Light is the main signal we receive from space. Almost everything we know about space has been found by measuring light.
One of the main ways to measure light is to look at the brightness. Measuring the brightness of something in space isn't as easy as you'd think though.
There are different ways to describe brightness, including luminosity and magnitudes.
(1) Luminosity
This is a more physical measurement. It measures the energy output of the object. This is done by looking at how much light is given out by the object over time.
The units of luminosity are more standard, given in Watts - much like a light bulb! A typical LED light bulb in a house might be 20 Watts - the Sun is 20 million billion billion (2 x 1025) times brighter!
- Luminous vs. Non-Luminous Objects
A luminous object is one that gives off light. In other words, it glows (or shines). To be able to glow, the object must have its own source of energy. For example, a torch shines because of the energy stored in its batteries. Stars shine using energy created by nuclear fusion in their cores. Both a torch and a star are considered to be luminous objects.
If an object is not luminous, we can only see it when it reflects light from somewhere else. Most of the objects we see, such as cars, clouds or even the Moon, are not luminous; it is just that they reflect light from the Sun.
(2) Magnitude
The magnitude of an astronomical object is simply a measure of its brightness. The magnitude unit of measurement is unusual.
The lower (or more negative) the value, the brighter the object is. Also, the scale is logarithmic, not linear. For example, a magnitude 1 star is not 2 times as bright as a magnitude 2 star. The difference in brightness between each magnitude is actually almost 2.512 times. This means that a star with a magnitude of 1 is 100 times brighter than a star with a magnitude of 6 (2.512 x 2.512 x 2.512 x 2.512 x 2.512 = 100).
- Types of Magnitude
There are two types of stellar magnitude: absolute and apparent.
- Absolute magnitude is how bright the object would be if it was a set distance from the Earth. A distance of 10 parsecs is used. A parsec is a distance unit used in astronomy. It is the same as 3.26 light years or 31,000 billion kilometres (19,000 billion miles). Absolute magnitudes let us compare the brightness of different objects.
- Apparent magnitude is how bright the object appears to be from Earth. When objects get further away from Earth, they appear fainter. A star more than 10 parsecs away from Earth will look fainter than its absolute magnitude. A star that is closer will be brighter than its absolute magnitude.
- Measuring Magnitudes
The human eye can detect a difference of 1 magnitude quite easily. Smaller differences are much harder to see, needing a lot of practice.
The night sky today is much harder to observe using only our eyes. Dim stars are often hard to see because of light pollution. Going somewhere far from towns and cities allows us to see many more stars.
CCDs on telescopes, like the Liverpool Telescope, can measure the number of photons detected. This value can be converted into an apparent magnitude or "instrumental magnitude." Be aware that you cannot directly compare this magnitude to those made by other people. This is because it depends on the instruments and software used to measure it.
To compare your values with others, you must calibrate your results. Calibration is usually carried out using a standard star.
Standard stars are stars that have had their magnitude measured very carefully by astronomers. You can measure the brightness of a standard star and use that information to work out the true magnitude of your object. It is then reasonable to compare your data with calibrated data collected by others.
- The Maths of the Magnitude System
The maths of magnitude can be summed up in the equation:
m₁ - m₂ = -2.5 × log (f₁ / f₂)
Where:
- m₁ and m₂ represent the magnitudes of two stars.
- f₁ and f₂ represent their relative fluxes. Flux is the brightness divided by the area used to collect the light (usually the telescope aperture).
For example, imagine two stars visible in the night sky. One star is 100 times brighter than the other. This value of 100 represents the ratio of the fluxes (f₁ / f₂). Since the log of 100 is 2, we can say that:
m₁ - m₂ = -2.5 × 2 = -5
This tells us that star 1 is 5 magnitudes brighter than star 2 (remember that the magnitude scale is inverse).
- The History of the Magnitude Scale
The magnitude scale was invented by an Ancient Greek astronomer named Hipparchus. He gave the brightest stars a value of 1 and the dimmest stars he could see a value of 6.
Since the invention of better telescopes, the magnitude scale has been extended. We now know of objects much fainter than the naked-eye limit of 6. We also include bright objects that have magnitudes less than 1.
For example, the star Vega is given the value of zero. So an object brighter than Vega will have a negative magnitude, and a fainter one will have a positive magnitude. Sirius, in Canis Major, is the brightest star in the night sky and has a magnitude of -1.5.
- Examples of the Magnitude Scale
Object Magnitude Comments Sun -27 Very bright
objectsFull Moon -13 Venus (planet) -4.4 Jupiter (planet) -2.7 Sirius (brightest star in sky) -1.5 Vega 0.0 Hipparchus's
original
brightness scale
(1 to 6)Betelgeuse (star in constellation of Orion) 0.5 Saturn (planet) 0.7 Regulus (star in constellation of Leo) 1.3 Uranus (planet) 5.5 Dimmest star seen with naked eye 6 Pluto (dwarf planet) 14 Objects
only seen
through telescopesDimmest object observable the with Liverpool Telescope 25 Hubble Telescope - Deep Field Observation 30