The Variable Stars Research Team was assigned the task of researching dwarf novae in outburst. With the help of the TIE program (Telescopes In Education), they were able to make observations and report their new findings to the AAVSO (American Association of Variable Star Observers). Some other projects included imaging a supernova and collecting a series of images of Hubble's Variable Nebula (NGC 2261). All three of these were projects involving variable stars.

Telescopes in Education is a program utilizing a 24" Cassegrain telescope located at Mount Wilson, California. This program allows various groups to observe objects. It is a special research opportunity for students, so that they may analyze variable stars, and attempt to understand them.

A variable star is a star whose luminosity changes with time. The purpose of studying these stars was to figure out why and how the luminosity changes. Some stars change more often than others. Pulsating variable stars are normal stars experiencing a brief period of instability as a natural part of stellar evolution. Scientists are attempting to figure out why this happens with stellar evolution, and what it means for the star.

The specific variable stars the team chose to research were dwarf novae. A normal novae usually occurs in a binary star system (a system in which two stars revolve around each other.) One star exchanges mass with the other, and the receiving star projects some of the gained mass. Thus, a nova occurs, taking place over a period of one to several hundred days. The magnitude (brightness) of the star can change in the period of outburst from 7 to 16 magnitudes. In the case of a dwarf novae, the outbursts last from 80 minutes to 14 hours; and they change in brightness by 3-8 magnitudes. A dwarf novae also consists of a binary star system, but with a white dwarf and a main sequence star.

A cataclysmic variable is a small white dwarf star that is devouring a larger companion star. Its variability is frequently caused by the change in brightness of the accretion disc (a doughnut of matter formed as material from the larger star is transferred to the smaller star) surrounding the white dwarf. This conception of the accretion disc is what we believe causes the changes in magnitudes.

Below: a cataclysmic variable star diagram.

NGC 2261 is a fan-shaped reflection nebula immediately north of R Monocerotis. It became known as "Hubble's Variable Nebula" after Edwin Hubble discovered that it varies in both shape and brightness month to month (Hubble 1916). L. M. Close et al. (1997) made high-resolution near-infrared images of R Mon and developed a model for the star and its optically thick envelope. They concluded that R Mon is surrounded by an accretion disk which is tilted 20 degrees toward us. A strong polar wind has cleared a "throat" through the envelope that surrounds the star and accretion disk. Infalling material is entrained into this wind, ejected through the throat and now forms a roughly parabolic dust shell (NCG 2261) north of R Mon. The walls of this nebula reflect and polarize the light from R Mon. Both Close and Lightfoot concluded that dust streamers forming 1-2 AU from the star cast shadows up through the throat and onto the walls.

NGC 2261 - Hubble's Variable Nebula

Supernova 0915+51SN1999by

This supernova was discovered on April 30, 1999. The AAVSO sent out an alert notice with a finder chart. On May 20, NHS students had a TIE observing slot scheduled and were able to image the supernova. The magnitude estimated by North students was 14.0. Worldwide averages for that night put its magnitude at 13.6.

All the information by observers is incorporated into the AAVSO database so future observers have access to the data. The AAVSO also provides an International Database so astronomers who really enjoy variable stars can share observations with astronomers all over the world. AAVSO services include real-time, up-to-date information on unusual stellar activity and collaborative statistical analysis of stellar behavior using long-term data. They also maintain e-mailing lists to distribute information. One way they send information that is quite convenient and helpful to observers are their News Flashes. They are issued when the circumstances warrant, usually 3-4 messages per week. This organization is helping both amateur and professional astronomers all over the world. Their web site is www.aavso.org.

Procedure

To construct an observing list they visited the AAVSO web page, where they checked the Alert Notices and recent News Flashes. An Alert Notice is news of a really important event (usually current) that happened. A News Flash is information about normal variable stars acting up, such as nova outbursts.

The team made a list of the things they could see in their small area of the sky. After that, the list was narrowed down to no more than 10 objects per hour. A map was made using Voyager II, plotting where the target variable stars were located in the sky.

There were three more very important rules followed so the researchers would know what order in which to view their candidate stars. The team organized them in this fashion: 1. Observe West to East, because the west side of the sky sets first. 2. Make the shortest jumps possible, not swinging the telescope from one position to another- it wasted observing time. 3. The most important stars got priority if time was running out.

Modem connections operate the telescope via remote control. The modem is set up through the Planetarium at Grosse Pointe North High School. A voice phone line is kept open so that the students talk with the operator in California. Different computer commands adjust the instruments, and a CCD camera images star fields with variable stars. These variable star images are then saved onto the school computer for further analysis.

The data table contains information for every observed star in the TIE research project in the last school year. It lists the dates and times the stars were imaged, and from that the stars' Julian dates can be calculated. Astronomers and chronologists use Julian Days. It’s a system of numbering days into a sequence of integers, such as -2, -1, 0, 1, 2, 3 ... etc. This system makes it easier to determine the number of days between two dates in history. All the astronomer has to do is subtract the two Julian Dates from each other. Next is the class estimated magnitude, the magnitudes of reference stars used on the finder charts, and the map designation, The designation is an AAVSO number that indexes the map and indicates scale. E scale maps are those used with the largest telescopes- they have a very small field of view and very dim stars.

Filters indicate what was used over the CCD camera when the image was taken. V means visual filter- it is green, which is where vision is most sensitive.

Analysis

The remainder of the data table lists analysis numbers. Range stands for the predicted range of high and low magnitude the star is known to historically exhibit. The fit column indicates whether or not our estimate was within the range. Next, light curve magnitudes were measured from the AAVSO database on the web page. These curves contain all reported observations. Their averages can be used to measure how close the class estimates were to worldwide observations. In some cases for stars with no light curves, News Flash magnitudes were researched. The column NF Time indicates how far a reported magnitude was from the time of the class observation. "-1d" means the NF magnitude was made one day before the class observing time.

When all the estimates were compared with others' estimates, the percent errors were calculated and averaged. Stars indicate errors derived from estimates that were not exactly comparable in time (a 1-3 day difference).

Conclusions and Extensions

There is no real end to this project, or a single conclusion that can be stated. Next year's observing time will be a continuation of some of these projects, and maybe opportunities to observe new sights in the heavens!