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Variable Star of the Month
March 2000: Z Ursa Majoris
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He who would scan the figured skies
Their brightest gems to tell
Must first direct his mind's eye north
And learn the Bear's stars
well.
Z UMa is located in the
bowl of the big dipper
near the star delta UMa.
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Located in the constellation Ursa Major and within the bowl of the big
dipper, lies the semiregular variable Z Ursae Majoris. Z UMa captivates
the attention of many because it is easy to find in a constellation that
is friendly to northern sky observers. Its semiregular periodicity is
exciting to follow and results from several different pulsation periods
within the star! This variable is perfect for beginning observers to
locate and monitor, and it is bright enough to be followed in small
telescopes or binoculars.
Dr. E.S. King, of the Harvard College
Observatory, discovered the variable Z Ursae Majoris in 1904 due in
large part to its peculiar spectrum. Z UMa's spectrum reveals delta and
gamma hydrogen lines in emission and the presence of peculiar molecular
absorption bands such as TiO (titanium oxide). Once inspired, Dr. King
had the star looked up on photographs in the Harvard collection, and on 22
Draper Memorial plates taken between the years 1897 and 1904. This led him
to discover that Z UMa was a variable star with a magnitude range of at
least 1.5 magnitudes.
The AAVSO historical light curve of Z UMa shows that the
mean magnitude range truly varies over time. Currently the mean range
appears to vary from 7.2 to 8.9 magnitudes. However, in the General
Catalogue of Variable Stars (GCVS) Z UMa is recorded to vary in
brightness from its faintest minimum magnitude of 9.4 to its brightest maximum magnitude of 6.2.
The GCVS classifies Z UMa as a semiregular variable (SR) of subtype B. SRB
stars are late type giant stars with an M, C, or S spectrum and poorly
defined periodicity or alternating intervals of slow irregular changes. In
the case of Z UMa, the spectrum is M5IIIE (a cool red giant spectrum with
molecular bands and emission lines), and the mean period is 195.5 days.
This is the mean period of the star, but the light curve indicates that
there is more than just one pulsation period for Z UMa.
And The Beat Goes On
...
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Click above for the AAVSO historical
light curve of Z UMa |
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Observational evidence suggests that Z UMa has more than just one
pulsation cycle, in fact it is thought that the star pulsates in high
modes and that beat and resonance phenomena occur within the star. Michael
K. Suchko (1980), a former AAVSO summer assistant who analysed AAVSO
observations, suggests that Z UMa has two periods; one primary period of
195.5 days (the current mean period listed in the GCVS) and a second
poorly defined period of about 205 days. He suggests that these two
periods are beating with each other to produce another beat with a period
of 4200 days. Isles (1998) also indicates that the irregularities seen in
the light curve of Z UMa are the result of the superposition of several
different pulsation cycles within the star.
These multi-periodicities can be seen in the AAVSO historical light
curve which displays data from 1934 to 2000.
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| Both images indicate the
variable Z UMa by a red arrow and a 5.9 magnitude star
by a yellow arrow. The bright star in the upper left corner of the images
is delta UMa and helps locate the variable Z UMa. The image
on the left shows Z UMa at minimum on April 5, 1995 JD 2449812.65, and is
indicated by the first red line on the light curve below.
The image on the right shows Z UMa at maximum on April 28, 1995 JD
2449835.73 and is indicated by the second red line on the
curve below. | |
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The light curve of Z UMa from March 1994 to December 1998. Each point is a
one-day mean of amateurs' visual observations from the AAVSO International
Database. Tick marks occur approximately every 195.5 days to show the
mean period. The first red line marks the point in time that the above
left image was taken, the second red line indicates the time of the above
right image. | |
The Research teams of L.L. Kiss (1999) and Percy and Polano (1998)
suggest that multiple periodicities of Z UMa result from simultaneous
pulsations of excited modes within the star. They indicate that Z UMa may
be pulsating in the fundamental mode, the first, and even higher
overtones of pulsation at the same time. Another researcher,
Barnbaum (1995), proposes that mechanisms other than pulsation may drive
the irregular light curve: mechanisms such as the presence of a stellar
companion, distorted stellar shapes, rotation, or star spots.
Can You Find Z
Uma?
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| This photograph shows a close-up yellow
outline of the big dipper with Z UMa located nearly 3 degrees
west-northwest of the star delta Ursae Majoris.
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Many veteran variable star observers began their careers by observing Z
UMa, so we invite you to give it a try also. Z UMa glows pale orange in
the northern circumpolar constellation, The Big Dipper or The Plow, and is
especially easy to find this month. Unlike many other semiregular
variables whose magnitude range is usually very small thus not very
suitable for visual observing, this red giant varies its light output by
close to two magnitudes.
To find Z UMa, look in the northern sky and locate the northern
circumpolar constellation Ursa Major, or "The Great Bear". The variable
star, Z UMa, sits in the bowl of the The Big Dipper, nearly 3 degrees
west-northwest of the star "delta", which connects the handle to the bowl
of the dipper. The AAVSO "A" chart is helpful in locating delta Ursae
Major, and then Z UMa. Once you have found Z UMa, the "B" chart assists
in getting a closer view of the variable and the surrounding comparison
stars.
| Click on either the AAVSO "A"
chart or "B" chart to assist in locating the variable Z UMa. Other
necessary star fields are available on the Star Charts Page. |
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Observing Tip:
When observing red variable stars, such as Z Uma, make
your magnitude estimate with a quick glance rather than with a prolonged
stare. Red stars tend to excite the retina more than blue stars when they
are viewed for a long time, making the red stars appear brighter than they
are (Purkinje effect). |
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With Z UMa in the field, make an estimate of its brightness by
comparing the variable to comparison stars in the surrounding area. Once
you have made your observation, you may submit it to the AAVSO
International Database to be incorporated into a permanent long-term light
curve for Z UMa. For more information on this process, visit our Submitting Observations page. A detailed
description of the magnitude estimating process can be found in section
8.1 "How to estimate a variable: Z Ursae Majoris and the AAVSO method", of
the book entitled Observing Variable Stars: A Guide for the
Beginner by David Levy, January 1998. Also, look for more information
on variable star observing in the revised, AAVSO Manual
for Visual Observing of Variable Stars, which will be published soon.
Semiregular
Variables
Z Ursae Majoris is a semiregular variable. Semiregular variables are
post main sequence cool (red) giant and supergiant stars. They are
classified as long period variables (LPVs), commonly called red variables,
and occupy a distinct zone in the far-right region of the
Hertzprung-Russel diagram. The long period variables are classified into
three categories: Mira (M), semiregular (SR), and slow irregular (L)
variables. The Mira variables are
the most well-defined and homogeneous subgroup. They have periods longer
than 60 days, amplitudes in the visual larger than 2.5 magnitudes, and
show rather periodic variability. The SR variables are a subgroup that
have smaller than 2.5 magnitude amplitudes and show significant
inhomogeneity in their division into SRA which have periods longer than 35
days, with periodic variability, and SRB (like Z UMa) which have
periods longer than 20 days, and less periodicity in their light
variation. Further divisions include the SRC types, which are more
luminous and the the SRD types, which are warmer variables. The slow
irregular variables (L) are a subgroup that shows light variations with no
evidence of periodicity, or any periodicity that is present is poorly
defined or appears only occasionally.
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| Mira and semiregular variables are
pulsating low and intermediate mass "red variables" that
occupy a distinct zone in the far right region on the HR diagram. They are
commonly found on the
asymptotic giant branch (AGB) where they
display characteristic properties of the red giant phase of
evolution Figure from Hands-On
Astrophysics, AAVSO 1998. |
The long period variables of low and intermediate mass red giants (like
Z UMa) located in the area of the HR diagram called the Asymptotic Giant
Branch (AGB) are critical to our understanding of stellar evolution
because they represent the final stages of stellar life. Observations of
these stars reveal significant pulsational behaviour that advances our
knowledge of the physical processes taking place within these evolved
stars. It is observed that, through their pulsations, these stars lose
significant amounts of mass and thus are primary sources for the
enrichment of the interstellar material. Continued observations of these
stars are important in understanding their long term behavior, and
correlating theories of stellar pulsation and stellar evolution.
For More
Information
- The Myths of Ursa Major
- Barnbaum, Cecilia; Morris, Mark; Kahane, Claudine; Astrophysical
Journal v.450, 1995, p.862.
- Isles, John E. "Big Dipper Variables", Sky and Telescope, June
1998, pp 98-100.
- Kiss, L.L., K. Szatmary, R.R. Cadmus Jr., J.A.
Mattei, "Multiperiodicity in semiregular variables", Astronomy and
Astrophysics, v.346, 1999, pp 542-555.
- Levy, David H. Observing Variable Stars: A Guide for the
beginner, Cambridge University Press, January 1998.
- Mattei, J.A., Foster,G., et al. "Classification of Red Variables",
Proceedings of the ESA Symposium Hipparcos-Venice '97, 13-16 May,
Venice, Italy, July 1997, pp 269-274.
- Percy, John R., Polano, S. "Pulsation Modes in M Giants", ASP
Conference Series, Vol. 135, 1998, pp 249-253.
- Suchko, Michael K. "The Periodicities of Z Ursae Majoris", The
Journal
of the AAVSO, vol.9, No.2, 1980, pp 74-80.
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