For the next ten or so posts, I will report on my experiences doing astronomical research at Brigham Young University this summer (2014). As I mentioned in a previous post, I have been selected to participate in a National Science Foundation program called Research Experiences for Teachers (RET). I have several objectives:
1. To do some actual original research in astronomy, where I make observations, reduce the data, and analyze it into something that could be worthy of a poster at a science conference (such as AAS next January). In other words, I want to be an actual astronomer for the summer and learn how it’s done through first hand experience.
2. To translate my experience back to my classroom, and actually teach my students how to do authentic astronomical research themselves. With the grant that comes with this program, I hope to purchase a decent telescope and camera and learn to do real astrophotography and photometery.
3. To pass on what I’ve learned, how I’ve learned it, and the entire experience to you who read this blog. I will utilize this knowledge in other potential future projects as well (maybe a book or a video – who knows? At least it will help me with the SOFIA video I’m working on). At least I want this to exemplify how one goes about doing real science.
4. To compliment my experiences with NITARP and provide relevant background knowledge to help me train the students who will be going with me to Caltech at the end of July.
I’ll take this approximately one week at a time rather than reporting daily like a diary. I don’t want this to sound like, “Today I did this, then this, then that.” I want to focus more on the why and the how instead of the what. Eventually I will write up all of the what, all of the small steps and details for the use of my students and others, but for now let’s just get the feel and the big picture of what I’m doing.
The First Day:
We started with a general meeting and breakfast on Monday, June 9. I didn’t know anything beyond the date; the precise time and place where not given in the e-mail. Communication prior to this experience was minimal, and I approached the whole thing with a bit of uncertainty and concern, because I didn’t know what to expect or what would be expected of me. But not really knowing what you’re doing is a common feeling in science. We’re all walking into the unknown. So I had confidence I’d work it all out.
I discovered there are three teachers in this program. Both of the others are new, with one having one year of classroom experience and the other just beginning. They both went through Duane Merrill’s program for science teacher training here at BYU. There is also a group of about 20 undergraduate students who are also doing research, called the Research Experiences for Undergraduates program (REU). Everyone else seemed to know more of what was going on than I did. But we probably all felt that way.
We introduced ourselves, and Dr. Steven Turley, who is over the program, told us more of what we would be doing as a group. We need to write up a prospectus of our research topic and plan by Friday, then during our fifth week we will give an interim presentation to the group on our progress. Then we will give a final report the last week as well as write up a paper for review. All of this must be done in cooperation with our mentor teachers. There will be group activities such as hikes and Tuesday mini-classes to attend.
We also toured the Eyring Science Center and had a general tour of BYU campus, which started at the Alumni House and took us in golf carts around campus. It was a strange feeling being back here again full-time. I haven’t been a student here since 1986, although I’ve visited campus many times to do research in the library or attend cultural events. Now I’ll be an actual Adjunct Research Faculty member, and get a coveted “A” Parking Permit and faculty ID, if only for 10 weeks.
I spent a total of six years as a student here, and touring campus brought back a flood of memories – of classes I took, of dates I’d been on, of experiences both wonderful and terrible. Of course I wondered what has become of all the people I used to know when I was here – where are they now? Have they made a name for themselves? Will I ever hear from any of them again? Even sitting in the remodeled cafeteria, which is very different that the old, brought back memories. I just hope the memories don’t distract me and stifle my ability to do useful research.
High-Mass X-Ray Binaries (HMXBs):
I am working with Dr. Eric Hintz, whom I had met in January at the AAS conference in National Harbor, Maryland. I sat down with him and two REU students, Angel Ritter and Olivia Mulherin, who are also doing astronomy research. He suggested a few projects based on his own research and areas where he has collected data but hasn’t had enough time to analyze it. Angel will be working more directly with Dr. J. Ward Moody and his research assistants, and Olivia and I will work with Dr. Hintz. Olivia decided she wanted to work on a project related to general relativity and gravity waves – a binary system where two stars are spiraling in and are slowing down as they radiate gravity waves.
My project will be to observe and analyze data from high-mass x-ray binary stars in open clusters in the constellation Cassiopeia, including NGC 663 and NGC 659. I will be analyzing data to look for periodicities – cases where the stars vary in a regular pattern and not chaotically. I spent most of the remaining week researching these stars, how they form and evolve, and where they are in these open clusters. Along the way, I found out some fascinating information.
HMXBs contain a highly compact, high density x-ray source orbiting around a large B type blue supergiant. For the x-ray source to be there, its original star must have been larger than the remaining B star. It must have been a type O star that has already gone supernova, smashing its remaining mass into a neutron star or black hole, which is now orbiting a center of mass between it and the blue giant. The blue giant is large enough and spinning rapidly enough that it is overflowing its Roche Lobe, throwing off material that forms a ring around the blue giant which is radiating infrared energy. Some of this ring material is pulled into a streamer toward the compact star. As it spirals in, the particles collide and heat up, eventually so hot that they give off x-rays from an accretion disk around the black hole or neutron star. Magnetic fields pull charged particles out of the disk and form jets that travel out along the magnetic poles, plowing through other material and producing radio waves. So HMXBs are messy, complex, dynamic systems that spew out much of the EM spectrum. The only EM band not represented is gamma rays, and even they might be produced occasionally as material falls onto the surface of the neutron star.
These systems are also very young, less than 20 million years. This means that these binary systems haven’t had time to move far from their stellar nurseries and can be used as standard candles to more accurately pin down the cluster’s age and distance. When the x-ray source went supernova, the shockwave was asymmetrical, which gave the whole system a kick to the side and pushed it out of its nebulous cocoon into interstellar space. Those binaries that stayed together now have eccentric orbits and show periodic changes in brightness at optical and other wavelengths.
Hydrogen Alpha and Be Stars:
The B-type stars in these systems that have not yet gone supernova show an unusual feature in their spectrum. Most stars have absorption spectra – the atmosphere of a star will absorb certain frequencies of light coming from the star, making a series of dark lines on the spectrum. This is how we identify the type of star it is – hotter stars have more absorption lines than moderately hot stars (Type A). Very cool red stars have many absorption lines and show a prominent double line for sodium, which hot stars do not show. There are particular series of lines called the Lyman and the Balmer series that represent the quantum leaps of the single electron in hydrogen atoms. One very prominent quantum leap is called the Hydrogen-alpha (Hα) transition, and occurs in the red end of the visible spectrum at 656.28 nm. It represents the absorption of just enough energy for the hydrogen atom’s only electron to jump from the second to the third quantum level (n = 2 to 3). These stars show a prominent, deep red hydrogen alpha absorption line. But right in the middle of the absorption dip is an emission spike. The hydrogen gas in the ring around the B star is being excited by energy from the star and is emitting light like a neon sign. The electrons in the gas ring are falling from the 3rd energy level back down to the 2nd, emitting exactly the same wavelength of energy that the star’s atmosphere is absorbing. These stars are called Be stars (B emission stars).
Deep in Cassiopeia:
The open clusters I will investigate are NGC 659 and NGC 663. Both are located inside Cassiopeia. I did some research on them and found that they are both part of a larger structure of hydrogen gas, dust, and stellar nurseries imbedded in the Perseus Arm of the galaxy, about 8000 light years away from us toward the outer rim. Our solar system is located on the inward edge of the Orion Spur, a branch of the inner Sagitarrius Arm that crosses from the area of Deneb and Cygnus across through Orion. Both clusters are just under the left leg of the Big W in Cassiopeia (the Throne asterism) to the left of Ruchbah and northwest of 44 Cassiopeia.
Dating a Cluster:
The larger a star is, the faster it consumes its nuclear fuel, converting hydrogen into helium through fusion in the star’s core. While this is going on, we say that the star is on the “Main Sequence” of the Hertzsprung-Russell Diagram, a chart comparing the temperature (or color) of a star versus its intrinsic brightness (or absolute magnitude or luminosity). These H-R Diagrams are also called Color-Magnitude Diagrams, or CMDs. An O-type super giant star is very hot (35,000 °K) and runs out of hydrogen 10-15 million years after forming. It then starts fusing helium into even heavier elements and migrates off the main sequence, becoming cooler and redder as it expands into a red supergiant, like Antares or Betelgeuse. Cooler B and A stars last longer before migrating off the main sequence.
If you chart a CMD for a cluster of stars such as M67 in Cancer, you will see a pattern similar to the diagram shown here. The bigger, bluer stars have already left the main sequence and migrated to the right. As time goes on, smaller and smaller stars migrate off. You can look at the “turn off” point on the H-R Diagram and get a good estimate of the overall age of the cluster. In the case of M67, the bluer stars are almost all gone except for a few “blue stragglers” that haven’t quite become red giants yet. These are probably stars that were in binary systems where two smaller stars have recently merged into a larger, bluer, hotter star. M67’s turn off point has progressed down into the A and F stars, and it is about 3.8 billion years old with quite a few yellow dwarfs similar in age and composition to our sun. It is unusual that we can still identify it as a cluster – by this time, the stars will usually disperse. NGC 663 and 659, however, are young clusters that are just beginning to turn off, probably about 15-20 million years old, and still rich with stars in the center of the cluster.
The Instability Strip:
Some stars, as they progress through larger and larger atoms in their cores, will become unstable and start to pulsate. This seems to happen in a particular range of temperature vs. magnitude on the H-R Diagram, a narrow rectangular area known as the Instability Strip. Dr. Hintz is very interested in these variable stars, because they tell us a great deal about stellar dynamics and nucleosynthesis (how new elements are formed). They are also extremely useful as standard candles for measuring distances. Regular variable stars that are truly variable for intrinsic reasons and are not just eclipsing binaries come in several varieties. They are classed according to the period of their variability and its amplitude (how many magnitudes it changes) as well as their size, age, and composition.
The shortest period stars are called “Delta Scooty” stars after the prototype star δ Scuti. They have very short periods on the scale of hours and magnitude changes of from .003 to .9 magnitudes. A well-known star of this type is Altair. Such stars are usually spectral type A to F white giants.
The next class is RR Lyrae stars (pronounced by astronomers here as “RR Laurie”). They are white stars of class A with short periods from .05 to 1.2 days and magnitude fluctuations of .3 to 2.0 v. They are also divided into two classes depending on the metallicity of the star.
The next class of variables is the Cepheids, named for δ Cepheus, with periods of 1 to 70 days and magnitude changes of 0.1 to 2 v. They are orange to yellow-white F to G or K giant stars. There are two types of Cepheids – the first are younger stars with higher metallicity and belong to Population I stars found mostly in the spiral arms of galaxies. Type II Cepheids are older, with less metals, and are usually found in globular clusters and galactic cores. They are sometimes called W Virginis stars. The relationship between the period and the magnitude change of these stars was first mapped out by Henrietta Leavitt and was used by Edwin Hubble to prove that the Andromeda Galaxy was a separate “island universe” from our own Milky Way. Because the intrinsic brightness of the star is related precisely to the period of its variability, and you can see the variability changes from long distances away using a large telescope, you can then work out the distance using a distance modulus formula based on the fact that the apparent brightness of a star varies inversely with the square of its distance.
Finally, there are variables such as RV Tauri, which are yellow to orange giants with periods of 30-150 days and magnitude shifts of 3.0 or more, and there are long period variables such as Mira, which are red giants with periods of 80-1000 days. As you go to brighter and larger stars, they also become cooler and more on the right side of the H-R Diagram. Because they change in brightness, they tend to be on one side or the other but not often inside the instability strip. This tends to produce what seems to be a gap in the H-R Diagram.
As for what the underlying mechanism is for pulsating stars, it comes down to the elements in the star and how they transmit or block light. In these older stars, helium has begun to build up as core fusion has progressed, and the helium forms layers in the star. As the helium is ionized, it changes opacity. In a normal star, the denser a layer is, the more transparent, so a star will stabilize at a particular size and energy flux. But if internal layers become opaque, as in the case with ionized helium, then the energy coming from core fusion can’t escape and it builds up under the layer. This causes the helium layer to expand and pushes the outer layers of the star with it, making the star larger, brighter, hotter, and bluer. As the built up heat escapes, the helium layer cools and ionization drops, making the layer more transparent and allowing more energy to escape. The layer then cools and shrinks, the gravity of the star compresses the outer layers and the star becomes smaller, dimmer, and redder. Then the cycle repeats. The pulsations are therefore effected by the mass, age, and composition of the star. This is called the Kappa Mechanism or sometimes the Eddington Mechanism, after Sir Arthur Eddington, who first proposed it as an explanation of variable stars.
It took me several days to do all of this research, and I took quite a few notes in my research journal. My job was then to distill all of this into a working proposal and write it up as a short prospectus, which I am including here: Prospectus-David_Black Dr. Hintz approved it and sent it on to Dr. Turley. The REU students actually get a bonus payment for submitting one, but we RETs do not, as it is worked into our contracts.
My biggest frustration the first week was dealing with actually getting a contract and getting hired, which I’ll talk about in later posts. I also missed my second day (Tuesday, Jun 10) because I was previously booked to give two conference presentations, one at the IT Educators conference at Granite Technical Institute in Salt Lake (I presented on ideas and projects for teaching Python programming) and one at the Utah Association of Charter Schools at the Davis Conference Center in Layton. I presented there on our STEM-Arts Alliance projects (see my other blog at: http://elementsunearthed.com.)
The Process of Science:
We tend to teach that science has a fixed “method” that all scientists use in exactly the same sequence of steps. This isn’t a very accurate picture. True, following this method will help ensure you’ve done the right things in the right order, but it doesn’t guarantee you’ll get any useful results. And, of course, real science is never that neat or cut and dried. No one sits down and writes up a formal hypothesis, etc. But you do start with a question and then follow your nose, and along the way, if you are thorough and think things through well enough, you can collect some useful data that actually means something and could answer your question. It’s a much more organic and messy process than what we teach kids in elementary and middle school and force them to follow on their science fair projects.
I hope through the next several posts to describe just what astronomy is like as a science and the kinds of activities astronomers do. They don’t spend all of their time staring through telescopes, and never did. I used to think to be an astronomer you needed to have good eyesight, and that misconception kept me from pursuing astronomy as a career. But all astronomy is done with cameras, not raw eyesight, and uses sophisticated software and some excellent thinking to compare one star with another.
So the first step of science is to know enough about your general area of study to know what is knowable, what is known, and what is not yet known. You’ve got to do some background research to understand what even constitutes a decent question. So that’s what I did this week – found out as much as I could about Be stars, HMXBs, and variable stars. I thought I already had a good knowledge of these things, but I’ve learned so much more. Now I can begin to formulate a question or objective for my research: Do HMXBs show periodicity, either extrinsically or intrinsically? Can I detect and characterize these variations? Beyond that, can I learn the process of astronomic observation, data reduction, and analysis? Can I translate what I’ve learned to the high school level so my students can share in this experience and do authentic research themselves?
I’m sure I will take detours and diversions along the way, following rich veins of possibility. Some of these will peter out and lead to dead ends, blind alleys, and box canyons of knowledge. I’ll have to backtrack and try a different direction. Perhaps some of the best things I’ll learn will be accidental or serendipitous – something I never anticipated and couldn’t have planned for but just happened to be at the right place and time to learn. I’ve already come across one of these that I hope to follow up on in the next few weeks. This entire experience will be a messy, uncertain, and at times frustrating process.
Welcome to real science.