Saturday, February 13, 2010
Identifying white dwarfs
My run on the Keck Telescope earlier this week involved looking for white dwarf stars and taking spectroscopic measurements of them. I got a couple of Twitter queries about how this works, and whether I had truly discovered previously unknown objects. So, I thought I'd explain the process of what I was doing a bit more. Another day I'll try and explain why I went through these pains.
Let's start with my goal. I wanted to find white dwarfs in an open star cluster, specifically the old open star cluster Messier 67. Messier 67 is about 4 billion years old, so only slightly younger than our Solar System and our sun. The stars in the star cluster have about the same amount of heavy elements as the sun. For this reason, studying Messier 67 gives us astronomers an idea of what "normal" stars the same age as our sun are like.
Some stars don't have enough fuel to last for 4 billion years. Those stars either exploded as supernovae eons ago, or they formed white dwarfs. White dwarfs are the white-hot ashes of the nuclear fusion that happened in a dead star. That fusion has ceased, so the white dwarf cools and fades, just like a hot poker taken from a fire or a red-hot heating element on an electric stove will slowly cool and fade from view.
So, the first step is to find white dwarfs in the star cluster. White dwarfs are very faint stars, and they tend to be blue or white in color. So, we take pictures with telescopes and look for things that are faint and blue. Other people have done this before in the star cluster Messier 67. However, most of these people did not publish the locations of the faint blue stars they found. So, I had to go take my own pictures and find faint, blue things for myself. I did that at the MMT Observatory outside of Tucson about 6 years ago. Here's a color picture I took during that observing. You won't see any white dwarfs in this picture, because this color picture was made from exposures too short to show the faint white dwarfs.
However, white dwarfs are not the only things that are faint and blue. In the field of Messier 67, we can see all kinds of faint galaxies (like these). Galaxies can usually, but not always, be separated from stars because galaxies are fuzzy, while stars are points of light. However, quasars also look just like points of light, and some dwarf galaxies are so small and so far away that they look just like points. One of the few ways to know for certain is to take a spectrum of the star. (Here are three previous posts where I discuss spectra in general: 1, 2 and 3). White dwarfs have unique spectra, so it's easy to be certain we are seeing a white dwarf and not something else. With a big telescope like Keck, we can securely identify the white dwarfs in the star cluster in just 15 minutes.
But I don't simply want to identify whether stars are white dwarfs, though. I want to figure out how hot the white dwarf is, so we know how long it has been cooling, and how massive the white dwarf is, so we know how much nuclear fuel the white dwarf's parent star used during its lifetime. This requires high-quality spectra, which means exposure times of two to four hours with the Keck telescope.
So, to summarize, before I went to the Keck telescope, I had to look for candidate white dwarf stars. Most of my candidates had probably been seen by other people, but they hadn't published catalogs of these candidates. I then used Keck to make sure that each candidate was indeed a white dwarf star and not something else that looks similar, like a quasar. Finally, for the real white dwarfs, I took a few hours of exposures so that the data have the quality I need for my science analysis.
So, did I really discover these white dwarfs? I wasn't the first to see them, but I hope to be the first to catalog them. I certainly was the first person to identify each star using spectra. And in science, we joke that the discoverer is the last person to find something. I certainly intend to be the last person to find these white dwarfs, so any future observers can then just look in my catalogs!