All text in black is from other sources, yellow-green is my commentary.

In 1900 Annie Jump Cannon, spectroscopist for the Harvard Observatory, undertook the classification of spectra from nearly 400,000 stars, published in the Henry Draper Memorial Catalog (Draper was the first astronomer to photograph the spectrum of a star)

She developed a classification scheme that is still in use today

Slide CO 16 Annie Jump Cannon_24b

Show spectra of stars

Slide 81a Spectral classification

Characteristics of Spectral Classes for Normal Stars

 

Born on July 4, 1868, she was the daughter of a Congregational minister. She graduated from Radcliffe College in 1892. In 1895 she accepted a position as a research assistant at Harvard College Observatory. During her time at Harvard, she worked with Annie Jump Cannon to measure the visual magnitudes of stars.

Special thanks to the Microsoft Corporation for their contribution to this site. The following information came from Microsoft Encarta: http://www.netsrq.com/~dbois/leavitt.html

Henrietta Swan Leavitt

Leavitt, Henrietta Swan (1868-1921), American astronomer, whose work made possible the first accurate determination of extragalactic distances. While working at the Harvard College Observatory on a survey of Cepheid variable stars (stars the luminosity, or brightness, of which varies in an extremely regular manner) she discovered [in 1908] (1912) that the Cepheids having the greatest average brightness also had the longest periods of variation. When, in 1913, the Danish astronomer Ejnar Hertzsprung accurately estimated the distances of a few Cepheids, the distances of all Cepheids could be calculated from Leavitt's period-luminosity correlation. This method of distance determination greatly increased the scientific knowledge of the physical universe.

"Leavitt, Henrietta Swan" Microsoft(R) Encarta.
Copyright(c) 1995 Microsoft Corporation.

 

Divide the class of N students into small discussion groups on Cepheids (15 min) {answers in brackets}. Each discussion group does one of the problems. Each member of the group should know how to do their calculation when the group is done.

1. In 1784 John Goodricke, a deaf, mute, 19-year-old English Amateur Astronomer discovered that the star Delta Cephei (the 4th brightest star in the constellation Cephius) varied in brightness. Stars of this kind are now called (Type I) Cepheid Variable stars. From the attached light curve for Delta Cephei, , determine its period of oscillation. P = {5.3661 days}

2. For a Type I Cepheid Variable star with a period of 5.4 days, use the Period-Luminosity relation to find its luminosity in "Solar Luminosities," defined as how bright the sun looks from 10 parsecs away. (One parsec is 3.26 light years.) L@10pc pc= {I read about 2200 LSolar, and the relation the graph was derived from was MV = -2.81 log P - 1.43 = -3.48, which is used in 2.512^{[Solar absolute visual magnitude - MV]} = 2.512^{[4.85 - (-3.48)]} = 2149 LSolar. }

3. Delta Cephei appears, on average, 2 1/4 times brighter than the sun would if both were 10 pc away from us.{This is from Feast's ground-based value 3.954 for the absolute visual magnitude V, which he says is more accurate than the Hipparcos value of 4.07, we get 2.28 LSolar = 2.512^{[4.85 - 3.954]}} If we know that it has a true luminosity of L@10pc = 2,150 LSolar, but it has an apparent luminosity of L@D = 2.28 LSolar, it must be further away than 10 pc. Use the inverse square law or to find how much farther than must it be away from us? D = {approximately Sqrt(1000) * 10 pc = 316 pc, or more exactly, Sqrt(2149/2.28) * 10 pc = 306 pc}

The distance to Delta Cephei as measured by Hipparcos was d=1000/3.32 = 301 pc

Harlow Shapley, Levitt’s advisor, studied a different kind of variable star, called RR Lyrae, after the prototype in the constellation Lyra. These are variable stars that have a shark’s tooth variation shape rather than the saw shape of the Cepheid’s wave. These stars all have the same period of about half a day and a luminosity of about 100. By 1915 he found a number of these in the 93 globe-shaped (globular) clusters that he studied outside of the milky-way band, and hence outside of obscuring dust, and determination that globular clusters appear to be centered on a point 8kpc away from the Earth.

Slide 105 Milky Way Galaxy layout

http://www.astro.virginia.edu/%7Eeww6n/astro/MilkyWayGalaxy.html

Our galaxy is 30 kpc (100,000 ly) in diameter, 0.3 kpc (2000 ly) thick in the disk at the distance of the sun, with a bulge about 5 kpc (15,000 ly) thick at the center. We are about 8 kpc (25,000 ly) from the galactic center, about 2/3 of the way to the edge.

led to the unambiguous conclusion that this object lies well beyond the limits of the Milky Way and must therefore be a separate galaxy. Hubble later made important discoveries about the properties of galaxies and what they tell us about the universe as a whole (see Chapter 19). (Niels Bohr Library, California Institute of Technology) http://universe.colorado.edu/universe/tango/figures.qry?function=number&number=18.01

B.Sc., 1979, M.Sc., 1980, Ph.D. (astronomy), 1984, University of Toronto

Cosmic distance scale; extragalactic astronomy; stellar populations of galaxies; cosmology

HST Key Project on the Extragalactic Distance Scale Team

Members

Principle Investigators

Wendy L. Freedman (Observatories of the Carnegie Institution of Washington, Pasadena, California)

Robert C. Kennicutt Jr. ( Steward Observatory, Univ. of Arizona)

http://oposite.stsci.edu/pubinfo/press-releases/94-49.txt

CONTACT: Ray Villard, STScI EMBARGOED UNTIL: 2:00 P.M. EDT (410) 338-4514 Wednesday, October 26, 1994

Dr. Wendy L. Freedman PRESS RELEASE NO.: STScI-PR94-49 Carnegie Observatories (818) 304-0204

HUBBLE SPACE TELESCOPE MEASURES PRECISE DISTANCE TO THE MOST REMOTE GALAXY YET

An international team of astronomers using NASA's Hubble Space Telescope announced today the most accurate measurement yet of the distance of the remote galaxy M100, located in the Virgo cluster of galaxies.

These results are being published in the October 27 issue of the journal Nature. The team of astronomers is jointly led by Freedman, Dr. Robert Kennicutt (Steward Observatory, University of Arizona), and Dr. Jeremy Mould (Mount Stromlo and Siding Spring Observatories, Australian National University).

Dr. Mould noted, "Those who pioneered the development of the Hubble Space Telescope in the 1960s and 1970s recognized its unique potential for finding the value of the Hubble Constant. Their foresight has been rewarded by the marvelous data that we have obtained for M100."

Using Hubble's Wide-Field and Planetary Camera (WFPC2), the team of astronomers repeatedly imaged a field where much star formation recently had taken place, and was, therefore, expected to be rich in Cepheids -- a class of pulsating stars used for determining distances. Twelve one-hour exposures, strategically placed in a two-month observing window, resulted in the discovery of 20 Cepheids. About 40,000 stars were measured in the search for these rare, but bright, variables. Once the periods and intrinsic brightness of these stars were established from the careful measurement of their pulsation rates, the researchers calculated a distance of 56 million light-years to the galaxy. (The team allowed for the dimming effects of distance as well as that due to dust and gas between Earth and M100.)

Many complementary projects are currently being carried out from the ground with the goal of providing values for the Hubble Constant. However, they are subject to many uncertainties which HST was designed and built to circumvent. For example, a team of astronomers using the Canada-France-Hawaii telescope at Mauna Kea recently have arrived at a distance to another galaxy in Virgo that is similar to that found for M100 using HST -- but their result is tentative because it is based on only three Cepheids in crowded star fields.

REFINING THE HUBBLE CONSTANT

These first HST results are a critical step in converging on the true value of the Hubble Constant, first developed by the American astronomer Edwin Hubble in 1929. Hubble found that the farther a galaxy is, the faster it is receding away from us. This "uniform expansion" effect is strong evidence the universe began in an event called the "Big Bang" and that it has been expanding ever since.

To calculate accurately the Hubble Constant, astronomers must have two key numbers: the recession velocities of galaxies and their distances as estimated by one or more cosmic "mileposts," such as Cepheids. The age of the universe can be estimated from the value of the Hubble Constant, but it is only as reliable as the accuracy of the distance measurements.

The Hubble constant is only one of several key numbers needed to estimate the universe's age. For example, the age also depends on the average density of matter in the universe, though to a lesser extent.

A simple interpretation of the large value of the Hubble Constant, as calculated from HST observations, implies an age of about 12 billion years for a low-density universe, and 8 billion years for a high-density universe. However, either value highlights a long-standing dilemma. These age estimates for the universe are shorter than the estimated ages of some of the oldest stars found in the Milky Way and in globular star clusters orbiting our Milky Way. Furthermore, small age values pose problems for current theories about the formation and development of the observed large-scale structure of the universe.

COSMIC MILEPOSTS

Cepheid variable stars rhythmically change in brightness over intervals of days (the prototype is the fourth brightest star in the circumpolar constellation Cepheus). For more than half a century, from the early work of the renowned astronomers Edwin Hubble, Henrietta Leavitt, Allan Sandage, and Walter Baade, it has been known that there is a direct link between a Cepheid's pulsation rate and its intrinsic brightness. Once a star's true brightness is known, its distance is a relatively straightforward calculation because the apparent intensity of light drops off at a geometrically predictable rate with distance. Although Cepheids are rare, once found, they provide a very reliable "standard candle" for estimating intergalactic distances, according to astronomers.

Besides being an ideal hunting ground for the Cepheids, M100 also contains other distance indicators that can in turn be calibrated with the Cepheid result. This majestic, face-on, spiral galaxy has been host to several supernovae, which are also excellent distance indicators. Individual supernovae (called Type II, massive exploding stars) can be seen to great distances, and, so, can be used to extend the cosmic distance scale well beyond Virgo.

As a crosscheck on the HST results, the distance to M100 has been estimated using the Tully-Fisher relation (a means of estimating distances to spiral galaxies using the maximum rate of rotation to predict the intrinsic brightness) and this independent measurement also agrees with both the Cepheid and supernova "yardsticks."

HST Key Projects are scientific programs that have been widely recognized as being of the highest priority for the Hubble Space Telescope and have been designated to receive a substantial amount of observing time on the telescope. The Extragalactic Distance Scale Key Project involves discovering Cepheids in a variety of important calibrating galaxies to determine their individual distances. These distances then will be used to establish an accurate value of the Hubble Constant.

----------------------------------------------------------------------

The Key Project Team on the Extragalactic Distance Scale consists of Sandra Faber, Garth Illingworth & Dan Kelson (Univ. of California, Santa Cruz), Laura Ferrarese & Holland Ford (Space Telescope Science Institute), Wendy Freedman, John Graham, Robert Hill & Randy Phelps (Carnegie Institution of Washington), James Gunn (Princeton University), John Hoessel & Mingsheng Han (University of Wisconsin), John Huchra (Harvard-Smithsonian Center for Astrophysics), Shaun Hughes (Royal Greenwich Observatory), Robert Kennicutt, Paul Harding, Anne Turner & Fabio Bresolin (Univ. of Arizona), Barry Madore & Nancy Silbermann (JPL, Caltech), Jeremy Mould (Mt. Stromlo, Australian National University), Abhijit Saha (Space Telescope Science Institute), and Peter Stetson (Dominion Astrophysical Observatory).

* * * * * * * * * * * *

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

The Wide Field and Planetary Camera 2 was developed by the Jet Propulsion Laboratory (JPL) and is managed by the Goddard Space Flight Center for NASA's Office of Space Science.