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For instance, a star with 2 solar radii is twice as large as the Sun. A star with 10 solar radii is 10x as large as the Sun, and so on.

Let's play around with the solar radius for awhile by talking about some popular stars.

How about we start with Polaris? I bet most of you have heard of the Polaris, the North Star. It is the brightest star in the constellation Ursa Minor (Little Dipper) and, because of its proximity to the north celestial pole, is considered the current northern pole star. Polaris is primarily used for navigation and has a solar radius of 30. That means, it is 30 times bigger than the Sun.

Alpha Centauri A, which is one of the nearest bright stars (at 4.39 light years), has a 1.227 solar radius. Those familiar with the game Sid Meyer's Civilization should be familiar with this star. Reaching Alpha Centauri is one of the ultimate goals of the game.

Of course, let's not forget Sirius which is easily the brightest star in the night sky. In terms of apparent magnitude, the second brightest star, Canopus, has only half that of Sirius'. No wonder it really stands out. Sirius is actually a binary star system, with Sirius A having a solar radius of 1.711 and B, which is much smaller, at about 0.0084.

In case you're not familiar with the term, a binary star system is a pair of stars that appear as one to an observer on Earth.

As you can see, the solar radius can be to describe not only stars larger than the Sun, but also those smaller than it. The Sun's radius is not the only physical quantity used as a reference for comparing stars.

Aside from the solar radius, one often used term is solar mass. By now, you should have guessed that it is simply the mass of the Sun and is used to compare the masses of other stars with. Just imagine how large the values would be if we used meters or even kilometers to describe the radii of stars.

The radius of the Sun is being discussed in an article here in Universe Today. Click on that link to know more about it.
Since we've been talking about stars, you might want to read about the smallest star.

Here's an article entitled star at NASA.

Check out this podcast at Astronomy Cast:

The Life of the Sun

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Here are a set of desktop wallpapers: stars that you can set as your computer background. Click on any image to see a larger version. Then right-click on the image and choose, "Set as Desktop Background".

Light Echo From Star V838 Monocerotis

This is an image of the star V838 Monocerotis captured by the Hubble Space Telescope. The dying star has blasted off its outer layers, and then flares on the star have illuminated the layers of gas and dust.

Ancient Globular Star Cluster NGC 6397

Here's a photo of stars at the center of the globular star cluster NGC 6397. This photo was taken by the Hubble Space Telescope.

Star Cluster in Nebula NGC 3603

There are thousands of stars in this image of the nebula NGC 3603. This is one of the most massive star forming regions discovered in the Milky Way.

Quasar

This is an artist's illustration of a quasar embedded within the center of a galaxy. A quasar is a supermassive black hole with millions of times the mass of the Sun.

Lighting up a Dead Star's Layers

Here's an image of the exploded star Cassiopeia A taken by the Spitzer Space Telescope. The faint blue glow around the dead star is material energized by the shock wave of the exploded star.

We've written many articles about stars for Universe Today. Here's an article about variable stars; stars which change their brightness. And here's an article about what stars are made of.

If you'd like more information on stars, check out NASA's World Book on stars, and here's the stars and galaxies homepage.

We've done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?

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Here are some cool star pictures. We're talking about actual balls of plasma here, not famous celebrities. Those are boring.

This is an image captured by the Chandra X-Ray Observatory of a star that might have been torn apart by an intermediate mass black hole. The blue colors in the image come from Chandra, while the other colors come from Hubble. Astronomers think this object is an ultraluminous X-ray object – they give off more x-rays than stars, but less than quasars.

Here's a picture of Cepheus B, a molecular cloud located about 2,400 light-years from Earth. This is similar to the nebula our own Solar System formed from within billions of years ago. Some event, like a nearby supernova explosion triggered the cloud to collapse into stars.

This is a Hubble Space Telescope picture of the supernova remnant RCW 86. This was once a very massive star that detonated as a supernova. It's possible that this was the supernova observed by Chinese astronomers in 185 AD. Don't worry, our own Sun won't go out like this, it doesn't have enough mass.

Here's another image of nebula that gives us a glimpse of what our Sun might have looked like in the early stages of its formation. The bright star in the nebula is the star L1157. It's thought to only be thousands of years old, just in the early stages of the main sequence phase of life.

This is the first image captured by NASA's WISE space telescope. It's an infrared image of a region of space near the Carina constellation. WISE is designed to take wide field observations of the sky in the infrared spectrum, building up views of the entire sky.

We've written many articles about stars for Universe Today. Here's an article about different types of stars. And here are some cool facts about stars.

If you'd like more information on stars, check out NASA's World Book on stars, and here's the stars and galaxies homepage.

We've done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?

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The nearest star system is Alpha Centauri. With the unaided eye, this looks like a very bright star, seen only from the southern hemisphere. But with a good telescope, you can actually see that Alpha Centauri is really two stars. They're known as Alpha Centauri A and B. And with a really good telescope, there's a third red dwarf star in the system, known as Proxima Centauri. Proxima Centauri is the closest star to Earth, and so the distance to nearest star Proxima Centauri is 4.243 light years away.

Proxima Centauri is a red dwarf star. It has only 1/8th the mass of the Sun, and a diameter of only 1/7th the Sun. It seems to be on a highly elliptical orbit around the two larger stars in the Alpha Centauri system, taking about 500,000 years to complete a single orbit. From here on Earth, we see Proxima Centauri about 4 times the width of the Moon away from Alpha Centauri AB.

Proxima Centauri has been the nearest star for about 32,000 years, and it will hold this record for another 33,000 years. It will make its closest approach to the Sun in about 26,700 years, getting to within 3.11 light-years of Earth. After 33,000 years from now, the nearest star will be Ross 248.

We've written many articles about Alpha Centauri for Universe Today. Here's an article about how to search for planets around Alpha Centauri, and here's an article about how Proxima Centauri might not be part of the system.

Here's a cool article from NASA about the nearest star.

We've recorded an episode of Astronomy Cast about what it might take to travel to the closest star. Listen here, Episode 145: Interstellar Travel.

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Alpha Centauri is the closest known star system to the Solar System. Also known as Rigil Kentaurus, Alpha Centauri is actually a multiple star system. It's certainly a binary star, with two sunlike stars orbiting one another. And there's also a red dwarf star, Proxima Centauri, which astronomers still argue about whether it's part of the system.

The closest star in the group is Proxima Centauri, located only 4.243 light-years from the Sun. And then the Alpha Centauri AB stars are located 4.37 light-years away.

With the unaided eye, Alpha Centauri looks like a single star. But then under the power of a telescope, it's possible to split them and see the individual stars separately. Alpha Centauri is only really prominent in the southern skies, and below the horizon to astronomers in the north.

Alpha Centauri A is slightly larger and more luminous than the Sun, while Alpha Centauri B is smaller and cooler than the Sun. But Proxima Centauri is a tiny red dwarf star, with only 1/8th the mass of the Sun.

We've written several articles about the Alpha Centauri system. Here's an article about how we might be able to detect Earthlike planets around Alpha Centauri, and here's an article about the sounds of Alpha Centauri.

Here's a cool image of Alpha Centauri at Astronomy Picture of the Day.

We've also recorded an episode of Astronomy Cast about what it might take to travel to Alpha Centauri. Listen here, Episode 145: Interstellar Travel.

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Progress in astrometic accuracy (Credit: ESA)

The furthest apart two locations on the Earth's orbit can be is 2 au (two astronomical units), as when observations of an object are taken six months apart. By simple trigonometry (geometry), the distance to the object being observed is just the length of the baseline divided by the tangent of the parallax angle (the angular difference in the two lines of sight) … and since parallax angles are extremely small for stars (less than one arcsecond), the tangent of the angle is the same as the angle. This gives a natural unit of distance for stars, the parsec … which is the distance at which an object has a parallax of one arcsecond when viewed from a baseline of one au.

There was a pretty hot competition, among astronomers, to be the first to measure the parallax of a star (other than the Sun), back in the 1830s; the race was won by Friedrich Bessell (remember Bessell functions?), in 1838, with a measurement of the parallax of 61 Cygni (0.314 arcsecs, in case you were wondering; two other astronomers measured the parallax of different stars in the same year).

To date, the most accurate parallaxes (~1 milli-arcsec) are the 100,000 or so obtained by the ESA's Hipparcos mission (which operated between 1989 and 1993; results published in 1997) … Hipparcos stands for High Precision Parallax Collecting Satellite, but is also a nod to the ancient Greek astronomer Hipparchus. The follow-up mission, Gaia (target launch date: 2012) will substantially improve on this (up to a billion stars, parallaxes as small as 20 micro-arcsec). Here's a fun fact: Gaia will measure the gravitational deflection caused the Sun … across the whole sky (and detect that due to Mars, for stars near the line sight to it)!

Universe Today has several stories on, or featuring, stellar parallax; here are a few: New Stellar Neighbors Found, Chasing an Occultation, and Happy Birthday Johannes Kepler.

Distance in Space is an Astronomy Cast episode on this very topic!

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Stars are kind of like people. They're born, they live their lives, and then they die. Let's take a look at the life of a star.

All stars start out a giant clouds of neutral hydrogen, which has been left over since the Big Bang. Some event, such as a nearby supernova explosion causes the cloud to collapse inward, and then gravity takes over. As the cloud collapses, it breaks up into different knots of material, each of which will go on to form a star.

As the cloud continues to collapse inward, the conservation of angular momentum from all the particles sets the cloud spinning. As gravity pulls it further inward, it begins spinning faster and faster and flattens out into a disk. The star forms from the concentration of material in the center of the protostellar disk, and the planets form out in the disk.

In the beginning, a star shines because of the heat of compression through gravity. But eventually the core of the star heats up to the point that nuclear fusion reactions can occur. At this point, the star blasts away the remaining dust and gas with its solar winds and enters the main sequence phase of life.

A star like our Sun will continue as a main sequence star for billions of years; slowly converting hydrogen into helium in its core. But it will eventually run out of easily usable hydrogen in its core. When this happens, the star collapses down a little and then starts to convert a shell of hydrogen into helium around the core. This additional heat puffs out the star into a red giant, causing it to become much larger.

A typical star will go through several phases of expansion and contraction as it burns through shells of hydrogen around its core. Larger stars will also switch to helium fusion in the core, and even go up the periodic table of elements, fusing heavier and heavier elements. Eventually they'll reach the limits of gravity, running out of fuel to burn. The star will then slough off its outer layers, creating the beautiful planetary nebulae we see from Earth.

And then the star will collapse inward, becoming a white dwarf star. This is a highly compressed object that can have the mass of the Sun, but only be as small as the Moon. It's still hot because of the residual energy it had when it was a true star, but it slowly cools down, eventually becoming a black dwarf; the same temperature as the background of the Universe.

Stars much larger than our own Sun can have a more dramatic finish. The largest stars will detonate as supernovae when they reach the end of their lives. Some will then collapse down to become neutron stars or black holes, while others explode with such energy that the entire star just blows itself apart.

We've written many articles about stars for Universe Today. Here's an article about the death of stars, and here's an article about the life cycle of stars.

If you'd like more information on stars, check out NASA's world book on stars.

We've also recorded several episodes of Astronomy Cast about stars. Here's a good one, Episode 12: Where Do Baby Stars Come From?

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Not a black dwarf ... yet (white dwarf Sirius B)

Details: Bessel – yep, the guy who Bessel functions are named after – analyzed data on the position of Sirius (Bessel was the one who first observed stellar parallax), in particular its proper motion, and concluded – in 1844 – that there was an unseen companion star (the same principle used to infer the existence of Neptune, around the same time). In 1862 Alvan Clark saw this companion, using the 18.5" refracting telescope he'd just built (quite a feat; Sirius B is ~10 magnitudes fainter than Sirius A, and separated by only a few arcseconds).

Sirius B is a white dwarf, one of the three "classics", discovered to be white dwarf stars in the early years of the 20th century (Sirius B was the second to be discovered – 40 Eridani B had been found much earlier, and Procyon B was also hypothesized by Bessel (in 1844) though not observed until much later (in 1896)). It is one of the most massive white dwarfs so far discovered; its mass is the same as that of the Sun (approximately). Like all white dwarfs, it is small (it has a radius of only 0.008, compared with the Sun's, which makes it smaller than the Earth!); like most seen so far, it is hot (approx 25,000 K).

Sirius B was likely a five sol B star as recently as 60 million years ago (when it was, coincidentally, approximately 60 million years old!), when it entered first a hydrogen shell burning, then a helium shell burning, stage, shed most of its mass (and enriching its companion with lots of 'metals' in the process), and shrank to become a white dwarf. There is no fusion taking place in Sirius B's degenerate carbon/oxygen core (which makes up almost all of the star; there is a thin, non-degenerate, hydrogen atmosphere … this is what we see), so it is slowly cooling (it cools so slowly because it has such a small surface area).

Packing such a large mass into such a small volume means that Sirius B's surface gravity is huge … so great in fact that it serves as an excellent test of one of the predictions of Einstein's theory of General Relativity: gravitational redshift (this was first observed in the lab in 1959, by Pound and Rebka). The most recent observation of this gravitational redshift was by the Hubble, in 2005, as described in the Universe Today article Sirius' White Dwarf Companion Weighed by Hubble.

Other Universe Today stories about Sirius B include White Dwarf Theories Get More Proof, and this 2005 What's Up This Week one.

Astronomy Cast has two episodes related to Sirius B, Dwarf Stars, and Binary Stars.

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X-Ray image of Proxima Centauri. Image credit: Chandra

A light year is the distance light travels in a single year – 9,460,528,000,000 kilometers. So the light we see coming from Proxima Centauri left the star 4.2 years ago. Even being the nearest star, Proxima Centauri is incredibly far away. It would take our fastest spacecraft more than 50,000 years to make the journey.

Proxima Centauri is a red dwarf star, approximately 14.5% the diameter of the Sun. It only has about 12.3% the mass of the Sun, and a lower surface temperature of only 3,042 kelvin. Its total brightness, or luminosity, is only 17% the brightness of the Sun.

The second nearest stars are also in the Centauri system. They're Alpha Centauri A and B, located about 4.3 light-years from Earth. Next comes Barnard's Star 5.96 light-years.

We've written several articles about Proxima Centauri for Universe Today. Here's an article about how Proxima Centauri might be flying alone through the Milky Way, and here's an article about Proxima Centauri viewed by the Chandra X-Ray Observatory.

Want more info on Proxima Centauri and the other close stars to Earth? Here's a list of the 26 closest stars, and here's a cool 3-d map of the closest stars.

We have also recorded an episode of Astronomy Cast about what it might take to travel to the nearest star. Listen here, Episode 145: Interstellar Travel.

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Want to update your computer desktop wallpaper with a stars background? Cool, here are some beautiful pictures that you can put on your desktop. If you see a picture you like, click it to open up a higher resolution image. Then right-click the image and choose "Set as Desktop". That will make the picture your desktop wallpaper.

Omega Centauri

Crab Nebula

This is a photo of the Crab Nebula. Also known as M1, this is a supernova remnant that detonated in 1054. The explosion was so bright that Arab and Chinese astronomers recorded it as a new star in the sky that faded after a few weeks. And now we can see this amazing remnant.

Cat's Eye Nebula

This is the Cat's Eye Nebula, a planetary nebula in the constellation of Draco. It was formed when a star larger than our Sun went into a red giant phase and blasted off its outer layers.

NGC 6397

This is a stars background image of the globular cluster NGC 6397, located in the constellation Ara. It's located about 7,000 light-years from Earth, making it one of the closest globular clusters.

NGC 3132

This is the planetary nebula NGC 3132. It also known as the Eight-Burst nebula because it kind of looks like a figure-8 through amateur telescopes. The nebula comes from gases puffed out by the dying star at the center.

We have written many articles about stars for Universe Today. Here's an article about hidden stars seen in the Southern Cross. And here's an article about newborn stars at the Milky Way core.

If you want more images of stars, you should probably check out the website for the Hubble Space Telescope.

We have recorded many episodes of Astronomy Cast about stars. How about Episode 12: Where Do Baby Stars Come From?

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Betelgeuse. Image credit: Hubble

With a mass of some 20 sols (= the mass of 20 Suns), Betelgeuse is evolving rapidly, even though it's only a few million years old. It's now a red supergiant, burning helium in a shell, and (very likely) burning carbon in another shell (closer to the nucleus), and (possibly) oxygen, silicon, and sulfur in other nested shells (like Russian dolls).

Betelgeuse is enormous … if it were where the Sun is, all four inner planets would be inside it! Because it's so big, and is only approx 640 light-years away, Betelgeuse appears to about 1/20 of an arcsecond in size; this made it an ideal target for optical interferometry. And so it was that in 1920 Michelson and Pease used the 100" Mt Wilson telescope, with a 20 m interferometer attached to the front, to measure Betelgeuse's diameter.

The Hubble Space Telescope imaged Betelgeuse directly, in 1995, in the ultraviolet (see above). Why the UV? Because ground-based telescopes can't make such observations, and because the Hubble's resolution is greatest in the UV.

Since the 1920s Betelgeuse has been observed, from the ground, by many different optical interferometers, at many wavelengths. Its diameter varies somewhat, as does its brightness (Herschel is perhaps the first astronomer to describe its variability, in 1836). It also has 'hotspots', which are ginormous.

Betelgeuse is also shedding mass in giant plumes that stretch to over six times its diameter. Although these plumes will certainly cause it to 'slim down', they won't be enough to stop its core turning to iron (when the silicon there is exhausted, if it hasn't already done so). Not long afterwards, perhaps within the next thousand years or so, Betelgeuse will go supernova … making it the brightest and most spectacular supernova visible from Earth in perhaps a million years. Fortunately, because we are not looking directly down on its pole, when Betelgeuse does go bang, we won't be fried by a gamma ray burst (GRB) which may occur (while a core collapse supernova can cause one kind of GRB, it is not yet known if all such supernovae produce GRBs; in any case, such a GRB is one of a pair of jets which rip through the poles of the dying star).

AAVSO has an excellent article on Betelgeuse, and COAST's (Cambridge Optical Aperture Synthesis Telescope) webpage on its observations of Betelgeuse gives a good summary of one interferometric technique (and some great images too!).

Universe Today has many stories on just about every aspect of Betelgeuse, from its varying size (The Curious Case of the Shrinking Star), the bubbles it's blowing and its plumes (Closest Ever Look at Betelgeuse Reveals its Fiery Secret), featured in What's Up This Week, to the bow shock it creates in the interstellar medium (The Bow Shock of Betelgeuse Revealed).

Astronomy Cast's The Life of Other Stars is a whole episode on the evolution of stars other than the Sun.

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The Arches Cluster, with young, massive stars, taken by the NACO on ESO’s Very Large Telescope.

The names of astronomical objects are given by the International Astronomical Union. They meet every 2 years to agree on names for objects and other classifications. For example, it was the IAU who recently decided that Pluto isn't a planet any more. And that's where the names for new objects are decided.

When you buy a star name from a company selling the service, they're just updating their own book. It's like paying to name a town or city after you. Some company might write down the new name and send you a satellite view of the town, but the residents in the town aren't going to change the street signs and recognize the new name.

If you understand that these services aren't actually renaming the star after you, just recording it and giving you a nice certificate, that's great. It's for entertainment purposes only. But if you want a legacy that other people will recognize, you should invest your money in something else.

Instead of buying a star name, why not adopt a star. There's a service that takes donations and lets you adopt a star. The proceeds go to the search for extrasolar planets.

We have written many articles about services that let you buy stars. Here's one of the most complete articles, entitled Name a Star – Real or Ripoff.

We have also recorded many episodes of Astronomy Cast about stars. Here's a good one, Episode 12: Where do Baby Stars Come From?

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Betelgeuse. Image credit: Hubble

Red giants start out as regular main sequence stars with masses between 0.5 solar masses and 4-6 solar masses. The stars convert hydrogen into helium at their cores, and this process happens for billions of years. Once the supply of hydrogen runs out in the core, hydrogen burning stops, and the star starts to contract inward from gravity. This heats up a shell just outside the core where there's still hydrogen, and this initiates fusion again.

This shell heats produces much more energy than the star did in its main sequence phase, increasing its luminosity by a factor of 1,000 to 10,000. The outer layers of the star expand outward, dramatically increasing its size. When our own Sun becomes a red giant, it might even consume the orbit of the Earth.

Even though the star is producing much more energy, it's also much larger, so that energy is spread out over a larger surface area. And so the overall temperature of the star decreases so that it appears red, instead of yellow or orange.

Red giants don't last long. After a few million years, they run out of fuel in this outer shell, and collapse down for good. They will blast off their outer atmospheres in a few final puffs, and then settle down to become slowly cooling white dwarf stars.

There are a few kinds of red giants. The most common variety are known as red giant branch stars (RGB stars). These are stars who have used up the hydrogen in their cores, but they're still fusing hydrogen into helium in their shells. The core of the star is filled up with inactive helium.

Other red giants are known as asymptotic giant branch stars (AGB). Instead of hydrogen, these stars are fusing together helium into carbon in their cores, in a process called the triple alpha process.

We have written many articles about red giants for Universe Today. Here's an article about a red giant that swallowed its planets. And here's an article that answers if the Earth will survive when the Sun becomes a red giant.

You can also learn more about red giants from NASA's Ask an Astronomer, and here's another question answered here.

We have recorded an episode of Astronomy Cast that investigates how stars die. Listen to it here, Episode 13: Where Do Stars Go When they Die?

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Betelgeuse. Image credit: Hubble

Well, there's a problem. Those services might let you give a star a name, and will take a fee to keep a record of it, but that name isn't officially recorded or recognized by the astronomical community.

Let me give you an example. Let's say I had a town naming service. Pay a fee and you can choose a town or city across the entire Earth to give a name to. Even though you might have renamed the city of Tokyo to Godzillatown, nobody in the city will recognize the name and use it.

The names of astronomical objects are given by the International Astronomical Union. It was at a recent meeting of the IAU in 2006 that they decided that Pluto was no longer a planet. Only this union can give stars their official names, so when you're paying a fee, and you think you're naming a star after someone, you're really just hiring a company to record the name in only their book.

Instead of buying to name a star after someone, why don't you "Adopt a Star" instead. With this service, you make a donation to an astronomical group that's using the funds to search for extrasolar planets. You're not actually naming the star, of course, but you're helping to find real planets orbiting other stars.

We have written about services like this in the past, like this story: Name a Star, Real or Ripoff.

We have also recorded episodes of Astronomy Cast dealing with stars. Listen to Episode 107: Elements from Stars.

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The Arches Cluster, with young, massive stars, taken by the NACO on ESO’s Very Large Telescope.

Let me give you an example. Imagine I ran a company called "Name a Town". For a fee of $20, I would let you pick any one of the thousands of towns and cities on Earth and give it whatever name you like. I'll record that name in my big book of names, and send you a world map with the location of the town that you named with your new name. Unfortunately, the citizens of that town aren't going to change the name of the town to the name you just gave them.

That's how it works for stars. The names of astronomical objects are given by the International Astronomical Union. Every two years they meet and agree on the new names for objects, like stars and newly discovered planets. It was at a meeting in 2006 where the IAU decided that Pluto wasn't a planet any more.

If you do decide that you want to "buy a star", there are a couple of services out there that you can use. One example is this service, called Adopt a Star. You're really donating money, and the proceeds will be used to help search for extrasolar planets. Another service lets you name a star for free. Once again, it's not an official name, but they'll give you a nice certificate showing you the location of the star.

We have written a more extensive article on the subject. Check out this article, Name a Star, Real or Ripoff.

There's another good article over at Wired Magazine that explains why you can't really buy a star.

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