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Star Luminosity

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Author: Janice VanCleave

What You Need to Know

Luminous means giving off light, and luminosity is a measure of how much light-energy is given off by something, such as a star. Apparent magnitude is the measure of a celestial body's apparent brightness, or how bright a celestial object appears to be as observed from Earth.

How Does Apparent Magnitude Work?

Over 2,000 years ago, the Greek astronomer Hipparchus of Nicaea (c. 190–120 B.C.) designed a number system for ranking stars according to their apparent brightness. The brightest stars were assigned magnitude 1; the faintest stars that can be seen with the unaided eye (that is, under perfect viewing conditions) were assigned magnitude 6. The diagram below of the constellation Aries, the Ram, shows the magnitudes of the stars in parentheses.

As indicated by their lower numbers, the stars Hamal and Sheratan are brighter than Mesarthim.

On the modern apparent magnitude scale, a first-magnitude star is exactly 100 times brighter than a sixth-magnitude star. The modern scale also measures magnitudes greater than 6 and less than 1. The larger the scale number, the dimmer the star. For example, a star with a magnitude of 9 is much dimmer than one with a magnitude of 7. Scale numbers less than 0 are negative. The higher the negative number, the brighter the star. For example, a star with a magnitude of –2 is brighter than a star with a magnitude of –1 or of +2.

What Does This Have to Do with Why Some Stars Look Brighter?

The diagram below shows how light spreads from a source and how bright a light spot would appear at different distances from the source. Each light spot has the same number of lines (six), indicating that the three lights have the same luminosity. The brightness of each light spot is indicated by how spread out the light is. The more the light is spread out, as indicated by the distance between the lines in the spot, the dimmer the light source will appear. In other words, as the distance from the light source increases, the less bright it appears. Because the higher the apparent magnitude number the dimmer the light, an increase in distance from Earth increases a star's apparent magnitude.

Fun Fact

The Sun, with a magnitude of –26.8, is the brightest light in the sky. The Moon and the planets are not luminous. Instead, they reflect the Sun's light. A full moon has an apparent magnitude of –12.6, and the planet Venus, at –4.5, has the greatest apparent magnitude of the rest of the celestial bodies.

Real-Life Science Challenge

The dimmest celestial bodies visible by the human eye under perfectly dark skies are around magnitude +6. With the aid of the Hubble space telescope (a telescope in orbit around the Earth), celestial bodies with a magnitude of +30 can be seen. The Hubble space telescope allows astronomers to view objects at a distance of 60 million light-years.

Experiment

Now, design an experiment to demonstrate how stars with the same luminosity can have different apparent magnitudes.

Hints

• Use identical flashlights to represent stars with the same luminosity.
• Design a way to measure the brightness of luminous bodies from different distances.