Date: January 2, 2011

Title: Deep Into the Dipper!

Podcaster: Rob Keown and Tavi Greiner

Links: http://askyfullofstars.com and http://astronomy.fm/radio

Description: We’re all familiar with the familiar shape of the Big Dipper, but how much do you know about the history and celestial surroundings of this famous asterism?

Bio: Rob Keown and Tavi Greiner have worked together in astronomy outreach for several years, originally as Slooh radio hosts and now as co-creators of A Sky Full of Stars website and productions. They are also among the original founders and directors of Astronomy.FM and remain involved with that website’s internet radio station, AFM*Radio. Both are passionate about observing the night sky and experimenting with night sky photography. You can find them sharing their experiences on twitter as @askyfullofstars, @keownrwk, and @tavigreiner. You might also like to browse Tavi’s night sky photo galleries, here and here, and their collective photo galleries here and here. A complete listing of all of their productions, including previous 365 Days of Astronomy podcasts, can be found here.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Wayne Robertson, who encourages you to join him in supporting this great podcast.

Transcript:

TAVI: One of the most famous objects in the night sky, the Big Dipper has guided both travelers and astronomers since days of old, with its famous pointer stars directing voyagers North, and that distinct handle escorting skygazers toward celestial points beyond.

ROB: Of course, we’re all familiar with the big square bowl and long curved handle of the Big Dipper. And most of us are aware that it is an asterism within the constellation Ursa Major, rather than a constellation itself.

TAVI: Some of us even know each of the dipper’s seven stars by name and the fact that if you look really close, there is an eighth star, right next to the middle star in the dipper’s handle.

ROB: Did you know that all but two of these stars, Alkaid and Dubhe, are white, class A members of the larger Ursa Major Moving Cluster – a sprawling, seemingly unrelated group that is, in fact, our planet’s closest star cluster.

TAVI: And that means that the Big Dipper won’t always be there. The fact that two of the stars are not members of the Ursa Major Cluster – they are actually moving away from the other stars – means that the Big Dipper will eventually become distorted so that future stargazers will not even recognize it as the giant ladle that it is today.

ROB: But, beyond those basics, intriguing as they are, what else is there to the Big Dipper? What history has it shaped with its guiding stars, and what celestial delights lie within its heavenly region?

TAVI: Well, let’s start with those seven bright stars that outline the dipper’s familiar shape – there are three in the handle and four along the bowl’s perimeter.

TAVI: The dipper’s handle is comprised of Alkaid, Mizar, and Alioth. For star-hoppers, the curve of these three stars “arcs” towards Arcturus – constellation Bootes brightest star – which then “speeds” to Virgo’s brightest star, Spica.

ROB: But these twinkling beauties are more than a star-hopping starting point. Alkaid, a class B star, sits at the handle’s tip. At seven-hundred times more luminous than our Sun, it is the second brightest star in Ursa Major and the thirty-fifth brightest in the night sky.

TAVI: Alkaid is also among the hottest stars visible to the unaided eye.

TAVI: In the middle of the handle, you’ll find Mizar, with its optical companion, Alcor. Together, these two stars are known as the Horse and Rider, and while they appear close to each other, three light years span the space between them.

ROB: Interestingly, Mizar does have a “real” companion; in fact, Mizar is a multiple star system comprised of four components, or two pairs of stars. Mizar’s B component, Theta 2, is readily visible through a telescope or larger binoculars – you’ll find it snuggled right up against Mizar A – but the other two stars can be detected only by spectroscopy.

TAVI: Of historical significance, Mizar A and Mizar B, are the first double star ever photographed – a feat accomplished in 1857, using the Harvard College Observatory’s 15-inch refractor.

ROB: Alioth, that third star in the handle – the one closest to the bowl – is a chemically-peculiar class A star four-times the diameter of our Sun. While designated an “epsilon” star, Alioth is actually the brightest in both the Dipper and Ursa Major.

TAVI: Alioth also exhibits a prominent dusty disk, but that material seems more related to the stars unusual evolution than the creation of any planets.

ROB: Moving along to the dipper’s bountiful bowl, you’ll find it represented by four bright stars: Megrez, Phecda, Merak, and Dubhe.

TAVI: In ancient Arabic astronomy, those four bowl stars were a coffin surrounded by mourners that were the three stars of the handle.

ROB: That corner star that secures the bowl to the dipper, or the coffin to the mourners, is represented by the star Megrez, the faintest of the Big Dipper stars. This fifty-million-year-old dwarf carries just twice the mass of our Sun and is only half as hot.

TAVI: Dropping straight down to the bottom of the bowl – that corner directly beneath Megrez – you’ll find Phecda, the thigh of the bear that is the constellation of Ursa Major, within which the Big Dipper resides.

ROB: Phecda is a rare class Ae star that is rotating at nearly one-hundred-seventy kilometers-per-second at its equator – that’s eighty-four times faster than our Sun!

TAVI: Phecda is also known as Phad.

ROB: Sliding across, to the outside edge of the bowl, you’ll find Merak, at the bottom, and Dubhe, at the top. Just as Megrez is the back leg of the great bear, Merak is the flank, or groin, and Dubhe is the back, or spine.

TAVI: Scientists have detected a dusty disk around Megrez, extending out to what would be the orbit of Saturn, but unlike Alioth’s extended material, Megrez’s disk may well be a harbinger of planets.

ROB: In visual, size, and temperature contrast, Dubhe is a cool, class K orange-giant measuring some thirty-times the radius of our Sun.

TAVI: Despite its “Alpha” designation, Dubhe is the third brightest star in Ursa Major.

ROB: And like Mizar, Dubhe is a quadruple system; it’s closest companion, at just twenty-four astronomical units, is a smaller sun-like star with an orbital period of forty-four years.

TAVI: For sky wanderers, Merak and Dubhe lead to Leo’s Regulus. More significantly, these two bright stars point the way toward Polaris – that north celestial pole star more commonly known as the North Star!

ROB: And it is this North Star, located by way of the Big Dipper, that has so effectively enabled celestial navigation throughout human history.

TAVI: Some of that Big Dipper and North Star navigation history includes Phoenician circumnavigation of the African continent,

ROB: and ancient Pacific Ocean trade routes;

TAVI: as well as Civil War Era slaves escaping southern states,

ROB: and Columbus’ expeditions to the Americas.

TAVI: And even today’s space-faring astronauts!

ROB: Peering a little deeper into the dipper, specifically with a telescope, you’ll find dozens of galaxies, most ranging from about 9th to 12th magnitude, and each with its own beautiful structure.

TAVI: Some of the even brighter, better-known galaxies populating the Big Dipper include M101 and M51, on either side of Alkaid;

ROB: M106, residing as the tip of an elongated triangle with Phecda and Alioth, in a direction towards Canes Venatici;

TAVI: M109, less than one-half degree south of Phecda; and M108, less than one-degree south of Merak.

ROB: And, of course, there is that famous pair, M81 and M82, along a line between Phecda and Dubhe.

TAVI: Oh, and we can’t forget that enigmatic little bird, the Owl Nebula, just one degree from Phecda!

ROB: The Big Dipper is also home to a special region of space viewed by the Hubble Space Telescope. Just a finger-width above Alioth and Megrez, there lies a patch of space uncluttered by any stars from our own Milky Way galaxy. From December 18 to December 28 – ten consecutive days in 1995 – Hubble stared at this one patch of sky and assembled a view some ten-billion light years into the past.

TAVI: Yes! In that small speck of sky, the equivalent width of a dime viewed at seventy-five feet, Hubble imaged over fifteen-hundred galaxies.

ROB: Many consider this work some of the most important images ever acquired, as it demonstrates the vastness of the cosmos. When we look up at the Big Dipper, we are reminded of this legacy of the Hubble Space telescope and of our exceedingly tiny place in the Universe.

TAVI: Too often, we are so entranced with the beauty of the sky as we see it, that we forget to look even further, to explore even greater beauty beyond.

ROB: Next time you’re outside looking up, think about those things you can’t see – the civilization-shaping histories and the hidden celestial treasures.

TAVI: You could even start, tonight, with the Big Dipper!

ROB: Thank you for listening. This is Rob Keown

TAVI: and Tavi Greiner

ROB and TAVI: wishing you all a dark sky full of bright stars!

End of podcast:

365 Days of Astronomy
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