Category: Astronomy

As the summer nights in the northern hemisphere grow short, Saturn fades quickly to the west and with it takes our great views of the solar system’s most serene and restful planet. If you were to walk out tonight and point a small telescope Saturn’s way, what would you see? Step right up the the StarGazer’s Telescope and let’s look…

Saturn, the sixth planet from the Sun, has the most spectacular set of rings in the solar system – ones which are easily seen in even the smallest of telescopes. Even at roughly 793 million miles away, we can still see this thin band of icy particles and rubble circling the magnificent planet. Maybe it was a comet that ventured too close… Perhaps Saturn’s moons collided… But no matter what the explanation is, the gentle yellow serenity of Saturn is something to behold.

Can you see Saturn’s gentle, yellow color? Much of the atmosphere of Saturn is made of hydrogen and helium, but there is also water vapor, methane, ammonia, and nitrogen. While it looks warm by nature, these cloudtops are quite cold, about –220° Fahrenheit. Winds in the upper atmosphere reach 500 meters (1,600 feet) per second in the equatorial region which is about five times faster than any hurricane force wind ever encountered here on Earth. These super-fast winds, combined with heat rising from within the planet’s interior, cause the yellow and gold bands visible in the atmosphere.

Did you know Saturn was the most distant of the five planets known to the ancients? In 1610, Galileo Galilei was the first to gaze at Saturn through a telescope and his view wasn’t even as close to as good as what we see here. In 1659, Christiaan Huygens, used a far more powerful telescope and announced that Saturn was surrounded by a thin, flat ring. A few years later in 1675, Jean-Dominique Cassini discovered a ‘division’ between what are now called the A and B rings. Thanks to modern science, we now know the gravitational influence of Saturn’s moon Mimas is what’s responsible for the black band known as Cassini Division. Can you see it peeking in and out when the atmosphere gets steady? Even from all these millions of miles away, we’re still able to resolve a feature which is 4,800 kilometers (3,000 miles) wide.

Do you want a real rush? While you’re looking at Saturn, click here. This is an audio file of radio emissions from Saturn. The Cassini spacecraft began detecting these radio emissions in April 2002, when Cassini was 374 million kilometers (234 million miles) from the planet, using the Cassini radio and plasma wave science instrument. Isn’t it creepy? It’s also kind of beautiful too… Like listening to whale songs.

Now move over… It’s my turn at the eyepiece.

Many people think that the answer to ‘what is the largest moon in the Solar System’ is our Moon. It is not. Our Moon is the fifth largest natural satellite. Ganymede, a moon of Jupiter, is the largest in this solar system. At 5,268 km at the equator, it is larger than Mercury, the dwarf planet Pluto, and three times larger than the Moon orbiting Earth. According to information from NASA,if Ganymede were to break free of Jupiter’s gravitational pull it would be classified as a planet.

Ganymede has an iron-metallic core that generates a magnetic field. The core is surrounded by a mantle of rock, which is, in turn, covered by a thick shell of ice and rock. The outer shell is up to 800 km thick. On top of the outer shell is a thin layer of material(accreted?) that is a mixture of ice and rock. In images taken by the Hubble Space Telescope in 1996, astronomers discovered a tenuous atmosphere of oxygen. The atmosphere would not support known life forms, but its very existence is interesting to science.

Through spacecraft observation, there are several facts known about the surface of Ganymede. Its surface shows two types of terrain. Forty percent of the surface is covered with highly cratered dark regions, while the remainder is covered by light grooved terrain. The light grooving forms intricate patterns across surface of the moon, some of which are thousands of km long. Sulcus, a term that means a groove or burrow, is often used to describe the grooving. This portion of the terrain was most likely formed by tensional faulting or the release of water from beneath the surface. Ridges as high as 700 m have been observed. The dark regions are heavily cratered, old and rough, and are believed to be the original crust of the moon.

Ganymede was discovered by Galileo on January 7, 1610. He made the discovery, along with three other Jovian moons. It marked the first time a moon was discovered orbiting a planet other than Earth. The discovery contributed to the acceptance of the heliocentric viewpoint over the geocentric that held sway prior to that.

Now you know the answer to ‘what is the largest moon in the Solar System’ and a few interesting facts about Ganymede. Jupiter has 63 moons, so there are plenty more facts for you to discover.

Here’s another article about Ganymede, and one about Ganymede’s lumpy interior.

Here’s a list of all the largest moons in the Solar System, and a listing of the largest moons and smallest planets at Solar Views.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.

References:
NASA Solar System Guide: Ganymede
Wikipedia: Ganymede
NASA Multimedia

Greetings, fellow SkyWatchers! Our weekend begins with one very hot look at a very cool area of the Moon – the lunar poles. Why is finding ice just so important? Hang on to your eyepiece because we’ll continue this lunar expedition with some mountain climbing and a look at the big lunar picture. Before our weekend is out, we’ll take a trip to the land of Yed and find out what’s coming up on Monday night. Time to slip into the night… Together.

Friday, July 11 – Tonight let’s take look at the lunar poles by returning to an old previous study crater, Plato. North of Plato you will see a long horizontal area of grey floor – Mare Frigoris – the “Cold Sea.” North of it you will note a “double crater.” This elongated diamond-shape is Goldschmidt, and the crater which cuts across its western border is Anaxagoras. The lunar north pole isn’t far from Goldschmidt, and since Anaxagoras is just about one degree outside of the Moon’s theoretical “arctic” area, the lunar sunrise will never go high enough to clear the southernmost rim. As proposed with yesterday’s study, this “permanent darkness” must mean there is ice! For that very reason, NASA’s Lunar Prospector probe was sent to explore here. Did it find what it was looking for? Answer – Yes!

The probe discovered vast quantities of cometary ice which has hidden inside the crater’s depths untouched for millions of years. If this sounds rather boring to you, then realize this type of resource may aid our plans to eventually establish a manned base on the lunar surface!

On March 5, 1998 NASA announced that Lunar Prospector’s neutron spectrometer data showed water-based ice was discovered at both lunar poles. The first results showed the “ice” mixed in with lunar regolith (soil, rocks and dust), but long-term data confirmed nearly pure pockets hidden beneath about 40 centimeters of surface material – with the results being strongest in the northern polar region. It is estimated there may be as much as 6 trillion kilograms (6.6 billion tons) of this valuable resource! If this still doesn’t get your motor running, then realize that without it we could never establish a manned lunar base because of the tremendous expense involved in transporting our most basic human need – water.

The presence of lunar water could also mean a source of oxygen, another vital material we need to survive. In order to return home or voyage onward, these same deposits could provide hydrogen which could be used as rocket fuel. So as you view Anaxagoras tonight, realize you may be viewing one of mankind’s future “homes” on a distant world!

Saturday, July 12 – Tonight let’s take an entirely different view of the Moon as we do a little “mountain climbing!” The most outstanding feature on the visible surface will be the emerging Copernicus, but since we’ve delved into the deepest areas of the lunar surface, why not climb to some of its peaks?

Using Copernicus as our guide, to the north and northwest of this ancient crater lie the Carpathian Mountains ringing the southern edge of Mare Imbrium. As you can see, they begin well east of the terminator, but look into the shadow! Extending some 40 kilometers beyond the line of daylight, you will continue to see bright peaks – some of which reach a height of 2072 meters. When the area is fully revealed tomorrow, you will see the Carpathian Mountains disappear into the lava flow that once formed them.

Continuing onward to Plato, which sits on the northern shore of Imbrium, we will look for the disjointed line of (1) Montes Recta – the “Straight Range.” Further east you will find the scattered peaks of (2) the Teneriffe Mountains. It is possible these are the remnants of much taller summits of a once stronger range, but only around 1890 meters of them still survive above the surface.

To the southeast, (3) Mons Pico stands like a monument 2400 meters above the grey sands – a height which places it level with Kindersley Summit at Kootenay Park in British Columbia. Further southeast is the peak of (6) Mons Piton – also standing alone in the barren landscape of Imbrium. Perhaps once a member of the (5) Montes Alpes to the east, Piton still towers 2450 meters above the surface with a base 25 kilometers in diameter still remaining in the lava flow. Yet look closely at the lunar Alpes, for (4) Mons Blanc is 3600 meters high!

Just north of shallow Archimedes stand (7) the Montes Spitzbergen whose remaining expanse trails away for 60 kilometers on the southern edge of a rille which begins at the small punctuation of crater Kirch to the north. While they only extend 1500 meters above the surface, that’s still comparable with the outer Himalayans!

Sunday, July 13 – So what if it’s the 13th? If you’re not superstitious, but only having bad luck in finding lunar features – then how about if we take a look at one that’s incredibly easy to find? We’ll continue our lunar mountain climbing expedition and look at the “big picture” on the lunar surface.

Tonight all of Mare Imbrium is bathed in sunlight and we can truly see its shape. Appearing as a featureless ellipse bordered by mountain ranges, let’s identify them again. Starting at Plato and moving east to south to west you will find the Alps, the Caucasus, and the Apennines (where Apollo 15 landed) at the western edge of Palus Putredinus. Next come the Carpathian Mountains just north of Copernicus. Look at the form closely: doesn’t it appear that perhaps once upon a time an enormous impact created the entire area? This was the Imbrium impact: compare it to the younger Sinus Iridum. Ringed by the Juras Mountains, it may have also been formed by a much later and very similar impact.

And you thought they were just mountains…

Now let’s have a look with our eyes first at Delta Ophiuchi. Known as Yed Prior (“the hand”), look for its optical double Epsilon to the southeast, symmetrically named Yed Posterior. Try using binoculars or a telescope at absolute minimum power for another undiscovered gem…

Delta Ophiuchi is 170 light-years from us, while Epsilon is 108 – but look at the magnificent field they share. Stars of every spectral type are in an area of sky which could easily be covered by a small coin held at arm’s length. Enjoy this fantastic field – from the hot, blue youngsters to the old red giants!

Now, keep a watch out for a certain old, red giant named Antares on Monday night, June 14. For many of us, the Moon and the Rival of Mars are going to be hauntingly close – close enough to be an occultation event for some lucky observers!

Have a great weekend….

This week’s images are: Lunar North Pole – Credit: NASA, Lunar Mountain Peaks – Image Credit: Greg Konkel Annotations: Tammy Plotner, Half and Gibbous Moon – Credit: Roger Warner, and Delta Ophiuchi – Credit: Palomar Observatory courtesy of Caltech.

Ever have one of those moments when you can’t remember how old you are? A group of astronomers may have felt they were having a “senior moment” when they couldn’t seem to figure out exactly the age of stars in the open star cluster NGC 6791, located in the constellation Lyra. Conventional thinking among astronomers is that stars in open clusters form at the same time, but in this particular cluster, researchers found stars at three different ages: one group of white dwarf stars appeared to be 4 billion years old, a second group of white dwarfs seemed to 6 billion years old, while the other regular stars were calculated to be 8 billion years of age. The astronomers say this dilemma may fundamentally challenge the way astronomers estimate cluster ages. Ivan King of the University of Washington and leader of the group using the Hubble Space Telescope to study this star cluster said: “This finding means that there is something about white dwarf evolution that we don’t understand.”

I just love it when astronomers say something like that, because it means they’ll return to their telescopes and the data in order to figure out the dilemma, and we’ll learn something new. And that’s just what they did. At least, partially.

“The age discrepancy is a problem because stars in an open cluster should be the same age. They form at the same time within a large cloud of interstellar dust and gas. So we were really puzzled about what was going on,” explained astronomer Luigi Bedin, who works at the Space Telescope Science Institute in Baltimore, Md.

After extensive analysis, members of the research team realized how the two groups of white dwarfs can look different and yet have the same age. It is possible that the younger- looking group consists of the same type of stars, but the stars are paired off in binary-star systems, where two stars orbit each other. Because of the cluster’s great distance, astronomers see the paired stars as a brighter single star.

Their brightness made them look younger.

Binary systems are also a significant fraction of the normal stellar population in NGC 6791, which contains over 10,000 stars, and are also observed in many other clusters. However, this would be the first time they have been found in a white-dwarf population.

“Our demonstration that binaries are the cause of the anomaly is an elegant resolution of a seemingly inexplicable enigma,” said team member Giampaolo Piotto the University of Padova in Italy.

Bedin and his colleagues are relieved that they now have only two ages to reconcile: an 8- billion-year age of the normal stellar population and a 6-billion-year age for the white dwarfs. All they need now is a process that slows down white-dwarf evolution.

Hubble’s Advanced Camera for Surveys analyzed the cooling rate of the entire population of white dwarfs in NGC 6791, from brightest to dimmest. White dwarfs are the smoldering embers of Sun-like stars that no longer generate nuclear energy and have burned out. Their hot remaining cores radiate heat for billions of years as they slowly fade into darkness. Astronomers have used white dwarfs as a reliable measure of the ages of star clusters, because they are the relics of the first cluster stars that exhausted their nuclear fuel.

White dwarfs have long been considered dependable because they cool down at a predictable rate. The older the dwarf, the cooler it is, making it a seemingly perfect clock that has been ticking for almost as long as the cluster has existed.

All right, astronomers, back to your telescopes to get this all figured out! And when they do, the rest of you can read about it on Universe Today. In the meantime, enjoy the lovely images above of star cluster NGC 6791.

News Source: Hubble press release