Section 1 of 5Luminosity
Luminosity is a property of the star itself and does not depend on where the observer is.
Definition
- Luminosity is the total amount of light energy a star emits each second, so it measures the power output of the star.
Dependence
- A larger and hotter star radiates more energy each second, so it has a higher luminosity than a smaller, cooler one.
Solar units
- Luminosities are often quoted relative to the Sun, so a star of luminosity 100 radiates one hundred times the Sun's power.
Not what we see
- Two stars with the same luminosity can look very different in the sky if they are at different distances from Earth.
Section 2 of 5Apparent Magnitude
Apparent magnitude describes how bright a star looks from Earth.
Apparent Magnitude
Definition
- Apparent magnitude is the brightness of a star as it appears to an observer on Earth.
Two factors
- It depends on the luminosity of the star and on how far away the star is from the observer.
Backwards scale
- The scale runs the opposite way to what is expected: the brighter the star, the lower its apparent magnitude number.
Limitation
- Apparent magnitude alone cannot show how powerful a star really is, because a dim nearby star may look as bright as a distant powerful one.
Section 3 of 5Absolute Magnitude
To compare stars fairly, astronomers imagine them all at the same distance.
Definition
- Absolute magnitude is how bright a star would appear if it were placed at a standard distance from Earth.
Standard distance
- The standard distance is 10 parsecs, which is about 32.6 light-years.
Purpose
- Removing the effect of distance means differences in absolute magnitude show real differences in luminosity.
Same scale direction
- A lower absolute magnitude means a more luminous star, in the same way that a lower apparent magnitude means a brighter-looking star.
| Quantity | Depends on | Use |
|---|---|---|
| Luminosity | Star only | True power output |
| Apparent magnitude | Luminosity and distance | How bright it looks from Earth |
| Absolute magnitude | Luminosity only | Fair comparison of stars |
Section 4 of 5Using the Ideas
1
Observe a star and record its apparent magnitude, which is how bright it appears from Earth.
2
Work out how far away the star is, because distance changes the apparent brightness.
3
Calculate what the brightness would be at the standard distance to find the absolute magnitude.
4
Compare absolute magnitudes of different stars to decide which has the greater luminosity.
Section 5 of 5Exam Checklist & Tips
5.2.4 – Key Points for the Exam
- Luminosity The total light energy emitted by a star per second.
- Apparent magnitude The brightness of a star as seen from Earth.
- Absolute magnitude The brightness a star would have at the standard distance of 10 parsecs.
- Light-year The distance light travels in one year.
- Parsec
- A unit of distance used in astronomy;
- 10 parsecs is about 32.6 light-years.
- A unit of distance used in astronomy;
Common Mistakes
- Assuming the star that looks brightest in the sky must be the most luminous, ignoring distance.
- Thinking a larger magnitude number means a brighter star; the scale is reversed.
- Confusing luminosity, which is an energy output, with apparent magnitude, which is an observation.
- Forgetting that absolute magnitude uses a fixed standard distance for every star.