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!

Looking For Gamma Rays

Image Credits: NASA / CBS

Yesterday, NASA launched a new telescope into orbit, The Gamma-Ray Large Area Space Telescope, or GLAST. I've been hearing about the preparation of this mission for at least a decade, so it is great to see it underway! My congratulations to the team.

But why would astronomers want to look at gamma rays? And don't we already have telescopes looking at gamma rays? Gamma rays are the most energetic form of light in the Universe. The wimpiest gamma rays are about 20,000 times more energetic than visible light, and there is no theoretical limit to how strong they can become (though there are many practical limits). This makes gamma rays very dangerous to living organisms; unsafe levels of exposure can cause all kinds of cancers and other nasty effects. (It was an overdose of gamma ray radiation that turned mild-mannered Bruce Banner into the Incredible Hulk at least one evening a week back in the late 1970s and early 1980s.)

Most gamma rays people encounter come from nuclear reactions and radioactive decay; gamma rays are the most dangerous form of radiation from nuclear waste. There are also gamma rays flying around you all the time from naturally-occurring radioactive elements. But, unless you are involved in a nuclear accident, the numbers of gamma rays on Earth are too low to cause much harm.

Many objects in space also produce gamma rays. Our atmosphere is opaque to gamma rays, so we are protected from this potentially dangerous radiation. But since gamma rays are produced by some of the most energetic and mysterious astronomical objects, like black holes, neutron stars, exploding stars, and the radioactive remnants of these exploding stars, we astronomers would like to study them. So we have to launch telescopes into space to look at gamma rays.

So, why don't we use the Hubble to look at gamma rays? Why spend lots of money on a totally different telescope? It's because gamma rays are so energetic, we can't look at them with normal mirrors. Gamma rays just pass right through the Hubble's mirror. So GLAST uses a very clever technique that relies on Einstein's most famous equation, E=mc².

Behind that famous equation is the idea that matter (electrons, protons, atoms, rocks, hamsters, etc.) is just another form of energy, like light, heat, and motion. And it is possible to change energy from one kind into another. Our car engines convert chemical energy from gasoline into the motion energy of travel, as well as into heat energy (which is why the engine gets hot!). On a sunny day, the interior of that same car converts the light energy from the sun into heat energy. Nuclear reactors change some of the matter in the atomic fuel into light and heat energy. And the GLAST telescope changes the light energy of gamma rays into matter: two or more subatomic particles (and any leftover energy is turned into motion energy of the particles). The telescope then tracks the position and speed of these particles, which, through some complex but well-understood physics, lets us surmise the original energy and direction of the gamma ray.

But gamma rays are rare, and gamma ray telescopes aren't very efficient at converting light into matter. So, it is important to make the telescope big ("large area") so we can detect as many gamma rays as possible.

GLAST is NASA's second big gamma ray telescope, after the Compton Gamma Ray Observatory, launched by the space shuttle in 1991. NASA has another gamma-ray telescope in orbit, the Swift Telescope, but Swift just looks for the mysterious flashes of gamma ray light called gamma ray bursts. GLAST can detect gamma ray bursts, but its primary mission is to look for other, steady sources of gamma rays.

Gamma rays are produced by matter about to fall into a black hole. The matter gets sped up to high speeds by the black hole's gravity, and before it falls into the black hole's event horizon it can emit gamma rays that we can detect here on Earth. Energetic jets spewing from the regions around black holes can also act as atomic particle accelerators, which can create all kinds of subatomic particles that then collide and release gamma rays. The remnants of exploding stars, such as Cassiopeia A, also glow in gamma rays from radioactive elements created in the giant explosion that destroyed a dying star.

Don't expect many spectacular pictures from GLAST. It's just not possible to make sharp, focused pictures. While the Hubble Space Telescope can see details as fine as 0.05 arcseconds (an angle something like the size of a penny seen from 50 miles away), GLAST can only see as sharp as 1 arcminute (600 times worse than Hubble). GLAST would have trouble resolving details the size of a penny about 380 feet away, a feat that sharp-eyed people can do in excellent conditions. But it is still much better than the Compton Gamma Ray Observatory, which could only resolve the said penny about 40 feet away, something anyone with normal vision can easily do.

But what GLAST can't do in sharpness, it can make up for in its field of vision. While Hubble can only look at a sliver of sky about 1/80th the size of the full moon, GLAST can look at 20% of the entire sky at once!

It'll probably be a year or two before the first GLAST science not dealing with gamma ray bursts comes out. Until then, GLAST will be staring hard, catching elusive gamma rays from deep space. Let's just hope that the scientists are nice to their telescope and don't make it angry. You wouldn't like the telescope when it gets angry.