Astronomy & Science

Dust Makes Cosmic Inflation Signal Iffy

Sky&Telescope -

A new analysis of Planck data provides the best measurements ever made of polarized dust emission across the sky — and bolsters the claim that the signal heralded as evidence for cosmic inflation is from dust instead.

Things are looking grim for the purported discovery of cosmic inflation’s fingerprints. Earlier this year, the BICEP2 team announced the detection of primordial B-modes, swirly polarization patterns imprinted on the cosmic microwave background (CMB) thanks to the hypothesized, split-second inflationary era. If inflation really did happen right after the universe’s birth, it should have spawned ripples in spacetime called gravitational waves, which in turn would have stretched and squeezed spacetime — and the plasma in it, ultimately fostering the creation of these polarization patterns in the CMB.

B-mode sky map

"Curly" B-modes of polarization in the cosmic microwave background. Each line is a measure of polarization at one point on the sky. This is an actual map of the sky near the south galactic pole, about 15° tall; the strongest curl patterns (emphasized with colors) are a couple of degrees wide, roughly the size of your thumb held at arm's length against the sky.

But other cosmologists quickly raised a red flag about BICEP2’s result, cautioning that the polarization detected might instead be from dust emission in the Milky Way itself. Irregular, charged dust grains interacting with our galaxy’s magnetic field can also produce the B-mode polarization patterns, and on the same angular scale as the theorized primordial ones.

Since the debate arose, the astronomy world has been waiting impatiently for the team of ESA’s Planck mission to cast the deciding vote. Planck’s all-sky CMB observations include polarization, and the mission’s researchers have been laboriously analyzing the data to separate the primordial signals from those originating closer to home. The team is being doubly careful because its preliminary data releases have inadvertently fueled the debate.

The galactic poles are the main regions of contention here. BICEP2 peered through the sparse galactic fog near the Milky Way’s south pole. More than a half dozen other B-mode experiments also focus on this Southern Hole. What everybody wants to know is, How much does dust emission contribute to the polarization signal in these galactic polar regions?

The Planck team has now released their preliminary analysis of the polarized dust emission near the galactic poles. The analysis doesn't include the detailed, multiple-frequency maps that will for certain settle the question, but it is still far and away the best measurement yet for this dust signal, says Planck scientist Charles Lawrence (JPL). What it shows is that dust is basically everywhere. "There is no escape from foregrounds, no part of the sky so clean that foregrounds can be ignored," Lawrence says.

Here's the punchline: the Planck team found that the strength of the dust signal is roughly of the same magnitude as the polarization signal reported by BICEP2.

No Final Answer Yet for B-modes

But we can’t write off the BICEP2 result just yet.

dust polarization emission at galactic poles

These kaleidoscope-esque maps show the estimated signal from polarized dust emission around the north (left) and south galactic poles, based on Planck data. Dark blue marks the “cleanest” areas. The maps are based on a fit to the angular power spectra, essentially a plot of how the strength of the dust signal changes with frequency across the sky. The black box in the south pole map roughly outlines the BICEP2 experiment’s field of view.
Planck Collaboration

First of all, the Planck analysis has sizable error bars, which the team is quick to point out. Second, there’s the ongoing issue of extrapolation. BICEP2 observed the CMB at a frequency of 150 GHz. The Planck analysis, on the other hand, uses 353-GHz data from the spacecraft’s High Frequency Instrument (HFI) and extrapolates down to what the emission should look like at 150 GHz. (See my detailed blog from June on why this is an important caveat.)

To do the extrapolation, the researchers used HFI data from 100, 143, and 217 GHz, as well as maps from various data subsets, to make sure they understood how the dust signal changes as they look at different frequencies. With this information in hand, they then took the 353-GHz data and created rough "150-GHz" maps based on how the dust signal's strength changes with frequency across the sky. The team stuck with 353 GHz instead of using the lower-frequency data to make the plots directly because at 353 GHz dust emission dominates over other polarization signals, meaning data at that frequency give the clearest picture of galactic dust.

The team stresses that the emission maps are estimates. But the analysis is exceptionally careful, and the results imply that polarized dust is the dominant signal in the BICEP2 field, says David Spergel (Princeton), who coauthored one of the cautionary analyses earlier this year. “There could be a weaker signal from gravitational waves, but I don’t think that the current data are good enough to separate out this weaker signal and make a statistically significant detection,” he sums up.

The Planck and BICEP2 teams are doing a joint analysis to get to the bottom of things. This analysis will hopefully be done in time for the big Planck conference in December, at which the Planck team will discuss the mission’s full temperature and polarization data (set to be released in November). These will include maps at all frequencies, Lawrence says, which will reveal features the averaging doesn't catch.

The new Planck analysis also picks out a few regions of sky that look the “cleanest” in terms of polarized dust emission. The largest region is, as expected, near the galactic south pole, as far from the dust-laden spiral disk as possible. Unfortunately, the BICEP2 field of view appears to have just missed the sweet spot. But other experiments — notably the Atacama B-mode Search (ABS) and the balloon-borne Suborbital Polarimeter for Inflation (SPIDER) — look right at it. (At least, if I’m doing my galactic-to-equatorial coordinate conversions correctly.)

Even if the BICEP2 result proves a gun-jumper, the experiment has helped to create the most sensitive polarization map for this sector of the sky, Spergel says. But it looks like it’s going to take even more sensitive measurements to discover primordial B-modes.


Reference: Planck Collaboration. “Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes.” Posted to September 19, 2014.

Read more about primordial B-modes and the race to detect them in our October 2013 issue.

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What’s Next for Inflation Cosmology – New Updates

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The July 2014 Sky & Telescope

The July 2014 Sky & Telescope cover

Our July 2014 cover story was the apparent discovery of gravitational waves from the instant of inflation when the Big Bang took shape. Just as the article was printed, a serious challenge to the discovery appeared: the researchers had underestimated the amount of interstellar dust that could be contaminating their data.

Here are more links regarding what may yet be the biggest cosmology discovery of the 21st century, and its dust-contamination problems, and how soon the finding might be confirmed or disproved.

  • The original BICEP2 2014 Results Release page — with the preliminary discovery papers, a public FAQ, image gallery, videos of the March 17th announcement at Harvard Observatory, and websites at the institutions involved.
  • Cosmology: Polar Star. Backstory on team leader John Kovac and the BICEP project, by Ron Cowen in Nature.
  • Multiverse Controversy Heats Up over Gravitational Waves, in Scientific American. Among some cosmologists, the multiverse is disreputable and politically touchy. And further evidence for inflation pushes it more into the spotlight.
  • Tabitha Powledge’s review of the immediate media coverage and scientific reaction, at her PLoS “On Science” blogsite.
  • BICEP isn’t the only project in the race. Here are ten B-mode searches underway in Antarctica, the Andes, the upper atmosphere, and in space, with links to their websites. Which will be the first to confirm or refute BICEP? Some of the searches will be much wider, deeper, higher-resolution, and/or multi-wavelength.
  • Added May 13: As the excitement died down, on May 9th the Kavli Institute held a roundtable discussion of where things stand and what comes next. Among the cosmologists in the discussion was skeptic Paul Steinhardt as devil's advocate. Transcript.
  • Added May 14: A Nature review of the picture now, and especially the future microwave-background projects that are being discussed — and their iffy funding prospects: Cosmology: First Light, by Joanne Baker (May 14, 2014).
  • Added May 29: Just dust?? Nature reports on two new papers that challenge the BICEP team's claim to have ruled out dust in the Milky Way as the cause of their polarization signal: “No evidence for or against gravitational waves,” subtitled "Two analyses suggest signal of Big Bang ripples announced in March was too weak to be significant," by Ron Cowen.
    The two papers are:
    – 1. Mortonson, M. M. & Seljak, U.; preprint at
    – 2. Flauger, R., Hill, J. C. & Spergel, D. N.; preprint at
    Also in Nature, the story of the supposedly misapplied dust correction: Gravitational wave discovery faces scrutiny (May 16).
  • Added May 30: Tabitha Powledge rounds up the evolving controversy as of May 23rd at her PLoS "On Science" news blog: Inflationary Universe data in question.
  • Added June 1: The dust-contamination situation. Science magazine has published the clearest account, as far as I've seen, of the possibility that much or all of the polarization signal may come from magnetically-aligned dust in the Milky Way filtering the cosmic microwaves. Unfortunately it's behind a $20 paywall, but you may be able to get free access through a library that subscribes. It's also in the print magazine, May 23rd issue, page 790.
  • Added June 5: Our own super-clear explanation: Big Bang Inflation Evidence Inconclusive, by Camille Carlisle.
  • Added June 21: The BICEP team's formal publication, with a description of their mistaken application of the preliminary Planck dust map, and why they still think most of their signal is cosmological: Detection of B-Mode Polarization at Degree Angular Scales by BICEP2 (June 19). With a link to a good review article by Larry Krauss. Here's a New York Times story on the paper's publication: Astronomers Hedge on Big Bang Detection Claim, by Dennis Overbye (June 19).
  • Added June 24: Does the Higgs boson rule out the BICEP claim? Two theoretical physicists say that if the inflation-era fluctuations of spacetime were as strong as the BICEP team originally announced (that is, r = 1.6 or so), the Higgs boson might have ended up in a different state that would have caused the universe to immediately recollapse: Electroweak Vacuum Stability in Light of BICEP2 (June 24). The official article is behind a paywall, but the preprint is free. Here's a press release explaining things.
  • Added Sept. 11: BICEP and Planck teams are collaborating and sharing data, and in her Backreaction blog, physicist Sabine Hossenfelder gets wind that Planck is going to announce something by the end of September about dust contamination... perhaps something inconclusive.
  • Added Sept. 22–24: Planck's announcement: Yes, lots of dust. But not necessarily enough to rule out a cosmological component in BICEP's signal. Clearer results may come later this year from the ongoing Planck-BICEP collaborative analysis. See our article, Dust Makes Cosmic Inflation Signal Iffy. Also, an excellent New York Times article (by former Sky & Tel staffer Dennis Overbye): Criticism of Study Detecting Ripples From Big Bang Continues to Expand. Here is the Planck team's paper. When asked for comment, Princeton cosmologist David Spergel told Sky & Telescope, "The Planck data implies that polarized dust is the dominant signal in the BICEP2 field. There could be a weaker signal from gravitational waves, but I don't think that the current data is good enough to separate out this weaker signal and make a statistically significant detection. Discovery of gravitational waves will await more sensitive measurements — fortunately, there are many groups pushing towards higher sensitivities."

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India’s Mars Orbiter Mission Arrives Safely

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On September 24th, after a convoluted, 10-month, 400-million-mile flight, India's first-ever interplanetary explorer fired braking rockets and slipped into orbit around the Red Planet.

MOM before launch

The Mars Orbiter Mission spacecraft, seen here prior to its launch in November 2013, employs a design used for other Indian spacecraft.

It's a historic day for India's maturing space program, as its first-ever interplanetary explorer successfully slipped into orbit around the Red Planet. The Mars Orbiter Mission, informally called Mangalyaan (Hindi for "Mars-craft"), became the planet's newest robotic satellite at 7:30 a.m. India Standard Time on September 24th (10:00 p.m. EDT on the 23rd).

MOM operated autonomously throughout the orbit-insertion sequence. At the time Mars was 139 million miles (224 million km) away, so radio confirmation of the craft's arrival took 12½-minutes to reach Earth. Adding further to the drama, the spacecraft was both out of view from tracking stations on Earth and hidden in the planet's shadow during portions of the engine firing.

Mission engineers and scientists, who'd gathered at the the Indian Space Research Organization's control center in the southern city of Bangalore, applauded when the main engine and eight smaller thrusters flared to life.

India's space control center

Applause erupts from flight controllers in Bangalore celebrate the arrival of India's Mars Orbiter Mission at its destination on September 24, 2014.

They cheered even louder when Doppler shifts in the arriving telemetry showed that the craft had slowed by nearly 2,500 miles per hour (1.1 km per second), exactly as planned. The initial orbit is a looping, polar-crossing 263-by-50,000-mile (423-by-80,000-km) ellipse that takes 3.2 days to complete.

MOM is the second craft to reach Mars this week (NASA's MAVEN arrived two days ago), and India now joins select company — along with the U.S., Russia, and the European Space Agency — to make the trek. Team members boasted that never before has a nation's spacecraft successfully reached Mars on the very first try. "History has been created today," prime minister Narendra Modi told the flight team.

Emboldened by the success of its 2008 lunar orbiter, Chandrayaan 1, ISRO's managers quickly set their sights on Mars. The MOM spacecraft follows that same basic design, utilizing a cube-shaped structure about 5 feet (1.5 m) on a side and a mass of about 1½ tons. Indian culture is imbued with the notion of jugaad — finding innovative but frugal solutions to life's challenges — and ISRO has taken that embraced that concept. Remarkably, MOM's entire mission cost is only about $75 million, a fraction of teh $670 million that NASA is spending on MAVEN.

MOM's path to Mars was not simple. After launch on November 13, 2013, the spacecraft made a half dozen trips around Earth in ever-larger loops, boosted higher and higher by repeated rocket firings. A final maneuver on December 1st pushed it free of Earth and sent it on a 10-month cruise toward Mars.

Success: Just Getting There

The mission's primary objective is not to make pioneering scientific discoveries about the Red Planet. Instead, it's simply to demonstrate the ability to design a craft capable of interplanetary travel. Science is secondary. But MOM's instrumental payload, while modest, can potentially deliver important discoveries.

A color camera will record the Martian surface. Also aboard is a photometer that will record Lyman-alpha emissions to deduce the relative abundance of deuterium in the planet's uppermost atmosphere. A spectrometer will determine the abundance of atmospheric methane — a gas at the center of a controversial debate — down to parts-per-billion levels. Rounding out the payload are a mass spectrometer (for atmospheric composition) and a thermal-emission spectrometer (for assessing surface composition and mechanical properties).

The mission is not without its detractors. Despite ISRO's jugaad-esque approach, India is a country trying to deal with endemic poverty. Still, science journalist Manoj Kumar Patairiya put the mission in perspective in a recent New York Times column. "We know how to embrace two ideas at once," he concludes, pairing "tradition and science, frugality and innovation — just as we can deal with issues like poverty at the same time as taking a giant leap into interplanetary space."

You can journey to Mars from the comfort of your favorite reading spot, thanks to our special issue, "Mars: Mysteries & Marvels of the Red Planet." It offers a timely, comprehensive look at this intriguing world next door.

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The 2014 Autumnal Equinox Arrives

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What is the "fall equinox" — and how do we know when it happens?

To those who’ve unpacked their winter coats, closed their windows at night, and felt that telltale crispness in the air, it seems that autumn has already started. Astronomically speaking, however, the fall season only comes to the Northern Hemisphere on Tuesday, September 23rd at 2:29 Universal Time (10:29 p.m. EDT on Monday, the 22nd). At that moment, the Sun shines directly on Earth’s equator, heading south as seen in the sky. For us northerners, this event is called the autumnal equinox.

This seems awfully precise for seasons that gradually flow from one to the next, but the reason astronomers regard this event as the “End of Summer” and “Beginning of Fall” is because it is marked by a key moment in Earth’s annual orbit.

Earth's spin axis isn't perpendicular to the plane of its orbit around the Sun. One consequence: the celestial-coordinate system is tilted 23½° with respect to the ecliptic (the path followed by the Sun through the stars over the course of a year). Equinoxes occur when the Sun crosses from one hemisphere to the other.
S&T / Gregg Dinderman

The apparent position of the Sun in our sky is farther north or farther south depending on the time of year due to the globe's axial tilt. Earth's rotational axis does not point straight up and down, like the handle of a perfectly spinning top, but is slanted about 23½° with respect to our orbit around the Sun.

Another way to think of this is that the plane defined by Earth's orbit around the Sun (called the ecliptic) is tilted with respect to the planet's equator. From our perspective, the Sun follows the ecliptic in its path through the sky throughout the year. Each day the Sun's daily arc moves northward or southward, depending on the time of the year. To observers at northern latitudes (in the U.S., Canada, and Europe, for example), the Sun appears to sneak higher in the sky from late December to late June, only to drop down again from late June through late December. The equinox occurs when the Sun is halfway through each journey.

This axial tilt also produces our seasons. When Earth is on one side of its orbit, the Northern Hemisphere is tipped toward the Sun and receives more direct solar rays (and more daylight hours) that produce the familiar climes of summer. Six months later, when Earth is on the opposite side of its orbit, the Northern Hemisphere is tipped away from the Sun. The slanting solar rays heat the ground less and daylight is shorter, producing the colder winter season.

The Sun's apparent motion during the year

The Sun rises due east and sets due west on the equinoxes in March and September. At other times of year it comes up and goes down farther north or south. This illustration is drawn for mid-Northern latitudes.
Sky & Telescope illustration

The same pattern occurs, but in reverse, for the Southern Hemisphere. There September’s equinox marks the beginning of spring, while March’s equinox signals the start of fall. Christmas is a warm holiday in Sydney, Australia. For those living in equatorial regions, however, there are usually only two recognizable seasons: wet and dry; and the days themselves vary less in length.

Several other noteworthy situations happen on the equinoxes:

  • Day and night are nearly the same length; the word “equinox” comes from the Latin aequinoctium, meaning “equal night,” according to the Oxford English Dictionary. However, a poke around your almanac will show that day and night are not precisely 12 hours each, for two reasons: first, sunrise and sunset are defined as when the Sun’s top edge — not its center— crosses the horizon. Second, Earth’s thick atmosphere refracts the Sun’s apparent position slightly when the solar disk sits very low on the horizon.
  • The Sun rises due east and sets due west, as seen from everywhere on Earth; the equinoxes are the only times of the year when this occurs.
  • Should you be standing on the equator, the Sun would pass exactly overhead at midday. Were you standing at the North Pole or South Pole, the Sun would skim completely around the horizon.

Curious about what's happening in the sky? Check out the 2015 Sky & Telescope Observing Wall Calendar!

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MAVEN Makes It to Mars

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NASA's latest interplanetary spacecraft has settled into orbit around the Red Planet. Its year-long atmospheric studies could reveal how and why Mars lost so much of its primordial atmosphere.

This image shows an artist concept of NASA's Mars Atmosphere and Volatile Evolution (MAVEN)  spacecraft, which reached the Red Planet on September 21, 2014.Lockheed Martin

This image shows an artist concept of NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which reached the Red Planet on September 21, 2014.
Lockheed Martin

When it comes to interplanetary exploration, you've got to trust your hardware. That was the case this evening, when the scientists and engineers for NASA's latest deep-space sortie could do little more than wait anxiously, fingers crossed, at a control center in Littleton, Colorado. Out there, 138 million miles (222 million km) and 12½ light-minutes from Earth — too far away to control directly — the Mars Atmosphere and Volatile Evolution spacecraft (MAVEN) successfully fired a cluster of six engines for 33 minutes and slipped into orbit around the Red Planet.

The spacecraft didn't exactly shout "I'm here" after the 10-month, 442 million-mile cruise that began last November 18th. But Doppler shifts in a weak radio beacon showed that the engines had reduced the approach velocity by about 4,000 feet (1.23 km) per second, slowing the craft enough for the planet's gravity to snare the spacecraft at 10:24 p.m. EDT (2:24 Universal Time on September 22nd). MAVEN had arrived at Mars.

For now, the spacecraft will follow a looping polar orbit that varies from 240 to 27,700 miles (380 to 44,600 km) in altitude. Over the next 6 weeks, the engines will fire again to shrink a 4½-hour-long orbit ranging from 95 to 3,850 miles, and then small thrusters will trim that further to a final, 3½-hour loop.

Unlike NASA's other Martian explorers, which have largely focused on the state of the planet's surface and its geologic evolution, MAVEN will study the Martian atmosphere exclusively. It carries eight instruments, six of which will measure charged particles, electromagnetic fields, and plasma waves in the solar wind as it sweeps past the planet. An imaging ultraviolet spectrograph and a mass spectrometer, both mounted on a steerable platform at the end of a short boom, will assess the upper atmosphere's chemical makeup.

What Happened to Mars?

Over the next year, flight controllers will command MAVEN to make five "deep dips", dropping it to altitudes as low as 77 miles (125 km) to sample directly the uppermost wisps of the planet's already tenuous air. These observations hope to answer a longstanding puzzle among planetary scientists. There's ample evidence that, early in its history, the Red Planet had a much denser atmosphere. Rain fell from its sky, and water coursed across its landscape.

But then something happened to the atmosphere: it basically vanished and, with it, the brief era when Mars might have been suitable as an abode for life. Mars quickly became the desolate, frigid world we see today. Researchers led by Bruce Jakosky (University of Colorado), MAVEN's principal investigator, want to know what happened to all that gas (most of it carbon dioxide) and, especially, to the ample water that once existed on the Martian surface.

One leading theory is that the gas escaped irrevocably to space, stripped away by the solar wind rushing past. Here on Earth, our planet's magnetosphere serves as an obstacle to the solar wind, keeping it from interacting directly with our atmosphere. But once Mars lost its global magnetic field, billions of years ago, the upper atmosphere became vulnerable.

MAVEN's spectrometers will attempt to determine if hydrogen atoms, torn from water molecules by ultraviolet sunlight, are escaping to space, and at what rate. "The stripping of gas from the atmosphere to space might have been the driving mechanism for climate change on Mars," Jakosky says.

For now, he and his team will ready the spacecraft to begin observations in early November. Results will not come quickly, he cautions, because it will take months to build up enough measurements to have a clear sense of what's going on — or going away.

However, one early, unexpected, and unprecedented opportunity will come relatively soon, when Comet Siding Spring (C/2013 A1) brushes within 82,000 miles of the Red Planet on October 19th. Because any cometary particles will strike at 35 miles (56 km) per second, there's some concern for the safety of MAVEN and other orbiters circling Mars. They'll be positioned on the back side of the planet during the time of greatest danger.

A few days before and after the comet's closest approach, MAVEN's ultraviolet spectrograph will measure both the abundance of gases within C/2013 A1's coma and also its effects on the Martian upper atmosphere (heating from cometary dust impacts or a temporary increase in water-vapor content). "We should have some pretty spectacular results," Jakosky promises.

Our special issue, "Mars: Mysteries & Marvels of the Red Planet," is loaded with spectacular photos and a must-read for anyone interested in this intriguing neighboring world.

The post MAVEN Makes It to Mars appeared first on Sky & Telescope.

This Week’s Sky at a Glance, September 19–27

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Moon and Jupiter at dawn

Look east at dawn for the waning crescent Moon passing Jupiter, then Regulus.

Friday, September 19

In early dawn Saturday morning, Jupiter shines upper left of the waning Moon in the east, as shown at right. How long has it been since you turned your scope on either Jupiter or the maria-covered waning crescent?

Saturday, September 20

In bright twilight, Mercury and fainter Spica are in conjunction 0.6° apart just above the west-southwest horizon. Use binoculars to scan for them about 20 minutes after sunset.

The eclipsing variable star Algol (Beta Persei) should be at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple of hours centered on 10:55 p.m. EDT.

In early dawn on Sunday the 21st, the waning crescent Moon shines far below Jupiter and closer to the right of Regulus, as shown above.

Sunday, September 21

Aquila's dark secret: If you're blessed with a really dark sky, try finding the big dark nebula known as "Barnard's E" near Altair in Aquila, using Gary Seronik's Binocular Highlight column and chart in the September Sky & Telescope, page 45.

And if you have a sky that dark, also use binoculars to investigate the big, dim North America Nebula and its surroundings near Deneb in Cygnus using the September issue's Deep-Sky Wonders article, page 56.

Monday, September 22

The September equinox comes at 10:29 p.m. on this date EDT (2:29 September 23rd UT). This is when the Sun crosses the equator heading south for the year. Fall begins in the Northern Hemisphere, spring in the Southern Hemisphere. Day and twilight-plus-night are nearly equal in length. The Sun rises and sets almost exactly east and west.

As summer ends, the Sagittarius Teapot is moves west of due south during evening and tips increasingly far over, as if pouring out the last of summer.

Tuesday, September 23

Arcturus is the bright star fairly high due west at nightfall. It's 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.

Algol is at minimum light again for a couple hours centered on 7:44 p.m. EDT.

Wednesday, September 24

Mars is within 4° of Antares (passing north of it) from this evening through the 30th. Mars is just a little brighter and almost the same color as its namesake star; "Antares" is Greek for "anti-Mars."

Thursday, September 25

With the coming of fall, Deneb slowly replaces Vega as the bright star nearest to the zenith just after nightfall (for mid-northern latitudes).

Friday, September 26

As early as 8 or 9 p.m. now look for Fomalhaut, the lonely 1st-magnitude Autumn Star, twinkling on its way up from the southeast horizon. It will be highest due south around 11 or midnight (depending on your location).

Saturday, September 27

Low in the southwest in twilight, Mars and Antares are passing 3° apart this evening and Sunday evening, as shown below. Meanwhile, off to their right, the waxing crescent Moon floats a couple degrees to the lower right of Saturn (for North America).

Moon passing Saturn, Mars and Antares

The waxing crescent Moon works its way eastward above the star-and-planet display low in the southwestern twilight. (These scenes are plotted for the middle of North America.)

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).

Pocket Sky Atlas

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's still less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae.

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 RoundupSaturn, Mars and Antares at dusk

Follow the Antares-Mars-Saturn lineup as Mars moves leftward from its position here day by day.

Mercury (magnitude 0.0) remains very deep in the sunset. Scan for it with binoculars just above the west-southwest horizon about 20 minutes after sundown. Fainter, twinklier Spica is right nearby. Mercury and Spica appear closest together, 0.6° apart, on Saturday evening the 20th.

Venus (magnitude –3.9) is barely above the horizon due east shortly before sunrise. Bring binoculars.

Mars (magnitude +0.8, in Scorpius) glows low in the southwest at dusk near similarly colored Antares (magnitude 1.0). They'll pass 3° apart on September 27th and 28th.

Jupiter (magnitude –1.9, in Cancer) rises around 3 a.m. and shines brightly in the east before and during dawn. It forms a roughly equilateral triangle with Pollux above it (by about two fists at arm's length) and Procyon to their right. Farther to the right or lower right of Procyon sparkles brighter Sirius.

Saturn (magnitude +0.6, in Libra) is sinking low into the afterglow of sunset. Look for it well to the right of the Mars-Antares pair, and perhaps a little lower depending on your latitude.

Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are high in the southeast and south, respectively, by 11 p.m. 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.


The post This Week’s Sky at a Glance, September 19–27 appeared first on Sky & Telescope.

Small Galaxy Boasts Big Black Hole

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Astronomers have detected a supermassive black hole in the center of a tiny galaxy — where it has no right to be.

Don't be fooled by the small size of the ultracompact dwarf galaxy M60-UCD1 — it harbors a supermassive black hole, according to research published in the September 18th Nature. The new finding makes M60-UCD1 the smallest and least massive galaxy known to contain such a gargantuan black hole and is the first concrete evidence for how ultracompact dwarf galaxies form.

ultracompact dwarf galaxy

This Hubble Space Telescope image shows the massive elliptical galaxy Messier 60, which overshadows the tiny dwarf galaxy M60-UDC1 (inset). M60-UCD1 is only 300 light-years across yet contains a black hole roughly 10 times more massive than the one that sits in the Milky Way’s center.
NASA / ESA / A. Seth (University of Utah)

M60-UCD1 is located in the Virgo Cluster, about 54 million light-years from Earth. It is one of the most massive and brightest ultracompact dwarf galaxies, objects that can shove up to 200 million solar masses into a radius 160 light-years or less. They are similar in size to globular clusters but are at least 10 times more massive.

Ultracompact dwarf galaxies have perplexed astronomers since they were discovered over a decade ago. Scientists were uncertain how these tiny galaxies formed. Two competing theories emerged: either they were unusually massive star clusters, or they were the remnants of larger galaxies that had been stripped down to their cores by the gravitational pull of massive neighbors.

The study is the first to provide observational evidence for the stripped-down theory, by revealing the presence of a supermassive black hole. Supermassive black holes have more than a million times the mass of the Sun and are found at the centers of large galaxies, but usually not in dwarf galaxies. But at 21 million times the mass of the sun, M60-UCD1’s black hole is “remarkably massive,” says study coauthor Anil Seth (University of Utah). “It’s about five times more massive than the Milky Way’s black hole, in spite of the fact that this object — this ultracompact dwarf galaxy — is 500 times smaller and a thousand times less massive” than the Milky Way.

The existence of the supermassive black hole indicates that M60-UCD1 was once a larger galaxy, and that many of its stars were later ripped away by interactions with its neighbor, the much larger elliptical galaxy M60. Given the black hole's mass, the original galaxy likely had a central bulge roughly 100 times more massive than the current galaxy's total stellar mass.

“They’ve really, finally, after a decade, got some solid evidence that it’s one model rather than the other, that at least some of them must have been at the center of a galaxy in the past,” says Michael Drinkwater (University of Queensland, Australia), who was not involved in the research.

M60-UCD1’s black hole is particularly impressive given the galaxy’s small size. It makes up a whopping 15 percent of the galaxy's total mass, whereas the black hole in our Milky Way makes up only a fraction of a percent of the total mass of the galaxy. Most galaxies follow the Milky Way’s pattern, although exceptions exist.

Characterizing such a tiny object is a difficult task. The astronomers used the Gemini North 8-meter telescope on Hawaii’s Mauna Kea to discern the motions of stars in the galaxy. They found that the stars in the center of the galaxy were orbiting at roughly 175 kilometers per second (390,000 mph), much faster than expected and indicating the presence of a black hole.

“Immediately, as soon as I saw the stellar motions map, I knew that there was something exciting,” says Seth. “It had a larger black hole than even we had considered as our maximum case.”

The result could have some broader implications, the astronomers say. There are many ultracompact dwarf galaxies — around 50 are known in the nearest galaxy clusters — and these objects have a common peculiarity. Many ultracompact dwarf galaxies are more massive than expected based upon their luminosities — a possible indication of a black hole, but not the only plausible explanation.

However, in the case of M60-UCD1, once the astronomers included the black hole in their calculations, the galaxy’s stellar mass matched what astronomers would expect, given its luminosity. This result, they argue, indicates that the same process is likely also the explanation for the high mass estimates for other ultracompact dwarf galaxies.

Another possible explanation for the unexpectedly large masses is that the average mass of stars in these dwarfs is much higher than in standard galaxies.

“Really, until we go and measure a few more, it’s hard to know the relative importance of the two origins,” says Drinkwater.

And measuring a few more will be the next step in this research. Most of the known ultracompact dwarf galaxies are too faint to study with this method, but the next generation of telescopes should allow astronomers to look for more supermassive black holes in other objects of this type.

The result indicates a new place for scientists to search for black holes. If the astronomers are correct and many ultracompact dwarf galaxies do contain black holes at their centers, then the team predicts that the true number of massive black holes in the local universe may be twice the current estimate.

This is still speculation at this stage, Drinkwater says, but it’s an idea that would be interesting to investigate.

“This certainly opens up that possibility that they’re there, and that’s very exciting,” said Seth.


Reference: A. C. Seth et al. "A supermassive black hole in an ultra-compact
dwarf galaxy." Nature. September 18, 2014.

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