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Identifying Long Period Variable (LPV) Stars Using Images from the Star Dial Observatory
Vanessa
Pebble
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Richwine ,
Tim
4
Slater
DOD (Tucson, AZ) 2U. Hawaii 3Riverwatch Middle School (Suwanee, GA) 4U. Wyoming
Magnitude vs. Julian Date
RA 1930 V1414 Aql
Magnitude vs. Julian Date
RA 1930 V0820 Aql
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Figure 2. A section of the night sky known as 1930. 1930
describes the Right Ascension (RA) location of this section of
the sky. The negative image is backwards from what is usually
seen at night (black sky with white stars). This is for the
purpose of blinking two images and trying to identify variable
stars.
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Magnitude vs. Julian Date
RA 1930 BZ Aql
Variable stars need to be systematically observed over decades
in order to: determine the long-time behavior of a star, provide
professional astronomers with data needed to analyze variable
star behavior, to schedule observations of certain stars, to
correlate data from satellite and ground-based observations, and
to make computerized theoretical models of variable stars.
Magnitude vs. Julian Date
RA 1930 V1541 Aql
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Results
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Research on variable stars is important because it provides
information about stellar properties, such as mass, radius,
luminosity, temperature, internal and external structure,
composition, and evolution. This information can then be used
to understand other stars. Professional astronomers have neither
the available time nor the unlimited telescope access needed to
gather data on the nightly brightness changes of thousands of
variable stars. Thus, it is amateur astronomers utilizing various
techniques who are making a real and highly useful
contribution to science by observing variable stars and
submitting their observations to the AAVSO International
Database. The aim of this project is to measure LPV star
magnitudes for several stars and to submit data obtained to the
international variable star database in the hopes of helping
astronomers learn more about objects in the sky.
Jim
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Magnitude
Variable stars are unique stars that periodically change in
brightness over time.The magnitude of a star is a number
assigned to a star’s brightness where the brightest stars are
known as magnitude 1 stars and the dimmest stars that can be
seen by the unaided eye are magnitude 6; therefore, the brighter
the star, the smaller its numerical magnitude. The magnitudes of
variable stars can change from a thousandth of a magnitude to
as much as twenty magnitudes over periods of a fraction of a
second to several years, depending on the type of variable star.
The type of variable stars being studied in this project are long
period variables (LPV) and are called so because their
periodicity is longer than 80 days. There are a number of
reasons why variable stars change in magnitude. Long period
variable (LPV) stars are pulsating stars and they swell and
shrink due to internal forces and their brightness changes with
their change in size. Over 30,000 variable stars are known and
catalogued, and many thousands more are suspected to be
variable.
Magnitude
Introduction
1STARBASE
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Figure 1. Photometric light curves for variable stars V0820 Aql, V1414 Aql, BZ Aql and V1541 Aql in RA 1930.
Methods
Observations of variable stars have been facilitated by the use
of an automated telescope, called Star Dial that takes nightly
images of the sky. The subsequent CCD images are then
analyzed with accompanying computer software to obtain
precise and accurate magnitude measurements over time. The
first step is to collect these images from Star Dial. Next, two
images of the sky from two different nights need to be aligned
and compared to determine if any stars are changing in
brightness. This method is known as blinking and is done using
a program called Astrometrica. Using this method, it is possible
to see changes in magnitude of several stars. After these stars
have been characterized by their Right Ascension and
Declination, an Internet database is used to further research
these stars for possible variable characteristics.
After several stars have been identified as possible variables
using the information gathered from these databases, it is
necessary to measure the changing brightness of a star for as
many nights as possible. This methodology is known as
aperture photometry. During this time, comparison stars, whose
brightness does not vary, are also identified. Comparison stars
are stars of known magnitude that are used to compare the
magnitude of the variable stars and calibrate the images. We
then use this comparison to determine the relationship between
brightness and time by creating a photometric light curve.
Finally, it is essential to contribute the light curve results to the
larger international database of variable star observations at the
American Association of Variable Star Observers.
This work was funded in part by the UA LAPLACE Astrobiology Center.
All four variable stars were analyzed using the aforementioned
methodology and the photometric light curves seen above were
created. The important piece of data that can be extracted from
the photometric light curve is the periodicity of the variable
star. The periodicity of any variable star describes the time it
takes to change from least magnitude to greatest magnitude and
back to least magnitude again. Using the data from the sky
region along the ecliptic of Right Ascension 19hrs 30 min, the
measured periodicity of all four variable stars are listed in the
table below. These numbers were determined by estimating the
number of days between several different peaks and taking the
average. The estimated periodicity values compare favorably to
the published periodicity values as shown in the table. The
published values were found in both the ASAS Catalog of
Variable Stars, Version 2005-09-05 and the Combined General
Catalog of Variable Stars Vol. I-III. The percent error between
the published values and the estimated values can also been
seen below. All error values are judged to be in the acceptable
range indicating that the Star Dial Observatory data is reliable
for determining the pulsation period for newly identified LPV
stars.
Variable Star
Estimated
Periodicity
Published
Periodicity
Percent Error
V0828 Aql
370
330
12.12
V1414 Aql
354
None found
N/A
BZ Aql
360
320
12.50
V1541 Aql
350
303
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