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H205 Cosmic Origins APOD Properties of Stars (Ch. 15) The Milky Way (Ch. 19) EP3 Due Wednesday Opportunities • Kirkwood Obs. Open April 1 (weather permitting) • Solar Telescope open April 4 (weather permitting) • Astronomy Club – Mondays at 7:30, Swain West 113 (Includes PIZZA!) • Remote Observing – April 9 & 10, 9:30-midnight, Swain West 403 • PBS on March 31 (Check times!): – Investigation into a possible comet strike • PBS on April 21? – 8 PM: 400 Years of the Telescope - narrated by Neil deGrasse Tyson – 9 PM: - A Sidewalk Astronomer - the story of John Dobson (now 91 years old!) Basic Properties of Stars distance brightness diameters The Hertzsprung-Russell Diagram Stars The Brightness of Stars • Apparent brightness – how bright does it look in the sky? Apparent magnitude - mV • Absolute brightness – how bright is it really?? Absolute magnitude - Mv • The apparent brightness depends on both a star’s distance and its intrinsic brightness The Inverse Square Law tells us how a star’s apparent brightness changes with distance • Brightness decreases as distance squared – something twice as far away will be four times fainter – something 10 times further away will be 100 times fainter – something 1000 times further away will be a million times fainter Apparent Brightness Luminosity 4π distance 2 How Far Away Are Stars? If we know a star’s apparent AND absolute brightness, we can calculate its distance brightness changes as 1/distance2 The inverse square law describes how the brightness of a source light (a star!) diminishes with distance But how do we get the distances to stars whose brightness we DON’T know? Measuring the distances to stars using Parallax Measuring the distances of stars Parallax: apparent change in the position of an object due to a change in the position of the observer Stellar parallax uses the Earth’s orbit as the baseline 1 AU sin (p) distance Parallax A parsec is the distance at which 1 AU subtends an angle of 1 arc sec 1 AU distance Angle = p 1 distance (in parsecs) p ( in arcsecs) What is a Parsec??? Parsec: the distance to an object with a stellar parallax of one arc second A star at a distance of 1 parsec shows a parallax of 1 arc second 1 parsec = 3.26 light years A parallax of ~0.001 arc seconds is the smallest we can measure How big is one arc second? The size of a dime at a distance of 2.3 miles! The parallax of Alpha Centauri = 0.76 arcseconds How Big Are Stars? We can’t see the stars’ diameters through a telescope. Stars are so far away that we see them just as points of light. If we know a star’s temperature and its luminosity, we can calculate its diameter. How do we determine a star’s temperature? Luminosity depends on…. TEMPERATURE the hotter a star is, the brighter it is. DIAMETER – the bigger a star is, the brighter it is. Stellar Radii We can’t see the stars’ diameters through a telescope. Stars are so far away that we see them just as points of light. If we know a star’s temperature, apparent magnitude, and distance, we can calculate its radius Temperature from Luminosity from parallax and apparent magnitude Luminosity 4R T 2 4 Stars range in size from about the size of the Earth to hundreds of times the Sun’s diameter Magnitudes • Astronomers use “magnitudes” to describe how bright stars are • Small numbers are brighter, large numbers fainter. • The brightest naked-eye stars are around magnitude zero. • The faintest naked-eye stars are around magnitude six • 5 magnitudes are a factor of 100 in brightness (a 6th magnitude star is 100 times fainter than a 1st magnitude star) The Nearest and the Brightest Goal: – to learn about types of stars – to explore the stars near the Sun and compare them to the stars we see in the sky Task: – plot a Hertzsprung-Russell diagram including both the nearest stars and the brightest stars in the northern sky The Brightest Stars in the Sky (no need to copy these down!) Star Distance (LY) Temperature (K) Absolute Magnitude Sun 0.000015 5800 4.8 9 9600 1.4 232 7600 -2.5 Alpha Cen A 4 5800 4.4 Arcturus 37 4700 0.2 Vega 25 9900 0.6 Capella 42 5700 0.4 Rigel 773 11000 -8.1 Procyon 11 6600 2.6 Achernar 144 22000 -1.3 Betelgeuse 427 3300 -7.2 Hadar 335 25000 -4.4 Acrux 321 26000 -4.6 Altair 17 8100 2.3 Aldebaran 65 4100 -0.3 Antares 604 3300 -5.2 Spica 263 2600 -3.2 Pollux 34 4900 0.7 Sirius Canopus Distance (LY) Temperature Absolute Magnitude Prox Cen 4 2800 15.53 Alp Cen A 4 5800 4.4 Alp Cen B 4 4900 5.72 Barnard’s 6 2800 13.23 Wolf 359 7.5 2700 16.57 Lal 21185 8 3300 10.46 Sirius A 9 9900 1.45 Sirius B 9 12000 11.34 Luyten 726-8A 9 2700 15.42 UV Ceti 9 2600 15.38 Ross 154 10 3000 13.14 Star The Nearest Stars Hertzsprung Russell Diagram -10 -5 Absolute Magnitude The HR Diagram Giants and Supergiants 0 5 Main Sequence 10 White Dwarf 15 20 30000 25000 20000 15000 10000 Temperature (K) 5000 0 Key Ideas – The HR Diagram • The intrinsic brightness or luminosity of stars depends on temperature and radius • if two stars have the same radius, the hotter one is brighter • if two stars have the same temperature, the bigger one is brighter • The Hertzsprung-Russell Diagram • relates the temperature and brightness of stars But only certain sizes and colors are allowed! Stars come in many sizes and colors HR Diagram Simulator BRIGHTNESS Most stars occur in these main groups in the luminositytemperature diagram TEMPERATURE Main Sequence Giants Supergiants White Dwarfs The Main Sequence BRIGHTNESS The sun is an ordinary, yellow main sequence star TEMPERATURE Giants and Supergiants are cooler and very large Supergiants BRIGHTNESS Giants White dwarfs are small and hotter TEMPERATURE The apparent brightness of a star in the sky depends on distance, luminosity, and temperature Most luminous stars: 106 LSun Least luminous stars: 10-4 LSun (LSun is luminosity of Sun) Most massive stars: 100 MSun Least massive stars: 0.08 MSun (MSun is the mass of the Sun) Main-Sequence Star Summary High Mass: High Luminosity Short-Lived Large Radius Blue Low Mass: Low Luminosity Long-Lived Small Radius Red Constructing an HR Diagram Apparent Magnitude 0 5 10 15 -0.5 0 0.5 B-V Color 1 1.5 2 What’s this B-V color? • Astronomers measure the brightness of stars in different colors – Brightness measured in blue light is called “B” (for “Blue”) – Brightness measured in yellow light is called “V” (for “Visual) • Astronomers quantify the “color” of a star by using the difference in brightness between the brightness in the B and V spectral regions • The B-V color is related to the slope of the spectrum The slope of the spectrum is different at different temperatures Most stars fall somewhere on the main sequence of the H-R diagram WHY WHY WHY ??? High-mass stars Low-mass stars Mass measurements of mainsequence stars in binary star systems show that the hot, blue stars are much more massive than the cool, red ones Main-sequence stars are fusing hydrogen into helium in their cores like the Sun massive mainsequence stars are hot (blue) and luminous Less massive stars are cooler (yellow or red) and fainter High-mass stars Low-mass stars The mass of a normal, hydrogenburning star determines its luminosity and temperature! Mass & Lifetime Sun’s life expectancy: 10 billion years Life expectancy of 10 MSun star: 10 times as much fuel, uses it 104 times as fast 10 million years ~ 10 billion years x 10 / 104 Life expectancy of 0.1 MSun star: 0.1 times as much fuel, uses it 0.01 times as fast 100 billion years ~ 10 billion years x 0.1 / 0.01 Off the Main Sequence • Stellar properties depend on both mass and age: those that have finished fusing H to He in their cores are no longer on the main sequence • All stars become larger and redder (and cooler) after exhausting their core hydrogen: giants and supergiants • Most stars eventually end up small and hot after fusion has ceased: white dwarfs Star Clusters Star Clusters Goals: – Understand how we learn about stellar evolution from the properties of stars in clusters – Understand how we can determine the distances of star clusters – Understand how we can determine the ages of star clusters “Globular" Clusters and “Open" Clusters Globular Clusters Open Clusters •104-106 stars •old! •compact balls of stars •high star density •10-104 stars •generally young •loose •low star density Properties of Stars in Clusters • Formed at the same time • Stars are the same age • All stars have the same composition • The stars are held in a group by their common gravity Cluster HR Diagrams Hotter stars are brighter in blue light than in yellow light, and have low values of B-V color, and are found on the left side of the diagram. Cooler stars are brighter in yellow light than in blue light, have larger values of B-V color, and are found on the right side of the diagram. hotter cooler Distances to Star Clusters The Sun has a “B-V” color of about 0.6. Stars like the SUN What would the apparent magnitude of the Sun be at the distance of the cluster Messier 6? Stars in Messier 6 with B-V colors of 0.6 have similar mass and luminosity to the Sun hotter cooler Ages of Star Clusters The “bluest” stars left on the main sequence of the cluster tell us the cluster’s age. As the cluster ages, the bluest stars run out of hydrogen for fusion and lose their “shine” hotter cooler Jewelbox The HR diagrams of clusters of different ages look very different 5 10 M 67 0 15 -0.5 0 0.5 B-V Color 1 1.5 2 Apparent Magnitude Apparent Magnitude 0 5 10 15 -0.5 0 0.5 B-V Color 1 1.5 2 Main Sequence Turnoffs of Star Clusters Here we see a series of HR diagrams for sequentially older star clusters that have been superimposed Burbidge and Sandage 1958, Astrophysical Journal Ages of Star Clusters Cluster NGC 752 M 67 Hyades Pleiades M 34 Jewelbox Turnoff Color 0.35 Age 1.1 billion years 0.45 2.5 billion years Thinking Question: Why with a 0.15 has a 800cluster million years turnoff color of B-V=1.0 -0.15 100 million years never been discovered? -0.10 180 million years -0.25 16 million years For Wednesday Chapter 19 – Milky Way EP3 Finished on Wednesday