COSMOS: UCI – Part 1

COSMOS Cluster 2 as Saturn and Its Moons

COSMOS Cluster 2 as Higgs Boson

This past month (June 24, 2012- July 21, 2012), I attended the COSMOS (California State Summer School for Mathematics and Science) at the University of California, Irvine, with brilliant minds from Northern and Southern California, as well as other states. The 152 students were divided into 8 clusters. I was part of Cluster 2: Astronomy and Astrophysics. With 22 other COSMOS students, I ventured into the world of astronomy and astrophysics unraveled by UCI professors Dr. Tammy Smecker- Hane, Dr. James Bullock, Dr. Aaron Barth, Dr. Erik Tollerund, TAs John Phillips, Liuyi Pei, and Shea Garrison-Kimmel, and Teacher Fellow Lisa Taylor. I discovered that all students shared a strong passion for astronomy and high aptitudes for learning. It has been my honor to learn with the students, listen to the professors’ lectures, and follow the TAs’ instructions for CLEA (Contemporary Laboratory Experiences in Astronomy) Labs.

For a cumulative final, the TAs divided the class into 8 groups for Project Labs:

“Deriving the Mass of Saturn” (By: Angel Guan, Francisco Terrones, and Luis Loza; Directed By: Liuyi Pei)

1. Deriving the Mass of Saturn

“Finding the Angular Velocity of an Asteroid” (By: Rachel Banuelos and Luis Salazar; Directed By: John Phillips)

2. Finding the Angular Velocity of Asteroids

3. Properties of an Eclipsing Binary Star System

  • By: Carlin Liao, Matthew Thibault, Sara Sampson; Directed By: Shea Garrison-Kimmel

4. Determining Stars’ Properties Using Stellar Spectra

  • By: Tina Liu, Noemi Urquiza, John Cabrera; Directed By: John Phillips

“Determining the Properties of Open Cluster M11” (By: Luzanne Batoon, Julian Rose, Janet Lee; Directed By: Tammy Smecker-Hane)

5. Determining the Properties of Open Cluster M11

“Determining the Properties of Globular Cluster M13” (By: Dennis Feng, Maricruz Moreno, Collen Murphy; Directed By: Tammy Smecker-Hane)

6. Determining Properties of Globular Cluster M13

7. Dark Matter in the Universe: Measuring the Rotation of Spiral Galaxies

  • By: Emma MacKie, Danny Tuthill, Michael Cox; Directed By: Shea Garrison-Kimmel)

8. Number Counts of Distant Galaxies and the Shape of the Universe

  • By: David Wong, Thomas Purdy, Joshua Heck; Directed By: Liuyi Pei

“Determining Stars’ Properties Using Stellar Spectra” (By: Tina Liu, Noemi Urquiza, John Cabrera)

 The red, white, yellow, and blue dots in the background represent stars of the H-R Diagram, including main sequence stars, red giants, and white dwarfs.

My Project: Determining the Properties of Stars Using Stellar Spectra (By: Tina Liu, John Cabrera, and Noemi Urquiza; Directed By: John Phillips)

ABSTRACT: Stellar spectra are fundamental in understanding properties — temperature, spectral type, chemical composition, and mass — of stars.  A spectrum is the amount of light that a star emits through narrow slit about 1 Angstrom in width. With the UCI Observatory’s 24-inch telescope and its photograph and ST-8 CCD camera, images of stars’ spectra — those of Arcturus, Vega (HD172167), and HD142780— were taken. Using the software program IRAF, the spectra were extracted, calibrated, and analyzed. Since stars are classified by spectral types, stellar spectra help distinguish a more massive and hotter star from a less massive and cooler star. Analyzing the strengths of absorption lines shows the stars’ compositions of elements such as hydrogen, helium, and calcium. While hotter stars such as Vega are more massive and have strong hydrogen absorption lines, cooler stars such as Arcturus are less massive and have strong neutral metals lines. Understanding stars’ properties leads to a better grasp of the past, present, and future of the Universe.

QUESTION: How can we use stellar spectra to determine the properties of stars such as spectral type, temperature, mass, and chemical composition?

Spectra of different elements including hydrogen, helium, and neon

BACKGROUND INFORMATION: Stars, actually infinitesimally small points of light, appear to twinkle because light refracts at Earth’s atmosphere. Held by gravity, stars shine due to nuclear fusion, its source of fuel. Their lifetimes depend primarily on mass; for their prime of life, stars, travel along the main sequence on the Hertzsprung- Russell, or Color- Magnitude Diagram. A stellar spectrum is the amount of light a star emits at a narrow wavelength interval (about 1 Angstrom, or 10^-10 meters). Each element has a distinguishable pattern of absorption lines (dark bands along the spectrum). Spectral types are a classification scheme developed by Annie Jump Cannon in the late 1800s and early 1900s. The spectral types are ordered in decreasing surface temperatures: O, B, A, F, G, K, M. Originally the classification scheme was A, B, C, D, etc. and stars were ordered according to the strengths of their Balmer (hydrogen) lines. Since stars with the strongest Balmer lines are not necessarily the hottest stars (hotter temperatures caused electrons to be excited and the atom to be ionized- lose electrons), the scheme was rearranged. The two stars analyzed were two variable stars: Arcturus of the constellation Boötes and Vega of the constellation Lyra.

MATERIALS:

  • 24-inch telescope, ST-8 CCD camera
  • Needed Files on Linux (software):
    • uciobs_fear_lowres.dat (from observatory website): List of arc lines used for wavelength calibration
    • arc_red.jpg (from website): Plots of arc images, with wavelengths of prominent lines
    • plotspec.pro : Plots the reduced spectrum and marks absorption lines that are found in LINES.UCI file
    • wave : File containing wave limits of reduced spectra. Needed to run plotspec.pro
    • LINES.UCI : Input file for plotspec.pro that contains prominent absorption lines to be marked on the final, reduced spectrum

PROCEDURES: Independent Variable: Wavelength (Angstroms); Dependent Variable: Intensity

  1. Take pictures of stars using a 24 inch telescope and ST-8 CCD camera
  2. Use DS9 on Linux to analyze and crop the portion of the image planned on using
  3. Edit parameters
  4. Label absorption lines according to reference, with each element specific to its wavelength
  5. Graph the spectrum
  6. Change “pixel” on the x-axis to “wavelength”
  7. Analyze the star’s properties by comparing them to predetermined spectral types.

RESULTS:

Arcturus Spectrum

Arcturus: “K” type star, 4,290 K, 1.5 solar masses, absence of hydrogen lines and abundance of neutral metal lines

Vega spectrum

Vega: “A” type star, 9,600 K, 2.14 solar masses, strong hydrogen lines

HD142780 spectrum

HD142780: “M” type star, 3,000 K, 0.2 solar masses, absence of hydrogen lines and abundance of neutral metal lines.

CONCLUSION: By analyzing the absorption lines on the stars’ spectra, we determined the spectral types of each, thus allowing us to find their respective properties. The absence of hydrogen lines and prevalence of neutral metals in Arcturus’ spectrum allowed us to identify it as a K type star (Figure 1) . Vega’s spectrum contained strong hydrogen and ionized metal lines. Therefore we classified it as an A type star (Figure 2). Because the spectrum revealed absent hydrogen lines and visible neutral metals, we classified it as a M type star (Figure 3).

DISCUSSION: WHY DO STELLAR SPECTRA MATTER FOR THE FUTURE OF ASTRONOMY?

  • Map galaxies
  • Map the Universe
  • Learn about the lifetimes of different stars
  • Use information on old stars to learn about conditions after the Big Bang
  • Learn about what has happened in the Universe since the Big Bang

REFERENCES:

Blumenthal, G., Burstein, D., Greeley, R., Hester, J., Smith, B., & Voss, H. G. (2007). Light, The Tools of the Astronomer, Taking the Measure of Stars. In 21st Century Astronomy. (2nd ed.). (pp. 92-128, 134-158, 380-385). New York, New York, U. S. A.: W. W. Norton & Company.

Kaler, J. B. (2010, July 30). Spectra. University of Illinois. Retrieved July 13, 2012, from http://stars.astro.illinois.edu/sow/spectra.html

Special Thanks to: COSMOS, UCI Professors and Graduate Students, our Teacher Fellow, and Cluster 2: Astronomy and Astrophysics!

COSMOS: UCI – Part 2

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Nebulae and Star Formation

Orion Nebula

Nebulae: a cloud of dust and gas that we see in light

  1. Emission Nebulae or Bright Nebulae: a glowing gas (hydrogen); e.g. Great Nebula in Orion, heated by the Trapezium
  2. Absorption Nebulae or Dark Nebulae: dark dust clouds; e.g. Horsehead Nebula
  3. Reflecting Nebulae: reflecting dust cloud; e.g. Pleiades in Taurus
  4. Planetary Nebulae: excited by central star; e.g. Dumbbell Nebula
  5. Cirrus

STAR FORMATION

Trapezium

Stars form in Giant Molecular Clouds about 100,000 to 1 million solar masses. A few thousand in the Milky Way Galaxy, Giant Molecular Clouds break into denser bits, contract, and eventually form stars. The Orion Molecular Cloud has about 500 stars. The Trapezium and the Orion Nebula have solar masses of matter with young stars.

  1. Non-stellar galactic objects reside in HII regions with molecular clouds of pre-main sequence stars and dense clumps of dust.
  2. Protostars and newborn stars about 1/2 to 1 solar mass reside in Molecular Clouds.

Interstellar Medium – The Material Between Stars

WHAT LIES BETWEEN STARS IN GALAXIES?

– Interstellar Medium

Interstellar Medium

Interstellar Medium is gas and dust between stars, nebulae, and giant molecular clouds (basic building blocks of galaxies in star formation). The four types of matter in interstellar medium are: interstellar dust, interstellar atoms, interstellar molecules, and interstellar snowballs.

Interstellar Dust

  • Interstellar Reddening: dust that scatters blue light and causes stars to look redder
  • Extinction of Obscuration: high dust content that diminishes the brightness of stars, by as much as 25 magnitudes
  • Can be smaller than smoke particles
  • Consists of graphite, silicates, or ices
  • In core of heavy elements (e.g. iron, magnesium), mantle of organic compounds (oxygen, carbon, nitrogen), and outer mantle of ice

RADIO ASTRONOMY

  • Radio waves = longest wavelength of electromagnetic waves
  • Brightest optical objects not necessarily the brightest radio objects
  • e.g. Taurus A (Crab Nebula) and Sagittarius A (center of the Milky Way Galaxy)
  • Radio Spectral Line: the frequency or wavelength at which radio noise is slightly more or less intense
    • Hydrogen: 21 centimeter line
    • Radio spectra lines of molecules
      • OH (hydroxide): 1963
      • H20 (water): 1968
      • NH3 (ammonia): 1968
    • Over 50 molecules in interstellar space
    • Gives information on temperature, density, and motion
    • Molecular absorption line in UV

Interstellar Molecules

  • Molecules: two or more atoms bound together (e.g. H2O, CO, CH4, OH, H2, NH3)
  • Give absorption or emission bands
  • Observable in very cold, low density interstellar environments

Interstellar Snowballs

  • Between the sizes of  grains and comets
  • Composed of water, carbon, silicates, and other molecules

Interstellar Regions

  1. HI region: 200 K
  2. HII region: 10,000 K
  3. Molecular clouds: 50% gas in our galaxy
  4. Hot interstellar medium: 1 million K, super-heated gas from expanding supernova blasts (up to 90% of total volume)
  • HI Region
    • High density of neutral hydrogen atoms about a million atoms per cubic centimeter (e.g. Orion Nebula)
    • ~ 200 K
  • HII Region
    • Hydrogen with electron removed; e.g. ionized hydrogen gas (in emission nebulae)
      • Average density of hydrogen elsewhere is 1 atom per cubic centimeter
    • ~ 10,000 K

Stellar Properties

Stars: Stellar Properties

Stars are balls of gas held together by force of gravity and generate energy and light by nuclear fusion.

A star’s “color,” or wavelength gives information on:

  1. Temperature
  2. Composition
  3. Conditions
  4. Motion (Doppler Shift)
  5. Classification Scheme

What to Measure and How to Classify Stars:

  1. Spectroscopy
    • To determine composition
      • Absorption line produced when an electron absorbs a photon; emission line produced when an electron emits a photon
      • Dual nature of light: light behaves as waves (electromagnetic waves) or as particles (photons)
      • High energy electromagnetic waves are high energy photons, low energy electromagnetic waves are low energy photons
      • Energy of a photon defined by: E = hf, where h is Planck’s constant (h = 6.63 x 10 ^ – 34 joules sec) and f is the frequency of the electromagnetic wave
      • Three Types of Spectra:
        • Continuous Spectrum: appears as a rainbow spectrum
        • Emission Spectrum: appears as distinct color lines, characteristic of chemical elements
        • Absorption Spectrum: appears as black lines on a rainbow background, reverse of emission spectrum
    • To determine temperature
      • All objects give off thermal radiation
      • Peak wavelength corresponds to maximum intensity of radiation
      • Peak wavelength of electromagnetic radiation is related to temperature
      • Wien’s Law: W = 0.00290/T
      • As temperature increases, wavelength decreases
      • The hotter an object, the bluer the radiation
    • To determine density
      • The thicker the spectral line, the greater the abundance of the chemical element present
    • To determine motion
      • Doppler shift of spectral lines
      • Red Shift = moving away
      • Blue Shift = moving closer
    • To determine distance
      • Measured in light years (ly) – distance light travels in one year and parsecs (pc) – one parsec is 3.26 light years
      • Parallax: the only direct measure of stellar distance, the angle across the sky that a star seems to move with respect to a background of distant stars) between two observation points at the ends of a baseline of one astronomical unit (A.U.); a star one parsec from Earth has a parallax of one arc second
  2. Brightness
    • Apparent Brightness
      • Affected By: absolute (true) brightness, distance, intervening space, Earth’s atmosphere, and eyes’ visual response
      • Measured by apparent magnitude “m,” relative brightness as seen on Earth; brightest star (m=1) to faintest (m=6); a 1st magnitude star is 100 times brighter than a 6th magnitude star
    • Absolute Brightness
      • Measured by absolute magnitude “M”
      • The magnitude of a star observed from a distance of 10 parsecs (1 parsec = 3.26 light years)
      • Stars further than 10 parsecs would “appear” brighter; M increases
      • Stars closer than 10 parsecs would “appear” dimmer; M decreases
    • m-M = 5 log (r/10)
  3. Distances
    • Distance as the primary factor in the decrease of stellar brightness as perceived on Earth, used to determine absolute brightness
    • Inverse Square Law: the intensity of light varies inversely with the square of the star’s distance from the Earth
  4. Mass and Size
    • MASS: For binary stars, both the period of revolution of one star orbiting the other and the distance between the two stars can be measured
    • SIZE: Diameters of stars can be determined from temperature and luminosity (calculated from absolute brightness) =>  L  =   σ  T^4  A, where L = luminosity, σ = distance, T = temperature, and A = absolute brightness
  5. Classification Scheme
    • Spectral Types: O, B, A, F, G, K, M; subtypes 0 to 9 (e.g. B1, A4, G2, and M0)
    • O stars are more than 10 times hotter than M stars
    • Developed by Annie Jump Cannon in the late 1800’s
  6. H-R Diagram: to study evolutionary tracks

What are Quasars?

Quasar

QUASARS (Quasi-Stellar Radio Source) [QSR]

  • Appear like stars
  • Emits strong radio signal
  • Distant active nuclei of galaxies
  • A compact region in the center of a massive galaxy surrounding its central supermassive black hole
  • Traveling away from Earth at tremendous speeds (largest Doppler red shifts known)
  • First discovered 20 years ago (3C273) by interaction of optical and radio astronomy
  • In 1963, Maarteen Schmid discovered a quasar with a 16% Doppler red shift (~3 billion light years away)
  • Since then, more than 1,500 quasars discovered with red shifts up to 473%
  • Provide information of the early phases of the Universe
  • Have been found in a cluster of galaxies (“host galaxies”)

Hubble’s Law: large redshift means large velocities of recession = great distances

Star Clusters: An Overview

Star Cluster

STAR CLUSTERS

  • Contain hundreds up to millions of stars
  • Held together by gravitational pull of the stars on one another
  • Stars formed nearly at the same time and the same age

Spiral GalaxiesAnatomy: bulge, disk, and halo

Open Cluster

Open Clusters

  • Contains typically hundreds of stars
  • Irregular shapes
  • Found in the disk region of our galaxy
  • Ages range few million years to few billion years
  • Some young clusters still contain diffuse gas and dust — the material from which the cluster formed

Globular Cluster

Globular Clusters

  • Very dense star clusters
  • Typically 10,000 to 1 million stars
  • Very old — up to about 12-13 billion years old
  • Have much lower abundances of heavy elements than the Sun
  • Found in the halo region of galaxies

*When plotted on the H-R Diagram, star clusters have different turnoff points, or the point where stars being to evolve and die; the turnoff point determines the age of the galaxy

  • Young clusters = turnoff point higher
  • Old clusters = turnoff point lower

Distance to Star Clusters

  • Apparent magnitudes and colors for many stars used to compare with a H-R Diagram that’s calibrated in terms of absolute magnitude

Variable Stars

  • Apparent brightness changes over time
  • Caused by eclipsing binaries or physical condition within a star itself
  • Certain kinds of stars pulsate, or regularly glow and go dark
  • In the “instability strip”: changes in temperature and luminosity, pulsating period ranges from hours to months
  • Light curves: used to plot a star’s luminosity
  • e.g. Mira: long period variable red giant – M3 to M9

Changes in Apparent Brightness of a Cepheid Variable

Cepheid Variables

  • Important class of variables
    • Very luminous super giants
    • Regular light curves with repetition periods of days or weeks
  • Henrietta Leavitt
    • Pulsation period is proportional to the mean absolute magnitude of the star
    • log P α absolute magnitude
      • More luminous Cepheids have larger pulsation periods

Useful in Determining Properties of Star Clusters

  • b = L/ (4∏d²) , where b = apparent brightness, L = intrinsic luminosity, d = distance
  • RR Lyrae Stars: metal-poor horizontal branch stars in the instability strip; common in globular clusters; average absolute magnitude = +0.6
  • Cepheid Variables: period-luminosity relationship; absolute magnitude = -2 to -8 magnitude
  • Type Ia Supernovae: peak luminosity related to the slope of the declining part  of the light curve; at peak of luminosity, absolute magnitude ranges from -17 to -19 magnitude