Stars: Intro & Classification Astronomy 1 Elementary Astronomy LA Mission College Spring F2015
Quotes & Cartoon of the Day The wonder is, not that the field of stars of so vast, but that man has measured it. Anatole France, The Garden of Epicurus, 189
Announcements SS Homework posted, due 11/19 Anybody see the. Midterm will debrief THURSDAY
Last Class Solar System Topics
This Class Intro to Stars Temperature, Color & Size Stellar Classification Intro to the HR Diagram LT HR Diagram
Stars, Temperature and Color Astronomy 1 Elementary Astronomy LA Mission College Spring F2015
Recall What a Star Is A sphere of hot gas mostly hydrogen & helium Interior hot enough to undergo nuclear fusion most commonly H > He above 107 K = 10 million K (18 million F) Held together by gravity
Fundamental Properties of Stars TEMPERATURE color, spectral properties LUMINOSITY inherent brightness amount of energy generated in the star and released as electromagnetic radiation SIZE Radius & Mass DISTANCE CHEMICAL COMPOSITION
Luminosity Luminosity is a measure of the rate at which a star puts out energy. Larger luminosity appears brighter measured in J/s or watts sometimes relative to the Sun, in solar luminosities L L stands for luminosity, stands for Sun The Sun has a luminosity of 3.846 1026 W
HOW HOT IS THAT STAR?
Recall: Why Stars Shine Visible Sun is a hot layer of gas about 5800 K (5525 C, 9980 F) not as hot as the center! Glows due to hot material Other stars have similar structure, range of surface temperature
Blackbody Radiation white light spectrum is continuous no gaps between colors the hotter you heat something, the bluer it appears Thermal (heat related) radiation from (relatively) hot bulk matter This is called blackbody radiation
Light diagnoses Temperature Colder feeling warm faint reddish glow brighter red glow bright orange very bright yellow extremely bright white bluish white http://i302.photobucket.com/albums/nn114/fiddlegirl89_photos/dave/dsc00596-1.jpg Warmer
Spectrum of the Sun Stars emit light as blackbodies to a large extent.
Shape of Blackbody Spectrum
Temperatue affects Color & Luminosity Hotter = greater luminosity appears brighter peak of curve higher technically greater luminosity per unit surface area Hotter = bluer peak of curve more to left more blue mixed in with red light
Let s Practice
A lump of lead is heated to a high temperature. Another lump of lead that is twice as large is heated to a lower temperature. Which lump of material appears bluer? A. The cooler lump appears bluer B. The hotter lump appears bluer C. Both lumps appear the same color D. Cannot tell which lump appears bluer
When something is red hot, it is hotter than something that is A. blue hot B. white hot C. neither of these D. both of these
SIZE, COLOR AND TEMPERATURE
Recall http://www.skinnerscience.com/year %2012,13/astrop3.jpg Hotter -> Brighter Hotter -> Bluer Bigger is also brighter! a star with a larger diameter has a greater luminosity
Analogy LED spotlight w/60 LEDs Total output ~1000 lumens comparable to 100-watt bulb each LED 1/60 of 1000 lumens ~16.7 lumens surface T gives you per-led luminosity not total for star
Stephan-Boltzman Law is brighter than is brighter than is brighter than could be brighter than could be brighter than
Let s Practice
Star Yoda is a small blue star and star Chewbacca is a large red star. Which is brighter? (in terms of light output, not intelligence...) A. Yoda B. Chewbacca C. They have the same brightness. D. It s not possible to tell.
Use the graph at right to determine which of the two stars (A or C) has a higher luminosity? A A. Star A B. Star C C. The two stars have the same luminosity. D. It is not possible to determine this.
Use the graph at right to determine which of the following best describes how Star A would appear as compared with Star B? A. Star A would appear more red than Star B. B. Both stars would appear the same color. C. Star A would appear more blue than Star B.
Use the graph at right to determine which of the two stars (A or B) is at a higher temperature? A. Star A is at a higher temperature than Star B. B. Both stars have the same temperature. C. Star B is at a higher temperature than Star A.
Use the graph at right to determine which of the two stars (A or C) is at a higher temperature? A A. Star A B. Star C C. The two stars have the same temperature. D. It is not possible to determine this.
Use the graph at right to determine how the size of Star A compares to the size of Star C. A A. Star A is smaller than Star C. B. Star A is larger than Star C. C. The stars are the same size. D. It is not possible to determine this
The graph at right shows the blackbody spectra for three different stars. Which of the stars is at the highest temperature? t A. Star A B. Star B C. Star C
Spectral Classification Astronomy 1 Elementary Astronomy LA Mission College Spring F2015
PRIMER ON SPECTRAL LINES
Spectroscopy http://www.ibsen.dk/technology/images/design/ modules/spectrometer-sketch.jpg Spectroscope or spectrometer astronomical instrument that breaks light into its component wavelengths Where light is & is not present provides information what an object is made of its physical properties
Blackbody Spectrum continuous light spread across all colors a continuous or continuum spectrum
Emission Spectrumn light is present only in selected parts of the spectrum emission lines bright bands on a black background
Absorption Spectrum light is missing only in selected parts of the spectrum absorption lines dark bands on a blackbody/continuous spectrum
Atomic Structure and Energy Levels http://odin.physastro.mnsu.edu/~eskridge/astr101/kauf5_20.jpg Electrons in an atom (this is Hydrogen) can be modeled as being in specific shells or orbits around the proton. Each shell has a specific energy level Further out & higher numbered shells have a higher energy
Atomic Transitions Electrons prefer to be in lower energy states can be kicked into a higher energy by absorbing a photon with the right energy. right energy, right wavelength E=hc/λ
Atomic Transitions Electrons prefer to be in lower energy states Drops back down by spitting out a photon with the right energy right energy, right wavelength E=hc/λ
Emission Spectrum This is a Hydrogen Emission spectrum Each line is due to a specific transition of electrons from a higher state to a lower state This pattern is specific to hydrogen, like a fingerprint Each element has a characteristic spectrum
Stellar Spectra (visible light portion)
Spectral Class Recall red stars are cooler than blue stars visual color isn t very precise Astronomers turned to spectral properties to classify stars detailed classification based on line pattern Various schemes were tried
PICKERING S HAREM & THE HARVARD SPECTRAL CLASSIFICATION SCHEME
From Cosmos http://www.amazon.com/gp/product/b00k0cdnoi/ ref=avod_yvl_watch_now From Sisters of the Sun Episode 8 7:45 to ~16:00
The Harvard Computers The director of the Harvard Observatory from 1877 to 1919, Edward Charles Pickering hired women to process astronomical data. They were cheaper than men earned less than a clerical worker Willamina Fleming had been his maid
The Harvard Computers "Pickering's Harem" or the Harvard Computers included several now-famous astronomers Annie Jump Cannon, Henrietta Swan Leavitt Antonia Maury
Modern Classification Harvard spectral sequence Developed by Annie Jump Cannon Characteristic absorption lines determine stellar class Note: in Astronomy metal means anything heavier than He
Modern Classification Cecilia Payne (Payne- Gaposchkin) discovered this was actually a temperature sequence More precise than BB peak or color
Spectral Classification From hot to cool: O B A F G K M Each spectral class is further divided into 10 ranges according to temperature. 0 = hot, 9 = cool therefore O0 is the very hottest, and O9 is slightly hotter than B0 and M9 stars are very cool This is still a temperature-based classification
Let s Practice
Star Rue is Type K, Star Peeta is Type B, Star Katniss is Type F and Star Primrose is Type M. Which star is cooler than Rue? A. Peeta B. Katniss C. Primrose D. None of them
You observe a very bright, bluish star. It s spectral classification is most likely. A. B B. G C. M D. More information is needed to determine this
H-R DIAGRAM
Spectral Class isn t everything Spectral Class is not sufficient to uniquely identify a type of star The supergiant Arcturus and the red dwarf Proxima Centauri are both Type M & 3500 K They are definitely not identical!
Hertzsprung and Russell http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_hrintro.html In 1911 Danish astronomer, Ejnar Hertzsprung, plotted the absolute magnitude of stars against their color Independently in 1913 American astronomer Henry Norris Russell plotted spectral class against absolute magnitude showed that the relationship between temperature and luminosity of a star was not random
The H-R Diagram H-R diagram plots Color and/or Temperature against Luminosity and/or Absolute magnitude Any data plotted like this is an H-R diagram, as is a theoretical version The H-R diagram is one of the most important tools in Astronomy
The Main Sequence The long strip from upper left to lower right is called the Main Sequence. (MS) Stars spend most of their existence on the MS 91% of nearby stars are MS stars. MS stars are fusing H into He in their cores.
Giants and Dwarfs
WARM-UP QUESTION
Star A has an absolute magnitude of -8.1 and belongs to spectral class B8. Star B has an absolute magnitude of 11.2 and also belongs to spectral class B8. Which star has the higher temperature? A. Star A B. Star B C. They have the same temperature. D. There is not enough information to determine which star is hotter.
LECTURE-TUTORIAL ON THE H- R DIAGRAM
Star A has an absolute magnitude of -8.1 and belongs to spectral class B8. Star B has an absolute magnitude of 11.2 and also belongs to spectral class B8. Which star has the higher temperature? A. Star A B. Star B C. They have the same temperature. D. There is not enough information to determine which star is hotter.
Let s Practice
A red giant of spectral type K9 and a red main sequence star of the same spectral type have the same. A. luminosity B. temperature C. absolute magnitude
What Type of Star is Aldebaran? A. Red Giant B. Main Sequence C. Supergiant D. White dwarf
What Type of Star is Vega? A. Red Giant B. Main Sequence C. Red Supergiant D. White dwarf
WRAP-UP
Topic for Next Class Stellar Evolution Measuring Distances (time permitting)
Reading Assignment Astro:8 Astropedia:13
Homework HW SS Posted, Due 11/19