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


Photo of the Week.A darkening sky from which emerge the stars.

Astronomy news for the week starting Friday, September 26, 2014.

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. Last week's Skylights is still available. 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 Harp in Stellar Stories.

The Moon starts our week as a thin waxing crescent just barely visible in western twilight the evening of Friday, September 26, and then grows steadily fatter as it climbs the sky toward first quarter, 90 degrees east of the Sun, the phase passed on Wednesday, October 1, with the Moon visible in the daylight sky. We then get to see a bit of the waxing gibbous as the week comes to an end. The evening of Saturday the 27th, the crescent will make a classical appearance just to the west of Saturn, looking like it is about to gobble the planet. And in a sense it will, as the Moon will actually occult Saturn as seen from various parts of the eastern hemisphere (but not here). In a remarkable coincidence, the Moon will also occult the asteroids Ceres and Vesta within a few hours of covering Saturn, again neither of the events visible from North America. The following night, that of Sunday the 28th, the Moon will appear to the right of Mars, while the evening of Monday the 29th it will lie atop both Mars and (farther down) Antares. On the evening of Wednesday, October 1, the growing crescent, now near first quarter, will glide north of the Little Milk Dipper of Sagittarius.

As is obvious from the lunar passage, Saturn is sinking out of sight, being lost to the brightness of twilight. It's been a good run over the past several months. Mars, though, falls only slowly behind the Earth in orbit, and is moving quickly to the east relative to the background stars. It not only stays quite visible throughout the remainder of the year, but even improves some relative to the end of twilight as winter approaches, the red planet now setting about an hour and a half after the sky gets fully dark. The big Martian event is its conjunction with the similarly-colored star Antares (the planet three degrees to the north) on Saturday the 27th. We are then planetless until Jupiter makes its striking appearance, rising around 2:30 AM Daylight Time in eastern Cancer well to the southeast of the Beehive Cluster. We can forget about Venus until it moves into evening twilight towards the end of the year.

With all the prominent constellations in the early autumn sky, we tend to overlook the smaller ones. About three- quarters of the way from bright Deneb (at the top of the Northern Cross) to Altair (immediately recognizable by its two flanking stars, which give it the appearance of flying across the sky) find exquisite Delphinus the Dolphin (which looks like a hand with a finger pointing more or less southward), and Sagitta, the Arrow, which pretty much looks like what it is supposed to be. Why Delphinus was not called simply "Manus" (the Hand)," is a mystery that is unlikely ever to be solved.

STAR OF THE WEEK: 1 DEL (1 Delphini). One might think that Number 1 in Delphinus, the Dolphin, would be right at the forefront of study. Number 1 and all that. But no, it's merely the westernmost naked eye (or close) star within the swimming constellation, Flamsteed numbers always going from west to east within a figure's rather flexible eighteenth century boundaries. Yet the star has an importance belied by its dim sixth magnitude (6.08) status. 1 Del is a class A (A1) "emission-line shell star," one with a surrounding equatorial disk that radiates the hydrogen spectrum. Most such stars are in class B, and constitute the populous set of "Be" stars epitomized by Gamma Cassiopeiae and Zeta Tauri. "Ae" stars on the other hand are rare. In a sense, they are the spillover into class A from hotter class B. But the bizarre world of astronomical nomenclature strikes again. Do not confuse them with Herbig Ae/Be stars like AB Aurigae, the class named after George Herbig, (1920-1913). While they too have equatorial disks, they are young protostars caught in the act of formation. Just under a second of arc away from 1 Del (last measured at 0.9 seconds) is an eighth magnitude companion. The magnitude difference and the original combined value yield respective magnitudes of 6.27 and 8.09 for A and B. In spite of their position in the Milky Way and distance of 743 light years (give or take a large uncertainty of 117), it's surprising that there is no significant dimming by interstellar dust. Respective but uncertain (again) temperatures of 9160 (measured?) and 9100 (estimated) Kelvin for 1 Del A and B, needed to account for ultraviolet light, plus distance give luminosities of 139 and 26 Suns and radii of 4.7 and 2.1 times solar. Be (and as here, Ae) stars are characterized by very rapid rotations, and 1 Del A is no exception, the equatorial spin speed given from 217 to 320 kilometers per second. That 1 Del is a shell star implies that the disk is more or less on edge (forming something of a shell around the star), so that the rotation axis should be fairly perpendicular to the line of sight, making the observed average rotation speed of 270 km/s probably close to the mark. 1 Del A then rotates in under 0.9 days. How rotation is linked to the Be (or Ae) phenomenon is not clear. Magnetic fields may be involved. Theory applied to luminosity and temperature gives a mass of two (2.1) Suns to 1 Del B, and show it to be a hydrogen-fusing dwarf, while 1 Del A, at three (3.0) solar masses, is apparently a subgiant that, with a dead helium core, is just beginning its journey into helium-fusing gianthood, after which it will toss away its outer layers to become a white dwarf of some 0.7 solar masses ("B" someday turning into a lighter one of 0.55 Suns). At a physical separation of at least 200 Astronomical Units, 1 Del B takes more than 1300 years to make a full circuit around 1 Del A. Much father off is 1 Delphini C. Over the past 120 years it has maintained a separation of just under 17 seconds of arc, and therefore seems to be keeping a nice pace with the inner AB pair. More than 3900 AU away from the close duo, it takes over 100,000 years to orbit. Maybe "number 1" is apt after all.

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.