trace amounts of heavier atoms like carbon, nitrogen, and oxygen into space. Some of these atoms may coalesce into dense clouds that form new stars and planets. In a way, this is how the galaxy recycles itself. In fact, some atoms of the lighter elements in your body may have been shed by a planetary nebula billions of years ago. At the center of a planetary nebula, the expose cored of the star glows with a temperature of 50-100,000 degrees. The core, which has stopped burning, will settle down as a dim, hot, Earth-sized lump of carbon and oxygen called a white dwarf.  These stars are very dim and hard to see, even with a small telescope. Most mid-sized stars in the galaxy will go on to become planetary nebula. So why can’t we see more of them in the sky? Because they don’t last very long. Computer models of stars show a planetary nebula lasts just 50,000 years, a tiny fraction of a star’s 1-10 billion life span.

Galaxies

All the stars, nebulae, and star clusters you see at night are part of a single galaxy: our Milky Way. But the Milky Way is just one of hundreds of billions of galaxies in the universe. The Hubble Space Telescope and large earth-based telescopes have mapped and catalogued galaxies out to a distance of some 12-13 billion light years. The light from these galaxies left just a couple of billion years after the formation of the universe.  Where stars are the constituents of galaxies, galaxies are the constituents of the universe.

Small telescopes don’t show the most distant galaxies, but hundreds of nearby galaxies lie within reach of a 3” or 4” scope.  A dozen or so can be seen with binoculars.  Though galaxies tend to be dim in small optics, a practiced eye can still detect a surprising amount of detail if the sky is dark and free of light pollution.

Face-on spiral galaxy Messier 101

Face-on spiral galaxy Messier 101 (image credit: Terry Hancock)

Galaxies come in a number of shapes and sizes. Our Milky Way is a slightly larger than average spiral galaxy, spread out in a flat plane of stars, with spiral arms winding like a pinwheel from a concentrated nucleus at the center. The spiral arms contain gas and dust out of which new stars are formed, which means diffuse nebulae, dark nebulae, and open star clusters also concentrate in the spiral arms. The image above of the spiral galaxy Messier 101 in Ursa Major shows reddish-pink emission nebulae and bright blue stars along the spiral arms. The Milky Way would look much like this if you could see it from high out of the plane of the spiral arms.

Seen from edge-on, a spiral galaxy reveals the dust lanes of its spiral arms and the bulge of its nucleus, which contains older yellow-red stars left over from the early days of the galaxy’s formation. The image of the galaxy NGC 891 below shows what the Milky Way might look like from the side. In fact, our Milky Way looks much like this in wide-angle photographs, since we see our galaxy edge-on from a point out near its edge, about 25,000 light years from the center.

Edge-on spiral galaxy NGC 891 in the constellation Andromeda

To form the spiral shape, a minimum mass seems to be necessary. Most spiral galaxies contain at least 50-100 billion stars and are roughly 100,000 light years across. Two nearby galaxies, the Andromeda galaxy and the Triangulum galaxy, are also spirals and are well within the reach of backyard telescopes and binoculars.

Elliptical galaxies have a much wider range of mass and size, and have a much less complex structure than spiral galaxies. A dwarf elliptical might have a few million stars, not much bigger than a globular cluster. A giant elliptical like M87 might have a trillion stars. Elliptical galaxies may have formed by gobbling up smaller elliptical and spiral galaxies after repeated gravitational encounters over billions of years. Because they lack the gas and dust of a spiral galaxy, elliptical galaxies don’t have much new-star formation going on.

The Magellanic Clouds, two nearby irregular galaxies visible only from the southern hemisphere

Irregular galaxies are neither spiral nor elliptical: they have an indeterminate shape. They are usually small galaxies, like the Magellanic Clouds, which are visible from the southern hemisphere, without the gravitational capacity to assume a regular form.  Or they may be a large galaxy like M82 in Ursa Major that’s been mangled by a major gravitational disturbance.