The past decade has seen so many incredible advances in astronomy that it would be hard to single out one as standing above the others. But near the top of my list is the work by Andrea Ghez (UCLA) and her colleagues to measure the mass of the Milky Way’s central black hole. In a paper posted on the Web last week, they derive a new and improved mass for our galaxy's monster in the middle.
Milky Way's central arcsecond
The orbits of stars within the Milky Way's central tenth of a light-year loop around the known location of the central black hole (yellow star symbol). The fuzzy blobs are diffraction-limited star images in an infrared adaptive-optics frame taken by a 10-meter Keck telescope in 2004. The frame is 1 arcsecond (0.13 light-year) square. While every star in this image has been seen to move, estimates of orbital parameters are only possible for those that show significant curvature so far.
The annual average positions for these stars are plotted as colored dots (which have increasing color saturation with time). Also plotted are the best-fitting orbit solutions. These orbits provide the clearest reading yet on the mass of the central black hole. Click for movie.
UCLA Galactic Center Group / Keck Observatory
The team’s pure measurement yields a mass of 4.1 million Suns with an uncertainty of only 0.6 million Suns. But if the black hole is assumed to be stationary with respect to the rest of the galaxy (and there's no evidence otherwise), its mass rises a bit to 4.5 million Suns with an uncertainty of only 0.4 million. This latter result is significantly higher than previous measurements made by Ghez’s team and by a European group led by Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics, Germany).
A black hole of that mass has a diameter of 0.1 astronomical unit (about 9 million miles).
Why is this measurement so amazing? The center of the Milky Way is 27,000 light-years away, and it's hidden behind thick clouds of gas and dust along our line of sight. How do you measure the mass of an invisible object tens of thousands of light-years from Earth when even its surroundings are obscured from view?
Ghez and her team had to employ all the resources of modern astronomy: a really big telescope, detectors that operate in the infrared, and the relatively recent technology of adaptive optics (AO for short). Oh yeah, they also needed a lot of patience.
Milky Way's central 10 arcseconds
This narrow-field image of the galactic center, taken at an infrared wavelength of 3.8 microns, resolves many stars around the position of the central black hole ("Sgr A*). The frame is 10 arcseconds tall. Keck's Laser Guide Star adaptive-optics system achieved a resolution of 82 milliarcseconds here, the equivalent of resolving a pair of headlights in New York from Los Angeles.
UCLA Galactic Center Group / Keck Observatory
For the past decade, they have been observing the galactic center with the 10-meter Keck I and II telescopes in Hawaii. Keck gives them the brute-force light-gathering power to see stars in the galactic center. They observe at infrared wavelengths, which can penetrate the thick clouds of gas and dust. And most of all, they use AO, which involves a laser-generated artificial guide star and flexible, deformable mirrors to compensate for the rapid-fire blurring effects of Earth’s atmosphere.
The combination of these techniques allowed the group to resolve dozens of individual stars near the galactic center. Incredibly, the team could trace the curving orbital motions of several of these stars over more than a decade, and actually create a movie of these motions. Astronomers of just 25 years ago would have considered this magic.
The high mass of the Milky Way’s black hole, known as Sagittarius A* (pronounced "A-star"), made this possible. Anything orbiting near such a massive object is going to move really, really fast. These stars are whirling around the black hole at speeds exceeding 4,500 km per second (10 million miles per hour). One star in particular, dubbed S0-2, has been clocked at nearly 8,000 km/sec. By using simple orbital laws dating back to Isaac Newton in the 1600s, Ghez could use these stellar velocities to derive the mass of the central gravitating object.
In their new paper (accepted for publication in the Astrophysical Journal), Ghez and her team took into account various effects, such as uncertainties in star positions, ignored by in previous studies. “It’s been a bit like teenagers making emphatic but uninformed statements,” explains Ghez. “In our new paper, we try to take an honest look at where the problems are. We’ve learned that things are more complicated. We’re growing out of our teenage years!”
Besides coming up with a more precise mass measurement, the latest observations refine the distance to the Milky Way’s center: 27,400 light-years, with an uncertainty of 1,300 light-years.
In addition, the group finds no evidence that the central black hole is being gravitationally yanked around by the mass of another. This argues in favor of the team’s higher mass measurement. This new, higher mass value is also more consistent with predictions based on the famous relationship between black-hole mass and the total mass in the spherical component of large galaxies.
Venus Returns
For more than a year, planetary observers in the Northern Hemisphere have been treated to a steady parade of our solar-system siblings particularly well placed at high declination.
That series has unfortunately come to an end with Saturn and Mars both receding into the evening twilight. Jupiter now commands the evening, but for most people in the U.S., Canada, and Europe, it's just too low in the sky to produce satisfying views.
Venus often has picturesque conjunctions with celestial objects, but none more frequently then the Moon.
S&T: Richard T. Fienberg
Fortunately, Venus has just passed superior conjunction, and is already shining low in the western horizon shortly after sunset. While mid-northern observers will find Venus very low in the evening twilight, there's still plenty of fun to be had pursuing the brightest planet in our skies.
One easy project anyone can do is to see how early in the evening you can spot Venus. I've managed to see it roughly a half-hour before sunset with only my unaided eyes by placing myself in the shadow of a tree or building, then searching just to the west of the Sun about two hand-widths away, roughly at the 10-o'clock position from the Sun. Venus is so bright that it shines through daylight skies easily, but you have to know exactly where to look to find it. For help, check out our interactive sky chart to help narrow down the area of your search. Clear, transparent skies makes this much easier to do.
If you're still having troubler finding it, don't worry. Once a month the Moon glides by the area, giving you a visual cue to both focus your eye on and skip over to Venus much easier than on other days. The Moon will be about 6° southwest of Venus on September 1st.
S&T editor Sean Walker imaged Venus throughout the 2007 western elongation with a monochrome video camera and color filters. This image was taken through an ultra-violet filter to reveal the planet's mysterious cloud features. Click on the image to see an animation of the planet's atmosphere as it churns through its four-day rotation cycle.
S&T: Sean Walker
Venus makes some beautiful pairings with other interesting objects throughout this apparition. On the evening of September 11, it passes roughly 20 arcminutes north of Mars. Eight days later, Venus passes just 2½° north of Spica.
Careful telescopic observing can reveal hints of the faint cloud bands that circulate around the planet roughly every four days. I've been able to detect these visually only a few times in the past, usually on days of excellent transparency and good seeing conditions. Observing through colored filters will enhance these features, and also dim the view enough to make these observations possible.
If you're into planetary imaging, these cloud bands can be recorded with monochrome cameras equipped with colored filters, and are easier to detect at shorter wavelengths. Due to a still poorly understood phenomena, the highest contrast features in Venuses cloud tops appear in the near-ultraviolet wavelengths around 365 nanometers. Special filters, such as the "U" filter in a UBVRI photometric filter set, will aid greatly in recording these ever-changing features.
Like all the other planets, your best photographic results will come when Venus is high in the sky, so I try to image it during daylight hours. Be extremely careful if you do this, as one glimpse of the Sun through a telescope or even a finderscope can cause permanent blindness.
Give any of these projects a try. You may just find yourself drawn back to our sister planet much more often than you expected to be.
For more than a year, planetary observers in the Northern Hemisphere have been treated to a steady parade of our solar-system siblings particularly well placed at high declination.
That series has unfortunately come to an end with Saturn and Mars both receding into the evening twilight. Jupiter now commands the evening, but for most people in the U.S., Canada, and Europe, it's just too low in the sky to produce satisfying views.
Venus often has picturesque conjunctions with celestial objects, but none more frequently then the Moon.
S&T: Richard T. Fienberg
Fortunately, Venus has just passed superior conjunction, and is already shining low in the western horizon shortly after sunset. While mid-northern observers will find Venus very low in the evening twilight, there's still plenty of fun to be had pursuing the brightest planet in our skies.
One easy project anyone can do is to see how early in the evening you can spot Venus. I've managed to see it roughly a half-hour before sunset with only my unaided eyes by placing myself in the shadow of a tree or building, then searching just to the west of the Sun about two hand-widths away, roughly at the 10-o'clock position from the Sun. Venus is so bright that it shines through daylight skies easily, but you have to know exactly where to look to find it. For help, check out our interactive sky chart to help narrow down the area of your search. Clear, transparent skies makes this much easier to do.
If you're still having troubler finding it, don't worry. Once a month the Moon glides by the area, giving you a visual cue to both focus your eye on and skip over to Venus much easier than on other days. The Moon will be about 6° southwest of Venus on September 1st.
S&T editor Sean Walker imaged Venus throughout the 2007 western elongation with a monochrome video camera and color filters. This image was taken through an ultra-violet filter to reveal the planet's mysterious cloud features. Click on the image to see an animation of the planet's atmosphere as it churns through its four-day rotation cycle.
S&T: Sean Walker
Venus makes some beautiful pairings with other interesting objects throughout this apparition. On the evening of September 11, it passes roughly 20 arcminutes north of Mars. Eight days later, Venus passes just 2½° north of Spica.
Careful telescopic observing can reveal hints of the faint cloud bands that circulate around the planet roughly every four days. I've been able to detect these visually only a few times in the past, usually on days of excellent transparency and good seeing conditions. Observing through colored filters will enhance these features, and also dim the view enough to make these observations possible.
If you're into planetary imaging, these cloud bands can be recorded with monochrome cameras equipped with colored filters, and are easier to detect at shorter wavelengths. Due to a still poorly understood phenomena, the highest contrast features in Venuses cloud tops appear in the near-ultraviolet wavelengths around 365 nanometers. Special filters, such as the "U" filter in a UBVRI photometric filter set, will aid greatly in recording these ever-changing features.
Like all the other planets, your best photographic results will come when Venus is high in the sky, so I try to image it during daylight hours. Be extremely careful if you do this, as one glimpse of the Sun through a telescope or even a finderscope can cause permanent blindness.
Give any of these projects a try. You may just find yourself drawn back to our sister planet much more often than you expected to be.
Solar Eclipse
On the Arctic Ocean
Rick Fienberg, Sky & Telescope's editor emeritus, led an eclipse expedition aboard the Russian icebreaker 50 Years of Victory in the Barents Sea west of Novaya Zemlya. "This trip has been awesome," he writes "— polar bears in the wild, a dip in the ocean at the North Pole — and we beat the odds and saw today's total eclipse, from a position near 76° north, 55° east.
"The day dawned foggy, but by 10:55 a.m. our time when the partial eclipse began, the fog had burned off and we were under scattered clouds with patches of blue sky all around. Excitement began to build among the approximately 100 passengers and 100 crew out on deck as we watched first contact through our solar filters. Almost immediately, though, the clouds merged into a single mass, hiding the Sun from view except for brief intervals during which we could see the progress of the partial eclipse, with the clouds as our filter.
"With about a half hour to go till totality we could see sunlight sparkling on the water in the distance, so the ship steamed at full power toward it. We reached an area where the cloud cover was thin enough to grant us a spectacular view of the eclipse from just before the start of totality through to the very end.
"We saw a beautiful 'diamond ring' as the corona emerged around the Sun along with several electric-pink prominences, most notably a large one at the 1 o'clock position for us. I was glancing occasionally at the horizon all around to experience the Moon's shadow washing over us — which was very dramatic out on the open water.
"Mercury immediately popped into view a few degrees to the Sun's left; I briefly looked further left to see brighter Venus shining too, about 15° to the Sun's left.
"The diamond ring at 3rd contact came too soon, as always. Within minutes of the end of totality the remaining cloud cover burned off, and by 4th contact an hour later we were sailing toward Murmansk under an almost completely clear, blue sky.
"According to the climatology, we had less than a 30% chance of seeing totality, yet I'm now 7 for 7 for total solar eclipses."