serving the tri-cities since 1975 · bay city, mi 48706-1243 subscription information subscription...
TRANSCRIPT
Volume 9, Issue 1
The Sunset Gaze t te Serving the Tri -Cit ies s ince 1975
Sunset Astronomical Society
Meeting information
Meetings are generally in the theater in the Delta College Planetarium in Bay City. The meetings will usually be on the 2nd Friday of each month at 7:00 PM. Watch the newslet-ter for changes in dates and times. Member-ship is not required to participate in meet-ings and activities. See last Page for this
month’s meeting site.
Membership Information
Our club has switched to e-mailing our newsletters. For those wishing to receive a hard copy mailed an additional dues of
$10.00 per year is required.
Student / Senior: (17 years & younger,
65+ years)
1 year - $15 (mailed Newsletter add $10)
2 year - $20 (mailed Newsletter add $10)
Regular: (18+ years)
1 year - $20 (mailed Newsletter add $10)
2 year - $30 (mailed Newsletter add $10)
Family:
1 year - $25 (mailed Newsletter add $10)
2 year - $40 (mailed Newsletter inclusive)
Membership includes voting privileges, the newsletter and free admission into Delta
College Planetarium shows.
Treasurer’s address for renewals and
subscriptions:
Tom Smith, 3423 Hidden Road,
Bay City, MI 48706-1243
Subscription Information
Subscription prices for “Sky and Telescope” Magazine or “Astronomy” Magazine are available at club rate with the purchase of individual or family membership. For prices please refer to the treasurer or the club’s
website:
http://www.sunsetastronomicalsociety.com/
SASMembership.htm
September, 2011
Measuring Astronomical Distances Over The Centuries
This new series is about how astronomers determined and measured distances to astronomi-
cal objects over the course of more than two thousand years. The series will span from the
beginnings in Greek antique to the latest development using earth and space based tele-
scopes.
In the last issue we started to talk about standard candles and how they are used: remember,
when an object has a known luminosity which is then compared to its observed brightness the
distance of the object can be determined using the inverse square law. These standard candles
are also known as physical distance indicators and in this issue and the next one we will talk
more about them (see below). You already know one very important physical distance indica-
tor, the Cepheids, which we covered in the last issue but there are many more. Over time
more and more different distance indicators have been developed by astronomers especially
with the availability of evermore powerful telescopes (Hubble telescope, 8 - 10 m class tele-
scopes). And it does not stop there: The prospect of telescopes in the 25 - 42 m class before
the end of this decade will truly push the detection limits further and further.
Physical distance indicators
The Dynamical Parallax method is computed from the estimated distance to a visual binary
star. Hereby the astronomer observes the angular semi-major axis of the orbit of the stars
around each other, together with their apparent brightness. Then, by using a combination of
Newton's and Kepler's third law, together with the mass-luminosity relation, it is possible to
determine the distance to the binary star.
Eclipsing Binaries is a direct method which has become feasible due to the arrival of the 8—
10 m class telescopes which makes them useful distance indicators up for distances which in-
clude the small and large Magellanic clouds, the Andromeda and Triangulum galaxies. With
the current telescopes the method is useful of up to 3 mega parsec with an improved 5% level
of accuracy.
RR Lyrae variables are periodic variable stars and it is thought that the mechanism of their
pulsation is similar to the Cepheids albeit the nature and history of these stars is rather differ-
ent. RR Lyraes are old, relatively low mass and metal-poor "Population II" stars commonly
found in globular clusters, which is quite in contrast to the Cepheids. Compared to Cepheids
their pulsation period is shorter, typically less than one day and sometimes ranging down to
seven hours. They are also much more common than Cepheids, but much less luminous with
an average absolute magnitude of 0.75. That is only 40 or 50 times brighter than our Sun. RR
Lyraes have evolved from stars with similar or slightly less mass than the Sun. RR Lyraes are
good standard candles for distances in our Milky Way but were difficult to observe in external
galaxies because of their intrinsic faintness. With the Hubble Space Telescope it was possible
to detect Lyraes in Andromeda's galactic halo and, more recently, in its globular clusters mak-
ing them now a good standard candle for nearby galaxies and globular clusters.
PAGE 2 THE SUNSET GAZETTE VOLUME 9, I SSUE 1
The next four methods all use stars in the old stellar
populations = so called Population II stars:
The Tip of the Red Giant Branch (TRGB) uses lumi-
nosity of the brightest red giant branch stars in a gal-
axy to estimate the distance to that galaxy. So what
exactly is the TRGB? When astronomers plot the stellar
luminosity versus the surface temperature for a popu-
lation of stars this is called a Hertzsprung-Russell or HR
diagram. During most of its lifetime a sun-like star will
burn hydrogen in its core and it will appear on the HR
diagram at a position along a diagonal band called the
main sequence. Once the star gas exhausted its hydro-
gen in the core, the star will still produce energy via
hydrogen fusion but from a shell around the core. The
center of the star will increasable accumulate the he-
lium from this fusion process. During this phase the
star will migrate along an evolutionary branch of the
HR diagram that leads toward the upper right. That
means that the stars surface temperature will decrease
while increasing its overall luminosity. Then at a certain
point the accumulated helium at the core of the star
will reach a pressure and temperature where it begins
to fuse helium to generate beryllium which will itself
fuse with another helium to form carbon - also called
the triple-alpha process. In our Sun and all stars with a
mass less than 1.8 Sun masses this change will cause
an event called the helium flash — definitely some-
thing where you do not want stick around in the solar
system. The Star will then settle into a new thermal
equilibrium and moves toward the left in the HR dia-
gram as the surface temperature starts to increase.
This move in the HR diagram results in a sharp break in
the evolutionary track of the star. This break or discon-
tinuity is called the Tip of the Red Giant Branch.
The Planetary Nebula Luminosity Function (PNLF) can be used to estimate the distance to a galaxy. The procedure for this
method is to first locate point sources within the galaxy that are visible at the OIII line (λ = 5007 Angstrom). This OIII line is the very
same emission line which your OIII filter lets through and enables you to see planetary nebula even under light polluted skies. These
points sources are candidates for planetary nebula, however, it is of course not quite so easy because there are three other types of
objects that would also exhibit such an emission line: these are HII regions, supernova remnants, and Lyα galaxies. After these have
been filtered out and the true planetary nebula have been determined, the fluxes of the OIII line have to be measured to estimate
the distance. This flux is then compared to the planetary nebula luminosity function (PNLF). The idea behind it is that all planetary
nebulae might have similar maximum intrinsic brightness, now calculated to be M = -4.53. From that the distance can be computed.
The Globular cluster luminosity function (GCLF) compares the luminosity of globular clusters (normally located in the halo of a
galaxy) from distant galaxies to that of the Virgo galaxy cluster. This assumes that all globular clusters have roughly the same lumi-
nosities and that the globular clusters in the Virgo galaxies have all the same distance and brightness. The method therefore carries a
considerable amount of uncertainty of ca 20 %.
Surface brightness fluctuation (SBF) is a method which uses the fact that some pixels of a CCD camera will pick up more stars of a
galaxy than others. When the distance of the observed galaxy increases the picture will get more and more smoother. An analysis
can describe the magnitude of the pixel-to-pixel variation which is directly related to a galaxy’s distance.
In the next issue we will hear more about standard candles, especially about Typ Ia supernova which played and still play
such an important role in determining distances on a cosmic scale and which led to the discovery of Dark Energy. Stay
tuned!
Stars tend to fall only into certain regions of the diagram. The most predominant is
the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler
and less bright), called the main sequence. In the lower-left is where white dwarfs
are found, and above the main sequence are the subgiants, giants and supergiants.
The Sun is found on the main sequence at luminosity 1 (absolute magnitude 4.8)
and B-V color index 0.66 (temperature 5780K, spectral type G2). Source Wikipedia.
PAGE 3 THE SUNSET GAZETTE VOLUME 9 , I SSUE 1
Pictures of the Astronomy Excur-sion by AU and SAS club members to the Detroit Observatory in Ann
Arbor on the 15th August.
All pictures courtesy of Ed Sederlund
Below: The Pister & Martin Meridian Circle
manufactured in Berlin 1854.
Right below: The 12” refractor manufac-tured by Henry N. Fitz in the 23’ dome mounted on a gorgeous equatorial mount
Above: The observatory building housing the 23’ dome on top and completed in
1854.
Left: AU and SAS club members on the
front steps of the observatory.
Martin Grasmann Secretary - SAS 6108 Summerset Drive Midland, MI 48640
SUNSET ASTRONOMICAL SOCIETY THE SUNSET GAZETTE SERVING THE TRI- CITIES SINCE 1975
UPCOMING EVENTS
1. Welcome, new members
2. Evening’s theme:
“TBD”
3. Club Stuff
Sep 3 Evening: 2.3 mag Scorpii very close
to the Moon. Binoculars or telescope
needed. For observers in the southern
and eastern parts of the USA the Moon
will cover the star.
Sep 4: 1st quarter Moon
Sep 9 Dawn: Faint Regulaus less than 1
deg away from Mercury in the east 30-45
min before sunrise. Binoculars!
Sep 12: Full Moon.
Sep19-22 Night: Jupiter can be found to
the right of the Moon.
Sep 20: Last quarter Moon
Sep 23 Dawn: Mars is on the upper left
of the waning crescent Moon.
Autumn begins in the Northern Hemi-
sphere.
Sep 29 Night: Good time to look for
Uranus who is at opposition this night.
Finder chart, binoculars or telescope
advisable.
Sep 25 - Oct 10 Pre-Dawn: Watch out
for zodiacal light ca 80 to 120 min before
sunrise. Dark locations needed.
What’s up in the Sky SAS Meeting
Start: 7:00 PM
Friday, Sep 9th, 2011
Delta Planetarium
September 22 - 25: Great Lake
Star Gaze 9 at River Valley RV Park.
Early registration up and till 2nd Sep-
tember!
http://www.greatlakesstargaze.com/r
egister.php
This issue can be accessed in color on the website of the SAS!!!
http://sunsetastronomicalsociety.com
Elected Officers for the SAS:
President, Tim Ross [email protected]
1. Vice President, Debra VanTol [email protected]
2. Vice President, Mohammed Khan [email protected]
Treasurer, Thomas Smith [email protected]
Secretary, Newsletter Editor, Martin Grasmann
Sep 25-26 Dawn: Extremely thin cres-
cent Moon visible a few deg over the
eastern horizon ca half hour before sun-
rise. Binoculars!
Sep 27: New Moon.
Sep 28 Dusk: Look for Saturn less than
2 deg above Venus very low in the west
15 min after sunset. A very thib crescent
Moon is settling 12 deg to their left.
Telescope!