The no-view nova

Image Credit: Contours: ESA/ XMM-Newton/ EPIC (adapted from A. Read et al.), Background: SSS

Most of the time, the stars and galaxies that we astronomers look do not change much, if at all, over a human lifetime. So, our only hurry in looking at a star is to do it before somebody else does. If the weather is bad or the telescope breaks, we can come back another night, or even another year, and there is little lost.

For some astronomical objects, though, time is critical. Supernova explosions, for example, are only visible for a few months or so before fading away from sight. Another, more common explosion, called a nova, only lasts a few nights. Glows from gamma-ray bursts last just a few hours. If one of these events occurs, we need to hop on it fast, or lose it forever.

The problem is, you've got to be looking in the right place at the right time to see one of these. At present, there are only a few small telescopes that take pictures of the entire sky on a regular basis. Such a search produces tremendous amounts of data, and on big telescopes, the biggest cameras can only image about one quarter of one ten thousandth of the entire sky in a single picture. So, much of the sky is not searched by professional astronomers for these time-critical events. Those who do search for these events tend to focus on tiny patches of the sky. Though they'll miss most explosions, they'll still see enough for their science. (The one exception are gamma ray bursts, because the gamma ray detectors in space actually can look at most of the sky in a single picture.)

Typically, this is where amateur astronomers step in. These men and women are often out conducting searches of their own, often using their own eyes and star charts to try and spot something out of place. It may be a comet, or it may be an explosion, or it could be some other event. Amateurs are pretty good at this, and are discovering comets, supernovae and novae all the time. They get a little bit of glory, and a lot of personal pride, out of beating us professionals. And they deserve it.

One of the big prizes is discovering something that will become bright enough to see with the naked eye (i.e., without a telescope or binoculars). Then people around the world will be able to go out and see your discovery, sometimes with your name attached (like Comet Hale-Bopp).

Still, even a small army of amateurs can't catch everything. This was proven in a press release last Friday from the European Space Agency's XMM-Newton X-ray telescope.

The X-ray telescope, like an optical telescope, points at interesting targets and takes pictures. When it moves from one target to another (which takes a long time in space), the cameras are usually turned off. But a group of scientists including Andy Read of the University of Leicester and Richard Saxton of the European Space Agency are running a project where, sometimes, the cameras are kept running as the telescope moves, allowing random objects to drift into the field of view.

Last October, a bright X-ray source popped into the XMM-Newton camera during one of these moves, but, according to catalogs, nothing should have been there. After some quick legwork and a few phone calls to big telescopes, it was determined that the X-rays were coming from a previously unknown nova.

A nova is a distant cousin of a supernova. In a supernova, an entire star explodes during a runaway nuclear explosion. In a nova, the outer layers of a white dwarf star explode like a hydrogen bomb, but the explosion is too weak to blow the entire star apart. As you might guess from the names, a supernova is many times brighter than a nova. But novae are actually more common, because there are a lot of white dwarfs in our galaxy. Several novae are found every year, and every few years, one is bright enough to see with the naked eye. As with bright comets, most novae are found by amateur astronomers, and not by professionals.

The odd thing about the XMM-Newton's discovery, though, is that novae don't make a lot of X-rays early on. So, the nova that XMM-Newton found was actually a few months old, but it had never been reported. So, the XMM-Newton team called up the operators of a robotic all-sky survey called ASAS. They combed through old data, and found that the nova had indeed been picked up by their optical telescopes on June 5, 2007. Not only that, but the nova had gotten bright enough that it would have been easily visible to the naked eye, the brightest nova in over a decade. And yet, not one human knowingly saw it!

How did everyone miss it? Well, the nova was in the constellation Puppis, which is not visible in most of the northern hemisphere (where most amateur astronomers live). And Puppis lies near the Milky Way, so it is full of stars -- only a trained eye would have been able to pick out the new one. But novae are found in Puppis by professional and amateur astronomers quite a bit. We just got unlucky with this one.

Discoveries like this make us wonder how many interesting things happen in the sky on time scales so short that nobody has a chance to see them. For that reason, astronomers are starting to build telescopes that will image the entire sky to very faint limits every few days. The ultimate data will come from the Large Synoptic Survey Telescope or LSST, which will soon be built in Chile. The mirror for this telescope is huge -- 8 meters across, making it one of the largest telescopes in the world. The telescope, in a single picture, can image an area of sky about 50 times the area of the full moon. A single 30 second exposure will be able to see objects about 2 million times fainter than what your eye can see.

Amazingly, the hard part of this project is not the telescope (though it will be one of the most complex mirrors and cameras ever built). The hard part will be the data volume: 30 terabytes of data every night. That's 30,000 gigabytes, or, if you were to put it on a normal DVD, about 6000 DVDs worth of information every single night. For five years. And we want to be able to analyze that data on the fly, so that interesting objects (like novae) can be observed with other instruments at other telescopes as soon as possible. To help with this, money and assistance from Google and the Bill and Melinda Gates Foundation (along with other technologically-oriented companies) are pouring into the project.

The LSST mirror is under construction in Tucson, Arizona, and the construction will soon start in Chile, with hopes of opening this new eye on the Universe in 2014. Hopefully no more novae will slip through the cracks!

39 years ago today...

Image Credit: NASA

39 years ago, humankind took its first tentative steps into the cosmos when astronauts Neil Armstrong and Edwin "Buzz" Aldrin walked on the surface of the Moon as part of the Apollo 11 mission. (Their moonwalk, while only 2.5 hours long, started about 11pm EDT on July 20 and finished in the early morning of July 21, so there is some ambiguity in the "date" of the moonwalk. But why not celebrate this amazing accomplishment over two days, instead of just one?)

At the end of the Apollo Moon program in December, 1972 (a full year before I was born), few people suspected that it would be nearly 50 years before we returned to the moon (and it could be longer than that, if the Orion project is significantly delayed). To some people, this is a travesty. Other people wonder why we are even considering going back.

I think it is in our nature to explore. From our early Homo sapien ancestors leaving the African continent to colonize new lands, to seafaring peoples of many nations and races that sailed the vast and unfriendly seas, to the astronauts/cosmonauts/taikonauts who risk their lives to sail the vacuum of space, the unknown seems to draw us onward. So, I suspect that it is just a matter of time before we humans are crawling across the face of Mars, and perhaps even considering one-way flights to explore new worlds around other stars. But this "time" may be hundreds or thousands of years from now, and there are many other challenges facing us right here on our home planet. So I think we can afford to be patient, as long as we don't take our eyes off the ultimate goal.

Sp, if you look out late this evening to see a big yellow moon rising, remember that we were there just 39 short years ago.

One really cool movie

Credit: D. Linder / EPOXI / NASA

The video above shows a very unique view of the Earth/moon system. It was taken by the EPOXI Mission, the name of NASA's Deep Impact spacecraft's new mission (now that it's completed its mission to explore Comet Tempel 1). The spacecraft turned its onboard camera toward the Earth in late May when it was 31 million miles away from Earth, and it captured (intentionally) the Moon passing in front of the Earth.

The video above is roughly true color. A cartoon globe in the lower left shows you what parts of the Earth are visible at any given time. If you look closely toward the center right of the Earth, you can see "sunglint," the reflection of sunlight off of the oceans in the direction of the spacecraft.

Notice the color difference between the Earth and the moon. The moon looks dark and reddish. That's the true color of the moon. It looks bright and silvery in our sky, but if we had another bright Earth near the moon in our sky, we'd see how reddish and dark the moon really is.

Also notice the relative size of the Earth and the moon (also correct); it can give you some idea of how small the moon is. The only thing this picture doesn't do is give you an idea of the relative separation of the Earth and moon; the moon is roughly 30 Earth-diameters closer to the spacecraft than the Earth is.

Another version of the video shows the same scene, but it includes some infrared imaging, which brings out vegetation (plants reflect a lot of infrared light). In this version, you can see parts of the continents even better. NASA's Landsat satellites use the same infrared colors to track changes in plant life on Earth, such as these pictures showing the damage caused by illegal logging in Indonesia over a 10 year period.

I find views of the Earth like this are very helpful in reminding myself of our place in the Universe, and how small and unique that place is. Some other views of our planet from outer space:

• The Earth seen from the Moon in the famous photo taken by Apollo 8 astronauts in December, 1968.
• A movie of the rotating Earth taken by the Galileo mission as it swung past the Earth on its way to Jupiter
• A view from Mars by the Mars Reconnaissance Orbiter
• A family portrait of the entire Solar System from the Voyager 1 spacecraft, taken from a distance of 8 billion miles.

Special thanks to Phil Plait for posting the EPOXI videos on YouTube and for bringing my attention to them.

writing about nothing

These days I am working on a fairly boring paper that I intend to publish in one of our professional astronomy journals. The reason it is boring is because it is about nothing. Or, at least, it is about us looking for something and not finding it.

A couple months ago, my colleagues and I announced that we has seen variations in the light of a special type of white dwarf star. This represented the discovery of a new type of variable star, and we continue to work on understanding that star.

That new variable star was discovered as part of a targeted search. We were looking for variations in the light coming from stars such as the one we observed. And we found those variations in one star. But we didn't see them in other stars, and now we need to write a paper describing these non-detections.

There may be many reasons why we didn't see variations in the other stars we targeted:

1. These other target stars aren't varying.
2. The other targets are varying, but at a very low level that we can't detect.
3. We messed up in our data analysis.

And there may be other reasons. Number 3 is fairly straightforward to check; I'm doing that now. I make sure that we were pointed at the right star, and that the other stars around it are acting pretty normal. I make sure that the weather wasn't too bad, and that the clouds weren't too thick. These checks are time consuming, but important. The hard part is choosing between possibilities one and two, and this is crucial for the science! If we claim (and we do) that these other target stars should not be varying, then we have to rule out variations to pretty low levels, better than one percent. And even that may not be good enough; one colleague of mine states that, "if you don't see a star varying, you haven't looked hard enough." And we have other reasons to thinking that variations smaller than the percent level would be something altogether different. But there is a big difference between saying, "it doesn't vary" and "it doesn't vary at a level larger than 1 percent."

A lot of astronomers don't bother publishing non-detections (or "null results," as we often call them). The papers are boring to write, because we usually aren't sure if we should have seen something or not, and it's not as fun as claiming to find something new. But, when it comes to testing theories, a null result can be just as important as finding something. If a theory were to predict that all of our stars should vary in brightness, but only one of a dozen does, than that theory can be ruled out.

Telescopes on the moon

Image Credit: NASA

We're going back to the moon. It may be 15 or 20 years until we get there, but NASA is headed that direction. Many people, including a lot of astronomers, are opposed to this new lunar exploration. It will be very expensive, and we aren't sure what the point of the exploration is.

A colleague of mine once argued that we astronomers should be pushing NASA to build a telescope on the moon once we return. His idea, a liquid-mirror telescope, is pictured above. Such a telescope would be quite expensive, and the science it could do may be able to be done more cheaply with orbiting telescopes. But the point was that NASA is much more likely to spend a lot of money on the moon, not on new space telescopes, and if astronomers want a piece of that money, the time to start lobbying is now. Otherwise, we could find ourselves out in the cold once lunar exploration ramps up.

It's sort of like a parent shopping at a high-end luxury store asking their thrifty teenager, "Hey, should we buy you this 2nd-generation iPhone for $500?" The teenager responds, "But we can go to the Apple Store and buy the new iPhone 3G, which is more functional and less then half the price!" To which the parent responds, "You're getting this phone or you're getting nothing." What should the kid do? I think most of us would be tempted to take the phone.

Today, Space.com posted an article on a new design for a lunar telescope that has been proposed. This telescope could be built from materials on the moon. I have no doubt that the cost would be very high and the design technologically challenging to build (as would be the liquid-mirror telescope my colleague proposed). I know very little about the new proposal, such as what wavelengths of light it would be best suited for, or what instruments would go on it, or what scientific question(s) it would address. And, no doubt, NASA would make sure it is a very nice telescope, in terms of these capabilities.

But I wonder if we astronomers need to start thinking much harder about this. Do we want to push for a telescope on the moon? If there is a trade-off between a lunar telescope and a more functional, less-expensive space-based telescope, then let's by all means choose the latter. But I do think it is likely that we might be given the choice of a big lunar telescope, or no big telescope at all. And if we wait 15 years to make up our minds, it will be too late. Who knows what the astronauts will be doing on the moon science-wise, but it certainly won't be astronomy. And we'll have missed out.

There is good astronomy that can be done on the moon. One of the most convincing ideas I've heard is to put a radio telescope on the far side of the moon; the moon will block out radio signals from Earth, allowing us to study signals from space that are currently swamped by our FM radios, our iPhones, our satellite TV, and most every other modern bit of wireless communication. Some types of astronomy, like optical astronomy and, perhaps, infrared astronomy, are better done in orbit so that we don't have to worry about astronaut dust and other activity upsetting the instruments. But both of these fields would not suffer from a lunar telescope.

Astronomers are beginning to assemble our "decadal survey," a 10-year look into the future needs and desires of astronomy. This survey is cited in our funding requests to Congress and NASA. Maybe this time around we should debate the various possibilities of lunar telescopes. If we wait until 2020 to ask, it may be too late.