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.