# Presentation: 887482

## Astronomers Observe

- Chris Stoughton
- March 16, 2002

## Observation

- Direct or indirect?
- Unaided or with instruments?
- Accidental or planned?
- Immediate or long term?
- Funding!

## First, a Tribute to William Herschel

**1738-1822 **

**Notes:**
William Herschel, destined to become one of the world's great observational astronomers, was born in Germany but lived most of his life in England. 
By the age of 34 he had established himself as a musician of note: teaching music, playing the organ at Bath, giving violin concerts, and composing military music, symphonies, and choral works. 
His leisure hours, however, were devoted to studying foreign languages, philosophy, and mathematics.
 At age 35 his simmering interest in astronomy was fired into action when he read Smith's Compleat System of Opticks and Ferguson's Astronomy. Before long he was constructing his own reflecting telescopes. 

By 1773, Herschel had built a telescope with a focal length of 5.5 ft, and he was now ready to begin observing. 

## Discovery of Infrared Radiation

- In the year 1800, Sir William Herschel “accidentally”discovered the existence of infrared radiation.
- NOTE:  you need to blacken the bulb of the thermometer to get this to work.

## Parallactic Shift

**Notes:**

Between 1774 and 1781, he recorded observations of numerous individual objects. His study of the sky led him to several important discoveries. 

Herschel was searching for double stars with the expectation that a parallactic shift of the brighter component might be detected relative to the fainter and presumably more distant component. Instead, Herschel discovered that one star actually orbited around the other, the first tangible proof that gravity extended to stars. Out of this work came a first in its field the Catalogue of Double Stars. 

The first accurate stellar parallax measurement of 0.32 seconds of arc for the star 61 Cygni was published by the F.W. Bessel in 1838. 

## Uranus

- It had actually been seen many times before but ignored as simply another star.
- The earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri.

**Notes:**
Another program Herschel initiated was the examination of every star in the standard star charts of his day. 

On the night of March 13, 1781, he made the historic discovery of a new planet in the constellation of Gemini, the planet Uranus. This discovery won him international fame and the royal patronage of King George III. After being knighted and awarded an annual stipend by the king, Herschel could devote all his time to astronomy, unhindered by the necessity of earning a living as a professional musician. 

One of Herschel's important discoveries was the Sun's motion in space. From the proper motions of only 13 stars he found that the Sun is moving in space relative to its stellar neighbors toward a point in the constellation Hercules not far from the bright star Vega. 

## Herschel’s Milky Way

**Notes:**
His most ambitious undertaking was an attempt to determine the structure of the Milky Way system. This involved a technique Herschel called "star gauging": making sample counts of the stars in the field of view of his telescope. 

By the time he finished, nearly 20 years later in 1802, he had counted over 90,000 stars in 2400 sample areas. 

Along the way Herschel noted many objects of interest for future astronomers: variable stars, binary stars, dark areas in the Milky Way that looked like holes, irregular bright nebulosities, clusters of stars, and several thousand small nebulous objects or nebulae. 

The last three categories of objects were the subject of his 1802 Catalogue of Star Clusters and Nebulae. 

The Milky Way system, he concluded from his star counts, had the shape of a disk, like a grindstone, having a thickness of about one-sixth its diameter. The Milky Way was marked by many irregularities, and the Sun was located near its center. Later studies confirmed Herschel's deduction that our Galaxy is disk-shaped, but found that the Sun is not near the center and that the system is considerably larger than Herschel supposed. 

## The Galaxy

## The Milky Way in Different Wavelengths

**Notes:**
Radio Continuum:  electrons moving through the interstellar magnetic field at nearly the speed of light. Shock waves from supernova explosions accelerate electrons to such high speeds; Cas A 1⁄4 from left shows diffraction spikes!

Atomic Hydrogen:  The 21-cm emission traces the "cold and warm" interstellar medium, which on a large scale is organized into diffuse clouds of gas and dust that have sizes of up to hundreds of light-years

Molecular Hydrogen:  Column density of molecular hydrogen inferred from the intensity of the J =1-0 spectral line of carbon monoxide, a standard tracer of the cold, dense parts of the interstellar medium. Such gas is concentrated in the spiral arms in discrete "molecular clouds." Most molecular clouds are sites of star formation. (Constant CO/H2)

Infrared:  Most of the emission is thermal, from interstellar dust warmed by absorbed starlight, including star-forming regions embedded in interstellar clouds.

Near IR:  Most of the emission at these wavelengths is from relatively cool giant K stars in the disk and bulge of the Milky Way. Interstellar dust does not strongly obscure emission at these wavelengths; the maps trace emission all the way through the Galaxy, although absorption in the 1.25 micron band is evident toward the Galactic center region. 

Optical:  Dark patches are due to absorbing clouds of gas and dust, which are evident in the Molecular hydrogen and Infrared maps as emission regions. Stars differ from one another in color, as well as mass, size and luminosity. 

X-ray:  In the Milky Way, extended soft X-ray emission is detected from hot, shocked gas. At the lower energies especially, the interstellar medium strongly absorbs X-rays, and cold clouds of interstellar gas are seen as shadows against background X-ray emission. 

Gamma Ray:  At these extreme energies, most of the celestial gamma rays originate in collisions of cosmic rays with hydrogen nuclei in interstellar clouds. The bright, compact sources near Galactic longitudes 185°, 195°, and 265° (right to left, from right) indicate high-energy phenomena associated with the Crab, Geminga, and Vela pulsars, respectively. 

## Herschel -- an Observer

- He made star counts, and found nebulae, etc.
- He analyzed these counts to make a “picture” of the Galaxy, as viewed from the outside – “indirect.”
- He used an instrument, in fact, built his own.
- A famous discovery was made by accident.
- That got him funding!
- His planned observations, which took 20 years, led to a more profound result.

## Annie Jump Cannon

**O**h, Be A Fine Girl--Kiss Me!
- This phrase has helped several generations of astronomers to learn the spectral classifications of stars.

**Notes:**
* Annie Jump Cannon was the eldest of three daughters of Wilson Cannon, a Delaware shipbuilder and state senator, and his second wife, Mary Jump. Annie's mother taught her the constellations and stimulated her interest in astronomy. At Wellesley, Annie studied physics and astronomy and learned to make spectroscopic measurements. 
* On her graduation in 1884, she returned to Delaware for a decade, but became impatient to get back to astronomy. After the death of her mother in 1894, Cannon worked at Wellesley as a junior physics teacher and became a "special student" of astronomy at Radcliffe. 
* In 1896, she became a member of the group that historians of science have dubbed "Pickering's Women," women hired by Harvard College Observatory director Edward Pickering to reduce data and carry out astronomical calculations. 
*Pickering's approach to science was thoroughly Baconian: "the first step is to accumulate the facts."* The accumulating was supported by a fund set up in 1886 by Anna Draper, widow of Henry Draper, a wealthy physician and amateur astronomer. 
*Pickering conceived the Henry Draper Memorial as a long-term project to obtain optical spectra of as many stars as possible and to index and classify stars by their spectra. While the measurements were difficult enough, the development of a reasonable classification scheme proved as much a problem in "theory" (which Pickering was slow to recognize) as "fact accumulation.“
*Bacon saw himself as the inventor of a method which would kindle a light in nature - "a light that would eventually disclose and bring into sight all that is most hidden and secret in the universe." This method involved the collection of data, their judicious interpretation, the carrying out of experiments, thus to learn the secrets of nature by organized observation of its regularities. Bacon's proposals had a powerful influence on the development of science in seventeenth century Europe. Thomas Hobbes served as Bacon's last amunensis or secretary. 

## At Work in  Cambridge, MA

**Notes:**
The analysis was begun in 1886 by Nettie Farrar, who left after a few months to be married. Her place was taken by Williamina Fleming, the first of Pickering's female crew to be recognized in the astronomical community at large. Fleming examined the spectra of more than 10,000 stars and developed a classification system containing 22 classes. The work was carried further by Antonia Maury, who developed her own classification system. The system was cumbersome by comparison with Fleming's, and Pickering could not sympathize with Maury's insistence on theoretical (what we would today call astrophysical) concerns that underlay her scheme. 

A group of women staff members directed by Mrs. Williamina Fleming, circa 1890. Standing--Fleming and Edward C. Pickering, sitting in front--Evelyn Leland, at rear left--Antonia Maura. (Courtesy Curator of Astronomical Photographs at Harvard College Observatory) 

It was left to Annie Jump Cannon to continue, beginning with an examination of bright southern hemisphere stars. To these she applied yet a third scheme, derived from Fleming's and Maury's, an "arbitrary" division of stars into the spectral classes O, B, A, F, G, K, M, and so on. It was as "theory-laden" as Maury's ordering, but greatly simplified. Her "eye" for stellar spectra was phenomenal, and her Draper catalogs (which ultimately listed nearly 400,000 stars) were valued as the work of a single observer. 

## Stellar Spectra

**Notes:**
In 1901, Annie Jump Cannon noticed that stellar temperature was the principal distinguishing feature among different spectra. 
Re-ordered the ABC types by temperature instead of Hydrogen absorption-line strength. 
Most classes were thrown out as redundant. 
Left with the 7 primary classes we recognize today, in the order: 
O B A F G K M 
Later work by Cannon and others added the classes R, N, and S.

Cecilia Payne-Gaposhkin Harvard graduate student in the 1920s. In 1925, her dissertation, published as the book Stellar Atmospheres was the breakthrough work in understanding stellar spectra. 
The first comprehensive theoretical interpretation of spectral spectra. 
It was based on the then new advances in atomic physics. 
Put our understanding of stellar spectra on a firm physical basis. Her work showed for the first time that all stars were made of mostly Hydrogen and Helium and small traces of all the other metals. 

## Cecilia Payne-Gaposhkin

**Notes:**
Cecilia Payne was born in Wendover, England in 1900. 

In 1919 while at Newham college at Cambridge, she became interested in astronomy after hearing a lecture by Professor Eddington about his eclipse expedition to Brazil.

Because astronomy continued to be seen as a branch of mathematics she was unable to change her major field of study to astronomy from physics. She continued however to attend Eddington's lectures. When she finally confessed her wish to become an astronomer to Eddington his response was, "I can see no insuperable objections." 

After graduating from Cambridge she became concerned about the future for women in astonomy careers in England. She chose to head toward the United States where she thought a woman might be more accepted. She received a fellowship to study at Harvard Observatory and so she headed across the seas to continue her career.

Payne quickly settled in among the women at the Harvard Observatory, working there under the director Harlow Shapley. She quickly began an investigation of the stellar spectra being compiled for the Henry Draper catalog. In 1925 Cecilia Payne became the first person, woman or man, to receive an Ph.D. in astronomy from Harvard.

Shapley had attempted to get her a Ph.D. in the already existing physics department, but the chair refused. To get around this roadblock she received her Ph.D. in astronomy instead. Her thesis, later published as the observatory's first monograph, Stellar Atmospheres, A contribution to the Observational Study of High Temperature in the Reversing Layer of Stars was labeled at the time and for many years afterwards as "the most brilliant Ph.D. thesis ever written in astronomy.“

In this thesis Payne calculated a temperature scale to match the classification system which Annie Cannon had developed. She also theorized about the composition of the stars. She suggested that the stars were mostly hydrogen. However, when Eddington heard this theory he told her that she was wrong because astronomers at the time felt that all celestial bodies had very similar compostions. As a result, Payne wrote in her thesis that her results were improbable and probably wrong. Today, of course, we know her results were actually fairly accurate. 

## Annie and her Colleagues

- Data Collection (direct observation) and Data Analysis (indirect observation) both have to be accomplished.
- Pickering had the observational horsepower to do “fact accumulation” but he did not appreciate the value of theory.
- “Astrophysics” brings down-to-earth physical laws to the sky.
- The flood of “accidental” observations turn out to make sense only after classification.
- Funding provided by Anna Draper led to the “Henry Draper Memorial.”
- Interpreting results using “astrophysics” yields enormous payback!

## The Big Bang

- A long time ago, the Universe we live in was incredibly small, dense and hot. Many different observations support this view and it now appears that the Universe underwent a very dramatic event right at the beginning, known as the **Big Bang**.
- Different estimates put the Big Bang some 13 to 17 thousand million (13 - 17 billion) years ago.
- Since then the Universe has expanded, cooled down and has become much less dense.

## Observational Pillars 

- Expansion of the Universe
- Origin of the cosmic background radiation
- Nucleosynthesis of the light elements
- Formation of galaxies and large-scale structure
- The Big Bang model makes accurate and scientifically testable hypotheses in each of these areas and the remarkable agreement with the observational data gives us considerable confidence in the model.

## Expansion of the Universe

**Velocity = H * distance**

**Notes:**
Vesto M. Slipher was taking spectra of spiral nebula in 1894, to see if they contained elements consistent with life!  (Percival Lowell hired him.)

Slipher took 40 spectra over 20 years!  (Another triumph of observing stamina.)
He noticed that ALL of the spectra where shifted to the red.
He announced this in 1914.

Hubble collaborated with Hilton Humason, who “dropped out of school in the 8th grade and began his astronomical career by driving a mule train up the trail on Mount Wilson to the observatory.

The units are in km/sec per Mpc
So Hubble got the following value:

H = velocity/distance = 1000 / 2 = 500

Well, this is close – a “modern” value is 70 +/- 7; previous measurements ranged from 50 to 100.

So now, if we accept that Hubble law, all we have to do is measure the redshift (recessional velocity) and we get a measure of the distance to a galaxy.

## Cosmic Microwave Background

**Notes:**
The case for an isotropic and homogeneous Universe became much stronger after Penzias and Wilson announced the discovery of the Cosmic Microwave Background in 1965. 

They observed an excess flux at 7.5 cm wavelength equivalent to the radiation from a blackbody with a temperature of 3.7+/-1 degrees Kelvin. 

The Kelvin temperature scale has degrees of the same size as the Celsius scale, but it is referenced at absolute zero, so the freezing point of water is 273.15 K.

A blackbody radiator is an object that absorbs any radiation that hits it, and has a constant temperature. 

Many groups have measured the intensity of the CMB at different wavelengths. Currently the best information on the spectrum of the CMB comes from the FIRAS instrument on the COBE satellite, and it is shown.

## Big Bang Nucleosynthesis

**Notes:**
Utilizing only the known particles (e.g., assuming three species of massless neutrinos) and assuming the correctness of General Relativity, the Big Bang Nucleosynthesis (BBN) predictions of the standard model depend on only one parameter, the ratio of nucleons to photons, or equivalently, the density of baryons since the number density of photons is known from measurements of the cosmic microwave background radiation. 

The quantitative success of the standard BBN model for the observed abundances of D, He-3, He-4, and Li-7, whose values span some nine orders of magnitude, has stimulated more detailed comparisons of predictions with data.

Among the goals of such comparisons, key is that of testing in detail the consistency of standard BBN - is there a unique value of baryon density such that the predicted abundances all agree with the observations? 

A related goal is to subject to the same detailed scrutiny alternate cosmological models (e.g., inhomogeneous nucleosynthesis) as well as variations on the standard model of particle physics (e.g., additional species of light neutrinos and/or the existence of massive, unstable exotic particles). 

## Observational Cosmology

- The observations we make are indirect – based on compiling a large number of direct observations.
- The types vary:  recessional velocities, spectrum of microwave “noise”, and the amount of Hydrogen, Helium, Deuterium, and Lithium.
- The instruments are a key here – developing instruments is an important, and at times neglected, endeavor.
- Slipher “accidentally” saw (but did not realize it) that the Universe is expanding, while looking for ET.
- Cosmological observations are planned, and long term.
- Is the funding worth it?

## Industrial Astronomy -- SDSS

**Cover 1⁄4 of the sky**
**Images of 300 million objects**
**Spectra of 1 million objects**

## 400 people; 12 institutions

- Data Collection will continue until June, 2005
- The “early data release” is available at skyserver.sdss.org,
- Along with educational modules
- Please, if you want to help, use these, evaluate them, and let us know
- How to improve them!

## SDSS Web Sites

- http://www.sdss.org/ -- general information
- http://www.sdss.org/gallery/gal_zqso.html -- how quasars change with redshift
- http://skyserver.fnal.gov  – outreach and education
- Please critique and send your question, comments, complaints, praise, and bribes to me, Chris Stoughton, stoughto@fnal.gov