M81 and M82 from HST

The galaxy pair M81 and M82 in Ursa Major must rank near the top of the list of best-loved objects for any Northern Hemisphere amateur astronomer.  So, to see such a familiar object as these in breathtaking Hubble Space Telescope detail is thrilling indeed:

Messier 81 from the Hubble Space Telescope – click on the image for a larger view
Messier 82 from the Hubble Space Telescope – click on the image for a larger view

M81 and M82 lie little more than a moon-width apart in the constellation Ursa Major, 11.8 million and 11.5 million light years, respectively, from Earth.  Check out this pretty pair with either binoculars or a telescope any clear evening during the next few days.  Both galaxies transit the meridian on April 14 at the end of evening twilight, so this is the perfect time to observe them at their highest in the sky.  You can find Bode’s Galaxy (M81) and the “Silver Sliver” (M82) by drawing an imaginary diagonal across the bowl of the Big Dipper, opposite (rather than along) the handle, and extending the diagonal beyond the bowl almost as far as the two bowl stars are apart. Or, using the chart I created below, draw an imaginary line between Dubhe and 24 UMa, then go about four-fifths of the way to 24 UMa.  M81 & M82 lie about 0.4° (a little less than a moon-width) perpendicular to that line on the Polaris side.  Bingo, you’ve got ’em!

Skyline to M81 (and M82)

 

Iapetus – Wow!

Saturn’s third largest moon, Iapetus (eye-AP-eh-tuss), was discovered at the then-new Paris Observatory in 1671 by Italian-French astronomer (and observatory director) Giovanni Domenico (Jean-Dominique) Cassini (1625-1712).  Upon further observation, Cassini noted that he could only see Iapetus when it was on the west side of Saturn, never the east.  His telescope was not large enough to detect Iapetus on the east side of Saturn because it was much fainter then.  He correctly reasoned that, “it seems, that one part of his surface is not so capable of reflecting to us the light of the Sun which maketh it visible, as the other part is.”  He also must have realized that Iapetus was locked in synchronous rotation—as is our Moon—with the same side facing Saturn all the time, with its rotation period being equal to its orbital period.  Today we know these periods to be 79.3215 days.

The leading hemisphere of Iapetus has a visual albedo of only about 5%, whereas most of the trailing hemisphere is much brighter, having an albedo around 25%.  Thus, when Iapetus is on the west side of Saturn, its apparent visual magnitude is around 10.2, but on the east side of Saturn Iapetus is 1.7 magnitudes fainter at 11.9.  Without a doubt, Iapetus is one of the most outlandish places in the solar system, and the Cassini Saturn orbiter flybys certainly amplified the strangeness.

Cassini made one close targeted flyby of Iapetus on September 10, 2007, passing within 762 miles of the surface.  Here are a few of the best photos of Iapetus from Cassini.

The first high-resolution glimpse of the bright trailing hemisphere of Saturn’s moon Iapetus
This is a raw, or unprocessed, image taken by the Cassini spacecraft during its close flyby of Saturn’s moon Iapetus on Sept. 10, 2007 showing its prominent equatorial ridge—still a mystery
The “Himalayas” of Iapetus
The Transition Zone
Closest View of Iapetus
Dark material splatters the walls and floors of craters in the surreal, frozen wastelands of Iapetus
May 30, 2017 – Cassini bids farewell to Saturn’s yin-and-yang moon, Iapetus

The dark material appears to have been deposited from elsewhere in the Saturnian system, but sublimation of water ice may also play a role.  In any event, the dark material is a relatively thin veneer, significantly less than a meter thick in many places.

The warm day on Iapetus sees a surface temperature of -227° F on the dark terrain and an even colder -256° F on the bright terrain.  Inhospitable, to say the least!