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 tjrastronomy@hotmail.com

1. Vice President, Debra VanTol debraj106@aol.com

2. Vice President, Mohammed Khan khan001@charter.net

Treasurer, Thomas Smith tom55net@att.net

Secretary, Newsletter Editor, Martin Grasmann

martin.grasmann@sbcglobal.net

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!