Friday, September 5
Saturn, Mars, Delta (δ) Scorpii, and Antares form an equally-spaced ragged line in the southwest at dusk, as shown at right. Delta Scorpii used to be a bit dimmer than Beta above it. Then in July 2000 it doubled in brightness. It has remained bright, with slow fluctuations, ever since.
Look high above the Moon this evening for Altair.
Saturday, September 6
This is International Observe the Moon night! Zoom in on the map to find an event near you. Or set up your own telescope for the public and add your event to the map so others can find you! The Moon is waxing gibbous, two days from full. (Just make sure it'll be in view from your site at the times you announce for people to come!)
Also, look to the right of the Moon, by a little more than a fist-width at arm's length, for two faintish (3rd-magnitude) stars: Alpha and Beta Capricorni, one above the other. Alpha is the one on top. With sharp vision, you can barely see that it's double. Binoculars resolve it easily.
Sunday, September 7
Look for bright Vega passing your zenith in late twilight, if you live in the world's mid-northern latitudes. Vega goes right through your zenith if you're at latitude 39° north (near Baltimore, Kansas City, Lake Tahoe, Sendai, Beijing, Athens, Lisbon). How closely can you judge this?
Monday, September 8
Full Moon (exact at 9:38 p.m. EDT). The Moon shines in dim Aquarius. To its upper left in the evening, the western side of the Great Square of Pegasus points down toward it (more or less). This as another perigean "supermoon," the third in a row.
Tuesday, September 9
Arcturus is the bright star fairly high due west at nightfall. It's an an orange giant 37 light-years away. Off to its right in the northwest is the Big Dipper, most of whose stars are about 80 light-years away.
Wednesday, September 10
The gibbous Moon, still big, rises in the east in late twilight. Look well above it for the bottom corner of the up-tilted Great Square of Pegasus.
Thursday, September 11
A winter preview: If you're up before dawn this week, the sky displays the same starry panorama as it will at dusk early next February: Orion stands high in the southeast, Sirius and Canis Major sparkle to Orion's lower left, Gemini occupies the east to Orion's left, and Jupiter shines far below Gemini's Castor and Pollux. Come February, Jupiter will still be near there.
Friday, September 12
Antares, Mars, and Saturn now form an almost equally spaced straight line, as shown at right. Mars continues to move east against its starry background. Watch for it to pass 3° north of Antares on September 27th and 28th.
This evening one of Saturn's moons, 10th-magnitude Rhea, will occult (cover up) a much brighter star, 7.8 magnitude SAO 159034, as seen from North Carolina northward. Telescope users farther south will witness a near miss. Details and illustrations are in our article, See Saturn’s Moon Rhea Hide a Star.
Saturday, September 13
How soon after sunset can you make out the big Summer Triangle? Vega, its brightest star, is nearly at the zenith (for skywatchers at mid-northern latitudes). Deneb is the first bright star to Vega's east-northeast. Altair shines less high in the southeast.
Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.
This is an outdoor nature hobby; for an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.
You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.
Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability, which means fairly heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."This Week's Planet Roundup
Mercury (magnitude 0.0) remains quite deep in the sunset, as it always does during its evening apparitions that happen in late summer and early fall. Scan for Mercury with binoculars just above the western horizon about 20 minutes after sundown. Don't confuse it with Spica to its upper left.
Venus (magnitude –3.9) and Jupiter (magnitude –1.8) shine in the east during dawn. They continue moving farther apart: Jupiter is moderately high now, and Venus is quite low (to Jupiter's lower left) as sunrise approaches.
Mars and Saturn (magnitudes +0.7 and +0.6, respectively) glow in the southwest at dusk, moving farther apart day by day. Saturn is the one on the right. To Mars's left are fainter Delta Scorpii, and then Mars's starry namesake, Antares.
Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are high in the southeast and south, respectively, by midnight. See our Finder charts for Uranus and Neptune online or in the September Sky & Telescope, page 50.
All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.
Eastern Daylight Time (EDT) is Universal Time (UT, UTC, or GMT) minus 4 hours.
“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”
— Neil deGrasse Tyson, 2014.
International Observe the Moon Night is an event that encourages people to "look up" and enjoy our nearest neighbor. This year's InOMN is Saturday, September 6th.
Here's a quiz: What astronomical object looks amazing no matter what the magnification, never looks exactly the same no matter how often you view it, and can be observed even when not a single star is visible in the sky?
Answer: The Moon!A crescent Moon joins Venus on the evening of September 11, 2010, as seen by skywatchers in Firuzkuh, Iran.
The Moon can be a wonderful viewing target no matter what your level of experience or how well you're equipped to view the night sky. Whether seen by eye, through binoculars, or with a telescope, it's always worth a look.
So here's your chance to join a "group viewing" of our closest neighbor world: this Saturday, September 6th, is "International Observe the Moon Night." It's the brainchild of several "Moon Units" within and related to NASA: the Lunar Reconnaissance Orbiter mission, Lunar Science Institute, Lunar and Planetary Institute, and Lunar Quest. Partnering with the Astronomical Society of the Pacific, these teams hope to rekindle public interest in observing and studying the Moon.
Participation couldn't be easier: just go outside on the 6th, and look up! You'll be greeted by a lovely, not-quite-full orb seemingly gliding across the southern half of the sky throughout the night. To take the next step, join one of the many organized activities being planned; check out the event map to find one near you. As just one example, Gianluca Masi plans a live webcast of telescopic viewing (starting at 20:00 UT, or 4 p.m. EDT) using his Virtual Telescope Project.InOMN
Or make your own event. Head for a pedestrian hotspot in your town with a telescope in tow, and crowds will surely gather for a look through the eyepiece. (Trust me: no matter how bad your light pollution, the Moon is still an arresting sight when seen telescopically.)
Even if your scope-less, the InOMN organizers have pulled together lots of entertaining online content for various sources, such as a lunar-exploration timeline, a guided tour, and a call for Moon-inspired poetry. The event's portal at NASA's Goddard Space Flight Center is here.
If you've got the time and inclination to view the Moon multiple times in the coming weeks, I highly recommend Patrick Thibault's can't-miss Month of Moonwatching.
Want to compare the Moon's nearside and farside for yourself? Check out Sky & Telescope's terrific new lunar globes. Choose either the natural-hued Moon you see by eye or the color-coded topographic version.
Measurements of thousands of galaxies’ motions toward and away from us reveal that the Milky Way sits on the edge of a gigantic supercluster of galaxies.
Astronomers have mapped the cosmic watershed in which our Milky Way Galaxy is a droplet. The massive structure, which the research team dubs the Laniakea Supercluster, extends more than 500 million light-years and contains 100,000 large galaxies.
The work, published in the September 4th Nature, is the first to trace our local supercluster on such a large scale. It also provides a physical way to define what a supercluster actually is.
Researchers have been working out the gravitational structure in our local universe for decades. Based on work by Gerard de Vaucouleurs in the 1950s, astronomers have thought of our galaxy as being on the edge of the so-called Local Supercluster, a structure about 100 million light-years wide that’s centered on the Virgo Cluster of galaxies.
But astronomers have also seen much larger structures in the universe, on the scale of several hundred million light-years, thanks to the Sloan Digital Sky Survey and other work. These maps have generally depended on calculating galaxies’ 3-D locations based on the galaxies’ cosmological redshifts, the shift in a galaxy’s spectral lines due to the galaxy’s apparent motion as the universe itself expands.
Brent Tully (University of Hawaii, Honolulu) and colleagues have taken a different approach. They used galaxies’ peculiar velocities, which are the galaxies’ motions due to the local gravitational landscape. Galaxies fall toward or away from one another in this landscape; the Milky Way and many others seem to be moving toward what’s called the Great Attractor, a dense region in the vicinity of the Centaurus, Norma, and Hydra clusters about 160 million light-years away.
Peculiar velocities are on the order of a few hundred kilometers per second, whereas the cosmic expansion velocities rise to 10,000 km/s roughly 130 million light-years away. (Due to the nature of cosmic expansion, a galaxy recedes faster the farther away it is.) There’s about 10-20% uncertainty in the peculiar velocity measurement for an individual galaxy, says Tully. So only for nearby galaxies is an individual system’s peculiar velocity high enough compared with its expansion velocity for astronomers to peg it confidently. Farther out, the data are sparser.
But the team found a way around this problem by using an analysis technique called Wiener filtering. This algorithm allowed the team to essentially take a step back and look at the big picture, revealing the large-scale flow patterns created by galaxies’ motions. With this wide-field view, the individual uncertainties don’t matter so much.
Last year, the team used this technique to map the local universe’s web of filaments, clusters, and voids, creating a fascinating video simulation of the flow patterns in the gravitational watershed. Now, they’ve taken a closer look using their Cosmicflows-2 catalog, which contains more than 8,100 galaxies. The new catalog reveals where the flows merge and diverge, unveiling a gargantuan structure on whose periphery the Milky Way sits. The Great Attractor is a central valley in this newly demarcated watershed.
The team calls this huge supercluster Laniakea, from the Hawaiian lani (heaven) + akea (spacious, immeasurable).
The analysis also reveals other structures, including a separate supercluster called Perseus-Pisces and a distant concentration named Shapley, which lies about 650 million light-years away and toward which Laniakea is moving.
The team put together an intro video, which I'm embedded below:
Cosmologist Elmo Tempel (Tartu Observatory, Estonia) says that, due to the nature of the analysis, he’s confident that the Laniakea structure exists. “However, the exact boundaries of Laniakea are not so well established, and they may change if more measurements are carried out,” he cautions.
Because stringent distance measurements (on which the peculiar velocity calculations depend) are much rarer beyond about 300 million light-years, it’s hard to make out what’s going on out there. Given that Laniakea is moving toward Shapley, our supercluster might indeed be only the trunk of the elephant, Tully says. For now, all he and his colleagues can say is that they’ve isolated a “local basin of gravitational attraction.” Finding out whether it’s an appendage of something larger will require accurate distance measurements that reach three times farther than the current catalog.
Nature has also created an in-depth video on the discovery, which I recommend watching.
R. B. Tully et al. “The Laniakea supercluster of galaxies.” Nature. September 4, 2014.
H. M. Courtois et al. “Cosmography of the Local Universe.” Astronomical Journal. September 2013.
Explore the galaxies in our local universe with our eBook Summer Deep Sky.
Watch as the moon Rhea steals a star from the sky for nearly a minute on September 12th.
Thumbing back through my memory, I can't recall ever seeing this rarity. Hmmm ... I must have some reason to fly to New York that evening.
For those in the right location, a clear sky and modest telescope are all you'll need that Friday evening.
Probably the biggest challenge will be to scout out an observing site with an open view to the southwest. At best, Saturn manages only 8-12° altitude. Stray clouds or dense haze might also be deterrents, but I trust you'll persevere.
10.2 magnitude Rhea covers 7.6 magnitude SAO 159034 for up to 58 seconds - depending on how far you are from the midline of the shadow path - at 8:38-39 p.m. Eastern Daylight Time. While Saturn's low altitude likely means soft images from air turbulence, because the magnitude drop is large, the occultation should be obvious in a 60-mm telescope and possibly even in binoculars.
The occultation map below shows the centerline crossing the heart of the U.S., Canada and Alaska. That would be fine if the Sun were far enough below the horizon before the event. Sadly, it won't be. Everyone outside the east will be thwarted by bright twilight or sunlight. For Midwesterners, the Sun sets around the same time as the occultation.
Still, all of us will get to see the Saturnian system temporarily invaded by a bright star posing as a moon. As both Rhea and Saturn go their separate ways, SAO 159034 will appear to slowly drift away from the planet, soon giving itself away as an intruder.
We know Rhea's diameter with precision thanks to 10 years of observations by the orbiting Cassini probe, but new information about the occulted star may come to light. Although SAO 159034 is listed as a single star, occultations can sometimes reveal that seemingly solitary stars have extremely close companions that reveal themselves as stepwise drops of the star's light.
However you enjoy the event, consider that Rhea, at 949 miles (1,528 km) across, will perform that favorite occultation sleight of hand where the little guy gets to eclipse the big guy, in this case a star estimated at some 121 million miles (195 million km) across.
Learn to navigate the sky with Sky & Telescope's Pocket Sky Atlas.
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A new analysis suggests that hot super-Earths might be the skeletal remnants of hot Jupiters stripped of their atmospheres.
Most alien planets are unlike any planet in our solar system. Hot Jupiters, for example, are broiling gas giants circling closer to their stars than Mercury orbits the Sun. Astronomers suspect that the star-planet tidal interaction will ultimately drag a hot Jupiter inward toward its doom.
More recently, astronomers have discovered a second class of star-hugging planets in the wealth of data from NASA’s crippled Kepler space telescope. These so-called hot super-Earths are rocky or icy planets that can be up to 10 times Earth’s mass and also orbit extremely close to their host stars.
Astronomers have speculated that the two classes may be related. But Francesca Valsecchi (Northwestern University) and her colleagues now take this a step further, suggesting that these odd planets are stripped hot Jupiters. Instead of forming as super-Earths, they are the skeletal remnants of gas giants peeled of their atmospheres.
The underlying theory is relatively simple. As the exoplanet spirals in toward its host star, the system will eventually reach the point where the two Roche lobes touch.
“The Roche limit or Roche lobe is the region around a planet or star (or moon or lump of bread dough) where that object's gravity dominates — it's a ‘sphere of influence’ so to speak,” explains coauthor Jason Steffen (also at Northwestern University).
When a fluffy star in a binary system overflows its Roche lobe, it can pour material down onto its smaller, denser companion star. In a similar way, when a hot Jupiter reaches the point where its Roche lobe and its star’s Roche lobe meet, the interaction opens a gravitational path along which mass can transfer from the exoplanet to the star. So the hot Jupiter inevitably starts shedding its gaseous envelope.
Valsecchi and colleagues modeled this transformation for several different cases. But they started early in the planet’s history, placing the hot Jupiter not under the glare of its bright star, but in the chilly outer reaches of the planetary system where astronomers think gas giants first form. Due to the star-planet tidal interaction, the planet migrates inward toward the star. But once the planet’s Roche lobe reaches the star’s Roche lobe, something interesting happens. The planet’s orbit reacts to the mass transfer by moving slightly outwards.
But this slight movement doesn’t stop it from shedding its entire atmosphere. Once the rocky or icy core is exposed, tidal forces take over again, causing the orbit to shrink once more and bring the planet close enough for the star to swallow it, explains Valsecchi.
“Broadly the idea makes sense,” says expert David Trilling (Northern Arizona University). “The only evidence will be indirect, so the question is really whether this theory explains the observational evidence better than all other competing theories.”
Trilling and colleagues first mentioned this idea briefly in a paper published in 1998. But at the time, only a few hot Jupiters had been detected and no hot super-Earths. We’re now in a much better position to understand how planets might transition between classes.
The research team did compare their results to observations, finding that most known hot super-Earths have similar orbital periods and masses to those modeled.
If the results hold, hot Jupiters might be about three times as common as what astronomers have inferred directly from observations, because the number of single super-Earths observed is nearly twice the number of hot Jupiters.
Francesca Valsecchi et al. “From Hot Jupiters to Super-Earths via Roche Lobe Overflow” Astrophysical Journal Letters, Accepted
Como julio y agosto han sido meses escasos de artículos, hemos compilado los dos en un solo número. Dado que es algo más corto de lo normal, está disponible para todo el mundo –mecenas o no– aquí mismo.
La versión PDF tiene unas 50 páginas A4, y además hay versiones epub, mobi, fb2 y html. Como siempre, las versiones epub/mobi/fb2 son gracias a la colaboración incansable de johansolo.
En el número de julio/agosto:
Las cuatro fuerzas - La gravedad (III)
Premios Nobel - Física 1920 (Johannes Stark)
[Matemáticas I] Vectores
¡Que lo disfrutéis!
The astronomical calendar says autumn arrives on September 22nd. It's a season of transition, with plenty of celestial comings and goings in the evening sky.
September’s equinox takes place on the 22nd at 10:29 p.m. Eastern Daylight Time. At that moment the Sun shines directly overhead as seen from the equator. Days and nights are both 12 hours long — that’s where the word equinox comes from — and no matter where you live the Sun rises due east and sets due west.
As darkness falls on September 1st, look for the planets Mars and Saturn to the Moon’s lower right. At month’s end, the Moon will have gone through nearly a complete cycles of phases and return to this stretch of sky. But by then Mars and Saturn will have shifted somewhat. Look for a very thin crescent Moon to Saturn’s right on the 27th. The ringed planet will soon sink from view.
Vega is right overhead at nightfall; use it to find the trapezoid-shaped head of Draco, the Dragon, about 1½ fists to Vega’s north. From there a long line of Draco's stars snakes along a curving line between the Big and Little Dippers. A second trapezoid, to the west of Vega, marks the Keystone of the constellation Hercules, a great hero in Roman mythology. Look beyond the Keystone toward west, a little more than one fist, to spot a lovely semicircle of stars called Corona Borealis, the Northern Crown.
Even if light pollution makes these groupings (and the Milky Way) difficult to see, the Summer Triangle — comprising the bright "alpha stars" Vega, Deneb, and Altair — is unmistakable overhead.
You can get more helpful guidance about stargazing in September 2014 by downloading the 6¾-minute-long audio podcast below.
And there's even more great skywatching advice in the September issue of Sky & Telescope magazine.