Thursday, April 30, 2009

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.

Thursday, April 23, 2009

On Hiatus This Week

This week, I and a few awesome astronomy colleagues are launching our exciting, IYA-inspired project: STAR-HOPPERS Weekend Workshops in Astronomy.

STAR-HOPPERS is a nonprofit program operating in partnership with the University of New Mexico Field Station on the Sevilleta National Wildlife Refuge in central New Mexico, USA.

Until next week, happy star trails to you!



Thursday, April 16, 2009

Speed on to Spica

Last year we learned how to arc to Arcturus using the handle of the Big Dipper. This year, let’s continue along that path and complete a well-known memory prompt for beginning stargazers that bids us “arc to Arcturus and speed on to Spica.”

1) About an hour after sunset, face east. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your back to the west, and you’ll be facing approximately east.

2) Tilt your head back and look at the zenith, the point in the sky that’s directly overhead. A little ways to the left (toward the northern horizon) and just east of the meridian, you’ll find the distinctive seven-star asterism (recognizable star pattern) known as the Big Dipper. The Big Dipper is in the constellation Ursa Major the Big Bear.

Now extend the curve of the Big Dipper’s handle— heading away from the Dipper’s bowl— to the next star along that arc that‘s brighter than the end star of the handle. You’ve arced to Arcturus! Arcturus is the brightest star in the constellation Bootes the Herdsman.


Star maps created with Your Sky



3) To speed on to Spica, simply continue the arc past Arcturus to the next bright star. This is the blue-white dwarf star Spica (SPY-kuh), which is a bit dimmer than Arcturus. Spica lies 260 light years away. A light year is the distance light travels in one Earth year, nearly six trillion miles.

Spica is the brightest star in the constellation Virgo the Maiden. The name Spica comes from the Greek word for ear of grain. The figure of Virgo is associated with the harvest, and she is often depicted carrying an ear of grain.


Virgo in Johann Bode’s 1782 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology



The ancient star figure of Virgo is not easily apparent to the modern stargazer. Virgo is one of those constellations characterized by a loose, sprawling collection of nondescript stars with no showy asterisms. If it weren’t for luminous Spica punctuating the arc from Arcturus, most of us would be hard-pressed to locate the dowdy maiden at all.

But now you can arc to Arcturus and speed on to Spica. This is an easy asterism to teach others, and best of all, it covers ports of call in three constellations!




Astronomy Essential: A telescope gathers light; it does not magnify.

The function of any telescope— whether it uses mirrors, lenses, or both— is to gather light and focus it into a clear, bright image.

The eyepiece— the accessory that is inserted into the focuser and through which the observer peers— is the optical element that magnifies the image produced by the telescope. The magnification of the image, or power as it is also called, changes each time a different sized eyepiece is inserted into the telescope.

Thursday, April 9, 2009

Shepherd Moon

Last week, I talked about starting an observing Life List and related how I’d recently checked off two items on my Life List. If you read that post, you know that both items are not for beginners. Rather they’re the elusive companions to a couple of familiar, naked-eye objects that are accessible to beginners.

The first tag-along object I checked off my Life List this year was Sirius B, the dim companion star to Sirius the Dog Star, brightest star in the night sky.

And now for the exciting conclusion…

Tag-along Object #2
On March 28 of this year, I gathered with fellow amateurs in my astronomy club for a much-anticipated Messier Marathon at a dark-sky site. Although I wasn’t “running” the Marathon this year, I was on a scavenger hunt of my own. I’d decided it was a good night to try for that most elusive moon of Saturn: Mimas. Since Saturn’s rings are nearly edge-on right now, it’s an excellent time to scout for the fainter moons. When Saturn’s rings are open, the planet reflects so much more light our way that faint moons get lost in the glare.

Diminutive Mimas (MY-muss) is only 243 miles in diameter—about the distance from Baltimore, Maryland to Norfolk, Virginia. Compare it to Saturn’s largest and brightest moon, Titan, which is 3,200 miles in diameter and which can easily be spotted when viewing Saturn through even a modest telescope. Titan looks like a little star, usually several ring diameters from the planet. If you view Saturn and see only one moon, chances are it’s Titan.




Saturn and the Cassini Division
Source: NASA, ESA and E. Karkoschka (University of Arizona)



Although small, Mimas has a powerhouse purpose. It’s a shepherd moon, so-called because its gravity tugs on the rock and ice particles in Saturn’s rubble-filled rings and herds them into formation. If you observe Saturn with a telescope when its rings are open, you may notice a thin black line inscribed on the surface of the ring plane, circling the planet. This is the Cassini Division, a gap in the rings that yawns nearly 3000 miles wide. The gravitational pull of Mimas is believed to be the force that keeps the Cassini Division clear.



Mimas the shepherd moon and Herschel the "Death Star" crater
Source: NASA



Close up, Mimas would look something like the Death Star from Star Wars, with its surface dominated by a huge impact crater, one third as wide as the moon itself. The crater is called Herschel, after Mimas’s discoverer, the eminent English astronomer William Herschel.

But on that cool spring evening, I was hoping for just a glimpse of a cagey little pinprick of light. Saturn was beckoning with its signature golden light, near the back leg of Leo the Lion. So I started my quest in our observatory dome, looking through a classic, 1950s Cave Astrola 16-inch reflector. The nearly edge-on Saturn looked like an olive on a toothpick.




Star map created with Your Sky



Bill, who was running the observatory that night, found Mimas’s position using the planetarium software on his laptop. A few more seasoned observers (I did tell you in my previous post that you have to be an instigator) climbed the tall ladder and yelled down that they thought they could see it, coming and going as it shimmered on the upper tip of the toothpick. I gave it a try and also thought I could see it dancing on the hairy edge of my vision.

We stumbled down onto the observing field and set upon Geoff, who was visiting from an astronomy club in northern New Mexico, but who had the most aperture on the field: a 24-inch-diameter reflector. Up the ladder to look through his graciously proffered cannon and…yes! There it was, a fleck of light, now separated from the toothpick and forming the bottom point of a diamond of moons. Comparing it to the other three moons in the diamond—Rhea, Dione, and Iapetus— I was shocked by how much dimmer Mimas appeared. It was an exhilarating revelation, because I realized how impossible it would be for me to spot such a faint fleck when Saturn’s rings are open. Sometimes in observational astronomy, timing is everything.

What’s next? Well, I’ve got a late spring/early summer rendezvous planned with astronomer buddy Dave (of Sirius B fame) to hunt down 3C 273 in the constellation Virgo.

3C 273 is the decidedly un-poetic name of the brightest quasar in the night sky. Although it appears dim to us and must be hunted with a telescope/eyepiece of substantial aperture and magnification, 3C 273 is believed to have a luminosity 100 times that of our entire Milky Way galaxy! Discovered in 1963, it was the first quasar found.

Quasars are distant energy sources that emit a tremendous amount of radiation. They are star-like in appearance due to their distance, but quasars are believed to be the bright, energetic cores of galaxies powered by supermassive black holes.

Note that 3C 273 and its quasar brethren are not in the Milky Way. In fact, at around two billion light years away, 3C 273 is considered one of the most distant objects an amateur astronomer can view. I simply can’t resist that challenge.




The quasar 3c 273
Source: NASA and J. Bahcall (IAS)



One final thought on the value of a Life List: there’s nothing to make you feel less like a novice than to find out that fellow observers with a lifetime of observing experience just saw the object on your Life List for the very first time also. Start your Life List…today!





Astronomy Essential: Sunspots are not spots.

Sunspots are irregularly-shaped dark regions on the Sun’s surface caused by intense magnetic activity. Sunspots vary in shape and size, and they’re often larger than the Earth. As the magnetic fields flux, existing sunspots may fade away and new ones may form elsewhere on the Sun.

The magnetic activity in those regions disrupts the flow of heat from the Sun’s core to its surface. Because of this, the sunspots are— although still intensely hot— much cooler than the solar surface that surrounds them.

Sunspots aren’t black either. The contrast in temperature with the blistering hot regions around them simply make them appear very dark in comparison.

Thursday, April 2, 2009

Life List

The typical beginning stargazer is not going to run out and look at the two celestial objects I’m about to describe. They should be considered targets for advanced observers, however, they are companions to familiar objects accessible to your naked eye. So you can run out and look at their “host” objects naked eye, and then live vicariously through my observing accounts of these tag-alongs.

Both objects were on my observing Life List and now have satisfying, indelible-ink checkmarks next to them. The lesson herein is to— if you haven’t already— start a Life List.

A Life List is the wish list of objects/phenomena you’d like to see during your observing lifetime. Generally a Life List contains challenge objects— objects that are difficult to see, objects that require a bit more effort, the right equipment, travel to a different latitude, and/or perfect sky conditions. A Life List is a fluid thing. I’m not sure you ever actually complete it. Check off a couple items, and more mysteriously appear.

How do you start a Life List? You might hear a more seasoned astronomer wax poetic about “the time I saw X after hiking 15 miles with a 60-pound scope on my back and waiting for the only ten-minute break in the clouds.” Or you might read something intriguing in an astronomy magazine or on a website, and think, “I’d like to see that!” And so a Life List begins.

Here are some of the items currently on my Life List:

Aurora borealis, aka the Northern Lights
The aurora is the colorful, undulating sky display that occurs primarily in arctic regions. Particles from the solar wind slam into Earth’s atmosphere at the poles and excite the gasses there.

I’ve only been in arctic regions once, on a trip to northern Scotland. Did I see the aurora? Of course not! Canada, here I come (some day).




The Northern Lights
Copyright 1995-2003 Jan Curtis




Pluto
The dwarf-planet-formerly-known-as-a-planet is still a goal, regardless of its status. Perhaps it’s a New Mexican thing, since Pluto’s discoverer, astronomer Clyde Tombaugh, made his home in New Mexico. Seeing Pluto requires a commitment of several nights of observing. Since the tiny rock looks like a star— surrounded by stars— you must watch it move relative to the background of stars over a few nights to be certain you are observing the real McCoy.




Clyde Tombaugh, discoverer of Pluto





Magellanic Clouds
These are two companion galaxies to our home galaxy, the Milky Way. They are dwarf galaxies, much smaller than our Milky Way, whose gravity has disrupted and distorted the little galaxies’ structures. Known as the Large Magellanic Cloud and the Small Magellanic Cloud, they are easily visible to the naked eye from the Southern Hemisphere. Yup, I’m saving my pennies for that trip to Australia.

Diamond Ring
As you know, diamonds are a girl’s best friend. But the sparkler I’m holding out for is in the sky— the daytime sky, as it were. The Diamond Ring is the effect seen just before and just after totality during a total solar eclipse. It’s caused by a last (or first) ray of sunlight peeking through a deep lunar valley. The soft glow of the Sun’s corona illuminating the perimeter of the Moon and the brilliant point of light combine to resemble a diamond ring.

A total solar eclipse is when the Moon passes between the Earth and the Sun and is aligned such that it completely covers the Sun’s disk. Totality is the period of time during which the Moon completely covers the Sun’s disk.

Anyone for mainland China in July?

Then there’s 3C 273. But more on that later.

Back to my original topic, an observing account of the two tag-along objects, in the order in which I saw them.

Tag-along Object #1
On January 31 of this year, I decided to try to see Sirius B. It had been on my Life List for awhile, and I’d already made several unsuccessful attempts to see it. What I had going for me this time were steady, clear sky conditions, and a tracking telescope with an occulting eyepiece.

The equipment was not mine. For those of us with one low-tech telescope to our names, working on the Life List is best accomplished as a communal activity. Be an instigator! Get everyone around you on the observing field worked into a froth about what it is you’re trying to see, and pretty soon you’ll have all sorts of people and equipment resources dedicated to the quest. Oops, I’ve divulged my secret strategy. The jig is up.

Sirius B is the incredibly dim companion star to Sirius, the Dog Star, the brightest star in the night sky. Sirius is prominent in the night sky in winter and spring, and you can easily spot the brilliant star by looking east and south of Orion the Hunter.







Sirius A (the bright one you see naked eye in the sky) and Sirius B are a binary system, that is, they are in orbit around one another. The elusive, hard-to-spot Sirius B is 10,000 times fainter than Sirius A, in large part due to its relatively diminutive size. Sirius B is slightly smaller than Earth, whereas Sirius A is 3.5 times larger than our Sun. To illustrate the scale of Sun and Earth, if the Sun were the size of a bowling ball, Earth would be the size of a peppercorn! So you can see that Sirius B is a smidge. Plus it tends to be hidden in the glare from blazing Sirius A.

Sirius A and B vary in their distance from one another due to their eccentric orbits. They are currently separating, with maximum separation predicted for the year 2019. So the next ten years or so is an excellent window of opportunity for spotting Sirius B.

On the fated night, at a dark observing site, we combined my observing partner Carl’s occulting eyepiece and observing buddy Dave’s tracking telescope. The occulting eyepiece was simply a 15mm eyepiece that Carl had modified with a thin piece of black electrical tape carefully affixed inside the open end. The result is a thin, opaque bar that bisects your field of view when you look through the eyepiece. It’s great for blocking out bright objects when trying to see dim objects hidden nearby in the glare. You simply position the scope so that the bright object is occulted under the opaque bar and then scan the area around it for the faint object.

Having the eyepiece in a tracking telescope can make all the difference. Otherwise, due to the Earth’s rotation, the bright object is constantly re-emerging from under the occulting bar and you’re constantly nudging the scope and twirling the eyepiece in the focuser in order to put it back underneath. It is, at best, annoying. At worst, you can’t focus long enough on any one spot to look for your target.

We used the same doctored eyepiece with great success to cover bright Mars and view its pinprick-sized moons, Phobos and Deimos, during Mars’ close approach in August 2003. Ah yes, Phobos and Deimos, former tenants of my Life List.

With the right eyepiece and telescope, it didn’t take the three of us long to spot the bright dust mote swimming near the edge of the glare. Success! It was the first glimpse of Sirius B for all three of us. Soon, others on the field came over for their first look also.

I know you’re just dying to know what Tag-along Object #2 was. Tune in next week, for the exciting conclusion!




Astronomy Essential: The solar system is much larger than the orbits of the planets.

When we think of the solar system, we think of the Sun and a series of roughly concentric planetary orbits, concluding with Neptune (or Pluto, depending upon when you went to elementary school).

In reality, the solar system is much larger. The term solar system means “system of the Sun,” that is, the area of space where the Sun exerts its gravitational influence.

Beyond the planets are the Kuiper Belt and the Oort Cloud. The Kuiper Belt (pronounced KIGH-purr) is a large band of small, rocky bodies beyond the orbit of Neptune. Beyond the Kuiper Belt is the Oort Cloud (pronounced ORT), an immense, sphere-like cloud of an estimated one trillion comets. The Oort Cloud marks the outer edge of the solar system.

The Oort Cloud is estimated to lie nearly one light year from the Sun. A light year is the distance light travels in one Earth year, nearly six trillion miles. By comparison, Neptune’s orbit lies only 2.7 billion miles from the Sun.