Category: Astronomy

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The equatorial radius of Neptune is 24,764 km.

That’s the quick answer. But you need to understand that things are a little more complicated. Like all of the planets in the Solar System, Neptune is spinning rapidly, completing a rotation in 16 hours and 6 minutes. This rapid rotation causes the planet to flatten out, so that the radius across the equator is bigger than the radius to the poles.

So here’s the more precise answer. The radius of Neptune, measured from the center to the equator is 24,764 km. And the radius of Neptune, measured from the center to either pole is 24,341 km. I’ll do the math for you. That means that the points on the equator are 423 km further away from the center of Neptune than either pole.

Need some comparison? Neptune’s radius is 3.9 times the radius of Earth. In other words, you could line up almost 4 Earths side by side to match the width of Neptune.

We have written many articles about Neptune for Universe Today. Here’s an article about the potential for liquid water deep down within Neptune. And here’s an article about how Neptune’s largest moon Triton might have been captured by Neptune’s gravity.

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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Neptune is so dim and distant that you can only see it with a moderately powerful telescope. But Neptune is the 4th largest planet in the Solar System. Let’s take a look at how big Neptune is, and compare it to some other objects that you might be familiar with.

Neptune is the 4th largest planet in the Solar System, after Jupiter, Saturn, and Uranus. It’s much larger than the terrestrial planets: Mercury, Venus, Earth and Mars.

The diameter of Neptune is 49,500 km. Need some comparison? That’s approximately 3.9 times the diameter of Earth. In other words, you could put almost 4 Earths side to side to match the diameter of Neptune.

The volume of Neptune is 6.25 x 10¹³ km³. That’s an enormous number, so once again, for comparison, that’s 57.7 times the volume of Earth. You could fit 57 Earths inside Neptune with room to spare.

The surface area of Neptune is 7.64 x 10⁹ km². That’s 15 times as much surface area as Earth; of course, Neptune doesn’t have a solid surface, so you wouldn’t want to live there.

The mass of Neptune is 1.02 x 10²⁶ kg. Again, for comparison, that’s the equivalent of 17.1 Earths.

So now, when you look through a telescope and see that tiny blue-green dot, you can get a better sense of the size of Neptune.

We have written many articles about Neptune on Universe Today. Here’s an article about a minor planet found near Neptune. And an article about how Neptune’s south pole is the hottest 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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The diameter of the luminous Milky Way is between 100,000 and 120,000 light years across, but that number could be much larger if you take into account dark matter. It all depends on where you measure the edge of the Milky Way to be…

If you are just measuring the normal matter we can see (in visible, infrared, X-ray and ultraviolet light), then the Milky Way is at least 100,000 light years across. The diameter is a bit larger (120,000 light years) if you take into account tidal streams – basically other galaxies the Milky Way is eating, such as the Sagittarius Dwarf Elliptical Galaxy.

But normal matter isn’t all that makes up the Milky Way. Simulations of our galaxy show that it has a “halo” of dark matter, which makes up about 10 times the mass of the visible matter in the Milky Way. Dark matter has never been directly observed, but is inferred due to its gravitational pull. This halo extends past the edge of the luminous part of the Milky Way, but the size of the halo has yet to be determined to a great degree of accuracy.

How do we measure the diameter of the Milky Way, given the fact that we live inside of it? We measure the distance to Cepheid variable stars. These are stars whose luminosity changes in a very predictable way because they puff up and shrink. Knowing the absolute luminosity of these stars allows us to measure their distance. Think of it this way: if you know how bright a flashlight is at 10 feet from you, and how that luminosity changes over distance, when it moves further away you can calculate that distance by determining how much dimmer the flashlight is.

Pamela talks about the diameter of the Milky Way, and how we measure it, in Episode 99 of Astronomy Cast. If you’re interested in learning more about variable stars – or even observing them – the American Association of Variable Star Observers is a great place to start.

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The constellation of Musca was originally created by Petrus Plancius from the stellar observations of Dutch sea navigators Pieter Dirkszoon Keyser and Frederick de Houtman when exploring the southern hemisphere. Musca’s star patterns became known when it appeared on a celestial globe in 1597 and was considered a constellation named Apis when it was added to Johann Bayer’s Uranometria catalog in 1603, but it was misinterpreted as a fly instead of a bee! In 1752 Nicolas Louis de Lacaille renamed it to Musca Australis, the Southern Fly to avoid confusion with Apus. Today the name is simply Musca and it has survived the years – and the confusion – to become one of the 88 modern constellations recognized by the International Astronomical Union. Located south of the ecliptic plane and covering only 138 square degrees of sky, Musca ranks 77th in size among its peers. It has 6 main stars in its asterism and 13 Bayer Flamsteed designated stars within its confines. Musca is bordered by the constellations of Apus, Carina, Centaurus, Chamaeleon, Circinus and Crux. It is visible to observers located at latitudes between +10° and ?90° and is best seen at culmination during the month of May.

Since Musca is a “new” constellation, there is no mythology associated with it – only a bit of folklore. When Petrus Plancius drew the insect on his celestial globe in 1598, for some reason he didn’t put a name on the critter, although Frederick de Houtman had referred to it in his native Dutch as “the fly”. In 1603, Johannes Bayer also added it to his star charts as well – but misinterpreted the insect as a bee, calling it Apis. Apparently it was also charted in another 1603 work by Willem Janszoon Blaeu, who being Dutch, understood the correct language and labeled it as a fly. By the time it reached the hands of Abbe Nicolas Louis de Lacaille in 1752 it was corrected yet again to Musca Australis – the southern counterpart of Musca Borealis. No wonder the IAU was invented! It was finally shortened to just Musca and adopted as an official constellation in 1930.

Let’s begin our binocular tour of Musca with a look at its brighter star – Alpha Muscae – the “a” symbol on our map. This class B giant star is located about 305 light years from Earth and shines over 4500 times brighter than our own Sun. Is it hot? You bet. Alpha Muscae runs a stellar temperature of around 21,900 degrees Kelvin – so hot that most of its light is emitted in the ultra-violet range. It whirls around at its equator at a speed of 114 kilometers per second, making a full rotation in about 48 hours. And… it has a Beta Cepheid variable star heartbeat. It pulses. About every 2.2 hours it changes its magnitude just ever so slightly!

Now, hop over to Beta Muscae – the “B” symbol on our map. Beta is a binary star of almost equal magnitude. This pretty blue star isn’t easy to split and will require high magnification in a telescope and a fine, steady night of seeing. For an easier double star, try Eta Muscae (13h 15.4 min RA -67 55 Dec). You’ll find it wide, bright and easy… With a bonus binary star in the field, too! Theta Muscae (13h 08.1 min RA -65 18 Dec) is also another fine binary star that shows an interesting color contrast.

For both binoculars and telescopes, try your hand at globular cluster NGC 4833 (12h 59.6 min RA -70 53 Dec). At not quite magnitude 7, Caldwell 105 is well compressed and shows some great resolution in larger instruments. It was first discovered by Abbe Lacaille during his 1751-1752 journey to South Africa, and catalogued in 1755 – then later observed and catalogued by James Dunlop and Sir John Herschel whose instruments could resolve it into individual stars. Located about 21,200 light years from our solar system, it would be a whole lot brighter if it weren’t for the Milky Way Galaxy’s dust!

Keep your binoculars and telescopes handy for NGC 4372 (12h 25.8 min RA -72° 40 Dec). This slightly brighter globular cluster is a very loosely constructed Class XII discovered by James Dunlop on April 30, 1826. It’s very metal poor and observations by the XMM-Newton Telescope have shown the presence of close binaries which are “thought to play an important role in the stability of the clusters by liberating energy and delaying the inevitable core collapse of globular clusters”.

Small telescopes will enjoy open star cluster NGC 4815 (RA 12h 57m 59.0s Dec -64° 57′ 36.0″). What it lacks in size, it makes up for in richness. Unlike the “Jewel Box”, this little cluster suffers greatly from interstellar absorption. Enjoy this relative of the Hyades!

Mid-to-large telescopes will enjoy planetary nebula NGC 5189 (13h 33.6 min RA -65° 59 Dec). Nicknamed the “Spiral Planetary Nebula”, this little gem was discovered by John Herschel in 1835. Located about 3,000 light years away from Earth, NGC 5189 has been studied for its kinematic structure and contains an unusual expanding ring of gas that we see nearly edge-on.

While in Musca, take a look for the southern extension of the Coal Sack – a dark nebula. Located about 600 light years away, this obscuration cloud was was known to the people of the Southern Hemisphere in prehistoric times and has even been referred to historically as the “Black Magellanic Cloud”.

Sources: SEDS, Wikipedia
Chart Courtesy of Your Sky.

The constellation of Monoceros was originally charted on a work done by Petrus Plancius in the early 1600s for its biblical references, but its first historical reference appears in Jakob Bartsch’s star charts created of 1624 where it was listed as Unicornu. There is also a possibility, according to Heinrich Wilhelm Olbers and Ludwig Ideler’s work with older astrological charts, that Monoceros could have been referred to as “the Second Horse” – while historian Joseph Justus Scaliger also makes reference to it in his (mid 1500s) work with Persian astrological records. Regards of its origins, Monoceros was adopted as one of the 88 modern constellations by the International Astronomical Union in 1930 and remains on the charts today. It is a relatively dim constellation that consists of 4 main stars in its primary asterism and contains 32 Bayer Flamsteed designated stars within its confines. Monoceros spans approximately 482 square degrees of sky and is bordered by the constellations of Canis Minor, Gemini, Hydra, Lepus, Orion and Puppis. It is visible to all observers located at latitudes between +75° and ?85° and is best seen at culmination during the month of February.

There is one annual meteor shower associated with Monoceros which peaks on or about December 10 of each year – the Monocerids: The radiant for this meteor shower occurs near the border of Gemini and averages about 12 meteors per hour at maximum fall rate. It is best viewed when there is little to no Moon to interfere with the faint streaks and activity is at its most when the constellation reaches the zenith.

Because Monoceros is a relatively “new” constellation, there isn’t any mythology associated with it – but the Unicorn itself has a long history of mystery. You’ll not find this creature mention anywhere in mythology, but everywhere else! The unicorn is mention in the Bible, in accounts of natural history, in Chinese lore, Ethiopian artwork, medieval stories and religious art. It is depicted as a one-horned horse, thought to have existed somewhere at the edge of the known Earth.. and it still exists roaming the edges of the celestial sphere just between the northern and southern ecliptic plane. Fable or folklore? No matter which, it’s filled with many great and starry delights!

Let’s begin our binocular tour of Monoceros with its primary star – Alpha Monocerotis – the “a” symbol on our map. Hanging out in space some 144 light years from Earth, it’s not the brightest star in the constellation, nor is it particular special. Alpha is just another orange/yellow helium-fusing giant star, not a whole lot different than ours. Averaging about 11 times larger than our Sun and putting out about 60 times more light, Alpha’s hydrogen fuel tank went to empty about 250 million years ago. Now it just waits quiety, waiting for its helium shell to fade away… ready to spend the rest of its life as just another dense white dwarf star.

Now, take a look at Beta Monocerotis – the “B” symbol on our map. If you think it’s slightly brighter – you’re right. That’s because Beta has some help from two other stars, too! Put your telescope Beta’s way and discover what Sir William Herschel called “one of the most beautiful sights in the heavens”. This fantastic triple star star system is located about 690 light years from our solar system. As you watch it slowly drift by the eyepiece, you’ll know the names of the stars by which leave sight first… from west to east they are A, B and C. In this circumstance, it is believed the B and C stars orbit each other and the A star orbits this pair. All three are about 34 million years old and all three are dwarf stars. Close to each other in magnitude, this trio of hot, blue/white B3 stars each run a temperature of about 18,500 Kelvin and shine anywhere from 3200 down t0 1300 times brighter than our own Sun and spinning on their axis up to 150 times faster. A real triple treat!

For binoculars, have a look at visual double star Delta Monocerotis – the “8” symbol on our map. Located 115 light years from our solar system, this cool pair is worth stopping by – just to see if you can resolve it with your eyes alone! Don’t forget to try Epsilon Monocerotis, too. The backwards “3” on our map. Larger, steady binoculars may separate it and it’s easy for a smaller telescope. This is a very pretty gold and yellow combination binary star, seperated by about two magnitudes. You’ll find it on a number of observing lists. While there, take a look just two degrees northwest of Epsilon for T Moncerotis. This is a great Cepheid variable star with a period of 27 days and a magnitude range of 6.4 to 8.0. Those are the kinds of changes you can easily notice!

Our first deep sky binocular and telescope target will be magnificent Messier 50 (RA 07:03.2 Dec -08:20). This splendid open star cluster averages around magnitude 6 and was logged on April 5, 1772 by Charles Messier in his catalog on deep sky objects. Located about about 3,200 light years from Earth, it spans about 20 light years of space and contains about 200 stars. Inside this 78 million year old cloud is at least one red giant star – located just a little bit south of central. Can you spot it? How about the smattering of yellows amid the blue/whites?

Now head for equally bright NGC 2301 (6:51.8 Dec +00:28). This easily resolvable chain of stars can be seen in binoculars, but requires a telescope to resolve its individual members. Smaller telescopes will notice at least 30 members, while larger aperture can detect many more from this 80 member galactic star cluster. Located about 2500 light years away, be sure to see if you notice color in the stars here, too. This intermediate age open cluster has been studied for short-term variable stars and chemically peculiar stars. You’ll find this one on many challenging observing lists, too!

Time to hop to NGC 2244 (RA 6:32.4 Dec +04:52). The “Rosette Nebula” is a fine target for either telescopes or larger binoculars at a combined magnitude of 5. But, remember, combined magnitude isn’t true brightness! You’ll find the nebula here is quite faint and requires a good, dark, Moon-less sky. NGC 2244 is a star cluster embroiled in a reflection nebula spanning 55 light-years and most commonly called “The Rosette.” Located about 2500 light-years away, the cluster heats the gas within the nebula to nearly 18,000 degrees Fahrenheit, causing it to emit light in a process similar to that of a fluorescent tube. A huge percentage of this light is hydrogen-alpha, which is scattered back from its dusty shell and becomes polarized. While you won’t see any red hues in visible light, a large pair of binoculars from a dark sky site can make out a vague nebulosity associated with this open cluster. Even if you can’t, it is still a wonderful cluster of stars crowned by the yellow jewel of 12 Monocerotis. With good seeing, small telescopes can easily spot the broken, patchy wreath of nebulosity around a well-resolved symmetrical concentration of stars. Larger scopes, and those with filters, will make out separate areas of the nebula which also bear their own distinctive NGC labels. No matter how you view it, the entire region is one of the best for winter skies.

Now for NGC 2264 (RA 6:41.1 Dec +09:53). Larger binoculars and small telescopes will easily pick out a distinct wedge of stars. This is most commonly known as the “Christmas Tree Cluster,” its name given by Lowell Observatory astronomer Carl Lampland. With its peak pointing due south, this triangular group is believed to be around 2600 light-years away and spans about 20 light-years. Look closely at its brightest star – S Monocerotis is not only a variable, but also has an 8th magnitude companion. The group itself is believed to be almost 2 million years old. The nebulosity is beyond the reach of a small telescope, but the brightest portion illuminated by one of its stars is the home of the Cone Nebula. Larger telescopes can see a visible V-like thread of nebulosity in this area which completes the outer edge of the dark cone. To the north is a photographic only region known as the Foxfur Nebula, part of a vast complex of nebulae that extends from Gemini to Orion.

Northwest of the complex are several regions of bright nebulae, such as NGC 2247, NGC 2245, IC 446 and IC 2169. Of these regions, the one most suited to the average scope is NGC 2245 (RA 6:32.7 Dec +10:10), which is fairly large, but faint, and accompanies an 11th magnitude star. NGC 2247 is a circular patch of nebulosity around an 8th magnitude star, and it will appear much like a slight fog. IC 446 is indeed a smile to larger aperture, for it will appear much like a small comet with the nebulosity fanning away to the southwest. IC 2169 is the most difficult of all. Even with a large scope a “hint” is all!

Now, get out there and capture NGC 2261 (RA 6:39.2 Dec +08:44). You’ll find it about 2 degrees northeast of star 13 in Monoceros. Perhaps you know it better as “Hubble’s Variable Nebula”? Named for Edwin Hubble, this 10th magnitude object is very blue in appearance through larger apertures, and a true enigma. The fueling star, the variable R Monocerotis, does not display a normal stellar spectrum and may be a proto-planetary system. R is usually lost in the high surface brightness of the “comet-like” structure of the nebula, yet the nebula itself varies with no predictable timetable – perhaps due to dark masses shadowing the star. We do not even know how far away it is, because there is no detectable parallax!

There are many other wonderful objects in Monoceros just waiting for you to discover them… So get a good star atlas and go hunting the Unicorn!

Sources: Chandra Observatory, Wikipedia
Chart Courtesy of Your Sky.

If you were going to throw a birthday party for the Milky Way, how many candles would you put on the cake? What is the age of the Milky Way? Well, even though this is a difficult question to answer, either way you slice the cake you need a lot of candles. If you were to put a candle for each year the Milky way has aged, then you’d need between 10 and 13.6 billion candles. That would be mighty difficult to blow out all in one go.

The oldest stars in the Milky Way are 13.4 billion years, give or take 800 million years. This is somewhat close to what the age of the Universe is (which hovers around 13.7 billion years). By measuring the age of these stars, and then calculating the interval between their formation and the death of the previous generation of stars, we can come to an approximate age of the Milky Way as 13.6 billion years. Here’s a good article on how this process works.

The age of the Milky Way is determined by measuring the amount of beryllium present in some of the oldest known stars in the Milky Way. Hydrogen, helium and lithium were all present right after the Big Bang, while heavier elements are produced in the interiors of stars and dispersed via supernovae. Beryllium-9, however, is produced by collisions of cosmic rays with heavier elements.

Since beryllium is formed in this way, and not in supernovae, it can act as a “cosmic clock” of sorts. The longer the duration between the first stars that created heavier elements and the stars that make up globular clusters in the early Milky Way, the more beryllium there should be from the exposure to galactic cosmic rays. By measuring the beryllium content of the oldest stars in the Milky Way, the age of the Milky Way can be approximated.

This method is kind of like using radioactive decay of carbon-14 on Earth to determine the age of fossils. Radioactive decay of uranium-238 and thorium-232 gives an age of the Milky Way as similar to that of measuring the abundance of beryllium.

The age of the Milky Way is a tricky question to answer, though, because we can say that the oldest stars are 13.4 billion years old but the galaxy as we know it today still had to form out of globular clusters and dwarf elliptical galaxies in an elegant gravitational dance. If you want to define the age of the Milky Way as the formation of the galactic disk, our galaxy would be much younger. The galactic disk is not thought to have formed until about 10 – 12 billion years ago.

Here’s an article on how the bulge in the Milky Way may have formed earlier than the rest. Also, we’ve recorded a show all about the Milky Way on Astronomy Cast.

Source: ESO News Release