Skylights featured on Astronomy Picture of the Day

Scout Report Selection Webivore Selection SpaceCareers Selection

Skylights featured nine times on Earth Science Picture of the Day: 1 2 3 4 5 6 7 8 9

Convective cloud

Photo of the Week. It's spring.

Astronomy news for the two weeks starting Friday, April 8, 2016.

Phone: (217) 333-8789
Prepared by Jim Kaler.

Clear skies and thanks to Skylights' blogger visitor reader.

Go to STARS for previous stars of the week. Access Skylights' Archive and photo gallery. From the Sun to the Stars: the OLLI Lectures provides a linked, illustrated introduction to astronomy.
The Constellations has a linked list with locations and brightest stars. Constellation Maps show the locations of the constellations. The 170 Brightest Stars lists them through magnitude 3.00. For more on stars and constellations, visit Stellar Stories.
Tour the Milky Way. Watch a total eclipse of the Moon and an annular eclipse of the Sun. Moon Light presents scenic photos of the Moon. Go to MoonScapes for labelled telescopic images of the Moon and other lunar information.
See the Moon move and pass just below Nu Virginis. Watch planets move against the background stars. See a classic proof of the curvature of the Earth with a "hull down" series. Visit Measuring the Sky to learn about the celestial sphere.
Admire sunsets, rainbows, and other sky phenomena in Sunlight. Read the illustrated Day Into Night on the phenomena of the sky See the The Aurora and the Midnight Sun. See and understand the ocean tides.
Enjoy Our Complex Universe: A Human Understanding through Art, with 12 illustrations. Advances in Astronomy, 1989-2011. Take a ride aboard Asteroid 17851 Kaler (1998 JK). Look for Books about the sky and stars.


ASPSupport science literacy by joining the Astronomical Society of the Pacific, an international organization that is among the world's premier providers of astro education. Get Mercury and a variety of other benefits.

Presenting three audio courses with 70 to 100-page study guides, narrated and written by Jim Kaler.
Heavens Above: Stars, Constellations, and the Sky from Recorded Books. Astronomy: Earth, Sky, and Planets, is available from Recorded Books. Astronomy: Stars, Galaxies, and the Universe, is also now available from Recorded Books.
Astronomy: Earth, Sky, and Planets is published as Vault of the Heavens: Exploring the Solar System's Place in the Universe by Barnes and Noble.

Enjoy Our Complex Universe:A Human Understanding through Art, with 12 illustrations.

Read "Heaven's Touch: From Killer Stars to Seeds of Life, How We Are Connected to the Universe," Princeton University Press.

SSTo learn about stellar spectra, read STARS AND THEIR SPECTRA: An Introduction to the Spectral Sequence, Second Ed., with two new chapters and 140 new illustrations, Cambridge University Press (UK or North America), 2011.

Read From the Sun to the Stars: the OLLI Lectures, which provides a linked, illustrated introduction to astronomy.

SSNEWEST! FIRST MAGNITUDE: A Book of the Bright Sky, World Scientific, 2013. Read the interview with Jim Kaler.

NEW! Read The Queen in Stellar Stories.

The next skylights will appear April 22.

Over the fortnight our Moon, our only natural satellite (unless you want to count some temporary ones that have drifted in from the asteroid belt), grows from a slim waxing crescent until it hits first quarter the night of Wednesday, April 13th, falling between the Castor-Pollux pair in Gemini and bright Procyon of Canis Minor, Orion's smaller hunting dog. The Moon then grows in the waxing gibbous phase until it reaches full the night of Thursday the 21st, the exact phase taking place about midnight in North America with the Moon to the east of Spica in Virgo. Moving slightly away from us during our two weeks, the Moon passes apogee, where it is farthest from Earth, on Thursday the 21st 11 hours before full, which will weaken tides at the coasts.

The night of Friday the 8th, the narrow crescent will appear up and to the left of Mercury. Two evenings later, on Sunday the 10th, the crescent will lie just barely east of Aldebaran and the Hyades cluster having occulted the star during daylight in the late afternoon. A telescope is needed to see the event. The night of Saturday the 16th the waxing gibbous Moon will fall right beneath the star Regulus in Leo. The following night it will glide just a couple degrees south of much brighter Jupiter.

Mercury, visible to the west in evening dusk, is making one its better appearances, passing greatest elongation to the east of the Sun on Monday the 18th. This is one of those rare times when Mercury will actually set after twilight ends. Obvious Jupiter transits the meridian about an hour and a half before midnight, followed about half an hour later by the rising of Mars, then after another half hour the rising of Saturn. Mars is making one its great appearances north of Antares in Scorpius, the star named for its Mars-like color ("Ares" the Greek version of Mars). Mars, which makes a fine triangle with Saturn and Antares, begins retrograde motion (westerly against the stars) in preparation for its opposition to the Sun on May 22. In the outer part of the solar system, Uranus is in conjunction with the Sun on Saturday the 9th, while Pluto begins retrograde motion northeast of the Little Milk Dipper of Sagittarius on Monday the 18th.

Follow the curve of the Big Dipper's handle to orange Arcturus, the brightest star in the northern celestial hemisphere, and then to blue-white Spica, the luminary of Virgo. To the south of Spica are the eastern stars of long Hydra, the Water Serpent, and yet farther south those of northern Centaurus, the Centaur. If you are far enough south, below say 25 degrees south latitude, you can catch Crux, the Southern Cross, and to the east of it Beta then Alpha Centauri, the latter the closest star to the Earth.

STARS OF THE WEEK: OMI-1 AND -2 CEN (Omicron-1 and Omicron-2 Centauri, another two-for-one special). Way down in southern Centaurus (the eponymous Centaur), in a tongue of the constellation that dips between Crux (the Southern Cross) and Carina (the Keel of Argo), lies what appears to be a naked-eye double made of two fifth magnitude stars, Omicron-1 (the western one) at magnitude 5.13 and Omicron-2 at 5.15. Most unusual is that they are both bright supergiants, class G (G3) Omi-1 separated from class A (A2) Omi-2 by only 4.6 minutes of arc. Omi-1 (which may be as cool as F7) has even been called a hypergiant. While they do not compose a true double, they are almost certainly related. Both stars are very far away, from their parallaxes 5720 and 5350 light years. While the statistical uncertainties are huge, half the sizes of the measures themselves, the similarity of the distances lend them some credence, so we'll adopt an average of 5530 light years for both. The motions, while not exactly the same, are similar too, both stars moving toward us but to the northwest against the background, Omi-1 at 43 kilometers per second, Omi-2 at 58 km/s. Both stars are considerably dimmed and reddened by interstellar dust. Averaging the observed colors suggests a dimming of 1.22 magnitudes, which again we'll apply to both. The measured (suggested?) temperatures for the stars are out of line with those derived from their classes, which are 5700 Kelvin for Omi-1, 8500 for Omi-2. Corrections for infrared or ultraviolet radiation then range from minimal to zero.

With all this out of the way, Omi-1 shines with the light of 68,000 Suns, Omi-2 with 61,000, which lead to respective radii of 270 and 115 solar radii. With a radius of 0.53 Astronomical Units, Omi-2 is some 35 percent bigger than the orbit of Mercury, while Omi-1 reaches out to 25 percent larger than that of the Earth. While there is no rotational information for Omi-1, Omi-2 (with a projected equatorial rotation velocity of 43 km/s) could take as long as a third of a year to spin just once. Theory gives masses of 17 Suns for Omi-1, 16 for Omi-2, both well above the lower limit of 8-10 solar masses beyond which stars explode as supernovae. Both have ceased fusing hydrogen in their cores and may be preparing to fire up the helium ash (if they have not done so already). They are too far apart to be an orbiting double, the angular separation and distance yielding a physical separation of at least 7.4 light years, one and three quarters the distance between here and Alpha Centauri. At that distance apart, they would take at least 55 million years to orbit, five times longer than the stars' ages of 10-12 million years. While they are likely not a gravitationally-bound binary, they are almost certainly members of some long-lost association of O and B stars (both born as roughly class O9) that is now expanding into the cosmic void (their membership in the Carina OB1 association now rejected). Not surprisingly, the larger of the two, Omicron-1, is slightly variable, wobbling irregularly by a few tenths of a magnitude with maybe an occasional 200-day period. It also has an 11th magnitude "companion" a dozen seconds of arc away that from its motion is surely just in the line of sight. Assuming Omi-1 and Omi-2 are at the same distance from us, from each the other would be about as bright as our quarter moon. If this magnificent pair were in the northern hemisphere, you could be sure they would have been were far more heavily studied.

Valid HTML 4.0! Copyright © James B. Kaler, all rights reserved. The written contents and (unless otherwise specified) the photograph are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes.