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Planetary nebula

A planetary nebula, abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The word “nebula” is Latin for mist or cloud, and the term “planetary nebula” is a misnomer that originated in the 1780s with astronomer William Herschel because, when viewed through his telescope, these objects resemble the rounded shapes of planets. Herschel’s name for these objects was popularly adopted and has not been changed. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.

Most planetary nebulae form at the end of the star’s life, during the red giant phase, when the outer layers of the star are expelled by strong stellar winds. After most of the red giant’s atmosphere is dissipated, the ultraviolet radiation of the hot luminous core, called a planetary nebula nucleus (PNN), ionizes the ejected material. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.

Planetary nebulae likely play a crucial role in the chemical evolution of the Milky Way by expelling elements to the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies, yielding useful information about their chemical abundances.

Stars greater than 8 solar masses (M⊙) will likely end their lives in dramatic supernovae explosions, while planetary nebulae seemingly only occur at the end of the lives of intermediate and low mass stars between 0.8 M⊙ to 8.0 M⊙.

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images: NASA/ESA, Hubble

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Hubble Sees Possible Runaway Black Hole Creating a Trail of Stars

There's an invisible monster on the loose, barreling through intergalactic space so fast that if it were in our solar system, it could travel from Earth to the Moon in 14 minutes. This supermassive black hole, weighing as much as 20 million Suns, has left behind a never-before-seen 200,000-light-year-long "contrail" of newborn stars, twice the diameter of our Milky Way galaxy. It's likely the result of a rare, bizarre game of galactic billiards among three massive black holes.

The black hole lies at one end of the column, which stretches back to its parent galaxy. There is a remarkably bright knot of ionized oxygen at the outermost tip of the column. Researchers believe gas is probably being shocked and heated from the motion of the black hole hitting the gas, or it could be radiation from an accretion disk around the black hole. "Gas in front of it gets shocked because of this supersonic, very high-velocity impact of the black hole moving through the gas. How it works exactly is not really known," said van Dokkum.

This intergalactic skyrocket is likely the result of multiple collisions of supermassive black holes. Astronomers suspect the first two galaxies merged perhaps 50 million years ago. That brought together two supermassive black holes at their centers. They whirled around each other as a binary black hole.

Credit: NASA

The Moon, Venus and Mercury at Dawn 1 - Feb 7, 2016

image credit: Joseph Brimacombe

Saturn and its moons

Image credit: NASA/JPL-Caltech

Siesta (by big andrei)

Ten interesting facts about Jupiter

Here is a list of some interesting facts about the planet Jupiter. A planet that catches the attention of all, by its size, storms and its surprising moons.

The mass of Jupiter is 318 times as massive as the Earth. In fact, Jupiter is 2.5 times more massive than all of the other planets in the Solar System combined.

Its gravity is so strong that a rocket would have to go an unthinkable 135,000 mph to leave.

The Great Red Spot on Jupiter is one of its most familiar features. This persistent anticyclonic storm, which is located south of its equator, measures between 24,000 km in diameter and 12–14,000 km in height. As such, it is large enough to contain two or three planets the size of Earth’s diameter. And the spot has been around for at least 350 years, since it was spotted as far back as the 17th century.

Jupiter’s rings were discovered in 1979 by the passing Voyager 1 spacecraft, but their origin was a mystery. Data from the Galileo spacecraft that orbited Jupiter from 1995 to 2003 later confirmed that these rings were created by meteoroid impacts on small nearby moons.

Extending up to seven million kilometers in the Sun’s direction and almost to the orbit of Saturn in the opposite direction, Jupiter’s magnetosphere is the largest and most powerful of any planetary magnetosphere in the Solar System, and by volume the largest known continuous structure in the Solar System after the heliosphere.

Jupiter has a total of 69 natural satellites. The four largest are: Io, Europa, Ganymede and Callisto. However, it is estimated that the planet has over 200 natural satellites orbiting it. Almost all of them are less than 10 kilometers in diameter, and were only discovered after 1975, when the first spacecraft (Pioneer 10) arrived at Jupiter.

Jupiter Has Been Visited 8 Times By Spacecraft. Jupiter was first visited by NASA’s Pioneer 10 spacecraft in December 1973, and then Pioneer 11 in December 1974. Then came the Voyager 1 and 2 flybys, both of which happened in 1979. This was followed by a long break until Ulysses arrived in February 1992, followed by the Galileo space probe in 1995. Then Cassini made a flyby in 2000, on its way to Saturn. And finally, NASA’s New Horizons spacecraft made its flyby in 2007. NASA’s Juno spacecraft is currently orbiting Jupiter.

Jupiter is the third brightest object in the Solar System, after Venus and the Moon.

Jupiter Is The Fastest Spinning Planet In The Solar System. For all its size and mass, Jupiter sure moves quickly. In fact, with an rotational velocity of 12.6 km/s (~7.45 m/s) or 45,300 km/h (28,148 mph), the planet only takes about 10 hours to complete a full rotation on its axis. And because it’s spinning so rapidly, the planet has flattened out at the poles a little and is bulging at its equator.

Jupiter Cannot Become A Star. Astronomers call Jupiter a failed star, but that’s not really an appropriate description. While it is true that, like a star, Jupiter is rich in hydrogen and helium, Jupiter does not have nearly enough mass to trigger a fusion reaction in its core. This is how stars generate energy, by fusing hydrogen atoms together under extreme heat and pressure to create helium, releasing light and heat in the process.

This is made possible by their enormous gravity. For Jupiter to ignite a nuclear fusion process and become a star, it would need more than 70 times its current mass. If you could crash dozens of Jupiters together, you might have a chance to make a new star. But in the meantime, Jupiter shall remain a large gas giant with no hopes of becoming a star. Sorry, Jupiter!

Sources: universetoday and wikipedia

Images credits: Wikimedia Commons, JAXA, NASA, ESA, Hubble, Wang Letian & Michael Carroll

What is a Wormhole?

Wormholes were first theorized in 1916, though that wasn’t what they were called at the time. While reviewing another physicist’s solution to the equations in Albert Einstein’s theory of general relativity, Austrian physicist Ludwig Flamm realized another solution was possible. He described a “white hole,” a theoretical time reversal of a black hole. Entrances to both black and white holes could be connected by a space-time conduit.

In 1935, Einstein and physicist Nathan Rosen used the theory of general relativity to elaborate on the idea, proposing the existence of “bridges” through space-time. These bridges connect two different points in space-time, theoretically creating a shortcut that could reduce travel time and distance. The shortcuts came to be called Einstein-Rosen bridges, or wormholes.

Certain solutions of general relativity allow for the existence of wormholes where the mouth of each is a black hole. However, a naturally occurring black hole, formed by the collapse of a dying star, does not by itself create a wormhole.

Wormholes are consistent with the general theory of relativity, but whether wormholes actually exist remains to be seen.

A wormhole could connect extremely long distances such as a billion light years or more, short distances such as a few meters, different universes, or different points in time

For a simplified notion of a wormhole, space can be visualized as a two-dimensional (2D) surface. In this case, a wormhole would appear as a hole in that surface, lead into a 3D tube (the inside surface of a cylinder), then re-emerge at another location on the 2D surface with a hole similar to the entrance. An actual wormhole would be analogous to this, but with the spatial dimensions raised by one. For example, instead of circular holes on a 2D plane, the entry and exit points could be visualized as spheres in 3D space.

Science fiction is filled with tales of traveling through wormholes. But the reality of such travel is more complicated, and not just because we’ve yet to spot one.

The first problem is size. Primordial wormholes are predicted to exist on microscopic levels, about 10–33 centimeters. However, as the universe expands, it is possible that some may have been stretched to larger sizes.

Another problem comes from stability. The predicted Einstein-Rosen wormholes would be useless for travel because they collapse quickly.

“You would need some very exotic type of matter in order to stabilize a wormhole,” said Hsu, “and it’s not clear whether such matter exists in the universe.”

But more recent research found that a wormhole containing “exotic” matter could stay open and unchanging for longer periods of time.

Exotic matter, which should not be confused with dark matter or antimatter, contains negative energy density and a large negative pressure. Such matter has only been seen in the behavior of certain vacuum states as part of quantum field theory.

If a wormhole contained sufficient exotic matter, whether naturally occurring or artificially added, it could theoretically be used as a method of sending information or travelers through space. Unfortunately, human journeys through the space tunnels may be challenging.

Wormholes may not only connect two separate regions within the universe, they could also connect two different universes. Similarly, some scientists have conjectured that if one mouth of a wormhole is moved in a specific manner, it could allow for time travel.

Although adding exotic matter to a wormhole might stabilize it to the point that human passengers could travel safely through it, there is still the possibility that the addition of “regular” matter would be sufficient to destabilize the portal.

Today’s technology is insufficient to enlarge or stabilize wormholes, even if they could be found. However, scientists continue to explore the concept as a method of space travel with the hope that technology will eventually be able to utilize them.

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Be Glad You Don’t Have to Dust in Space!

Throw open the windows and break out the feather duster, because spring is here and it’s time to do a little cleaning! Fortunately, no one has to tidy up the dust in space — because there’s a lot of it — around 100 tons rain down on Earth alone every day! And there’s even more swirling around the solar system, our Milky Way galaxy, other galaxies and the spaces in between. 

By studying the contents of the dust in your house — which can include skin cells, pet fur, furniture fibers, pollen, concrete particles and more — scientists learn a lot about your environment. In the same way, scientists can learn a lot by looking at space dust. Also called cosmic dust, a fleck of space dust is usually smaller than a grain of sand and is made of rock, ice, minerals or organic compounds. Scientists can study cosmic dust to learn about how it formed and how the universe recycles material.

“We are made of star-stuff,” Carl Sagan famously said. And it’s true! When a star dies, it sheds clouds of gas in strong stellar winds or in an explosion called a supernova. As the gas cools, minerals condense. Recent observations by our SOFIA mission suggest that in the wake of a supernova shockwave, dust may form more rapidly than scientists previously thought. These clouds of gas and dust created by the deaths of stars can sprawl across light-years and form new stars — like the Horsehead Nebula pictured above. Disks of dust and gas form around new stars and produce planets, moons, asteroids and comets. Here on Earth, some of that space dust eventually became included in living organisms — like us! Billions of years from now, our Sun will die too. The gas and dust it sheds will be recycled into new stars and planets and so on and so forth, in perpetuity!

Astronomers originally thought dust was a nuisance that got in the way of seeing the objects it surrounded. Dust scatters and absorbs light from stars and emits heat as infrared light. Once we started using infrared telescopes, we began to understand just how important dust is in the universe and how beautiful it can be. The picture of the Andromeda galaxy above was taken in the infrared by our Spitzer Space Telescope and reveals detailed spirals of dust that we can’t see in an optical image.

We also see plenty of dust right here in our solar system. Saturn’s rings are made of mostly ice particles and some dust, but scientists think that dust from meteorites may be darkening the rings over time. Jupiter also has faint dusty rings, although they’re hard to see — Voyager 1 only discovered them when it saw them backlit by the Sun. Astronomers think the rings formed when meteorite impacts on Jupiter’s moons released dust into orbit. The Juno spacecraft took the above picture in 2016 from inside the rings, looking out at the bright star Betelgeuse.

Copyright Josh Calcino, used with permission

And some space dust you can see from right here on Earth! In spring or autumn, right before sunrise or after sunset, you may be able to catch a glimpse of a hazy cone of light above the horizon created when the Sun’s rays are scattered by dust in the inner solar system. You can see an example in the image above, extending from above the tree on the horizon toward a spectacular view of the Milky Way. This phenomenon is called zodiacal light — and the dust that’s reflecting the sunlight probably comes from icy comets. Those comets were created by the same dusty disk that that formed our planets and eventually you and the dust under your couch!

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Everything You Need to Know About the Aug. 21 Eclipse

On Aug. 21, all of North America will experience a solar eclipse.

If skies are clear, eclipse-watchers will be able to see a partial solar eclipse over several hours, and some people – within the narrow path of totality – will see a total solar eclipse for a few moments.

How to Watch

It’s never safe to look at the Sun, and an eclipse is no exception. During a partial eclipse (or on any regular day) you must use special solar filters or an indirect viewing method to watch the Sun.

If you have solar viewing glasses, check to make sure they’re safe and undamaged before using them to look at the Sun. Make sure you put them on before looking up at the Sun, and look away before removing them. Eclipse glasses can be used over your regular eyeglasses, but they should never be used when looking through telescopes, binoculars, camera viewfinders, or any other optical device.

If you don’t have eclipse glasses, you can still watch the eclipse indirectly! You can make a pinhole projector out of a box, or use any other object with tiny holes – like a piece of cardstock with a hole, or your outstretched, interlaced fingers – to project an image of the partially eclipsed Sun onto the ground.

Of course, if it’s cloudy (or you’d just rather stay inside), you can watch the whole thing online with us at nasa.gov/eclipselive. Tune in starting at noon ET.

If you’re in the path of totality, there will be a few brief moments when it is safe to look directly at the eclipse. Only once the Moon has completely covered the Sun and there is no light shining through is it safe to look at the eclipse. Make sure you put your eclipse glasses back on or return to indirect viewing before the first flash of sunlight appears around the Moon’s edge.

Why do eclipses happen?

A solar eclipse happens when the Moon passes directly between the Sun and Earth, casting its shadow down on Earth’s surface. The path of totality – where the Moon completely covers the Sun – is traced out by the Moon’s inner shadow, the umbra. People within the Moon’s outer shadow, the penumbra, can see a partial eclipse.

The Moon’s orbit around Earth is tilted by about five degrees, meaning that its shadow usually doesn’t fall on Earth. Only when the Moon lines up exactly between the Sun and Earth do we see an eclipse.

Though the Sun is about 400 times wider than the Moon, it is also about 400 times farther away, making their apparent sizes match up almost exactly. This is what allows the Moon to block out the Sun’s bright face, while revealing the comparatively faint, pearly-white corona.

The Science of Eclipses

Eclipses are a beautiful sight to see, and they’re also helpful for our scientists, so we’re funding eleven ground-based science investigations to learn more about the Sun and Earth.

Total solar eclipses reveal the innermost regions of the Sun’s atmosphere, the corona. Though it’s thought to house the processes that kick-start much of the space weather that can influence Earth, as well as heating the whole corona to extraordinarily high temperatures, we can’t study this region at any other time. This is because coronagraphs – the instruments we use to study the Sun’s atmosphere by creating artificial eclipses – must cover up much of the corona, as well as the Sun’s face in order to produce clear images.

Eclipses also give us the chance to study Earth’s atmosphere under uncommon conditions: the sudden loss of solar radiation from within the Moon’s shadow. We’ll be studying the responses of both Earth’s ionosphere – the region of charged particles in the upper atmosphere – and the lower atmosphere.

Learn all about the Aug. 21 eclipse at eclipse2017.nasa.gov, and follow @NASASun on Twitter and NASA Sun Science on Facebook for more. Watch the eclipse through the eyes of NASA at nasa.gov/eclipselive starting at 12 PM ET on Aug. 21. 

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