Download M = 5.5 - The Millstone

Spring 2012 Astronomy Course
Mississippi Valley Night Sky Conservation
The Sky Around Us
Program developed by
Mississippi Valley
Conservation Authority
Royal Astronomical Society
of Canada
Ottawa Astronomy Friends
Instructors:
Pat Browne
Stephen Collie
Rick Scholes
Course Assistant
Amy Booth
Earth Centered Universe
software for illustrations –
courtesy David Lane
III Star Clusters in and around our Galaxy
WHERE
Locating Star Clusters
Where are they within or around the galactic plane
Which clusters live where?
WHEN
When did they form? (Stellar Evolution)
Are they visible?
M3
WHAT
Types of star clusters
Clobular Clusters
Open Clusters
WHO
Supernova SN1987a discoverer
Ian Shelton – Cdn astronomer
Pioneers in star cluster analysis
Helen Sawyer Hogg
(Canadian Astronomer)
Henrietta Leavitt
M44 BeeHive
F
Stellar Properties
Lecture 2 presented some of the physical properties
that can be gleaned from visual observing of
individual or binary star systems– notably colour
and magnitude. Using starlight spectrum analysis,
stars can be classified according to their peak
wavelength intensity (colour temperatures) and the
absorption lines superposed over the continuum of
the spectra.
The classification of stellar spectra fits into roughly
groups OBAFGKM. We can classify specific stars
according to their Spectral Class and therefore their
Effective Temperatures.
On the left, the scale of Absolute Magnitude reflects
the true luminousity of the star. In order to determine
Absolute Magnitude, we must have a measure of the
stellar distance (by other means).
Absolute magnitude, M, expresses the brightness
of a star as it would be if it were placed 10 parsecs
away. Since all stars would be placed at the same
distance, absolute magnitudes show differences in
actual luminosities. It is a measure based on stellar
analysis and distance determinations.(The sun is
absolute Magnitude 4.3 roughly. Sirius is 1.4 (much
brighter!)) Luminosities are measured with respect to
solar luminosity.For Main Sequence Stars within our
galaxy Note: Apparent magnitude is what we use in
our observations, a visual scale that ranges from
roughly -2 to 6. The scale is also logarithmic – so
that a 2nd magnitude star is 2.5 x brighter than a
3rd magnitude star. A difference of 5 magnitudes is
100. (2.5 ^ 10). You can show apparent (visual)
magnitudes in ECU.
A
B
O
-5
0
+5
+10
K
M5
Stellar Evolution – Red Giants, White Dwarfs and Supernova Remnants
Evolutionary pathways are shown here for stars 1, 5,10Solar
Masses. As a newly formed star stabilizes, it drops down on
the H-R diagram and takes up a place on the main
sequence. Just where it settles depends on its initial mass.
On the main sequence, a star fuses hydrogen to helium in
its core. A star spends most of its lifetime on the main
sequence.
When Stars move off the Main Sequence, they become Red
Giants, White Dwarfs or Supernova Remnants.
Solar Mass Stars:
Once the core has exhausted its supply of hydrogen, it
contracts and heats up. The star brightens and its outer
layers expand, and it moves up and off the main sequence
to become a giant. Larger radius, cooler temperature.
High Mass stars: complex nuclear fusion transformations
that can lead to core collapse when Iron core requires
energy rather than releases it in nuclear fusion
We can explore the evolutionary tracks
http://rainman.astro.illinois.edu/ddr/stellar/intermediate.html
http://outreach.atnf.csiro.au/education/senior/astrophysics/stellare
volution_postmain.html#postmainevotrack
Evolutionary Path – Solar Mass Stars off the Main Sequence
Case 1 Stars = 1 Solar Mass -> Red Giant -> White dwarf
Stars such as our Sun move off the main sequence and enter the
red giant branch (RGB), when the core hydrogen is exhausted.
With no thermonuclear fusion in the core, the star contracts . An
outer shell of hydrogen continues to burn and the radius
expands, but the temperature decreases – Red giant – lower
temperature, higher luminosity.
Horizontal Branch
Hydrogen fusion in the shell produces more helium. This gets
dumped onto the core, adding to its mass, causing it to heat up
even more. When the core temperature reaches 350 million K,
the helium nuclei now have sufficient kinetic energy to overcome
the strong coulombic repulsion and fuse together, forming
carbon-12 in a two-stage process
White Dwarf Evolution  Planetary
Nebula
In an AGB star, if the helium fuel in the Heburning shell runs low, the outward radiation
pressure drops off.
As this was previously holding out the shell
of hydrogen gas this shell now contracts,
heats up and ignites, converting hydrogen
to helium. This helium "ash" in turn falls onto
the helium shell, heating it up till it is hot
enough to re-ignite in a helium-shell flash,
producing a thermal pulse.
Increased radiation pressure now causes
the hydrogen shell to expand and cool,
shutting down H-shell burning.
Once shell temperature is sufficient, helium
shell burning starts and the star moves up
into the asymptotic giant branch (AGB).
This is accompanied by a core of
degenerate matter where a higher
temperature does not correspond to an
increase in pressure. So the core is tiny and
remains so.
Mass Loss: Over time the outer layers of
the AGB star are almost totally ejected and
may initially appear as a circumstellar shell.
With the ejection of the outer layers of the
star, its hot, dense core is left exposed. It is
initially so hot that the intense ultraviolet
radiation it emits ionises the expanding,
ejected shell. This results in the cloud
glowing, similar to an emission nebula. Such
objects are called planetary nebulae after
their initial description by Herschel in the
Instant Expert
As the balance of the reaction shifts, the
star executes a series of ‘blue loops’ that
take it zig-zagging up the diagram.
> 5 Solar Masses are believed to
produce iron-rich cores that eventually
collapse, triggering a supernova
explosion. This is because the fusion of
elements < Fe (Iron) give off energy
whereas it takes energy to fuse iron.
Hence at this point the gravitational
contraction overcomes the radiation
energy of fusion and the star oscillates
until explosion in a supernova event
Summary – Post Main
Sequence Stellar
Evolution for Sun-like
Stars
Courtesy Zelik and Smith
Introductory Astronomy
and Astrophysics
For AGB enthusiasts here is an
excellent reference:
https://www.eeducation.psu.edu/astro801/con
tent/l6_p3.html
Supernova
Massive Stars > 10 Solar Masses
SN1987A discoverer
Dr. Ian Shelton, U of T
.
Massive stars evolve and produce iron-rich cores that
eventually collapse, triggering a supernova explosion This
is because the fusion of elements < Fe (Iron) give off
energy whereas it takes energy to fuse iron.
Hence at this point the gravitational contraction overcomes
the diminishing energy of fusion and the star oscillates until
explosion in a supernova event.
As the balance of the reaction shifts, the star executes a
series of ‘blue loops’ that take it zig-zagging up the
diagram.
Nucleo-synthesis of elements above helium is less efficient so
that each successive reaction produces less energy per unit
mass of fuel. Statistically they are very low in numbers as they
are less likely to form than lower-mass stars and their lifetimes
are so short anyway.
Stellar Populations
Population 1
disk stars
Population II
halo stars
Main Sequence
Evolved off main sequence
metal rich (~ sun)
metal poor
open clusters
globular clusters
Typically we speak of 2 extreme populations: the young “metal”
rich Population 1 and the old metal poor Population II. We
examine their properties by plotting them on the Main Sequence.
We analyze their spectral types. We observe …
The earliest spectral types are in the region of F2.. in some –
as late as G5.
Their spectra are deficient in metal lines, showing that they
are sub-dwarfs
(luminosity class VI) formed before the recycling of stellar
material in such processes as supernova explosion had
properly begun.
Where : Observing Open Clusters in our Galaxy and around our Galaxy
Open clusters are groupings of 20-50 star sin
a region 10-60 light years across. Most OCs
are found close to the plane of the galaxy. It
is possible to find the age of the cluster by
identifying the spectral type of the earliest
Main Sequence member.
Example: Beehive Cluster M44 – young 730
million years, close, 577 light years, sparse <
1000 members
Observations:
Globular clusters are generally metal-poor
Open clusters are generally more metal-rich
There is some correlation between age and metallicity
in the Galaxy:
Older things tend to be more metal-poor, but this is
not a rule.Clusters with Z > .001 are metal poor .
Globular clusters VERY dense… 50,000 to 1M stars in a region <
150 light years diameter. They appear to be orbiting the galactic
center in a spherical halo at a typical distance of 60000 lys.
Example: M3 – further away than the center of our Milky Way
34,000 ly, Absolute Mag = -8  luminousity of 300,000 suns,
8Billion years old
fraction by mass
solar value
hydrogen
content
X
0.70
helium content
Y
0.28
everything else
(C, O, Mg, Si,
Fe, etc:
"metals")
Z
0.02
http://burro.astr.cwru.edu/Academics/Astr222/Galaxy/Structur
Stellar and Cluster Distances – How do we know the distance?
Because stellar colours and spectral types are
roughly correlated , and for Main Sequence stars,
we know the Absolute Magnitudes of nearby stars
with a degree of precision, we can compute the
distances to unknown stars or star clusters using the
relationship between apparent visual magnitude m
and Absolute Magnitude M.
From a stars spectrum (on the main sequence), we
determine its spectral type. This fixes a position on
the H-R diagram, from which we can read off its
Absolute Magnitude M. From the observed visual
magnitude m we compute a distance modulus:
m – M = f(distance in pc) based on
m/M ~2.5 Log (d /(10^2))
Redder
We can use a fitting technique for clusters of stars
shifting the test cluster up and down along the
calibrated sequence .
3000
Here the best fit m – M = 5.5
m – M = 5 log d – 5
Example: 5.5 + 5 = log d
d = antilog (10.5/5) = 10 ^2.1 = 126 pc.
Cluster distances well into the region of globular
clusters were made possible by the calibration of
variable stars called Cepheid Variables.
Courtesy Introductory Astronomy & Astrophyisics p.
207, 241
M
m
Open Clusters looking West (setting)
( Spring time Northern Hemisphere)
WHAT:
Open clusters:
Widely-spaced groupings
of easily resolvable stars
Also called Galactic Clusters
because they lie in the
galactic disk
Looking west
Auriga: Clusters,M38,M36,
M37 (West)
Monoceros: M46, M47
Looking just right of
theMeridian high
up…
*Cancer : M44 Beehive,
M67
M44 – ‘Beehive’
Modest neighbour
M67
Auriga
cluster
M38,
M36,M37
Meridian
Monoceros
Cluster M46,
M47
Winter (west) Milky Way
From a true, dark sky,
nothing can compare to a
naked eye view of the Milky
Way. During the winter
months in the Northern
Hemisphere, we face away
from the furiously busy core
of our home galaxy and look
outward, through its more
tenuous periphery.
Despite being more delicate,
this slice of the Milky Way is
still rich with structure.
http://www.perezmedia.net/b
eltofvenus/archives/0
01397.html
Open Clusters and Nebulous
Regions in Constellation Auriga
Auriga
Auriga contains an nteresting variety:
many open clusters and nebulous
regions simply because the
Milky Way runs through it.
3 Open clusters in/out of
pentagon of Constellation
Auriga south of Capella.
M37 the richest cluster containing over
500 stars spread across 20
arcminutes and is the brightest of
the three with an apparent magnitude +5.6.
M36 - 60 stars with an angular width of 12
arcminutes.M38 100stars and is the
dimmest of the three at magnitude+6.4.
All three of these clusters, 4000 light-years
away, can be seen with a small telescope.
Courtesy - Dave Garner teaches astronomy at
Conestoga
Observing Log Book
Suggesteed Recording Format
(Do what’s comfortable for you)
Header:
Observation Number
Observation Date and Time
Observing Instrument
Telescope/EyePiece Combination
Observing Conditions – Temperature, Wind, moon
phase
References – Books, Sky Charts,etc
Body:
Guests or observing companions
Each object – Designations commonly include those
found in in the RASC Observers Handbook : Messier
NGC
David Levy Gems
Methodology for Finding the Object
Impressions of the object
This log book won the RASC Ottawa Center Observer of the Year
Award 2004 . Lack of neatness is forgiven in favour of
persistence in recording (even after a long night).
Introduction to Star Cluster Observing
What’s up ? Is the Moon up?
Where’s our meridian?
What can we see when the
Moon is up…
For clusters of stars, or special
nebulous stellar bodies, or
galaxies, the moon , like light
pollution obscures the photons
emitted from these objects.
Where’s our meridian?
Galaxies galore coming up close to
our local meridian…
Open Clusters setting in the West…
Globular Clusters in the East
When the Moon is UP!
First Quarter Moon in the
West – Waxing Crescent
– sets after midnight!
This makes it difficult to
see Deep Sky Objects
because they are awash
in moonlight. However,
we can now turn our
attention to the Moon at
First Quarter… one of
the best times to make
observations as Stephen
Collie will explain…
Now go do the Lunar Observing Exercise!
Conserving NightSky Environment -> Solutions
Milky way only visible wth moderately dark skies
Faint objects like clusters of stars, and even galaxies
can be naked eye objects with very dark skies not even
visible in a telescope from moderately dark skies.
When we are in the phase of the moon from First
Quarter to Full moon, we can see how much light (even
natural light) can obscure the fainter celestial objects
Good Neighbour Lighting
Shielded lighting directs light towards buildings and
ground, on the surface not in the sky. No glare. Like
using a lampshade outdoors as well as indoors.
Light goes where it is needed reducing electricity by
30% for the same results
Flat glass fixtures also good because the bulb is
recessed
Mississippi Mills By-law for Outdoor Illumination
Light pollution abatement Conservation of the night sky
This is a canadian video describing what to do to
stop light ing up the night. You can download
this:http://millstone.typepad.com/files/dark
-sky-campus.mp4
GOOD NEIGHBOUR LIGHTING = SHIELDED LIGHTS
Simple solution – no up-lighting, sky is protected, ground surface
visible
Shielded lights – like
lampshades – no
bulb exposed
Flat Glass Cobra
Street light
Observing Brightness and Size of objects
:
Given a dark location reasonably free of unshielded
lighting (referred to as "light pollution"), this
scale describes what is shown when you query ECU
about visual magnitudes:
http://www.mpas.asn.au/MembersInfo/viewing/smohr/ApparentMag/ApparentMag.htm
Magnitudes on a Sky Chart
and in the sky…
So that when we see Mars is at
magnitude -0.2 with an
angular width of 10.7”
we know, it’s bright, and
can be seen in binoculars ,
but better yet in a telescope.
Image courtesy Rolf Meier - RASC
Observing naked eye
and with optical aids…
Constellation Cancer –
easiest to see the
large
cluster rather than the
stars that make up the
constellation)
Compare the size and
magnitude of the
Beehive cluster vs. the
other Open Cluster in
(mag 3.1):
M67 (much smaller,
fainter (mag 6.9), and
one of the oldest star
clusters known…
!
Beehive Cluster – Praesepe –
size 95’ (> deg) Magnitude 3.1
M67: size 29 ‘ (1/3 deg)
Magnitude 6.9
Observing Tips
Binoculars and Telescopes:
Know your field of view in the binoculars corresponding to the
arc-distance in the sky
Know the size (arc-units) and brightness (visual magnitude) of
the object you are looking for
Telescopes:
Use a finder or low power eyepiece to find the object relative to
its surroundings
Find your field of view in your telescope by
1.
Comparing your view through binoculars or
telescope with a chart like this.
2.
Eyepieces have a certain magnification:
EP magnification = Focal Length
Scope/
Example : 6inch F8 =F.L = 48” or 1200 mm
EP PWR for 20 mm = 1200/20 = 60 x
Focal Length EP
Distances in the sky are Arc measures
1-2°
• Use your hand as a scale
• Finger: between 1 and 2
degrees
– Fist: about 10degrees
– Spread fingers: about
20degrees
– Works for any hand since the
bigger the hand, the longer the
arm, and the angles are about
the same
The Moon around ½ deg
The Pleiades 2-3 deg
Find your field of view in your telescope by
1.
Comparing your view through binoculars or
telescope with a chart like this.
2.
Eyepieces have a certain magnification:
EP magnification = Focal Length
Scope/
Focal Length EP
Example : 6inch F8 =F.L = 48” or 1200 mm
EP mag for 20 mm = 1200/20 = 60 mm
Star Chart Courtesy Sue French:
Celestial Sampler
Now go do the
OpenCluster exercise