Presentation on theme: "Instructors: Pat Browne Stephen Collie Rick Scholes Course assistant"— Presentation transcript:
1Spring 2012 Astronomy Course Mississippi Valley Night Sky Conservation The Sky Around Us Instructors:Pat BrowneStephen CollieRick ScholesCourse assistantAmy BoothAprilAnnouncements:Errata – M46, M47 in the constellation PuppisCourse Assistant Amy BoothCourse Group online:invitations pending…Program developed byMississippi Valley Conservation AuthorityRoyal Astronomical Society of CanadaOttawa Astronomy FriendsEarth Centered Universe software for illustrations –courtesy David Lane
2II Stars in our Milky Way Galaxy WHERELocating stars on the Celestial Sphere -Constellations,Aligning our telescopes to track the starsWHENDo they rise and set on our local horizonWHATStellar properties, stellar designation,classificationasterisms clusters of starsWHOPioneers in stellar astronomy:Annie Jump CanonHelen Sawyer Hogg (Canadian)Ejnar Hertzprung- Henry Russell
4Objects on our Celestial Sphere = Stars in our Milky Way Galaxy Celestial Sphere – AprilRecall: What we see in the sky dependsDateTimeWhat our latitude is which sets our local horizonDemo first on the planisphere then on the celestial sphere (local horizon)Lets do this for April 20…The stars rise 4 minutes earlier each daybecause the earth has also movedthrough its orbit as it has rotatedaround from night to day to night.
5Star Time – Sidereal Time A year on earth in star time… Sidereal Time = our time measurementwith respect to the stars..1 Day = 1/365th of a circle~ about one degree around the Sun.Earth rotates on its axis as well as rotates around the sun.So, the time for a star to return to the same place in our sky the following evening is only 23 hours, 56 minutes and 4 seconds (not 24)This is called a sidereal day ( 1 revolution of the earth with respect to the stars)Do the earth rotating dance around the sun then with respect to the stars infinitely far away…
6Say ‘goodbye” to winter constellations Lets do this for Apr 20…Say ‘goodbye” to winter constellations
7Observations from Last week Open Clusters Nebula and Stars Globular Clusters Galaxies
8As the Earth Turns –Tour of the Night Sky April 13 2012, 9pm EDT MarsAs the Earth Turns –Tour of the Night Sky April , 9pm EDTN/S line - MeridianM44M67When planning your are observingsession , start with the things that aregoing to set first – Westward HO!Here is the ECU view of the celestialsphere showing the western sky,You can see this on your planisphere.But your planisphere does not record theplanets because they change from year toyear. ECU can program the planets in…Jupiter, nearly set…Venus (the brightest object)We shall see a phase on VenusConstellation ObjectTaurus M1 Crab Nebula – Supernova remnantTaurus M45 – the Pleaides – setting…Gemini M35 – Open ClusterAuriga M37,M36,M38 OCsOrion M42 Orion Nebula Emission, M78 Reflection NebulaMonoceros M46, M47 OCsCancer M44 Beehive Cluster , M67We finish the Western tour with ruddy Marswhich is culminating on our meridian.M37M35M36M46,M47M38M1M78VenusVenusM42M45line of the planets (ecliptic)horizon (west)Jupiter
9We went after the western Winter sky – objects that would soon set. What We ObservedRecall:We went after the western Winter sky – objects that would soon set.These objects were mostly in the Winter Milky Way althouch you couldn’t tell that because the sun was still lighting up the horizonWe saw lots of Open Clusters . Their sizes/brightness differences were obvious in Puppis (not Monoceros) M47 vs M46Cancer Beehive M44 vs M67We saw Emission and Reflection Nebula like M42 and M43 which are in fact illuminating proto-starsWe saw a Supernova Remnant, the Crab Nebula in Taurus, … the Cosmic Dust Bunny!Particular observations?This excellent image of M43 shows well the dark lane separating it from its larger neighbor, M42, the Orion Nebula. It was taken with the KPNO 0.9-meter telescope on the night of December 20th 2002 UT. The central star is a young irregular variable designated NU Orionis or HD As in many deep images, this star looks elliptical here, presumably due to the material surrounding it (at least, we hope that's the explanation). Notes on M43 This excellent image of M43 shows well the dark lane separating it from its larger neighbor, M42, the Orion Nebula. It was taken with the KPNO 0.9-meter telescope on the night of December 20th 2002 UT. The central star is a young irregular variable designated NU Orionis or HD As in many deep images, this star looks elliptical here, presumably due to the material surrounding it (at least, we hope that's the explanation). Notes on M78itself as an eerie ghost with two eyes. 146x made the nebula brighter. But the eerie feeling turned into panic at 220x when all of the surrounding stars were no longer seen and the ghost simply was staring at me with two bright eyes. At this point, I got the feeling someone was looking at me behind my
12Distance Graph and Brightness Graphs of what we saw GalaxiesGlobularsDistance dimmingLogWhen we look at Open Clusters, we are looking into the disk of the Milky Way between 500 – 1000 light years distance.It turns out we are looking at two different spiral arms – Auriga Open Clusters are in the Perseus Arm, whereas the Orion/Puppis clusters are in the Orion ArmWhen we look at Globular Clusters we are looking 10x more deeply out of the disk of the galaxy in a halo around it – M3 is one exampleFinally when we look at Galaxies, we are looking outside of our own galaxy > 10,000,000 light yearsThe brightest objects are the smaller magnitudes! !Surface brightness depends on the concentration of thematerial as well as the distance to the object.M1, the Crab nebula is considered a difficult object inthe city because of its low surface brightness.It is also on the higher end of the distance for theseasterisms.
13Practical Procedures – when thinking about Telescopes What we practically need to know is howto set up our scopes if we have anequatorial mount. …Setting up our equatorial mount isjust like setting our local horizon on thecelestial sphere…
14pointing to the Pole star Polaris. To set the scope polar axis to the celestial polar axis, the wedge is rotated to match the altitude of polaris at your latitude. This is the same thing as setting the altitude of the polestar equal to our latitude (45 deg)Point the telescope North andLook up the polar axis.Se the altitude of the wedge toyour Latitude. To line us up on the axis. We do this bypointing to the Pole star Polaris.Polaris should be centered in theeyepiece
15Celestial Coordinate System = Equatorial Mount Coordinates Once we are aligned, we only have to nudge the Right Ascension axis (around the polar axis), in order to keep the object centered in the eyepiece.Because when we are aligned with our polar axis we track the sky.Polaris in not on the zenith but roughly 45 degrees up = our latitude above the equatorMeridian facing northLines of Right AscensionParallels of DeclinationCelestial EquatorThe equatorial mount has the same axes asthe celestial sphere. It is an alt-azimuthmount that has been tilted up to the pole starso that one axis can be turned with the earthturning.
16Back to what’s out there in … the Night Sky … Star hopping to find objects does not require fancy mountsDifferent scopes without equatorial mounts
17When we observe…Always dress warmly as if it were still winter.Standing around in the springtime can get chilly because you are not movingAllow your eyes to adapt to what you are seeingLearn not to stare into the eyepiece but let your eye relax and allow the peripheral vision to see things tooUse a red flashlight to consult charts if you are trying to hunt something downKeep an observing Log! and record observations even if you’re tired“If you don’t keep a logbook you’ll always be a beginner.”
18Celestial Sphere Earth Centered Universe Computed for our location Given our geographical position and time on the earth: our latitude, our time zone and our Time of Day, ECU displays an accurate description of our celestial sphere for our position on the earth.We can use a manual planisphere, set it for our time of year and day for our location to determine whether the object is above our horizon, what our L.S.T is, to place it on our meridian, etcWe are ready to plan our observing session and view not only stars, but star clusters, galaxies,etc.But everything, stars, asterisms, constellations, galaxies have a time and a season… according to sidereal time.M65, M65 Galaxies
19Constellations: Area of sky identifiable by star pattern Ursa MajorConstellations and asterisms are not necessarily closeto each other in space. Everything is at a nearly‘infinite’ distance on our celestial sphere within ourMilky Way. This is to assign properoordinates to them.Historically, the brightest stars on were groupedtogether into constellations and asterisms and thebrightest stars gained proper names.Extensive catalogues of stars have been assembledby astronomers, which provide standardized stardesignations.Greek Letters (Bayer Catalogue) order by relativebrightness so that Alpha Leonis is brighter thanGamma.Their absolute positions in RA and DEC wererecorded at special Meridional telescopes fixed towatch stars culminating on the meridian.The ancients grouped those constellationsthat traveled along the ecliptic(the path of the planets) into the ZodiacThere are 4 zodiacal constellations here…Gemini, Cancer, Leo, Virgo12 Zodiacal Constellations out of 88 modernones (including Southern Hemisphere).Looking South then pan east or west of our meridianClick to see the major constellationsBootesGeminiLeoCancereclipticMarsVirgoSaturnCorvusHydraExercise 1:Go out and observe these constellations. How many bright stars can you see in them. Number them…Optional – DVD Chapters 4,5,6,7,8,9,11,12
20When we observe stars naked eye … Starlight and Spectra (some clues) What visual clues tell us?BrightnessColourBrightness doesn’t really tell usthe distance (parsecs or lightyears) because we need to knowtheir intrinsic brightnessColour – will tell us somethingabout their temperatureOther Propertiesluminosity (intrinsic brightness)and spectra (relative abundanceof spectral lines in the light fromthe star),Tell us about-the age (> millionsof years)- the distance to the object-chemical compositionof the stellar object.Without its spectral type a star is a meaningless dot.Add a few letters and numbers like "G2V“ and the star suddenly gains personality and character
21WHAT is a star… The Sun is a Star Sun is below our horizon at 10 pm along the path of the plane of the eclipticA star is a massive, luminous sphere of plasma held together by gravityAt the end of its lifetime, a star can also contain a proportion of degenerate matter.The nearest star to Earth is the Sun, which is the source of most of the energy on Earth.In a plasma gas, a certain portion of the particles are ionized. This is because the gas is heated to high temperatures at which point a gas may ionize its molecules or atoms (reduce or increase the number of electrons in them), thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions.This figure shows some of the more complex phenomena of a plasma. The colors are a result of relaxation of electrons in excited states to lower energy states after they have recombined with ions. These processes emit light in a spectrum characteristic of the gas being excited.
22Visual Star Colour and Star Spectra using Spectroscope When a star is brought into the field of view and the spectroscope is properly focused and adjusted, you will see a beautiful spectrum with the colors of the rainbow spread out along its length. Depending on the spectral type and luminosity class of the star, and your particular setup, you may see hydrogen lines cutting perpendicular across the spectrum, or many fine lines of metals, or wide absorption bands of molecules. These lines and bands in stellar spectra have been called the "fingerprints of the stars" because their patterns identify the elements in a star's atmosphere and indicate a star's temperature. These spectral features are easy to see in some classes of stars and more difficult to see in others.The image below was taken with the Visual / Photo / CCD Star Spectroscope: How are spectral lines formed?By electrons jumping between different energy levels in the atoms in thestar's outer layers. Bound electrons can absorb and emit energy onlyin certain discrete amounts.When an electron absorbs a photon of light with just the right amount ofenergy, it jumps to a higher energy level. When the electron spontaneouslyjumps back to a lower energy level, a photon is emitted. Enough electronsjumping between any two given energy levels of a given element will result ina spectral emission or absorption line at a characteristic wavelength.For example, the strongest spectral line in a hot main-sequence star like Vegalies in the blue-green part of the spectrum. It is a dark orabsorption line resulting from electron jumps fromthe second to the fourth energy level of the neutral hydrogen atom, and is known as hydrogen beta (in the Balmer series).
23Color Star Atlas or Color Stars in ECU The main reason why stars are differently coloured is that some are hotter than others.Deep in their interior all stars are enormously hot (measured in millions of degrees),but their temperature lessens towards their outer layers, and the coolest starpours out most of their visible radiation in the red part of the spectrum.Hotter stars like the Sun appear yellow, still hotter stars appear white, and the hottestappear blue.The spectral type of a star is not the same thing as its intrinsic colour althoughthe two are closely related. When starlight passes through a spectograph ( a prism orglass grating) it is split into the colors of the rainbow, a spectrum. Most importantlythere are spectral absorption lines that give a clue to the temperature and thechemical composition of that star Almost all starlight spectra can beassigned to one of seven main types (OBAFGKM).A great deal about the nature of the star can be inferred from its spectrum :how bright it really is, how massive it is, whether it is a compact main sequence star(see next slide) or a swollen giant. Broadly speaking, we can tell how old it is,and what is happening to it with respect to its hydrogen, helium orheavier element combustion process.Coma Star Cloud – and star colours!
24Stars and their Spectra Hertsprung-Russell Diagram to classify Stars according to their Spectral ClassStars and their SpectraMost stars gather in certain narrow regions of the H-R diagramaccording to their masses and ages. Stars arrive on what's called the main sequencesoon after they are born, and this evolutionary trackis where they spend most of their lives.Massive stars blaze brightly on the hot, blue end ofthe main sequence. They burn up their nuclearfuel in only millions or tens of millions of years.Stars with lower masses comprise the yellow,orange, and red dwarfs on the lower-right part ofthe main sequence, where they remain for billionsof years.As a star begins to exhaust the hydrogen fuel in itscore, it evolves away from the main sequencetoward the upper right and becomes a red giant orsupergiant. Stars that began with more than eighttimes the Sun's mass then evolve left and rightthrough complicated loops on the H-R diagram asif in a frenzy to keep up their energy production.Then they finally explode assupernovae. Less massivegiants evolve to the left and then down tobecomewhite dwarfs; this is the track the Sun willtrace through the H-R diagram
25ClassTemperature (kelvins)Conventional colorApparent colorMass (solar masses)Radius (solar radii)Luminosity (bolometric)Hydrogen linesFraction of all main sequence starsO≥ 33,000 Kblue≥ 16 M☉≥ 6.6 R☉≥ 30,000 L☉Weak~ %B10,000–33,000 Kblue to blue whiteblue white2.1–16 M☉1.8–6.6R☉25–30,000 L☉Medium0.13%A7,500–10,000 Kwhitewhite to blue white1.4–2.1M☉1.4–1.8R☉5–25 L☉Strong0.6%F6,000–7,500 Kyellowish white1.04–1.4M☉1.15–1.4R☉1.5–5 L☉3%G5,200–6,000 Kyellow0.8–1.04M☉0.96–1.15R☉0.6–1.5 L☉7.6%K3,700–5,200 Korangeyellow orange0.45–0.8M☉0.7–0.96R☉0.08–0.6 L☉Very weak12.1%M≤ 3,700 Kredorange red≤ 0.45 M☉≤ 0.7 R☉≤ 0.08 L☉76.45% Stellar classification is a classification of stars based on their spectral characteristics. The spectral classof a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure of the temperature in this chromosphere. Light from the star is analyzed by splitting it up by a diffraction grating, subdividing the incoming photons into a spectrum exhibiting a rainbow of colors interspersed by absorption lines, each line indicating a certain ion of a certain chemical element. The presence of a certain chemical element in such an absorption spectrum primarily indicates that the temperature conditions are suitable for a certain excitation of this element. If the star temperature has been determined by a majority of absorption lines, unusual absences or strengths of lines for a certain element may indicate an unusual chemical composition of the chromosphere.Most stars are currently classified using the letters O, B, A, F, G, K, and M (usually memorized by astrophysicists as "Oh, be a fine girl, kiss me"), where O stars are the hottest and the letter sequence indicates successively cooler stars up to the coolest M class. According to informal tradition, O stars are called "blue", B "blue-white", A stars "white", F stars "yellow-white", G stars "yellow", K stars "orange", and M stars "red", even though the actual star colors perceived by an observer may deviate from these colors depending on visual conditions and individual stars observed
26We can show this in the lab: Stellar Spectra tells us surface temperature, chemical composition atmospheric pressure and surface gravity, total luminousity (energy pouring out)Whether in a star's atmosphere or in a laboratory, absorption lines are produced when a continuous rainbow of lightfrom a hot, dense object (top left) passes through a cooler, more rarefied gas (top center). Emission lines, by contrast, come from an energized, rarefied gas such as in a neon light or a glowing nebula.When I look at a star, why do I see dark absorption lines rather than bright emission lines?Gas under high pressure produces a continuous spectrum, a rainbow of colors.Continuous radiation viewed through a low density gas results in an absorption-line spectrum.What's happening here is that radiation emitted by gas under high pressure deepwithin the star is being absorbed by low density gas in the star's outer layers.We can show this in the lab:Using a slit and prism, physicists discovered that when a solid, liquid, or dense gas is heated to glow, it emits a smooth spectrum of light with no lines: a continuum. A rarefied hot gas, onthe other hand, glows only in certain colors, or wavelengths: bright, narrow emission lines instead of a rainbow band. If a cooler sample of the same gas is placed in front of a glowing object emitting a continuum, dark absorption lines appear at the wavelengths where the emission lines would be if the gas were hot.What kinds of deep sky objects have emission-line spectra?A low density gas shows an emission-line spectrum, when not observed against a background of continuous radiation. Thus emission lines are found in the spectra of planetary and diffuse nebulae, and in some stars. In the latter case the lines often arise from gas clouds ejected from the star by strong stellar winds.
27Harvard College Observatory Astronomer Annie Cannon 1863 –1941Harvard College Observatory Astronomerapplied her own scheme which resulted inthe famous OBAFGKM classification which is still used todayThe Sun's spectrum was marked by many narrow, black lines of various intensities.These dark lines stayed at exactly the same places in the colorful band from day to dayand year to year. This solar spectrum — a 'rainbow' of sunlight with thin,dark absorption lines at numerous discrete wavelengths.Each chemical element creates its own unique set of spectral lines.Similar spectral lines showed up in laboratoryThe sun is a G2 star representing 7.2% of the statistical population within 10 pcs.The (much abused) Copernican PrincipleIn its modern form, the Copernican Principle has becomesomething that would have entirely horrified Copernicus.This shift in interpretation aside, the Copernican Principle isgenerally expressed in a form that asserts the non-favouredlocation and non-special viewpoint of humanity5 withinthe Universe. That is, we are not privileged or even uniqueobservers of the cosmos. The idea behind the principle is ofgeneral importance in the practice of science, and, for example,in the field of cosmology, appears to be demonstrably true: theUniverse, on the large scale, is isotropic and homogeneous, andour general viewing circumstances are no different from thoseof any other potential observer in the Universe. Be all thisas it may—the point is, we would argue, that an unguardeddevotion to the Copernican Principle in the modern era hasresulted in wrong conclusions being drawn about the Sun.The argument apparently runs along the lines that since,by the Copernican Principle, humanity, as observers of theUniverse, is not specially located, so the star (the Sun) aboutwhich the Earth orbits, and from which humans observe,cannot be special, and therefore it must be an average sort ofstellar object in a non-special, “nondescript” location withinthe Milky Way Galaxy. Such conclusions, as far as the Sungoes, are not logically propagated and, more to the point, aredemonstrably wrong.ClassTemperature (kelvins)Conventional colorApparent colorMass (solar masses)Radius (solar radii)Luminosity (bolometric)Hydrogen linesFraction of all main sequence starsO≥ 33,000 Kblue≥ 16 M☉≥ 6.6 R☉≥ 30,000 L☉Weak~ %B10,000–33,000 Kblue to blue whiteblue white2.1–16 M☉1.8–6.6R☉25–30,000 L☉Medium0.13%A7,500–10,000 Kwhitewhite to blue white1.4–2.1M☉1.4–1.8R☉5–25 L☉Strong0.6%F6,000–7,500 Kyellowish white1.04–1.4M☉1.15–1.4R☉1.5–5 L☉3%G5,200–6,000 Kyellow0.8–1.04M☉0.96–1.15R☉0.6–1.5 L☉7.6%K3,700–5,200 Korangeyellow orange0.45–0.8M☉0.7–0.96R☉0.08–0.6 L☉Very weak12.1%M≤ 3,700 Kredorange red≤ 0.45 M☉≤ 0.7 R☉≤ 0.08 L☉76.45%
28Stellar Spectra Summary surface temperaturechemical compositionatmospheric pressure and surface gravitytotal luminousity (energy pouring out)*The temperature sets the star's color and determines its surface brightness:how much light comes from each square meter of its surface.The atmospheric pressure depends on the star's surface gravity and therefore, roughly, on its size —telling whether it is a giant, dwarf, or something in between.The size and surface brightness in turn yield the star's luminosity (its total light output, or absolute magnitudeand often its evolutionary status (young, middle-aged, or nearing death).)The luminosity (when compared to the star's apparent brightness in our sky)also gives a good idea of the star's distanceT=5500Note also that the colour of the star is related to thecorresponding peak wavelength emittedof the continuous radiation:λmax = b/ Twhere λmax is the peak wavelength, T is the absolute temperatureof the black body, and b is a constant of proportionalitycalled Wien's displacement constant, value).Knowing the suns temperature, we infer a particularcolour expressed as a wavelength in the visible …T = 5000λ = λ max
29Stellar Classification - Who Annie J. Cannon discovered that nearly all stars' spectra can be fit into one smooth, continuous sequence. The sequence matched the stars' color temperatures, from the hottest, blue-white stars at one end to relatively cool, orange-red ones at the cool end . The basic sequence ran O B A F G K M from hot to cool.
30Planning your Observations Get a book from the library or a magazine that features a particular selection of objects visible from your location at the current dateYou can use ipod type devices but plan what you are doing beforehand so that you don’t just stare at the ipodBetter to plan indoors first . Use a planetarium program like ECU. We can do a lab showing how to set the time, place, information detail, catalogues…Make sure you are comfortable at the eyepieceYou can sit down when you get tired.Plan your session.Choose an area to work on and pick from a listof different things:stars with colour/ colour contraststar clustersstar nebulae and nursuriesgalaxiessupernovae remnantsclusters of galaxiesECUEarth Centered Universe
31Looking up – Spring Night Sky exploration What binary stars can you see –pick some famous onesWhat color contrasts can youobserve?Blue and yellow??1. How do you use stellar‘landmarks’ to hop to non-stellar objects such as theVirgo Cluster of Galaxies(hint – Find Epsilon Virgo andBeta Leonis)… or the cluster of galaxies in2. Leo, M65,M66?3What does the M stand for…when we talk about Messierobjects?4.What kinds of Mobjects are there?5.What kind of object is M44?(The Beehive cluster)
32Star- Hopping to find Star Clusters and Clusters of Galaxies To find the Markarian Chain ofGalaxies in the Virgo cluster, locateEpsilon Virginis and Beta Leo . They liehalf-way along the lineTo find M65, M65 dropdown from Theta LeonisTo find M3 (Globular Cluster) locateArcturus (Alpha Bootes) and AlphaCanes Venatici (not shown) . M3 is 1/3of the way from Alpha BootesSee ObservingGalaxies.ppt on the Millstone Website for more informationAlpha Bootes