Category: Guide to Space

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The diameter of Neptune is approximately 49,500 km. This makes Neptune the 4th largest planet in the Solar System, after Jupiter, Saturn and Uranus.

I say approximately because the diameter of Neptune changes depending on where you measure it. Neptune is rotating on its axis, completing a full day once every 16 hours or so. This rapid rotation flattens Neptune out slightly so that the diameter measured from pole to pole is less than the equatorial diameter.

Neptune’s polar diameter is 48,682 km. While its equatorial diameter is 49,528 km. In other words, points on the equator are 423 km more distant from the center of Neptune than the poles.

Want some comparison? The diameter of Neptune is about 3.9 times the diameter of Earth.

We have written many stories about Neptune for Universe Today. Here’s an article about how Neptune’s south pole is the warmest part of the planet.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

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A day on Neptune is 16 hours, 6 minutes and 36 seconds.

Wait, not so fast! Here’s the problem. Neptune isn’t a single solid object like the terrestrial planets, so different parts of the planet rotate at different speeds. This is a process that astronomers call differential rotation. Neptune’s equatorial zone takes about 18 hours to complete a rotation – that’s slower than the planet’s averate 16.1 hour rotation period. And the polar regions can take just 12 hours to rotate; much more quickly than the average.

This big difference in rotational rate between the equatorial regions and the planet’s poles means that Neptune has a strong latitudinal wind shear. This helps to generate the strongest winds in the Solar System. Astronomers have clocked winds on Neptune going as fast as 2,400 km/hour (1,500 miles/hour).

We have done several stories about Neptune on Universe Today. Here’s an article about movies of Neptune captured by Hubble. These show its rotation.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

The color of Neptune is a bright azure blue. During its flyby in 1989, NASA’s Voyager 2 revealed the bright blue color, different from the pale blue color of Uranus. So why does Neptune have this color?

The answer to Neptune’s color comes from its cloud tops. The upper atmosphere of Neptune is made up of 80% hydrogen, 19% helium with a trace 1% amount of methane and other ices, like ammonia and water. Methane absorbs light at 600 nm, which is the red end of the spectrum of visible light.

Like all the planets in the Solar System, the light we see coming from Neptune is actually reflected light from the Sun. These methane clouds absorb the red end of the spectrum, and allow the blue end of the spectrum to bounce back out. So when you see the color of Neptune, you’re seeing reflected sunlight with the red light stripped out.

From a distance, Neptune looks just like a blue ball, but as you get closer you can see variations in its clouds. Lighter clouds of methane hang above the lower cloud deck. Powerful storms whip across the surface of Neptune; the fastest storms in the Solar System are on Neptune, with winds exceeding 2,400 km/hour. Neptune has a large dark storm, similar to the Great Red Spot on Jupiter.

We have written many articles about Neptune on Universe Today. Here’s an article about “movies” of Neptune captured by Hubble. And some additional images captured by Hubble that really show the color of Neptune.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

The atmosphere of Neptune is similar to all the large planets in the Solar System; it mostly consists of hydrogen and helium, with trace amounts of methane, water, ammonia and other ices. But unlike the other gas planets in the Solar System, Neptune’s atmosphere has a larger proportion of the ices. It’s the methane in the planet’s upper atmosphere that give it its bright blue color.

At the highest altitudes, where the Neptune’s atmosphere touches space, it consists of about 80% hydrogen and 19% helium. There’s also a trace amount of methane. The light we see from Neptune is actually the reflected light from the Sun. Although the entire spectrum of light hits Neptune. This trace amount of methane absorbs light from the red end of the spectrum, while allowing the blue light to bounce back out. The color of Neptune’s atmosphere is brighter than Uranus, which has a similar atmosphere; astronomers aren’t sure why there’s such a dramatic color difference.

The upper level clouds on Neptune occur at the point where pressures are low enough for methane to condense. Astronomers have photographed these high altitude clouds forming shadows onto the lower cloud deck below. Deeper down inside Neptune, temperatures should get up to 0 C, where clouds of water might form.

As with the other planets, the atmosphere of Neptune is broken up into distinct bands of storms. In fact, the fastest moving winds in the Solar System occur at Neptune – winds have been clocked at 2,400 km/h (1,500 miles per hour). Some storms can grow large and remain for long periods of time. Neptune has its own Great Dark Spot, similar to the Great Red Spot on Jupiter.

We have written many articles on Universe Today about the atmosphere of Neptune and its storms. Here’s an article about the weather in springtime on Neptune, and how Neptune’s south pole might be the warmest place on the planet.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

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When we look at the Moon, we see a landscape shaped by impact craters. But scientists only discovered the true cause of crater in the last hundred years. Before that, they believed that there were many volcanoes on the Moon, and this is what caused the craters we see today.

Now we know that craters come from meteorite impacts, that still doesn’t answer the question: are there volcanoes on the Moon?

There used to be volcanoes on the Moon. The Moon is much smaller than the Earth. Although it was molten after its formation, it cooled down relatively rapidly. Scientists think that the Moon’s interior remained hot enough to produce magma for about a billion years after the Moon formed. The lava that came out of the Moon cooled quickly, and formed fine-grain, dark rocks called basalt. The Apollo astronauts sampled this material when they landed on the Moon.

When you look at the Moon, you see lighter and darker regions. The lighter regions are the mountainous highlands. The darker regions are vast “seas” of basalt lava that erupted out of the Moon billions of years ago.

Are there volcanoes on the Moon today?

There is recent evidence that there were volcanoes on the far side of the Moon much longer than on the near side. While the near side of the Moon shut down more than 3 billion years ago, there seems to be evidence that there were volcanoes on the surface of the Moon as recent as about a billion years ago.

Some researchers believe there are still vents that blast out volcanic gasses, but there are no longer active volcanoes on the Moon.

Want more information about the Moon? Here’s NASA’s Lunar and Planetary Science page. And here’s NASA’s Solar System Exploration Guide.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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Created as one of Ptolemy’s original 48 constellations and positioned just south of the celestial equator, Lepus has endured the test of time to become one of the 88 modern constellation recognized by the IAU. Spanning 290 square degrees of sky, it ranks fifty-first in size and contains only 2 bright stars, yet has 8 stars in its major asterism. Within the confines of Lepus you will also find 20 stars with Bayer/Flamsteed designations. It is bordered by the constellations of Orion, Monoceros, Canis Major, Columba, Caelum and Eridanus. Lepus is visible to all observers at latitudes between +63° and ?90° and is best seen at culmination during the month of January.

In mythology, or perhaps more correctly story and legend, Lepus represents the hare at Orion the Hunter’s feet. It is believed the winged messenger god, Hermes honored the hare for its speed, giving it a place amongst the stars. It is also believed that Canis Major, Orion’s dog, forever pursues Lepus across the sky. The Egyptians saw this constellation as associated with Osiris and fertility… Of course, there is no more fertile creature than a rabbit!

Let’s start our binocular tour of Lepus with Alpha Leporis – the “a” symbol on our map. Its name is Arneb and it literally means “hare” in Arabic. Arneb is an older, dying star that may have already passed through a supergiant phase and is now contracting and heating up in the latter phases of stellar evolution, or perhaps is still expanding into the supergiant phase. With a mass of likely less than 10 times that of the Sun, it will likely end its life as a hot white dwarf, although if it is at the heavier end of its estimated mass it may end in a spectacular stellar explosion known as a supernova. Positioned about 1300 light years from Earth, Arneb may be dying… But it still has a few years of light left for you to enjoy!

Stay with binoculars and head south for Beta Leporis – the “B” symbol on our map. Beta’s proper name is Nihal – the “drinking camel”. Somewhat similar to our Sun, this unusual 159 light year distant dwarf star outshines Sol by 165 times. Why? Probably because it’s 16 times larger. Inside it has a rapidly evolving helium core and in less than a million years it will brighten as it begins to fuse its internal helium into carbon. Now take a look in a telescope. That’s right, Nihal is a binary star. About 2.5 seconds of arc away you’ll find a companion star that’s sometimes as bright as stellar magnitude 7 and sometimes as dim as 11. So what’s going on here? Chances are the companion star is an eclipsing double, much like an Algol-type. What’s more, the primary star – Nihal A – is also a bright X-ray source, which means it has strong stellar magnetic properties. According to research, it has a high content of yttrium and the rare earths praseodymium, neodymium, and samarium – chemicals that occurred because it began life just a little hotter than usual!

Now hop to Gamma Leporis – the “Y” shape on our chart. Gamma is a multiple star system which is about 29 light-years from Earth and consists of 2 or possibly 3 stars. What’s so cool about another multiple system? This one is on the move! Gamma is part of the Sirius Moving Group Of Stars. These stars are all about the same distance away and part of a larger collective of stars known as the Ursa Major Moving Group. Based upon its stellar characteristics and distance from Earth, Gamma Leporis, a main-sequence white-yellow dwarf star, is considered a high-priority target for NASA’s Terrestrial Planet Finder mission as well!

Point your binoculars or telescope at R Leporis – better known as “Hind’s Crimson Star”. Very few places in the sky will you find such ruby beauty! This well-known variable star is right on the border of Eridanus, but since the border doesn’t show on the sky, simply use bright Rigel to help you locate it. Named after famous British astronomer J.R. Hind, who observed it in 1845, you’ll find the most excellent carbon star varies from around magnitude 5 to 12 in about 427 to 432 days. In other words, you basically observe it from one year to the next! Hind’s Crimson star is the most beautiful when it is a minima, displaying an incredible smoky red color, which turns almost garnet as it brightens the following year. Enjoy this annual favorite!

Now keep those binoculars and telescopes handy as we drop a little less than four degrees south/southwest (a binocular field) of Beta and go for Messier 79 (RA 05:24.5 Dec -24:33). This 7th magnitude globular cluster was originally discovered by Pierre Mechain and later added to the Messier Catalog. Located about 40,000 light years from our solar system, the huge ball of stars spread across 118 light years of space an incorporates tens of thousands of distant suns. What’s unusual about it? Chances are, M79 is an import to our Milky Way Galaxy. From what we can tell through recent studies, this globular cluster may have actually belonged the the Canis Major dwarf galaxy at one time and became part of our galaxy through a galaxy collision! For double star fans, look another half degree southwest where you’ll see fifth magnitude ADS 3954 and its seventh magnitude companion. A nice same field bonus!

Sources: SEDS, Wikipedia
Chart Courtesy of Your Sky.

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Leo Minor is a very small and dim constellation which was created by Johannes Hevelius in 1687 and later recognized as one of the 88 modern constellations. While Leo Minor did not belong to any older star catalogs such as those drawn up by Ptolemy in the 2nd century AD, this set of stars became part of the Firmamentum Sobiescianum, a 56 sheet atlas created by master astronomer Hevelius in an attempt to update star catalogs using what (was then) considered modern equipment. Leo Minor was one of seven new constellations and endured to become officially recognized by the International Astronomical Union. It possesses no bright stars and has only 2 main stars in its asterism, yet there are 34 Bayer/Flamsteed designated stars within Leo Minor’s confines. It spans 232 square degrees of sky and is bordered by the constellations of Ursa Major, Lynx, Cancer and Leo. Leo Minor is visible to all observers at latitudes between +90° and ?45° and is best seen at culmination during the month of April.

Since Leo Minor, the “Little Lion” is consider a new constellation, it has no ancient mythology associated with it. As you may have noticed by looking at the chart, it curiously has no Alpha star. When it came to making charts, Hevelius was great – but he didn’t label stars. It wasn’t until the 19th-century when English astronomer Francis Baily had a go at Leo Minor that he assigned the stars with their Greek letters and he simply overlooked the Alpha designation! Leo Minor is just another example of how constellation names and figures can sometimes repeat themselves, like Ursa Major and Minor, Canis Major and Minor, Pegasus and Equuleus… Hydra and Hydrus. Half the challenge to this constellation is simply finding it!

Break out your binoculars and let’s have a look at Beta Leonis Minoris – the “B” shape on our map.. This is a very rapid binary star – not in terms of movement through space – but in orbit of its companion star. Believe it or not, the 6th magnitude companion completes a full orbit in less than 40 years. That’s just a little bit slower than Saturn takes to orbit our Sun and over twice as fast at it takes Neptune!

Now head east for 46 Beta Leonis Minoris. By all rights, this should have been the Alpha star and it’s the only Bayer/Flamsteed numbered stellar designation to have a proper name – Praecipua. As stars go? Well, Praecipua is actually pretty ordinary. Just another orange giant star hanging out in space around 98 light years from Earth. It is happily radiating away about 32 times brighter than our Sun and it is around 9 times bigger. One of the coolest things about this star is just how well we know it! According to Jim Kaler’s excellent information; “Recent accurate measures of angular diameter by the Navy Interferometer show it to be 0.00254 seconds of arc across (the separation of car headlights seen from a distance of 80,000 kilometers, 20 percent of the way to the Moon), which gives it a physical diameter 8.2 times that of the Sun, the agreement with the previously calculated diameter showing that we know the size, temperature, luminosity, and distance very well.”

Now, get out your telescope and let’s go on a galaxy hunt. Our first target is NGC 3486 (RA 11:00.4 Dec +28:58). At magnitude 10, this barred spiral galaxy discovered by Sir William Herschel is around 33 million light years away and it has attitude. Even in a small telescope, observers will note a bright, sharp nucleus and larger instruments will reveal a strong central bar and patchy structure that is the signature of a Seyfert galaxy.

Next up is a large telescope challenge – NGC 3344 (RA 10:43.31 Dec +24:55). Located much closer to the Milky Way Galaxy at 25 million light years in distance, this 13th magnitude grand design spiral galaxy is a face-on presentation, and only about half the size of our own galactic home. Like our preceding observation, it, too, has a central bar – but don’t be fooled by the foreground stars! According to studies done by Verdes-Montenegro (et al), the bar is exponential and dominates the central parts, while the bulge component is small. This makes this faint customer belong to the classification of a “ringed galaxy”.

Sources: SEDS, Wikipedia
Chart Courtesy of Your Sky.

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The constellation of Lacerta is unusual, because it did not belong orignally to those created by Ptolemy – but to the works of Johannes Hevelius. Lacerta was included in Firmamentum Sobiescianum, a 56 page atlas created by Hevelius, which outlined seven new constellations which survived time – and many which did not. Positioned north of the ecliptic plane, it spans 201 square degrees of sky and contains 5 main stars in its asterism and 17 Bayer/Flamsteed designated stars within its boundaries. Lacerta is bordered by the constellations of Andromeda, Cassiopeia, Cepheus, Cygnus and Pegasus. It is visible to all observers at latitudes between +90° and ?90° and is best seen at culmination during the month of December.

Since Lacerta is considered a “modern” constellation, there is no mythology associated with it – although the stellar pattern was very visible to the ancient Greeks and Romans. At the time, Native American culture was highly regarded and the Chusmash of the California region referred to this area of the sky as the “Lizard”. Perhaps Hevelius honored their many stories and their culture by adopting the Latin term for lizard – Lacerta – and placing it upon this constellation.

Although Lacerta contains no bright stars, once you pick out its dim lightning bolt pattern of stars you’re well on the way to exploring with binoculars or a telescope. A sure way to help locate it is to wait for a dark night and scan the sky between Cassiopeia and Cygnus. When you’re ready, let’s take a look at Alpha Lacertae – the “a” symbol on our map. While it is just a rather ordinary A-class star residing about 102 light years away from our solar system, Alpha is about twice the size of our Sun and shines about 27 more brightly. Take a look through a telescope and you will see that Alpha appears to have a companion, but it is only an optical double star. The 11.8 magnitude line of sight interloper is really almost 2600 more light years away!

Now hop to Beta Lacertae – the “B” symbol on our map. Located about 170 light years from Earth, Beta is a giant yellow star, similar in some ways to our own Sun, but far more massive. If you’re seeing a field of stars to the west/southwest of Beta in binoculars, you’d be correct. Positioned about 2.6 degrees away from Beta is loose open cluster NGC 7243, also known as Best 59 or Caldwell 16. It contains about 40 stars and is spread out over a very large area which makes it a nice binocular object. If you’ve got the magnification power of a telescope on it, be sure to check out the brightest star in the cluster. Its name is Struve 2890 and it’s a great double star! For a telescope viewing challenge, look about 2 degrees west/northwest of Beta for IC 1434 – another open cluster. At magnitude 10, the small compressed beauty is meant for larger optics!

For another great rich field telescope treat, aim your sights towards NGC 7209 (RA: 22h 05m 12.0s Dec.:+46 29’ 59”). At a comfortably bright magnitude 7.7, this galactic star cluster is well compressed and very rich in stars. Also known as Collinder 444 and Melotte 238, this stellar beauty has been studied photometrically for reddening and metallicity, as well as the presence of suspected binary stars. Viewable in binoculars as a dim, hazy patch and well resolved in the telescope.

For binary star fans, have a look at 8 Lacerta (RA 22h 35m 52.28s Dec.: +39d 38’ 03.6”). Here you’ll find a beautiful multiple star system that’s also on the Astronomical League 100 list. In the telescope eyepiece, look for a 5.7 magnitude primary star accompanied by a 6.5 secondary star separated by about 22″. Further away you’ll find the 7.2 magnitude C star separated by about 82″. It’s very worthy of your time and attention!

Source: Wikipedia
Chart Courtesy of Your Sky.

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The constellation of Indus was originally created by Petrus Plancius from the observations of Dutch sea navigators Pieter Dirkszoon Keyser and Frederick de Houtman when exploring the southern hemisphere. Indus’ stellar patterns became known when it appeared on a celestial globe in 1597 and was considered a constellation when it was added to Johann Bayer’s Uranometria catalog in 1603. It survived the years to become one of the 88 modern constellations recognized by the International Astronomical Union. Indus is located south of the ecliptic plane and covers approximately 294 square degrees of sky. It consists of three main stars in the primary asterism and has 16 stars with Bayer/Flamsteed designations. Indus is bordered by the constellations of Microscopium, Sagittarius, Telescopium, Pavo, Octans, Tucana and Grus.

Since the constellation of Indus wasn’t created until late in the sixteenth century, there isn’t any ancient mythology associated with its stellar patterns. However, Indus is meant to represent as a native – perhaps one met by the Dutch explorers on their travels in the Indies or Africa. It is also believed that Johannes Bayer wish to honor the native American Indians as well, so Indus was thus included in his works.

For observers, let us begin with binoculars the brightest star – Alpha Indi – the “a” symbol on our map. Who know exactly how stars sometimes get their names, but you’ll often find this star is called ” the Persian” on some lists. Located about 101 light years from our solar system, Alpha is super-metal-rich, K-type orange giant star that outshines our own Sun by about 62 times. Have a telescope? Be sure to take a look at “the Persian”. You’ll find it has a pair of 12th and 13th magnitude red dwarf star companions!

Now hop to the center of this Y-shaped asterism and have a look at Theta Indi. That’s right! Another binary star. Located about 91 light years from Earth, you’ll find a very nice double star here, with components that are easy to separate with a small telescopes. The primary is fifth magnitude and the secondary is magnitude seven.

Time to follow the branch of the Y southwest and have a look at Beta Indi – the “B” shape on our map. While the rest of the stars we’ve look at so far were fairly near – Beta isn’t. Located at minimum of 600 light years away, Beta Indi is a very massive and luminous star of the orange K-type classification. Take a look through the telescope, too… Because you’ll find that Beta also has a 12th magnitude visual companion whose distance is unclear. At the southeast end of the Y branch is Delta, whose orbital mechanics have been closely studied.

Now drop south for Epsilon – the backward “3”. Epsilon Indi is one of the closest stars to Earth, approximately 11.82 light years away. Epsilon is a dwarf star – only about 75% the size of our own Sun – and very similar in respects to movement, corona and gravity. Even its photosphere and metallicity is a close comparison. In 1847, Heinrich Louis d’Arrest was the first to notice that Epsilon had moved right along compared to its charted 1750 position, and it has been measured about every 100 years since. Astronomers have since placed it in what is called the Epsilon Indi Moving Group of Stars – a stellar association of about 16 members that quite likely formed about the same time in the same location.

Of course, being so close means Epsilon was also the object of many signal studies, including radio signals and lasers – but unfortunately, no signals were ever returned. Even though we haven’t gotten a reply, it still leads the list of 17,129 nearby stars most likely to have planets that could support complex life. With good reason! In January 2003, astronomers announced the discovery of a brown dwarf with a mass of 40 to 60 Jupiter masses in orbit around Epsilon Indi at a distance of at least 1500 astronomical units… And what’s more, it’s actually a binary brown dwarf star! Although measurements of the radial velocity of Epsilon Indi appear to show the presence of a planetary companion with an orbital period of more than 20 years, so far no space telescope yet has been able to prove its existence.

Now it’s time to take a telescope tour of Indus. Our first object is IC 5152 (RA 22:02.9 Dec -51:17). Hanging out about 3 million light years away, this irregular dwarf galaxy could very well be an outlying member of our own Milky Way local group of galaxies. At roughly magnitude 11, look for some patchy details, including a line of sight star caught on its edge.

Next up? NGC 7090 (RA 21:36.5 Dec -54:33). Even though billed at near magnitude 11, this soft spoken spiral galaxy is low surface brightest to the eye – but an astrophotographer’s dream. It has a fantastic h-alpha halo! Look for several scattered stars in the same field, including a very wide equal optical pair lying to the east.

Hop now to NGC 7083 (RA 21:35.7 Dec -63:54). At magnitude 12, this galaxy is meant for larger telescopes, but this barred galaxy is also highly studied for its spiral galaxy structure. It is considered a grand design and well worth taking some time on!

Last for now? NGC 7049 (RA 21:19.0 Dec -48:34). Although on the small side, NGC 7049 is a bit brighter than our last study at magnitude 11. It is an early-type spiral galaxy and also a target of the Hubble Space telescope, which studied it for its inner polar disc properties. You’ll find it about about 15′ east of a bright, yellowish star (6.5 magnitude) and the surface brightness will allow you to do a little more serious studying!

Source: Wikipedia, ESO
Chart courtesy of Your Sky.