A Whirldwind Tour of the Night Sky
Let’s begin with the end in mind: here’s a quick survey of what you can see from your backyard or favorite observing site with your unaided eye, a good pair of binoculars, or a small telescope. As you’ll see, there’s a lot more up there than just stars…
Aside from the Sun, the Moon is the brightest and most recognizable object in the sky. At about 1/4 the diameter of the Earth and some 250,000 miles away, the Moon spans about 1/2 degree of sky… about half the width of your pinky finger held at arm’s length.
Even your unaided eye reveals the dark lava seas, or maria, on the Moon’s surface. These dark patches are the newest parts of the Moon. They were flooded with lava after a period in the early solar system when the Moon was bombarded with asteroids that formed thousands of craters. Still, the lunar surface is extremely old. The Moon has no air or liquid water to erode the surface, so most features are unchanged over the last 2-3 billion years.
A pair of binoculars reveals perhaps a dozen large craters. Near the south-central part of the Moon (“south” is “down” for observers in the northern hemisphere), the crater Tycho has a series of rays that shoot out in all directions, giving the Moon the appearance of a peeled orange, with Tycho as the “pip”. The rays are material ejected when Tycho was created by a large asteroid impact about 110 million years ago.
A crescent moon at low magnification
Even a modest telescope reveals thousands of sights, including craters of all shapes and sizes, arcing mountain ranges that tower thousands of feet above the lunar surface, and cracks and fault lines and strange domes that give some hint of geological activity.
Many skilled amateur astronomers have observed the Moon in detail for 40 years or more, and never tire of its stark beauty. Some even actively help in lunar research by monitoring the surface for strange flashes of light and blurred clouds of gas called transient lunar phenomena, the nature of which remains mysterious.
The Moon cycles through phases once every 27 days or so. As it waxes from its “new” phase to full, it’s visible in the evening or night sky; as it wanes back to new, it’s mostly visible in the daytime.
Beginners often believe the best time to observe the Moon is when it’s full. But that’s almost always the worst time. You’ll get the best views of the Moon along the terminator, the line that separates night from day. On the terminator, the craters and mountains stand out more clearly in the long shadows of lunar sunset or sunrise.
The planets are a favorite among stargazers. The two planets closer to the sun, Mercury and Venus, appear bright and fast moving in the sky. You’ll usually see them around sunrise or sunset. To a telescopic observer, Mars, Jupiter, and Saturn reveal an astonishing array of detail on their disks. And Uranus and Neptune can be resolved into featureless white disks without detail; it’s an accomplishment for a beginner to see these distant giants at all. Pluto, recently demoted from planet status, is too faint to see in all but the largest backyard telescopes.
Because the solar system was formed by a spinning disk of material, the planets lie in a nearly flat plane in space. So from Earth, all planets lie in a narrow band called the “ecliptic” that runs through the familiar twelve constellations of the zodiac. If you’re looking for Jupiter, for example, look in one of these constellations. Don’t bother looking in Orion, or Centaurus, or any constellation off the ecliptic. The planets never go there.
The planets distinguish themselves by moving with respect to the background stars from day to day and week to week. The apparent motion is caused the the revolution of the planets around the Sun, and the motion of the Earth with respect to the planet.
With the unaided eye, you can see Mercury, Venus, Mars, Jupiter, and Saturn. If you know just where to look, you might see Uranus too. Binoculars are not powerful enough to reveal much detail, though they will reveal the four largest moons of Jupiter that move around the planet from night to night. To see any surface detail, or to see the phases of Mercury and Venus, you will need a telescope.
Saturn as it might look in a small telescope
Stars are the most numerous objects in the sky, other than perhaps mosquitoes from time to time. On a clear dark night, you’ll see about 3,000 stars with your unaided eye. Binoculars and a telescope reveal tens of thousands more. Every star you see belongs to our own galaxy, the Milky Way.
Take a look at the stars on the next clear night. Aside from the difference in brightness from star to star, you will also see difference in color. Some stars, like Rigel in the constellation Orion, are blue. Others, like Altair in Aquila, are white. Arcturus, a bright star in the northern spring sky, is yellow-red. Yet others, like Betelguese in Orion or Antares in Scorpius are a deeper red.
These color differences are real. Just like hot coals in a fire glow red, and hotter coals glow white, the color of a star depends on its temperature. Red stars are coolest. They have a temperature of 4,000-5,000 degrees at their surface. Yellow-white stars like our Sun are hotter… about 6,000-10,000 degrees. And blue stars are the hottest of all, with surface temperatures of 15,000 to 30,000 degrees.
Blue stars are massive… some 5-10x the mass of our own sun. Massive stars burn hottest and emit most of their light in the blue and ultraviolet part of the spectrum. White and yellow stars are young-to-middle-aged middleweights. Most of the red stars you can see were once massive blue stars that are nearing the end of their lives. And most of the stars of the galaxy are dim red stars, much less massive than our sun, and too faint to see without a large telescope
Some stars look bright because, well, they ARE bright. Others appear bright because they are close to us. Deneb, the brightest star in Cygnus, is one of the brightest stars known… about 60,000x brighter than our sun. It’s 1,500 light years away (one light year is the distance light travels in one year, about 9.5 trillion kilometers). Sirius, on the other hand, appears a little brighter than Cygnus. But it’s only 25x brighter than our sun and only 0.0004x as bright as Deneb. But it appears brighter than Deneb because it’s 175 times closer… just 8.6 light years away.
Stars form out of clouds of dark dust and gas that pull together under by the force their own gravity.
Stars usually form in groups of dozens or more out of clouds of gas and dust along the plane of the Milky Way galaxy. As the gas cloud condenses into dark globules, it heats up and eventually starts a process called nuclear fusion that releases a huge amount of heat and light. When fusion starts, each globule within the immense cloud of gas and dust becomes a star, and dozens of stars are born at roughly the same time in a small cluster.
These groups of newly-born stars are called galactic star clusters, or open star clusters since they form a fairly loose collection of stars eventually disperses into the galaxy. Open clusters are one of the most beautiful sights in the sky for binoculars or a small telescope. You may have seen at least one already: the star group called the Pleiades is an open cluster of new stars less than 1/10 the age of our own Sun. Hundreds more such clusters lie within the reach of a modest backyard telescope.
Globular star clusters are another favorite for stargazers. Unlike open clusters, globular clusters are immensely old, some 10-13 billion years. That’s nearly as old as the universe. Globulars contain tens of thousands of stars or more… many more than open clusters. But they’re located far away in a vast halo around the nucleus of our galaxy, so most don’t appear very bright. Only three are visible to the naked eye: M13 in Hercules, 47 Tucanae, and omega Centauri. The latter two are seen only from the southern hemisphere. A small telescope will show you dozens more of these ancient relics.
The globular star cluster M13 in the constellation Hercules
Even a random search with a small telescope along the plane of the Milky Way reveals hazy patches of silver-white light that look like glowing patches of fog against the dark sky. These are the diffuse nebulae. Made from gas and dust set aglow by stars formed in their midst, diffuse nebulae are the nurseries of the galaxy, where new stars form out of collapsing clouds of dark gas and dust. The new stars excite the atoms of gas that remain in the cloud, and the atoms relax again by emitting the light you see. In time, as the gas clears, an open star cluster will take the place of the nebula.
The sword of the constellation Orion contains one of the brightest and most famous such nebula. You can see it as a fuzzy patch of light. A telescope reveals an astoundingly beautiful view of the Orion nebula that holds many stargazers in its spell for decades. You can see many more such nebula, such as the Swan, Lagoon, Trifid, and eta Carina nebulae even with a pair of binoculars.
A planetary nebula is another common sight for stargazers. Quite different from diffuse nebula, and having been named after their appearance as small disks that somewhat resemble a planet, these nebulae are the last gasp of life from mid-sized stars that have exhausted their fuel. The stars at the center of each planetary nebula are ejecting their atmosphere before they settle down as their final stage of life as a white dwarf. Our sun will end its life as a planetary nebula in some 5 billion years. No planetaries can be seen with the naked eye, but hundreds are visible in a telescope, including the famous Ring Nebula in Lyra and the Dumbbell Nebula in Vulpecula.
The “Swan” Nebula (from the telescopes at LightBuckets.com)
All the objects we’ve explored so far in our quick tour lie within our own Milky-Way galaxy. Beyond lies the rest of the universe, with untold billions more galaxies as far as the largest telescopes can see. Each distant galaxy contains tens or hundreds of billions of stars. But the galaxies are so far away that they appear quite faint in our sky. With your unaided eye, you can only see two nearby galaxies in the constellations Triangulum and Andromeda. These are the farthest things you can see without optical aid; the light left the Andromeda galaxy more than 2 million years ago. Southern observers can see two small satellite galaxies of the Milky Way called the Magellanic clouds.
With a good-sized amateur telescope and dark sky, you can see thousands of galaxies in the northern and southern hemispheres. You’ll get the best view of galaxies in the spring and fall, when the night sky of the Earth points out of the obscuring plane of our own galaxy and into intergalactic space. Some, like the Whirlpool galaxy, have the same beautiful spiral shape as our Milky Way. Others, like the monstrous M87, are round or elliptical and featureless, yet contain more than a trillion stars.
Spiral galaxy M64
Some sights in the sky are visible for a short time. Meteors, pebble-sized pieces of space debris, fall through the Earth’s atmosphere and burn up in a few seconds, igniting a sudden trail of light across the sky. Look up on any night and you might see 2 or 3 meteors each hour. But sometimes, on fixed dates throughout the year, the Earth passes through the path of a comet which has left behind small clouds of debris as it orbit the Sun. When this happens we’re treated to a meteor shower during which you may see dozens or even hundreds of meteors each hour. Perhaps the finest meteor shower each year occurs around August 12 when the Earth passes through the path of Comet Swift-tuttle. This shower is called the Perseids, since the trails of meteors all appear to originate in the constellation Perseus.
Comets are more rare than meteors. There might be a half-dozen comets visible each year, though they are usually faint enough to require a telescope to see well. Every 10 years, on average, a comet appears that’s bright enough to be easily seen with the naked eye. Comet Hale-Bopp in 1997, and Comet McNaught in 2007 were examples. Comets are likely remnants of the formation of the solar system, dark balls of ice and dust that linger beyond the orbit of Pluto, occasionally plunging into the inner solar system where they’re set aglow by the heat and radiation from the Sun.
Comet McNaught in early 2007
Even more rare are stars that suddenly flare up by hundreds or thousands of times. These stars are called nova, which are caused by a number of mechanisms, including the deposit of material from one star onto another until the latter suddenly ignites and brightens. A supernova is an even more spectacular event. It’s caused by the detonation of a massive star that’s run out of fuel and collapsed upon itself. The explosion of such as star often outshines an entire galaxy for a period of a few weeks. Astronomers estimate a star in our own galaxies explodes every 50 years. Several supernova in other galaxies are visible each year in large telescopes; occasionally, these extragalactic explosions are discovered by backyard astronomers.
A Lifetime of Fascinating Sights…
So you see, there’s lots to see up there. This brief tour just touches on the basics. With the right know-how and a modest telescope, you will have thousands of fascinating sights to see in our own Milky-Way galaxy and beyond… You’ll never be bored again!