Tuesday, September 30, 2008

KAIT and the High-Z Supernova Search

I have been and always will be a space geek. Ever since I got glow in the dark stars and comets taped to my ceiling as a kid, I have dreamed of being an astronaut (something I fully intend to apply for). I read voraciously about the space program and got through my fair share of science fiction. When I came to college, I had the opportunity to take an astronomy class for the first time, and one in particular came highly recommended from older students. So, in my rookie semester, I took Astro. C10 with Professor Filippenko (Alex, as he likes to be called). It was a class designed to be accessible to science students and non-science students alike, and that fact combined with Alex's unique enthusiasm for the subject has made it one of the most popular courses at Cal. The course covered a wide range of topics from the behavior of light to stellar evolution to the shape of the universe. As someone who has read a lot about space on my own, much of the information was familiar to me, but I learned a lot regardless. Towards the end of the semester, Alex announced that he would be taking 10-15 (of the ~800(!) people in the class) students to Lick Observatory near San Jose, CA to get a tour of the facilities and a little insight into the nuts and bolts of modern astronomy research.

I wrote the optional paper and was selected for the trip. After a long and winding drive into the mountains, we arrived at the top of Mt. Hamilton. We got a tour of the historic observatory (it was the first permanently-occupied observatory on a mountaintop) and got to look at a few interesting objects through the 120-year-old 36 inch refractor. Part of the tour included Alex's personal invention: KAIT. From his lectures it was clear that Alex worked on supernovae, but those of us on the trip got to see exactly how he did it. I asked Alex a lot of questions about the telescope, and he invited me become one of his undergraduate assistants on the project. I jumped at the chance to participate in astronomical research and I have been working for him ever since. I want to share some information about this fascinating project, but some background is needed.

What's a Supernova?
There are many different objects out there in the observable universe, and different people study different things. Professor Filippenko and his team mainly study supernovae, which most people know as exploding stars. These objects, or rather the violent explosions their so-called progenitors produce, are interesting to study for a variety of reasons, but what exactly are they? Nova means "new" in Latin, because originally these events appeared as a new star in the night sky that would fade over time. Amazingly, supernovae were seen by humans long before the telescope was invented. Extragalactic supernovae aren't bright enough to be visible with the naked eye, so these events arose from stars in the Milky Way exploding. In 185 AD, Chinese astronomers recorded that a new star appeared and stayed visible for 8 months. The only other Milky Way supernovae that have been recorded since were in 1006, 1054, 1572 and lastly in 1604. A galaxy the size of ours should theoretically have one supernovae every 50-100 years, and the objects give off such a tremendous amount of energy that they should be obvious in the night sky. However, there is a large amount of dust in our galaxy, and it's very possible that it has blocked the light of supernovae from reaching us, especially one on the far side of the galaxy.

All supernovae are massive explosions, but they are theorized to occur in a few different ways. The "classic" supernovae, known as a Type 1A, happens when something called a white dwarf (basically a very old star) gets extra mass added to it from a companion star or via a merger with another white dwarf. Once the total mass of the object gets over a certain limit, the star rapidly undergoes nuclear fusion and explodes. When I say rapidly, I mean something around the size of Earth with a mass around that of the sun converting most of its mass into pure energy in a matter of seconds. The result of this is a supernova, the most energetic event in the known universe. To put this in perspective, lets look at SN1006. This supernova occurred when a star 7200 lightyears away (over ten quadrillion miles) exploded. The result of this was an object appearing in the sky half the size of the moon with enough brightness to light the ground at night for months. In fact, if a supernova goes off close enough to Earth, it could easily destroy all life on the planet. One could have taken out the dinosaurs. These things don't mess around.

All that's left of SN1006 is a rapidly-expanding shell of gas and dust. Over the last 1000 years, it has gotten to be trillions of miles wide. Here's what it looks like today, courtesy of the Hubble Space Telescope:


So why do we look for these things, aside from the fact that they are awesome?
It is fairly well known that Edwin Hubble discovered that the universe was expanding in the 1920s. By looking at special stars called Cepheid variables, Hubble found a relationship between the distance of a galaxy and how fast it was moving away from us. This observation, confirmed over and over again since, is one of the fundamental pieces of evidence in support of the Big Bang theory. Supernovae are kind of like modern Cepheid variables in that they can both be used as 'standard candles'. Basically this means that you can use clever tricks figure out exactly how much light they are giving off in total, and thus deduce their distance. For example, lets say you look out to sea on a dark night and see a single light from a boat. It could either be a faint light that is close by or a bright light that is far away. Both would look the same, and you would have no way of knowing which was the case. However, if you knew the exact brightness of the bulb (the "absolute magnitude"), you can compare that to the amount of light you actually see ("apparent magnitude") and calculate a distance. The speed of recession is a little more complicated, and has to do with something called the Doppler effect. We have all experienced this when ambulances drive past us. They sound more high-pitched as they approach and more low-pitched as they drive away. From these two effects, one can determine both distance and recession velocity, which is exactly what Hubble did. Supernovae are especially useful for this kind of work because of their incredible brightness. They easily outshine the galaxies they reside in when they explode, and we can thus get data about very distant parts of the universe.

The Big Bang theory took a while to get accepted by most members of the scientific community, but it is now basically indisputable. Questions turned from "if" to "how", and cosmology was born. People became very interested in the 'shape' of the universe, which is a concept so abstract many people who study astronomy for the first time struggle with. It became a common conception that the universe expanded very rapidly at first, but thanks to gravity this expansion was slowing over time. Then, in the late 1990s, an astounding result came out of the high-z (meaning high redshift, thus very distant) supernovae research field. Two groups, both of which Alex was associated with, reported that the most distant supernova were dimmer than they ought to be. The logical conclusion of this observation was that the expansion of the universe was in fact accelerating. This would mean that some hitherto-unseen force was not only working against gravity (antigravity, anyone?), but enough so that it actually reversed gravity's effects. Thus dark energy was born. Not since the discovery of the expansion itself had the astronomy community been turned on its head so dramatically. To this day, nobody has a clue what dark energy actually is, but they have calculated that it must account for around 3/4 of all of the mass-energy of the entire universe in order for it to be having this kind of effect.

Needless to say, supernova research is a pretty hot field in astronomy right now, and not just because of the acceleration discovery. Alex designed and built KAIT, the Katzman Automatic Imaging Telescope, to make finding these events easier and more efficient. It is a relatively small telescope that operates almost completely autonomously. Every night with good weather, KAIT takes a look at several hundred locations in the sky. Using software specially written for the purpose, it identifies new objects and flags them to be double-checked by a researcher. That's where I and several other students come in. Every night, these images are downloaded from the telescope in the mountains onto a computer in Berkeley, and someone goes through each and every one of them. Usually the software tags a cloud, a satellite, or something called a cosmic ray, but every once in a while you get something good. It is my job to separate the wheat from the chaff, and find the occasional supernova candidate. If these candidates appear in the exact same place again on another night, they are very likely supernovae. Several hundred supernovae are discovered worldwide every year, and KAIT is one of the most successful supernova projects ever. I have personally discovered around 10 so with KAIT since I began working with Alex in 2007. Here is one of the more recent ones from March of this year, SN2008BF:

The pair of large fuzzy dots are actually galaxies on a collision course, each containing billions of stars. The arrow is pointing to the supernova. One can see from this image how 'big' the supernova looks, which is an effect of its brightness saturating the camera on the telescope. Below is a much more beautiful image of a SN Filippenko's team discovered in 1994, again from Hubble, showing very clearly how bright the supernova (lower right corner) is compared to the host galaxy:


Working on this project has been an amazing experience, and I hope to continue it until the day I graduate. I am very grateful to Alex and to the Berkeley scientific culture in general which encourages undergraduate participation in real research. Thanks to this trust, I have been able to learn a lot more than can be gleaned in a lecture while contributing to the scientific process. Sometime in the future I will write something about my biology research, which has been equally rewarding.

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