Presentation on theme: "Program developed by Mississippi Valley Conservation Authority"— Presentation transcript:
1 Spring 2012 Astronomy Course Mississippi Valley Night Sky Conservation The Sky Around Us Program developed byMississippi Valley Conservation AuthorityRoyal Astronomical Society of CanadaOttawa Astronomy FriendsInstructors:Pat BrowneStephen CollieRick ScholesCourse AssistantAmy BoothEarth Centered Universe software for illustrations –courtesy David Lane
2 III Star Clusters in and around our Galaxy WHERELocating Star ClustersWhere are they within or around the galactic planeWhich clusters live where?WHENWhen did they form? (Stellar Evolution)Are they visible?WHATTypes of star clustersClobular ClustersOpen ClustersWHOSupernova SN1987a discovererIan Shelton – Cdn astronomerPioneers in star cluster analysisHelen Sawyer Hogg(Canadian Astronomer)Henrietta LeavittM3M44 BeeHive
3 M5 K F A B O -5 +5 +10 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 roughlygroups 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.BO-5+5+10
6 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 fusionWe can explore the evolutionary tracks
7 Evolutionary Path – Solar Mass Stars off the Main Sequence Case 1 Stars = 1 Solar Mass -> Red Giant -> White dwarfStars 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 BranchHydrogen 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
8 Instant ExpertWhite Dwarf Evolution Planetary NebulaAs 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 eventIn an AGB star, if the helium fuel in the He-burning 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 18th century.
9 Summary – Post Main Sequence Stellar Evolution for Sun-like Stars Courtesy Zelik and SmithIntroductory Astronomy and AstrophysicsFor AGB enthusiasts here is an excellent reference:https://www.e-education.psu.edu/astro801/content/l6_p3.html
10 Massive Stars > 10 Solar Masses SupernovaMassive Stars > 10 Solar MassesSN1987A 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.Dr. Ian Shelton of U of T discovered SN 1987A in the LMC!
11 Stellar PopulationsPopulation Population IIdisk stars halo starsMain Sequence Evolved off main sequencemetal rich (~ sun) metal pooropen clusters globular clustersTypically 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.
12 everything else (C, O, Mg, Si, Fe, etc: "metals") Where : Observing Open Clusters in our Galaxy and around our GalaxyOpen clusters are groupings of star sin a region 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 membersObservations:Globular clusters are generally metal-poorOpen clusters are generally more metal-richThere 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 .fraction by masssolar valuehydrogen contentX0.70helium contentY0.28everything else (C, O, Mg, Si, Fe, etc: "metals")Z0.02Globular 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 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
13 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 onm/M ~2.5 Log (d /(10^2))We can use a fitting technique for clusters of stars shifting the test cluster up and down along the calibrated sequence .Here the best fit m – M = 5.5m – M = 5 log d – 5Example: = log dd = 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, 241Redder3000Mm
14 Open Clusters looking West (setting) ( Spring time Northern Hemisphere) M44 – ‘Beehive’Modest neighbour M67WHAT:Open clusters:Widely-spaced groupingsof easily resolvable starsAlso called Galactic Clustersbecause they lie in thegalactic diskLooking westAuriga: Clusters,M38,M36,M37 (West)Monoceros: M46, M47Looking just right of theMeridian high up…*Cancer : M44 Beehive, M67Auriga cluster M38, M36,M37MeridianMonoceros Cluster M46, M47Open clusters range from very sparse clusters with only a few members to thousands of stars. The core is typically about 3–4 light years across, with the corona extending to about 20 light years from the cluster centre. Typical star densities in the centre of a cluster are about 1.5 stars per cubic light year; the stellar density near the sun is about stars per cubic light year.Open clusters are often classified according to a scheme developed by Robert Trumpler in The Trumpler scheme gives a cluster a three part designation, with a Roman numeral from I-IV indicating its concentration and detachment from the surrounding star field (from strongly to weakly concentrated), an Arabic numeral from 1 to 3 indicating the range in brightness of members (from small to large range), and p, m or r to indication whether the cluster is poor, medium or rich in stars. An 'n' is appended if the cluster lies within nebulosity.
15 Winter (west) Milky Way From a true, dark sky,nothing can compare to anaked eye view of the MilkyWay. During the wintermonths in the NorthernHemisphere, we face awayfrom the furiously busy coreof our home galaxy and lookoutward, through its moretenuous periphery.Despite being more delicate,this slice of the Milky Way isstill rich with structure.
16 Auriga contains an nteresting variety: many open clusters and nebulous Regions in Constellation AurigaAurigaAuriga contains an nteresting variety:many open clusters and nebulousregions simply because theMilky Way runs through it.3 Open clusters in/out ofpentagon of ConstellationAuriga south of Capella.M37 the richest cluster containing over500 stars spread across 20arcminutes and is the brightest ofthe three with an apparent magnitude +5.6.M stars with an angular width of 12arcminutes.M38 100stars and is thedimmest of the three at magnitude+6.4.All three of these clusters, 4000 light-yearsaway, can be seen with a small telescope.Courtesy - Dave Garner teaches astronomy at Conestoga
17 Observing Log Book Suggesteed Recording Format (Do what’s comfortable for you)Header:Observation NumberObservation Date and TimeObserving InstrumentTelescope/EyePiece CombinationObserving Conditions – Temperature, Wind, moon phaseReferences – Books, Sky Charts,etcBody:Guests or observing companionsEach object – Designations commonly include thosefound in in the RASC Observers Handbook : MessierNGCDavid Levy GemsMethodology for Finding the ObjectImpressions of the objectThis log book won the RASC Ottawa Center Observer of the Year Award Lack of neatness is forgiven in favour of persistence in recording (even after a long night).
18 Introduction to Star Cluster Observing What’s up ? Is the Moon up?Where’s our meridian?What can we see when theMoon is up…For clusters of stars, or specialnebulous stellar bodies, orgalaxies, the moon , like lightpollution obscures the photonsemitted from these objects.Galaxies galore coming up close to our local meridian…Open Clusters setting in the West…Globular Clusters in the East
19 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!
20 Conserving NightSky Environment -> Solutions Milky way only visible wth moderately dark skiesFaint 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 objectsGood Neighbour LightingShielded 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 by30% for the same resultsFlat glass fixtures also good because the bulb is recessedMississippi Mills By-law for Outdoor IlluminationLight pollution abatement Conservation of the night skyThis 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
21 GOOD NEIGHBOUR LIGHTING = SHIELDED LIGHTS Simple solution – no up-lighting, sky is protected, ground surface visibleShielded lights – like lampshades – no bulb exposedFlat Glass Cobra Street light
22 Observing Brightness and Size of objects: Given a dark location reasonably free of unshieldedlighting (referred to as "light pollution"), thisscale describes what is shown when you query ECUabout visual magnitudes:
23 Magnitudes on a Sky Chart and in the sky… So that when we see Mars is at magnitude -0.2 with anangular width of 10.7”we know, it’s bright, andcan be seen in binoculars ,but better yet in a telescope.Image courtesy Rolf Meier - RASC
24 Observing naked eye and with optical aids… Constellation Cancer –easiest to see the largecluster rather than thestars that make up theconstellation)Compare the size andmagnitude of theBeehive cluster vs. theother Open Cluster in (mag 3.1):M67 (much smaller,fainter (mag 6.9), andone of the oldest starclusters known…!Beehive Cluster – Praesepe –size 95’ (> deg) Magnitude 3.1M67: size 29 ‘ (1/3 deg)Magnitude 6.9Messier 67 (also known as M67 or NGC 2682) is an open cluster in the constellation of Cancer. M67's Trumpler class is variously given as II 2 r, II 2 m, or II 3 r. It was discovered by Johann Gottfried Koehler in 1779. Age estimates for the cluster range between 3.2 and 5 billion years, with the most recent estimate (4 Gyr) implying stars in M67 are younger than theSun. Distance estimates are likewise varied and typically range between pc. Recent estimates of 855, 840, and 815 pc were established via binary star modelling and infrared color-magnitude diagramfitting, accordingly.M67 is not the oldest known open cluster, but there are few Galactic clusters known to be older, and none of those are as close as M67. M67 is an important laboratory for studying stellar evolution, since the cluster is well populated, obscured by negligible amounts of soot, and all its stars are at the same distance and age, except for approximately 30 anomalous blue stragglers, whose origins are not fully understood. - wikipedia
25 Observing Tips Binoculars and Telescopes: Know your field of view in the binoculars corresponding to the arc-distance in the skyKnow the size (arc-units) and brightness (visual magnitude) of the object you are looking forTelescopes:Use a finder or low power eyepiece to find the object relative to its surroundings
26 Binocular/Finder vs Telescope Field of View Find your field of view in your telescope byComparing your view through binoculars or telescope with a chart like this.Eyepieces have a certain magnification:EP magnification = Focal Length Scope/ Focal Length EPExample : 6inch F8 =F.L = 48” or 1200 mmEP PWR for 20 mm = 1200/20 = 60 x
27 Distances in the sky are Arc measures Use your hand as a scaleFinger: between 1 and 2 degreesFist: about 10degreesSpread fingers: about 20degreesWorks for any hand since the bigger the hand, the longer the arm, and the angles are about the same1-2°10°20°The Moon around ½ degThe Pleiades 2-3 deg
28 Binocular/Finder vs Telescope Field of View Find your field of view in your telescope byComparing your view through binoculars or telescope with a chart like this.Eyepieces have a certain magnification:EP magnification = Focal Length Scope/ Focal Length EPExample : 6inch F8 =F.L = 48” or 1200 mmEP mag for 20 mm = 1200/20 = 60 mmStar Chart Courtesy Sue French:Celestial SamplerNow go do theOpenCluster exercise