One really strange star cluster

Image Credit: NASA, ESA, and L. Bedin (STScI)

Sometimes in astronomy we come across a really odd object. And the hard part is knowing whether the object is telling us something fundamental about physics and astronomy, or whether the object is just unique, or some combination of those. Today's example is based on a new Hubble Space Telescope press release (that I was griping a bit about yesterday).

The object in question is an open star cluster in our Milky Way galaxy called NGC 6791. First, some background. Star clusters are groups of stars born at the pretty much same time (a new star cluster is being formed right now in the Orion Nebula) and held together by their own gravity. Star clusters are not individual galaxies, but are parts of our own Milky Way galaxy.

There are two types of star clusters. Globular star clusters (like Messier 13 in the constellation Hercules) are groups of millions of stars. All globulars seemed to have been born in the very early universe, about 12 or 13 billion years ago. Globular star clusters also are very poor in iron and other metals relative to the sun. While almost 2% of the sun is made out of iron, uranium, nickel ,oxygen, carbon, silicon and other elements heavier than hydrogen and helium, only about 0.02% of a globular cluster star is made out of these elements. The reason for this is that stars are factories for the elements, and have been constantly making new elements, so the fraction of those elements goes up over time.

Open star clusters are the other kind of star cluster. They tend to have only a few thousand stars, they tend to be relatively young (most are under 1 billion years old), and they tend to have about the same amount of metal as the sun, give or take a little bit.

But NGC 6791 is, frankly, quite odd. It is old; it's stars seem to be about 8 billion years old (more on that later). This is much younger than the globular clusters, but older than any other open star cluster. Since it is so old, you might expect that its stars have a metal content between that of the globular clusters and the sun, but in fact it has over twice as much metal as the sun! And NGC 6791 has many more stars than any other open cluster, despite the fact that open clusters tend to lose stars over time. Lastly, NGC 6791 has a lot of really strange types of stars in it. So, it has always been viewed as odd, and its true nature has always been a source of controversy.

A group of scientists led by Italian astronomer Luigi Bedin looked at NGC 6791 with the Hubble Space Telescope in order to study its white dwarfs. White dwarfs are the ashes of dead stars; over time, a white dwarf cools and slowly fades away (just like what happens when you turn off the heat on an element on an electric stove, or when a blacksmith pulls red-hot metal out of a fire). We can estimate how old a white dwarf is by looking at how cool it is and using models to tell us when the star that made the white dwarf died (this is some of my own research).

When Bedin and his collaborators did this for NGC 6791, they found something odd. First, there were two groups of white dwarfs, one that seemed to be 4 billion years old, and another that seemed to be 6 billion years old. But, as I said above, the still-living stars in the star cluster look to be 8 billion years old. What's going on? We don't really know. But we do have some ideas.

First, we use models to get ages of stars and ages of white dwarfs. Those models are not perfect, and there are some uncertainties about them. In stars, a process called convection (which is like boiling or the turbulence that creates thunderstorms) mixes the stars, and that mixing can bring some new fuel to the star's nuclear furnace. We don't really know exactly how much of this mixing happens, so we guess. And that while these are highly educated guesses, they might be wrong. So, the 8 billion year age for the cluster may be wrong by as much as a billion years.

Also, the white dwarf ages have some uncertainties, especially for old white dwarfs. Old white dwarfs, as they cool, start to crystallize, forming giant, ultra-dense diamonds in space. That process of crystallization is not fully understood. Also, old white dwarfs may undergo a process called fractionation, where the heavier metals sink toward the center of the white dwarf. That sinking process can slow down the white dwarf's cooling rate, making an old white dwarf appear younger than it really is. This process (which may not even happen; it is very controversial) would be more likely to happen in white dwarfs with more metal. And, remember, this star cluster has more metal than any other star cluster!

So, I don't think that the 6-billion year age of one set of white dwarfs necessarily disagrees with the 8-billion year age from the stars. It would be kinda like looking at two separate people, saying, "Mr. X looks to be between 50 an 60 years old, and Mr. Y looks to be between 60 and 70, so I'll guess that Mr. X is 55 and Mr. Y is 65." Then you find out that they are both 60 years old. I think the same thing is happening here.

The bigger question is with the second group of white dwarfs, those that look only 4 billion years old. Here there are two ideas. Bedin and collaborators have suggested that these white dwarfs may be binary stars. Binary stars look like a single star from the Earth, but are really two stars orbiting each other closely enough that we can't tell them apart. The main effect of binary white dwarfs would be to produce a single point that looks brighter than a lone white dwarf would be. Since we get white dwarf ages from how bright they look (because they fade over time), we would mis-interpret binary white dwarfs as younger white dwarfs. In most open star clusters, about half of the stars are in binaries, and if that holds true in NGC 6791, then maybe the "young" white dwarfs are really just old twins.

But there is another option. An acquaintance of mine, Jason Kalirai at the University of California Santa Cruz, looked in detail at the brightest white dwarfs in this star cluster. He found that these white dwarfs were kinda wimpy, only about 2/3 as massive as they should be. (Most white dwarfs are about 60% the mass of the sun; these were about 40% the mass of the sun). White dwarfs with masses this low cool differently than normal white dwarfs (for reasons I don't want to go into). If you analyze the apparently younger white dwarfs as if they were merely featherweight white dwarfs, you would find that their ages are about 6 or 7 billion years, the same as the other white dwarfs.

But these featherweight white dwarfs can only exist if their parent stars ended their lives differently from the way we think most stars do. Some people (such as another acquaintance, astronomer Brad Hansen at UCLA) have suggested that the high amount of metals in the stars could cause the stars to lose a lot more matter at the end of their lives. There are other ideas that might work, though.

In short, this star cluster is a really odd one. It is very old, but it has more metal than any other star cluster (and we would think that newer star clusters should have more metals). The cluster has more stars than any other open star cluster, although it has been losing stars over its entire, long lifetime. The cluster has some really weird stars (which I haven't talked about here). The ages of the dead stars in the cluster may be different from the ages of the living stars. And even the dead stars disagree on how old they are, or they maybe they formed in multiple and unexpected ways. The jury is still out! Is this cluster just weird, or is it telling us that we don't really understand the life cycles of stars as well as we thought we did?The only thing for certain is that this cluster will continue to be studied for years to come.

Seeing stars explode in real time

Image Credit: NASA/Swift Science Team/Stefan Immler

When a star ends its life by exploding, it tends to be a while before we see anything on Earth (and this is ignoring the millions of years it takes light to get to Earth). Because we don't know in advance which star in the Universe is going to explode when (as there is no "Upcoming Attractions" posting on the Universe's blog, and psychics continually fail to warn us of these things), we tend to see explosions after the fact. But even if we knew when a star was going to explode, it would be hours after the actual explosion before we saw any light on Earth.

When a massive star nears the end of its life, its core engine (a nuclear fusion reactor) is busily fusing silicon into iron and nickel and cobalt. These elements have absolutely no energy value, so they form a lump of inert ash at the center of the star. When the lump gets big enough, the forces between atoms can no longer counteract gravity, and the core collapses into a neutron star or a black hole. Suddenly, the star finds it has no support in its middle, and the star begins to collapse inward. All of the inward falling material collides, causing a shock wave to go rushing outwards toward the surface at the star at speeds of 10,000 miles or more per second. This shock wave is also probably driven by energy from a stream of subatomic particles called "neutrinos" that are formed by the collapse of the core of the star. When the shock wave reaches the surface of the star, it breaks free in a blinding flash of X-rays and ultraviolet light, as the first energy from the star being ripped apart is released into the empty vacuum of space.

But even though the shock wave is going at these very high speeds, it can take the shock wave a long time to reach the edge of the star. The stars that go supernova can be almost a billion miles in diameter. A shock wave starting at the star's center can take 14 hours to reach the surface of such a star! So, for at least half a day after a star explodes, we on Earth have no clue (in the form of light) that the star has exploded.

Even after the explosion, it often takes days for us to notice anything on Earth. The supernova explosion gets brighter for several days as the shrapnel from the star expands outward, exposing more and more of the bright debris to view. Then the debris starts to cool, and the supernova begins to fade away (though radioactive decay from elements created in the explosion help to keep the star from completely fading away in a matter of days). On Earth, the supernova appears as a point of light that didn't used to be there, and someone has to be looking in the right direction to see it. Because of this, most supernovae are discovered only around the time that they reach their brightest point, which can be days after the explosion.

Yesterday, NASA announced that their Swift X-ray telescope had discovered the break-out flash of a supernova. The telescope was looking at a galaxy when a bright X-ray "flash" was observed. Since part of Swift's mission is to look for flashes of X-rays (most of which come from gamma-ray bursts), the telescope immediately alerts interested astronomers around the world that a flash has gone off. After the alert, many professional telescopes went and looked at the spot of the flash, and were able to catch some of the earliest light ever to come from a supernova explosion. The picture above shows the X-ray picture of the supernova (top) and the optical-light picture of the explosion (bottom).

This research is interesting, because it allows astronomers to explore some of the earliest stages of a supernova. It was also very lucky, because the telescope happened to be looking in the right place at the right time, and the typical galaxy only has a star explode every few decades or so. But is the discovery important (the press release calls this supernova the "Rosetta Stone" for understanding exploding stars)?

Probably this is not going to be a crucial piece of data in understanding supernova explosions. The X-ray flash was expected, and now it has been observed, which does confirm one part of the theory of exploding stars. But it is hard to see that we will learn anything new from a single event. The theory of exploding stars seems to be pretty solid, and what we tend to learn from the earliest stages of a supernova is mostly what the star's outer-most layers looked like, and we already study the outer layers of stars (it's what we see when we look at them). Yes, there are things to be learned, but these are almost certainly just details, not grand over-arching themes. But a cool and lucky find, nonetheless.

This discovery would be great for someone interested in the psychology of astronomical research. Since many different research groups got notification of the X-ray flash, they all scrambled to produce papers and get credit for the discovery. There are grumblings under the surface about the group getting credit in this press release, but there are also good arguments for why they got credit -- I don't know enough to have a well-informed opinion. And the personalities in competing groups are always continually clashing, so there are some, um, colorful opinions floating around. Anyone who claims that scientists are a completely dispassionate people are wrong, and this particular discovery is a great piece of evidence that human psychology plays a large role in science.

When nothing means something

Image Source: European Southern Observatory / João F. Alves

Sometimes, you can learn a lot by what you cannot see. For example, if you are a night watchman at the Tower of London and you find you cannot see the Crown Jewels, you learn that you had better start looking for a a new job and a good lawyer. Or maybe you learn that the batteries in your flashlight have gone dead. However, if you go running out claiming that the Crown Jewels have been stolen when really you just needed new batteries, you are in a heap of trouble.

In astronomy, understanding a "non-detection" (in other words, understanding what you are not seeing) is often a crucial piece of evidence in solving a puzzle. The key is knowing when you could have seen something if it were there.

One easy example of this is the picture above. In the middle of the picture, you don't see any stars, while on the edges, you see hundreds of stars. If there were stars in the middle of the picture, could we have seen them? The answer is "Yes!" (or perhaps "Duh!") We know this is true because we can see stars on the edges of the picture. So, either there aren't any stars in the spot in the middle of the picture, or something is blocking the light from those stars. The answer is the latter -- a thick cloud of dust and gas is blocking the starlight. If we look at infrared light, which can pass through dust, we can see lots of stars all over the same patch of sky. And, by careful analysis of these pictures, we can learn a lot about the cloud of dust and gas, and the infant star or stars being formed inside it.

Okay, now a harder example. We astronomers are pretty sure we know how gravity works. After all, we use Newton's Law of Gravitation and Einstein's General Relativity to send space probes all over the Solar System. But when we apply these laws to clusters of galaxies, we run into a problem. The galaxies move around much faster than we think they should, at least when we add up all of the light that we see. This effect was first noticed by astronomer Fritz Zwicky in 1933, which led Zwicky to propose that there is additional material there that we can't see.

In the 1970s and 1980s, X-ray telescopes detected a glow of X-rays from these clusters of galaxies, indicating that the clusters had million-degree gas swirling around in them! This very hot gas would not be visible in optical light, and when astronomers added up how much gas was in this invisible hot phase, it ended up being more mass than in visible light. But even then, it was still not enough to account for the speeds at which the galaxies move around. For this reason (and others that I won't go into), astronomers came up with the idea that some other type of matter, "dark matter", must exist. There is still some controversy as to whether dark matter exists, or whether we just don't fully understand gravity (though most astronomers, myself included, are pretty sure that some invisible form of matter exists).

Another example of a non-detection being used is in black holes. In all cases we suspect there is a black hole, light from gas or stars in orbit around the black hole tells us that there is something big and unseen there. For example, this movie shows the observed (and predicted) positions of stars at the center of our Milky Way Galaxy. From the laws of gravity, we are certain that, at the center of the galaxy there is an object with a mass over two million times that of the sun. But, as you see from this movie, we don't see anything there! The only thing astronomers know about that is dense enough to have this much material and yet not be seen is a black hole. In the past, some other possibilities remained, like a dense cluster of white dwarfs or neutron stars -- these would be faint enough not to see. But as more evidence comes in, we have constrained the size of any such cluster to be so small that gravity would cause such a cluster to collapse into a black hole anyway.

But we have to be careful not to be fooled. Astronomy literature is full of mistakes people make when they messed up in calculating what they could see. For example, several years ago people were claiming that, in earlier ages of the Universe, there were not as many barred spiral galaxies as there are today. As it turns out, these claims are most likely wrong. When you look at distant galaxies, they appear smaller, so what is clearly a bar in a nearby galaxy can be invisible when seen from far away. And the people doing the study had a little too much confidence in their ability to be able to see these bars. In short, it's hard to see things that are far away, and, if you are wrong about how hard it is to see them, you make mistakes.

This week, I was reading a paper by an acquaintance who was looking for light from planets and brown dwarfs (stars that are too small to power themselves by nuclear fusion) around white dwarfs. He didn't find any. And so, the question becomes, why not? Are they too faint for us to see? Or were there never planets there? Or were any planets that were there get swallowed by the star as it grew into a red giant during its death throes? These are questions that the paper tried to address, and it all hinges on how well we know what we can and cannot see. If the planets are simply too faint to see, then not seeing them doesn't mean much. But if they should be visible and we don't see them, then we'll learn something about planets and the fates of solar systems. We'll just have to wait and see.

ECHO...Echo...echo

Image Credit: NASA/CXC/Rutgers/J.Warren, J.Hughes

We all have experienced sound echoes. Sound waves from an event (preferably loud and short) bounce off of distant walls and travel back to our ears. Because of the finite time it takes sound to travel, we can hear individual echos. Sound echos are used quite often by living beings -- The Navy and fishermen use sonar to find fish (or submarines) under water; bats and dolphins use echolocation to get their food.

Light also travels at a finite speed, though much, much faster than sound. Humans have learned to make use of "light echoes" for all sorts of clever things -- we call this radar (for radio waves) or lidar (for laser light).

There are some events in space that make nice, short bursts of light, such as supernova explosions or eruptions from the surface of a star. The neat thing (at least to me) is that we are now able to detect these "light echoes" from astronomical sources, as light from the event bounces off of dust or gas and toward the Earth.

A press release from the Chandra X-ray Observatory shows one cool example of a light echo detected in the Large Magellanic Cloud (LMC), one of the Milky Way's companion galaxies. About 400 years ago, a supernova exploded. The supernova was probably quite easy to see from the Earth, but I'm not aware of records from it. This wouldn't be too surprising, as the LMC is only visible south of the equator, and there just aren't a lot of written astronomy records from civilizations down under during that time.

Anyway, astronomers studying this supernova's remains (an X-ray picture of them is at the top of this blog post; I think it looks like a celestial pufferfish) also looked in optical (visible) light at the area surrounding the supernova, and they found an echo of light from the supernova itself! This page has movies showing the movement of the light echo over a period of five years (be warned, the movies are large. If they are too big for your internet connection, you can look at the individual pictures). Pretty neat!

What can we learn from the light echo? First, astronomers have been able to analyze the light from the echo and determine the type of supernova that made the explosion. There are two types of supernova explosions -- those that come from a massive star ending its life, and those that come from white dwarf stars. From the light echo, astronomers were able to confirm that the explosion was from a white dwarf. This knowledge helps us understand the X-ray light the Chandra observatory sees.

We can also use the light echos to get a three-dimensional picture of the dust and gas in the region surrounding the supernova. You can see the shape of the echo change slightly from picture to picture in the movie, which tells us that the gas and dust in the LMC galaxy are clumpy.

Perhaps astronomy's most famous light echo is from an eruption on the star V838 Monocerotis, shown in this video from the Hubble Space Telescope. Over a period of 4 years, the light from the eruption lit up swirling dust surrounding the star. While the light echo in the LMC pictures is not quite as dramatic, it serves the same purposes!

(Thanks to Jason Harris for pointing this story out)

Super-Asymptotic Giant Branch Stars

As I promised in my last post, here's an attempt to explain why in the world I would travel to London for a one-day visit. The challenge is to see if I can both explain the science and keep this post reasonably short.

First, we need to talk briefly about how stars work. Stars shine by nuclear fusion. They spend most of their lives turning hydrogen into helium. But eventually the hydrogen fuel runs low, and the star starts to burn helium into carbon. Almost all stars bigger than half the sun's mass go this far.

For stars up to about 7 times the mass of the sun, this is as far as the star gets. Once it has burned all of its helium, it ends its life, throwing off its outer layers as a planetary nebula and leaving behind a glowing lump of carbon ash called a white dwarf.

For stars more massive than about 10 times the sun's mass, the star is big enough to start fusing the carbon into oxygen, neon and magnesium, which then ignites and fuses into silicon and related elements, which then fuse to make iron and nickel and related atoms. And then the star explodes as a supernova star, spewing most of these elements out into the universe.

My story above, though, has a bit of a hole in it. I said that stars smaller than 7 times the mass of the sun make carbon white dwarfs, and stars bigger than 10 times the mass of the sun explode after fusing elements up to iron. But what happens to stars that are 8 or 9 times the mass of the sun? These stars are big enough to burn carbon into oxygen, neon and magnesium, but they are not big enough to fuse these elements into silicon and iron.

From the standpoint of theory, it is possible to make a white dwarf out of oxygen, neon and magnesium. It is also possible to have oxygen, neon and magnesium explode in a special kind of supernova explosion. But we don't know which of these scenarios happen.

Our meeting in London was therefore to discuss these stars. We talked about what we know about these stars between 7 and 10 times the mass of the sun, and what we don't know about these stars. Frankly, there are a lot of mysteries, and not a lot of answers. What happens to these stars depends a lot on how fast the stars are rotating, how quickly they are shedding their outer layers, and how much the inside of the star is mixed up by the slow boiling that happens inside many stars. But these mysteries are what makes the science interesting, and will keep me working for years to come!

I talked about the white dwarfs I have been studying, and a British group talked about their studies of the types of stars that explode as supernovae. If we combine our two areas of study, it seems that these stars must explode and not make white dwarfs. But there is a lot of careful study we need to do before blindly combining our work! I may make different assumptions than the other group which can affect the outcome of our data, or maybe one or both of us have made mistakes in our analysis. Only time will tell!

Crunching numbers

Image credit: Popular Science, Modern Mechanix

The trick to finding a needle in a haystack is not brute force (like Jim Moran above), but a clever sorting mechanism -- whether a metal detector, or some sifting machine, or another unique scheme.

In astronomy, some of the most interesting objects are some of the rarest. This is often because the interesting objects are those that are changing rapidly (at least in a cosmic sense). For example, stars like the sun are everywhere in the sky, because they live a middle-age life for ten billion years, nearly the entire age of the Universe. But stars ending their lives, making beautiful planetary nebulae, are very rare -- the phase only lasts ten thousand years, a blink in the cosmic eye. So, compared to stars like the sun, planetary nebulae are rare. Another rare object is a supernova, or exploding star. These are only visible for a year or so before fading from sight, and the stars that make them are rare. So, in our skies right now, there are no supernovae in the Milky Way galaxy. We have to look at distant galaxies to find supernovae.

But planetary nebulae and supernovae are pretty easy to find, relatively speaking. Planetary nebulae are big and glow in very specific colors of light, so you can design a search to take pictures of the sky in those colors, and you'll find lots of planetaries. Supernovae are very bright, and so can be seen far away. So, we just look at more and more distant galaxies until we see a supernova -- in essence, we are searching hundreds of billions of stars at once, looking for a "new", bright star in a galaxy. Both of these are clever ways of searching through the haystack of the sky for that elusive needle.

I am part of a group at the University of Texas Astronomy Department that is looking for a specific kind of white dwarf star in a specific patch of the sky. White dwarfs are faint, and bright ones are rare (because they have to be close by). And the patch of sky we are looking in is large, by astronomy standards -- about the size of both of your hands held at arms' length. A typical astronomical camera can only image part of the sky as big as a part of your pinky finger's fingernail. Out of the hundreds of thousands of stars in that patch of the sky, we expect to find about ten of these white dwarfs. So, how can we find them?

The brute force method, like searching each strand of hay for the needle, is long and complex. We would have to take pictures of the entire region of the sky (dozens of nights on telescopes available to us). Then we would analyze the images and look for stars that have just the right colors. Then we go back to the telescope and take spectra of all of those stars, splitting the light up into its component colors, and determining what each one is. We then would have to analyze each spectrum. And, lastly, we have to return to the telescope to double-check each of our best candidates. So, we're talking a few weeks (at least!) on the telescope, and hundreds of person-hours to find ten objects.

Thankfully, there is an easier way -- an automated sorting machine known as the Sloan Digital Sky Survey. The Sloan survey has taken images of one-quarter of the sky, and automated software analyzed each object. Interesting objects are targeted for follow-up work, such as the spectra, which automated routines then analyze. As of right now, the Sloan database has image analysis of 280 million stars and galaxies, and spectra of 1.2 million of those objects.

So, yesterday afternoon I sat at a coffee shop and used the Sloan database query tools to find a dozen interesting white dwarfs in our patch of sky. A grand total of less than one person-hour went into my search. Granted, Sloan has used hundreds of nights of telescope time and countless thousands of person-hours to create and maintain the database. But that enormous undertaking allows small teams such as the team I'm part of to do searches that used to be prohibitively time-intensive. Computers and clever software allow a single person to search through 280 million objects for the dozen he or she is interested in, and those searches only take minutes, not years.

So, I found the needles I was looking for in the haystack of the sky, all thanks to computers and a giant team dedicated to making this possible.

Making pretty pictures

Early next month, astronomers from across the U.S. (and even further abroad) will converge on Austin, Texas for our annual winter meeting of the American Astronomical Society. Several thousand astronomers will be prowling the streets and crowding into the convention center, each presenting their own research.

I'm preparing a poster with some of my research to show off at the meeting. As my poster will be competing for attention with a few hundred other posters, I'm trying to whip up some flashy graphics to draw people in. It's a little sad that flashiness and not science is part of the draw, but that's the way things are.

Anyway, the picture above is a near-true color image of the center of the star cluster Messier 67 that I put together. The images were taken with the 6.5-meter MMT telescope south of Tucson, Arizona. Messier 67 itself is about 2800 light-years away in the constellation Cancer. The stars in the star cluster are about 4 billion years old, or just a little younger than our Solar System.

If you look at the large version of the image (click on the image above), you can see several types of stars. The brighter, orangish stars are red giants, stars that have exhausted their hydrogen fuel, and have swollen up from the size of our sun to a star larger than the Earth's orbit around the sun.

At the lower right, you can see a bright, bluish star. This star is known as a "blue straggler." Based on the star's color and brightness, it should be much younger than the star cluster. But we also know that it is part of the star cluster, and we know that all stars in a star cluster are the same age. So, this star is thought to be the result of two normal stars colliding and merging into a single star.

Meanwhile, most of the faint stars you see are stars like the sun, and some of these may even have planets around them (though, so far, we haven't found any). So, when I look at this cluster, it is possible that some alien astronomer is looking back at us.

Stars don't make good Christmas presents

"While astronomy is a relatively safe hobby, keep in mind that stars are hot and will burn for millions of years if left unattended." --- The Onion

As the holiday shopping season reaches its peak in the next few weeks, you may be wondering what to buy that space fanatic for Christmas. One item that is available from some retailers and that seems to remain fairly popular is the "buy a star" or "name a star" gift. Typically, these services, in exchange for a moderate sum of money, give you the "rights" to name a star and provide you with a few goodies, such as a nice certificate, a booklet with information on stars, a sky map with your star indicated, and/or a few other personalized items.

What these companies don't go out of their way to tell you is that the name you give a star is not official. These names are not recognized by any organization of professional astronomers, nor will they ever be used by any astronomer or star catalog (except, perhaps, a catalog produced by the retailer that no astronomer will ever look at). And you get no legal rights to that star or any money that may come from that star (say someone wins a government grant or Nobel prize for studying "your" star -- don't expect to see a red cent).

In fact, I once had a relative buy me a star to name. I appreciated the gesture, I named the star after myself, and I got the little gift packet in the mail. What annoyed me was that the packet was full of a lot of false information (probably not intentionally so, but still quite wrong). I was given my star's coordinates, but there is no star there. The star that came indicated on a star chart was nearly one degree away on the sky! I also received a booklet on the "science" of stars that was full of wildly inaccurate "facts" on stars and their lives.

I would have no problem with companies that want to "sell" stars, if they would clearly state up front that the gift is not official in any manner, and if they would not provide false scientific "facts" about the stars. Neither of these things is difficult, and given the price these retailers charge; it should be insulting to the consumer that these retailers do not take any apparent effort to provide a product worth even a fraction of the cost. And these products do the science of astronomy a disservice by giving people materials that are anti-educational.

If you have already purchased or named a star through such a company, though, don't feel ashamed -- one thing these retailers do accomplish is good advertising for their product. It's just a shame that the product is a sham.

But there are similar products you can buy for that space fanatic. Why not try a framed picture from the Hubble Telescope? Several companies offer them, or you can download high-resolution pictures from Hubblesite and have the picture printed at a photo shop. And, in case your developer is worried, this is legal -- NASA pictures are in the public domain (You may want to print out this letter to give to your developer in case they are worried). Hubblesite even provides a nice step-by-step guide to this -- and there are hundreds more Hubble images than the few dozen the step-by-step guide offers; just download the highest-quality images from the gallery. Put the print in a nice frame, and you have a museum-quality Hubble picture for a gift!

Of course, there are other options for space gifts -- I just wanted to point out one that can be inexpensive and yet result in a high-quality gift, without having to shell out over $50 for bogus naming rights.

CSI: Universe --- Who set off that explosion?

Image credit: Jon Morse (University of Colorado) and NASA

In 2004, astronomers reported a possible supernova (the explosion of a dying star) in the galaxy UGC 4904, a barred spiral galaxy about 75 million light-years away. However, the apparent explosion was awfully faint for a supernova, and it faded away too quickly for a supernova. And so, the "transient" (as such events are called) was forgotten.

On the night of October 9, 2006, amateur astronomers in Japan detected another transient in the same galaxy, a transient that was confirmed as a supernova several nights later. Not only that, but the supernova seemed to be coming from the same part of the galaxy as the first transient. What was going on? And just recently, European astronomers were able to do a careful alignment of the images of both events, which confirms that they are coming from the same spot. What's going on?

Very massive stars, those nearly 100 times the mass of the sun, live short and violent lives. These stars live their lives on the brink between gravity holding the star together and the radiation from the nuclear reactions at the star's center ripping the star apart. Sometimes these very massive stars become unstable, and can rapidly lose large amounts of material, many times our sun's mass. As that material flies off in a massive eruption, the star can get significantly brighter -- just like the first transient in UGC 4904. Typically, these stars seem to settle back down after the eruption, just like a little burp can make you feel better after you've eaten too much. And we think (or thought) that these stars would go on to live for another 200,000 years or more.

But the supernova throws that into question. Did the same star that erupted a few years ago then go supernova, meaning it had used up all of its nuclear fuel much faster than astronomers thought?

Maybe, maybe not. Massive stars tend to be born in clusters of stars, with many other very massive stars around. And many of these massive stars have companion stars in tight orbits. Because we don't have pictures of this galaxy taken with the Hubble Telescope, we can't see individual stars in this galaxy, so we can't know if the eruption of the star in 2004 and the explosion of a star in 2006 came from the same star. Based on the coincidence and the close timing, it would make sense that they are related. But maybe this just was two separate stars, and the timing was a coincidence.

If eruptions of material from massive stars often results in a supernova shortly thereafter, we should see this occurrence more often. It's only been in the last decade that astronomers have been diligently searching nearby galaxies for supernovae, so more time is needed before the book can be closed on this case.

But maybe we don't have to look too far away. In the southern hemisphere, the star Eta Carina is a massive star, 120 times the mass of the sun. In 1843, the star temporarily became the brightest star in our sky after the sun, despite being 8000 light-years away. Then the star rapidly became fainter than the human eye can see, and it has slowly gotten a little brighter since. This is thought to be the same type of eruption that was seen in UGC 4904 in 2004. The picture above shows a Hubble Space Telescope picture of Eta Carina -- the star is buried in the middle of two giant, expanding bubbles of material. Those bubbles were probably created in the eruption 160 years ago. So, will Eta Carina go supernova soon? 160 years seems a lot longer than 2 years, but in astronomical terms, they are both almost instantaneous. But maybe we will have to wait 200,000 years to see Eta Carina explode. If Eta Carina were to explode in the next several decades, then we would have to re-think these massive eruptions.

A Snowball in hell?

A news story making the rounds for the past few days contains claims that astronomers have found a hot planet (called "GJ 436b") made out of ice. How can this be?

First, let me state that I am far from convinced about this claim. What do we really know about the planet? We know its mass, which we have measured by the planet's gravitational pull on its parent star. We also know its diameter, because the planet passes in front of its star as we see it from Earth, and we can measure how much light it blocks out. Because we know its diameter and its mass, we also know its average density. In these ways, the planet is very close to a twin of Neptune.

But this is really all we know for certain. We can guess how hot GJ 436b must be based on how close it is to its parent star (this works well in our solar system for Mercury and Mars, it's close for Earth, and fails miserably for Venus). But we have no direct data on what the planet is made out of. The claim that the planet must be made out of "hot water ice" is a guess. An educated guess, but just a guess. So don't start buying beachfront real estate on GJ 436b yet.

Why would astronomers guess that the planet is made of "hot ice?" First, in our solar system, the planets Uranus and Neptune, the planets closest in size and average density to GJ 436b, are thought to be made of ices (including water ice) covered by an extraordinarily thick atmosphere of methane. So, it is not unreasonable to guess that all planets the size and density of Neptune and Uranus look like Neptune and Uranus. But, again, this is just an educated guess.

How can ice exist if the planet is hot? Look at the phase diagram for water below, which shows what form water will take as a function of its temperature and its pressure. For temperatures and pressures on the Earth (about 250-310 degrees Kelvin and a pressure of about 100,000 Pascals), water is near the point where all three phases of water -- gas (tan), liquid (green), and ice/solid (blue) can exist. On the diagram, notice that even at extremely hot temperatures, like 600 Kelvin (by chance, about 600 degrees Fahrenheit), water can be in the form of ice if it is at high pressures -- at least 10 million times the pressure on Earth's surface.

Image credit: Martin Chaplin / London South Bank University

So, if GJ 436b has a very thick atmosphere, like Uranus and Neptune, the pressure inside the planet can be high enough to make water turn into ice, even though it would be very hot! This ice is not quite like ice on Earth -- the structure of ice crystals on Earth is not the same structure of ice crystals at high pressure. And, if this hypothesis is right, most likely the atmosphere of the planet would go through a gradual transition from air to liquid to solid. This is quite unlike the Earth, where there is an abrupt transition from the atmosphere to the sea and again from the sea to the sea floor. On GJ 436b, there is probably no real "surface" to the planet.

So, as I've said before, take this new press announcement with a grain of salt. Until we have observations showing us what a planet around another star is made out of, we can only make educated guesses. And the makeup of this "snowball in hell," GJ 436b, remains just that -- an educated guess.

How old is it?

Just like stars in Hollywood, stars in the heavens don't like you to know how old they really are. There are a few clues -- we know that the biggest stars don't live more than a few billion years, and when a star is getting ready to die, we know because it begins to swell up into a red giant star. But for most stars, we can only make educated guesses.

But sometimes we get lucky. Anna Frebel, a postdoc here at the University of Texas, announced yesterday that she has discovered a star nearly as old as the Universe itself. Frebel's luck was in finding a star where she could detect radioactive elements she could use as clocks to measure the age of the star. Her skill comes in recognizing the utility of those elements and being able to make precise measurements, which is very difficult to do.

The heaviest elements in the Universe, elements like uranium, lead, gold, and mercury, are made in the death throes of dying stars. Some are made by slow processes in the atmospheres of red giants, while others are made quickly during supernova explosions. Some elements can be made both ways, while others are only made by one process or the other. But the amazing thing is that the relative amounts of each element produced one way or another doesn't change from one star to the next. For example, for every three atoms of the element europium in a star, you will find one atom of barium.

So, if you can find and measure radioactive elements in a star and can predict how much of that material the star must have started with, you can determine how old the star is. This is just like radioisotope dating on the planet Earth. It's not used that much in stars because the radioactive elements are hard to find -- their "fingerprints" in the light from the star are usually buried by fingerprints of other metals, especially iron. But in some of the first stars, not much iron had been created in the lifetime of the Universe, so those radioactive lines are buried.

What Frebel did was measure the amount of uranium and thorium in her star. These are both radioactive elements, and they decay at different rates. She then compared the amount of those two elements with three elements that are not radioactive: europium, osmium, and iridium. She then calculated how much time had to pass for the "missing" amounts of uranium and thorium to have decayed. And the answer, which gives the age of the star, is: 13.2 billion years.

How accurate is this measurement? There are ways to make mistakes in measuring how much of each element, and there are some uncertainties in the exact ratios of each element that the star would have started with. For any single element, the error is pretty large. But, because Frebel had six measurements (two radioactive elements to compare to each of three stable elements), those errors get reduced in size. So, the age of the star is very likely within a billion years of being correct.

This measurement is an important one. Since this star is in the Universe, we know that it has to be younger than the age of the entire Universe. But because the star has so little iron and other metals, we think it must have been made early in the Universe. Once our galaxy started producing iron, it produced it at a very fast rate.

From studying the echoes of the Big Bang, astronomers have estimated the age of the Universe to be 13.7 billion years. This agrees well with Frebel's age for her star of 13.2 billion years. This is comforting to us as astronomers. Two completely different lines of reasoning give roughly the same answer for the age of the Universe. This makes astronomers more confident that our understanding of the physics behind the formation and early history of the Universe is correct. And there are other even more different observations that give a similar age to the Universe, giving us yet more confidence in this age.

(Mis-)Naming Stars

Today I saw that the satirical newspaper The Onion had this radio news "story" about the International Star Registry accidentally re-naming the Sun "Margaret."

Several companies claim to let you buy or name a star for a fee, and we astronomers are often by customers of these companies if we will use their names or take a picture of their star. The answer is always "no," for various reasons.

Although companies "selling" stars or star names likely do keep track of who has named which star what, these names are unofficial. Research astronomers call objects after names approved by the International Astronomical Union. Outside of objects in our Solar System and stars with common names (like "Polaris" or "Betelgeuse"), stars and galaxies don't get anything approaching real names. They get names based on either the catalogs that list the stars and galaxies (such as Gleise 581, the star recently announced to have an Earth-like planet) or based on their coordinates (like PG 1115+080, a gravitational lens near the coordinates RA=11hr 15min, Dec = +8.0 deg).

When you send money to a company selling stars, you are buying a product from them. Often it includes a certificate, a sky map with your star marked, and maybe some information on stars. If you give the company a name, they keep it on record, sometimes promising to publish the names in a book. But you are not doing anything official, and often what you are getting may not be worth the money.

Let me give you an example. One year, a family member thought it would be neat to give me, an astronomer, a star. I thanked them profusely, and then named the star something like my "Totally Bogus Star." I refuse to say which company it was from, though.

The certificate I received came with coordinates of the star, as well as a star map. So, for fun, I went and looked the coordinates up in our catalogs. There was nothing at those coordinates! I tried several variations on the coordinates (perhaps the coordinates were old, or perhaps the star was moving fairly rapidly), but nothing came up. Finally, I tracked down the star circled on the star chart. It was pretty far away from the stated coordinates! And, as I knew it would, it already names from several catalogues produced in the late 19th and early 20th century.

I can guess what happened to the coordinates. The sky chart was printed out from some popular astronomy software; the software allows you to see coordinates of a spot by moving your mouse around on a star map on the screen. I suspect somebody just read those coordinates off. They were close, but not nearly as accurate as real star positions are known to be.

But the only thing that angered me was a booklet on stars sent with the certificate and star chart. It was full of horrible factual errors. It was clear that whoever wrote that booklet did little if any research into what they were writing, and certainly had no editor checking their facts. I find this upsetting, because (a) many of the errors are so outrageous that the tiniest bit of research would catch them, and (b) most of the people who read the schlock are not going to know that it is wrong, and in fact are probably more likely to believe it because it comes from an "authority," a company that deals in stars.

If you have "bought" a star or are thinking of doing it, I won't tell you not to do it. But realize that what you are getting is not official naming rights for a real star. In fact, much of what you are getting is likely very cheaply produced with little oversight or care about quality. For the price that these stars cost, you deserve better.