Photometry: Measuring the Brightness of Stars

Photometry involves measuring the brightness or flux of electromagnetic radiation from stars and other celestial objects. By using filters astronomers can obtain values that allow them to determine the colour and temperature of a star. In this section will will examine how the brightness of stars is measured, how this can be used to compare stars and the value of filters. We will also look at how the technology used to record celestial details has evolved from the naked-eye astronomy of antiquity, to the importance of photography in the nineteenth and twentieth centuries and the incredible efficiency and power of modern photoelectric techniques.

Syllabus Requirements

4. Photometric measurements can be used for determining distance and comparing objects

• define absolute and apparent magnitude
• explain how the concept of magnitude can be used to determine the distance to a celestial object
• outline spectroscopic parallax
• explain how two-colour values (ie colour index, B-V) are obtained and why they are useful
• describe the advantages of photoelectric technologies over photographic methods for photometry
• solve problems and analyse information using:
  M = m-5 log (d/10)
  and
  I _A/ I_B=100^(m_B-m_A)/5

to calculate the absolute or apparent magnitude of stars using data and a reference star

• perform an investigation to demonstrate the use of filters for photometric measurements
• identify data sources, gather, process and present information to assess the impact of improvements in measurement technologies on our understanding of celestial objects

Physics Stage 6 Syllabus © Copyright 2002 The State of New South Wales (Board of Studies).