Sporadics

Last week I witnessed one of the brightest and most colorful meteors (aka "shooting stars") I’ve ever seen. It was the sort that makes you–and everyone around you–whoop like a rowdy fan at a sporting event.

A meteor or shooting star is the streak of light caused by a bit of space dust or debris (meteoroid) burning up as it encounters Earth’s atmosphere. The pyrotechnical display owes less to the size of the meteor than it does to the velocity at which it slams into our atmosphere. In fact, most meteors are merely the size of grains of sand or rice. The real whoppers–known as bolides–that explode and light up the night environment like flashbulbs, might be only the size of peas or pebbles. The occasional big chunk of rock that survives its screaming descent through our atmosphere to impact the ground (or an unfortunate vehicle) is called a meteorite.

I’m accustomed to seeing color in meteors. It’s not untypical to see yellow-colored or green-hued shooting stars, along with run-of-the-mill white ones. Some meteor showers are even known for reliably exhibiting a particular color. The Perseid meteor shower in August, for example, tends to produce meteors with a gold-colored trail.

My recent meteor of note, however, was big, bright, and unexpectedly blue–neon blue. It took my breath away. Color in meteors is caused by both the composition of the meteoroid and the composition of the air it encounters. The blue may have indicated a high magnesium content in the meteoroid.

The blue meteor was most likely a sporadic, a meteor that isn’t associated with a particular meteor shower. A meteor shower is a display that occurs annually around the same time, when the Earth encounters a specific cloud of debris left behind by a comet whose orbit brought it close to the Sun.

To determine if a meteor you spot is a sporadic, first check to see if a meteor shower is occurring. I use the calendar on the American Meteor Society’s website and check the column marked "Activity Period." If there’s no current meteor shower, then chances are any meteors sighted are sporadics.

If, however, you are within a meteor shower’s period of activity, observed meteors may be either part of the shower or sporadics. To determine which, ask yourself two questions:

1) Is the radiant–the spot in the sky from which the meteors of that shower appear to emanate–visible?
The radiant is typically located within the constellation for which the meteor shower is named (e.g., the Lyrids for Lyra and the Perseids for Perseus), and it moves slightly over the course of the shower. A number of websites, such as this one, offer star maps with the radiant location plotted for the duration of the shower.

The radiant may not, in fact, be visible at the time you spot the sporadic because the constellation where the radiant’s located may not yet have risen over the eastern horizon. It’s typical for a meteor shower radiant to rise after midnight, making midnight to morning twilight the best time to view showers. If you see a shooting star before midnight, there’s a good chance it’s a sporadic.

2) If the radiant is above the horizon, can you trace the trajectory of the meteor back to the radiant?
Observe the starting point, ending point, and direction of the meteor observed. Trace the streak of light from the ending point back through the starting point and beyond to see if that path crosses the meteor shower’s constellation of origin. If so, it’s probably associated with the shower. If not, it’s most likely a sporadic.

There are anywhere from two to sixteen sporadic meteors per hour that strike our atmosphere nightly. The farther away from city lights you get, the more shooting stars you’ll see.

As my memorable blue meteor proved, sometimes one random bit of wayward space dust is just as visually exciting as the peak of a major meteor shower.






Astronomy Essential: A comet is a dirty snowball that orbits the Sun.

A comet is a solar system body composed primarily of ice, gas, and dust trapped in the ice, in other words, a dirty snowball. The snowball begins to melt a bit if the comet’s orbit takes it close enough to the Sun. This melting of the ice releases some of the dust, which creates a "tail" of dust streaming out in the comet’s wake. Looking at the dust tail shows us the comet’s path.

A comet has a second tail, which is usually not as easily visible as the dust tail. Electrically charged gas particles streaming from the comet are blown back by the solar wind, a continuous stream of particles given off by the Sun. Therefore, this ion tail, as it is called, always points away from the Sun.

Giving Thanks

These are a few of my favorite things.


Dance of the Planets
Watching the naked-eye planets (Mercury, Venus, Mars, Jupiter, Saturn) rhythmically huddle and disperse is like watching an intricate dance. These planetary alignments are the result of our earthbound view of the naked-eye planets moving in their respective orbits around the Sun. Some are traveling faster than Earth; some are traveling slower. The resulting variety of configurations gives us a sense of the “clockwork of the universe” and reminds us that everything is in motion.

Solar system diagram created with Solar System Live

Be sure to look southwest after sunset on Monday, December 1, when Venus, Jupiter, and the crescent Moon will bunch up in the twilit sky. Venus will be the brighter of the two star-like objects you see, and Jupiter the fainter. The waxing (growing) crescent Moon will be about 15 percent illuminated.

While you're enjoying the alignment, look for earthshine on the Moon. Earthshine is sunlight reflecting off the surface of the Earth and illuminating the dark part of the Moon. Even though the bright crescent is the only part of the Moon’s face being directly illuminated by the Sun, if you look carefully, you’ll see that the rest of the Moon’s face is glowing faintly with reflected glory: earthshine.

Shooting Stars
Wishing on a “falling star” is one of the guilty pleasures of the stargazer. Even after you know that falling stars, aka "shooting stars," are meteors, you can still appreciate this charming holdover from early folkloric traditions.

A meteor is the streak of light we see in the sky when a bit of dust or space debris hits Earth’s atmosphere at high velocity. A meteor shower occurs when Earth encounters a stream of debris left behind by a comet’s close approach to the Sun. Sporadics are meteors not associated with a particular shower.

Meteor showers occur reliably around the same time every year, as Earth returns to the point in its orbit where the debris field lies. Whenever you like, you can find out what meteor showers are currently active by visiting this website and scrolling down to the Meteor Shower Calendar.

Meteor storm - 1889 engraving by Adolf Vollmy

If you’re lucky, you might someday see a meteor storm, a very intense meteor shower with a high frequency of meteors. One of my top ten stargazing experiences ever was viewing the 2001 Leonid meteor storm. The meteors came so fast and furious, it was a bit like watching a fireworks show. I was with a large group of fellow amateur astronomers at a dark-sky site, and the oohs and aahs from the crowd as particularly fine meteors blazed across the sky just added to the ambiance of the event. We all knew we were seeing something rather spectacular.

Cosmic Dust Bunnies
Interplanetary space is not empty. Our solar system contains a vast number of minute particles, some left over from the time of planet formation and some ejected from passing comets or asteroid collisions. Light from our Sun illuminates the myriad particles lying in the solar system plane, and sometimes we see the reflected sunlight as a luminous pillar of light called the zodiacal light.

The zodiacal light (zoh-DYE-uh-kull) is a large, roughly triangular or cone-shaped glow in the night sky, extending upwards from either the eastern or western horizon. The base of the triangle or cone— the widest part of the glow— is at the horizon. The zodiacal light is typically seen in the spring above the western horizon after sunset and in autumn above the eastern horizon before sunrise.

If you're fortunate enough to view the zodiacal light when transparency (atmospheric clarity) is good and the sky is inky black, you may notice the light pillar has a yellowish cast. Our Sun is, after all, a yellow star, so it shines with a golden light.

Cone-shaped glow of zodiacal light above sunset glow
Image source: ESO

The zodiacal light is subtle and can be quite challenging for the beginner to spot. A dark site away from urban and suburban night lighting is critical for success, as is avoidance of a moonlit night.

Believe me, this phenomena is worth a little extra effort. It’s quite a rush when you spot it, especially when you then start thinking about what you’re looking at— vintage dust from when the solar system formed. The goofy image that always pops into my head is the Sun as a clean freak with a big flashlight, showing us what lurks under the furniture.


The Way
My personal pet name for our home galaxy, the Milky Way, is simply “the Way,” because it represents for me a way of being and a way of seeing. I learned an important lesson a number of years ago on a camping and observing trip in southern Arizona. I had spent the better part of a night looking through the eyepiece of my telescope, hunting various galaxies, globular clusters, and nebulas on an observing list and methodically ticking off those “faint fuzzies” as I found them. Straightening up from the eyepiece to stretch my back, I faced south and audibly gasped. The woods south of the clearing where I and my partner were observing were on fire!

To be continued…

The Pursuit of Perseids

The Perseids (pronounced PURR-see-yidds) are a major, reliable meteor shower and one of my personal favorites. It simply wouldn’t be August if I didn’t try to spot some of those long, yellow-tailed beauties streaking across the sky.

Sometimes a big Moon will thwart or at least hamper my attempt. That’s the case this year. The peak of the meteor shower--when the number of meteors per hour is greatest--is predicted to occur in the early morning hours of Tuesday, August 12. As this is less than one week before Full Moon, the waxing (growing) Moon will be in a gibbous phase. Gibbous means the Moon is greater than 50% illuminated without being fully illuminated. In other words, a big Moon.

A big Moon tends to wash out the sky, reducing the contrast needed to see faint streaks of light. No Moon is optimum. Since the Moon doesn’t set until two to three hours after midnight (depending on location) on the morning of the 12th, you’ll have a narrow window of opportunity to spot peaking Perseids before morning twilight lightens the sky. But the reward will be great: the predicted peak rate is 100 meteors per hour!

By all means, don’t get too hung up on peak predictions. I’ve seen some awesome meteor shower displays on nights prior to and after the so-called peak. In some cases, I was pleasantly surprised to discover they bested the peak display. The Perseids are active this year through August 24. Pick any night from now until then, go out after midnight when the Moon’s not around, and you’re bound to see some dreamy Perseids.

There’s a reason why I specified “go out after midnight” above. Perseids emanate from, and are named for, the constellation Perseus (pronounced PURR-see-yuss). The show can’t begin until the radiant in Perseus rises above the northern horizon. The radiant is the point in the constellation where the Perseids appear to originate. The higher the radiant rises in the sky, the more likely you'll see meteors. The radiant is well above the horizon--and rising--from midnight through dawn.

The radiant is marked on the map below. Despite their origination point, you can see Perseids in any part of the sky, in any direction you happen to be looking. However, knowing the radiant will help you identify which “shooting stars” are Perseids and which are sporadics, meteors not associated with a particular meteor shower. If you can trace a meteor’s trajectory back to Perseus, it’s most likely a Perseid.

Chart created with Your Sky

1) Go to as dark of a site as you can, with no line-of-sight outdoor lights. Take extra layers of clothing or blankets with you. Even though it’s summer, you won’t be moving around much and could get chilled.

2) Take a lounge chair or a blanket for the ground. Reclining is really the best way to watch a meteor shower, because it allows you to see the most sky at one time. Although the meteors originate from the radiant, you can see them fizzling anywhere in the sky. I normally lie with the top of my head pointed toward the rising radiant (which will be in the northeast) so that I can be scanning the rest of the sky.

3) A meteor is the streak of light caused when a bit of interplanetary dust, comet debris, or manmade space junk slams into Earth’s atmosphere at high velocity. Typically, the meteoroid--the bit of debris--vaporizes upon impact with the atmosphere. But occasionally, a large chunk may survive its screaming 100,000-plus mph entry and hit the ground, at which point we refer to it as a meteorite.

As you watch the fireworks, keep in mind that meteors are typically no bigger than a grain of sand. The really bright ones that go off like Roman candles may be no bigger than a grain of rice. It is their high entry velocity that causes the wonderful light show.

4) A meteor shower occurs when Earth encounters a stream of debris left behind by a comet’s close approach to the Sun. Comet Swift-Tuttle takes 135 years to complete its orbit of the Sun. Each time it approaches, the Sun's heat releases dust trapped in the comet's icy nucleus or frozen center. This periodic renewal of the discarded dust stream ensures us cosmic fireworks every August when we move through it.

Meteor showers are one of the supreme naked-eye treats for lounge-chair astronomers everywhere, beginners to veterans. Do make the pursuit of Perseids part of your summer recreation. Do make a memorable evening with a friend or family member. Don’t forget to make a wish.