Methods of Observational Astronomy

• Concerned with recording data about the observable universe

  Observational Astronomy
• A ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time


  Observable Universe
• The practice and study of observing celestial objects with the

  use of telescopes and other astronomical instruments.

  Observational Astronomy
• Designed mainly to gather light and reveal more detail than can be seen with the naked eye

  Optical Telescopes
• Require at minimum two lenses, the objective and the eyepiece.


  Optical Telescopes
• Making the objective larger in diameter can increase the amount of light a lens can gather.

  Light gathering power
• Concerns how well it can discern two distant objects whose angular separation is small, like a pair of twin stars.

  Resolving power
• Field of view must be considered
• Two Types of Optical Telescope:

  Refracting Telescopes

  Reflecting Telescopes
• Use one or more lenses to collect and focus the light from objects in space, forming an image.

  Refracting Telescopes
• An eyepiece lens enlarges, or magnifies, the image of the object.

  Refracting Telescopes
• The basic principle of upon which lenses work is their ability to refract light.

  Refracting Telescopes
• This principle of refraction when applied to lenses results in the formation of images.

  Refracting Telescopes
• Flaw called when the spherical lenses produced in the real world are

  not the same as the ideal thin lenses of physics, resulting in less than perfectly sharp images.

  Spherical Aberration
• It is the result of light of different wavelengths (that is, colors) refracting differently.

  Chromatic Aberration
• Use mirrors to help astronomers see more clear far-away objects in space. A mirror collects light from objects in space forming

  the image.

  Reflecting Telescopes
• This first mirror, which can be very wide, reflects the image to another mirror.

  This smaller mirror reflects the light to an eyepiece lens, which enlarges, or magnifies, the image of the object.

  Reflecting Telescopes
• All modern optical telescopes used by professional astronomers are

  reflectors because:

  there is no chromatic aberration;

  only one mirror needs to be precise; and

  the mirrors do not sag because they can be supported not only on the sides but on the back as well.

  Reflecting Telescopes
• The largest reflecting telescope

  Subaru-Japan National Large Telescope located on Mauna Kea in Hawaii
• Used to produce images of the sky objects by processing the radio ways emitted by them.

  Radio Telescopes
• They collect weak radio light waves, bring it to a focus, amplify it and make it available for analysis.

  Radio Telescopes
• These specially-designed telescopes observe the longest wavelengths of light, ranging from 1 millimeter to over 10 meters long.

  Radio Telescopes
• Telescope in outer space used to observe astronomical objects.

  Space Telescopes
• From space one can view the part of the electromagnetic spectrum

  obstructed by the Earth's atmosphere (infrared, ultraviolet, x-rays,

  and gamma-rays)
• There is no light pollution in space

  
  

  The lack of an atmosphere results in a lack of light distortion due to

  atmospheric turbulence
• Takes sharp pictures of objects in the sky such as planets, stars and galaxies.

  Hubble
• Produced the deepest and sharpest infrared image of the distant universe to date.

  NASA’s James Webb Space Telescope
• By collecting the radiation stars emit astronomers can determine the

  brightness of an object and the spectra of that object (e.g., with a visible light telescope one can determine the color spectrum).
• The brightness can tells us the distance of a star, while the spectrum tells us temperature, mass, chemical make-up, diameter, and distance.
• Information-rich measurement that astronomers do not usually directly look through their large telescopes.

  Spectra
• Collect light which itself is connected to a computer for data analysis.

  Spectroscope
• Instrument that consists of a prism or a grating spreads the incoming beam of radiation into its different wavelengths and some kind of screen to project the spectrum

  Spectroscope
• Device that measures the spectrum of light. Early versions had a slit, a prism, and a screen with markings to indicate various wavelengths or frequencies; later versions were calibrated to electronic detectors.

  Spectroscope
• A smooth gradient of electromagnetic radiation without any gaps e.g., the spectrum of incandescent solids.

  Continuous spectra
• Incomplete spectrum with missing gaps (which appear as dark lines) due to the absorption of a continuous electromagnetic radiation by a cooler medium, like a gas.

  Absorption spectra
• Such absorbed energy can be re-emitted, but the absorbed energy is essentially removed from a telescope’s view.

  Since the cooler, outer gaseous surface of a star tends to absorb the radiation produced in the hotter, inner part, the spectra of most stars are absorption spectra.

  Absorption spectra
• A spectrum that represents all the wavelengths emitted by atoms or molecules.

  Emission spectra
• Astronomers take advantage of something from physics called Wien’s Displacement Law, a mathematical relationship that basically says that the hotter a body (like a star) is, the shorter the wavelength of light will be emitted from it:

  λpeak T = 2.898 x 10-3 m K.

  where λpeak is the peak (i.e., maximum) wavelength that the star emits and T is the star’s surface temperature.
• Hence, a red star, with a maximum wavelength of 966 nanometers, has a surface temperature of only 3000 Kelvin while a blue star, emitting at a maximum wavelength of 290 nanometers, has a surface temperature of 10, 000 Kelvin.
• Stars have been classified into spectral classifications (labeled by a letter) based on their surface temperature. These spectral types also organize the stars by their chemical make-up and their main sequence lifetimes, that is, the lifetime of the star based on calculations of its available fuel and the rate at which it is consuming that fuel (as interpreted by its luminosity).