Leaving Uranus, we now move deeply into the outer Solar System and encounter that last gas giant planet, Neptune. With a gassy atmosphere full of methane, and (we believe) a deep ocean underneath, Neptune also has the swiftest winds of any planet in the Solar System...
Triton, largest of Neptune's eight known moons, is a place where you'll find terribly low temperatures and diverse landscapes. Here we see the so-called cantaloupe terrain.
We think Triton may look quite a bit like Pluto, the most remote planet in the Solar System, but we have never been there. Even the Hubble Space Telescope is incapable of showing us much of tiny Pluto's surface. It is that far away and that small, measuring smaller than seven planetary moons!
Not to be outdone, however, Pluto has a moon named Charon ("KAIR-en"). Not too bad for such a small planet, huh?
Hold On! We're Leavin' the Solar System
Our journey continues as we leave the comfort of the Solar System, picking up speed and changing direction to intercept the Sun's closest stellar neighbors, the triple-star, Alpha Centauri system. Visible from the southern hemisphere back on Earth, the system rushes past.
We still have a long distance to cover before we leave the Milky Way Galaxy -- tens of thousands of lightyears, as a matter of fact -- so let's stop by some interesting objects on the way.
Here we come to the Pleiades star cluster, a grouping of a couple of hundred stars, all relatively young. The Pleiades move through the Galaxy together, and here we see them passing through a cloud of interstellar gas and dust; note how their light is scattered by particles in the cloud, an example of nebulosity.
Since "nebulae" are my favorite objects, we'll stay here a minute. The Pleiades cluster contains an example of a reflection nebula; there are other kinds of nebulae.
The planetary nebula NGC 3132. Image courtesy of AURA/STScI and NASA.
Planetary nebulae are made of small shells of gas thrown off by dying, low-mass stars. The Sun will produce a planetary nebula as it dies in about six billion years. Here we see the Helix nebula, a good example of a planetary nebula; note the small bluish star at the very center of the nebula -- the dead star's core!
Emission nebulae are composed of gases (mostly hydrogen) that have been excited by stars' emitted radiation. When gas atoms get excited, they glow like a neon sign. The Great Nebula in Orion is 1500 lightyears away, but the young stars inside the Nebula excite the gas so much that the Orion Nebula is bright even to the naked eye.
Finally, dark nebulae are clouds of gas and dust which block the luminosity from any objects behind them. The Horsehead Nebula, then, also in Orion, appears as it does because dark material in the cloud completely blocks light from the stars beyond it.
Another dark example is the FANTASTIC Eagle Nebula. Visit each image in turn.
Oops, it's time to move on! We've seen young stars in the Orion Nebula, older ones in the Pleiades, and still older ones in our own backyard (pssst -- the Sun is about five billion years old). But what about the oldest stars we can see anywhere? To find those, we'll need to head into the outer reaches of the Milky Way Galaxy -- into its tenuous halo.
Home to some of the oldest stars in the Universe, globular cluster NGC 6093 is striking in this Hubble Space Telescope image. Image courtesy of AURA/STScI and NASA.
Up ahead is a globular star cluster. Composed of a few thousand to a few million of the very oldest stars in the Universe, globular clusters are beautiful markers to our galaxy's limits.
Whew! That was exhausting. But we have reached the edge of the Milky Way Galaxy. What lies beyond, moving swiftly through the practical emptiness of the Universe, are other galaxies -- some like ours, some not. The vast majority are, like the Milky Way Galaxy and the Andromeda Galaxy (shown here), organized into clusters of galaxies. And each galaxy, you must recall, is made up of star clusters, nebulae, solitary stars, and planets.
Where to Travel Next?
You can spend a lifetime studying our one planet and not see it all. Maybe a number of lifetimes to study only a small portion of our Solar System. All these planets make up just one solar system, around one star, in one galaxy. Our medium-sized galaxy has a hundred billion stars, and we believe there are a few tens of billions of galaxies in the observable Universe. Where does this continuum of enormity end? We don't know, but we do know that there are countless voyages left to take.
To continue the virtual voyage beyond our galaxy, go to Kent Cheatham's "A Walk in the Stars" at http://www.ionet.net/~cheatham/astro/astrotour.shtml
Kent Cheatham is an avid amateur astronomer who lives just outside the lights of Harrah, Oklahoma. Although in his job he controls the state's electrical grid for Oklahoma Gas & Electric, his life centers around his wife and two children. His email address is firstname.lastname@example.org.
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