Be sure to mark your calendar for tomorrow night. For lucky observers in northeastern North America, eastern Canada, and western Europe, the evening of September 19-20, 2008 is your opportunity to watch the face of the peaceful gibbous Moon glide across the ancient blue beauty of the Plieades…
On Friday night, September 19, 2008, observers in northeastern North America, eastern Canada need to be outdoors and ready when the Moon begins to rise low in the east-northeast. If you live in Western Europe, the event takes place high in the sky just before dawn on the morning of September 20th. Don’t come alone, bring binoculars or a telescope with you, because this is your chance to see the stars first disappear behind the Moon’s bright limb and then reappear on the dark side.
Although we rarely call the Pleiades by name, let’s try them on for size. The brightest is Alcyone at magnitude 2.86, followed by Atlas at 3.62, Electra at 3.7, Maia at 3.86, Merope at 4.17 and Taygeta at magnitude 4.29. While it may be a bit difficult for a novice to read the occultation information, please check this occultation information at IOTA where you can get precise times for your location for the disappearance and reappearance of each individual star.
To help you understand a bit further, let’s choose Alycone. Click on the September 20th category for the US and let’s choose Cleveland, Ohio. Alycone would disappear behind the bright limb of the Moon at 02:14:09 UT (which would be 10:14:09 pm, September 19th). Alycone will re-emerge from behind the dark edge of the Moon at 02:45:47 UT (or 10:45:47 pm). See? It’s not that hard! The most difficult part is simply figuring out the universal time difference is all it takes, and there’s even a website for that!
While you can watch the event without any optical aid, the Moon will overpower the stars. Use a card or something held at arm’s length to help diminish the glare. However, if you’re serious? A telescope is the key to really enjoying this event. It is great fun to keep tabs on each star as it slowly approaches the lunar limb and then just winks out! Timing of these events is critical, because it helps astronomers to further understand lunar features. How? By assessing times, astronomers are able to determine if there are peaks and valleys that we simply do not know about. For example, a bright star may wink off and on several times before it finally disappears behind a hidden lunar mountain… and the same holds true when it reappears.
Although the Moon frequently encounters the Pleiades on each monthly journey across the ecliptic plane, it’s infrequent that we have such a great opportunity to watch several occultation events in a well-placed area of the sky during a comfortable time of the year. Enjoy this great event…
Images accompanying this article are the Pleiades Occultation by John Cudworth and the annotated image of the Plieades by David Malin, courtesy of the Anglo-Australian Observatory/University of Edinburgh.
[/caption]Mercury, the planet closest to the Sun, is a study in extremes and offers many surprises. The extremes of the planet have made it an understudied body in our Solar System, though the MESSENGER mission is trying to change that as you are reading this article.
In addition to being the planet closest to the Sun, Mercury is also the smallest by mass. If you ignore the former planet Pluto, it is also the smallest by surface area, as well. The planet has the most eccentric orbit: at perihelion it is 46,001,200 km from the Sun and at aphelion it is 69,816,900 km. The planet’s short orbital period(87.969 Earth days) and slight axial tilt combine to make the day on Mercury(116 Earth days) longer than the year.
The average temperature on the planet is 442.5°K. Because of the planet’s thin atmosphere there is a wide temperature range, 100°K to 700°K. The temperature at the equator can be as much as 300°K more than the temperature at the poles. Despite its proximity to our central star, the poles of the planet are thought to have water ice hidden within impact craters. Claims for water ice are substantiated by observations by the 70 m Goldstone telescope and the Very Large Array. There are areas of very high radar reflection at the pole areas so, since water is highly reflective of radar, astronomers believe that water ice is the most likely cause of this reflection.
Due to its size and average temperatures, the planet’s gravity can not retain a significant atmosphere over a long period. It does have a negligible surface-bounded exosphere that is dominated by hydrogen, helium, oxygen, sodium, calcium, and potassium. Atoms are continuously being lost and replenished from this exosphere. Hydrogen and helium atoms are thought to derive from the solar wind that buffets the planet. These elements diffuse into Mercury’s magnetosphere before escaping back into space. Radioactive decay within the crust is a source of helium, sodium, and potassium.
Mercury has been explored by two mission: Mariner 10 and MESSENGER. Mariner 10 was able to map 40-45% of Mercury’s surface through more than 2,800 photos. It revealed a more or less moon-like surface, a slight atmosphere, a magnetic field, and a large iron rich core. MESSENGER was launched in August of 2004. After a 31/2 year flight, it made its first flyby in January 2008 and arrived in orbit on March 18, 2011. So far, the probe has discovered large amounts of water in the exosphere, evidence of past volcanic activity, and evidence of a liquid planetary core.
As the MESSENGER mission continues, the closest planet to the Sun should continue to reveal more surprises for the scientists at NASA. It appears a new age of discovery has begun for Mercury.
We have an extensive section just on Mercury on Universe Today. And did you know there’s a spacecraft visiting Mercury called MESSENGER? You can read news about this mission here.
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I’m sure you know that we live in the Milky Way galaxy, but where is the Sun located? And how did astronomers figure out where the Sun is located, since we’re living inside the galaxy?
The Milky Way is a grand spiral galaxy, which astronomers think has four major spiral arms: Perseus, Cygnus, Scutum-Crux, Sagittarius. Some astronomers think we might actually just have two arms, Perseus and Sagittarius. The Sun is located in the inner rim of the Orion Arm, which is thought to be an offshoot of the Sagittarius Arm. The Sun is located about 26,000 light-years away from the center of the galaxy.
Before telescopes, the Milky Way just looked like a bright area in the sky, but when Galileo first turned his telescope on the region in 1610, he realized that it was actually made up of faint stars. The astronomer Immanuel Kant correctly guessed that this might be a cloud of stars held together by gravity, like the Solar System.
The famous astronomer William Herschel attempted to map out the stars in the Milky Way to get a sense of the galaxy’s size and shape, and determine the Sun’s position in it. From Herschel’s first map, it appeared the Sun was at the center of the Milky Way. It was only later on that astronomers realized that gas and dust was obscuring our view to distant parts of the galaxy, and that we were actually in the outer region of the Milky Way.
The astronomer Harlow Shapley accurately determined where the Sun is in the MIlky Way in the early 20th century by noticing that globular clusters were uniformly located above and below the Milky Way, but they were concentrated in the sky towards the constellation Sagittarius. Shapely realized that many globular clusters must be blocked by the galactic core. He created one of the most accurate maps of the Milky Way.
It wasn’t until the 20th century, with the development of larger and more powerful telescopes that astronomers could see the shape of other spiral galaxies, located millions of light-years away. In 1936, Edwin Hubble used cepheid variables as yardsticks to measure the distances to many galaxies, and prove conclusively that the Universe was filled with galaxies, each with as many stars as our own Milky Way.
Here’s an article about the Great Debate that Harlow Shapley had about the nature of the Milky Way. And here’s Shapley’s obituary, published in Nature in 1972.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.
September 26, 2007 APOD - Saguaro Moon by Stefan Seip
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Actually, the official time was September 15 at 5:13 a.m. EDT, but missing the exact time isn’t going to stop a little beauty from happening tonight. What else can I say except “Come a little bit closer.. Hear what I have to say. Just like children sleepin’… We could dream this night away. But there’s a full moon risin’… Let’s go dancin’ in the light. We know where the music’s playin’… Let’s go out and feel the night…”
I love Harvest Moon time – mainly because there’s so much folklore and legend attached to it. Here in the heartland, we associate it with tractors in the field, working late into the night gathering the harvest by the light of our nearest astronomical neighbor. It’s a romantic and fanciful thought – especially since modern tractors just combine the stuff down with headlights approximately bright enough to land a Boeing 747. However, it’s still a lot of fun to think about old cultures like the Norse folks who believed the Moon granted them Loki’s blessing for plenty. I’ve heard it called the Singing Moon, too… A time for rest after harvest, sit around, sing some songs, smoke a peace pipe. Or, you can celebrate with the Celtics. They called it the Wine Moon, eh? No matter what your choice may be, the whole object is to be mellow.
But, hey! What’s mellow without a little science behind it? What constitutes the Harvest Moon most of all is that it’s closest to Equinox. Just this little tidbit and the change of seasons ought to give you a clue of what’s going on. Most of time during the year, the Moon comes along about 50 minutes later each night, but as the tilt of our Earth is gradually changing, that time is a bit shorter – by around 20 minutes for several evenings in a row. Why? The answer is easy enough. The ecliptic – or plane of Earth’s orbit around the sun – makes a narrow angle with respect to the horizon in the evening in autumn.
Is it really more orange or yellow than normal? How about larger? Oh, yes. You want those science facts, don’t you? Sure! Why not… Oftentimes we perceive the Harvest Moon as being more orange than at any other time of the year. The reason is not only scientific enough – but true. Coloration is caused by the scattering of the light by particles in our atmosphere. When the Moon is low, as it is now, we get more scattering effect and it truly is a deeper orange. The very act of harvesting itself also produces dust and oftentimes that color will last through the whole night. As for larger? Well, that’s just an illusion. Everyone knows the Moon looks larger on the horizon, but did you know this is a psychological phenomenon and not a physical one? Prove it to yourself by looking at the rising Moon upright…it looks larger, doesn’t it? Now stand on your head, or find a comfortable way to view it upside down…now how big is it?
Go on out tonight and enjoy the Harvest Moon… “Harvest moon… I see the days grow shorter. I feel the nights grow cold. Harvest moon… Young people feeling restless. Old people feeling old. Harvest moon… I sense the darkness clearer. I feel the presence here. Harvest moon… A change in the weather. I love this time of year. Harvest Moon….”
An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)
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In ancient times, everyone thought the Earth was the center of the Universe – it was obvious to anyone who just looked up. The Sun, Moon, stars and planets were thought to be attached to crystal spheres that turned around us. We now know that the Earth goes around the Sun, but how do we know this?
In astronomy, putting the Sun at the center of the Solar System is known as heliocentrism, while putting the Earth at the center is called geocentrism. As astronomers put in more and more time studying the heavens, they realized that this model didn’t match reality. The Sun didn’t follow an exact path every day, and the planets didn’t move how they were supposed to.
It wasn’t until the 16th century that the Polish astronomer Copernicus developed a model that placed the Sun at the center of the Solar System.
Until that point, astronomers had developed very complicated models that tried to explain the motions of the planets. At times they appeared to move backwards in the sky, and then go forwards again. Astronomers had developed the thought that there were spheres within spheres that could explain these motions. Copernicus simplified things, and showed that all the planets were orbiting the Sun, and the strange motions of the planets was then easy to understand as the Earth caught up and then passed them in orbit.
In 1610, Galileo used his first rudimentary telescope to observe that Venus went in phases just like the Moon. This went against the theory that everything orbited the Earth, and was further evidence that it goes around the Sun. Galileo also observed how Jupiter has 4 major moons that orbit it. This broke the previous believe that all objects orbit the Earth.
More precise measurements followed, and Johannes Kepler created his three laws that explained that the planets were actually following elliptical orbits around the Sun. He was the first astronomer to accurately predict a transit of Venus, where the planet was seen to pass directly in front of the Sun.
The motion of the Earth as it goes around the Sun is well calculated today. Space agencies use these calculations to launch spacecraft to explore the other planets in the Solar System. If everything went around the Earth, we’d know by now.
Known as Caldwell 20 to some, NGC 7000 to others and the North America Nebula to most, this diffuse emission/reflection nebula near Deneb can frequently be seen by the unaided eye from a dark location, but the sheer size of this 1600 light year distant gas cloud often confuses people as to the reality of what they are seeing. Let’s take a look at just a few of the bricks in the “Wall”..
Hey you, out there beyond the wall… Is there anybody out there?
In this image taken by Kent Wood, we are looking a just a close-up of the region shaped like the Gulf of Mexico and often referred to as the “Cygnus Wall”. It is here that light from young, energetic stars is taking the surrounding cold gas fields and warming them, causing an ionization front to form – filled with dense and delightfully delicate filaments. This highly energized “shock front” stands out in bold relief against the complex dark gases and streaking dark dustlanes.
What shall we use… To fill the empty spaces? What shall we use… To complete the wall?
Let’s try star formation, eruptive variables, flare stars and T-Tauri types. According to G.W. Marcy: “A slitless spectrographic search for H..cap alpha.. emission stars in NGC 7000 has revealed 18 new examples, most of which are presumably T Tau stars. An examination of all known T Tau stars in these fields has uncovered no events of the FU Ori type, except for that of V1057 Cygni.” All of these make themselves at home in the warm ionized gas in the local interstellar medium. However, it is the properties of this ionized gas that are so curious to study. In this case, in the faint optical emission lines of hydrogen alpha.
Hey you, don’t help them to bury the light…
Along the bright rim of the wall is where the action is at. According to the work of Koji (et al), it is here where most of the star forming action is going on. “We have found small clusters of near-infrared sources having young stellar object (YSO) colors in some of these objects; most of the cluster members are considered to be older than the IRAS point sources and to be pre–main-sequence stars such as T Tauri stars. In at least six bright-rimmed clouds, the clusters are elongated toward the bright-rim tip or the exciting star(s) of the bright rim with the IRAS sources situated near the other end. There is a tendency for bluer (i.e., older) stars to be located closer to the exciting star(s) and for redder (i.e., younger) stars to be closer to the IRAS sources. This asymmetric distribution of the cluster members strongly suggests small-scale sequential star formation or propagation of star formation from the side of the exciting star(s) to the IRAS position in a few times 105 yr, as a result of the advance of the shock caused by the UV radiation from the exciting star(s).”
And all in all it was just a brick in the wall…
But some of the true beauty is the dust and soot laced clouds filled with PAHs. We learned about those Polycyclic Aromatic Hydrocarbons, not long ago and just what they mean. And, we know the Cygnus X region is one of the richest star formation sites in the Galaxy. But what about this structure? This Wall?
The Wall- NGC 7000 (Panorama) by Kent Wood
A distant ship, smoke on the horizon…. You are only coming through in waves.
Believe it or not, NGC 7000 was imaged from the lunar surface during the 1972 Apollo 16 mission and continues to be studied for its polarization properties and scattering in h-alpha wavelengths. It has even had its electron temperature taken to prove that interstellar dust is masking the light we see. However, what we do see may be an illusion. From the studies of R.J. Reynolds; “According to photoionization models of the warm ionized medium, these [O i]/Ha ratios suggest that most of the Ha originates from density-bounded, nearly fully ionized regions along the lines of sight rather than from partially ionized H i clouds or layers of H ii on the surfaces of H i clouds.”
Hey you, out there beyond the wall… Is there anybody out there?
Venture into the dark cloud and find out. According to Laugalys (et al) “Magnitudes and color indices of 430 stars down to V Ëœ 17.5 mag in the eight-color Vilnius + I photometric system were obtained in four areas of diameter 20′ within the dark cloud L935 separating the North America and Pelican nebulae. Spectral types, interstellar color excesses, extinctions and distances of stars were determined from the photometric data. The plot of extinction vs. distance shows that the dark cloud begins at a distance of 520±50 pc. About 40 stars in the cloud, mostly K and M dwarfs, are suspected to have Hα emission; these stars also exhibit infrared excesses. Four of them are known pre-main-sequence stars. Our star set contains J205551.3+435225 (V = 13.24) which, according to Camerón and Pasquali (2005), is the O5 V type star ionizing the North America and Pelican nebulae. If this spectral type is confirmed, the star would have an extinction AV between 9 and 10 magnitudes (depending on the accepted extinction law) and a distance which is not very different from the dust cloud distance.”.
How shall I fill the final places? How should I complete the wall?
I guess the last words would be the illuminating source. In a study done by Comerón and Pasquali; “We present the results of a search for the ionizing star of the North America (NGC 7000) and the Pelican (IC 5070) nebulae complex. The application of adequate selection criteria to the 2MASS JH KS broad-band photometry allows us to narrow the search down to 19 preliminary candidates in a circle of 0o 5 radius containing most of the L935 dark cloud that separates both nebulae. Follow-up near-infrared spectroscopy shows that most of these objects are carbon stars and mid-to-late-type giants, including some AGB stars. Two of the three remaining objects turn out to be later than spectral type B and thus cannot account for the ionization of the nebula, but a third object, 2MASS J205551.25+435224.6, has infrared properties consistent with it being a mid O-type star at the distance of the nebulae complex and reddened by AV ≃ 9.6. We confirm its O5V spectral type by means of visible spectroscopy in the blue. This star has the spectral type required by the ionization conditions of the nebulae and photometric properties consistent with the most recent estimates of their distance. Moreover, it lies close to the geometric center of the complex that other studies have proposed as the most likely location for the ionizing star, and is also very close to the position inferred from the morphology of cloud rims detected in radio continuum. Given the fulfillment of all the conditions and the existence of only one star in the whole search area that satisfies them, we thus propose 2MASS J205551.25+435224.6 as the ionizing star of the North America/Pelican complex.”
All in all… It’s just another brick in the wall.
We would like to thank AORAIA member, Kent Wood for the splendid image and the great research challenge!
Sun with a huge coronal mass ejection. Image credit: NASA
There are so many beautiful pics of the Sun, it’s almost too difficult to know where to start.
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This is a picture of the Sun captured by NASA’s SOHO spacecraft. It would be a typical day on the Sun, except for the enormous coronal mass ejection blasting out of the upper right-hand side of the Sun. When the Sun is at its most active state, it can release 5-6 of these a day.
STEREO's image of the Sun. Image credit: NASA
This photograph of the Sun was one of the first captured by NASA’s STEREO mission. These twin spacecraft were launched in 2006. One is leading the Earth in orbit, while the other has fallen behind. With both observing the Sun, scientists are given a 3-dimensional view of the Sun.
Sun seen from TRACE. Image credit: NASA
This pic of the Sun shows our star on a calm day, believe it or not. When you look close, this is what the surface of the Sun is doing all the time. The TRACE spacecraft was launched in 1997, and helps scientists study the Sun’s magnetic field – and to take beautiful photos like this.
Ultraviolet view of the Sun. Image credit: SOHO
This picture of the Sun was captured by the EIT instrument on board the NASA/ESA SOHO spacecraft. It reveals the normally invisible ultraviolet radiation streaming from the Sun. It’s actually a composite of three different Sun photos captured at different parts of the ultraviolet spectrum and then merged together.
Picture of the Sun in 3-D. Image credit: NASA
You’re going to need a set of 3-D glasses to get the most out of this Sun photograph. It’s an image of Sun captured by NASA’s twin STEREO spacecraft. Images like this help scientists understand how the Sun interacts with its local environment, and better predict space weather.
You take your clock for granted today, but it’s only been in the last couple of centuries that machines (and electronics) have been accurate enough to be used for timekeeping. Before that, people had to use other ways to tell the time of day. One of the most useful and easy to make is a sundial.
In its simplest form, a sundial consists of a style – a thin rod or sharp straight edge – that casts a long shadow onto a flat surface. As the Sun moves in the sky, the shadow moves as well in a perfectly predictable way. By putting marks on the flat surface, you can know what time it is by the position of the shadow.
For a sundial to work, it must be aligned with the axis of the Earth’s rotation. The style must be pointed towards North, and the style’s angle with horizontal must be equal to the sundial’s latitude.
NASA’s Mars Exploration rovers are equipped with miniature sundials on top of their color calibration targets. Scientists use these to fix colors in images based on the known colors in these calibration targets. The sundials are decorative, but they also help locate the Sun’s direction compared to the rovers.
This article on Universe Today talks about the sundial attached to NASA’s Mars Exploration rovers.
Would you like to make your own sundial? NASA has a cool page that shows you how to construct and use your own sundial. This page gives you a template so you can construct your own sundial (warning, it’s a PDF document, not a web page).
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.
Position of the Sun in the Milky Way. Image credit: NASA
Everything’s orbiting something it seems. The Moon goes around the Earth, and the Earth orbits the Sun. But did you know that the Sun orbits the Milky Way galaxy?
Astronomers have calculated that it takes the Sun 226 million years to completely orbit around the center of the Milky Way. In other words, that last time that the Sun was in its current position in space around the Milky Way, dinosaurs ruled the Earth. in fact, this Sun orbit has only happened 20.4 times since the Sun itself formed 4.6 billion years ago.
Since the Sun is 26,000 light-years from the center of the Milky Way, it has to travel at an astonishing speed of 782,000 km/hour in a circular orbit around the Milky Way center. Just for comparison, the Earth is rotating at a speed of 1,770 km/h, and it’s moving at a speed of 108,000 km/h around the Sun.
It’s estimated that the Sun will continue fusing hydrogen for another 7 billon years or so. In other words, it only has another 31 orbits it can make before it runs out of fuel.
Are you interested in more articles about the Sun? We have written plenty for Universe Today. Here’s an article that shows how some stars take an erratic journey around the Milky Way, and another article about a ring of stars orbiting the Milky Way.
Here’s an article that describes the process astronomers used to determine the orbit around the Milky Way.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.
Sun with a huge coronal mass ejection. Image credit: NASA
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We owe everything we have to the Sun. If it weren’t for the Sun, there’d be no life on Earth. The relationship between Sun and Earth has gone back for 4.6 billion years, and should last for another 7 billion years or so.
As you probably know, the Sun is just a giant sphere of gas. At the core of the Sun, huge quantities of hydrogen are squished together in the intense pressure and temperature of this extreme environment. Hydrogen is converted to helium, and this reaction releases a tremendous amount of energy.
How much energy? Astronomers calculate that there are 600 million tons of hydrogen fused every second. 4 million tons of matter is converted to pure energy every second. This releases 3.86×1026 joules of energy every second. Although most of this energy heads off into space, plenty still falls onto the Earth. In fact, there’s enough energy coming from the Sun to deposit 342 Watts of energy onto every square meter of the Earth (averaged over the year, over the whole planet).
From our perspective, Sun and Earth go hand in hand. This energy from the Sun heats up the planet, preventing us from cooling down to near absolute zero temperatures of space. Our atmosphere traps the energy as heat, keeping the whole planet a nice comfortable temperature.
Plants have been soaking up this energy for millions of years. When you burn gasoline in your car, it comes from oil, which is energy from the Sun that planets have been storing for millions of years.
Sun and Earth are locked in a gravitational dance as well. The mass of the Sun is 2 × 1030 kilograms. This is enough to reach out across space and keep the Earth (and the rest of the planets) locked in orbit around it. We even experience tides from the gravity of the Sun.
Were you wondering how far away the Earth is from the Sun? And the Sun isn’t always trying to help us. Sometimes it’s throwing monster flares at us as well.
Although we rarely call the Pleiades by name, let’s try them on for size. The brightest is Alcyone at magnitude 2.86, followed by Atlas at 3.62, Electra at 3.7, Maia at 3.86, Merope at 4.17 and Taygeta at magnitude 4.29. While it may be a bit difficult for a novice to read the occultation information, please check this occultation information at IOTA where you can get precise times for your location for the disappearance and reappearance of each individual star. 
