Presentation on theme: "Spring 2012 Astronomy Course Mississippi Valley Night Sky Conservation The Sky Around Us Program developed by Mississippi Valley Conservation Authority."— Presentation transcript:
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
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? 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 III Star Clusters in and around our Galaxy M3 M44 BeeHive
Stellar Properties -5 0 +5 +10 O B A F K M5 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.
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
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 Evolutionary Path – Solar Mass Stars off the Main Sequence
In 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. 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 White Dwarf Evolution Planetary Nebula Instant Expert
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.e- education.psu.edu/astro801/con tent/l6_p3.html
. 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! Massive Stars > 10 Solar Masses SN1987A discoverer Dr. Ian Shelton, U of T Supernova
Stellar Populations 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 … Population 1 Population II disk stars halo stars Main Sequence Evolved off main sequence metal rich (~ sun) metal poor open clusters globular clusters 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.
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 fraction by masssolar value hydrogen content X0.70 helium contentY0.28 everything else (C, O, Mg, Si, Fe, etc: "metals") Z0.02 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. http://burro.astr.cwru.edu/Academics/Astr222/Galaxy/Structure /metals.html Where : Observing Open Clusters in our Galaxy and around our Galaxy 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
Redder 3000 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)) 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 = 5.5 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 Stellar and Cluster Distances – How do we know the distance? Mm
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 Meridian Open Clusters looking West (setting) ( Spring time Northern Hemisphere) Auriga cluster M38, M36,M37 Monoceros Cluster M46, M47 M44 – Beehive Modest neighbour M67
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 whats 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 Whats up ? Is the Moon up? Wheres 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. Wheres our meridian? Galaxies galore coming up close to our local meridian… Open Clusters setting in the West… Globular Clusters in the East
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… When the Moon is UP! Now go do the Lunar Observing Exercise!
Conserving NightSky Environment -> Solutions 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 http://millstone.typepad.com/files/dark -sky-campus.mp4 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
GOOD NEIGHBOUR LIGHTING = SHIELDED LIGHTS Simple solution – no up-lighting, sky is protected, ground surface visible Flat Glass Cobra Street light Shielded lights – like lampshades – no bulb exposed
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, its 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
Binocular/Finder vs Telescope Field of View 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 PWR for 20 mm = 1200/20 = 60 x
Distances in the sky are Arc measures 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 1-2° 10° 20° The Moon around ½ deg The Pleiades 2-3 deg
Binocular/Finder vs Telescope Field of View 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 Now go do the OpenCluster exercise Star Chart Courtesy Sue French: Celestial Sampler