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Mercury Through a Telescope

enhanced image of mercury

enhanced image of mercuryMercury is the closest planet orbiting the sun in our solar system.  It’s also the smallest of the eight planets, being about two-fifths the size of earth – smaller than some moons.

Being closer to the sun than the earth, Mercury is known as an inferior planet.  Like all the planets of the inner solar system it’s made up of rock & metal, with a proportionally large molten core.

The surface of mercury is much like the moon, covered with impact craters.  This is because there is no atmosphere and no geological activity to change the surface.  It is a place of extreme temperatures, with a surface temperature around 427°C (800°F) during daylight to -173°C (-280°F) at night near the equator.  The poles are always freezing, due to the fact that Mercury’s axial tilt is almost non existent, the least of any planet in the solar system.

A day on Mercury lasts a long time – 59 earth days. Yet the year is very short – Mercury takes just 88 earth days to orbit the sun.  This extremely slow spin is unique in our solar system, with the other planets spinning at a much faster rate.  These figures mean that Mercury has just 3 of its days every 2 of its years!

Another feature of Mercury is its accentuated orbit. While all the planets orbits are technically ellipses, they’re very close to circular.  Only Mercury has a highly elliptical orbit.  Mercury’s distance from the sun varies from 46,000,000 km to 70,000,000 km (28,580,000 mi to 43,500,000 mi). As Mercury’s orbit brings it closer to the sun it also speeds up, travelling faster in its orbit on the close side of the ellipse and slower on the far side. This, coupled with the very slow axial rotation, gives the sun a retrograde motion for part of the Mercurian day – the sun will actually go backwards in the sky.

Observing Mercury

Mercury is not as easily observed as the other planets due to its proximity to the sun.  It will always be near the sun, appearing low in the sky just before sunrise or just after sunset, depending which side of the sun it’s on relative to us.  For much of the year it’s not visible at all, and when it is it’s often not for long and not fully dark.  Optimal viewing rarely occurs due to the number of factors that need to be right. This makes it a difficult target for beginning observers. Through a telescope Mercury will appear as a bright disc showing its phase with little other details observable.

Mercury goes through phases just as the moon and Venus do.  It’s best viewed when it’s at its furthest distance from the sun as viewed from earth, for at this time it will be higher in the sky and more likely visible in full dark.  This is known as greatest elongation.  This will mean it will be in the first or last quarter phase, as from our perspective it will be directly out to either side of the sun.  Mercury’s angular height above the horizon varies depending on which part of its orbit it’s in when it’s at greatest elongation.  The highest will be when it’s at aphelion.  This will occur at the March equinox for greatest western elongation or the September equinox for greatest eastern elongation.

Mercury’s orbit is at a 7° angle to the plane of earths orbit (the ecliptic) and this also has an effect on best viewing times and locations.  Southern Hemisphere viewers have a better view of Mercury due to the angle Mercury will be at when at ephelion and the time of year it occurs – early autumn for greatest eastern elongation or late winter for greatest western elongation.  For most northern hemisphere viewers Mercury will never be visible when it’s fully dark.

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Gravitational wave kicks monster black hole out of galactic core

This image, taken by NASA’s Hubble Space Telescope, reveals an unusual sight: a runaway quasar fleeing from its galaxy’s central hub. A quasar is the visible, energetic signature of a black hole. Black holes cannot be observed directly, but they are the energy source at the heart of quasars — intense, compact gushers of radiation that can outshine an entire galaxy.
The green dotted line marks the visible periphery of the galaxy. The quasar, named 3C 186, appears as a bright star just off-center. The quasar and its host galaxy reside 8 billion light-years from Earth. Credit: NASA, ESA, and M. Chiaberge (STScI and JHU)

Astronomers have uncovered a supermassive black hole that has been propelled out of the centre of a distant galaxy by what could be the awesome power of gravitational waves.

Though there have been several other suspected, similarly booted black holes elsewhere, none has been confirmed so far. Astronomers think this object, detected by NASA’s Hubble Space Telescope, is a very strong case. Weighing more than 1 billion Suns, the rogue black hole is the most massive black hole ever detected to have been kicked out of its central home.

Researchers estimate that it took the equivalent energy of 100 million supernovae exploding simultaneously to jettison the black hole. The most plausible explanation for this propulsive energy is that the monster object was given a kick by gravitational waves unleashed by the merger of two hefty black holes at the center of the host galaxy.

First predicted by Albert Einstein, gravitational waves are ripples in space that are created when two massive objects collide. The ripples are similar to the concentric circles produced when a hefty rock is thrown into a pond. Last year, the Laser Interferometer Gravitational-Wave Observatory (LIGO) helped astronomers prove that gravitational waves exist by detecting them emanating from the union of two stellar-mass black holes, which are several times more massive than the Sun.

Hubble’s observations of the wayward black hole surprised the research team. “When I first saw this, I thought we were seeing something very peculiar,” said team leader Marco Chiaberge of the Space Telescope Science Institute (STScI) and Johns Hopkins University, in Baltimore, Maryland. “When we combined observations from Hubble, the Chandra X-ray Observatory, and the Sloan Digital Sky Survey, it all pointed towards the same scenario. The amount of data we collected, from X-rays to ultraviolet to near-infrared light, is definitely larger than for any of the other candidate rogue black holes.”

Chiaberge’s paper will appear in the March 30 issue of Astronomy & Astrophysics.

Hubble images taken in visible and near-infrared light provided the first clue that the galaxy was unusual. The images revealed a bright quasar, the energetic signature of a black hole, residing far from the galactic core. Black holes cannot be observed directly, but they are the energy source at the heart of quasars – intense, compact gushers of radiation that can outshine an entire galaxy. The quasar, named 3C 186, and its host galaxy reside 8 billion light-years away in a galaxy cluster. The team discovered the galaxy’s peculiar features while conducting a Hubble survey of distant galaxies unleashing powerful blasts of radiation in the throes of galaxy mergers.

“I was anticipating seeing a lot of merging galaxies, and I was expecting to see messy host galaxies around the quasars, but I wasn’t really expecting to see a quasar that was clearly offset from the core of a regularly shaped galaxy,” Chiaberge recalled. “Black holes reside in the center of galaxies, so it’s unusual to see a quasar not in the center.”

The team calculated the black hole’s distance from the core by comparing the distribution of starlight in the host galaxy with that of a normal elliptical galaxy from a computer model. The black hole had traveled more than 35,000 light-years from the center, which is more than the distance between the sun and the center of the Milky Way.

Based on spectroscopic observations taken by Hubble and the Sloan survey, the researchers estimated the black hole’s mass and measured the speed of gas trapped near the behemoth object. Spectroscopy divides light into its component colors, which can be used to measure velocities in space. “To our surprise, we discovered that the gas around the black hole was flying away from the galaxy’s center at 4.7 million miles an hour,” said team member Justin Ely of STScI. This measurement is also a gauge of the black hole’s velocity, because the gas is gravitationally locked to the monster object.

The astronomers calculated that the black hole is moving so fast it would travel from Earth to the moon in three minutes. That’s fast enough for the black hole to escape the galaxy in 20 million years and roam through the universe forever.

The Hubble image revealed an interesting clue that helped explain the black hole’s wayward location. The host galaxy has faint arc-shaped features called tidal tails, produced by a gravitational tug between two colliding galaxies. This evidence suggests a possible union between the 3C 186 system and another galaxy, each with central, massive black holes that may have eventually merged.

Based on this visible evidence, along with theoretical work, the researchers developed a scenario to describe how the behemoth black hole could be expelled from its central home. According to their theory, two galaxies merge, and their black holes settle into the center of the newly formed elliptical galaxy. As the black holes whirl around each other, gravity waves are flung out like water from a lawn sprinkler. The hefty objects move closer to each other over time as they radiate away gravitational energy. If the two black holes do not have the same mass and rotation rate, they emit gravitational waves more strongly along one direction. When the two black holes collide, they stop producing gravitational waves. The newly merged black hole then recoils in the opposite direction of the strongest gravitational waves and shoots off like a rocket.

The researchers are lucky to have caught this unique event because not every black-hole merger produces imbalanced gravitational waves that propel a black hole in the opposite direction. “This asymmetry depends on properties such as the mass and the relative orientation of the back holes’ rotation axes before the merger,” said team member Colin Norman of STScI and Johns Hopkins University. “That’s why these objects are so rare.”

An alternative explanation for the offset quasar, although unlikely, proposes that the bright object does not reside within the galaxy. Instead, the quasar is located behind the galaxy, but the Hubble image gives the illusion that it is at the same distance as the galaxy. If this were the case, the researchers should have detected a galaxy in the background hosting the quasar.

If the researchers’ interpretation is correct, the observations may provide strong evidence that supermassive black holes can actually merge. Astronomers have evidence of black-hole collisions for stellar-mass black holes, but the process regulating supermassive black holes is more complex and not completely understood.

The team hopes to use Hubble again, in combination with the Atacama Large Millimeter/submillimeter Array (ALMA) and other facilities, to more accurately measure the speed of the black hole and its gas disk, which may yield more insight into the nature of this bizarre object.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

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Stars Born in Galactic Wind

Astronomers have found newborn stars in gas pouring out of a galactic nucleus.

stars forming in galactic outflow

ESO / M. Kornmesser

This artist’s illustration depicts stars forming in gas streaming out of the center of a galaxy. Outflows are a natural part of galaxy development, powered by bursts of starbirth or maniacally accreting black holes (or both) in galactic cores. Astronomers expect such outflows to ignite star formation. Although they’ve seen outflow-triggered star formation before, for example in cold gas condensing around bubbles inflated by black hole outbursts, it’s been difficult to conclusively spot stars forming in the winds themselves.

Reporting March 27th in Nature, Roberto Maiolino (University of Cambridge, UK) and colleagues say they’ve finally managed to find such stars. The team studied the system IRAS F23128−5919, a mishmash of (what was once two) merging galaxies in the far southern constellation Tucana, the Toucan. The system’s southern nucleus has a big stream of gas coming out of it.

The astronomers detected emission in this outflow that matches what’s seen in star-forming regions. They also found a population of young stars — just a few million years old — that’s moving with the gas at speeds up to 100 km/s (2 million mph). This rate is actually less than half the gas’s speed, but that’s expected: when the stars form, they feel the galaxy’s gravitational attraction and slow down; the gas, on the other hand, is driven onward by outward-shoving pressures. The team even sees a hint of stars losing the fight with gravity and beginning to fall back toward the disk.

The team estimates that about 15 Suns’ worth of stars form each year in the part of the outflow they can see. (The flow has a receding component that’s obscured by dust.) That’s more than 10% of the total estimated starbirth for this system.

Read more about the discovery in the European Southern Observatory’s press release.

Reference: R. Maiolino et al. “Star formation inside a galactic outflow.” Nature. March 27, 2017.

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