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Astronomical Measurements and Observations Revision Notes

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22.7Astronomical Measurements and Observations


In these revision notes for Astronomical Measurements and Observations, we cover the following key points:

  • What are the most common methods for measuring long distances in the sky?
  • Which method is used to measure short/average/long astronomical distances?
  • What are the four units used to measure astronomic distances? What are the conversion factors between them?
  • What are the instruments used to obtain information from celestial bodies?
  • How big is the visible universe?
  • Which optical instruments operate in the spectrum of visible light?
  • Which instruments use the invisible light to obtain useful info from cosmos?
  • How does a telescope work?
  • What is space telescope? What advantages does it have compared to other telescopes?

Astronomical Measurements and Observations Revision Notes

There are three methods used by scientists to measure astronomical distances in the sky. When ordered according to the distance of application they are: radar method (for short astronomical distances), parallax method (for average astronomical distances) and Cepheid method (for long astronomical distances).

The Radar method is used to measure distances that are no longer than the dimensions of our Solar System. It is based on the measurement of the time needed for a short light pulse to make a cycle of one-dimensional round-trip. The Radar method is used to measure distances not longer than Solar System dimensions.

The Parallax method is based on the change in the observation angle of a star in two different periods of year due to the revolution of the Earth around the Sun. The best measurement is taken during solstices (December 22 and June 22) when observed at the same time of the day, as in this case the Earth is at the ends of the large axis of ellipse. The equation used to calculate the distance d of a star from Earth through parallax method is

a/2 = d ∙ tan p

where a/2 acts as the opposite legs of the angle p in the right triangle involved and d is the hypotenuse of this triangle. We know from trigonometry that for small angles, we can use the approximation

tan p ≈ p

where the angle p is given in radians. Hence, we can write

a/2 = d ∙ p

A Parsec (pc) is a unit of distance used to measure very long astronomic distances. In scientific terms, one parsec is the distance that corresponds to a parallax angle of 1 second. There are four units available for measuring distances in the sky: kilometres (km), astronomic units (au), light years (l.y.) and parsec (pc). The conversion factor between all of them is

1 pc = 2.063 × 105 au = 3.09 × 1013 km = 3.26 l.y.

From the definition of a parsec, we can directly compute the distance of a star in parsecs using the formula:

d(pc) = 1/p

The Cepheids method is used for measuring very long astronomical distances. It uses the period-absolute magnitude relationship in Cepheid stars to measure their distance from each other and from Earth. The equation involved in this method is

log d = 1/5 (m - M) + 1

where d is the distance of the given star from Earth (in parsecs), m is the apparent magnitude and M the absolute magnitude of Cepheid star.

Observation of the sky is based on detection of EM radiation incident from sources that produce and emit it. Through various tool, we are able to detect some of this radiation and then process the information to draw relevant conclusions regarding the position, temperature, age, size and additional properties related to stars and other celestial bodies.

Observations of the sky are grouped into two main categories: observations made in the visible spectrum and those made in other spectra of EM radiation. The first category includes observations ranging from those made using the naked eye to modern telescopes operating at visible spectrum, where the most famous are mirror telescopes and lense telescopes.

As for observations made outside of the visible spectrum, they belong to relatively new sub-branches of astronomy developed in the last century. They include:

The first sub-branch of astronomy operating in radio frequencies is known as "radio-astronomy". It was developed between 1932-1936 and allows scientist to study the solar photosphere as well as other celestial phenomena. Telescopes that operate at radio frequencies are known as "antenna" - an analogy to the antennas used to receive radio waves in communication.

"Infrared astronomy" was developed during the 1960s. IR telescopes allow us to observe interstellar dust and clouds in a given galaxy.

Other types of astronomy based on the study of invisible EM waves use satellites to raise telescopes above the atmosphere to avoid disturbances caused by environmental factors. Today there are UV astronomy, X-rays astronomy and Gamma rays astronomy - all sub-branches developed after the 1960s. The study of UV spectrum allows us analyse the spectra of many chemical elements that form various celestial bodies including stars and galaxies. X-ray astronomy detects very hot regions and the environment between two stars that move in pairs, where one of them is a white dwarf, neutron star or black hole. This sub-branch of astronomy is the key factor that allows us to identify black holes in the universe. Gamma astronomy on the other hand, deals with very energetic processes in the universe that are able to emit high frequency radiation.

The Hubble telescope, launched into space in 1990, operates in the visible and UV spectrum. The main part of this telescope consists on a spherical mirror which has a 2.4 m diameter

Another technology used in this way is spatial probes. These cosmic machines carry all the necessary tools on-board for the observation and analyse of celestial objects. They usually make a one-way journey from Earth to other celestial bodies and transmit images and videos obtained during their flight. This occurs for several year until they are lost in space and can no longer send information to laboratories on Earth.

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