Motion
and Distances
As technology permitted it was found that stars positions are not fixed and that stars move at various speeds measured in changes of direction in fractions of a second of arc per year. This second of arc is the angular size of a pinhead that is 183 m away (wow, now thats precise, you would think). Many of the fainter stars almost seem to not move at all though because they are so far away and thats why we use them as reference stars to measure others and scientists call this proper motion. A Parallex is another apparent motion of nearby stars caused by the Earth's orbit around the sun. The star seems to shift as the Earth goes from 150 million km on one side of the sun to the other. We also use these Stellar Parallexes to determine astronomical distances. If the shift is 1s of arc both ways, then the star is about 32 million illion km from an observer. This distance is known as the Parsec and is equal to 3.26 light years.
Courtesy of: Peter Garnavich (Notre Dame), 1.2-m Telescope, Whipple Observatory
As technology permitted it was found that stars positions are not fixed and that stars move at various speeds measured in changes of direction in fractions of a second of arc per year. This second of arc is the angular size of a pinhead that is 183 m away (wow, now thats precise, you would think). Many of the fainter stars almost seem to not move at all though because they are so far away and thats why we use them as reference stars to measure others and scientists call this proper motion. A Parallex is another apparent motion of nearby stars caused by the Earth's orbit around the sun. The star seems to shift as the Earth goes from 150 million km on one side of the sun to the other. We also use these Stellar Parallexes to determine astronomical distances. If the shift is 1s of arc both ways, then the star is about 32 million illion km from an observer. This distance is known as the Parsec and is equal to 3.26 light years.
Courtesy of: Peter Garnavich (Notre Dame), 1.2-m Telescope, Whipple Observatory
Brightness,
Luminosity, and Composition
Star's brightness was initially measured by the eye and placed into magnitudes, but around 1900 photographs were also used to measure brightness and it was found that blue stars that appeared to have the same brightness as a red star to the eye, was actually brighter in a photograph. From that point on two magnitudes of brightness were recorded: the scales of visual magnitude and photographic magnitude. After using several different filters and special emulsions, astronomers foudn several other magnitudes of brightness including ultraviolent and infrared. Once photoelectric detectors were introduced, the brightness of stars could be measured with a photoelectric photometer at the focus of a telescope.
The composition of stars was not actually known until the invention of the spectrascope. This invention was used to difract the light rays into different wave lengths. Knowing that each element reflects light rays only at a specific wave length we were able to determine what each star is composed of. Our star, like many others is composed on the surface of mostly hydrogen and helium, iron and calcium all at temperatures of several thousand degrees.
Courtesy of: NASA, ESA, S. Beckwith (STScI) and the HUDF Team
Spectral Type
and Surface Temperature
During the early 20th century a woman by the name Annie J Cannon ecamined throusands of tellar spectra without concern for the atmospheric gases or temperatures in order to classify each intoa a spectrum as A, B, C, ...S depending on the numer of absorption lines found.where A would be the lowest of strong lines and M would be much higher. Later studies found that Cannon's measurements were a measure of surface temperature in the sequence O, B, A, F, G, K, M, R, N, S. These measurements were based upon Plank's formula, which gives the relative emissions of various colors for a heated body. They found that cool stars emitted a red light and that hot stars emitted a blue light. Using a ratio of blue to red light of a star, scientists could determine a relative temperature. The blueest stars were found to be about 30,000 k and red stars could be as low as 3000 k. The sun has a temperature of about 6000 k.
Star's brightness was initially measured by the eye and placed into magnitudes, but around 1900 photographs were also used to measure brightness and it was found that blue stars that appeared to have the same brightness as a red star to the eye, was actually brighter in a photograph. From that point on two magnitudes of brightness were recorded: the scales of visual magnitude and photographic magnitude. After using several different filters and special emulsions, astronomers foudn several other magnitudes of brightness including ultraviolent and infrared. Once photoelectric detectors were introduced, the brightness of stars could be measured with a photoelectric photometer at the focus of a telescope.
The composition of stars was not actually known until the invention of the spectrascope. This invention was used to difract the light rays into different wave lengths. Knowing that each element reflects light rays only at a specific wave length we were able to determine what each star is composed of. Our star, like many others is composed on the surface of mostly hydrogen and helium, iron and calcium all at temperatures of several thousand degrees.
Courtesy of: NASA, ESA, S. Beckwith (STScI) and the HUDF Team
During the early 20th century a woman by the name Annie J Cannon ecamined throusands of tellar spectra without concern for the atmospheric gases or temperatures in order to classify each intoa a spectrum as A, B, C, ...S depending on the numer of absorption lines found.where A would be the lowest of strong lines and M would be much higher. Later studies found that Cannon's measurements were a measure of surface temperature in the sequence O, B, A, F, G, K, M, R, N, S. These measurements were based upon Plank's formula, which gives the relative emissions of various colors for a heated body. They found that cool stars emitted a red light and that hot stars emitted a blue light. Using a ratio of blue to red light of a star, scientists could determine a relative temperature. The blueest stars were found to be about 30,000 k and red stars could be as low as 3000 k. The sun has a temperature of about 6000 k.