Astronomy 162:
Introduction to Stellar, Galactic, & Extragalactic Astronomy
Lecture 6: "Starlight, Starbright"
Stellar Brightness
Key Ideas:
- Luminosity of a star:
- total energy output
- independent of distance
- Apparent Brightness of a star depends upon
- Inverse Square Law of Brightness
- brightness decreases as d2
How "Bright" is an Object?
We must define "Brightness" quantitatively.
Two ways to quantify brightness:
Intrinsic Luminosity: Measures the Total Energy Output.
Apparent Brightness: Measures how bright it appears to be as seen
from a distance.
Luminosity
Luminosity is the total energy output from an object.
- Measured in Power Units: Energy/second emitted by the object
(e.g., Watts)
- Independent of Distance
Important for understanding the energy production of a star.
Apparent Brightness
Measure of how bright an object appears to be to a distant observer.
What we measure here on earth ("observable").
Measured in Flux Units:
- Energy/second/area received from the object.
Depends on the Distance to the object.
- Energy spreads out over a larger area the further it travels
from the source
Inverse Square Law of Brightness
The Apparent Brightness of a source is inversely proportional
to the square of its distance:
In Words:
- 2-times Closer = 4-times Brighter
- 2-times Farther = 4-times Fainter
Apparent Brightness of Stars
The apparent brightness of a star depends upon:
- How far away it is (Distance).
- How bright it really is (Luminosity)
Appearances can be deceiving...
Does a star look "bright" because
- it is intrinsically very luminous?
- it is intrinsically faint but located nearby?
To know for sure, you must know:
- the distance to the star, or
- some other, distance-independent property of the star
that clues you in.
Flux-Luminosity Relationship:
Relate Apparent Brightness (Flux) and Intrinsic Brightness (Luminosity)
through the Inverse Square Law of distance:
Measuring Apparent Brightness
The process of measuring the apparent brightnesses of objects
is called Photometry.
Two ways to express apparent brightness:
- Stellar Magnitudes
- Absolute Fluxes (energy per second per area)
Magnitude System
Traditional system dating to classical times (Hipparchus of Rhodes, c. 300BC).
Rank stars into 1st, 2nd, 3rd, etc. magnitude.
- 1st magnitude are brightest stars,
- 2nd magnitude are next brightest,
- and so on...
Faintest naked eye stars are 6th magnitude.
Modern System
Modern version quantifies magnitudes as:
5 steps of magnitude = factor of 100 in Flux.
Examples:
- 10th mag star is 100x fainter than a 5th mag star.
- 20th mag star is 10,000x fainter than a 10th mag star.
Computationally convenient, but somewhat obtuse.
Flux Photometry
Measure the flux of photons from a star using a light-sensitive
detector:
- Photographic Plate
- Photoelectric Photometer (photomultiplier tube)
- Solid State Detector (e.g., photodiode or CCD)
Calibrate the detector by observing a set of "Standard
Stars" of known brightness.
Measuring Luminosity
In principle you just combine
- the apparent brightness (flux) measured via photometry, and
- the distance to the star
using the inverse-square law.
The biggest problem, as usual, is finding the distance...